DOCSLIB.ORG

[Sqrt]Snn=2.76 Tev with the ATLAS Detector at The

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
[Sqrt]Snn=2.76 Tev with the ATLAS Detector at The Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead Collisions at [sqrt]sNN=2.76 TeV with the ATLAS Detector at the LHC The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Aad, G. et al. (ATLAS Collaboration) "Observation of a Centrality- Dependent Dijet Asymmetry in Lead-Lead Collisions at √sNN=2.76 TeV with the ATLAS Detector at the LHC." Phys. Rev. Lett. 105, 252303 (2010) [18 pages] © 2010 CERN, for the ATLAS Collaboration. As Published http://dx.doi.org/10.1103/PhysRevLett.105.252303 Publisher American Physical Society Version Final published version Citable link http://hdl.handle.net/1721.1/62796 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. Selected for a Viewpoint in Physics week ending PRL 105, 252303 (2010) PHYSICAL REVIEW LETTERS 17 DECEMBER 2010 Observation of a Centrality-Dependent Dijet Asymmetry in Lead-Lead pffiffiffiffiffiffiffiffi Collisions at sNN ¼ 2:76 TeV with the ATLAS Detector at the LHC G. Aad et al.* (ATLAS Collaboration) (Received 25 November 2010; published 13 December 2010) By using the ATLAS detector, observations have been made of a centrality-dependent dijet asymmetry in the collisions of lead ions at the Large Hadron Collider. In a sample of lead-lead events with a per- nucleon center of mass energy of 2.76 TeV, selected with a minimum bias trigger, jets are reconstructed in fine-grained, longitudinally segmented electromagnetic and hadronic calorimeters. The transverse ener- gies of dijets in opposite hemispheres are observed to become systematically more unbalanced with increasing event centrality leading to a large number of events which contain highly asymmetric dijets. This is the first observation of an enhancement of events with such large dijet asymmetries, not observed in proton-proton collisions, which may point to an interpretation in terms of strong jet energy loss in a hot, dense medium. DOI: 10.1103/PhysRevLett.105.252303 PACS numbers: 25.75.Bh Collisions of heavy ions at ultrarelativistic energies are The LHC heavy ion program was foreseen to provide an expected to produce an evanescent hot, dense state, with opportunity to study jet quenching at much higher jet temperatures exceeding 2 Â 1012 K, in which the relevant energies than achieved at the Relativistic Heavy Ion degrees of freedom are not hadrons but quarks and gluons. Collider. This Letter provides the firstp measurementsffiffiffiffiffiffiffiffi of In this medium, high-energy quarks and gluons are ex- jet production in lead-lead collisions at sNN ¼ 2:76 TeV pected to transfer energy to the medium by multiple inter- per nucleon-nucleon collision, the highest center of mass actions with the ambient plasma. There is a rich theoretical energy ever achieved for nuclear collisions. At this energy, literature on in-medium QCD energy loss extending back next-to-leading-order QCD calculations [6] predict abun- to Bjorken, who proposed to look for ‘‘jet quenching’’ in dant rates of jets above 100 GeV produced in the pseudor- proton-proton collisions [1]. This work also suggested the apidity region jj < 4:5 [7], which can be reconstructed by observation of highly unbalanced dijets when one jet is ATLAS. produced at the periphery of the collision. For comprehen- The data in this Letter were obtained by ATLAS during sive reviews of recent theoretical work in this area, see the 2010 lead-lead run at the LHC and correspond to an Refs. [2,3]. integrated luminosity of approximately 1:7 bÀ1. Single particle measurements made by Relativistic For this study, the focus is on the balance between the Heavy Ion Collider experiments established that high highest transverse energy pair of jets in events where transverse momentum (pT) hadrons are produced at rates those jets have an azimuthal angle separation Á ¼j1 À a factor of 5 or more lower than expected by assuming 2j >=2 to reduce contributions from multijet final QCD factorization holds in every binary collision of nu- states. In this Letter, jets with Á>=2 are labeled as cleons in the oncoming nuclei [4,5]. This observation is being in opposite hemispheres. The jet energy imbalance is characterized by measurements of RAA, the ratio of yields expressed in terms of the asymmetry AJ: in heavy ion collisions to proton-proton collisions, divided by the number of binary collisions. Dihadron measure- ET1 À ET2 AJ ¼ ; Á> ; (1) ments also showed a clear absence of back-to-back hadron ET1 þ ET2 2 production in more central heavy ion collisions [5], strongly suggestive of jet suppression. The limited rapidity where the first jet is required to have a transverse energy coverage of the experiment, and jet energies comparable to ET1 > 100 GeV, and the second jet is the highest trans- the underlying event energy, prevented a stronger conclu- verse energy jet in the opposite hemisphere with ET2 > sion being drawn from these data. 25 GeV. The average contribution of the underlying event energy is subtracted when deriving the individual jet trans- verse energies. The event selection is chosen such that the *Full author list given at the end of the article. first jet has high reconstruction efficiency and the second jet is above the distribution of background fluctuations and Published by The American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distri- the intrinsic soft jets associated with the collision. Dijet bution of this work must maintain attribution to the author(s) and events are expected to have AJ near zero, with deviations the published article’s title, journal citation, and DOI. expected from gluon radiation falling outside the jet cone, 0031-9007=10=105(25)=252303(18) 252303-1 Ó 2010 CERN, for the ATLAS Collaboration week ending PRL 105, 252303 (2010) PHYSICAL REVIEW LETTERS 17 DECEMBER 2010 as well as from instrumental effects. Energy loss in the covering the full azimuth between 2:09 < jj < 3:84 and medium could lead to much stronger deviations in the divided into eight sectors and two sectors. reconstructed energy balance. Coincidences in the zero degree calorimeter and luminos- The ATLAS detector [8] is well-suited for measuring ity measurement using a Cherenkov integrating detector jets due to its large acceptance, highly segmented electro- were also used as primary triggers, since these detectors magnetic and hadronic calorimeters. These allow efficient were far less susceptible to LHC beam backgrounds. These reconstruction of jets over a wide range in the region triggers have a large overlap and are close to fully efficient jj < 4:5. The detector also provides precise charged par- for the events studied here. ticle and muon tracking. An event display showing the inner In the offline analysis, events are required to have a time detector and calorimeter systems is shown in Fig. 1. difference between the two sets of minimum bias trigger Liquid argon technology providing excellent energy and scintillator counters of Át<3nsand a reconstructed ver- position resolution is used in the electromagnetic calorime- tex to efficiently reject beam-halo backgrounds. The pri- ter that covers the pseudorapidity range jj < 3:2. The mary vertex is derived from the reconstructed tracks in the hadronic calorimetry in the range jj < 1:7 is provided inner detector, which covers jj < 2:5 by using silicon by a sampling calorimeter made of steel and scintillating pixel and strip detectors surrounded by straw tubes. tiles. In the end caps (1:5 < jj < 3:2), liquid argon tech- These event selection criteria have been estimated to ac- nology is also used for the hadronic calorimeters, matching cept over 98% of the total lead-lead inelastic cross section. the outer jj limits of the electromagnetic calorimeters. To The level of event activity or ‘‘centrality’’ is character- complete the coverage, the liquid argon forward calo- ized by using the total transverse energy (ÆET) deposited rimeters provide both electromagnetic and hadronic energy in the forward calorimeters (FCal), which cover 3:2 < measurements, extending the coverage up to jj¼4:9. jj < 4:9, shown in Fig. 2. Bins are defined in centrality The calorimeter ( and ) granularities are 0:1 Â 0:1 for according to fractions of the total lead-lead cross section the hadronic calorimeters up to jj¼2:5 (except for the selected by the trigger and are expressed in terms third layer of the tile calorimeter, which has a segmentation of percentiles (0%–10%, 10%–20%, 20%–40%, and of 0:2 Â 0:1 up to jj¼1:7) and then 0:2 Â 0:2 up to 40%–100%) with 0% representing the upper end of the jj¼4:9. The electromagnetic calorimeters are longitudi- ÆET distribution. Previous heavy ion experiments have nally segmented into three compartments and feature a shown a clear correlation of the ÆET with the geometry much finer readout granularity varying by layer, with cells of the overlap region of the colliding nuclei and, corre- as small as 0:025 Â 0:025 extending to jj¼2:5 in the spondingly, the total event multiplicity. This is verified in middle layer. In the data-taking period considered, ap- the bottom panel of Fig. 2, which shows a tight correlation proximately 187 000 calorimeter cells (98% of the total) between the energy flow near midrapidity and the forward were usable for event reconstruction. ÆET. The forward ÆET is used for this analysis to avoid The bulk of the data reported here were triggered by biasing the centrality measurement with jets. using coincidence signals from two sets of minimum bias Jets have been reconstructed by using the infrared-safe trigger scintillator detectors, positioned at z ¼3:56 m, anti-kt jet clustering algorithm [9] with the radius parame- FIG.
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 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.
    [Show full text]