1 / 32

Results from the ATLAS experiment

Results from the ATLAS experiment. Introduction LHC and ATLAS in 2010 The LHC physics landscape ATLAS results …from common to rare processes QCD and jet physics, W/Z/photons, top, SUSY and Higgs searches

alima
Download Presentation

Results from the ATLAS experiment

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Results from the ATLAS experiment • Introduction • LHC and ATLAS in 2010 • The LHC physics landscape • ATLAS results …from common to rare processes • QCD and jet physics, W/Z/photons, top, SUSY and Higgs searches • Rediscovering the Standard Model, challenging theoretical predictions, looking for surprises • Can only cover a fraction of the results, and not touch all topics • Some will be covered in more detail by later ATLAS talks • Much more at https://twiki.cern.ch/twiki/bin/view/AtlasPublic Richard Hawkings (CERN / ATLAS collaboration) XXIX Workshop on Recent Advances in Particle Physics and Cosmology, Patras, Greece, 14/4/11 Richard Hawkings

  2. LHC and ATLAS running in 2010 • Main LHC goals for 2010 • Commission machine at 7 TeV step-by-step • Stored energy in beam beyond 1MJ (beyond HERA, Tevatron, potential for damage from uncontrolled losses) • Up to 100s bunches per beam, bunch trains • Deliver data to experiments – commissioning detectors and first physics results • Alternating periods of commissioning/data-taking • Aimed to get to L=1032 cm-2s-1 by end 2010 • Achieved – got to ~2 1032 cm-2s-1 and 48 pb-1 total • Also a short Heavy Ion (Pb+Pb) run at end • ATLAS goals for 2010 • Commission the detector, trigger and computing • Understand the detector performance • ‘Rediscover’ the Standard Model and take first steps into new territory • For both precision physics and searches Richard Hawkings

  3. The LHC physics landscape • LHC detectors looking for needles in haystacks • Rare processes hiding in an overwhelming background of ‘bread and butter’ physics • Rates for some SM processes at nominal LHC performance – 14 TeV and L=1034 cm-2s-1 • Early work: understanding these ‘backgounds’ • Many interesting things to be learned on the way • Important for calibrating detectors LHC@7 Integrated luminosity / time Richard Hawkings

  4. Initial running compared to Tevatron • Also much early ‘discovery potential’ • Rather than comparing with ultimate LHC performance, compare with Tevatron • Searches typically ~5 fb-1 at Ecm=1.96 TeV • Gain in energy (27 TeV) … • Loss in int. L (5-10  0.05-1 fb-1 in 2010-11) • Gain/loss depends on initial state (gg or qq) and required invariant mass MX • E.g. HWW at 160 GeV, gains factor ~15 from increased ECM (gg dominated) • E.g. ttbar production (X=2×175 GeV) gains a factor ~20, becomes gg dominated • E.g. 1 TeV Z’ (qq) gains factor ~100 • Plenty of potential for opening up new territory, even with 2010 dataset • See what has happened, starting from the highest cross-section processes W Stirling Richard Hawkings

  5. Total inelastic cross-section measurement • Total cross-section for ppXY divided into • Dominant non-diffractive, colour flow between protons • Single (SD) and double-dissociative (DD) processes • Fractions of SD, DD model-dependent • (elastic contribution pppp) • Measure using MBTS scintillators • Front-face of endcap calo (2.1<|h|<3.4) • RSS=#(hit 1 side)/#(hit both sides) • Constraints f(SD+DD) in models – acceptance • Result 60.3±0.05(stat)±0.5(syst)±2.1(lumi) pb • For Mx/s>5 10-6 - can also extrapolate • Many other results on minimum bias events • Multiplicities and UE event studies (track,calo) Richard Hawkings

  6. Measuring the luminosity • Cross-section measurements need int. luminosity • Relative luminosity as function of time from e.g. • LUCID – dedicated Cherenkov detectors 17m from IP • Hit counting and timing from MBTS scintiallators • Counting events with reconstructed primary vertex • Good consistency between them (0.5% over time) • Absolute calibration from van der Meer scans • Calculate instantaneous luminosity from LHC params • Beam sizes Sx and Sy • Measure from separation scan (vdM scan) • Total uncertainty of 3.4%, limited by bunch charge (3.1%) and shape of luminous region (0.9%) vdM scan Richard Hawkings

  7. Hard QCD – jets studies • Jet production from quark and gluon scattering dominant high pT process • Now done for 20<pT<1500 GeV; |y|<4.4 • Good agreement with NLO QCD predictions • Many detailed studies – e.g. rapidity gaps • Study gap between two highest pT jets • Gap fraction – no third jet above pT>Q0 • Varying success of MC/calculations • Important for jet-veto studies in VBF Higgs Richard Hawkings

  8. Highest pT jet event • Highest pT jet from 2010 data: pT=1.5 TeV (2nd jet 1.0 TeV) Richard Hawkings

  9. Determination of jet energy scale • Jet energy scale uncertainty is critical • Very good understanding with 2010 data • Based on ‘single particle’ response in data • Measure E/p for tracks up to ~ 20 GeV • Translate to effects on jets using MC • Various uncertainties – noise modelling, dead material, MC modeling and tune • Valid for central region – transport to forward using dijet balance central/fwd jets • Validation comparing calibrated jets to various references ‘in-situ’ • Jet-photon direct balance: pT<250 GeV • Photon vs. missing ET (MPF) pT<250 GeV • High-pT jet vs. multi jets – pT up to ~1 TeV • Calorimeter vs. track jets up to ~1 TeV • Factor 2 better than in summer 2010 • Ultimate aim to get to ~1% Richard Hawkings

  10. New physics in dijets? • Dijet events also used to search for new physics • Resonance in dijet mass: excited quarks (q*), axigluons, RS gravitons, quantum black holes • ‘BumpHunder’ used to search for narrow or wide resonances – nothing found • Limits: mq*, axigluon>2.1 TeV, QBH M>3.7 TeV • Also set model-independent limits for resonance • Information also from angular distribution • In QCD, t-channel gluon dominant, cos q*~1 • Gives flat distribution in c=exp(|y1-y2|) • Define F(c) fraction with c<3.32vs c<30, binned in dijet mass • Again set limits on additional contributions • e.g. contact interactions L>9.5 TeV (exp. 5.7) Richard Hawkings

  11. b-flavoured jets • b-jet production – test of QCD pred. • 1st step to understanding Wbb, Zbb • Measure using b-jets selected with secondary vertex algorithm • Long lifetime/ high mass of b hadrons • Select displaced vertices and fit vertex mass to obtain b-fraction • Systematics from JES, b-tag efi • Data falls more quickly at high pT than POWHEG or PYTHIA predict Richard Hawkings

  12. W and Z production • Dominant process with high-pT isolated leptons (e/m) • Wln - missing ET from undetected neutrino • Transverse mass gives clear peak around W mass • Zll – two high pT leptons – very clean signal • Now also using forward electrons in FCAL (2.5<|h|<4.9) – more challenging environment • 260k W and 25k Z observed in 2010 data • Experimental uncertainties 2-3% in measured region central-forward central-central Richard Hawkings

  13. W / Z results • 5% uncertainties on s when extrapolating to full range • Compare with predictions – sensitive to PDFs at x≈10-2 • Good agreement for cross-sections (also W+ vs W-) and ratios • Further constraints on PDFs from W asymmetry • Ratio measurement – many systematics cancel • Crucial to control acceptance effects (e.g. trigger and recon biases) that are different for m+/m- • Redundant m measurements from inner detector and muon spectrometer help Richard Hawkings

  14. Calibrating the detector with Zs • Large sample of Zee/mm used to calibrate • Z-mass very precisely known from LEP • Width of peak sensitive to detector resolution • Zmm closer to MC expectation after reprocessing data after end of 2010 LHC run • Zee statistics allow calorimeter calibration in 28 separate regions – good to 3% • Z-topology also used for ‘tag-and-probe’ • Measure lepton trigger and reconstruction efi • Very good agreement with MC expectations m efi Richard Hawkings

  15. New physics in leptonic events? • Many models predict heavy bosons W’ or Z’ • W’-like objects would appear in tail of MT distrib with additional Jacobian peaks • Background dominated by tail of SM W - level sensitive to understanding of lepton pT and missing ET resolution at high pT • Z’-like objects would appear in tail of mll distrib • background dominated by SM Z/g* • No excess of events – limits from LHC now similar to or exceeding those from Tevatron Richard Hawkings

  16. Diboson production • Can produce pairs of bosons (e.g. WW, WZ, ZZ, Wg, Zg) Richard Hawkings

  17. WW and W/Z+g production • WW production sensitive to TGC vertex • Events characterised by 2 high-pT leptons, missing ET and low jet multiplicity • Low number of events due to Wen/mn BR • Measure sWW=40+20-16±7pb, agrees with SM • W/Z+g: add. photon in W/Z • Contributions from FSR g from lepton, and from fragmentation • Require isolated g • No TGC ZZg / Zgg vertex • Resuts agree with expectations Richard Hawkings

  18. Photons and photon searches • Photon prodn is another window on LHC physics • Colourless probe of hard scattering, info on PDFs • Important for searches – e.g. Hgg or gg+missing ET • Inclusive isolated prompt photons from 20-400 GeV • In two steps due to evolving luminosity and triggers • Need to understand fake and fragmentation contribs • Good agreement with NLO theoretical predictions • Searches for narrow resonance in gg spectrum • Sensitive to Randall-Sundrum gravitons • Limits set at 545 / 920 GeV for k/MPL=0.02 (0.1) • Also searches with gg+MET for e.g. UED Richard Hawkings

  19. Diphoton candidate event • Highest invariant mass pair for two ‘tight’ photons: mgg=679 GeV Richard Hawkings

  20. W/Z+jets • Large samples of W/Z+n-jets available • Up to W+5 jets, Z+4 jets (each jet costs ~as) • Challenge for QCD calculations and background for top and new physics • Expt uncertainties dominated by jet energy scale • Study multiplicity and jet pT distributions • Good description by state-of-the art NLO calculations and LO multi-parton generators Richard Hawkings

  21. Top quark production • Heaviest SM particle – interesting in its own right, and important background • Top decay tWb; Wln or qq: tt final states with 2, 1 (0) leptons, ETmiss and (b)-jets e-m candidate Richard Hawkings

  22. Top cross-section measurements I • Simplest measurement from dilepton channel • ttWbWblνblνb with l=e or μ (5% of tt events) • Relatively clean final state – backgrounds from Z/g*+jets or fake leptons estimated from data control samples • Signal in ll+≥2 jets (no b-tagging), especially e-m channel • O(100) events in 2010 data – statistics limited • Higher statistics in semileptonic channel • ttWbWblνbjjb, l=e/μ (30%) • Lepton+≥4 jets selection, reconstruct mtop from 3 jets • Backgrounds from W+jets, and QCD (fake lepton) • B-tagging increases S/B • Combined analysis of 3 and ≥4 jet samples with 0,1,2 b-tags to maximise sensitivity Richard Hawkings

  23. Top cross-section measurements • Combine l+jets (b-tag) and dilepton results • Including correlated systematics • QCD prediction for s(tt) depends on mtop • Interpret s(tt) measurement as measurement of top quark mass – complementary to direct recon • Directly sensitive to top quark pole mass • Analysis repeated with different MC templates • Result mtop=166.4+7.8-7.3 GeV Richard Hawkings

  24. Direct measurement of the top quark mass • Enough data to start on mtop measurement • Fundamental SM param, relation to Higgs? • Measured to 1.1 GeV precision at Tevatron • Measurement in lepton+jets channel • Reconstruct mass of 3 jets (ttlνbjjb) • Major uncertainty due to jet energy scale • Known to 4-6% at this stage in ATLAS • Reduce using known W mass (tWbjjb) and measure R32=mtop/mW • Measure mtop=169.3±4.0±4.9 GeV • Systematics dominated by residual JES and b-jet uncertainties, plus ISR/FSR • Alternative methods - simultaneous kinematic fits of mtop and JES • Statistical error below 1 GeV for 1 fb-1 • Then techniques based on lepton pT, decay length, tbJ/ybecome viable Richard Hawkings

  25. Top quark properties • Investigate helicity of W in top decay • θ* between W in top frame, l in W frame • Assume massless b-quark, fit F0 (FL=1-F0) • Fit F0=0.59±0.10±0.07 (SM value 0.70) • Search for new physics sources of top • E.g. top partner pairs TTtA0tA0 • A0 a stable neutral scalar – missing ET • Look for excess missing ET in lepton+jets • Tail dominated by tt dileptons with missed second lepton – veto loose second lepton • Exclude models around mT of 300 GeV e/m+jets channel Richard Hawkings

  26. Single top production – a first look • Tops also produced singly • Substantial cross-sections, but b/g from W+jets and top pairs • t-chan: e/m, b-jet, Etmiss, fwd jet • Further cuts on h-jet, reco mtop • Signal concentrated in t not tbar • Fitted value st=53+46-36 pb • Limit of st<162 pb @ 95% CL • Wt: l+jets (Wqq), dilepton (ln) • Combined limit sWt<158 pb • More data needed … t-chan: 66pb Wt: 15pb t-chan Wt dilepton Richard Hawkings

  27. Searches for supersymmetry • R-parity conserving SUSY at LHC • squark/gluino pair production giving cascades (jets/leptons) ending in LSP (missing-ET) • Characterised by Meff=ΣETjets+ΣETlep+ETmiss • 0-lepton analysis with jets and missing-ET • 2 and 3 jet regions for sensitivity to squark and gluino pairs – no excesses seen • 1-lepton analysis with 3 jets and missing-ET • Lepton gives trigger and reduces QCD b/g, but reduces signal acceptance • For both analyses, backgrounds measured separately in control regions Richard Hawkings

  28. Supersymmetry limits • Null results – interpret in various simplified SUSY models • m0 vs m1/2 plane in MSUGRA/CMSSM framework, tanb=3, A0=0, m>0 • For equal squark and gluino mass,0+1 lepton combination excludes up to 815 GeV • Alternatively, set m(c10)=0, exclusion limits in (squark/gluino) mass plane Richard Hawkings

  29. More exotic supersymmetry scenarios • Searches also performed in 2-leptons,3+ leptons, leptons+bjets, em dileptons … • Improves exclusion in various corners of SUSY parameter space • R-parity violating SUSY may give stable massive particles • Forming R-hadrons (bound state SUSY+SM) • Slow high-pT tracks with anomalous dE/dx (pixels) and timing for b=v/c<1 (TileCal) • Used to estimate candidate masses • Background estimated from data, using independence of pixel and tile results • Limits set on stable sbottom (294 GeV), stop (309 GeV), gluino (562-586 GeV) • Latter depends on model for interaction of gluino-gluon bound state with matter Richard Hawkings

  30. The Standard Model Higgs • Full range of Higgs searches on 35pb-1 • Close to Tevatron limits on HWWlnln • Reaching close to 10x SM for 200-600 GeV • No hints of signal so far … • Interesting prospects for 2011-2012 run • With 1 fb-1 at 7 TeV, 50% chance to exclude 130-460 GeV if nothing there • With 5 fb-1 at 8 TeV, 3s for any mH<500 GeV • Only weakly dependent on the Ecm choice HZZllnn/llqq HWWlnqq HWWlnln Richard Hawkings

  31. Higgs beyond the Standard Model • Inclusive search for MSSM A/H/ht+t- with lepton+hadronic tau decays • Signal reconstructed in tt visible mass, require low MT to reduce W, top b/g • Backgrounds estimated from data – QCD and W+jets using opposite/same sign • Zt+t-+jets from Zm+m-replacing muons with ‘embedded’ simulated t decays • No excess found – exclusion in (mA,tanb) plane extended beyond Tevatron Richard Hawkings

  32. Summary and outlook • The LHC era has begun – already a huge harvest of physics results • LHC machine commissioning in 2010 was a spectacular success • 2011 running started mid February – ATLAS already recorded 26 pb-1 • Detector and trigger commissioning with high pileup, field-off runs for alignment • Expect ≥1fb-1 data this year if all goes well, more in 2012 • Many beautiful ATLAS physics results from 2010 data • ~25 physics papers submitted, >50 new preliminary results at winter conferences • Standard Model ‘rediscovered’ at LHC energies • Very detailed and precise studies in soft and hard QCD, W,Z and photon production • Becoming sensitive to diboson production • Top physics program started – already close to theory precision in ttbar cross-section • First steps into unexplored territory at the energy frontier • SUSY and other new physics searches beyond previous experiments • Higgs searches starting to become interesting • This is only the beginning – much more to come in 2011… Richard Hawkings

More Related