Search for 750 GeV Resonance in MonoZ channel at CMS Experiment

by

Kamal Lamichhane, B.S.

AThesis

In

Physics

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of

MASTER OF SCIENCE

Approved

Dr. Shuichi Kunori Chair of Committee

Dr. Nural Akchurin

Dr. Luis Grave de Peralta

Mark A. Sheridan Dean of the Graduate School

December, 2016 c 2016, Kamal Lamichhane Texas Tech University, Kamal Lamichhane, December 2016

ACKNOWLEDGEMENTS

First and foremost, I would like to express my kudos to my mentor and supervisor Shuichi Kunori. I can’t express in words how much he means to me for all the pain he has gone through while grooming my knowledge and understanding of underlying fundamental principles. A big thank you for his e↵orts to tag the skills of physics analysis on me. This analysis is quite unusual in a sense that it had a very short life span (less than a year) content, and he picked the channel that no one in the collaboration chose. In addition, this work had a strict time constraint as there was a very high competition with it both inside and outside of collaboration. Dr. Kunori’s desire to pursue this challenge was the major reason for the existence of this thesis, and I extend to him a big thank you for the trust he put in me in this regard and for the freedom he provided, even to test his own ideas. Of course, in the end I was wrong every time; but this freedom helped me to learn tremendously. His trust, support, and guidance has been very inspiring to me. It was an honor to work for such a first-rate individual, and I hope he is pleased with this work. I am very thankful to Nural Akchurin for his teaching, support, guidance, and encouragement both in the professional and personal sector. It was a great blessing to be a student of a person with such great idealism, knowledge, inspiration, clear-thinking, and calming influence. I am blessed to have him as a committee member of this thesis. IwouldliketoexpressmysinceregratitudetoLuisGravedePeraltaforkindly accepting my request that he become a thesis committee member. I have not yet had a chance to work with him directly; however, many of his students shared with me how enlightening he is at teaching and mentoring in both professional and personal matters. I would also like to thank Sung-won Lee, Igor Volobouev, Richard Wigmans, and the entire HEP group at TTU. Thank you Chris, Jordon, and Federico for your help. Thanks to my colleagues Tyler and Tielige for their friendship and all those hilarious moments of jokes and chats, including Manchester City and Arsenal’s football game. I would also like to thank Phil Harris from CERN, Adish Vartak

ii Texas Tech University, Kamal Lamichhane, December 2016 from UCSD, Zeynep Demiragli from MIT, Thiago Tomei from SPRACE, and Yalcin Guler from Cukurova University for their help and support. Thank you to the entire TTU physics department for all the support and to my undergraduate students, whom I had the honor to help and teach. I make a special mention to Mahdi Sanati: without your help, I am not sure if I would still be in the graduate school. Thank you for taking the pains to overcome my weakness and helping me to learn physics in an enthusiastic way, which helped me to pass the Ph.D prelim exam. A big thanks to Joel Walker (SHSU), and David Toback (TAMU); without you I don’t think I would be in the field of particle physics. I am indebted to your contribution in my career and life. Thank you for all the support you have provided. Finally, I am thankful to my family for their every support and understanding. Thanks to my wife Usha for all the support, love, encouragement, and especially for asking - “Is HEP just to run the codes?” - you are always inspiring me to push the extent of the knowledge I possess. Bless you for coming into my life.

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TABLE OF CONTENTS

Acknowledgements ...... ii Abstract...... vi ListofFigures...... vii ListofAbbreviations...... ix 1. Introduction and Motivation ...... 1

2. Theoretical Overview ...... 5

2.1 Historicalperspective ...... 5 2.2 The Standard Model ...... 6 2.3 Comments on Constants ...... 9 3.ExperimentalApparatus ...... 11

3.1 TheLargeHadronCollider ...... 11 3.2 The CMS Detector ...... 13 3.2.1 The Coordinate System ...... 15 3.2.2 Tracking System ...... 15 3.2.3 Electromagnetic Calorimeter ...... 16 3.2.4 HadronicCalorimeter...... 17 3.2.5 Muon System ...... 18 3.2.6 TriggerSystemandDataAcquisition ...... 19 4. Experimental Tools and Objet Reconstruction ...... 22

4.1 JetsandJetReconstruction ...... 22 4.2 JetEnergyCorrection...... 23 4.3 BoostedObjectandJetSubstructure ...... 24 4.3.1 JetSubstructure ...... 26 4.4 Missing Transverse Energy and Transverse Mass ...... 30 5. Data/Monte Carlo Samples, and Event Selection/Rejection ...... 31

5.1 Data and Monte Carlo Sample ...... 31 5.1.1 Private RS Graviton Sample ...... 32 5.2 Event Selection ...... 32

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5.3 Various Kinematic Distribution from Signal Selection . . . . . 36 6. Background Estimation, Results, and Systematics ...... 44

6.1 Background Estimation ...... 44 6.1.1 Error on Background Estimation, and Excess events . . . . 47 6.2 Limit Calculation on Production Cross Section ...... 47 6.3 Results ...... 49 6.4 Systematics ...... 51 7. Summary ...... 52

Bibliography ...... 53

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ABSTRACT

A search for the 750 GeV resonance on ZZ channel is presented with the 2016 1 ICHEP dataset of 12.9fb from pp collision at CMS experiment at LHC with the center of mass energy of 13 TeV. One Z decays to hadrons (jets), and the other decays to neutrinos (Missing energy). This work was motivated by the observation of the hint of resonance of 750 GeV mass on diphoton channel with 2015 data from both CMS and ATLAS experiment. Since the Z decays to a merged jet, jet substructure techniques are utilized for Z-tagging, which results in better signal selection.

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LIST OF FIGURES

1.1 Diphoton excess results from CMS on 2015 ...... 3 1.2 DiphotonexcessresultsfromATLASon2015 ...... 3 1.3 BranchingratioofHiggsfromref[32] ...... 4 2.1 Radioactivity Experiment from ref [40] ...... 6 2.2 Standard Model of Particle Physics ...... 7 3.1 DiagramoftheCERNAcceleratorComplex ...... 12 3.2 Diagram of the CMS Detector ...... 14 3.3 CMS Tracker in rz plane ...... 16 3.4 GeometricviewofCMSECAL ...... 17 3.5 GeometricviewofCMSHCAL ...... 18 3.6 Geometric view of CMS Muon System ...... 19 3.7 Overview of CMS Trigger and DAQ system ...... 20 4.1 OverviewoffactorizedapproachofJECinCMS ...... 24 4.2 Overview of Boosted and resolved scenario from ref [needed] . . . . . 25 4.3 Schematic Representation of Boosted scenario ...... 26

4.4 Analytics of ⌧2 vs ⌧1 ...... 29 5.1 Eciency for ak8 vs ak4 jets for boosted object ...... 35 5.2 EciencyforjetReconstructionforZcandidate ...... 35 5.3 NumberofVertex...... 36 5.4 NumberofAK8Jets ...... 37

5.5 Leading jet PT @(N-1)cut ...... 37

5.6 Leading Jet PT @Ncuts...... 38 5.7 PrunedMass@(N-1)cut ...... 38 5.8 Pruned Mass @ N cuts ...... 39

5.9 L1 Jet ⌧21 @(N-1)cut ...... 39

5.10 ⌧21 vs MPruned @(N-1)cutforsignal ...... 40

5.11 ⌧21 vs MPruned @(N-1)cutforBG ...... 40 5.12MET@(N-1)cut...... 41 5.13 MET @ N cut ...... 41 5.14MT@(N-1)cut...... 42

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5.15 MToutsideofZselection@(N-1)cut ...... 42 5.16 MT @ N cut ...... 43 6.1 Zbar before and after correction ...... 45 6.2 Comparision of shape for Data vs BG ...... 45 6.3 Investigating (S-B), and S/B ...... 46 6.4 Exclusion of various production cross-section of the RS graviton . . . 49 6.5 Excess on channelin2015 ...... 50 6.6 Disappearance of excess in 2016 ...... 50

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LIST OF ABBREVIATIONS

ALICE: A Large Ion Collider Experiment ATLAS: A Toroidal LHC ApparatuS CERN: European Organization for Nuclear Research CMS: Compact Muon Solenoid CSC: Cathode Strip Chamber DAQ: Data AcQuisition system DT: Drift Tube EM: ElectroMagnetic ECAL: Electromagnetic CALorimeter EWK: Electromagnetic and WeaK interaction ES: Endcap preShower PF: Particle Flow HB: Hadronic Barrel HCAL: Hadronic CALorimeter HE: Hadronic Endcap HF: Hadronic Forward HLT: High Level Trigge HO: Hadronic Outer ICHEP: International Conference on High Energy Physics JEC: Jet Energy Correction JES: Jet Energy Scale JER: Jet Energy Resolution L1: Level-1 Trigger LO: Leading Order LHC: Largh Hadron Collider LHCb: Large Hadron Collider Beauty experiment LINAC: Linear Acceleartor MC: Monte Carlo MET: Missing Transverse Energy NLO: Next-to-Leading Order

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NNLL: Next-to-Next-to-Leading-Log NNLO: Next-to-Next-to-Leading Order PAT: Physics Analysis Tool PDF: Parton Distribution Function PF: Particle Flow POG: Particle Object Group PS: Proton Synchrotron PU: PileUp QCD: Quantum ChromoDynamics QED: Quantum ElectroDynamics QFT: Quantum Field Theory RECO: o✏ine RECOnstruction RPC: Resistive Plate Chamber SM: Standard Model SUSY: SUperSYmmetry TEC: The two End Cap detector system TIB: The Inner Barrel detector system TID: The Inner Discs detector system TOB: The Outter Barrel detector system

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CHAPTER 1

INTRODUCTION AND MOTIVATION

From the cosmological perspective, the composition of universe is categorized into three types, Dark energy ( 68%), Dark Matter ( 27%), and the ordinary matter ⇠ ⇠ ( 5%) [1]. The ordinary matter or the known part are ultimately made up of the ⇠ fundamental elementary particles which are experimentally discovered. These elementary particles are very well accommodated in the most promising and successful model of particle physics so far i.e. Standard Model. Standard model was built from the unification of QCD, flavor structure of the fermions, and the Electroweak theory proposed by Salam, Glashow, and Weinberg [2-9]. One after another discovery of the particles incorporated has made it inevitable. The last constituent of it was Higgs boson which was discovered on 2012 at the giant Large Hadron Collider experiment at CERN near Geneva[ 10-11], Switzerland. This discovery bagged the Nobel prize a year later[12], which made this model more elegant and particle physics subject a public fascination. Most of the particles of standard model are discovered at collider experiment. The current biggest accelerator facility is LHC with giant detectors [34]. The necessity of such a giant accelerator and detector to detect such tiny particles is explained on experimental apparatus section. LHC comprise four di↵erent detector with di↵erent purpose: the two biggest detectors, ATLAS [35] and CMS [36] are designed for Higgs boson discovery and dark matter, ALICE [37] is for primordial cosmic plasma at early universe, and LHCb [38] for matter-antimatter di↵erence. Newtonian physics talks about mass in terms of weight, and Einstein’s theory of relativity talks about mass in terms of energy; however, neither of these historic physics explain the origin of mass. Since, particles have mass, and we need the explanation not just to answer the existence of those particles, but ourselves also [13]. Robert Brout, Francois Englert, and Peter Higgs independently presented the phenomena of spontaneously symmetry breaking (SSB) as an answer for the source of the mass of particles[2, 14-16]. SSB requires the existence of another scalar particle which is now widely known as Higgs boson or god particle to non-physicist, the candidate for mass to the elementary particles. Higgs mass was an open window

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(free parameter), which theorists initially predicted that it should be about 1000 GeV, but the ambitious experimentalists with the improved experimental conditions studied its coupling to other particles as a function of mass and concluded that the Higgs mass should be less than 200 GeV [13]. In 2012 this mystery was solved as the Higgs was discovered with mass of 125 GeV. This observed Higgs mass ⇠ m m + ,where is for correction, and it turned out that the h,observed ⇡ h,bare mh mh observed one is the di↵erence of two 30 decimal numbers resulting m 125 h,observed ⇡ GeV. This correction is known as hierarchy problem [16] or naturalness or fine tuning [2]. Standard model (SM) is simply overwhelming; however, it is not enough to understand the universe, nature, and the phenomena associated. SM could not provide answer to the following [13]:

dark matter, dark energy, matter-antimatter asymmetry, and gravity. • It now answers the origin of mass, but doesn’t answer the origin of matter, • We know from the neutrino oscillation that they have mass; however, SM • incorporates neutrinos as massless particle.

the cosmological observation of the enormous size of the universe, and some • more.

There is no any discovery after the Higgs boson to support any of the theoretical models available to explain these issues. Last December, ATLAS and CMS experiment [17-19] both reported the observation of some excess (local significance of 3.6,and2.6 respectively, and we need at least of 5 excess to claim a discovery) of 750 GeV signal on diphoton () channel as shown in figure 1.1 from ref [33] and 1.2 from ref [17]. Since, both experiments have observed this excess, it had brought some excitement in the community, and many people worked on it if we can get 5 excess. I recalled a joke made by G. Isidori on 21 April 2016 at his presentation during CMS Week: ‘Some very negative conclusions have been derived in the last 2-3 years due to the absences of New Physics signals: PHD (= Post-Higgs

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Figure 1.1. Diphoton excess results from Figure 1.2. Diphoton excess results from ATLAS CMS on 2015 on 2015

Discovery) depression syndrome ..but the situation is changing rapidly, thanks to a small DOSE (Diphoton Over- excitement for a Small Excess) of Run-II data’. We know photon is a spin 1 particle, so the 750 GeV resonance can’t be spin 1 particle as Landau-Yang theorem forbids the spin 1 particle to decay to two other spin 1 particle [20-22]. Hence, the options for 750 GeV object to exist is either a spin 2, which is graviton or spin 0, which is Higgs boson. 750 GeV became very enthusiastic because of the following reasons:

If this is spin 0 i.e. Higgs like particle then this could help to answer the • hierarchy problem mentioned above, as the theoretically expected Higgs mass was about 1000 GeV.

It may open an additional window on dark matter hunt, as some studies were • done to extend this to dark matter side [23-28].

It may also help on SUSY manifestation [29-30] or extra-dimensions [31]. • Final inevitable case is, it may even require something completely new • phenomena and physics to justify its existence.

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If 750 GeV would be real spin 0 i.e. Higgs like particle, then from the known the menu of probability of higgs decaying to other particle (branching ratio), at 750 GeV mass from figure 1.1, H ZZ decay should be more than 103 times than the ! H decay [2, 32]. This was the motivation of my work, search for 750 GeV ! resonance on ZZ channel to test if it is higgs like particle. Furthermore, probability of Z boson decaying to hadrons (jets) is 70%, and to neutrinos (⌫)is 20% [2]. ⇡ ⇡ Hence, we require one Z to be invisible i.e. decay to ⌫⌫ or Missing Transverse Energy, and another Z to jets. So, we are directly measuring only one Z i.e. monoZ.

Figure 1.3. Branching ratio of Higgs from ref [32]

In this document, historical perspective of particle physics, and the theoretical overview of particle physics is explained on chapter 2, LHC experiment, and CMS detector are explained on chapter 3, experimental tools and objects of interest such as jets production, and substructure techniques are overviewed on chapter 4, chapter 5comprisedofdataandMontecarlosamplesused,Montecarlotechniquesforprivate production, and signal selection. Background estimation, results, limit calculation, and systematics are discussed on chapter 6, and a summary on chapter 7.

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CHAPTER 2

THEORETICAL OVERVIEW

In this chapter, first the historical perspective of particle physics and then the current framework of particle physics i.e. Standard Model will be discussed.

2.1 Historical perspective A general question that we want to always know in detail is“What particle physics is about?”. It is about particles, and a very ancient question ( 2500 years ⇠ old): Is nature discrete i.e. does matter come with discrete (indivisible) unit which Greeks called “atom”? If so, everything in universe is made up out of something discrete called particles (perhaps a proper name for something discrete). On the other hand, the counter of this question is also ancient and equally legible: Is nature continuous i.e. Is matter uniformly smear out or continuously distributed in nature, which we called field. Field is basically a function in a given space, for example: density of a material, electric field, magnetic field in a given region of space. In some sense (quantum mechanics), neither and both turned out to be right [39]. The first evidence of the matter is discrete came from Chemistry not Physics, which was John Dalton’s idea of mass of a given molecule(or a mole of a given material) comes in discrete multiple of a mass of a mole of hydrogen. Almost everything is made up of a unit of hydrogen in terms of mass. This was one motivation of the periodic table in Chemistry. Evolution of particle physics began in 1896 with the radioactivity experiment of Henri Becquerel as shown in figure 2.1. While observing the radiation from a radioactive source in presence of magnetic field, he noticed the three distinct pattern, one completely undeflected (electrically neutral gamma radiation), two deflected asymmetrically on each side of the neutral one which are positively charged (alpha radiation), and negatively charged (beta radiation). Also, these are the first beam of particles in particle physics. This experiment also served to see the discreteness of particle. On the other hand, the wave nature of the particle was strongly demonstrated by the interference experiment with the interference pattern observed. In the recent era, we know atoms are divisible, and they are made up of protons,

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Figure 2.1. Radioactivity Experiment from ref [40] electrons, and neutrons. Proton and neutron are made up of quarks. Hence, the quarks, electron (leptons) along with force carrier particles (gauge bosons) are the fundamental elementary particle which explains the latest understanding of universe (Standard Model).

2.2 The Standard Model Like the periodic table of elements in Chemistry, we have a categorical collection of elementary particles, governed by some rules which is called Standard Model of Particle Physics. As seen in figure 2.2 [41], SM comprise of two main categories of particles: fermions, and bosons. Fermions are half integer spin particles which are described with Fermi-Dirac statistics, and comes in three generations. On the other hand, bosons are integer spin particles and follow Bose-Einstein statistics. In addition, one can see from the spin perspective that a combination of two fermions give a boson, two boson give another boson, but a fermion with a boson give a fermion. Fermions are the fundamental constituents of matter whereas bosons are the force carriers and responsible for the interactions. Fermions are further categorized to quarks and leptons of six types each. There are three “up type” quarks which are up (u), charm

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2 (c), and top (t) each with electric charge of + 3 ,andthree“downtype”whichare down (d), strange (s), and bottom (b) each with electric charge of 1 .Similarly, 3 among the six leptons: electron (e), muon (µ), and tau (⌧), are negatively charged with charge of 1, and their corresponding neutrinos i.e. electron neutrino (⌫ ), e muon neutrino (⌫µ), and tau neutrino (⌫⌧ ) are electrically neutral. On the boson side, there are gluons (g) for strong interaction, photon ()forelectromagnetic interaction, W ±,andZ for short ranged weak interaction, and lastly the scalar (spin 0) Higgs (H) for the source of mass to other fundamental particles. As seen in the figure, graviton; the force carrier of gravitational force, resides outside of the standard model as gravity is very weak, and we don’t know why.

Figure 2.2. Standard Model of Particle Physics

Generally, the model building in particle physics is done with the input of some conditions, which are symmetries (expressed in terms of mathematical group

7 Texas Tech University, Kamal Lamichhane, December 2016 representation) and fields to a framework(Lagrangian) and the resulting Lagrangian is the formalism of the model. So, using the fields and SU(3) SU(2) U(1) C ⇥ L ⇥ Y symmetries, we get the resulting Lagrangian for SM up to dimension 4 as: = + + ,where is the Lagrangian for the gauge L LKinetic LHiggs LYukawa LKinetic interaction, for the Higgs field/boson, spontaneous symmetry breaking, and LHiggs giving mass to other particles, and is for the mixing of quarks, and for mass LYukawa of fermions via Yukawa interaction in Higgs condensate [2, 16]. One exception is about the mass of neutrinos which standard model assumes massless. This is L basically the concise physical description of Standard Model. So far there is no evidence of the single isolated quark, and the combination or mixing of quark are generally expressed in terms of a 3 3unitarymatrixcalled ⇥ “Cabibbo-Kobayashi-Maskawa (CKM) matrix”. The eigenvalues of CKM matrix sets the mass of the individual quarks [2, 4]. Discussing about the symmetry which is the main ingredient of standard model; each symmetry corresponds to respective interactions such as SU(3) for strong interactions/force mediated by color (red, blue, and green) charged bosons called gluons. Since, gluons are color charged particles, so they interact with each other. Eight 3 3unitaryGell-Mannmatrices,whicharethefundamentalsofthequark ⇥ model are the generators of SU(3) group. In principle, although there are 9 possible combinations; however, the requirement of determinant of SU(special unitary) matrices to be 1 kills the one possibility, also the trace of that one killed matrix is not 0, meaning no any physical significance. This follows the number of generators of a SU(n) to be n2 1; leaving 8 independent Gell-Mann matrices representing the di↵erent color combinations of gluons [2, 4, 42, 43]. As the name represents “Quantum Chromodynamics (QCD)” is the study of the color particles i.e. quarks, gluons and their strong interactions. Furthermore, SU(2) U(1) is for the unified L ⇥ Y electromagnetic and weak interaction called “Electroweak (EWK) interaction” which was introduced by Glashow-Weinberg-Salam [6-8]. The interaction of photon with electron is termed as “Quantum Electrodynamics (QED)” represented by U(1) symmetry [44]. Photons have no charge, hence doesn’t interact with each other. Neutral bosons (Z, g, & ) when coupling with fermions do not change the flavor which is explained mathematically from the resulting diagonalized matrix whereas

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the charged boson W ± contributes on flavor change and the phenomena is explained as “Flavor Changing Charged Current”. In addition, particles are also categorized as left, and right handed. W boson only couples with left handed fermions, this is why standard model is often called a left handed one, but some theorists object to call the standard model a left handed model defending it as just a cause of vector interaction. Bosons in gauge theory are required to be massless as photon is for electromagnetic interaction on U(1) gauge symmetry; however, the W ±,andZ are quite heavy. This is due to SU(2) or Electroweak symmetry breaking mediated by the Higgs field, and this phenomena is widely known as Higgs mechanism. The broken symmetry, mass of W ±, Z, and the existence of Higgs boson are sourced from the massless scalar Nambu-Goldstone boson. Out of four components of the Nambu-Goldstone boson created from Higgs field, three are linked to the Z, W +, and W giving them mass, and the last one is represented as the Higgs boson with spin 0 [2, 16]. One final thing I would like to mention is about a concept in gauge theory i.e. “coupling constant” or the strength of an interaction which is analogous to charge. In microscopic quantum mechanics, charge is a dimensionless number measured in the unit of charge of an electron, and often called as an amplitude of an interaction. The square of amplitude is nothing but a probability. For e.g. e2 is the | | probability for a charge emitting at cathode and absorbed at anode, this is how charge e is explained. In QED, the interaction strength ↵,alsocalled‘Sommerfeld 2 constant’ or fine structure constant is given by ↵ = e ,whichis 1 .Thisis 4⇡ ⇠ 137 simply the probability of emitting one electron when 137 photons hit a cathode tube. This also hint that electromagnetic interaction or force is a weak force [39]. Also, the range of the force is inversely related to the mass of the force carriers; the photon is massless so the range of electromagnetic force is infinite and the W, Z bosons are heavier so the range of the Weak force is very small.

2.3 Comments on Constants In particle physics equations, couple of constants such as speed of light (c) and the planck’s constant (~)aresetto1.Thereasonforthisare:nothingcanmovefaster than light, so c is the upper limit constraint and from the uncertainty principle, ~ is

9 Texas Tech University, Kamal Lamichhane, December 2016 the maximum precision with which we can measure any parameter precisely [39].

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CHAPTER 3

EXPERIMENTAL APPARATUS

2 h In particle physics, E = ~! = mc ,andp = are of very high importance, as they help to answer why particle physics is the way it is, and also why we need to build giant experiments to see tiny particles. Roughly speaking, size of accelerator is inversely related to the size of the particles we want to see. As particles have very small wavelength (), they have very high momentum (p). Hence, they require very high energy to get detected, which can be achieved by accelerating particles close to speed of light in those gigantic accelerators such as LHC at CERN near Geneva, Switzerland.

3.1 The Large Hadron Collider LHC is a gigantic accelerator to accelerate the protons to very high energy and collide them as it happened right after big bang, the starting point of our universe. The schematic of CERN accelerator complex is shown in figure 3.1 from ref [45]. All this starts with a highly compressed hydrogen in a cylinder which are injected in a precisely controlled rate to the source chamber of linear accelerator (LINAC2), where electrons are stripped o↵from the hydrogen leaving only proton. These protons are now accelerated with the electric field. By the time protons leave 1 LINAC2, they are now traveling with the speed of 3 rd of speed of light (c) [34, 46, 47]. From LINAC2, protons goes to booster, and in order to maximize the intensity of beam, protons are split into 4 booster rings. They are accelerated in booster by repeatedly circulating in the presence of the magnetic field (from powerful electromagnet to keep them in circular track) and pulsating electric field at a certain point (to accelerate) analogous to pushing a child in a swing every time he reaches to a certain point. At booster protons are accelerated up to 91.6% of c,and squeezes them closer together. After recombining the packet together they are now sent to proton synchrotron (PS). Let’s consider just two of such proton packets, they are circulated at PS for 1.2 seconds and gain the speed 99.9% of c.Thisspeedis ⇡ very close to the ultimate speed i.e. ‘c’sotheyreachedthepointoftransition,and the energy added from pulsating electric field now can not raise the speed. Instead

11 Texas Tech University, Kamal Lamichhane, December 2016 the increased energy contributes to increase the mass of protons (E = mc2).

Figure 3.1. Diagram of the CERN Accelerator Complex

Here, protons reach the energy of 25 GeV i.e. protons are now 25 times heavier than they are at rest. They are now sent to Super Proton Synchrotron (SPS) ring where protons are more energized to 450 GeV, and finally launched to the orbit (ring) of LHC which lies at 100 meters underground, and has 27 kilometers of circumference. At LHC ring, two vacuum pipes containing protons are traveling in opposite direction. The incoming protons are synchronized with those already circulating by the kickers magnet near injection point at ring. So, one vacuum pipe has clockwise circulating protons, and another has anti-clockwise circulating protons. These counter rotating beam cross over in four places at detector caverns (ATLAS, CMS, ALICE, LHCb), where they can be made to collide, and debris are tracked in detector. SPS injects protons for about half an hour and finally we will have 2808 packets of protons. At LHC ring, some extra energy is added to each

12 Texas Tech University, Kamal Lamichhane, December 2016 protons, whose speed is now so close to c, that it goes round the 27 Km ring over 11,000 times each second, getting a boost of energy from the pulsating electric field. Finally each proton has an energy of 6.5 TeV (although designed one is 7 TeV). Magnets are used to keep the protons in circular ring of LHC, and in order to run this set up properly, the magnets needs to be colder than outer space i.e. magnets become superconducting [48]. The steady magnet finally brings to the collision mode giving the center of mass energy (ps) of 13 TeV. Furthermore, the tubes where particles go around the accelerator are circulating in a vacuum which is similar to interplanetary space. The pressure on beam pipe is about ten times lower than the pressure on moon. We need that low pressure to make sure that the protons go around and collide to other proton rather than hitting some molecules of gas as they go around. Also, our cooling system is cooler than outer space. Our magnet cooling system is only 1.9 degrees above absolute zero temperature whereas the outer space temperature as measured in cosmic microwave background (CMB) radiation is 2.7 degree above the absolute zero degree. So, it is 0.8 degree cooler than outer space (cosmology) with the exception at collision point where the e↵ective temperature is very high.

The rate of events (Ne)forphysicsprocesswithcrosssection is related to instantaneous luminosity(L) of the accelerator by: kN2f Ne = L;andL = ;wherek=numberofbunches,N=numberofproton 4⇡x⇤y⇤ 11 per bunches (1.15 10 ), f = revolution frequency (11.25 kHZ), and ⇤,⇤ is beam ⇥ x y sizes at collision point (horizontal, vertical) = 16 µm.ThedesignedLis1 1034 ⇥ 2 1 cm s , but recently LHC is running so smoothly that it slightly exceeded it’s design luminosity.

3.2 The CMS Detector CMS as seen in figure 3.2 from ref [50] is one of the biggest detectors at LHC. It was primarily designed for Higgs boson discovery, which is now already achieved and the next target is to discover the dark matter candidate [36, 49, 50]. CMS detector is 21 m long, 15 m wide, and 15 m high, which is like a filter where, each layer stops, track or measure varieties of particles resulting from collision. Measuring the energy and momentum of those particles helps for the identification

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Figure 3.2. Diagram of the CMS Detector of particle. The goals of CMS detector are: a high performance system to detect and measure muons as the name stands ‘Compact Muon Solenoid’, high resolution method to detect and measure electrons and photons which is why we have electromagnetic calorimeter, great quality of central tracking system for the precise momentum measurement, and a “hermetic” (air tight) hadron calorimeter to prevent the particles from escaping out. These motivates the necessity of a very powerful magnet, as the curve on the trajectory of a charged particle goes down with higher momentum. Hence, in order to have a precise measurement of even the very high momentum particles like muons, the necessity of the strong magnet is inevitable. This also facilitates on muon momentum measurement both inside the coil by tracking device, and outside by muon chambers. As the name suggests, CMS has the largest solenoidal magnet ever constructed, and the solenoid is a cylindrical coil of superconducting cable cooled to 268.50 C

14 Texas Tech University, Kamal Lamichhane, December 2016 which creates the magnetic field with the supply of the electricity. This solenoid has the dimension of 13 meters in length and 7 meters in diameter; and it produces the magnetic field strength of 4 Tesla which is 100000 Earth’s magnetic field ⇠ ⇥ strength. The beauty of this magnet is, the calorimeters and tracker can be placed inside the coil resulting the overall detector to be “compact”, this is how the name “Compact Muon Solenoid” stands. Beside the solenoidal magnet, CMS detector comprise of tracker, calorimeters (electromagnetic and hadronic), and muon system, and the data from these detector parts are handled through trigger and data acquisition system which are summarized in the respective sections below.

3.2.1 The Coordinate System CMS has the right handed coordinate system with origin at the collision point, x-axis points towards the center of LHC ring, y-axis points up perpendicular to the LHC plane, and the z-axis along the counter-clockwise beam direction [49]. In terms of polar coordinates, radial distance (r) is measured from the origin, the polar angle (✓)ismeasuredfromthepositivez-axis,andtheazimuthalangle()ismeasuredin the x-y plane. Pseudorapidity (⌘)isthemostcommonlyusedcoordinateandis defined as ⌘ = ln(tan( ✓ )). Generally used coordinates are (⌘,). 2

3.2.2 Tracking System As mentioned earlier, a good performance of tracker is necessary for the precise momentum measurement. In addition, with the increasing luminosity and pile up, the precise measurement of the primary interaction point (vertex) makes the role of tracking system crucial. Also, the tracker is helpful to reconstruct the trajectory of not only muons, electrons, and hadrons but also the tracks from decay of short lived particles such as ‘b quarks” which is the major candidate for the B-Physics, to study the matter-antimatter di↵erence in universe. CMS tracker [49, 51, 52] as shown in figure below is of cylindrical shape with dimension of 5.8 meters in length and 2.5 meters in diameter, and sits inside the solenoidal magnet covering the range up to ⌘ =2.5. It is all silicon detector with | | sensitive area of 200m2,andcompriseofpixeldetectoratcoresurroundedbystrip detector.

15 Texas Tech University, Kamal Lamichhane, December 2016

Figure 3.3. CMS Tracker in rz plane

The pixel has three cylindrical barrel layers at a distance of 4 centimeters (cm), 7 cm, and 11 cm respectively with two endcaps at either end. Each layer is split to segments each of 100µm 150µm.Ithas1440moduleswithabout65million ⇥ channels. The strip detector surrounds the pixel, and has 10 layers. It has about 10 millions strips in 15,200 module. Strip detector comprise of 4 inner barrel (TIB) layers configured in shells with 2 inner endcaps (TID) each with 3 small discs. Outer barrel (TOB) comprise of six concentric layers, and lastly the two endcaps (TEC) close the tracker.

3.2.3 Electromagnetic Calorimeter The purpose of ECAL [53] is to measure the momentum and energy of the electromagnetic particles like electrons and photons ()primarilytargetedtodetect the ‘Higgs boson’ (H ). It sits in between the tracker and HCAL. ECAL is ! made up of lead tungstate (PbWO4), and it produces the scintillation light in proportion to the energy of the particle that hits it. ECAL as shown in figure 3.4 above, it comprises of barrel and two endcaps between tracker and HCAL. In addition, it also contains preshower (ES) sitting infront of endcap for additional spatial precision measurement. Barrel comprise of 61, 200 trapezoidal crystals, and on each side (+/ ) consists of 18 ‘supermodules’

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Figure 3.4. Geometric view of CMS ECAL located at 200 interval on and each consisting of 1700 crystals. Barrel covers ⌘ = | | 1.48 with granularity of ⌘ =0.0175 0.0175 , and endcap covers 1.4 < ⌘ ⇥ ⇥ | | < 3withgranularityof⌘ =0.05 0.05. ⇥ ⇥

3.2.4 Hadronic Calorimeter The purpose of HCAL [49, 54, 55] is to absorb/detect the hadron (things made up of quarks and gluons). It also gives the information about the direction of the neutrinos by measuring the missing transverse energy as described in section 4.4. It has 4 parts; barrel (HB) at ⌘ =1.3, endcap (HE) at 1.3 < ⌘ < 3, forward at | | | | 3 < ⌘ < 5.2, and finally outer (HO) which sits outside the magnet as a supplement | | to HB. The purpose of HO is to catch the left over shower that escapes out of HB; that is why it is also called the tail catcher. HB, and HE are the sampling calorimeters (i.e. finds the energy, position, and arrival time of particle using alternative layers of absorber and scintillator material which produce the light pulse as the particle hits and pass through it) made up of brass absorber plates and scintillator sheets as the active medium. Hybrid photo diodes (HPD) is used as the readout instrument to read signal (light) from scintillator. HF has the high radiation damage risk as it receives the most particle flux close to the beam line, so

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Figure 3.5. Geometric view of CMS HCAL it uses steel as absorber and quartz fiber as an active material benefitting from their radiation hardness. HCAL has granularity of ⌘ =0.087 0.087 for ⌘ < 1.6, ⇥ ⇥ | | and ⌘ =0.17 0.17 for ⌘ 1.6. ⇥ ⇥ | |

3.2.5 Muon System CMS muon system [49, 56] are located outside of the calorimeters and magnet as shown in figure 3.6. Since muons have long life, and also the hadrons and electromagnetic particles are already absorbed by calorimeters, so having it outside of magnet helps to get the precise detection of muons and trajectory. Muon system covers up to ⌘ =2.4andconsistsoffollowingthreeparts: | | Drift Tube (DT): DT is located in barrel region up to ⌘ =1.3, and has 4 • | | stations (MB1-MB4).

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Figure 3.6. Geometric view of CMS Muon System

Cathode Strip Chamber (CSC): CSC is located in endcap region up to ⌘ = • | | 2.4, and has 4 stations (ME1-ME4).

Resistive Plate Chamber (RPC): RPC are in both barrel and endcap region. • RPCs are gaseous plate chambers, and are very beneficial as they give good spatial and time resolution, as well as faster timing for signal.

3.2.6 Trigger System and Data Acquisition The purpose of the experiment is to get data to study physics processes. As the data rate and size of the events is very high hence, it is very challenging to store every single data. At CMS the interaction rate is of 1 GHz, the bunch crossing rate is 40 MHz, and average event size of 1MB. CMS has two level trigger system [57-59] which accept and output the data of interest at certain rate after some filtration or

19 Texas Tech University, Kamal Lamichhane, December 2016 selection cuts are applied to those events. Those events are categorized in terms of physics objects such as energy, momentum of electrons, photons, jets, taus, muons, missing energy, etc.

Figure 3.7. Overview of CMS Trigger and DAQ system

Level 1 (L1) trigger system is based on hardware systems i.e. local pattern recognition and energy evaluation on prompt macro-granular information form calorimeters and muon detectors, and these are linked to global trigger. L1 info is sent to detector frontends and read outs by TTC system, and the L1 output rate is 100kHz. The high level trigger (HLT) system which is a part of DAQ is software based. It gets the input from L1 and selects the events with higher purity for o✏ine storage. The daily data production rate is about 1TB. Data Acquisition system (DAQ) manages the overall flow of data. It comprise of detector frontend, readout modules, event builder, and monitoring systems. It

20 Texas Tech University, Kamal Lamichhane, December 2016 assembles the data fragments from the detector fragments with the trigger information and ultimately sends to the data storage center such as tier-0 site at CERN.

21 Texas Tech University, Kamal Lamichhane, December 2016

CHAPTER 4

EXPERIMENTAL TOOLS AND OBJET RECONSTRUCTION

4.1 Jets and Jet Reconstruction Proton consitutes of quarks and gluons which are also known as partons, and the partons together make hadrons. Since, partons have color charge so they cannot exist freely, and hence after the process of fragmentation or hadronisation they combine with other partons to form color neutral hadrons like ⇡,K giving the narrow conical like shape of spray of the hadrons called Jets. The definition of jet is not unique. Since at LHC, we have proton-proton collision, so jet is a de-facto object in physics analysis at LHC [60]. Jets are reconstructed with algorithms, and the most commonly used algorithm at LHC is anti-kt algorithm (one of the sequential recombination algorithm) which is bottom up (from small separation to higher separation of particles) reconstruction approach [61-63]. The main criteria for the algorithm is safety test i.e. algorithm should be infrared safe, meaning if a soft emission or a collinear splitting of a parton (usually gluons) occurs within a jet then the hard jets still remain unchanged. Failing the safety test allows the soft emission to cause the di↵erent (could even be infinite) sets of final jets which lacks the cancellation of infrared divergences from loop integrals by phase space integrals and voids the Kinoshita-Lee-Nauenberg (KLN) theorem [64]. Sequential Clustering Algorithm is based on the distance measurements such as distance between the two (i, and j) particles (dij), distance between a particle i and 2k 2k R2 2k the beam (diB), and is formulated as: dij = min(pti ,ptj ) R2 ;anddiB = pti where R2 =(⌘2 ⌘2)+(2 2), p is the transverse momentum of the particle, and R i j i j t is the radius of the cone of jet. For low pt case we generally consider R =0.4which is called AK4 jet, and for high pt case we use R =0.8 which is AK8 jet and often called merged or fat jet. The parameter k sub-classify the jet algorithm as k =1is k algorithm, k = 0 is Cambridge-Aachen (CA) algorithm, and k = 1isanti k t t algorithm. Since anti k algorithm is the one which is predominantly used at LHC t physics, so I will focus on it. In anti k algorithm, the recombination scheme for N particles in final state is as t

22 Texas Tech University, Kamal Lamichhane, December 2016 follows:

first we calculate the distances d ,andd as mentioned above, and compare. • iB ij Find the minimum distance: If d is minimum than d ,declaretheith • iB ij particle as one jet and then repeat the same first step for other N-i particle.

If d is the minimum to d then combine those two particles, and repeat the • ij iB step 1 again. If we have more than one pair satisfying this condition, then the

one with higher pt gets the priority for combination.

These steps are repeated till all the particles are clustered into jet of desired R. • This algorithm does not restrict the number of jets and overlapping of jets as the distance between two jets dR has the least separation of dR2 = R2.Inorderto justify the safety test let’s say there is an emission of new soft particle i.e. p 0, t ! and since the low pt particles are clustered at the end so either it is reconstructed as asingle0pt jet which don’t have any physical importance or even if combined to the last jet which is the lowest pt jet. Hence, in any case it doesn’t e↵ect the hard jet which is reconstructed first. If we have collinear particles, then they have R2 =((⌘2 ⌘2)+(2 2)) 0, and will be clustered first as suggested from the i j i j ! recombination scheme above without making any e↵ect on jets. These two justification proves that anti k algorithm is always infrared safe. Furthermore, the t recombination scheme has the another important aspect which is the addition of the four vectors of the particles in jet gives mass to the jet, even if the particles of jet are massless. This is significantly noticed in the boosted jet case, where pt is very large, hence the mass of jet seems to be suciently large as well.

4.2 Jet Energy Correction Jet is reconstructed following the procedure as mentioned above; however, the jet energy we get from reconstruction does not necessarily reflect the precise energy of the particles in jets. This is because of various factors such as; pile up (additional semi-hard interactions from di↵erent proton-proton interactions in the same bunch crossing), underlying events (additional semi-hard interactions in the same proton-proton interaction), nonlinearity nature of detector, and some detector noise.

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Hence, CMS has a factorized correction scheme with each sub correction for di↵erent detector and physics cause, and each level of correction are responsible for the remedy of di↵erent e↵ect [65-67]. Each level of correction corresponds to the

Figure 4.1. Overview of factorized approach of JEC in CMS implementation of a scale factor i.e. correction to the four momentum of the jet, and the scale factor depends on the di↵erent quantities such as energy o↵set, pt, ⌘, etc. The level of correction follows sequential order as seen in figure 4.1, where the output of one step is the input to the next. The first correction applied on the reconstructed jet is the L1 correction which corresponds to the pile up and detector noise removal, and are done on an event-by-event or jet-by-jet basis. The second correction applied is combination of L2L3 (often individually called as L2 relative, and L3 absolute), is the correction on measured pt the jet response ( actual )tomakeituniformasafunctionof⌘ (L2 relative), and pt pt (L3 absolute). In this step, jet energy is corrected to particle level with most probable value in barrel region ( ⌘ < 1.3), so that the corrected jet p is equal on | | t average to the pt of the jet from simulation (GenJet). L2 corrections are obtained from pt balance in monte-carlo (MC) QCD back to back dijet sample, and L3 is determined from /Z + jet and multijets samples. The final mandatory JEC at CMS is L2L3 Residuals which is used to correct the jet response di↵erence in data and MC.

4.3 Boosted Object and Jet Substructure Heavy objects (mostly beyond standard model prediction such as Extra-Dimenisons of TeV scale) are favored to decay to the standard model electro-weak bosons. These bosons are often boosted (produced with high pt). The

24 Texas Tech University, Kamal Lamichhane, December 2016 comparison of boosted or non-boosted (at rest) is as follows: for non-boosted scenario it is quite simple to resolve jets (from decaying bosons) and calculate the invariant mass but signals are predominantly swamped under background; whereas, for boosted scenario although the cross-section is reduced but the acceptance is very good. Moreover, the implementation of techniques (jet substructure) helps to reduce the background, and QCD contamination quiet well giving much better signal S background ratio ( B ) for boosted objects. Lets put the light on this with the following example; a heavy object (X of mass MX )isproducedfromppcollision, and then decays to a pair of bosons (Y of mass MY ), which further decay to hadrons (jet) or leptons (z) as shown in fig 4.2 [64].

Figure 4.2. Overview of Boosted and resolved scenario from ref [needed]

MX Final states depend on the ratio of mass rM = .IftherM is large, then the Y 2MY will have high pt from the contribution of MX ,henceYareboostedandtheirdecay products (z) appear to be closer to each other. If z is jet then we call this a merged or fat jet i.e. two z are reconstructed with in the same jet cone of bigger radius

(usually AK8). On the other hand, if rM is small then the Y are approximately at rest, and their decay products (z) are cleanly well separated on detector. This scenario is called resolved case, and jets are well reconstructed even with the small jet cone radius (mostly AK4). Anaiveexampleofboostisshowninfigure4.3[68],whereaparticleofmass(m)

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Figure 4.3. Schematic Representation of Boosted scenario when gets Lorentz boosted ()decaystodaughterparticlesmakingsomeangle between them. Since R is the measure of the angular separation in (⌘,)plane,so for the boost, R = 2m , and the decay products are contained in a single fat jet. min pt If the daughter particle are quark or gluon jets (QCD), then quark and gluons individually are considered massless, but the energy of mother particle is actually contained in the angle of separation.

4.3.1 Jet Substructure Generally the decay of a boosted objected to two prongs (X 1, 2) substructure ! within a radius R is characterized by R m 1 ,wherez is the energy sharing pt pz(1 z) pt1 2m fraction . The smallest R to have two prongs within a jet is given by Rmin pt ⇠ pt [69-74]. The jet is highly contaminated by the many radiations from the splitting of quarks and gluons giving additional prongs like structure in the jet. Mass of the ↵sCF,A 2 2 QCD jet is given by: ⇡ ptjetR ;where↵s is the strong coupling 4 ⇡ constant, and CF = 3 for quarks, and CA =3forgluonsarethecolorfactors N 2 1 (“Casimir”). Gluons having higher color factor (given by 2N for quarks, and N 4 for gluons in SU(3) symmetry, so for SU(3), CF = 3 ,andCA =3)tellsusthatthe splitting of gluons and its contamination is way much greater than that of quarks. Jet mass is one of the important discriminating variables of jet substructure. This is

26 Texas Tech University, Kamal Lamichhane, December 2016 the first discriminating variables we generally use in analysis, for example if we are looking for Z (mass 91 GeV), then making the mass cut let’s say 60 110 GeV not ⇠ only helps to reduce the background, but also reduce the possibility of contribution from the fake jets. In general, substructure study needs to toss out the soft wide angle radiation (QCD dominance), measure the energy fraction (z), identify and test for the number of prongs, and these tasks are performed with algorithm [74]. In 2008, jet substructure technique was introduced to study pp ZH(b¯b)by ! Jonathan Butterworth, Adam Davidson, Mathieu Rubin, Gavin Salam which is widely known as BDRS method [70, 74]. Although the substructure method was in literature before than this; but this one turned out to be the more pronounced and handy. Higgs search on b¯b with this method turned out to be very robust and since then BDRS is the fundamentals of jet substructure techniques. BDRS is basically a recursive, angle based algorithm to de-cluster the boosted object to subjets. This in principle works on three steps as follows:

recursively cluster a jet with nearest neighbor combination of four-vectors in • ⌘ plane till the specified R is reached (CA algorithm).

E2 unwind the jet by requiring : a) with E2 (ycut) z (energy sharing • E1 ⇠ max(m1,m2) factor), b) m < (µcut)(knownasmassdrop) The purpose of mass drop is to divide the jet into two pieces, and to make sure most of the mass is coming from the angle between the jets, not from the object themselves. This is in some sense to test the presence of hard prongs with relatively small or no invariant mass, which is desired to have.

If those requirement fails, throw out the softer subjet (known as gromming), • and continue recursion on harder one. If the test succeeds, we found our jet.

In summary, BDRS method is an algorithmic procedure to unfold, unwind, and find the core boosted object like Z or W or H that might be buried underneath the fat jet, and grooming is a technique to improve mass resolution or signal, and also getting rid of the background that is mostly generated by soft radiation. In current days, the BDRS method is being developed in varieties of manipulations, where grooming techniques like filtering, trimming, soft-drop, pruning and the tagging tools like n-subjettiness are very commonly used. In this work, I used the pruning

27 Texas Tech University, Kamal Lamichhane, December 2016 for jet grooming and n-subjettiness for tagging purpose so I will discuss these two in next subsections. Jet grooming means the active removal of soft wide angle radiation. Softness is tested on parton level instead of hadron level as it is sensitive on hadronization. For example, let’s say we have a pion from pile up (PU) or initial state radiation (ISR), and if we apply some energy cut to remove this pion then this applies to the pion from non-PU also. Hence, the softness test is done on parton level being consistent with collinear and IR safety of QCD.

P runing : The main purpose of pruning is to remove the contamination on jet • mainly due to pile up along with underlying events, and initial state radiation (ISR) e↵ect [71-72]. Pruning follows the similar procedure as in BDRS method i.e.

1. Clustering of jet (originally reconstructed from anti k algorithm) with t CA algorithm as in first step of BDRS method explained before. While

clustering at each step we should check if: zRmin; where z = min(pti,ptj ) ,R = 2m ,andtheprescribedvalueforz is 0.1 pt min pt cut for CA algorithm. [Rmin is also called Dcut in many literature which is basically a distance parameter on jet algorithm] 2. If this is true, do not recombine the softer branch, just remove them. This is called ‘pruning’, and the resulting jet is called pruned jet. Two

parameters : zcut checks for the soft cluster, and Rmin checks the wide angle cluster, and requiring both cut helps to remove soft and wide angle cluster.

N Subjettiness : N-subjettiness (⌧ )teststhesub-leadingradiationor • N number of prong structure [69, 73-74]. Unlike BDRS and pruning; it is not an algorithm; it is an observable, and the value of the observable are obtained

from the four-vector. ⌧N is expressed as:

1 ⌧ = p min R , R , , R N d ti { i,1 i,2 ··· i,N } 0 i X where N is number of prongs, d0 is the normalization factor to make ⌧N

dimensionless, i represents the all particles in jet, and Ri,N is the distance of

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the particle from the subjet (N) axis. This is simply taking all the particles in

ajetwiththeirpt,andweightingtheirpt by the distance to a subjet axis. As, ⌧ is normalized so it’s value lies between 0 and 1. If it is big or 1, then N ⇠ distance to any subjet axis is big as Ri,N is big which is the case of radiation all over the place i.e. more than the number of subjets but if it is small or 0, then the radiations are well aligned along the subjet axis. The ⇠ prescription is to consider all set of subjet axis and choose the particular set of

subjet axis that minimizes ⌧N such that we can find the axis as good as we can lined up in the direction or along the dominant amount of radiation in our

jets. In general, ⌧N 1 should be much larger than ⌧N ,butifthejetaxis doesn’t lie along the prong then this order is reversed or screwed. In case of

QCD, there is no any clear multiple prongs, so ⌧N value is ambiguous. So, if

Figure 4.4. Analytics of ⌧2 vs ⌧1

we plot the ⌧2 vs ⌧1 as in figure 4.4, then, we see the QCD sits along the diagonal line (QCD often has the ambiguous value for both ⌧); whereas, two

prong object like Z, W, or Higgs has ⌧1 >> ⌧2 so sits at the base. Hence, having some proper ⌧ = ⌧2 cut shown by the purple dotted line in figure 4.4, 21 ⌧1 we can remove the QCD contamination and tag the target object with high

precision. Current tradition is to use ⌧21cut < 0.45, which is widely known as ‘high purity’ case. Unlike BDRS and pruning; it doesn’t test for energy

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sharing. One final comment about why to use the ratio ⌧ = ⌧2 instead of 21 ⌧1 individual ⌧N ;thisisbecausetheratioisquasi-boostinvariant,andifwe

boost to di↵erent frame then the individual ⌧N may change dramatically but the ratio of Lorentz boost in numerator and denominator roughly cancels out.

Hence, the ratio ⌧21 shows the quasi-boost invariant property and because of this it ends up being the favorable and better observable in practice.

4.4 Missing Transverse Energy and Transverse Mass

Missing transverse energy(MET) and transverse mass (MT )areimportantobject not only for this work but also in many other new physics searches. MET as the name suggests is the absence of energy in the detector or the energy which we can not and don’t measure directly from the detector. This is because as neutrinos are the carriers of MET and they do not interact with ordinary matter (in this case detector), so we have no means of measuring them directly. Hence, MET is estimated as the negative vector sum of pt of all visible or measured particles in detector [75]. !MET = p ! ti i X Similar to jets, corrections are applied to MET also: type 0 correction is applied for PU removal by doing charge hadron subtraction (CHS), and type 1 correction is propagated from the jets. Furthermore, the data events could have other sources of MET such as beam backgrounds, beam halo, mis-reconstruction, detector noise, and these issues are quiet well taken care by implementing the recommendations of JetMET POG [76] which are: global halo filter, good vertex filter (at least one good primary vertex), HBHE noise filter, HBHEIso noise filter, EE bad SC filter, and ECAL TP filter. Transverse mass is an observable whose invariant value is calculated as [77]:

M = E2 p2 = 2p (MET)(1 cos()) T T t t qX X p

30 Texas Tech University, Kamal Lamichhane, December 2016

CHAPTER 5

DATA/MONTE CARLO SAMPLES, AND EVENT SELECTION/REJECTION

This work is done using the data collected by CMS experiment for the 2016 ICHEP (roughly from the beginning of 2016 till middle of July) at 13 TeV center of 1 mass energy. ICHEP datasets corresponds to the 12.9fb (2016 B to D dataset). The Monte-Carlo (MC) simulation samples used are mainly produced in “Spring16” campaign with Mad-Graph generator [78].

5.1 Data and Monte Carlo Sample The data used are the MINIAOD’s of /MET/Run2016B, /MET/Run2016C, and 1 /MET/Run2016D, which are jointly known as ICHEP dataset with 12.9fb of integrated luminosity. Several MC samples listed in table 5.1 are used to understand their contributions on background. Unless otherwise mentioned, MC samples are produced with Mad-Graph generator for leading order (LO) in QCD, and these samples which include jets are produced bin by bins with respect to the sum of hadronic transverse energy (HT) corresponding to the respective cross-sections for each bin as listed in the table 5.1. Since, this analysis is based on the CMS monojet, monoV analysis framework, so the samples, and the prescriptions for the analysis are followed accordingly [79-80].

Z(⌫⌫)+ jets: is the major background for most of the analysis based on • MET, including this one. This is an irreducible background. Since the “Spring 16” campaign of these samples were not available for all HT bins at the time I settled with the analysis; hence, for only this sample “Spring 15” campaign samples are used instead.

W (l⌫)+ jets: this background is generally reduced by doing lepton veto; • however, it turns out to be irreducible background in the case when leptons are lost beyond detector acceptance. This is the second most dominant background in this work.

Z(ll)+ jets: this background is also known as Drell-Yan (DY) and is quite •

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tricky as the leptons not detected by detector beyond its acceptance mimics the signal events.

+ jets: this background is although not very dominant but has reasonable • contribution.

top (single + tt¯ ): top quark mostly decays to W which then decays to • lepton, and hence contributes reasonable MET on background. This contribution is bigger than the diboson sample. Single top sample is generated with the Powheg generator at NLO, and the tt¯ samples are generated with aMC@NLO and Powheg generator.

Diboson: dibosons such as ZZ, WW, WZ decays one boson to leptons and • other to hadrons (jets) hence, the final state jet + MET contributes to the background in this analysis.

QCD: Since QCD doesn’t have pronounced MET, so they are simply not • considered in this analysis.

5.1.1 Private RS Graviton Sample Randall-Sundrum (RS) graviton decaying to ZZ is used as the motivation for signal interpretation [77, 81]. RS graviton sample for the mass 750 GeV was privately produced using MadGraph5 aMC@NLO [78] and processed through pythia8 [82] during gen-sim process for hadronization. The leading order (LO) production cross-section was 60 pb. Although the production cross-section depends on the width of the RS graviton, it turned out that the acceptance is same for both 6 2 narrow ( =1.4 10 )andwidewidthcase( =5.6 10 )as our final state has M ⇥ M ⇥ MET, and the reconstructed transverse mass distribution with the MET is broad. I used the wide width case with the cross-section of 60 pb.

5.2 Event Selection In this work, the selection criteria is same to the one used by CMS Monojet analysis team for the search of dark matter in MET + jets channel [79-80]: Selection Level-1: Events passing the trigger threshold in the dataset.

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Table 5.1. List of the Monte-Carlo samples

Monte-Carlo samples list MC sample Name Cross-section Cross-section (pb) Order in QCD /ZJetsToNuNu HT-100To200 13TeV-madgraph 280.5 LO /ZJetsToNuNu HT-200To400 13TeV-madgraph 77.7 LO /ZJetsToNuNu HT-400To600 13TeV-madgraph 10.71 LO /ZJetsToNuNu HT-600ToInf 13TeV-madgraph 4.098 LO /WJetsToLNu HT-100To200 TuneCUETP8M1 13TeV-madgraph 1343.0 LO /WJetsToLNu HT-200To400 TuneCUETP8M1 13TeV-madgraph 359.6 LO /WJetsToLNu HT-400To600 TuneCUETP8M1 13TeV-madgraph 48.85 LO /WJetsToLNu HT-600To800 TuneCUETP8M1 13TeV-madgraph 12.05 LO /WJetsToLNu HT-800To1200 TuneCUETP8M1 13TeV-madgraph 5.501 LO /WJetsToLNu HT-1200To2500 TuneCUETP8M1 13TeV-madgraph 1.329 LO /WJetsToLNu HT-2500ToInf TuneCUETP8M1 13TeV-madgraph 0.03216 LO /DYJetsToLL M-50 HT-100To200 TuneCUETP8M1 13TeV-madgraphMLM- 148.0 LO pythia8 /DYJetsToLL M-50 HT-200To400 TuneCUETP8M1 13TeV-madgraphMLM- 40.94 LO pythia8 /DYJetsToLL M-50 HT-400To600 TuneCUETP8M1 13TeV-madgraphMLM- 5.497 LO pythia8 /DYJetsToLL M-50 HT-600ToInf TuneCUETP8M1 13TeV-madgraphMLM-pythia8 2.193 LO /GJets HT-100To200 TuneCUETP8M1 13TeV-madgraphMLM-pythia8 9235.0 LO /GJets HT-200To400 TuneCUETP8M1 13TeV-madgraphMLM-pythia8 2298.0 LO /GJets HT-400To600 TuneCUETP8M1 13TeV-madgraphMLM-pythia8 277.6 LO /GJets HT-600ToInf TuneCUETP8M1 13TeV-madgraphMLM-pythia8 93.47 LO /TTJets TuneCUETP8M1 13TeV-madgraphMLM-pythia8 831.76 NNLO /ST s-channel 4f leptonDecays 13TeV-amcatnlo-pythia8 TuneCUETP8M1 3.4 NLO /ST t-channel top 4f leptonDecays 13TeV-powheg-pythia8 TuneCUETP8M1 44.1 NLO /ST t-channel antitop 4f leptonDecays 13TeV-powheg-pythia8 TuneCUETP8M1 26.2 NLO /ST tW top 5f inclusiveDecays 13TeV-powheg-pythia8 TuneCUETP8M1 35.6 NNLO /ST tW antitop 5f inclusiveDecays 13TeV-powheg-pythia8 TuneCUETP8M1 35.6 NNLO /WW TuneCUETP8M1 13TeV-pythia8 118.7 NNLO /WZ TuneCUETP8M1 13TeV-pythia8 47.2 NLO /ZZ TuneCUETP8M1 13TeV-pythia8 16.6 NLO

Level 1 Trigger: L1 ETM70 which is MET 70 GeV trigger. • High Level Trigger: The HLT path for dataset are: HLT PFMET170 *, • HLT PFMETNoMu[X] PFMHTNoMu[X] IDTight where [X] is for the di↵erent threshold (90, or 100, and an additional backup of 110 GeV) depending on the di↵erent instantaneous luminosity.

Selection Level-2: MET + jets event with no charged leptons

reject the events which does not pass the MET filters mentioned in section 4.4. • require the ak4 leading jet p > 100 GeV and ⌘ < 2.5 • t | | reject the ak4 leading jet events not passing the the cleaning cuts i.e. charge •

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hadron fraction cut, and neutral hadron fraction cut (CHF > 0.1, NHF < 0.8), to remove the residual backgrounds from beam and detector noise.

(jet,MET) > 0.5inordertosuppresstheQCDevents. • veto the leptons (loose electrons, muons or taus) to suppress the EWK • background.

veto the photon (loose one) to suppress the EWK backgrounds • Z(⌫⌫) + jets, W (l⌫) + jets.

veto the b-jet, to reduce the contribution from the top quark background. • MET cut > 200 GeV (consistent with trigger turn on). • Selection Level-3 : Z(⌫⌫)+Z(FatJet) event selection

require the ak8 leading jet p > 250 GeV and ⌘ < 2.4 • t | | ak8 leading jet pruned mass between 60 GeV and 110 GeV •

⌧2 N-Subjettiness ratio (⌧21 = < 0.45). Monojet Analysis uses (⌧21 < 0.6), but • ⌧1 as this work is motivated for resonance research, so I used this cut value to focus on high purity case only.

Selection Level-4 : ZZ Resonance event selection

Signal zone : Transverse mass (MT) of 600 to 800 GeV • Since, this analysis is based on boosted topology, so the ak8 jets is preferred over ak4 jets. The eciency to reconstruct the high pt object with ak8 jet is overwhelmingly high over ak4 jet. The comparision of these eciency for a boosted Wisshowninfigure5.1fromref[79]wheretheblackcolorisforak8andredoneis for ak4 jet. Figure 5.2 shows the Z reconstruction eciency which was calculated as the ratio of the pt for quark jets from Z with respect to the pt of generated events using the RS graviton signal sample i.e. efficiency = Ztag . Zall

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Figure 5.1. Eciency for ak8 vs ak4 jets for boosted object

Figure 5.2. Eciency for jet Reconstruction for Z candidate

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5.3 Various Kinematic Distribution from Signal Selection The distribution plot for the various physics objects are listed in this section. Background samples, and RS signal sample are normalized to the integrated luminosity. The RS graviton sample in the plots corresponds to only 10% of the RS graviton production cross-section. This is because the amplitude/peak of the whole RS graviton sample is very high; hence, either it is suppressing the background of interests, or it is going out of the canvas. This is just because of the cosmetic issue.

Here, N cuts refer to all cuts applied, and (N-1) cuts for all cuts except ⌧21 cut.

Figure 5.3. Number of Vertex

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Figure 5.4. Number of AK8 Jets

Figure 5.5. Leading jet PT @ (N-1) cut

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Figure 5.6. Leading Jet PT @Ncuts

Figure 5.7. Pruned Mass @ (N-1) cut

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Figure 5.8. Pruned Mass @ N cuts

Figure 5.9. L1 Jet ⌧21 @ (N-1) cut

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Figure 5.10. ⌧21 vs MP runed @ (N-1) cut for signal

Figure 5.11. ⌧21 vs MP runed @ (N-1) cut for BG

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Figure 5.12. MET @ (N-1) cut

Figure 5.13. MET @ N cut

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Figure 5.14. MT @ (N-1) cut

Figure 5.15. MT outside of Z selection @ (N-1) cut

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Figure 5.16. MT @ N cut

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CHAPTER 6

BACKGROUND ESTIMATION, RESULTS, AND SYSTEMATICS

In this analysis, background estimation is done with shape based data driven method. Data-driven method is used in almost every analysis these days due to insucient data to tune physics event generators for proton-proton collisions at 13 TeV and harsh conditions to model the experiment background at the high luminosity LHC at this early stage of the data analysis at the new energy of 13 TeV.

6.1 Background Estimation The transverse mass (MT) distribution for signal and background are shown in figure 5.16, and 5.15 respectively. The distribution of the MT for signal drops rapidly above the mass of hypothetical resonance of 750 GeV, and the distribution for background drops exponentially above 500 GeV. The shape of the background is estimated using the following quantities:

Zbar :eventsaftertheMET+jetsselection(selectionlevel2fromsection • Data 5.2), excluding Z(⌫⌫)+Z(Jet)candidateeventsindata.

Zbar : same selection as above in Monte Carlo background sample. • MC Z Monte Carlo events satisfying the ZZ selection (selection level 3 from • MC section 5.2).

ZbarData is dominated by background. Some of jets in the background events satisfy the Z(Jet) identification criteria and fake Z(⌫⌫)Z(Jet) sample. Shape of the background (BG) is calculated as:

Z shape(BG)=shape of Zbar MC Data ⇥ Zbar ✓ MC ◆

Figure 6.1 shows ZData, ZRSgraviton after selection level 4, and ZbarData before and after the shape correction (calculation). Figure 6.2, is basically the same plot to 6.1 but in linear scale without ZbarData before correction. We take a region of MT > 825 GeV as signal free region, and normalize the shape(BG) to data after ZZ

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Figure 6.1. Zbar before and after correction

Figure 6.2. Comparision of shape for Data vs BG selection (selection level 3 from section 5.2) in 825 < MT < 1500 GeV. The numbers of events in the region are 12852 (702) events before (after) the Z selection in MC, i.e. in ZbarMC (ZMC). In data, we observed 492 events in the same MT range after the Z selection. Normalizing the the shape of estimated BG from MC to the data, the estimated background in the signal region, 600

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Figure 6.3. Investigating (S-B), and S/B

124 events. The error is dominated by a statistical error in normalization process ± (5% from 1 ). p492 The distribution after the subtraction of the estimated background (ZbarData corrected in plot 6.2) from the ZZ data sample (Z Data in plot 6.2) is shown in figure 6.3. Two red lines at the mass of 600 and 800 GeV represents our choice of signal zone, where the 200 excess events are observed. The excess corresponds to 1.6,whichisnotsignificantasasignaturefornewresonance. While the signal peaks around 700 GeV, the excess events increase as MT decrease. The shape of the excess is not consistent with the expected shape for signal. Therefore, the excess is not likely to be the evidence of new resonance. Also, from the ratio plot ( Z part in figure 6.3) we can say that the shape for the Data is Zbar quite consistent with that of BG in the signal zone, without reflecting any indication of resonance. In addition, as MT decrease, it drops rapidly below 550 GeV due a cut on AK8 jets pt of 250 GeV. We attribute the excess to the imperfect modeling of data near the edge of the pt cut in Monte Carlo simulation.

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6.1.1 Error on Background Estimation, and Excess events Error on background estimation is calculated from the normalization part. Background is normalized in signal zone (A) [600 to 800 GeV] with respect to the event outside of signal zone i.e. 825 to 1500 GeV (B). So, the normalization ratio (R) = A/B, and the error propagation is estimated as [83]:

R 2 A 2 B 2 = + R A B ✓ ◆ ✓ ◆ ✓ ◆ R 1 1 + ; since, A = pA R ⇠ pA pB Since, the number of events on the signal zone i.e. A is 2474.5 events, and for B is 492 events, hence the error on background estimation is 5.0% ⇠ Therefore, number of events(1)correspondingtoRis5.0% of 2474.5whichis 124 events. So, now when we say that means 1 =124events,andanything ⇠ below 5 excess is refuted in particle physics. In this work, 200 excess events were observed, and now comparing to the error ⇠ on BG estimation, this corresponds to the 1.61 excess. This now leads to the limit calculation.

6.2 Limit Calculation on Production Cross Section First the observed cross-section of this process based on the observed events is calculated as:

observed cross-section = production cross-section branching ratio acceptance ⇥ ⇥ =60pb 0.28 0.13 = 2.184pb ⇥ ⇥

For the branching ratio (BR); branching ratio of Z boson to hadrons (jets) is 70%, and to neutrinos is 20%. So, total branching ratio for Z(⌫⌫)Z(jets) is ⇠ ⇠ 2 0.2 0.7=0.28. Acceptance(✏)iscalculatedastheratioofthenumberof ⇥ ⇥ events after all cuts on signal zone to the number of events before any cuts over entire range. This is calculated for signal sample, and in this case the RS graviton

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1.674 104 for which, it turned out to be 1.296⇥105 =0.13. As mentioned earlier on section 5.1.1, ⇥ here the acceptance is independent of the width (narrow vs wide). Although the width of the resonance at the generator level is di↵erent, but we are looking for the Jacobian peak on transverse mass which is calculated from the missing energy. So, we have the wide width of MT as MET can not be measured properly from our detector, and hence the acceptance becomes independent of width. However, if we consider di↵erent channel without MET then the width matters, and the acceptance becomes di↵erent for narrow and wide width cases. The commonly used width in 4 CMS for 750 GeV resonance search with graviton is 1.4 10 %fornarrowwidth ⇥ resonance, and 5.6% for wide resonance. The number of observed events is given by the following equation:

observed cross-section dt BR ✏ = number of events ⇥ L ⇥ ⇥ Z 1 Here, dt is the integrated luminosity which is 12.9fb . Using this equation for L cross-sectionR limit (limit)insteadofobservedcross-section,weget: number of events number of events = = limit dt BR ✏ 469.56pb 1 L ⇥ ⇥ IusedRooStats[86-87]tocalculatethecross-sectionlimitat95%confidenceR Level (CL). The input error values to RooStat for the corresponding parameters are: 6.2% for luminosity [79], 25% on acceptance (theory), and 5% for the background estimation from section 6.1. The 95% CLlimit for the two di↵erent cases considered are:

Case 1: Assuming 0 excess events. 95% CL =0.51 pb • limit Case 2: Assuming 200 excess events. 95% CL =0.90 pb • limit In figure 6.4, y-axis is the number of events expected for the corresponding production cross-section of signal in x-axis. The 0.075, 0.15, 0.3, 0.6, 1.2, and 1.8 pb in x-axis represents the 0.125, 0.25, 0.5, 1, 2, and 3 percent of the production cross-section (60 pb) of RS graviton. Two 95% CLlimit lines are drawn for the case of assuming no excess events, and 200 excess events. The 95% CLlimit line intersects

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Figure 6.4. Exclusion of various production cross-section of the RS graviton the fit (parabolic fuction) at 0.5 pb, and 0.9 indicating it as the limiting cross-section for each case of this work. Hence, the cross-sections above these are excluded.

6.3 Results The total number of excess events on the signal zone 600 to 800 GeV is 200 ⇠ events which corresponds to the 1.61 excess. This leads to our conclusion that there is no resonance of 750 GeV with the current experimental data we have. Also, independent to this analysis, the possibility of resonance seen in diphoton channel with 2015 data was also gone with more data from 2016 ICHEP dataset. This turned out to be just a statistical fluctuation [84]. Figure 6.5, is the diphoton mass distribution with 2015 data from ATLAS experiment, where we clearly see the sign of resonance around 750 GeV mass, and Fig 6.6 is again the same distribution but with 2016 data from CMS, and there the sign of resonance got lost. Comparing figure 6.6 to figure 6.2, and 6.3 on our work, we can see there is no any hint of resonance, also the figure 6.6 shows some excess of roughly 2 close to 600 GeV. Hence, the result of this analysis seems to be consistent with the results on the diphoton channel from CMS.

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Figure 6.5. Excess on channel in 2015

Figure 6.6. Disappearance of excess in 2016

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6.4 Systematics There could be the various sources of discrepancies or errors in this work which are listed below:

There is a CMS wide concern that 2016 ICHEP dataset have “HIP” (heavily • ionizing particle) problem. HIP problem is basically due to the hadronic interactions in the silicon tracker that produce huge pulses that saturate the analog front-end causing the readout channel to go unresponsive/high impedance for several bunch crossings [85].

One of the most challenging thing in this analysis was to have a good sideband • to select the events for normalization. Since the minimum pt cut is 250GeV, and the MET cut is 200 GeV which sits just at the edge of the left side band, making impossible to use this part which has quiet good statistics over the right side band beyond 800 GeV mass zone. This has certainly contributed on some way, but I am not sure about the strength of the contribution.

Here in order to have the proper shape of the background, it was corrected • with MC, and the discrepancy on MC is another source of systematic uncertainty.

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CHAPTER 7

SUMMARY

I performed the analysis for the search of the possibility of resonance of 750 GeV mass on ZZ channel where one Z decays to hadrons (jets), and the other decays to neutrinos. The data collected by the CMS experiment for proton-proton collision at 1 13 TeV with 12.9fb (2016 ICHEP dataset) was used. This work was motivated by the hint of the resonance on diphoton channel at the same mass with 2015 data as reported by both CMS and ATLAS experiment at LHC. No excess signal is observed in this channel. The observed limit on production cross-section at 95 % CL is 0.9 pb with an observed excess of 200 events, and 0.51 pb with null excess hypothesis. The ATLAS excess in 2015 data corresponded to the cross-section O(10 fb) and our limit is O(1 pb). For the standard model Higgs, the branching ratios are 07 01 1.8 10 for H and 2.9 10 for ZZ for Higgs mass of 750 GeV [88]. ⇥ ! ⇥ Therefore, we expected O(100 pb) for ZZ channel, if the excess were true and the couplings of the new resonance to the standard model particles are similar to those of the Higgs particle. This analysis clearly excludes such possibility. The ATLAS and CMS Collaboration updated the result on the resonance at the ICHEP conference in August 2016. The hint of the resonance in the diphoton channel observed in 2015 has now been disappeared with 2016 data. The original excess was most likely due to statistical fluctuation. I applied jet substructure techniques : pruning and NSubjettiness in this analysis. Those are new techniques and important to analyze high mass resonances decaying into boosted H, Z, W or top quarks. Also it was a key tool to search for higgs boson from b¯b channel previously. It was a good experience for me to learn the performance of these tools on improving signal over background ratio, and Z tagging. The LHC plans to increase the beam luminosity and accumulate more data, about a factor of 100, in a decade. These technique and further improvement of such technique will be critical in searches and studies of new heavy resonances at the LHC. I plan to develop and apply the boosted object reconstruction in searches for new physics at the LHC.

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