A review of some of the exclusive measurements of the SUSY masses and parameters using the reconstruction of events with 2 leptons+missing energy+jets with the ATLAS detector is presented

De Sanctis, U

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Split, 29/09/2008 Physics at LHC 2008U. De Sanctis1Measurement of SUSY parameters using events with dileptons with ATLAS.U. De SanctisUniversity of Milan & INFNSplit, 29/09/2008 Physics at LHC 2008U. De Sanctis2Outline– Reminder of SUSY and mSUGRA framework;– Topology of the SUSY events;– Leptons identification;– Measurement of masses and other properties of SUSY particles in the 2-lepton channel.– Extracting masses and parameters from measurementsSplit, 29/09/2008 Physics at LHC 2008U. De Sanctis3Adds to each SMfermion (boson) abosonic (fermionic) partner.Par. Descriptionm0 Common scalar massm1/2 Common gaugino massA0 Common trilinear termsign(µ) µ from Higgs sectortanβ Ratio of Higgs vev R-parity can be conserved (RPC) or violated(RPV)  RPC implies:- SUSY particles produced in pairs- stable and neutral lightest SUSY particle (LSP)- no proton decay LSP is a good candidate for cold Dark MatterSLBR 2)(3)1( +−−=SUPERSYMMETRY REMINDERMSSM Lagrangian depends on 105 parametersmSUGRA requires only 5 parameters- Also other SUSY models exist: GMSB, AMSB, …Split, 29/09/2008 Physics at LHC 2008U. De Sanctis4mSUGRA benchmark pointsSU1SU3SU2SU6SU4SU8SUSY benchmark points chosen in the (m0, m1/2) plane for different tanβvalues:9 Systematically exploring phenomenological signatures9 Scanning the parameter phase space constrained by latest experimental data and Cold Dark Matter abundance. Split, 29/09/2008 Physics at LHC 2008U. De Sanctis5SUSY signatures at an hadronic colliderlqqlg~ q~l~χ02~χ01~p p• Assuming R-parity conservation• Strongly interacting sparticles (squarks, gluinos) should dominate production unless very heavy.• Cascade decays to the stable, weakly interacting lightest neutralino follows.• Event topology: • high pT jets (from squark/gluino decay)– Large ETmiss signature (from LSP) – High pT leptons, b-jets, τ-jets (depending  on model parameters).A typical decay chain:Particle mass (GeV)Cross section (pb)Several other possibilities exist, but our effort has to be as more “model independent” as possible.Split, 29/09/2008 Physics at LHC 2008U. De Sanctis6– Reduces the signal because of (model dependent) leptonic BRs;– Heavily suppresses the background: top is the dominant one;– Statistical significance is smaller but S/B ratio larger. – The Same Sign channel has the best S/B ratio – but limited by signal rate 2-lepton channel: strengths and weaknesses1 fb-1 1 fb-1ATLAS PreliminaryATLAS PreliminaryOpposite SignSame SignBaseline selection :• Jet multiplicity ≥ 4,  pT1st > 100GeV,  pTothers > 50GeV• ETmiss > max(100GeV, 0.2xMeff) ;  Transverse sphericity > 0.2 .Split, 29/09/2008 Physics at LHC 2008U. De Sanctis7Electron & Muon selections for 2-leptons channel• Pt > 10 GeV,  |η|<2.5;• Calorimetric isolation < 10 GeV in a 0.2 radius cone;• If an electron is found in the 1.37 < |η|< 1.52 region, the event is rejected (ID services and ECAL barrel-extended barrel transition worsen the performances);• Overlap removal procedure.Say ∆R (e, jet) the distance electron-jet in (η,φ) plane:- if ∆R < 0.2 Æ jet discarded- if 0.2 < ∆R < 0.4 Æ electron discarded.• Pt > 10 GeV,  |η|<2.5;• Calorimetric isolation < 10 GeV in a 0.2 radius cone;• Combined muons (e.g. using information from both the muon spectrometer and the Inner Detector)• Overlap removal procedure.Say ∆R (muon, jet) the distance muon-jet in (η,φ) plane:- if ∆R < 0.4 Æ muon discardedSplit, 29/09/2008 Physics at LHC 2008U. De Sanctis8Di-Lepton Edge mass measurement (1)• In case of a discovery of SUSY, particle propertiescan be measured to verify that they are indeed SUSY partners• Edge(s) of di-lepton invariant mass correlated with slepton and neutralino masses• Impossible to reconstruct peaks because       (LSP) escapes detection, more complicatedrelations between masses of particlesinvolved.~~χ02 ~χ01l l±l±±~ ~χ02 → l l → χ01 l+ l-~χ019Uncorrelated (SUSY+SM) background (two leptons from independent chains) removed by flavour subtraction:e+e- + β2 µ+µ- – β (e+µ--e-µ+) , β=εe/εµ9 Leptons can also be combined with jets of the full decay chain to look for other kinematical edges (Mllj or Mlj)Split, 29/09/2008 Physics at LHC 2008U. De Sanctis9Di-Lepton Edge mass measurement (2)Flavour subtraction at work….Flavour Subtraction1 fb-11 fb-1SU3, 1 fb-1Edge: (99.7±1.4) GeVTruth: 100.2 GeVFitting function:Triangle smeared with a Gaussian with σ = 2 GeV (to take into account experimental resolution)ATLAS PreliminaryATLAS PreliminaryATLAS Preliminary ATLAS Preliminary1 fb-11 fb-1Split, 29/09/2008 Physics at LHC 2008U. De Sanctis10Di-Lepton Edge mass measurement (3)Flavour Subtraction1 fb-10.5 fb-1SU4, 0.5 fb-1Edge: (52.7±2.4) GeVTruth: 53.6 GeVFitting function:Theoretical three body decay function in the limit of large slepton mass, smeared by the experimental resolution withσ = 2 GeV.ATLAS Preliminary0.5 fb-1ATLAS PreliminaryATLAS Preliminary ATLAS Preliminary0.5 fb-1For SU4, the slepton is heavier than       Æ The decay is: −+→ ll0102 χχ02χSplit, 29/09/2008 Physics at LHC 2008U. De Sanctis11Assuming that the squarks decays originate the two hardest jets of the event, one can use the qll combinations. Each combination has a minimum or a maximum whichprovides one constraint on the masses of  χ01 χ02 l q .~~ ~~ATLAS TDR ATLAS TDRlqq lg~ q~l~χ02~ χ01~p pLepton+jets combinationFormulas in Allanach et al., hep-ph/0007009EDGEllqm ⇒1llq2llqKeep the minimumKeep the maximumTHRESHllqm2llq1llq ⇒Split, 29/09/2008 Physics at LHC 2008U. De Sanctis12Lepton+jets combination (2)llq edgesATLAS TDR ATLAS TDRSU3SU4ATLAS PreliminaryATLAS Preliminary1 fb-1 0.5 fb-1Fit formula: 2 straight lines (for signal and background) smeared by a Gaussiandistribution to take into account the experimental resolution.Edge: 517±30±10±13 GeVTruth: 501 GeVEdge: 343±12±3±9 GeVTruth: 340 GeVSplit, 29/09/2008 Physics at LHC 2008U. De Sanctis13Lepton+jets combination (3)ATLAS TDRllq thresholdsSU3SU4ATLAS PreliminaryATLAS Preliminary1 fb-1 0.5 fb-1Fit formula: 2 straight lines (for signal and background) smeared by a Gaussiandistribution to take into account the experimental resolution.Edge: 265±17±15±7 GeVTruth: 249 GeVEdge: 161±36±20±4 GeVTruth: 168 GeVSplit, 29/09/2008 Physics at LHC 2008U. De Sanctis14Extracting masses and parametersUsing the previous measurements (with also ql edges and thresholds) , a global fit is performed in order to extract the value of the masses of the particles involved: mSUGRA parameters determinationMasses of SUSY particlesWith 1 fb-1 the uncertainties on the masses and on the mSUGRA space parameters are very big Æ more statistics is needed.Split, 29/09/2008 Physics at LHC 2008U. De Sanctis15Measurement of neutralino spin (1)spin 0spin 1/2Important to measure the spin of new particles: it’s the fundamental check to ensure that what we have discovered is SUSY!!The charge asymmetry is diluted because:1. Usually it is not possible to discriminate the near and far leptons: we sum m(qlfar) and m(qlnear) invariant masses 2. The charge conjugated cascade decay (from the anti-squark) gives the opposite asymmetry. However, cancelation is not exact because at LHC a larger number of squarks than anti-squarks is produced (pp collider)Split, 29/09/2008 Physics at LHC 2008U. De Sanctis1601L02L χ~lqll l~ qχ~ qq~ +→→→ ± m0102~ ~ ~ ~ χχ +→→→ ± mlqlllqqq RL264 255 137soft hard264 154 137hard soft0102~~~~ χχ +→→→ ±± mlqlllqqq RL    219 155 118SU3 point: 19.3 pb x 3.8%Ratio squarks/anti-squarks ~3SU1 point: 7.8 pb x 1.6%Ratio squarks/anti-squarks ~3.5Asymmetry SU3 OSSF-OSOFL=30 fb-1L=100 fb-1• Cuts on missing energy and jet pt to reject SM background• 2 Opposite Sign, Same Flavour (OSSF) electrons or muons. • Subtract background from independent decay chains with the combination meee ±−+−+ −+ µµµIn SU3 point, 5÷10 fb-1are already enough to exclude charge symmetryMeasurement of neutralino spin (2)Split, 29/09/2008 Physics at LHC 2008U. De Sanctis17Conclusions• A brief review of the search strategies for SUSY in the 2-leptons channels with ATLAS has been presented;– New discoveries possible with early LHC data (O(100)pb-1)• Accurate knowledge of SM physics and of detector performance needed for any new discovery– First data taking period devoted to understanding of detector– After that, di-lepton channel could be competitive in the early LHC phase because its clear signature.• Relations among masses can be determined with a 2-5% precision already with 1 fb-1 of “well understood” data.• Larger statistics needed to measure the neutralino spin and to use the relations above to constraint the parameter space of mSUGRA and eventually to discriminate among the various SUSY models.Split, 29/09/2008 Physics at LHC 2008U. De Sanctis18BACKUP SLIDESSplit, 29/09/2008 Physics at LHC 2008U. De Sanctis19Electron & Muon PerformancesElectrons Efficiency  vs |η|TOPATLAS PreliminaryATLAS Preliminary|η|SU3Mean value of PT  resolution vs |η| |η|ATLAS PreliminaryATLAS PreliminaryMuons (PTRECO- PTTRUTH) / PTRECOTOPMuons Fake rate vs. efficiencyTOPElectrons Efficiency vs. rejection

Measurement of SUSY parameters using events with dileptons with ATLAS

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