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Standard Model QCD

Standard Model QCD. Outline Group Group convener background Group resources List of mayor institutions in the group Overview of the main analyses TDR vs current status Understanding of triggering Understanding of backgrounds Group goals for 2007. ATLAS-IFAE Meeting

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Standard Model QCD

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  1. Standard Model QCD Outline Group Group convener background Group resources List of mayor institutions in the group Overview of the main analyses TDR vs current status Understanding of triggering Understanding of backgrounds Group goals for 2007 ATLAS-IFAE Meeting December 21-22 2005 Sigrid Jorgensen

  2. Standard Model working group • This group studies all SM topics excluding those for which there are dedicated WG (Higgs, top and beauty physics). The studies are divided into Electroweak physics (see Carolina’s talk) and QCD physics. • There is a proposal to split the group in EW, QCD and in situ calibration. • Convenors: • Marteen Boonekamp (SPP, Service Physique des Particules) • Craig Buttar (Glasgow University) • Group Resources: • http://atlas.web.cern.ch/Atlas/GROUPS/PHYS/ICS/SM/sm.html#Mailing • List of major institutions in the group: • BNL • Glasgow University • SPP, France • University of Michigan

  3. Standard Model QCD • QCD topics: • Determination of parton densities of the proton • Measurement of the strong coupling constant • Definition of objects for QCD studies • QCD dynamics and event properties • Forward physics

  4. Determination of parton densities of the proton • Using di-jet production • Using direct photon production (based onTDR ch15 and on I. Hollins talk on SM phone meeting 10/10/05) • Using Drell-Yan and W/Z boson production (based onTDR ch15 and on A.Tricoli talk in Rome for W study and on M. Verducci talk in Rome for Z+b-jet study) • Using heavy quark production • Using any combination of the above signatures • Determine the most efficient way to verify our understanding of the PDFs in the scenario of an early discovery • Quantify the expected impact of LHC measurements on the accuracy of our knowledge of parton distributions • Motivation: • Deliver quantitative results on the expected precision on parton densities obtained from the various possible measurements. • Include statistical uncertainty and estimates of systematic uncertainties, due to detector and theoretical sources

  5. Determination of parton densities of the proton • Z+b-jet • Motivations: • Measurement of the b-quark PDF. • Background to Higgs search. • bb->Z is 5% of Z production. Knowing σZ to 1% requires a b-pdf precision of the order of 20% (HERA measurements far from this precision). • 30M event/year at low luminosity. • Signal cross section at LHC 80 times larger than at Tevatron and background 5 times smaller. • Summary: • Z+b measurement in ATLAS will be possible with high statistics and good purity of the selected samples. • It will be possible to control systematic errors related to b-tagging at the few-% level over the whole pT range. • Precision on this measurement will be dominated by systematic errors on luminosity, jet reconstruction and energy resolution. • Overall precision will be comparable to uncertainty on theoretical prediction. • Jet + photon • Motivation: • Constrain gluon PDF using direct photon production. • Study background to establish the degree to which signal can be seen. Investigate methods to reduce background. • 3T event/year at low luminosity • Current topics: • Understand how to reject against QCD jets in the full simulation to obtain good photon identification. • Investigating sensitivities to different pdf sets.

  6. Largest contribution Determination of parton densities of the protonusing W production • Motivation: • Improve on PDF uncertainties. • W total and differential cross sections theoretical calculations are very robust: known to NNLO in QCD perturbation theory, input EW parameters known to high accuracy. • Main theoretical uncertainty comes from PDFs. • At Q2~M2W the sea is driven by the gluon at low-x. • Measurement of W → lepton rapidity distribution can increase our knowledge of the gluon PDF. • Abundance of W’s: 300M event/year at low luminosity. • Method: • Evaluate systematic errors (statistical are negligible) • Simulate realistic experimental conditions. • Generate W+- → e+- events using HERWING 6.505 and MC@NLO 2.3 with different PDFs (MRST02, CTEQ61, ZEUS2002) and full quoted PDF uncertainty. Kinematic regime for LHC much broader than currently explored

  7. W-→ e- Rapidity Distributions at Generator Level for different PDFs using HERWIG. • Determination of parton densities of the proton using W production Error boxes are the full PDF uncertainties At y=0 the total uncertainty is ~ ±6% from ZEUS ~ ±4% from MRST01E ~ ±8% from CTEQ6.1 ZEUS to MRST01 central value difference ~5% We need to be more accurate than this (~3%) to discriminate between different pdfs MRST02 CTEQ61 W- -> e- W- -> e- h • Can we reduce the PDF errors by including the W rapidity distributions in global PDF Fits? • Generate data with CTEQ6.1 PDF, pass through ATLFAST detector simulation and then include this pseudo-data in the global ZEUS PDF fit. • Central value of prediction shifts and uncertainty is reduced W+ to lepton rapidity spectrum data generated with CTEQ6.1 PDF compared to predictions from ZEUS PDF AFTER these data are included in the fit W+ to lepton rapidity spectrum data generated with CTEQ6.1 PDF compared to predictions from ZEUS PDF

  8. Determination of parton densities of the protonusing W production • Event selection criteria (TDR selection cuts): • Electrons: |ŋ| < 2.4, Pt > 25 GeV • Missing Et > 25 GeV • To reject QCD bkg & high pT W and Z due to ISR: No reconstructed jets with pT > 30 GeV • Background to W+-→ e+-νe: • W →τν (→ eνν) • Z →τ+τ- (→ e+νν+ e-νν ) • Z → e+e- • QCD events (all 2 → 2 processes involving q, qbar, g) • Generated with HERWIG + CTEQ5L • Summary: • W rapidity distributions are good observables to constrain PDF’s at LHC. • LHC can significantly constrain the gluon distribution. • d/u ratio measurement is under investigation. • We are not limited by statistic but by systematic uncertainties. To discriminate between conventional PDF sets we need to achieve an accuracy ~3% on rapidity distributions. • Herwig & k-factors are a good approximations of NLO. • Need to explore various sources of systematic uncertainties: detector misalignments, detector efficiency, backgrounds etc.

  9. Measurement of the strong coupling constant • Via inclusive jet production(based on ATL-PHYS-2001-003, Determination of alpha_s using jet cross section parameterizations at hadron colliders, Stenzel, H. and on C. Gwenlan on SM meeting 10/10/2005) • Via the ratio of 3-jet to 2-jet production • Using jet shape observables • Via direct photon observables • Via the ratio of W+1-jet to W+0-jet production • Motivation: • Quantify the possibility of precision measurement of the strong coupling constant with ATLAS, which should cover a wide range in scales up to TeV.

  10. Measurement of the strong coupling constantvia inclusive jet production • Motivation: • Quantify the possibility of precision measurements of the strong coupling constant. • Aim for 10% accuracy. • Large uncertainties: PDF and missing higher orders • Inclusive jet production: • Being studied the inclusive jet cross section. • PDF uncertainties and scale uncertainties. • Uncertainties for final HERA data have been estimated. • Method: • Precise measurements of single inclusive jet cross section have been performed by TEVATRON and will be provided at LHC to larger values of transverse energy. • Theoretical predictions of the cross section at NLO in perturbative QCD depend both on the PDF parameterization set and on the value of the strong coupling constant. • The method uses PDFs with variable values of αs which allows for a measurement of αs independent of the input αs of the PDFs. • The cross section predictions obtained with different PDF families and for variable αs values are parameterized. The result of the parameterization are simple analytical functions f(ET,αs(ET)) which return the cross section for a given range of jet transverse energy and αs. The inverse of these functions can be used to obtain from a cross section measurement the coupling constant as function of the jet ET. • Parameterization of the total cross section is of the form σtot(αs) = a0 + a1αs or σtot(αs) = Aαs2+ Bαs3. Systematic uncertainties are derived from a variation on PDFs and renormalization scales. • All studies are made at parton level.

  11. Measurement of the strong coupling constantvia inclusive jet production • The figures represent the total cross section for central jet production at large ET calculated with different PDFs as function of αs(MZ). A linear dependence has been fitted to the calculated cross sections, including either all PDF sets or individual subsets (CTEQ4M, MRSAP, GRV94). • Summary: • Single inclusive jet cross section is sensitive to the strong coupling constant. • Used sets of PDFs to calculate jet cross section at NLO. • Dependence on αs(ET) parameterized with simple analytical functions. • This parameterization can be used to extract αs(ET) from a given cross section measurement. • Systematic uncertainties comes from uncertainties on PDFs, de difference between a linear and a quadratic form and from perturbative origin, which is the most important. They are found to be of ±10% at TEVATRON and ±7% at LHC. • A measurement of αs (MZ) up to scales of order TeV from jet production at LHC with a 10% accuracy seems achievable. • For the future: • Study correction with gluon PDF. • Investigate hadronization aspects.

  12. Definition of objects for QCD studies • How to define a jet?, clustering algorithms • How to define an isolated photon? (based on TDR ch15 and on I. Hollins talk on SM phone meeting 10/10/05) • Motivation: • Find an optimal algorithm which can be applied at the detector, hadron and parton level. • Investigate and quantify the relation between the different levels

  13. Definition of objects for QCD studiesHow to define an isolated photon? • Phonon + jet • Motivation: • Use direct photons to constrain the gluon pdf • Need to identify photons at high ET (ET > 100 GeV) • Reject against QCD jets • Summary: • Identified key variables and cut strategy to separate photons from the background. • Studied only unconverted photons. • Separation using EM calorimeters and isolation cuts. • Study done for five different eta bins from 0 to 2.47. • Reached a photon efficiency of 92% and a S/N for ET>150 GeV of 9.7

  14. QCD dynamics and event properties • Search for manifestations of BFKL effects in signatures as two jet production at large rapidity separation, W+2jet production • Measurement of properties of minimum bias events (based on TDR ch 15 and on A. Moraes, C.Buttar and D.Clements in Rome) • High multiplicity signatures • Motivation: • Understanding minimum bias is very important for all physics studies as well as a powerful diagnostic tool on the performance pf the detector.

  15. LHC QCD dynamics and event properties Measurement of properties of minimum bias events • Motivations: • Improve our understanding of QCD effects, multiple interactions, total cross-section,... • Understand pile-up, backgrounds,... • Early measurements with minimum bias data • dNch/dŋ • dNch/dpT • Started looking at dNch/dŋ at ŋ = 0. Only require several thousand events and the inner tracker

  16. 1000 events dNch/d  1000 events dNch/d dNch/dpT QCD dynamics and event properties Measurement of properties of minimum bias events • Events generated with Athena 9.0.4 and reconstructed with 10.0.1. • Only a fraction of tracks reconstructed due to limited rapidity coverage and and because can only reconstruct track pT with good efficiency down to ~500 MeV • Investigating for different scenarios: • Full inner detector track reconstruction • Track reconstruction using only SCT barrel • Track reconstruction with reduced (or zero) solenoid field (don’t need track pT for dNch/dŋ) • dNch/dŋ does not require TRT at ŋ = 0 • If only require angle ŋ for dNch/dŋ measurement, don’t need magnetic field • Also starting to look at pion reconstruction efficiency Black = Generated (Pythia6.2) Blue = TrkTrack: iPatRec Red = TrkTrack: xKalman 1000 events dNch/d SCT only PIXEL+SCT+TRT Black = Generated charged tracks Blue = Reconstructed: NO TRT, NO solenoid Red = Reconstructed: NO TRT, WITH solenoid

  17. LHC dNchg/dη ~ 15 Tevatron QCD dynamics and event properties Underlying events <Nch> • UE are defined as the transverse region of the event and everything in the collision except the hard process. dNchg/dη ~ 30 Average multiplicity of charged particles in the underlying event associated to a leading jet pT leading jet (GeV) • Jet samples used (recon with 10.0.1): J1-J8 in bins from 17 GeV to >2240 GeV. • Selection: Njets > 1, |ŋjet| < 2.5, ETjet > 10 GeV. • Reconstructed vs. MC jet samples:

  18. QCD dynamics and event propertiesMinimum bias and underlying events • Triggering: • Minimum bias: • Scintillators mounted on front face of LAr end-cap cryostat: 20cm < R < 130cm • Use during early running when luminosity very low • Need to study triggering efficiency • Few hours of data taking should provide enough Statistics. • Jets: • At low luminosity, LVL1 trigger: single jet, ETjet > 180 GeV (TDR) • Summary: • There are sizeable uncertainties in LHC predictions for minimum bias and the underlying event. Need to understand better how to tune the energy dependence of the event activity. • Measurements with min-bias data should be amongst first with pp collisions. dNch/dŋ at ŋ=0 only requires several thousand events. • Candidate measurement for pp commissioning phase? • Robust measurement, not dependent on full ID reconstruction • Early measurements (low luminosity) of jets are likely to allow a reasonable good measurement for the UE and consequently should allow the tuning of MC models. There will be enough statistics in the first days of data taking. • Reconstructed track distributions for the underlying event reproduce the MC event generator predictions • Need to continue investigation into how well we can make such measurement for several detector running scenarios

  19. Forward physics • Measurement of the total proton-proton cross-section • Measurement of elastic scattering • Measurement of soft and hard diffractive scattering (based on ATL-PHYS-2000-004, Studies of Hard Diffraction in ATLAS, Battistoni, G and Tapprogge, S) • High t diffraction • Exclusive production of central states: pp → p+X+p • A lot of these measurements will only be feasible when additional detector components would be added to ATLAS, allowing to tag and measure leading protons, to extend the coverage for (charged) particles beyond the limit of |ŋ| = 5 and possibly for detection of leading neutrals.

  20. Forward physicsMeasurement of soft and hard diffractive scattering • Motivation: • Studies on the expected cross section • Investigate the sensitivity of some observables (Pomeron) to the partonic structure • Method: • Using PHOJET MC, samples of single diffractive and double diffractive events have been generated. • Selection of central diffractive scattering requires two leading protons, both with xF>0.9 and 0.01<|t|<1GeV • The expected cross-section for central diffraction as a function of the invariant mass of the diffractive system have been studied for three cases: no requirements on detection of leading protons (solid line), requiring two leading protons according to the selection mentioned above (dashed line) and requiring in addition at least two jets with |ŋjet|<3.2 and ET>10GeV. • To study the sensitivity to the partonic structure of the Pomeron, different partonic distributions were used and lead to different shapes of the pseudorapidity distribution. • Summary: • A measurement of single diffractive di-jet production should allow to constrain the Pomeron structure. • Study was done for particle level without taking into account detector effects. • This investigation have to take the geometry and performance of the ATLAS detector into account (for |ŋ|<5) and could be extended beyond |ŋ|>5 for possible additional detectors to be added to ATLAS.

  21. Summary • Current SM QCD activities: • MB and UE • Tuning UE and MB for MC (new model in PY6.3) • How to measure MB and UE during early running • PDFs and jets • Studies using Ws • Inclusive jet rates • Photon + jet • Z+b-jet • General motivations: • Test predictions of QCD. • Perform precision measurements allowing additional constraints to be established or providing measurements of the strong coupling constant. • Study in depth QCD processes since they represent a major part of the background of other SM or new physics signals. • Goals for 2007: • Continue current analyses and start new ones. • Evaluate SM model benchmarks and uncertainties (How to tune MC) • Detector studies (triggers and calibration, just starting)

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