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

2. Maarten Boonekamp 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.

3. Maarten Boonekamp 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 ~ 2.10 33 / 10 34 cm -2 s -1  Production cross section and dynamics are largely controlled by QCD.  Reach : E T up to ~ 5 TeV  Test QCD predictions and perform precision measurements. Processσ (nb)Events/10 fb -1 ) Inclusive bb5 x 10 5 ~ 10 13 Inc. jets, E T > 0.2 GeV1~ 10 11 Inclusive tt0.8~ 10 7 Inc. jets, E T > 2 TeV~10 -8 ~ 10 3

4. Maarten Boonekamp Precision measurements with Atlas 4 ATLAS: A Toroidal LHC AparatuS Inner Detector (tracker) : Si pixels & strip detectors + TRT; 2 T magnetic field; coverage |η|< 2.5. Calorimetry : LAr EM calorimeter (| η |< 3.2); Hadron calorimeter ( | η |< 4.9). Trivia : 7000 tons, L ~ 44 m,  ~ 22 m, ~10 7 electronics channels. Muon Spectrometer : air-core toroidal system, | η | < 2.7.

5. Maarten Boonekamp Precision measurements with Atlas 5 ATLAS: Status

6. Maarten Boonekamp Precision measurements with Atlas 6 LHC Parton Kinematics  The kinematic acceptance of the LHC detectors allows to probe a new range of x and Q 2 ( ATLAS coverage: |η| < 5 ).  Q 2 up to ~10 8  x down to ~10 -6

7. Maarten Boonekamp Precision measurements with Atlas 7 Jets  Measure triple differential dijet cross-section : d  /dE T d  1 d  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. ● 0 < |η| < 1 ○ 1 < |η| < 2 ■ 2 < |η| < 3 dσ/dE T [nb/GeV] E T Jet [GeV] Q 2 [GeV 2 ] Log(1/x) Jet E T N events > 1 TeV4 x 10 5 > 2 TeV3 x 10 3 > 3 TeV40

8. Maarten Boonekamp Precision measurements with Atlas 8 α s : scale dependence  measurements of α S (M Z ) will not compete with precision measurements from e + e - /DIS  BUT we can measure its running, up to the highest energies:  α S = 0.118 at E T = 100 GeV  α S ~ 0.082 at E T = 4 TeV  30% effect  Method : with A,B computed using pdf’s (caveat: they contain an assumption for α S )  Expected uncertainties :  pdf accuracy  Jet energy scale & detector resolution : a very small (<1%) non-gaussian part, together with d  /dE T ~ E T -8, can easily mimic a spectacular violation of QCD ( -1.5 < η jet < 1.5 )

9. Maarten Boonekamp Precision measurements with Atlas 9 Direct photon production |η γ | < 2.5  Production mechanisms: qg→γq (dominant) qq→γg  Potentially very useful for f g determination E T >40 GeV (Q 2 >10 3 GeV 2 ) allows to reach x ~5x10 -4 (for |  |<2.5).  Statistics again not a problem : 2x10 4 events with E T >500 GeV are expected for 30 fb -1  The fragmentation background is very dangerous and difficult to control  Rejection against  0 ’s (from jets) : ~ few 10 3   (  -jet)/  (dijet) : ~ 10 -3 (100 < E T < 500)

10. Maarten Boonekamp Precision measurements with Atlas 10 W&Z production  e and  channels : 10 8 W and 10 7 Z events/year each, at low luminosity  Small background  Z events contrain quark pdf’s at low p T, and also the gluon at large p T. Typical range : 3x10 -4 < x< 0.1 at Q 2 ~ 8x10 3 GeV 2  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

11. Maarten Boonekamp 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)   c, b pdf   Range : p T γ > 40 GeV, p T μ ~ 5-10 GeV  0.001 < x c (x b )< 0.1  c- and b-jet E scale again affects results (pdf’s, top quark mass) Processσ (nb)Events/year ( L = 10 fb -1 ) cc, bb~ 10 5 ~ 10 12 ( tt0.8~ 10 7 )

12. Maarten Boonekamp 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  m)  Special optics :  * = 2650 m, L ~ 10 27 cm -2 s -1  Combined fit of dN/dt to L,  tot, , b  Goal : precision  2%  compare to 5-10% from machine  More details : see talk in Diffraction session Fit Results (χ 2 /NDF=1442/1467): σ tot = 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%) 10 27 cm -2 s -1 (1.09×10 27 ) (5M events generated (  90 hrs), ~4M reconstructed, beam optics assumed perfectly known)

13. Maarten Boonekamp 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

14. Maarten Boonekamp Precision measurements with Atlas 14 Setting the absolute scales - Jets  Jet scale == p parton / E jet  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 m jj = m W  obtain average scaling factors vs. E, cone size… Advantage : ~5.10 4 events / year Problems : combinatorial bg, overlapping jets Achieves ~1% precision above ~75 GeV, ~3% below

15. Maarten Boonekamp Precision measurements with Atlas 15 Setting the absolute scales - Jets  Z + jet events : still a few 10 5 for 10 fb -1  jet = gluon (~30%), light quarks (~50%), c-quarks (~13%), b-quarks (~7%)  Exploit the expected p T (Z) vs. p T (jet) balance Advantages : statistics again ; events are less crowded ; p T (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? Jet pT range20-60 GeV60-120 GeV>120 GeV All events16.3%6%1.1% Back-to-back7.4%3%0.5% Additional jet veto4.9%1.5%0.4%

16. Maarten Boonekamp Precision measurements with Atlas 16 Setting the absolute scales - EM  Electron & photon scale, from Z  ee, and Z  ee ,   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,  conversions… lengthy!  FSR is a theoretical ingredient  Remember target : 0.02% ConditionResulting scale uncertainty ID material known to 1%~0.01% FSR known to 10%~0.01%

17. Maarten Boonekamp 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