CMS will use dijets to search for physics beyond the standard model during early LHC running. The inclusive jet cross section as a function of jet transverse momentum, with 10 pb$^{-1}$ of integrated luminosity, is sensitive to contact interactions beyond the reach of the Tevatron. The dijet mass distribution will be used to search for dijet resonances coming from new particles, for example an excited quark. Additional sensitivity to the existence of contact interactions or dijet resonances can be obtained by comparing dijet rates in two distinct pseudorapidity regions

Bhatti, Anwar

Bollen, Benjamin

Cardaci, Marco

Chlebana, Frank

Esen, Selda

Harris, Robert M

Jha, Manoj

Kousouris, Konstantinos

Mason, David

Zielinski, Marek

CERN Document Server

Available on CMS information server CMS NOTE-2008/019The Compact Muon Solenoid ExperimentMailing address: CMS CERN, CH-1211 GENEVA 23, SwitzerlandCMS Note18 July 2008CMS Search Plans and Sensitivity to NewPhysics using DijetsMarco Cardaci and Benjamin BollenUniversiteit Antwerpen, Antwerp, BelgiumSelda EsenBrown University, Providence, RI, USAManoj K. JhaUniversity of Delhi, India and INFN, Bologna, ItalyFrank Chlebana, Robert M. Harris, Konstantinos Kousouris and David MasonFermilab, Batavia, IL, USAMarek ZielinskiUniversity of Rochester, Rochester, NY, USAAnwar BhattiRockefeller University, New York, NY, USAAbstractCMS will use dijets to search for physics beyond the standard model during early LHC running. Theinclusive jet cross section as a function of jet transverse momentum, with 10 pb−1 of integrated lumi-nosity, is sensitive to contact interactions beyond the reach of the Tevatron. The dijet mass distributionwill be used to search for dijet resonances coming from new particles, for example an excited quark.Additional sensitivity to the existence of contact interactions or dijet resonances can be obtained bycomparing dijet rates in two distinct pseudorapidity regions.The Large Hadron Collider at CERN will produce many events with two energetic jets resulting from proton-protoncollisions at√s = 14 TeV. These dijet events result from parton scattering, produced by the strong interaction ofquarks (q) and gluons (g) inside the protons. This paper discusses plans to use dijets in the search for two signalsof new physics: contact interactions and resonances decaying into dijets. This generic search is applied to twomodels of quark compositeness, that are used as benchmarks of sensitivity to new physics. The rst model is acontact interaction [1] among left-handed quarks at an energy scale Λ+ in the process qq → qq, modeled with theeffective Lagrangian Lqq = (±2pi/Λ2)(qLγµqL)(qLγµqL) with + chosen for the sign. The second model is anexcited quark (q*) [2] in the process qg → q∗ → qg, detectable as a dijet resonance. All processes presented herehave been simulated using PYTHIA version 6.4 [3].A detailed description of the Compact Muon Solenoid (CMS) experiment can be found elsewhere [4, 5]. TheCMS coordinate system has the origin at the center of the detector, z-axis points along the beam direction towardthe west, with the transverse plane perpendicular to the beam. We dene φ to be the azimuthal angle, θ to bethe polar angle and the pseudorapidity as η = − ln(tan[θ/2]). The central feature of the CMS apparatus is asuperconducting solenoid, of 6 m internal diameter. Within the eld volume are the silicon pixel and strip tracker,and the barrel and endcap calorimeters (|η| < 3): a crystal electromagnetic calorimeter (ECAL) and a brass-scintillator hadronic calorimeter (HCAL). Outside the eld volume, in the forward region, there is an iron-quartzber hadronic calorimeter (3 < |η| < 5). The HCAL and ECAL cells are grouped into towers, projecting radiallyoutward from the origin, for triggering purposes and to facilitate the jet reconstruction. In the region |η| < 1.74these projective calorimeter towers have segmentation ∆η = ∆φ = 0.087, and the η and φ width progressivelyincreases at higher values of η. The energy in the HCAL and ECAL within each projective tower is summed tond the calorimeter tower energy. Towers with |η| < 1.3 contain only cells from the barrel calorimeters, towersin the transition region 1.3 < |η| < 1.5 contain a mixture of barrel and endcap cells, and towers in the region1.5 < |η| < 3.0 contain only cells from the endcap calorimeters.Jets are reconstructed using both the iterative and midpoint cone algorithms [5], with indistinguishable resultsfor this analysis. Below we will discuss three types of jets: reconstructed, corrected and generated. The re-constructed jet energy, E, is dened as the scalar sum of the calorimeter tower energies inside a cone of radius√(∆η)2 + (∆φ)2 = 0.5, centered on the jet axis. The jet momentum, ~p, is the corresponding vector sum:~p =∑Eiuˆi with uˆi being the unit vector pointing from the origin to the energy deposition Ei inside the cone.The jet transverse momentum, pT , is the component of ~p in the transverse plane. The E and ~p of a reconstructedjet are then corrected for the non-linear response of the calorimeter to a generated jet. Generated jets come fromapplying the same jet algorithm to the Lorentz vectors of stable generated particles before detector simulation. Onaverage, the pT of a corrected jet is equal to the pT of the corresponding generated jet. The corrections estimatedfrom a GEANT [6] simulation of the CMS detector increase the average jet pT by roughly 50% (10%) for 70 GeV(3 TeV) jets in the region |η| < 1.3. Further details on jet reconstruction and jet energy corrections can be foundelsewhere [5, 7]. The jet measurements presented here are within the region |η| < 1.3, where the sensitivity tonew physics is expected to be the highest, and where the reconstructed jet response variations as a function of ηare both moderate and smooth.The dijet system is composed of the two jets with the highest pT in an event (leading jets), and the dijet mass isgiven by m =√(E1 + E2)2 − (~p1 + ~p2)2. The estimated dijet mass resolution varies from 9% at a dijet mass of0.7 TeV to 4.5% at 5 TeV.CMS will record events that pass a rst level trigger followed by a high level trigger. For an instantaneous luminos-ity of 1032 cm−2s−1, we consider three event samples collected by requiring at least one jet in the high level triggerwith corrected transverse energy above 60, 120 and 250 GeV, prescaled by factors of 2000, 40 and 1, respectively.For an integrated luminosity of 100 pb−1, the three event samples will effectively correspond to 0.05, 2.5, and100 pb−1. The rst event sample will be used to measure the trigger efciency of the second sample. The secondand third event samples will be used to study dijets of mass above 330 and 670 GeV, respectively, for which thetrigger efciencies are expected to be higher than 99% [8].Backgrounds from cosmic rays, beam halo, and detector noise are expected to occasionally produce events withlarge or unbalanced energy depositions. They will be removed by requiring 6ET /∑ET < 0.3 and∑ET < 14TeV, where 6ET (∑ET ) is the magnitude of the vector (scalar) sum of the transverse energies measured by allcalorimeter towers in the event. This cut is estimated to be more than 99% efcient for both QCD jet events andthe signals of new physics considered. In the high pT region relevant for this search, jet reconstruction is fullyefcient.CMS plans to search for contact interactions using the jet pT distribution. Figure 1 shows the inclusive jet differen-tial cross section as a function of pT , for jets with |η| < 1. Considering rst the QCD processes, the reconstructed2and corrected quantities are compared with the QCD prediction for generated jets. After corrections, the recon-structed and generated distributions agree. The ratio of the corrected jet cross section to the generated jet crosssection varies between 1.2 at pT = 100 GeV and 1.05 at pT = 500 GeV, remaining roughly constant for higher pT .The deviation of this ratio from 1 is attributed to the smearing effect of the jet pT resolution on the steeply fallingspectrum. The measured spectrum in data could be further corrected for resolution smearing, and this ratio fromsimulation is an estimate of the size of that correction. The measurement uncertainties are predominantly system-atic. The inset in Fig. 1 shows the effect on the jet rate of a 10% uncertainty in the jet energy correction. Thislevel of uncertainty could be expected in early running, for an integrated luminosity around 10 pb−1. This exper-imental uncertainty is roughly ten times larger than the uncertainties from parton distributions, as estimated usingCTEQ6.1 ts [9]. Figure 1 shows that the effect of new physics from a contact interaction with scale Λ+ = 3 TeVis convincingly above what could be expected for measurement uncertainties with only 10 pb−1. For comparison,a Tevatron search has excluded contact interactions with scales Λ+ below 2.7 TeV [10].CMS plans to search for narrow dijet resonances using the dijet mass distribution. Figure 2 shows the differentialcross section versus dijet mass, where both leading jets have |η| < 1, and the mass bins have a width roughly equalto the dijet mass resolution. Considering rst the QCD processes, the cross section for corrected jets agrees withthe QCD prediction from generated jets. To determine the background shape either the Monte Carlo prediction or aparameterized t to the data can be used. The inset to Fig. 2 shows a simulation of narrow dijet resonances with a q*production cross section. This is compared to the statistical uncertainties in the QCD prediction, including triggerprescaling. This comparison shows that with an integrated luminosity of 100 pb−1 a q* dijet resonance with a massof 2 TeV would produce a convincing signal above the statistical uncertainties from the QCD background. Forcomparison, a Tevatron search has excluded q* dijet resonances with mass, M, below 0.87 TeV [11]. The heaviestdijet resonances that CMS can discover (at ve standard deviations) with 100 pb−1 of integrated luminosity, usingthis search technique and including the expected systematic uncertainties [12, 13], are: 2.5 TeV for q*, 2.2 TeV foraxigluons [14] or colorons [15], 2.0 TeV for E6 diquarks [16], and 1.5 TeV for color octet technirhos [17]. Studiesof the jet η cut have concluded that the optimal sensitivity to new physics is achieved with |η| < 1.3 for a 2 TeVspin 1 dijet resonance decaying to qq¯ [18].CMS plans to search for both contact interactions and dijet resonances using the dijet ratio, r = N(|η| <0.7)/N(0.7 < |η| < 1.3), where N is the number of events with both jets in the specied |η| region. The di-jet ratio is sensitive to the dijet angular distribution. For the QCD processes, the dijet ratio is the same for correctedjets and generated jets, and is constant at r = 0.5 for dijet masses up to 6 TeV [18]. Figure 3 shows the dijet ratiofrom contact interactions and dijet resonances, compared to the expected statistical uncertainty on the QCD pro-cesses, for 100 pb−1 of integrated luminosity, including trigger prescaling. The signal from a contact interactionwith scale Λ+ = 5 TeV rises well above the QCD statistical errors at high dijet mass. Systematic uncertainties inthe dijet ratio are expected to be small, since they predominantly cancel in the ratio as previously reported [12, 19].Using the dijet ratio, CMS can discover a contact interaction at scale Λ+ = 4, 7 and 10 TeV with integrated lumi-nosities of 10, 100, and 1000 pb−1, respectively [18]. The signal from a 2 TeV spin 1/2 q* produces a convincingpeak in the dijet ratio, because it has a signicant rate and a relatively isotropic angular distribution compared tothe QCD t-channel processes. Fixing the cross section of the 2 TeV dijet resonance for |η| < 1.3 at 13.6 pb (fromthe q* model), the dijet ratio in the presence of QCD background increases by approximately 6% when consider-ing a spin 2 resonance decaying to both qq¯ and gg (such as a Randall-Sundrum graviton [20]), and the dijet ratiodecreases by approximately 4% when considering a spin 1 resonance decaying to qq¯ (such as a Z′, axigluon, orcoloron) [18]. Hence, the sensitivity to a 2 TeV dijet resonance depends only weakly on the spin of the resonance.To measure the spin, we need both the dijet ratio and an independent measurement of the cross section of theresonance, for example, from the dijet mass differential cross section. Nevertheless, with sufcient luminosity, thissimple measure of the dijet angular distribution, or a more complete evaluation of the angular distribution, can beused to see these small variations and infer the spin of an observed dijet resonance.In conclusion, CMS plans to use measurements of rate as a function of jet pT and dijet mass, as well as a ratio ofdijet rates in different η regions, to search for new physics in the data sample collected during early LHC running.With integrated luminosity samples in the range 10100 pb−1, CMS will be sensitive to contact interactions anddijet resonances beyond those currently excluded by the Tevatron.We thank the technical and administrative staffs at CERN and other CMS Institutes, and acknowledge supportfrom: FMSR (Austria); FNRS and FWO (Belgium); CNPq and FAPERJ (Brazil); MES (Bulgaria); CERN; CASand NSFC (China); MST (Croatia); University of Cyprus (Cyprus); Academy of Sciences and NICPB (Estonia);Academy of Finland, ME and HIP (Finland); CEA and CNRS/IN2P3 (France); BMBF and DESY (Germany);GSRT (Greece); NKTH (Hungary); DAE and DST (India); IPM (Iran); UCD (Ireland); INFN (Italy); KICOS3(Korea); CINVESTAV, CONACYT and UASLP-FAI (Mexico); PAEC (Pakistan); SCSR (Poland); FCT (Portugal);JINR (Armenia, Belarus, Georgia, Ukraine, Uzbekistan); MST and MAE (Russia); MSEP (Serbia); OCT (Spain);ETHZ, PSI, University of Zurich (Switzerland); NSC (Taipei); TUBITAK and TAEK (Turkey); STFC (UnitedKingdom); DOE and NSF (USA).References[1] E. Eichten, K. Lane and M. E. Peskin, Phys. Rev. Lett. 50, 811 (1983).[2] U. Baur, I. Hinchliffe and D. Zeppenfeld, Int. J. Mod. Phys. A2, 1285 (1987).[3] T. Sjostrand, L. Lonnblad, S. Mrenna, hep-ph/0108264.[4] CMS Collaboration, The CMS experiment at the CERN LHC (accepted for publication in JINST).[5] CMS Collaboration, Physics TDR Volume I, CERN-LHCC-2006-001.[6] S. Agostinelli et al. (GEANT4 Collaboration), Nucl. Instrum. Meth. A506, 250 (2003).[7] CMS Collaboration, CMS Physics Analysis Summary JME-07-003 (2008). athttp://cms-physics.web.cern.ch/cms-physics/public/JME-07-003-pas.pdf .[8] S. Esen and R. Harris, CMS Note 2006/069.[9] J. Pumplin et al. (CTEQ Collaboration), JHEP 07, 012 (2002).[10] B. Abbott et al. (D0 Collaboration), Phys. Rev. Lett. 82, 2457 (1999).[11] K. Hatakeyama et al. (CDF Collaboration) recent public results athttp://www-cdf.fnal.gov/physics/exotic/r2a/20080214.mjj resonance 1b/.[12] CMS Collaboration, J. Phys. G: Nucl. Part. Phys. 34, 995 (2006).[13] K. Gumus, N. Akchurin, S. Esen and R. Harris, CMS Note 2006/070.[14] J. Bagger, C. Schmidt and S. King, Phys. Rev. D37, 1188 (1988).[15] E. Simmons, Phys. Rev. D55, 1678 (1997).[16] J. Hewett and T. Rizzo, Phys. Rep. 183, 193 (1989) and references therein.[17] K. Lane and S. Mrenna, Phys. Rev. D67, 115011 (2003).[18] CMS Collaboration, CMS Physics Analysis Summary SBM-07-001 (2007) athttp://cms-physics.web.cern.ch/cms-physics/public/SBM-07-001-pas.pdf .[19] S. Esen and R. Harris, CMS Note 2006/071.[20] L. Randall and R. Sundrum, Phys. Rev. Lett. 83, 3370 (1999).4 (GeV)TJet p0 500 1000 1500 2000 2500 3000 3500 (pb/GeV)T/dpσd-710-610-510-410-310-210-110110210310410 (GeV)TJet p200 400 600 800 1000 1200 1400Number of Jets / 50 GeV110210310410510610CMS SimulationFigure 1: The inclusive jet pT differential cross section expected from QCD for |η| < 1, for generated jets (points),reconstructed jets (triangles), and corrected jets (open circles). The inset shows the number of generated jetsexpected in 50 GeV bins for an integrated luminosity of 10 pb−1. The standard QCD curve (solid) is modied by asignal from contact interactions with scale Λ+ = 3 TeV (dotted) and 5 TeV (dashed). The shaded band representsthe effect of a 10% uncertainty on the jet energy scale.5Dijet Mass (GeV)0 1000 2000 3000 4000 5000 6000 7000/dm (pb/GeV)σd-610-510-410-310-210-110110210310410CMS SimulationDijet Mass (GeV)0 1000 2000 3000 4000 5000 6000 7000(Signal-QCD)/QCD-0.100.10.20.30.40.50.60.7Figure 2: The dijet mass differential cross section expected from QCD for |η| < 1 from generated jets (points),reconstructed jets (triangles), and corrected jets (open boxes). The inset shows dijet resonances reconstructedusing corrected jets, coming from q* signals [13] of mass 0.7, 2, and 5 TeV. The fractional difference (histogram)between the q* signal and the QCD background is compared to the statistical uncertainties in the QCD prediction(vertical bars) for an integrated luminosity of 100 pb−1.6Dijet Mass (GeV)500 1000 1500 2000 2500 3000 3500|<1.3)η|<0.7) / N(0.7<|ηDijet Ratio = N(|0.30.40.50.60.70.80.911.1CMS SimulationFigure 3: The dijet ratio for corrected jets expected from QCD (horizontal line), with statistical uncertainties(vertical bars) for an integrated luminosity of 100 pb−1, is compared to QCD + contact interaction signals with ascale Λ+ = 5 TeV (dashed) and 10 TeV (dotted), as well as to QCD + dijet resonance signals (histogram) with q*masses of 0.7 and 2 TeV.7

CMS Search Plans and Sensitivity to New Physics using Dijets

CMS Search Plans and Sensitivity to New Physics using Dijets

Abstract

Similar works

Full text

Available Versions

CERN Document Server