Seminar atlas 1103.6208


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HCOL seminar in KEK. Review of recent results from LHC.

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Seminar atlas 1103.6208

  1. 1. Search for an excess of events with an identicalflavour lepton pair and signicant missing transverse momentum * ATLAS collaboration * ArXiv:1103.6208 * Submitted to EPJC — * √s=7TeV ∫Ldt=35pb-1 were collected between March and November | | | | | > t 2009 2010 2011
  2. 2. Aimsa search for the supersymmetric (SUSY) particles in events withexactly two leptons of identical flavour (e or μ) and oppositecharge, and signicant missing transverse momentum lepton pair invariant mass > { distribution } l+/- l+ > ~0 ~0 l- > χ2 l-/+ χ2 • • ~ l • ~0 ~0 χ1 > χ1 miss > ET
  3. 3. Decay chains - q ~0 χ1 q > • > - q • ~± • > > χ1 W± q ~ • - ~ g q > q q > • > • > ~0 χ2 l+ ~ • > qg • > > ~ l • > l- q ~0 χ1
  4. 4. End-points in mll distribution (m 2̃ −ml2̃ )(m 2̃ −m2̃ ) χ0 l χ 0 2 edge R R (m ) ll = 2 2 1 m l̃R OSOF subtraction In signal region Model:SPS1a* Gjelsten, Hisano, Kawagoe, Lytken, Miller, Nojiri, Osland & Polesello (in LHC/IC study group) 04* SUSY Parameter and Mass Determination at the LHC, C. Sander, Cambridge Phenomenology Seminar, 2010
  5. 5. Features* one of the best routes to model-independent measurements of themasses of SUSY particles via end-points in the lepton pair invariantmass distribution* Standard Model background is (almost) equal for lepton pairs ofidentical and different flavour (in the signal region): Bg(e+e-) = Bg(μ+μ-) = Bg(e+μ-)= Bg(μ+e-)* SM bg. can be removed with a ‘flavour subtraction’ procedure: Signal(e+e-Vμ+μ-)=Data(e+e-)+Data(μ+μ-)-Data(e+μ-Vμ+e-)
  6. 6. Monte-Carlois used to develop analysis procedure and estimate residual SM bg. }* QCD jets PYTHIA, LO-PDF: MRST2007LO* >* Drell-Yan }* top quark pairs MC@NLO, NLO-PDF: CTEQ6.6 >* single top* W and Z/γ* production > ALPGEN }* Diboson production (WW, WZ, ZZ) > HERWIG* Fragmentation and hadronization* Underlying event > JIMMY* Parameter tune > ATLAS MC09* Detector simulation > GEANT4
  7. 7. Electron identification (general) ATLAS, JHEP12(2010)060starts in the high-granularity liquid-argon sampling electromagnetic (EM)calorimeters. Further, there are three reference sets of requirements:“Loose”: uses EM shower shape information and discriminant variables fromhadronic calorimeters“Medium”: full information from EM + some from the inner tracking detector (ID)(track quality variables + cluster-track matching variable)“Tight”: exploits the full electron identication potential of the ATLAS detector (fullinformation from ID and EM)
  8. 8. Electron identification (ArXiv:1103.6208)* pass “tight” electron selection criteria Pseudorapidity: η =-lnTan(θ/2)* have pT>20GeV and |η|<2.47* additional E/P cut (E is the shower energy in the EM and p the trackmomentum in the ID) and TRT (transition radiation tracker) cut to provideadditional rejection against conversions and fakes from hadrons.* pass isolation criteria: total transverse energy within a cone ΔR=√(Δη)2+(Δφ)2around the electron is less than 0.15 of the electron pT* Veto if a “medium” if a medium electron is found in the transition regionbetween the barrel and end-cap EM : 1.37<|Δη|<1.52
  9. 9. Muon identification* have pT>20GeV and |η|<2.4 There are two possibilities“Combined muons” are identified in matching between an extrapolated IDboth the ID and MS (muon track and one or more track segmentsspectrometer) systems in the MS* ID track quality test – several (or at least one) silicon pixel detector hits,silicon microstrip detector (SCT) hits, TRT hits* a good match between ID and MS tracks* pT measured by these two systems must be compatible within the resolution* isolation: ΣpT<1.8GeV for other ID tracks above 500 MeV within ΔR<0.2 aroundthe muon track* Δz<10mm between the primary vertex and the extrapolated muon track
  10. 10. Jet identification* have pT>20GeV and |η|<2.5* reconstruction using the anti-kT-algorithm with a distance parameter D=0.4* Jets are corrected for calorimeter non-compensation, material and other effectsusing pT- and η-dependent calibration factors obtained from Monte Carlo andvalidated with test-beam and collision-data studies
  11. 11. Common criteria* identified “medium” electrons or muons are only considered if they satisfyΔR>0.4 with respect to the closest remaining jet* if a jet and a “medium” electron are both identified within a distance ΔR<0.2 ofeach other, the jet is discarded (?)* ET is the modulus of the vector sum of pT of the reconstructed objects (jets withpT>20GeV but over the full calorimeter coverage |η|<4.9, and selected leptons), anyadditional non-isolated muons, and the calorimeter clusters not belonging toreconstructed objects.
  12. 12. Signal region* events that contain a lepton pair of identical or different flavour* signs of the leptons are opposite* invariant mass mll>5GeV* missing ET>100GeV in order to reject SM Z+jets events whilst maintainingeffciency for a range of SUSY models.* events must also possess at least one reconstructed primary vertex with at leastfive associated tracks (?)
  13. 13. Flavour subtractionUsing the quantity S defined as follows: N(e+e-) βN(μ+μ-) N(e+μ-Ve-μ+) S= + - β(1-(1-τe)2) (1-(1-τμ)2) (1-(1-τe)(1-τμ))* electron plateau trigger efficiency τe=(98.5±1.1)%* muon plateau trigger efficiency τμ=(83.7±1.9)%* the ratio of electron to muon effciency times acceptance β=0.69±0.03* the value of S obtained from selected identical-flavour and different-flavourlepton SM events is expected to be small but non-zero, due primarily to Z/γ*boson production
  14. 14. Data vs MC N(e+e-) βN(μ+μ-) N(e+μ-Ve-μ+) + and β(1-(1-τe ) 2) 1-(1-τμ )2 1-(1-τe)(1-τμ)* weighted invariant mass distribution ofe+e- or μ+μ- pairs prior to applying themissing ET requirement* the distribution for different flavourpairs* in the region with mll < 100 GeV, thedominant contributions to the differentflavour data events are expected to come -from tt, QCD and Z/γ*+jets events
  15. 15. Data vs SM background e+e- e+μ- or e-μ+ μ-μ+ dominated, but cancel out in S,Data 4 13 12 but signal-free RMS dominated by stat. >Z/γ*+jets 0.40±0.46 0.36±0.20 0.91±0.67 fluctuations in number of tt eventsDiboson 0.30±0.11 0.36±0.10 0.61±0.10 > Actually dominated in Stt 2.50±1.02 6.61±2.68 4.71±1.91Single top 0.13±0.09 0.76±0.25 0.67±0.33 > negative is an artifactFakes 0.31±0.21 -0.15±0.08 0.01±0.01Total SM 3.64±1.24 8.08±2.78 6.91±2.20 Sobs=1.91±0.15(β)±0.02(τe)±0.06(τμ) SSM=2.06±0.79(stat.)±0.78(sys.)
  16. 16. Contibution to S from SM* from single top and diboson events are estimated using the MC samples describedabove, scaled to the luminosity of the data sample* from Z/γ*+jets, tt and events containing fake leptons (from QCD jets and W+jetsevents) are estimated using MC samples normalised to data in an appropriatecontrol region - Z/γ* tt fake leptons* (ET)miss<20GeV * “top-tagged” lepton * electron with pT>30GeV,* 81<mll<101GeV pair (ET)miss<60GeV, * 60<(ET)miss<80GeV Δφ<0.1 between a jet and the (ET)miss vec. * ≥2 jets with * muon with pT<40GeV, (ET)miss<30GeV, pT>20GeV mT(μ,ET)<30GeV* A loose-tight matrix method is used to estimate the number of events withfake leptons in the signal region
  17. 17. Consistency between Sobs and SSM* generating pseudo-experimentsusing the estimated mean numbersof background events from Table 1as input* The resulting total mean number ofbackground events in each channel isthen used to construct a Poisson * The probability of observing adistribution from which the observed value of S at least as large as Sobsnumber of events in that channel is is 49.7% and hence no evidencedrawn of an excess of identical flavour events beyond SM expectations is* 106 pseudo experiments observed.
  18. 18. Model-independent constrains on Ssignal* adding signal event contributions to the input mean numbers ofbackground events in each channel* assumption about the relative branching ratio of new physics events intoidentical flavour and different flavour channels* new set of signal-plus-background pseudo-experiments* If Brnew physics(eμ)=0, then * If Brnew physics(eμ)=½Brnew physics(ee+μμ),Ssignal<8.8 at 95% confidence level Ssignal<12.6 at 95% confidence level
  19. 19. Model-dependent constrains* mean numbers of signal events added to each channel are sampled according tothe expectations from each point in the parameter space of the model togetherwith the uncertainties in these expectations* 24 parameter MSSM model: mA=1TeV, μ=1.5minP(mq,mg), tanβ=4, At=μ/tanβ,Ab=Al=μtanβ. The masses of the 3d generation sfermions are set to 2 TeV, andcommon squark mass and slepton mass parameters are assumed for the first twogenerations* Two grids in the (mq,mg) plane are considered (MSSM PhenoGrid2): “compressed spectrum”: “light neutralino”: m χ̃ =M −50GeV 0 m χ̃ =M −50 GeV 0 2 2 m χ̃ =M −150GeV 0 m χ̃ =100GeV 0 1 1 ml̃ = M −100 GeV L m l̃ =M /2 L M =min( mq , m g ) ̃ ̃ M =min( m q , m g ) ̃ ̃
  20. 20. For “compressed spectrum” (“light neutralino”) models andmg = mq + 10 GeV, the 95% confidence lower limit on mq is 503 (558) GeV
  21. 21. Conclusion* a flavour subtraction technique has been used to search for an excessbeyond SM expectations of high missing transverse momentum eventscontaining opposite charge identical flavour lepton pairs* no signicant excess has been observed, allowing limits to be set on themodel-independent quantity Ssignal, which measures the mean excess fromnew physics taking into account flavour-dependent acceptances andeffciencies.