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Recent results from LHC/ATLAS

2011 IPMU-YITP School and Workshop on Monte Carlo Tools for LHC 10 September 2011 Yuji Yamazaki (Kobe University). Recent results from LHC/ATLAS. LHC (Large Hadron Collider) at CERN. For finding Higgs, understanding EW symmetry breaking mechanism and physics beyond the Standard Model

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Recent results from LHC/ATLAS

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  1. 2011 IPMU-YITP School and Workshop on Monte Carlo Tools for LHC 10 September 2011 Yuji Yamazaki (Kobe University) Recent results from LHC/ATLAS

  2. LHC (Large Hadron Collider) at CERN • For finding Higgs, understanding EW symmetry breaking mechanismand physics beyond the Standard Model • Highest energyexperiment Mont Blanc Geneva 27km circumference Design beam energy: 7TeV Currently 3.5 GeV = 7 GeV CMS energy(3.5 times design energy) 40MHz collision frequency Design luminosity Lac Léman Swiss CERN France

  3. LHC accelerator operation • 2011: ? • 2012: continueing 7 TeV run, aiming for total 10 for 7 TeV • 2013-14 repairing super-conducting magnets for ~14 TeV run 2.62 as of 2011/09/09 2010 ~ 2.660already

  4. List of topics in this talk • Higgs search result as of LP2011 • W and Z boson production • + jets • Diboson • Top quark production • SUSY and other searches • QCD • Soft physics • jets Subjects are chosen from ones something to do withQCD part of Monte Carlo simulation / theoretical calculation

  5. Higgs search

  6. SM Higgs boson production @ LHC gluon fusion • Inclusive search • VBF (+ 2 jets) just started • WH, ZH only for Vector-boson fusion (VBF) Associated WH, ZH Associated ttH, bbH

  7. SM Higgs decay • mH > 2mW:  WW, ZZ (, tt) • mH 2mW: WW(*) ~ 100% • mH < 2mW bb, WW*, ZZ* ττ (< 10%) gg (2  10-3) • Golden channel: ZZ4l , gg • Sensitivity at 1-2: 200 500

  8. ATLAS result @ LP2011 • Individual channel upper limit already close to SM cross sections • Insignificant excess in channel seen

  9. Upper limits combined • Excess observed at around 130-150 GeV @ EPS2011 (July 2011):now with less significance • GeV pretty much excluded CMS results mostly with 1.6-1.7 , ATLAS mostly 1.0 but 4lepton and WW

  10. High-mass Higgs • Higgs decaying into 4leptons, and • : high-sensitivity at high mass • Small background

  11. Low-mass Higgs (1) : • The channel for low mass • Current limit: 2-4 times SM • some signal may be seen next year with

  12. Low-mass Higgs (2) • W tagged by lepton + Etmiss, Z by dilepton • Large cross section but huge background • Key point: good resolution in mass reconstruction • using subjet technique mass reconstructed!v

  13. Reconstructing the mass of heavy objects from “fat jet” • Boosted heavy object may be reconstructed as a jet with large radius parameter (e.g. R = 1.2) • Need to remove objects not from the decay • From QCD radiation not associated to the decay • Multi-parton interaction and pileup • Procedure • Splitting into two objects where mass after recombination becomes very large • Reclustering each of the small objects with small radius (e.g. R = 0.3) • Remove jets away from high-momentum partons (filtering)

  14. Jet mass distributionbefore and after the treatment • Model dependence of mass distribution disappeared • All models show good agreement to data • Also much smaller dependence to pileup (not shown) After splittingand filtering

  15. The channel • Has lead excitement in July • Still cross section tend to be high • No mass peak, counting experiment • Events selection mainly by • 2 opposite sign lepton • Large missing Et • #of jets and b-tag to control bckgnd (mainly top) • Low and small assuming Higgs is scalar

  16. # of jets, and • Background from • 0-jet: WW • 1-jet: top, WW • Insignificant excessin 0-jet sample

  17. # of jets, and : CMS • Similar degree ofinsignificantexcess observed

  18. Background uncertainty • Background estimated by control samples • low mass region GeV uses: • WW: < 1.3and cuts removed • + jets: Etmiss distribution from data used to estimate large fake Etmiss events • top: normalised by b-tag efficiencyfrom data • W+jets: jet lepton misID from data • MC samples used in ATLAS • WW: MC@NLO (syst: ALPGEN) • : MC@NLO (syst: POWHEG) • In CMS: • WW: MADGRAPH • : MADGRAPH

  19. Diboson production

  20. Diboson production at the LHC colour flow • Motivation: • Sensitivity to (anomalous) triple gauge boson coupling (TGC)shown up as cross section enhancement at high • Background to other searches • Example from

  21. WW, WZ, ZZ cross sections • Clear signal with of data • Very small background • Consistent with prediction

  22. WW distribution in detail • Plots with jet-veto (no jet with GeV, • Overall agreement OK, some shift in some distribution • More events in small as in the Higgs search(not shown) • OK; transverse mass of 2leptons + Etmiss some shift

  23. Top quark physics

  24. LHC: top factory • Already events with • More statistics than in Tevatron • Sensitivity to new physics • Single top cross section is also much larger • Mass of top quarks: still better measured at Tevatron (hence not shown today) • How to find • ~100%b-quark tagging for most of analysis • (leptonic) or (hadronic)

  25. Top quark cross sections • Good agreement with NNLO • More precision: stringent testof pQCD in top sector NEW: L+jets w/o b-tagging (∫Ldt = 0.72011):

  26. Single top production t-channel: large s 64pb • Sensitive to and new physics • Cross section @ LHCalmosttwo order of magnitude larger than at the Tevatron • Clear signal by cut-based analysis tWproduction moderate s 17pb s-channel: large s 4.6pb From EPS 2011/ talk by Frederic Deliot

  27. High-pt top pairs: resonance search • Many models have enhanced coupling to top quark • e.g. KK excitation of graviton/gluon • Using dilepton channel for high purity (ATLAS)muon + jet from boosted top (CMS)

  28. Search for SUSY and other BSM models

  29. SUSY search strategy • Detailed mass spectrum depends on models • Common strategy: • Coloured produced • Cascade decay:e.g. • Many models give leptons • Most models give many jets • Neutralino is LSP: R-parity conserved • Missing Events with Multi-Jet(4) + MissingEt (+ lepton(s))

  30. Result 0/1-lepton: ATLAS and CMS • No excess – giving limits • Already exhausting sensitivity with 7 TeV beam for simple scenario

  31. More exclusively: 1-lepton+btag • Other exclusive search for investigating scenarios such as • LSP not escaping detector (either long-lived NLSP or R-parity violation) • Many leptons Consistent w/ background, with (unfortunate) excess Limit from 35to

  32. Generic resonance search (1) dilepton • Z’, etc. • Limit up to 1-1.5 TeV

  33. Dijet resonance • Various models:excited quarks (q*), heavy W/Z,RS graviton, axigluon, E6 diquark… • Limit: 1.5-4 GeV

  34. Monojet + missing • Graviton goes away from brane to bulk (into extra-dimension) • > 3.39 TeV (n = 2)ADD model • planck scale in 4+n dimension

  35. W + dijet excess@ CDF • Not observed in LHC • Large W+jets backgroundwith this cut

  36. W, Z production and QCD

  37. Drell-Yan process • Electroweak process • Precise prediction • Standard candle for detector understandings invariant mass with onlycomponent of momentum

  38. W/Z “visible” cross section • No extrapolation to total cross section – smaller uncertainty • Comparison to calculations with various pdfs • LHC data will be sensitive to PDFs with improved precision

  39. Z and W: rapidity dependence • Rapidity dependence of cross sections • Sensitivity to pdf • W charge asymmetry vs rapidityfor ATLAS/CMS/LHCb data • Wide range of coverage • Strong discrimination power for pdfs

  40. of : higher order in QCD • Sherpa, ALPGEN, Pythia: good • cannot explain: need • Fixed order+parton shower models show large deviation

  41. W + jets • Often the background for searches • LO+PS cannot reproduce jet multiplicity ratio • ALPGEN/Sherpa, fixed order calculation show good agreement

  42. Z, W + b-jets • W + bjets show good agreement, with slight tendnecy of cross section being higher • High statistics study awaited

  43. Jets and QCD

  44. Jet production • Inclusive jet double-differentialcross sections • Comparison to NLO calculations Good agreement in central rapidity

  45. CMS result • Good agreement • With slight tendency of being low (also for ATLAS) Anti-kT, R = 0.5PDF4LHC = centre of the envelope ofMSTW08, CTEQ6.6, NNPDF2.0 • Jet energy scale (JES)key for precision JES (particle flow) 2.5 – 4% uncertainty

  46. Forward jets • Need to understandand reduce systematicerror • Interesting tendency ATLAS JES central rapidity: similar to CMS JES in forward rapdity:large uncertaintyat low pTgood at high-pT: 3%

  47. Multi-jet events • Providing good test on MC simulation / QCD calculation • Good description of modelswithin experimental uncertainties Four-jet events Scalar sum of jet

  48. Gap between jets • See if there is any jet (> 20 GeV) between two leading jets separatedin rapidity by • Sensitive to non-orderedQCD radiation • POWHEG tend to givemore radiation if gap rapidity is large • HEJ too few radiation– parton shower may improve the situation

  49. Soft QCD Very selected example

  50. Underlying event studies • Energy/particle flow measured w.r.t.the leading particle • Azimuthal profile quite flat • Pythia with MC09 or Perugia0 tune ~ OK

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