Results from the ATLAS experiment

1. Summer Institute SI2010 at Fuji Calm, Japan 17/08/2010 Yuji Yamazaki (Kobe Univ.) Results from the ATLAS experiment

2. LHC (Large Hadron Collider) at CERN • Objective: origin of mass (Higgs in SM), physics beyond standard model • The largest acceleratorwith the highest energy Mont Blanc City of Geneva 27km circumference (reusing the LEP tunnel) 7TeV proton-proton collisionsNominal CMS energy: 14 TeVTevatron  7 2010-2011 run: collisions at 7 TeV Construction started in 1996First physics run in March 2010 Lac Leman Switzerland ATLAS CERN France Results from the ATLAS experiment

3. 2010/3/30 First 7TeV collision ALSO in the ATLAS detector! Results from the ATLAS experiment

4. and people are finally happy … Results from the ATLAS experiment

5. … and busy days afterward:Luminosity vs time in ATLAS • Current lumi: 1.x pb–1 • Gradual increase in intensityfor safty of the accelerator • Linst ~ 4  1030 s–1 cm–2 • 2010/11 : 7 TeV • 2010： aiming for Linst ~ 4  1032 s–1 cm–2 • 2011： 1 fb–1 (Linst > 1032) • > 2013 : 14 TeV operation • After repairing joints in SC magnet Results from the ATLAS experiment

6. Where we are 7TeV • Reviewed today:presented at ICHEP10-280 nb−1 • We see: • Collision/jets/b-quark • W and Z • some top quarks • Higgs is not even produced … We are here Results from the ATLAS experiment

7. Today’s contents • Introduction • Re-discovery of our old friends • W and Z • top quarks • (high-pT) jets • Understanding basics in hadron-hadron collisions • Soft QCD • Status of searches and outlook * all results from ICHEP2010 or earlier release, see also: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/RESULTS/summer2010.html Results from the ATLAS experiment

8. ATLAS detector status • Active channel: 97-100% • Central tracker (semiconductor) • Calorimetry • Muon system • Trigger • Understanding inactive material All working fineWorking for understanding in detail Results from the ATLAS experiment

9. Hard collisions Standard-model processes: EW boson production, top, high-ET jets Results from the ATLAS experiment

10. Hadron-hadron collisions proton remnant a proton • Hard collision: viewed as a parton-parton collision • More theoretically(QCD factorisation theorem) • Proton remnant may collide each other high-pT particles fromhard scattering Results from the ATLAS experiment

11. W/Z production: Drell-Yan process • s-channel production of W/Z à la e+e • (best process for detector study) • Sensitive to quark part of PDFs(parton density functions) • Higher order QCD process:background for searches • Possibly new resonancedecaying into dilepton Z prod. O(10nb) Drell-YanW prod. O(100nb) W+jets Results from the ATLAS experiment

12. W production: signal extraction • Selection criteria: • Large missing from neutrino • Transverse mass close to MW Example from electron channel – almost background free Results from the ATLAS experiment

13. W production cross section • consistent with NNLO • Theoretical uncertainty ~ 4% • W less than W+as expected(more ud than du) using 17nb −1 Results from the ATLAS experiment

14. e/μ, charge asymmetry • All consistent each othermore statistics → systematic error dominant (challenges!) Results from the ATLAS experiment

15. Z0 → ee, μμ observation • consistent with NLO using 225nb −1 Results from the ATLAS experiment

16. Top quark at the LHC etc. • LHC produces a lot of top quarks • High energy collision→ large effective gluon luminosity • Much more events than Tevatron,even at 7 TeV topthreshold Results from the ATLAS experiment

17. Top decays and physics • t → Wb (~ 100%) • W → lν [1 b-jet, 1 charged lepton]semileptonic decay • W → cs, ud etc. [1 b-jet, 2 jets]hadronic decay • tt tagged by b-jets and • 2 semileptonic or • 1 semileptonic + 1 hadronic • Top: strong coupling to HiggsNew physics may emergee.g. t → H+b decay Results from the ATLAS experiment

18. 1-lepton sample electron channel muon channel using 280nb −1 • lepton + Emiss+ ≥ 3 jets (≥ b-tagged) • mjjj: clear peakfewer backgroundthan at Tevatron • Events are wherethey should be9 events observed Results from the ATLAS experiment

19. Results from the ATLAS experiment

20. 2-lepton sample • One candidate forboth ee and eμ • No candidate for μμ • b-tagged jet notrequired, but foundin both events • Again events are foundwhere they should be2 events observed Results from the ATLAS experiment

21. Results from the ATLAS experiment

22. Statistically not conclusive, but ATLAS seems to find top quarks properly Results from the ATLAS experiment

23. QCD hard scattering – jet production • Cross section = (PDF1)(PDF2)(ME) • Matrix element: Multijet eventBackground for searches • Effect on uncertainty in parton-density functionsGluons in particularDGLAP evolution valid? • Other issues • Heavy flavour production • Effect from soft QCD … will be covered later Results from the ATLAS experiment

24. Inclusive jetcross sections • Algorithm: anti-kT • jet area: cone-like • R = 0.4, 0.6 • Comparison w/ MCgood agreement • Systematic error(purple band)mainly fromJet Energy Scaleuncertainty Results from the ATLAS experiment

25. Comparison with NLO (R = 0.4) • Very good agreementalso on rapidity distribution Data / Theory Results from the ATLAS experiment

26. Main uncertainty: jet energy scale (JES) • large uncertainty in hadronic shower response simulation • Checked by • track momentum vs calorimeter cluster energy • dijet balance Results from the ATLAS experiment

27. JES uncertainty summary • 7% for ET > 100 GeV • ~ 10% at 20 GeV • Rome was not build in a day • need more study/data to narrow down the uncertaintyTentative goal : a few % Results from the ATLAS experiment

28. How high in pTor Mjjare we?– up to 600GeV / ~ 2 TeV • Good agreement with NLO (no surprise yet …) Results from the ATLAS experiment

29. Tentative conclusion on SM processes • Basic 2 → 2 processes investigated • Drell-Yan W/Z • High-ET jets • Successful re-discovery and quantitative confirmationon the standard model • Statistically limited • ATLAS seems to be able to measure top quarks properly No apparent trouble in understanding SM eventsTheory (pQCD, in particular) is healthy also at 7 TeV Results from the ATLAS experiment

30. Softer QCD results b-quark, J/ψ, inclusive particle spectrum, and low-ET jets Results from the ATLAS experiment

31. Heavy-flavour production LO NLO • Not easy to calculate • Important background for EM/searches • QCD MC used in ATLAS seems overestimating background in W study charm excitation bb pair not necessarilyback-to-back gluon splitting Tevatron “excess” on B meson productionover NLO QCD, need resummation Results from the ATLAS experiment

32. First result through J/ψ production • Two production mechanism • Direct J/ψ production • through B-meson decays • Two component separatableby tagging b-meson decay vertex direct(prompt) Results from the ATLAS experiment

33. J/ψ yield • Shape well described, but PYTHIA with colour-octet diagram gives factor-10 too high singlet “breaching”colourby a gluon Octet +softemission B-decay/prompt ratio described by the model→ both may have less octet state? Results from the ATLAS experiment

34. Soft QCD study • Most of the hadron-hadron collisions are with low pT • αS very largeperturbative calculation of quark-gluon interaction not possible • hadronic object behaves like “quark-gluon matter”collision occurs when two objects overlap • partons not visible – no clear jetparticles emitted from “colour string” forward-goingproton remnant proton remnant Results from the ATLAS experiment central production

35. Soft QCD is still important at high energies • Proton remnant may collideeach other, or • more than one partons maycollide at a proton crossing • because of high density of partons in proton at high energies  changing particle flow between high-pT objects affecting measurements • Pile-up (more than one collision in a bunch crossing) • One is high-pT (triggered) • Others would be low-pT (large cross section ~ 100mb)Need to know the nature of it Results from the ATLAS experiment

36. Charged particle spectrum in “minimum bias” events • Minimum bias • Requiring ≥ 1 charged track with ≥ 500 MeV • close to inelastic collisions but not exactly the same • (marginally) more tracks at 7 TeV than 900 GeV collisions inelastic single diffraction Results from the ATLAS experiment

37. Charged particle multiplicity in η:comparison with models (7 TeV) • Does not reproduce data • some are quite close, though • Didn’t these models tuned to SpS/Tevatron minbias data?Answer: not only. • also to particle flow between hard jets in hard events • showing deviation alsofrom 0.9 GeV and 2.36 GeV data • models do not have infinite number of free parameters! Results from the ATLAS experiment

38. PTdistribution,‹PT› vs. nch • Many models predict harder spectrum • need to retune the models … Results from the ATLAS experiment

39. After retuning (ATLAS MBT1) • Tuning mainly the parameters of multiple interactione.g.transverse distribution of partons in the proton Results from the ATLAS experiment

40. Softer side of hard jets – fragmentation • Investigation on lowest possible ET jets • ET > 4 GeV, using only charged particles • Should be described by pQCD (hard scattering)but how low? • Jet properties investigated • Charged multiplicity nch • Fragmentation function f(z) • pTjet, track spectrum Charged multiplicity6 < pTjet, track < 10 GeV Less tracks in the model Results from the ATLAS experiment

41. pT spectrum, fragmentation function f(z) • pT spectrum softer thanmost of models • Fragmentation function is also softer → particle spectrum is softer than expected at 7 TeV Results from the ATLAS experiment

42. Searches at 7 TeVLHC 2010-2011 • Aiming for accumulating 1fb−1 of data • Rather difficult: • SM Higgs • Sensitivity even at 7 TeV:to light particles of • SUSY • Extra dimension • Z’ etc. (dilepton) Results from the ATLAS experiment

43. Searches Dijet resonance and some initial studies for other channels Results from the ATLAS experiment

44. Today reviews … • Dijet resonance with full data available before ICHEP (new limit!) • SUSY background study • Outlook for 2010/2011 with 7 TeV • SUSY prospect • Dilepton resonance • Higgs prospect Results from the ATLAS experiment

45. Dijet resonance – excited quark search • Search focused to q* (excited quark) • qg → q* → qgdijet resonance • also sensitive to Technicolor, extra dimension, GUT bosons, etc. etc. • produced at central rapidityapplying |η1|, |η2| < 1.3 Results from the ATLAS experiment

46. Limit on q* • Excluding < 1.2 TeV • Tevatron limit< 870 GeV (w/ 1.13fb−1) First limit alreadybetter than Tevatron! Results from the ATLAS experiment

47. Searching for contact interactionthrough dijet angular distribution χ • Contact interaction limit :> 900 GeV • Quantum black hole may decay to dijet, spherically • Already excluded: for nextra = 6 and M < 800 GeV Results from the ATLAS experiment

48. Searching SUSY at the LHC • SUSY produced by squarks/gluinos • Normally the heaviest • Cascade decay to LSP + standard model particles→ Multi-jet events with large total mass • Leptons and/or heavy flavour quarks may be produced • Most of the SUSY study assumes R-parity conservation • Weakly interacting neutral LSP (dark matter candidate)escaping the detector → missing ET Multijet + Missing EToption: lepton(s), … Results from the ATLAS experiment

49. Background study (1) 4-jet 0-lepton channel • Distribution reproduced by QCD MC • High-mass event observed • at 1.6 TeV, Emiss ~ 100 GeV Missing ET Meff (scalar sum of pT’s, ETmiss) ETmiss > 40 GeV, ETmiss/Meff > 0.2ETmiss vector not aligned to jets Results from the ATLAS experiment

50. Results from the ATLAS experiment