DOCSLIB.ORG

Search for New Phenomena in Dijet Angular Distributions at S = 8 and 13

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Search for New Phenomena in Dijet Angular Distributions at S = 8 and 13 SEARCHFORNEWPHENOMENAINDIJETANGULARp DISTRIBUTIONSAT S = 8 AND 1 3 TEV lene bryngemark Thesis submitted for the degree of Doctor of Philosophy CERN-THESIS-2016-007 18/03/2016 2016 ABSTRACT A new energy regime has recently become accessible in collisions at the Large Hadron Collider at CERN. Abundant in hadron collisions, the two-jet final state explores the structure of the constituents of matter and the possible emergence of new forces of nature, in the largest momentum transfer collisions produced. The results from searches for phenomena beyond the Standard Model in the dijet angular distributions are presented. The data were collected with the ATLAS detector in proton-proton collisions at centre-of-mass energies of 8 and 13 TeV, corresponding to integrated luminosities of 17.3 fb−1 and 3.6 fb−1, respectively. No evidence for new phenomena was seen, and the strongest 95% confidence level lower limits to date were set on the scale of a range of suggested models. This work details the limits on the compositeness scale of quarks in a contact interaction scenario with two different modes of interference with Standard Model processes, as well as on the threshold mass of quantum black holes in a scenario with 6 extra spatial dimensions, and on the mass of excited quark states. It also includes new exclusion limits on the mass of a dark matter mediator and its coupling to fermions, as derived from the contact interaction limits using an effective field theory approach. The performance in ATLAS of the jet-area based method to correct jet measurements for the overlaid energy of additional proton-proton collisions is also presented. It removes the dependence of the jet trans- verse momentum on overlaid collision energy from both simultaneous interactions and those in the neighbouring bunch crossings, and was adopted as part of the jet calibration chain in ATLAS. iii POPULÄRVETENSKAPLIG SAMMANFATTNING Hur förklarar vi vad som hände efter Big Bang? Hur är det möjligt att titta 14 miljarder år tillbaka i tiden, isolera de första miljarddelarna av en sekund, och studera vad som pågår precis där och då? Svaret är såklart, att vi inte kan det. Det vi kan göra, är att om och om igen återskapa några viktiga aspekter av de förhållanden som rådde då, och studera vad som händer. Metoden vi använder är att omvandla energi till partiklar med massa — precis som vi föreställer oss hände i universums begynnelse. Det gör vi genom att accelererera upp stora mängder av protoner — atomkärnor av universums lättaste och vanli- gaste grundämne, väte — till höga energier i en partikelaccelerator, och sedan låta dem kollidera. Runt kollisionspunkten placeras en detektor. I kollisionerna kan tunga partiklar bildas eftersom det finns så mycket energi tillgänglig. De tunga partiklarna sönderfaller sedan till lättare partiklar, ibland i långa sönderfallskedjor fram till de lätta partiklar som är det som universum består av idag. De växelverkar med materialet i detektorn och ger upphov till elektriska signaler som läses ut och används till att rekonstruera vad som bildades i kollisionen. På så sätt kan vi få en glimt av vad som kunde hända med den stora tillgängliga energi som fanns koncentrerad i en mycket liten volym i universums begynnelse. Den vanligaste typen av växelverkan mellan protonernas bestånds- delar — kvarkar och gluoner, eller med ett gemensamt namn: partoner — är den så kallade starka kraften. I protonkollisionerna växelverkar två partoner, och ett möjligt utfall är att nya partoner bildas med hög energi, och slungas ut från kollisionspunkten. Men en egenskap hos den starka kraften är att en parton aldrig kan isoleras! Istället bildas kontinuerligt nya kvark/antikvark-par i dess kölvatten, av rörelsee- nergin hos partonen, som successivt bromsas in av energiförlusten. Resultatet blir en riktad skur av partiklar — en jet — som tillsammans har den energi och de andra kvantmekaniska egenskaper som parto- v nen fick i kollisionen. Genom att mäta jetens egenskaper kan vi säga något om egenskaperna hos partonen som bildades. Den här avhandlingen beskriver dels en metod för att noggrant kunna mäta jetenergier även när mätningen påverkas av energi från andra kollisioner, och dels hur vinklarna mellan jetpar kan användas för att leta efter fenomen som inte beskrivs av den rådande teori som beskriver vilka naturlagar (krafter och fundamentala partiklar) som finns. Vi vet att det behövs en mer fundamental teori än den nuvaran- de, eftersom det finns observationer som den inte förklarar, t ex den stora skillnaden mellan massorna av olika typer av kvarkar. En av den experimentella partikelfysikens viktigaste uppgifter just nu är därmed att finna tecken på avvikelser från den rådande teorin, så att vi kan börja ana på vilket sätt vi bättre kan beskriva universums bestånds- delar och krafter. Avhandlingen visar att vid de mest högenergetiska kollisioner vi hittills kunnat åstadkomma i en accelerator, har vi ännu inte observerat några avvikelser från den rådande teorin. vi LISTOFPUBLICATIONS Some of the original work described in this thesis has appeared previ- ously in the following publications: [1] ATLAS Collaboration. Search for New Phenomenap in Dijet Mass and Angular Distributions from pp Collisions at s = 13 TeV with the ATLAS Detector. Physics Letters B, 754:302 – 322, 2016. [2] ATLAS Collaboration. Search for New Phenomena inp the Dijet Angular Distributions in Proton-Proton Collisions at s = 8 TeV with the ATLAS Detector. Phys. Rev. Lett., 114:221802, 2015. [3] ATLAS Collaboration. Performancep of pile-up mitigation tech- niques for jets in pp collisions at s = 8 TeV using the ATLAS detector. To be published in EPJC, 2015. arXiv:1510.03823. [4] ATLAS Collaboration. Search for new phenomenap in the dijet mass distribution using p-p collision data at s = 8 TeV with the ATLAS detector. Phys. Rev. D, 91:052007, 2015. [5] ATLAS Collaboration. Search for New Phenomena in Dijetp Mass and Angular Distributions with the ATLAS Detector at s = 13 TeV. Technical Report ATLAS-CONF-2015-042, CERN, Geneva, Aug 2015. [6] ATLAS Collaboration. Pile-up subtraction and suppression for jets in ATLAS. Technical Report ATLAS-CONF-2013-083, CERN, Geneva, Aug 2013. [7] ATLAS Collaboration. Search for New Phenomena in the Dijet −1 Massp Distribution updated using 13.0 fb of pp Collisions at s = 8 TeV collected by the ATLAS Detector. Technical Report ATLAS-CONF-2012-148, CERN, Geneva, Nov 2012. vii ACKNOWLEDGEMENTS It takes a village to write a thesis. Countless are the people who have contributed in one way or another to the work presented here, with efforts ranging from building detectors to cooking and caring for me when work has taken too much of my time. I will nevertheless attempt to list a few I want to thank. Torsten, who suggested the research topic of dijet distributions and became my main supervisor. Always online, always there for physics or coffee or both, just waiting to make a discovery (I’m sorry we didn’t yet). It’s safe to say that without you, this thesis would not have been. Else, Johan, my co-supervisors, who gave me the blessing of not worrying, but being encouraging and available when I needed it. A special thanks to them and David for thesis draft reading. Ariel, who talks of jets with contagious enthusiasm and knows there is so much more we can do to extract all their secrets. I could not have had a better start in ATLAS than I got from working with you. I also thank John, who regrettably left physics, for patiently discussing and condensing all my (and Ariel’s) wild pile-up ideas to a useful conclusion and strategy. Caterina. Dijets, jets in ATLAS, just ATLAS would not be the same without you. Sharing code and expertise and cat pictures, you thoroughly showed me what working in a team can be. You’re ready to both give advice and get your own hands dirty; Lund is better with you. Tuva. You have to admit that this is one crazy adventure we started together. Our paths from starting a Bachelor’s project to finishing a PhD were increasingly parallel, but queerly similar — I can but don’t want to imagine this time without you. All my shorter and longer term colleagues (and you know I count students) who have made the division kitchen such a lively place! Anders, Bozena, Evert, Peter, the steady coffee and lunch crowd, always up for a debate, and for support over the years. Florido and ix my fellow PhD students (Alejandro, Anders, Anthony, Martin, Sasha, Tuva, Vytautas, and more recently Ben, Edgar, Trine and Katja) — for coffee and beer, physics and politics, for lunches and dinners and the opportunity to vent our fair share of collaboration frustration. That same collaboration for making it all possible. The speedy dijet team, striking thrice within a year, ready to solve anything and on two occasions making our results public within weeks after data taking finished. The numerous delightful people in the university’s most charming LGBTQ network for employees, you know who you are. Hanno and Kroon and Laurie, for feeding and reading, for diverting my attention and listening to everything. Karin for laughter and all too rare long breakfasts and for always letting me leave the dishes. Pauline, for housing me during my year at CERN, for warm friendship and inviting me to join anything from dinners to running before dawn. Lotte for in turn housing my cat that year, and Julius-Lisa, that same cat for — I dare say — love and support. The pursuit of a PhD is the hardest on, and least gratifying for, the persons closest to you. Elina, words will never be enough. Thank you for letting me get absorbed, and waiting for me to come out again.
Load more

Recommended publications
  • ATLAS EXPERIMENT 15 3.1 the Large Hadron Collider
    THE UNIVERSITY OF CHICAGO SEARCH FOR NEW PHENOMENA IN DIJET TOPOLOGIES FROM PROTON-PROTON COLLISIONS AT pS = 13 TEV A DISSERTATION SUBMITTED TO THE FACULTY OF THE DIVISION OF THE PHYSICAL SCIENCES IN CANDIDACY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS BY JEFFREY ROGERS DANDOY CHICAGO, ILLINOIS SEPTEMBER 2016 For my family TABLE OF CONTENTS ABSTRACT xvii 1 INTRODUCTION 1 2 THE STANDARD MODEL 3 2.1 Quantum Chromodynamics . .4 2.1.1 Hard Scatter . .8 2.1.2 Parton Shower . 11 2.1.3 Hadronization . 11 2.2 Motivation for New Physics . 12 3 THE ATLAS EXPERIMENT 15 3.1 The Large Hadron Collider . 15 3.1.1 LHC Operation . 17 3.2 The ATLAS Detector . 20 3.2.1 Inner Detector . 20 3.2.2 Calorimetry . 25 3.2.3 Electromagnetic Calorimeters . 27 3.2.4 Hadronic Calorimeters . 29 3.2.5 Muon Spectrometer . 35 3.2.6 Data Acquisition . 37 4 EVENT SIMULATION 39 4.1 QCD Simulation . 40 4.1.1 The Pythia Generator . 40 4.1.2 Monte Carlo Production . 42 4.2 Signal Models . 44 4.2.1 Excited Quark . 44 4.2.2 Dark Matter Mediators . 44 4.2.3 Heavy Boson . 46 4.2.4 Quantum Black Holes . 47 4.3 Monte Carlo Uncertainties . 49 iii 5 JET RECONSTRUCTION AND PERFORMANCE 52 5.1 Jet Reconstruction . 52 5.1.1 Topo-clusters . 53 5.1.2 Jet-finding . 54 5.2 Jet Calibration . 57 5.2.1 In-situ Jet Calibration . 60 5.2.2 Single Particle Response . 62 5.2.3 Corrections for 2015 data .
    [Show full text]
  • 5.12. Ratio of the Inclusive B-Jet Measurement with the POWHEG+Pythia 6 Prediction
    Two b or not two b-jets Measurements of inclusive and dijet b-jet differential cross-sections with the ATLAS detector Stephen Paul Bieniek Supervisor: Prof. Nikos Konstantinidis University College London Submitted to University College London in fulfilment of the requirements for the award of the degree of Doctor of Philosophy, 21st June 2013. I, Stephen Bieniek confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. 1 2 Acknowledgements There are many people who I would like to thank for helping me get through my PhD and I don’t have the space to list all of them. I would first like to thank my supervisor, Nikos Konstantinidis [1], for providing me the support, motivation and direction for this project. I would like to thank Eric Jansen [2] for being there to answer my questions, show me the ropes while I was learning everything and always being there for bouncing ideas off. I would like to thank Lynn Marx [3] who provided the competition I needed to motivate me to produce my best during my time at CERN and Sarah Baker [4] for keeping me company when working late. I’d like to thank my fellow PhD students Erin Walters [5] and Bobby Xinyue [6] who allowed me to keep things in perspective when the work was hard. I would also like to thank Daniel Lattimer [7] for helping me ease back into London life after my time away. As a last note, I would like to dedicate this thesis to Francis Corry.
    [Show full text]
  • Search for New Physics in Dijet Resonant Signatures and Recent Results from Run 2 with the CMS Experiment
    138 Proceedings of the LHCP2015 Conference, St. Petersburg, Russia, August 31 - September 5, 2015 Editors: V.T. Kim and D.E. Sosnov Search for new physics in dijet resonant signatures and recent results from Run 2 with the CMS experiment GIULIA D’IMPERIO Universit`aLa Sapienza and INFN Roma On behalf of the CMS Collaboration Abstract. A search for narrow resonances in proton-proton collisions at a center-of-mass energy of √s = 13 TeV is presented. The dijet invariant mass distribution of the two leading jets is measured with the CMS detector using early data from Run 2 of the Large Hadron Collider. The dataset presented here was collected in July 2015 and corresponds to an integrated luminosity of 42 1 pb− . The highest observed dijet mass is 5.4 TeV. The spectrum is well described by a smooth parameterization and no evidence for new particle production is observed. Upper limits at a 95% confidence level are set on the cross section of narrow resonances with masses above 1.3 TeV. When interpreted in the context of specific models the limits exclude: string resonances with masses below 5.1 TeV; scalar diquarks below 2.7 TeV; axigluons and colorons below 2.7 TeV; excited quarks below 2.7 TeV; and color octet scalars below 2.3 TeV. Introduction Deep inelastic proton-proton (pp) collisions often produce two or more energetic jets when the constituent partons are scattered with large transverse momenta (pT ). The invariant mass of the two jets with the largest pT in the event (the dijet) has a spectrum that is predicted by quantum chromodynamics (QCD) to fall steeply and smoothly with increasing dijet mass (m jj) [1].
    [Show full text]
  • THE DIJET CROSS SECTION MEASUREMENT in PROTON-PROTON COLLISIONS at a CENTER of MASS ENERGY of 500 GEV at STAR Grant D
    University of Kentucky UKnowledge Theses and Dissertations--Physics and Astronomy Physics and Astronomy 2014 THE DIJET CROSS SECTION MEASUREMENT IN PROTON-PROTON COLLISIONS AT A CENTER OF MASS ENERGY OF 500 GEV AT STAR Grant D. Webb University of Kentucky, [email protected] Recommended Citation Webb, Grant D., "THE DIJET CROSS SECTION MEASUREMENT IN PROTON-PROTON COLLISIONS AT A CENTER OF MASS ENERGY OF 500 GEV AT STAR" (2014). Theses and Dissertations--Physics and Astronomy. Paper 20. http://uknowledge.uky.edu/physastron_etds/20 This Doctoral Dissertation is brought to you for free and open access by the Physics and Astronomy at UKnowledge. It has been accepted for inclusion in Theses and Dissertations--Physics and Astronomy by an authorized administrator of UKnowledge. For more information, please contact [email protected]. STUDENT AGREEMENT: I represent that my thesis or dissertation and abstract are my original work. Proper attribution has been given to all outside sources. I understand that I am solely responsible for obtaining any needed copyright permissions. I have obtained and attached hereto needed written permission statement(s) from the owner(s) of each third-party copyrighted matter to be included in my work, allowing electronic distribution (if such use is not permitted by the fair use doctrine). I hereby grant to The nivU ersity of Kentucky and its agents the irrevocable, non-exclusive, and royalty- free license to archive and make accessible my work in whole or in part in all forms of media, now or hereafter known. I agree that the document mentioned above may be made available immediately for worldwide access unless a preapproved embargo applies.
    [Show full text]
  • An Introduction to Jet Substructure and Boosted-Object Phenomenology
    Looking inside jets: an introduction to jet substructure and boosted-object phenomenology Simone Marzani1, Gregory Soyez2, and Michael Spannowsky3 1Dipartimento di Fisica, Universit`adi Genova and INFN, Sezione di Genova, Via Dodecaneso 33, 16146, Italy 2IPhT, CNRS, CEA Saclay, Universit´eParis-Saclay, F-91191 Gif-sur-Yvette, France 3Institute of Particle Physics Phenomenology, Physics Department, Durham University, Durham DH1 3LE, UK arXiv:1901.10342v3 [hep-ph] 17 Feb 2020 Preface The study of the internal structure of hadronic jets has become in recent years a very active area of research in particle physics. Jet substructure techniques are increasingly used in experimental analyses by the Large Hadron Collider collaborations, both in the context of searching for new physics and for Standard Model measurements. On the theory side, the quest for a deeper understanding of jet substructure algorithms has contributed to a renewed interest in all-order calculations in Quantum Chromodynamics (QCD). This has resulted in new ideas about how to design better observables and how to provide a solid theoretical description for them. In the last years, jet substructure has seen its scope extended, for example, with an increasing impact in the study of heavy-ion collisions, or with the exploration of deep-learning techniques. Furthermore, jet physics is an area in which experimental and theoretical approaches meet together, where cross-pollination and collaboration between the two communities often bear the fruits of innovative techniques. The vivacity of the field is testified, for instance, by the very successful series of BOOST conferences together with their workshop reports, which constitute a valuable picture of the status of the field at any given time.
    [Show full text]
  • ATLAS Document
    EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN) Submitted to: Phys. Rev. Lett. CERN-EP-2018-033 April 11, 2018 Search for low-mass dijet resonances using trigger-level jets with thep ATLAS detector in p p collisions at s = 13 TeV The ATLAS Collaboration Searches for dijet resonances with sub-TeV masses using the ATLAS detector at the Large Hadron Collider can be statistically limited by the bandwidth available to inclusive single- jet triggers, whose data-collection rates at low transverse momentum are much lower than the rate from Standard Model multijet production. This Letter describes a new search for dijet resonances where this limitation is overcome by recording only the event information calculated by the jet trigger algorithms, thereby allowing much higher event rates with reduced storage needs. The search targets low-mass dijet resonances in the range 450–1800 GeV. The analyzed dataset has an integrated luminosity of up to 29.3 fb−1 and was recorded at a center-of-mass energy of 13 TeV. No excesses are found; limits are set on Gaussian-shaped contributions to the dijet mass distribution from new particles and on a model of dark-matter particles with axial-vector couplings to quarks. arXiv:1804.03496v1 [hep-ex] 10 Apr 2018 © 2018 CERN for the benefit of the ATLAS Collaboration. Reproduction of this article or parts of it is allowed as specified in the CC-BY-4.0 license. 1 Introduction If new particles beyond those of the Standard Model (SM) are directly produced in proton–proton (pp) collisions at the Large Hadron Collider (LHC), they must interact with the constituent partons of the proton, and can therefore also decay into the same partons, resulting in two-jet final states.
    [Show full text]
  • Triple-Differential Dijet Cross Sections and PDF Constraints
    ETP-KA/2020-15 Measurement of the Ratio of inclusive Jet Cross Sections in Events with and without a high-pT Z Boson Zur Erlangung des akademischen Grades eines MASTER OF SCIENCE von der Fakultät für Physik des Karlsruher Instituts für Technologie (KIT) genehmigte MASTERARBEIT von B.Sc. Bettina Schillinger aus Welschensteinach Tag der Abgabe: 27. April 2020 Referent: Priv.-Doz. Dr. Klaus Rabbertz Korreferent: Asst. Prof. Dr. Mikko Voutilainen Contents 1 Introduction 3 2 Theoretical Foundations 5 2.1 Quantum Chromodynamics..........................5 2.1.1 Factorisation Theorem.........................8 2.1.2 Parton Distribution Functions....................9 2.2 Dijet Production................................ 10 2.3 Z Boson Production at Hadron Colliders................... 12 2.3.1 Significance for Calibration...................... 13 2.4 Dijet versus Z+jet: Differences and Similarities............... 14 2.5 Fixed-Order Theory Calculations....................... 15 2.6 Monte Carlo Event Generators........................ 16 2.7 Jet Clustering Algorithms........................... 19 3 Experimental Setup 21 3.1 The Large Hadron Collider.......................... 21 3.2 CMS Experiment................................ 21 3.2.1 Jet Reconstruction........................... 24 3.2.2 Jet Calibration............................. 26 3.2.3 Trigger................................. 28 4 Cross Section Measurement 29 4.1 Observables of Triple Differential Measurement............... 29 4.2 General Considerations............................. 32 4.3 Dijet Analysis.................................. 36 4.3.1 Trigger Studies............................. 36 4.3.2 Event Selection............................. 39 4.3.3 Comparison to Monte Carlo Simulation and Theory Predictions.. 40 4.3.4 Dijet Cross Section Results...................... 42 4.4 Z+Jet Analysis................................. 46 4.4.1 Z Boson Reconstruction........................ 47 4.4.2 Event Selection............................. 48 4.4.3 Comparison to Monte Carlo Simulation and Theory Predictions.
    [Show full text]
  • Chapter 5 EVENT RECONSTRUCTION 5.1 Jets
    Chapter 5 EVENT RECONSTRUCTION The goal of particle physics experiments is to reconstruct and measure the outgoing particles produced in proton-proton collisions to describe the hard scatter process. After an event is accepted by the ATLAS trigger systems to be recorded to disk, the objects of interest such as electrons, muons, and jets must be reconstructed from the low-level detector signals. These complex objects, meant to be representative of the true Standard Model (SM) particle, are built from some of the low-level detector signals, such as muon spectrometer tracks or energy depositions in the electromagnetic or hadronic calorimeters. As the Large Hadron Collider (LHC) is a hadron collider, the LHC tends to produce colored final states through the collisions of gluons. Many Beyond the Standard Model (BSM) physics models contain these hadronic objects which are crucial to reconstruct accurately, amidst the initial and final state radiation and multiple simultaneous proton-proton collisions. Once reconstructed, the measured properties of these objects may be calibrated to a particular energy scale. This chapter describes in detail the methods of standard ATLAS event reconstruction used for jets (section 5.1), b-jets (section 5.2), muons (section 5.3), electrons and photons (sec- tion 5.4), taus (section 5.5), and missing transverse momentum (section 5.6). 5.1 Jets The first question with a particle physics detector is how to measure the hadronic final state. The difficult in reconstructing quark and gluons is because, due to the nature of QCD described in ??, colored particles cannot be observed directly in the detector.
    [Show full text]
  • DEUTSCHES ELEKTRONEN-SYNCHROTRON Measurement of The
    DEUTSCHES ELEKTRONEN-SYNCHROTRON Ein Forschungszentrum der Helmholtz-Gemeinschaft DESY-THESIS-2017-038 September 2017 Measurement of the Total and Differential b Cross Sections at HERA and CMS Tracker Alignment at LHC by Nazar Stefaniuk ISSN 1435-8085 NOTKESTRASSE 85 - 22607 HAMBURG Measurement of the total and differential b cross sections at HERA and CMS tracker alignment at LHC Dissertation zur Erlangung des Doktorgrades an der Fakult¨atf¨ur Mathematik, Informatik und Naturwissenschaften Fachbereich Physik der Universit¨atHamburg vorgelegt von Nazar Stefaniuk aus Iv.-Frankivsk/Kiew (Ukraine) Hamburg , 2017 Gutachter in/der Dissertation: PD Dr. Achim Geiser Prof. Dr. Elisabetta Gallo Mitglieder der Pr¨ufungskommission: PD Dr. Achim Geiser Prof. Dr. Elisabetta Gallo Prof. Dr. Caren Hagner Prof. Dr. Gudrid Moortgat-Pick Dr. Katarzyna Wichmann Tag der Disputation: 08 May 2017 Vorsitzender des Pr¨ufungskommission: Prof. Dr. Caren Hagner Vorsitzender des Promotionsausschusses: Prof. Dr. Wolfgang Hansen Dekan des MIN-Fakult¨at: Prof. Dr. Heinrich Graener Abstract This thesis is logically divided into two main parts. The first part present a ZEUS analysis which was performed on the data obtained from electron-proton collisions measured by the ZEUS detector for the 2003-2007 running period. The full HERA-II integrated luminosity of 376 pb−1 is used. The ZEUS detector is sensitive to the full phase space of beauty production, since it has cylindrical geometry, covers a wide rapidity range and was able to measure low transverse momentum muons. In this part a measurement of beauty production was studied via the process ep ! e0b¯bX ! e0µµX. Making full use of the HERA-II detector upgrade, secondary vertex information was used to constrain the beauty and charm contribution to this process.
    [Show full text]
  • Study of Dijet Events with a Large Rapidity Gap Between the Two
    EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCH (CERN) CERN-EP-2017-141 2018/03/26 CMS-FSQ-12-001 Study of dijet events with a large rapidityp gap between the two leading jets in pp collisions at s = 7 TeV The CMS Collaboration∗ Abstract Events with no charged particlesp produced between the two leading jets are studied in proton-proton collisions at s = 7 TeV. The jets were required to have transverse jet jet momentum pT > 40 GeV and pseudorapidity 1.5 < jh j < 4.7, and to have val- ues of hjet with opposite signs. The data used for this study were collected with the CMS detector during low-luminosity running at the LHC, and correspond to an inte- −1 grated luminosity of 8 pb . Events with no charged particles with pT > 0.2 GeV in the interval −1 < h < 1 between the jets are observed in excess of calculations that assume no color-singlet exchange. The fraction of events with such a rapidity gap, amounting to 0.5–1% of the selected dijet sample, is measured as a function of the pT of the second-leading jet and of the rapidity separation between the jets. The data are compared to previous measurements at the Tevatron, and to perturbative quantum chromodynamics calculations based on the Balitsky–Fadin–Kuraev–Lipatov evolu- tion equations, including different models of the non-perturbative gap survival prob- ability. We dedicate this paper to the memory of our colleague and friend Sasha Proskuryakov, who started this analysis but passed away before it was completed. His contribution to the study arXiv:1710.02586v2 [hep-ex] 22 Mar 2018 of diffractive processes at CMS is invaluable.
    [Show full text]
  • Search for Dark Matter in Dijet Events Using Quark/Gluon Tagging
    Search for Dark Matter in Dijet Events using Quark/Gluon Tagging NeˇzaRibariˇc Supervising Professor: Iain Bertram September 2019 Abstract This thesis explores the possibility of improving the limits set on the Dark Matter mediator Z0 obtained from the dijet invariant mass spectrum using a technique called quark/gluon tagging. The main discriminator between quark and gluon initiated jets is the number of tracks within the jet, however both linear and logarithmic functions of jet pT and invariant mass with various starting parameters were tested. The new limits obtained from the sub-samples were compared to the un-tagged dijet data sample generated to mimic 2015/2016 data of 37fb−1 collected with the ATLAS detector at the LHC. The samples are generated using Pythia 8 with the leading-order NNPDF 2.3 PDFs and A14 tune and the detector response is modeled in Geant 4. It was shown that for an ideal separation (based on parton truth ID), the improvement in significance can range up to 50%. However, the best separation functions based on jet pT and mjj show improvements in the range of 2-5%. The dependence on mjj shows potential for significant improvements in the mass range lower than the one analyzed in this thesis. Declaration of Authorship This thesis is a presentation of original research performed by the author. It has not been previ- ously submitted for the award of a higher degree. The thesis would not be possible without the Quark/Gluon tagger group, dijet and di-b-jet group and exotics group from the ATLAS collabora- tion.
    [Show full text]
  • Search for New Phenomena in Dijet Events with the ATLAS Detector at S = 13 Tev Attilio Picazio
    Search for New Phenomena in Dijet Events with the p ATLAS Detector at s = 13 TeV Attilio Picazio∗ (on behalf of the ATLAS Collaboration) University of Massachusetts, Amherst E-mail: [email protected] During the last two years the LHC produced pp collisions at the record center-of-mass energy of 13 TeV. The sensitivity of searches for new phenomena with a high mass scale greatly benefited from the energy increase with respect to the LHC Run 1 data. Events with two hadronic jets in the final state are of particular interest: new phenomena produced in parton collisions are likely to produce final states with (at least) two partons. In this document several searches performed by the ATLAS Collaboration are presented. The very high mass and the low mass regions have both been investigated, by exploiting dedicated signatures and, in case of the latter, new techniques to overcome trigger limitations. Results and perspectives for these searches are presented. ATL-PHYS-PROC-2017-198 10 October 2017 The European Physical Society Conference on High Energy Physics 5-12 July Venice, Italy ∗Speaker. c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). http://pos.sissa.it/ p Search for New Phenomena in Dijet Events with the ATLAS Detector at s = 13 TeV Attilio Picazio 1. Introduction The existence of new heavy resonances is predicted in many extensions of the Standard Model (SM). If a new particle is produced in hadronic collisions delivered by the Large Hadron Collider (LHC), it is likely to produce final states with (at least) two partons.
    [Show full text]