The study of QCD processes at the LHC will serve two main goals. First the
predictions of QCD will be tested and precision measurements will be performed.
Second QCD processes represent a major part of the background to other Standard
Model processes and signals of new physics at the LHC and thus need to be
understood precisely in the new kinematic region available here. Furthermore,
the production cross-sections for almost all processes are controlled by QCD.Comment: 4 pages, 4 figures in eps, talk given at XXXI International Symposium
  on Multiparticle Dynamics, Sept 1-7, 2001, Datong China. URL
  http://ismd31.ccnu.edu.cn

Stavropoulos, Georgios D.

English

arXiv

The study of QCD processes at the LHC will serve two main goals. First the predictions of QCD will be tested and precision measurements will be performed. Second QCD processes represent a major part of the background to other Standard Model processes and signals of new physics at the LHC and thus need to be understood precisely in the new kinematic region available here. Furthermore, the production cross-sections for almost all processes are controlled by QCD

Stavropoulos, G D

CERN Document Server

QCD Processes at the LHC

Crossref

QCD PROCESSES AT THE LHC

arXiv:hep-ph/0111420v1  29 Nov 2001QCD PROCESSES AT THE LHCGEORGIOS D. STAVROPOULOSCERNThe study of QCD processes at the LHC will serve two main goals. First thepredictions of QCD will be tested and precision measurements will be performed.Second QCD processes represent a major part of the background to other Stan-dard Model processes and signals of new physics at the LHC and thus need to beunderstood precisely in the new kinematic region available here. Furthermore, theproduction cross-sections for almost all processes are controlled by QCD.1 IntroductionThe Large Hadron Collider (LHC) is a proton-proton collider with 14 TeVcenter of mass energy and design luminosity of 1034 cm−2s−1. Beam crossingsare 25 ns apart and at design luminosity there are on average 23 interactionsper crossing. 10 fb−1 of integrated luminosity are expected to be collected inone year of data taking at the initial low (1033 cm−2s−1) luminosity and 100fb−1 for one year of data taking at the nominal one.A detailed understanding of QCD is important for almost all the physicsprocesses to be studied at the LHC, as the production mechanisms are mostlycontrolled by QCD. LHC will allow QCD studies to be performed at very highenergy, including precision tests and measurements in an as yet unexploredkinematic region.2 Jet PhysicsThe measurement of jet production cross-section at LHC will provide a strin-gent test of perturbative QCD in an energy regime never probed so far. InFigure 1(a) the inclusive jet cross-section1 is shown as a function of the trans-verse energy of the jet, ET , for three different bins in η. The expected statisticsfor an integrated luminosity of 30 fb−1 amounts to 4·105 events with EjetT >1 TeV, demonstrating the fact that at LHC the statistical uncertainties onthe jet cross-section measurement will be small. More important in this mea-surement are the systematic errors, which can arise from the jet algorithm,the knowledge of the jet energy scale and the understanding of the overallcalorimeter response, the knowledge of the jet trigger efficiency, the knowl-edge of the luminosity for the overall normalization, and the effect of theunderlying event. The expected statistics at large jet ET values implies a0 < |h|  < 11 < |h|  < 22 < |h|  < 3ET Jet [GeV]ds/dE T [nb/GeV] NLO QCD10-1110-910-710-510-310-110 1000 2000 3000 4000 500010 510 610 710 80 1 2 3log(1/x)Q2 [GeV2 ]Figure 1. (a): Inclusive jet cross-section as a function of the transverse energy of the jet.(b): Range in 1/x and Q2 for the di-jet differential cross-section measurement.desirable control of the systematic uncertainties to a precision of better than1% for ET < 1 TeV and to about 10% for ET of about 3 TeV. The jet cross-section measurement can be used for the determination of αs. It was shown2that at LHC a measurement of αs(MZ) up to scales of order TeV can beachieved with a 10% accuracy.The measurement of di-jet events and their properties can be used toconstrain the parton densities inside the proton, in various kinematic regionsof Q2 and x. In Figure 1(b) the expected range in x and Q2 accesible withthe di-jet differential cross-section measurement is shown. For a transverseenergy threshold of 180 GeV , most of the events have Q2 values of about 105and values of x between 2·10−3 and 0.5.The di-jet invariant mass and angular distributions can be used to searchfor new physics. Studies have shown that the di-jet angular distribution hasan excellent discovery capability for quark compositeness. An integral lumi-nosity of 300 fb−1 will allow excluding quark substructure at 95% CL, if theconstituent interaction constant is of the order of 40 TeV (see figure 2(a)).3 Photon PhysicsDirect photon measurements can provide important constraints on partondistributions, especially on the gluon distribution in the proton. The detectionof photons at hadron collider is a challenging task due to the large backgroundfrom jet production, where fluctuations can mimic the signature of a photon.02.557.51012.5-10 -5 0 5 10hdNch/dh PYTHIA 6.122 - APYTHIA 6.122 - Model4PYTHIA 5.724 - ATLASPHOJET 1.12HERWIG 5.9ISAJET 7.32Figure 2. (a): Deviation of the di-jet angular distribution from the Standard Model predic-tions, for di-jet mass above 5 TeV for 300 fb−1. (b): Charged particle density in minimum-bias events at LHC energies as a function of pseudorapidity, for six model predictions.As an example in ATLAS it was found that for a photon efficiency of 80% arejection factor against jets of larger than 3x103 can be achieved for ET >40 GeV, thus providing a good signal-to-background ratio. The selection atATLAS will be based on a trigger demanding an isolated photon of ET > 60GeV within |η| < 2.5. This gives a value of the Bjorken-x of 5·10−4.4 Lepton PhysicsThe measurement of the Drell-Yan lepton pair production and the productionofW and Z bosons (with a leptonic decay to electrons or muons) will allow oneto constrain the quark and anti-quark densities of the proton at a scale givenby the invariant mass of the lepton pair (W/Z mass respectively) over a widerange in Bjorken-x. For example, the measurement of Z boson productioncross-section allows one to cover the range of 3·10−4 < x < 0.1 in Bjorken-xat Q2 = 8·103 GeV2.5 Hard Diffractive ScatteringDuring the last few years the understanding of diffractive phenomena has re-ceived revived attention due to the observation of hard diffractive events inseveral experiments. Thus, the study of hard diffractive scattering at LHCbecomes an important topic of the physics program of the involved exper-iments. An extension of the LHC detectors in the forward region (beyondpseudo-rapidities of |η| = 5) is presently under study, in order to increase theacceptance for charged particles from inelastic interactions (possibly includ-ing a momentum measurement) and to provide tagging and measurement ofleading protons from elastic and diffractive interactions.6 Properties of minimum-bias eventsAt the LHC, minimum-bias events will make up the 23 interactions per bunchcrossing at high luminosity. In order to understand precisely their contributionto the measured quantities for the hard scattering events of interest, a detailedknowledge of the structure of the minimum-bias events is required. In Figure2(b) the charged particle density in minimum-bias events at LHC energies isshown as a function of pseudorapidity. Six models, tuned on CDF data, wereused for this calculation. There is a difference of a factor 2 in the expectedcharged particle density in the central region, for models which are able todescribe CDF data. Each minimum-bias event is expected to contribute fromcharged particles about 0.5 to 1 GeV per unit rapidity and unit azimuth tothe transverse energy.7 ConclusionsThe LHC will provide a large sample of events with high pT signatures forQCD studies. It will extend the kinematic range to values of Q2, the hardscale of the partonic process, of the order of TeV2. The fraction of the protonmomentum attributed to a parton will reach values below 10−5, while keepingthe hard scattering scale above 100 GeV2.A precise knowledge and understanding of QCD processes is importantfor the studies of the Higgs boson(s) and for searches for new physics beyondthe Standard Model, where QCD represents a large part of the background.References1. ATLAS Detector and Physics Performance Technical Design Report,CERN/LHCC 99-14 and 99-15 (1999).2. H. Stenzel, Determination of αs using jet cross-section parameterizationsat hadron colliders, ATL-PHYS-2001-003, March 26, 2001.

QCD Processes at the LHC

Abstract

Similar works

Full text

Available Versions

CERN Document Server

Crossref

CERN Document Server