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Поиски хиггсовского бозона, суперсимметрии и тяжелых векторных бозонов в эксперименте АТЛАС PowerPoint Presentation
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Поиски хиггсовского бозона, суперсимметрии и тяжелых векторных бозонов в эксперименте АТЛАС

Поиски хиггсовского бозона, суперсимметрии и тяжелых векторных бозонов в эксперименте АТЛАС

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Поиски хиггсовского бозона, суперсимметрии и тяжелых векторных бозонов в эксперименте АТЛАС

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  1. Поиски хиггсовского бозона, суперсимметрии и тяжелых векторных бозонов в эксперименте АТЛАС Научная сессия-конференция секции ЯФ ОФН РАН«Физика фундаментальных взаимодействий» ИТЭФ, ноябрь 2011 г А.М.Зайцев, ИФВЭ, Протвино

  2. Scope • ATLAS • Searches: • Higgs boson • Supersymmetry • Gauge bosons • ATLAS upgrade

  3. The ATLAS detector 4 Superconducting magnets: Central Solenoid (B= 2T) 3 Air core Toroids HAD calorimetry (|η|<5): segmentation, hermeticity Fe/scintillator Tiles (central), Cu/W-LAr (fwd) Trigger and measurement of jets and missing ET E-resolution: σ/E ~ 50%/√E ⊕ 0.03 Muon Spectrometer (|η|<2.7) Muon trigger and measurement with momentum resolution < 10% up to Pμ ~ 1 TeV EM calorimeter: Pb-LAr Accordion e/γ trigger, identification and measurement E-resolution: σ/E ~ 10%/√E InnerDetector (|η|<2.5, B=2T): Si Pixels, Si Strips & Transition Radiation detector tracking and vertexing, e/π separation, dE/dX Momentum resolution: σ/pT ~ 3.8x10-4 pT (GeV) ⊕ 0.015

  4. Forward detectors B. Di Girolamo

  5. Data Taking Efficiency the luminosity delivered (between the declaration of stable beams and the LHC request to turn the sensitive detectors off) Efficiency = --------------------------------------------------------------------------------- the luminosity recorded by ATLAS

  6. Operational fraction

  7. Luminosity • The maximum instantaneous luminosity: 3.65x1033 cm-2 s-1 • Delivered Luminosity: 5.61 fb-1 • ATLAS Ready Recorded: 5.25 fb-1 Absolute luminosity calibration by van der Meer scans ΔL/L = ±3.4% (2010, prel) ΔL/L = ±3.7% (2011, prel)

  8. Pile-up • 50 ns bunch trains for ~all 2011 data • Substantial in- and out-of-time pileup • Much progress understanding impact on performance, with data & simulation Z→μμ event with 11 primary vertices

  9. Highest-mass dijet event observed so far Mjj = 4 TeV Jet 1: pT = 1.8 TeV, η = 0.3 Jet 2: pT = 1.8 TeV, η = -0.5 Etmiss= 100 GeV

  10. Search for Higgs boson (SM and MSSM)

  11. Higgs cross-sections • H→γγ: rare channel, clean • signature, the best for low mass • H→WW(*): • →lνlν: very important in the • intermediate mass range • → lνqq: highest rate, important • at high mass • H→ZZ(*): • → 4l: golden channel • → llνν: good for high mass • →llbb: also high mass • H→ττ: good signal/background, important at low mass, rare • Associated prod. H→bb-bar • ttH, WH, ZH • It is useful for the discovery • It is very important for Higgs • property studies if SM Higgs is • discovered L=1.08 fb-1 L=1.70 fb-1 L=1.04 fb-1 L = 2.2 fb-1 L=1.04 fb-1 L=1.04 fb-1 L=1.06 fb-1 L=1.04 fb-1 Events expected to be produced with L=1 fb-1

  12. H→γγ arXiv:1108.5895v1 [hep-ex] Two high-quality isolated high-pTphotons pT1 > 40 GeV; pT2 > 25 GeV |η12| < 1.37 and 1.52 < |η12| <2.37 isolation A pointing method (using the first two samplings of the LAr calorimeter , or the first sampling and the conversion point for converted photons) is used to determine the vertex position → Its resolution is ~1.6 cm No indication of an excess is found. Limits on SM Higgs production cross section are set. Excluded: ~ 4 the SM production cross-section * BR

  13. Other Light Higgs searches Z (W) + Higgs →bb̄ ATLAS-CONF-2011-103 Select events with Z or W boson in the leptonic final state, and with exactly two jets btaggedwith pT>25 GeV Highest rate at low higgs mass Large background H→ττPromising channel for mH=110-140 GeV H→ττ̄→ll+4νATLAS-CONF-2011-133 H→ττ̄→lτhad +3νATLAS-CONF-2011-135 collinear approxi-mation τ τ + 1 high pTjet ET miss > 30 GeV Both channels exclude ~ 10 the SM production cross-section *BR

  14. H→WW(*)→lνlνATLAS-CONF-2011-134 The most sensitive process for 130 < mH< 200 GeV But also one of the most challenging channels: complete reconstruction of the invariant mass of this final system not possible. Largest background is the irreducible WW SM production. Also Drell-Yan and top process when looking to final states associated to one jet . Select events with two high-pT opposite sign leptons and large transverse missing energy (ETmiss)

  15. H→WW(*)→lνlν – event selection eμ ee μμ Distributions of ET, relmiss after lepton pTcuts. Background distributions normalized by the data from control samples Man Backgrounds - WW, tt-bar, single top, Z/γ*+jets, diboson (WZ,ZZ,Wγ): from MC - W+jets: from data using loosened identification and isolation criteria Data/MC agreement better than 10%, within systematic uncertainties

  16. H→WW(*)→lνlν – exclusion limit The expected (dashed) and observed (solid) 95% C.L. upper limits on the cross-section, normalized to the SM cross-section, as a function of the Higgs boson mass. (the jump in the expected and observed limits at 220 GeV is due to the change in the selection at that point) A Standard Model Higgs boson with 154 <mH <186 GeV is excluded at 95% C.L. Expected exclusion mass range is 135<mH <196 GeV The observed limit is within 2σ the expected one in the mass range 130 – 150 GeV

  17. H→ZZ(*)→4l arXiv:1109.5945v1 • The “gold-plated” channel. • Very clean, but small rates. • Background: • irreducible ZZ(*); • reducible Z+jets (particular:Zbb̄), tt̄ • Event selection: • Inclusive high-pt electron or muon; • Two isolated same-flavour opposite charge • lepton pairs • Reconstruct the Z (mass window cut) • Veto low invariant mass pairs • For m4l <2mZ require also small lepton impact parameter (rejects Zbb̄ and tt̄)

  18. 2μ2e candidate m=209.7GeV

  19. H→ZZ(*)→4l A total of 27 events are selected by the analysis algorithm: 6ee, 9eμ, 12μμ Expected: 28±4 Invariant mass leading (left) and sub-leading (right) lepton pair • Main Backgrounds: • ZZ(*): from MC theoretical uncertainty (15%) • tt-bar: from MC theory normalization uncertainty (10%) • Z+jets: normalized using data (control sample regions) uncertainties 20-40%

  20. H→ZZ(*)→4l- exclusion limits Very close the SM cross-section. SM Higgs boson excluded at 95%CL in the mass ranges 191-197, 199-200 and 214-224 GeV

  21. ZZ→llννATLAS-CONF-2011-148 Best channel at high mass • Identify Z→ll: • High pT e/μ • 2 SF opposite sign isolated leptons (e/μ) • with|mll–mZ|<15 GeV • Identify Z→νν if mH >200 GeV: • Reject event with Δφ(pTmiss,pTjet)<0.3 to • reduce bkg with fake ETmiss • ET miss>66 (82)GeV in low (high) mass region • Events with one or more b-tagged jets • are rejected (to reduce top bkg) • Main Backgrounds • Diboson, Z, tt̄: from MC • W : normalization obtained data/MC of like-sign lepton pair events with high ETmiss • QCD multi-jet: using data sample with loosened electron selection SM Higgs boson excluded at 95%CL in the mass range 310- 470 GeV

  22. ZZ→llqqATLAS-CONF-2011-150 • Selection: • 2 on-shell Z’s (if mH>2mZ) • - For isolated leptons pT>20 GeV with |mll-mZ|<15 GeV • -2 jets pT>25 GeV with 70 <mjj<105 GeV and mjj constrained to Z mass • - mH>300 GeV: Δφll,jj<90°& 2 jets pT>45 GeV • ETmiss <50 GeV (to reduce bkg from tt̄) • Categories: 2b-tagged jets and untagged jets • Main Backgrounds: • - Z+jets: shape from MC an normalization from data • using sidebands • - top: shape from MC and normalization from data • using sidebands • - ZZ, WZ, W+jets: from MC • - QCD multijet: using data sample with loosened • electron selection SM-like Higgs boson with a production rate of 1.2 to 12 SM cross section is excluded at 95%CL

  23. WW→lνjjarXiv:1109.3615v1 [hep-ex] Largest σ*BR • 2 on-shell W’s (if mH>2mW) • Exactly one reconstructed isolated lepton (e/μ) • with pT>30 GeV , ET miss >30 GeV • Exactly 2 jets (H+0jet) or 3 jets (H+1jet) with • pT>25 GeV and the closest mass to W of the • jet pair has to satistfy 71<mjj<91 GeV • Events with one or more b-tagged jets are • rejected (to reduce top bkg) • Main Backgrounds • - W+jets: from MC • - Z+jets, tt-bar, diboson: from MC • - QCD multijet: using data sample with • loosened electron selection The upper limit at mH=400 GeV is 2.7 SM cross section

  24. ATLAS SM Higgs CombinationATL-CONF-2011-135 The expected (dashed) and observed (solid) cross-section limits for the individual search channels, normalized to the Standard Model Higgs boson cross-section, as functions of the Higgs boson mass. Correlated uncertainties (Jet Energy Scale, Luminosity, etc) taken into account. In other cases, e.g. data driven background estimates, the uncertainties are uncorrelated. Careful treatment of theory uncertainties: - Higgs boson cross-section uncertainties in QCD scale and PDF+αs - PDF uncertainty is fully correlated among different channels and it is included in the combination

  25. ATLAS SM Higgs Combinations Combined upper limit on the SM Higgs production cross section divided by the SM expectation. This is a 95% CL limit using the CLs method in the entire mass range Standard Model Higgs boson mass excluded at 95% C.L.: 146< mH<232, GeV 256 < mH< 282 GeV 296 < mH< 466 GeV

  26. Perspectives A 95% CL exclusion for the whole mass range could be achieved by combining ATLAS-CMS results for an integrated luminosity of about 4-5 fb-1 2012 (with hopefully additional 10 to 20 fb-1 ) should bring more answers

  27. Combination ATLAS+CMSG.Rolandi, HCPS 2011, Paris, November 14-18 SM Higgs boson excluded at 95%CL or higher in the mass range 141- 476 GeV 114 < MH < 141 ???

  28. MSSM Higgs Searches MSSM H/A/h→ττ ATL-CONF-2011-132 Production: gg→A/H/h and bb̄A/H/h Decays: eμ 4ν, e/μτ had 3ν, τ had τ had 2ν H+→τ had+ + ν ATLAS-CONF-2011-138 • Selection: • - τ jet with pT>35 GeV. • -no other τ • -ET miss >40 GeV • -at least one b-tagged jet

  29. SUSY modeling > 100 free parameters in general MSSM • Constrained models: • variousSUSY breaking scenarios • assume unification at GUT scale • → only a few free parameters • Toy models (phenomenological models): • assume mass and hierarchy of sparticles • assume specific decay chain • Model independent sensitivity: • cross-section x efficiency x acceptance (σAε) can be used to test other models • R-Parity conserving (RPC): • SUSY particles produced in pairs • lightest SUSY particle (LSP) stable • → missing transverse momentum (ETmiss) R-Parity: introduced to suppress violation of leptonic and baryonic numbers R-Parity violating (RPV): - long-lived particles → displaced vertices → particles reaching muon spectrometer - resonance searches

  30. Searches for SUSY At the LHC sparticles are pair produced, dominantly squarks and gluinos via the strong interaction (σ ~ 1 pb). They decay via cascades into the stable LSP (neutralino or gravitino), assuming R-parity conservation. Common signature: multiple, high energetic jets and transverse missing momentum Distinguish final states by additional particles: zero, one, two, .. leptons (e, μ), two photons, b-jets Incomplete event reconstruction due to LSP No mass peak SUSY is in the tails of the distributions SM backgrounds (top, W/Z+jets, QCD) are taken from/verified in control regions

  31. SUSY searches

  32. SUSY search in Jets + ET,missarXiv:1109.6572 Select events with jets and missing ET Veto events with pTof e(μ) > 20(10) GeV Different jet multiplicity sensitive to different squark or gluino pair production. Optimize cut on meff= H T + ETmiss and ETmiss for each jet multiplicity (HT = scalar sum of all jet ET). Combine channels to optimize search for different topologies Background sources W+jets Leptons reconstructed as jets Z/γ+jets γ/leptons as jets, Z+jets→νν+jets Top Hadronic tau decay QCD Mis-measurement of jets or ν from heavy flavor decay

  33. SUSY limits from Jets + ET,miss Phenomenological model: m(X̃01) = {0, 195, 395} mSUGRA ( tanβ=10, A0=0, μ>0 ) → m(g̃) = m(q̃) > 1075 GeV → m(g̃) = m(q̃) > 980 GeV

  34. Large Jet Multiplicity + ETmissarXiv:1110.2299v1 Use jets + missing ET analysis and increase number of jets: Njets > 6. Signal region defined by number of jets and MET/√HT. Main background: multijet production. QCD control region defined by lower number of jets. Other background estimated from MC and validated in different data control regions. mSUGRA exclusion limit (tanβ=10, A0=0, μ>0) : competitive to 0lepton (2-4 jets) for high m0 → no excess over SM expectation found → m(g̃) > 520 GeV

  35. ETmiss+ b-jets + 0-leptonATLAS-CONF-2011-098 Analysis similar to jets+0 lepton channel but requires at least one b-jet. Sensitive to 3rd generation squarks. Phenomenological MSSM model m(g̃) > 720 GeV for m(b̃1) < 600 GeV General simplified model: - Squarks assumed to be heavy - Gluino-gluino pair production m(g̃) > 660 GeV for m(X̃10) < 200 GeV

  36. ETmiss+ 1-lepton arXiv:1109.6606 ET,miss+ 2-leptonsarXiv:1110.6189 Leptons in the decay chains of squarks and gluions (e.g. via sleptons or W) • Limit on simplified model: • g̃g̃ production • decay to LSP via Chargino • (mheavy-mLSP)= 2*(mheavy-mchargino) • → m(g̃) > 600 GeV for m(X̃10) < 200 GeV Event selection: - Exactly 1 high pTlepton (e or μ) - 3 jets (>60, >25, >25 GeV) - ETmiss>125 GeV, ETmiss>0.25*Meff - Meff>500 GeV Leptons in the decay chains of squarks and gluions(e.g. via sleptons or W). Direct gaugino production. Search for ET miss+2-leptons events with - Same-sign leptons - Opposite-sign leptons - Opposite-sign, identical flavor Model: Charginos with masses up to 200 GeV are excluded

  37. Diphoton + ETmiss arXiv:1111.4116v1 In gauge-mediated SUSY breaking (GMSB) models, the LSP is the gravitino Signature: 2γ + ETmiss +X Generalized model of gauge mediated SUSY breaking (GGM) with a bino-like lightest neutralino σ<0.02 – 0.04 pb (95 % C.L.) m(gluino)>776 GeV

  38. SUSY Searches result summary

  39. Search for heavy resonances • Predicted by many BSM theories: • GUTs, extra dimensions, alternative EWSB… • Famous historical precedents • Clean signatures: • bump in invariant mass • - well controllable background • Experimental challenges: • -no mass predictions; • -measured objects O(1 TeV), • - resolution and efficiency based on extrapolation and simulation.

  40. Dilepton resonances (Z’, spin 1) arXiv:1108.1582 Sequential Standard Model (SSM)Z’ as a benchmark. GUT inspired E6 model leading to 6 Z’ candidates. General strategy: -calibrate resolution and scale -background control and evaluation of systematic uncertainties -search for an excess above background Electrons: - ET1,2 > 25 GeV -|η| < 2.47 (TR excl.) - Isolation - ΣET (R <0.2) < 7 GeV - Pixel B-layer Acceptance ~ 65% Muons: -pT1,2 > 25 GeV -Reconstructed in ID and muon spectrometer -Isolation: pT (R < 0.3)/pT (μ) < 0.05 Acceptance ~40%

  41. Z’ limits Background: Z/γ∗, diboson, W+jets, tt̄ → MC@ NLO(NNLO) normalized to the Z peak, QCD background – from data 95% CL intervals on fitted N(Z') converted into limits on σB(Z' → ll) using the cross- sectonratio between Z/Z’: m(Z’)>1.83 TeV

  42. Dilepton resonances: technimesonsATLAS‐CONF‐2011‐125 Low Scale Technicolor: - QCD ‐ like spectrum - ω0Tand ρ0Tdegenerate in mass. - Narrow spin 1 resonances: ω0T, ρ0T, a0T ‐> dileptons - Searching for low mass is also relevant (lower than highest mass limits as Z’) 95% CL exclusion 130‐480 GeV

  43. W’ (lepton + MET) arXiv:1108.1316 Using SSM W’ as a benchmark, decay to an electron or a muon and a neutrino Transverse mass as the discriminant variable No excess found, observed (expected) mass limit from combination of both channels: mW’ >2.15 TeV 95% C.L. limit

  44. Conclusion 1 fb-1 → 3000 fb-1 0.03% → 100% Good prospects for discoveries Tentative upgrade schedule

  45. Spare

  46. Spare

  47. Aplanarity: the smallest eigenvalue of the momentum tensor • HT;3p : the transverse momentum of all but the two leading jets, normalized to the sum of absolute values of all longitudinal momenta in the event • The anti-kt algorithm constructs, for each input object (either energy cluster or particle) i, the quantities dijand diB as follows: where kti is the transverse momentum of object i with respect to the beam direction. A list containing all the dij and diB values is compiled. If the smallest entry is a dij, objects i and j are combined (their four-vectors are added) and the list is updated. If the smallest entry is a diB, this object is considered a complete ‘‘jet’’ and is removed from the list.

  48. Combined performanceTRT Operation At High Occupancy (MEPhI, MSU) Momentum resolution as a function of centrality. Default reconstruction and new reconstruction parameters J/PSI-> mmmass spectrum for the tracks with TRT extensions:96.5%of all the events. For these events mass resolution is by 10% better than for standard reconstruction. New reconstruction parameters do not compromise track reconstruction but allow to get complete TRT information on the particle track at any occupancy.

  49. Combined performance Photon efficiency study based of FSR selection in Z decay processes (MEPhI) The main idea is to obtain a photon sample with maximum purity with the method decoupled from the standard analysis methods. 3 body mass spectrum with constrains on the mass of two leptons (Data and MC) Purity=Signal/(Signal+Background) ET()>15 GeV Mmm <82 GeV Etcone: 20<3 GeV nucone20=0 Photon Et [GeV] Developed approach allows to obtain a sample of probe photons with a purity of about 97% for studies of the ATLAS photon response. Applying 3 body cut one can select the signal