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Physics at hadron collider with Atlas 2nd lecture

Physics at hadron collider with Atlas 2nd lecture. Simonetta Gentile Università di Roma La Sapienza, INFN on behalf of Atlas Collaboration. Outline. Introduction to Hadron Collider Physics LHC and ATLAS detector Test of Standard Model at LHC Parton distribution function

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Physics at hadron collider with Atlas 2nd lecture

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  1. Physics at hadron collider with Atlas 2nd lecture Simonetta Gentile Università di Roma La Sapienza, INFN on behalf of Atlas Collaboration Simonetta Gentile Gomel School of Physics 2005

  2. Outline • Introduction to Hadron Collider Physics • LHC and ATLAS detector • Test of Standard Model at LHC • Parton distribution function • QCD + jet physics • Electroweak physics (Z/W –bosons) • Top physics • Search for Higgs boson • Supersymmetry • Conclusions 1st 2nd 3rd 4th Simonetta Gentile Gomel School of Physics 2005

  3. Cross Section of Various SM Processes • Low luminosity phase • 1033/cm2/s = 1/nb/s • approximately • 200 W-bosons • 50 Z-bosons • 1 tt-pair • will be produced per second! • The LHC uniquely combines the • two most important virtues of • HEP experiments: • High energy 14 TeV • and high luminosity • 1033 – 1034/cm2/s Simonetta Gentile Gomel School of Physics 2005

  4. K.Jacobs Simonetta Gentile Gomel School of Physics 2005

  5. Detector performance requirements • Lepton measurement: pT GeV  5 TeV • ( b X, W’/Z’) • Mass resolution (m ~ 100 GeV) : •  1 % (H gg, 4) •  10 % (W  jj, H  bb ) • Calorimeter coverage : || < 5 • (ETmiss, forward jet tag) • Particle identification : e,, , b Simonetta Gentile Gomel School of Physics 2005

  6. Three crucial parameters for precise measurements • Absolute luminosity :goal < 5% • Main tools: machine, optical theorem, rate of • known processes (W, Z, QED pp  pp ) •  energy scale : goal 1‰ most cases • 0.2‰ W mass • Main tool: large statistics of Z   (close to mW , mH) • 1 event/l/s at low L • jet energy scale: goal 1%(mtop, SUSY) • Main tools: Z+1jet (Z  ) , W jj from top decay • 10-1 events/s at low L Simonetta Gentile Gomel School of Physics 2005

  7. LEP • Mw is an important parameter • in precison test of SM • MW=80.425 ± 0.034 GeV. • 2007 Mw 80…± 20 MeV • (Tevatron Run II) Improvement at LHC requires Control systematic better 10-4 level Simonetta Gentile Gomel School of Physics 2005

  8. s (pp  W + X)  30 nb e, mn ~ 300  106events produced ~ 60  106 events selected after analysis cuts one year at low L, per experiment W production process ~ 50 times larger statistics than at Tevatron ~ 6000 times larger statistics than at LEP Simonetta Gentile Gomel School of Physics 2005

  9. mW= 79.8 GeV mW= 80.3 GeV lepton neutrino ΔΦ mTW (GeV) hadronic recoil  ETmiss Method of mass measurements SincepLnnot known (onlypTncan be measured throughETmiss), measuretransverse mass, i.e. invariant of  perpendicularto the beam : mTW distribution is sensitive to mW  fit experimental distributions with SM prediction (Monte Carlo simulation) for different values of mW  find mW which best fits data Simonetta Gentile Gomel School of Physics 2005

  10. W mass MC thruth Estimated with W recoil • Isolated lepton PT>25 GeV • ETmiss>25 GeV • No high pt jet ET<20 GeV • W recoil < 20 GeV Full sim. MTW (MeV) Sensitivity to MW through falling edge c2 (data-MC) Compare data with Z0 tuned MC samples where input MW varies in [80-81] GeV by 1 MeV steps  Minimize c2(data-MC): 2 MeV statistical precision Input MW (GeV) Simonetta Gentile Gomel School of Physics 2005

  11. W mass • Uncertainties • Come mainly from capability of Monte Carlo prediction to reproduce real life: • detector performance: energy resolution, energy scale, etc. • physics: pTW, W,, backgrounds, etc. Dominant error (today at Tevatron, most likelyalso at LHC): knowledge of lepton energy scale of the detector:if measurement of lepton energy wrong by 1%, then measured mW wrong by 1% • Trailing edge of distribution is sensitive to W-mass • Detector resolution smears the trailing edge of mT distribution Simonetta Gentile Gomel School of Physics 2005

  12. Atlas MW Most serious challenge • Take advantage from • large statistics • Z  e+e, + • Combine channels & • experiments •  mW  15 MeV Simonetta Gentile Gomel School of Physics 2005

  13. Calibration of the detector energy scale • E measured = 100.000 GeV for all calorimeter cells → • perfect calibration • To mesaure Mw to ~ 20 MeV need a enegy scale to 0.2 ‰, • ( Eelectr = 100 GeV then 99.98 GeV < E measured< 100.02 GeV) Simonetta Gentile Gomel School of Physics 2005

  14. Calibrations strategy. • Calorimeter modules are calibrated with test beam of known energy • In Atlas calorimeter sits behind Inner Detector: • electrons lose energy in material in front of calorimeter (inner detector) calibration “in situ” using physics sample Z → e+e- with the constrain m ee = mz known to ≈ 10-5 • same strategy for muon spectrometer, using Z → m+m- Simonetta Gentile Gomel School of Physics 2005

  15. q e, /Z pT > 6 GeV || < 2.5 e+, + q Inversion of e+e qq at LEP Drell-Yan Lepton-Pair Production • Total cross section • pdf • search for Z, extra dim. , ... • Much higher mass reach as • compared to Tevatron Z pole Simonetta Gentile Gomel School of Physics 2005

  16. Drell-Yan Lepton-Pair Production • Forward-backward asymmetry • estimate quark direction • assuming xq > xq • Measurement of sin2W effective • 2005: LEP & SLD • sin2W = 0.2324  0.00012 • AFB around Z-pole • large cross section at the LHC • (Z  e+e)  1.5 nb • stat. error in 100 fb-1 • incl. forward electron tagging • (per channel & expt.) • sin2W  0.00014 • Systematics (probably larger) • PDF • Lepton acceptance • Radiative corrections Atlas [%] Zpole Asymmetry → sin2W Controlled at required level For the significance of measurement Simonetta Gentile Gomel School of Physics 2005

  17. Di - Boson production Measuring Triple Gauge Couplings (TGC) & Testing gauge boson self couplings to SM • WWg WWZ vertices exist → 5 parameters: in SM g1Z,kg, kZ = 1;lg, lZ= 0 • ZZg , ZZZ do NOT exist in SM: 12 couplings parameters hiV,fiV (V=g,Z) Charged & Neutral TGS’s • Some anomalous contributions ( -type) increase with s • high sensitivity at LHC • Sensitivity from : • -- cross-section (mainly -type)and pT measurements • -- angular distributions (mainly k-type) Simonetta Gentile Gomel School of Physics 2005

  18. WW Couplings Atlas Charged TGS’s Test CP conserving anomalous couplings at the WW vertex  and  • W final states • W  e and  • pT spectrum of bosons Wg Sensitivity to anomalous couplings from high end of the pT spectrum ATLAS 30 fb-1 pTg(GeV) Simonetta Gentile Gomel School of Physics 2005

  19. Atlas ZZ Couplings • PTZ distribution ATLAS 30 fb-1 WZ pTZ (GeV) Simonetta Gentile Gomel School of Physics 2005

  20. Sensitivity to WW Couplings 30 fb-1 LEP 2004 Atlas Charged TGS’s • At LHC limits depend on energy scale  • Large improvement wrt LEP • in particular on  due to higher • energy Simonetta Gentile Gomel School of Physics 2005

  21. Triple Gauge Couplings Neutral TGS’s • ZZZ vertex doesn’t exist in SM • gZZ vertex does exist in SM Analysis: search for ZZ → 4 leptons (e±, µ±) Main background - real ZZ events (σ=12pb) - Z+jet Sensitivity: ~7·10-4 (100fb-1 and ΛFF=6 TeV ) Simonetta Gentile Gomel School of Physics 2005

  22. ZZ Couplings Neutral TGS’s Example: Couplings at the ZZ vertex hi • Z final states • Z  e+e– and +– • pT spectrum of photons or Z and mT() Zg Simonetta Gentile Gomel School of Physics 2005

  23. SM anomalous QGC Atlas Triple-Boson Production Sensitive to quartic gauge boson couplings (QGC) 30 W signal events in 30 fb-1 Simonetta Gentile Gomel School of Physics 2005

  24. Triple gauge couplings • SM allowed charged TGC in WZ, Wg with 30 fb-1 • ≥1000 WZ (Wg) selected with S/B = 17 (2) • 5 parametersfor anomalous contributions scale with √ŝ for g1Z,ks and ŝ for s • Measurements still dominated by statistics, but improve LEP/Tevatron results by ~2-10 • in SM g1Z,kg, kZ= 1;lg, lZ = 0 • SM forbidden neutral TGC in ZZ, Zgwith 100 fb-1 • 12 parameters,scales with ŝ3/2 orŝ5/2 • Measurements completely dominated by statistics, but improve LEP/Tevatron limits by ~103-105 Z,g Z,g Z,g • Quartic Gauge boson Coupling in Wggcan be probed with 100 fb-1 Simonetta Gentile Gomel School of Physics 2005

  25. Status of SM model • High precision measurements→ Test of Standard Model • 1000 data points combined in 17 observables calculated in SM TOP MASS • a em(precision 3 10-9) (critical part Dahad) • GF (precision 9 10-6) (→MW) • Mz(precision 2 10-5)from line-shape • as(Mz) precision 2 10-2 hadronic observable Mtop and MHiggs Simonetta Gentile Gomel School of Physics 2005

  26. Measurement of the Top Mass: Motivation t W+ W+ b H W,Z 1-r  (1- )(1-rW) rW  (mt2-mb2)  Top mass from Tevatron (2005) : rW  log mH Simonetta Gentile Gomel School of Physics 2005

  27. Combination of Measurements Only best analysis from each decay mode, each experiment. Expected precision in 2007 at Tevatron: ± ~1GeV EPS95 KojiSato Simonetta Gentile Gomel School of Physics 2005

  28. tt production • 87% gluon fusion13% quark annihilation • Approx. 1 tt-pair per second at 1033/cm2/s • LHC is a top factory! Top Physics Inverseratio of production mechanism as compared to Tevatron • Top decay:  100% t  bW • Other rare SM decays: • CKM suppressed t  sW, dW: 10-3 –10-4 level • tbWZ: O(10-6) • difficult, but since mt mb+mW+mZ sensitive to mt • & non-SM decays, e.g. t  bH+ Simonetta Gentile Gomel School of Physics 2005

  29. Top Decays • the tt pair cross section is ~ 600 pb • Br (t→Wb) ~ 100% • no top hadronization • Di-lepton channel • Both W’s decay via W→n ( = e or m ;5%) • Lepton-jet channel • One W decays via W→n (= e or m ; 30%) • All hadronics • Both W decay via W→ qq (44%) tt final states(LHC,10 fb-1) Signature: Leptons Missing transverse energy b-jets • Full hadronic(2.6M): 6 jets • Semileptonic(1.7M):  + n+ 4jets • Dileptonic (0.3M): 2 + 2n+ 2jets Simonetta Gentile Gomel School of Physics 2005

  30. Tagging b-quarks Soft lepton tag Silicon vertex tag displaced tracks Search for non-isolated soft lepton in a jet B mesons travel ~ 3mm before decaying – search for secondary vertex Simonetta Gentile Gomel School of Physics 2005

  31. Atlas Top Mass from Semi-Leptonic Events • Easiest channel tt  bb qq l • Large branching ratio • Easy to select tt  bb qq  events from ATLAS Simonetta Gentile Gomel School of Physics 2005

  32. decay (29.6%) Measurement of mtop • Selection Require at least one e or µ PT > 20 GeV/c in central detector 2 jets 2 b-jets • Efficiency: ~65% • Systematics Dominant: Final-state radiation • jet energy calibration: 1% especially b-jet calibration Simonetta Gentile Gomel School of Physics 2005

  33. j1 W j2 t b-jet Atlas Top Mass from Semi-Leptonic Events Reconstruct mt from hadronic W decay Constrain two light quark jets to mW 70% top purity - efficiency 1.2 % • Isolated lepton PT>20 GeV • ETmiss>20 GeV • 4 jets with ET>40 GeV DR=0.4 • >1 b-jet (b60%, ruds102, rc101) Background: <2% W/Z+jets, WW/ZZ/WZ Simonetta Gentile Gomel School of Physics 2005

  34. Golden channel BR 30% and clean trigger from isolated lepton Important to tag the b-jets: enormously reduces background (physics and combinatorial) Hadronic side: W from jet pair with closest invariant mass to MW Require |MW-Mjj|<20 GeV Ligth jet calibrated with Mw constraint Assign a b-jet to the W to reconstruct Mtop Leptonic side Using remaining  +b-jet, the leptonic part is reconstructed |m b -<mjjb>| < 35 GeV Kinematic fit to the t t hypothesis, using MW constraints Br(ttbbjj  )=30%for electron + muon MTop from lepton+jet SN-ATLAS-2004-040 • Isolated lepton PT>20 GeV • ETmiss>20 GeV • 4 jets with ET>40 GeV DR=0.4 • >1 b-jet (b60%, ruds102, rc101) Simonetta Gentile Gomel School of Physics 2005

  35. Atlas Atlas Top Mass from Semi-Leptonic Events • Linear with input Mtop • Largely independent on Top PT Simonetta Gentile Gomel School of Physics 2005

  36. Top mass systematics • Systematics from b-jet scale: • 3.5 million semileptonic events in 10 fb-1 • (first year of LHC operation) • Error on mt  1 – 2 GeV • Dominated by • Jet energy scale (b-jets) • Final state radiation Simonetta Gentile Gomel School of Physics 2005

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