Precision measurements (?) with Atlas, at the LHC (emphasis on QCD)

1. Precision measurements (?)with Atlas, at the LHC(emphasis on QCD) Maarten Boonekamp CEA-Saclay (on behalf of the ATLAS collaboration) DIS 2004 Precision measurements with Atlas

2. Outline • LHC and ATLAS. Tests & measurements in an unexplored kinematic region. • Jets, direct photons, W&Z, heavy quark measurements and their uncertainties • Luminosity measurement, minimum bias trigger • Mass scale and detector resolution function • Conclusions. Precision measurements with Atlas

3. LHC (Large Hadron Collider): • p-p collisions at √s = 14TeV • bunch crossing every 25 ns (40 MHz) • low / high luminosity : L ~ 2.1033 / 1034 cm-2s-1 • Production cross section and dynamics are largely controlled by QCD. • Reach : ET up to ~ 5 TeV • Test QCD predictions and perform precision measurements. Precision measurements with Atlas

4. ATLAS: A Toroidal LHC AparatuS Trivia : 7000 tons, L ~ 44 m,  ~ 22 m, ~107 electronics channels. Muon Spectrometer : air-core toroidal system, | η| < 2.7. Calorimetry : LAr EM calorimeter (| η|< 3.2); Hadron calorimeter ( | η|< 4.9). Inner Detector (tracker) : Si pixels & strip detectors + TRT; 2 T magnetic field; coverage |η|< 2.5. Precision measurements with Atlas

5. ATLAS: Status Precision measurements with Atlas

6. LHC Parton Kinematics • The kinematic acceptance of the LHC detectors allows to probe a new range of x and Q2( ATLAS coverage: |η| < 5 ). • Q2 up to ~108 • x down to ~10-6 Precision measurements with Atlas

7. Jets ● 0 < |η| < 1 ○ 1 < |η| < 2 ■ 2 < |η| < 3 • Measure triple differential dijet cross-section : ds/dETd1d2 •  pdf’s • A few numbers : (L = 30 fb-1) • Statistical uncertainty small (<1% up to 1 TeV) • Systematics : • Influence of jet definition • calorimeter response & trigger efficiency • jet energy scale (goal of 1%), • luminosity (dominant when known to 5% -10%) • the underlying event. dσ/dET [nb/GeV] ET Jet [GeV] Q2 [GeV2] Precision measurements with Atlas Log(1/x)

8. αs : scale dependence • measurements of αS(MZ) will not compete with precision measurements from e+e-/DIS • BUT we can measure its running, up to the highest energies: • αS= 0.118 at ET = 100 GeV • αS~ 0.082 at ET = 4 TeV  30% effect • Method : with A,B computed using pdf’s (caveat: they contain an assumption for αS) • Expected uncertainties : • pdf accuracy • Jet energy scale & detector resolution : a very small (<1%) non-gaussian part, together with ds/dET ~ ET-8, can easily mimic a spectacular violation of QCD ( -1.5 < ηjet < 1.5 ) Precision measurements with Atlas

9. Direct photon production • Production mechanisms: qg→γq (dominant) qq→γg • Potentially very useful for fg determination ET >40 GeV (Q2 >103 GeV2) allows to reach x ~5x10-4 (for ||<2.5). • Statistics again not a problem : 2x104 events with ET >500 GeV are expected for 30 fb-1 • The fragmentation background is very dangerous and difficult to control • Rejection against p0’s (from jets) : ~ few 103 • s(g-jet)/s(dijet) : ~ 10-3 (100 < ET < 500) |ηγ| < 2.5 Precision measurements with Atlas

10. W&Z production • e and m channels : 108 W and 107 Z events/year each, at low luminosity • Small background • Z events contrain quark pdf’s at low pT, and also the gluon at large pT. Typical range : 3x10-4 < x< 0.1 at Q2 ~ 8x103 GeV2 • W mass measurement : several methods available, all familiar from the Tevatron. Goal : ~20 MeV. Main uncertainties/limiting factors : • Uncertainties in pdfs, W width and radiative decays contribute 10 MeV each • Energy/momentum scale should be known to 0.02%, to contribute less than 15 MeV Precision measurements with Atlas

11. Heavy flavour production • Again copious : • tag using soft muons or displaced tracks • Production mechanisms : • gg  cc, bb ( gluon pdf) • c(b)g  c(b)g ( c, b pdf) • Range : pTγ > 40 GeV, pTμ ~ 5-10 GeV  0.001 < xc (xb)< 0.1 • c- and b-jet E scale again affects results (pdf’s, top quark mass) Precision measurements with Atlas

12. Luminosity measurement • From elastic scattering, in the Coulomb region ( Totem) • Roman pots at ~240 m of the IP; scintillating fibre detectors (position res. ~ 25 mm) • Special optics : b* = 2650 m, L ~ 1027 cm-2s-1 • Combined fit of dN/dt to L, stot, r, b • Goal : precision  2% • compare to 5-10% from machine • More details : see talk in Diffraction session Fit Results (χ2/NDF=1442/1467):σtot = 98.7±0.8 mb (100)ρ = 0.148±0.007 (0.15)B = 17.90±0.12 GeV-2(18)L = (1.11±1.6%) 1027cm-2s-1 (1.09×1027) (5M events generated (90 hrs), ~4M reconstructed, beam optics assumed perfectly known) Precision measurements with Atlas

13. Minimum bias & underlying event • Pedestals to all physics measurements. Predictions not precise enough • Aim : pin down this uncertainty at start-up • Problem : ATLAS can trigger only on jets and leptons. Random trigger will pick up only noise (start-up lumi) • Recent idea : add scintillator planes at both ends of ID (temporarily), divert a few channels from the TileCal to read-out == interaction trigger Precision measurements with Atlas

14. Setting the absolute scales - Jets • Jet scale == pparton / Ejet • Want to determine indepedently of Monte-Carlo (as much as possible) • W decays in semi-leptonic top events : 2 light-quark jets, 2 b-jets, a lepton • in events with 2 b-tagged jets, assume the 2 other ones are from W • rescale their energy so that mjj = mW • obtain average scaling factors vs. E, cone size… Advantage : ~5.104 events / year Problems : combinatorial bg, overlapping jets Achieves ~1% precision above ~75 GeV, ~3% below Precision measurements with Atlas

15. Setting the absolute scales - Jets • Z + jet events : still a few 105 for 10 fb-1 • jet = gluon (~30%), light quarks (~50%), c-quarks (~13%), b-quarks (~7%) • Exploit the expected pT(Z) vs. pT(jet) balance Advantages : statistics again ; events are less crowded ; pT(Z) very well measured Problem : theoretical justification? cf. fractional imbalances at parton level (Pythia): • Main source : ISR! How to account for it without reintroducing model-dependence? Precision measurements with Atlas

16. Setting the absolute scales - EM • Electron & photon scale, from Z  ee, and Z  eeg, mmg • But the Z isn’t at the Z, because of a mixture of effects : mainly material in front of the calorimeters and radiative decays • ID material ultimately known from E/p, g conversions… lengthy! • FSR is a theoretical ingredient • Remember target : 0.02% Precision measurements with Atlas

17. Conclusions • LHC will probe unexplored regions, OK • Jets, photons, dileptons, heavy quarks will be produced in copious quantities • The samples are complementary and probe many aspects of strong and EW interactions with high precision • … in principle. • Although statistical precision will almost always be <10-3, systematics are most often ~1-5% • Dominating : • Luminosity  recent new perspectives • Minimum bias, underlying event  recent new perspectives • Mass scales, knowledge of detector resolution  work needed, clearly Precision measurements with Atlas