Dark Matter Hunting in the Large Missing Energy and Multijet Signature at the Large Hadron

Transcription

1 Dark Matter Hunting in the Large Missing Energy and Multijet Signature at the Large Hadron Kenichi Hatakeyama Baylor University Particle-Astrophysics-Cosmology Seminar Texas A&M University April 6th, 2011

2 Outline Introduction Apparatus Expectations from Run2010 Analysis Strategy Estimation of Backgrounds Invisible Z Top and W QCD Results & Significance Limits Summary

3 Dark Matter Various astrophysical observations suggest the presence of Dark Matter Galactic rotation curves Galaxy clusters and gravitational lensing Cosmic microwave background

4 Dark Matter Evidence Rotation Curves from Galaxies A gravitationally bound system of many particles obeys the virial theorem Observed flat rotation curve V c G N M r R 1 Expected from luminous matter in the disk Luminous matter does not account for enough mass to explain rotational velocities of galaxies ==> Dark Matter halo around galaxies

5 Dark Matter Evidence Collision of Galactic Clusters Dark Matter (inferred by gravitational lensing) Chandra X-ray Observatory and Hubble & Magellen Telescopes 1 2 Hot gaseous (ordinary) matter 3 4

6 Dark Matter Evidence Measurement of the Cosmic Microwave Background Fluctuations From Wilkinson Microwave Anisotropy Probe (WMAP) The manner in which structure grows depends on the amount and type of dark matter present. All viable models are dominated by cold (relativistically slow moving) dark matter.

7 What Is Dark Matter? Dark Matter: Stable, Neutral, and Cold Many well motivated candidates in particle physics, but none within the Standard Model (SM) The SM has no suitable candidates: quarks, leptons, force carriers, hadrons: interactions two strong neutrinos: too light Most suitable candidates beyond the Standard Model: Axions Weakly interacting massive particles (WIMPs) with masses and interaction cross sections of order of the electroweak scale Most models of EWSB ==> add an extra discrete symmetry ==> the lightest newly introduced particle is stable: Models of EWSB predict a WIMP WIMP candidates: LSP ==> lightest SUSY particle, neutralino, with R-parity conservation. LKP ==> lightest extra dimensional KK particle w/ KK parity conserved. LTP ==> Lightest T-odd heavy photon in Little Higgs models w/ T-parity conserved.

8 Supersymmetry Supersymmetry is a fundamental global symmetry between fermions and bosons. ±

9 Supersymmetry (SUSY) SUSY is a fundamental global symmetry between fermions and bosons. Each fermion has a boson superpertner, and vice versa Higgs mass stabilizes against loop correction (fine tuning problem) Modifies running of SM gauge couplings just enough to give Grand Unification at single scale SUSY is broken (sparticles are not seen) MSSM: Simple SUSY model consistent w/ SM R-parity conservation R=(-1) 2S+3B+L Sparticles produced in pairs, decay to an odd number of Lightest Supersymmetry Particle (LSP) LSP is a dark matter candidate SUSY breaking msugra, GMSB,

10 Experimental Signature Signature: MultiJets + MET Squarks & Gluinos cascade decays: produce a number of quarks and gluons, leptons and possibly weakly interacting stable neutral particles (WIMP). In the detector, WIMPs / Lightest SUSY particles appears as the momentum imbalance in the transverse plane (Missing ET!)

11 Apparatus

12 The Large Hadron Collider Lake Geneva CMS LHCb ALICE ATLAS

13 First Collision at 7 TeV, March 2010 Building 40 at CERN main site CMS Control Room near detector CMS Center at CERN main site

14

15 Expectation for 2010 Data msugra : parameter space described in terms of 5 quantities m0, m1/2, tan β, Trilinear coupling A, sign ( μ ) m 0 : Mass for spin 0 particles at the GUT scale m 1/2 : Mass for spin 1/2 particles at the GUT scale Sensitivity will go into the unexplored region very quickly

16 Analysis Strategy

17 Analysis Strategy Aim for an all hadronic inclusive analysis Main variables used in this study HT = scalar sum of JetPt (selecting large s-hat production) MHT = neg. vector sum of JetPt (presence of a WIMP) Event selection:. HT Trigger 3 jet with pt>50 GeV & η <2.5 ( central production) Veto events with isolated electrons & muons ( suppress EWK background) ΔΦ(MHT, Jets123) > (0.5, 0.5, 0.3 ) (reduce QCD background) HT>300 & MHT > 150 => baseline selection Evolved Selection: High MHT (MHT>250 GeV): High background rejection High HT (HT>500 GeV): High eff. for signals with long cascade decay chains

18 Particle Flow (PF) Algorithm In this search, all physics objects (jets, leptons, HT, MHT etc) are reconstructed with the particle flow algorithm Basic idea: Reconstruct and identify all different types of particles Apply corresponding calibrations The list of particles is given to the jet clustering and missing ET (MET) reconstruction algorithm

19 Particle Flow Algorithm Charged hadrons ~65% of jet energy Use the high resolution tracker ~1% at 100 GeV

20 Particle Flow Algorithm Photons ~25% of jet energy Use high resolution / good granularity ECAL Granularity: 0.02 ( η ϕ) Energy resol: ~2%/ E

21 Particle Flow Algorithm Neutral hadrons ~10% of jet energy Use HCAL Granularity: 0.1 ( η ϕ) Energy resol: ~100%/ E

22 Particle Flow Algorithm Particles clustered in jets Jet Charged hadron (solid) Photon (dashed line) Neutral hadron (dotted line)

23 PF Jet and MET Performance Jet energy response Calorimeter jet PF jet Jet energy resolution PF jet Calorimeter jet Particle flow algorithm improve the performance of jet and missing ET reconstruction significantly

24 PF Jet and MET Performance Teruki has been a key person of CMS Missing ET reconstruction Particle flow algorithm improve the performance of jet and missing ET reconstruction significantly

25 Data vs Standard Model MCs An out-of-box comparison of Data vs MC for search variables HT and MHT Baseline selection w/o MHT cut Baseline selection High MHT High HT

26 Data-Driven Background Estimates Invisible Z ( ν ν) + Jets Remove the identified boson (photon/w/z) to mimic neutrino Photon+Jets : high event yield (use photon/z correction from theory) Z l l + Jets (straightforward prediction but limited by statistics) Top / W + Jets Top/W ( lost lepton + ν ) + Jets : Lepton is not identified or is outside detector acceptance. Estimated from a sample with inverted lepton veto Top/W ( hadronic τ + ν )+ Jets : Estimated by replacing μ with τ using a τ response template QCD MultiJets (jet mis-measurements resulting in imbalance) R+S : rebalance event and smear rebalanced sample with jet resolutions Factorization : extrapolate two-variable correlation to signal region

27 Invisible Z Background Z/γ* ν Missing ET / MHT ν

28 Invisible Z background Z(νν) + multijets: Irreducible background in this search Three different methods using boson+jets were employed to obtain the data-driven estimates of this background (substitute boson with MHT) remove remove remove Z/γ* μ - γ μ + W - μ_ - ν μ Lower statistics than γ and W Suffer from Br(Z μμ)/ Br(Z νν)=1/6 Similar event topology Higher stat than Z νν (W+jets rate is about x 2.5 of Z+jets) Similar to Z+jets at large Pt (MHT) High stat (no branching ratio) Cross check of different channels is important as they have different sensitivities to potential new physics signal

29 Z Invisible From Photons Only direct photons are related to Z production; fragmentation photons and photons from meson decays are background Z + jets / photon + jets from MC Benefit from about 5x higher statistics of photon sample to estimate Z FRAG: Correct for fragmentation photons basedon JETPHOX PUR: Correct for photons from light mesons using shower shape

30 Z Invisible From Photons

31 Zinvisible from Z Method using leptonic W samples N( Z νν ) = N A obs W W N ε ε W bkg W b veto R Z W νν lν ε W = ε Iso ε RECO ε HLT R Z W νν lν Method using leptonic Z samples 6 10 N( Z νν ) = R N Z Z obs Z N A ε Z Z bkg Z R νν = 5.95 ± ll Z Z 0.02 νν ll ε ε lepton Z = ε = ( ε where ε Iso leptopn trig ε ) 2 RECO ε trig ε, trig = 1 (1 ε HLT ) 2

32 Zinvisible from Zll Baseline selection Simulation: From Z ee : Z μμ : combined: Consistent with Z νν from photons and with simulation. Z νν from photon results best precision: therefore solely used for limit calculation

33 W and Top Background W ν l Missing ET / MHT Veto leptons (e,μ) to suppress BGs, still occasionally fail to find and veto leptons. Hadronically-decaying taus also constitute BG

34 W/Top + (Lost Leptons) + ν + Jets Leptons failing the lepton veto contribute to background There can be 3 reasons to lose leptons the lepton is not reconstructed not isolated out of acceptance Start with a control sample of events with exactly one muon Measure the identification and isolation (in)efficiencies from data Scale the control sample according to the measured (in)efficiencies from data

35 Top / W + hadronic tau + ν + Jets Start with a muon+jets sample Replace the muon by tau response template derived from MC Recalculate HT and MHT including this expected energy from Tau Correct for muon acceptance Trigger efficiency, Reco efficiency BR(W Tau)/BR(W mu) * BR(Tau->Hadrons)

36 QCD Jet Missing ET / MHT Ideal Reality (Occasional mis-measurement)

37 QCD MultiJets + MHT A QCD Multijet event balanced in pt at parton level may end up in a reconstructed event with significant imbalance due to Physics effects: semi-leptonic decay of heavy flavor quarks Detector effect: dead channels (a significant contribution from ECAL) intrinsic jet energy resolution

38 Holes in ECAL CMS high performance ECAL has only ~1% of masked/mulfunctional cells Still we may lose a significant amount of energy there, leading to fake missing ET or MHT η/φ distribution of dead/masked ECAL channels in barrel and endcaps Dead/Masked in Readout Masked in Reconstruction Masked in Trigger Masked in Trigger + Readout ECAL Barrel ECAL Endcap Will Flanagan worked on this study

39 MET Tail Due to ECAL Holes DiJet MC Events p T -jet 1(2) > 50(25) GeV Colored: Require a jet with: Red ΔR < 0.2 to masked channels Green Barrel endcap boundary Blue Endcap/Forward boundary Δφ(MET, jet)<0.2 ~20% of high MET events due to ECAL holes causing fake MET. If we consider only fake MET events, ~30% of high MET events due to ECAL holes. Others come from heavy flavor jets, and intrinsic jet resolution Data and MC

40 QCD ReBalance + Smear Jet Response functions can be used to smear a sample of perfectly balanced events and get back the QCD sample as measured in Data. Requirement : Full jet response (including tails) measured from data to rebalance and resmear the multijet events. Resolution is measured using Gamma+Jets events (low pt) and DiJet events (high pt)

41 QCD: ReBalance + Smear Step 1 : Rebalance the data events (jets with Pt>10 GeV) using jet Pt resolutions by maximizing likehood, LJets subject to constraint MHT=0 ===> create the pseudo-particle-level QCD events Step 2 : Smear rebalanced jets with Pt>10 GeV with resol. functions R+S predicts full event kinematics (jet Pt and angular distributions) Contamination by SM & Signal processes with real MHT is negligible as such events are made QCD like by rebalancing

42 QCD: ReBalance + Smear Step 1 : Rebalance the data events (jets with Pt>10 GeV) using jet Pt resolutions by maximizing likehood, LJets subject to constraint MHT=0 ===> create the pseudo-particle-level QCD events Step 2 : Smear rebalanced jets with Pt>10 GeV with resol. functions R+S predicts full event kinematics (jet Pt and angular distributions) Contamination by SM & Signal processes with real MHT is negligible as such events are made QCD like by rebalancing

43 Results

44 Results No excess of data events over expected Standard Model prediction observed Setting limits.

45 Within the constrained MSSM tanβ = 10, m 0 m 1/2 plane tanβ = 10, m(gluino) m(squark) plane Gluino masses up to ~600 GeV are excluded. Sensitivity greater than ATLAS at high m 0. High HT search region was effective. Sensitivity lower than ATLAS at high m1/2. Need to look at 2 jet events (currently only >=3 jets)

46 Simplified Model Focus on topology instead of underlying physics model (physics is not understood anyway) Provide ability to characterize data in model independent and more comprehensive ways Provide intuitive guidance for investigation Allow one to factorize the key elements potentially present in a new signal in order to answer specific questions. Set limit on cross section (σ.br) Any model with same topology (parent particle mass, decay chain, duaghters mass) can be easily compared with experimental results.

47 Within Simplified Model m(gluino) m(lsp) m(squark) m(lsp)

48 For Future Excitement MHT = 693 GeV HT = 1132 GeV Meff = MHT+HT = 1.83 TeV No b-tagged jet No isolated lepton Incompatible with W or top mass Invisible Z???

49 SUSY Search with Tau The CDM candidate needs to interact in the early big bang just right in order to produce the observed dark matter relic density. This SUSY phase space (stauneutralino co-annihilation region) predicts abundance of final states containing Tau: Arnowitt, Dutta, Gurrola, Kamon, Krislock, Toback, PRL100 (2008) Searching for SUSY signal in events with tau (Texas A&M group et al) will be performed on the 2011 data

50 Conclusion Dark matter pauses one of the most important question of current physics. LHC is a great place for dark matter hunting! CMS has performed an inclusive search for physics beyond the standard model in the multi-jets and missing transverse energy final state using the full 2010 data. CMS has established solid data-driven techniques to predict the SM expected yields with minimal information from MC. Several novel techniques More than one method is used for each background. The 2010 data are in good agreement with the data-driven SM predictions. Exclude the parameter space in the cmssm plane. Present the upper limit on σ.br using ``Simplified models Cross-section limits pb, exclude m(gluino)< 600GeV CMS is eagerly waiting for data to arrive, which will enhance our sensitivity to SUSY discovery significantly. A big excitement may be ahead of us!