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Search for SUSY at ATLAS

Search for SUSY at ATLAS. Tommaso Lari Università and INFN Milano On behalf of the ATLAS collaborat i on. Outline. Introduction Search for a generic susy signal (excess of events over SM contribution)

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Search for SUSY at ATLAS

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  1. Search for SUSY at ATLAS Tommaso Lari Università and INFN Milano On behalf of the ATLAS collaboration

  2. Outline • Introduction • Search for a generic susy signal (excess of events over SM contribution) • Rather than review all the possible search strategies, I will focus on Jet+EtMiss search and discuss the background a little bit. • Reconstruction of the mass of SUSY particles using selected decays • Basic techniques, some full simulation results • Some special models T. Lari SUSY searches at ATLAS

  3. ATLAS studies Supersymmetry physics one of the priorities of on-going ATLAS studies. In the past (ATLAS Physics TDR 1998) • Fast simulation studies (physics process+parametrized detector response) • Focus on discovery potential, reconstruction of s-particle masses for a few selected benchmarks Now • Detector commissioning and systematic (detailed simulation studies) • Background estimation (use/validate latest MC, techniques to measure background from data) • New models and measurement techniques (in fast simulation first) Huge variety of models being studied. In this talk will concentrate on mSUGRA. Cannot do justice to the topic in 15 minutes. T. Lari SUSY searches at ATLAS

  4. p p ~ c01 ~ ~ ~ q ~ c02 l g q q l l How SUSY looks like(to an experimental physicist) A typical decay chain: SUSY particles: Scalars (s-quarks, sleptons) Gaugino (gluino, 4 neutralinos, 2 charginos) 5 Higgsbosons • Strongly interacting sparticles (squarks, gluinos) dominate production. • Heavier than sleptons, gauginos etc. : cascade decays to LSP. • Long decay chains and large mass differences between SUSY states • Many high pT objects observed (leptons, jets, b-jets). • If R-Parity conserved LSP (lightest neutralino in mSUGRA) stable and sparticles pair produced. • Large ETmiss signature • Closest equivalent SM signature top pair production • Top physics important for commissioning (and important background) T. Lari SUSY searches at ATLAS

  5. SUSY search strategies Jets + ETmiss + 0 leptons ATLAS Physics TDR • Best strategy for mSUGRA is usually jets + ETmiss + n-leptons. • The “Effective Mass”: discriminates SM and SUSY and has a maximum strongly correlated with the mass of the s-particles produced in the pp collision. ATLAS 1 TeV SUSY 10 fb-1 Meff=S|pTi| + ETmiss. SM Meff (GeV) • SUSY selection cuts used in the pictures: • 1 jet with pT >100 GeV, 4 jets with pT>50 GeV • ETMISS > max(100 GeV,0.2Meff) • Transverse sfericity ST>0.2 • No isolated muon or electron with pT>20 GeV SUSY mass scale 10 fb-1 ATLAS Meff peak (GeV) T. Lari SUSY searches at ATLAS

  6. SUSY search ATLAS 10 fb-1 2.5 TeV g M1/2 (GeV) • A parameter scan is performed to evaluate • the discovery potential and the trigger • efficiency of different signatures. • 10 fb-1 of data should allow discovery • for mSUSY < 2 TeV • Caveats: • Statistical errors only. • Need to understand detector and SM • background first • SM background was simulated with shower • MC (multi-jet xSection too low) Charged LSP 2.5 TeV q 1 TeV g 1 TeV q No EWSB LEP excluded M0 (GeV) T. Lari SUSY searches at ATLAS

  7. Backgrounds to SUSY searches • Recent (2005) study with AlpGen + Pythia (MLM match) + Atlfast • Background increases • Discovery of 1 TeV SUSY still easy if systematic smaller than statistical errors • More hard jets in background process but not ETMISS • Missing energy crucial for SUSY searches • 1-lepton mode better than 0-lepton • Better S/B, dominant background is top (more controllable than multi-jets) Phys. TDR study (0 leptons) ATLAS Preliminary (0 leptons) ATLAS Preliminary (1 leptons) Meff (GeV) All plots for a 1 TeV scale mSUGRA model T. Lari SUSY searches at ATLAS

  8. Background from data ATLAS Preliminary Full Simulation 0.5 fb-1 • Top mass reasonably uncorrelated with ETMISS • Select events with m(lj) in top window (with W mass constraint – no b-tag used). Estimate combinatorial background with sideband subtraction. • Normalize to low ETMiss region (where SUSY small) • Procedure gives estimate consistent with top distribution also when SUSY is present Blue: tt Green: SUSY Dots: top estimate T. Lari SUSY searches at ATLAS

  9. p p ~ c01 ~ ~ ~ q ~ c02 l g q q l l SUSY mass spectroscopy mSUGRA • After discovery: reconstruction of SUSY masses. • Two undetected LSP: no mass from one specific decay. Measure mass combinations from kinematics endpoints/thresholds. With long enough decay chain, enough relations to get all masses. • A point in parameter space is chosen, and decay chains are reconstructed. • Analysis should be applicable whenever the specific decay do exist. • Leptonic (e/m) decay of χ02 “golden channel” to start reconstruction. • But Higgs and t decays can also be used. • Some benchmark points favoured by cosmology studied in detail • Masses can be extracted also by combination of informations from different events • (mass relation method, …) T. Lari SUSY searches at ATLAS

  10. ~ ~ c02 ~ c01 l l l Dilepton Edge Polesello et al., 1997 • Clear signature, easy to trigger: starting point of many mass reconstruction analyses. • Can perform SM & SUSY background subtraction using OF distribution e+e- + m+m- - e+m- - m+e- • Position of edge (LHC Point 5) measured with precision ~ 0.5% (30 fb-1). e+e- + m+m- - e+m- - m+e- e+e- + m+m- 5 fb-1 FULL SIM Point 5 ATLAS ATLAS 30 fb-1 atlfast Modified Point 5 (tan(b) = 6) Physics TDR SUSY backg Mll (GeV) Mll (GeV) SM backg T. Lari SUSY searches at ATLAS

  11. p p ~ c01 ~ ~ ~ q ~ c02 l g q q l l lq edge llq edge 1% error (100 fb1) 1% error (100 fb-1) Mass reconstruction: a typical decay chain The invariant mass of each combination has a minimum or a maximum which provides one constraint on the masses of c01 c02l q ~ ~ ~ ~ ATLAS Fast simulation,LHCC Point 5 ATLAS TDR ATLAS TDR ATLAS TDR ATLAS TDR llq threshold ll edge Formulas in Allanach et al., hep-ph/0007009 T. Lari SUSY searches at ATLAS

  12. Sparticle Expected precision (100 fb-1) qL 3% 02 6% lR 9% 01 12% ~ ~ ~ ~ Model-independent masses • Combine measurements from edges of different jet/lepton combinations to obtain ‘model-independent’ mass measurements. • LSP mass uncertainty large, all other masses strongly correlated with it. A future Linear Collider measurement of c01mass would improve the precision on all masses. ~ ~ ~ c01 lR ~ ~ ~ ATLAS ~ ~ c02 qL T. Lari SUSY searches at ATLAS

  13. Mass peaks ATLAS SPS1a 300 fb-1 CMS 1 fb-1 ~ ~ m(g) m(q) = (536 ± 10) GeV • The 4-momentum of the c02 can be reconstructed from the approximate relation p(c02) = ( 1-m(c01)/m(ll) ) pll valid when m(ll) near the edge. • The c02 can be combined with b-jets to reconstruct the gluino and sbottom mass peaks from g→bb→bbc02 m(cbb) (GeV) ~ ~ ~ ~ m(g)-m(b) CMS 10 fb-1 ~ m(g) = (500 ± 7) GeV ATLAS SPS1a 300 fb-1 SPS1a, 300 fb-1, stat. errors only: ~ m(g)-0.99m(c01) = (500.0 ± 6.4) GeV ~ ~ m(g)-m(b1) = (103.3 ± 1.8) GeV ~ ~ m(g)-m(b2) = (70.6 ± 2.6) GeV m(cbb)-m(cb) (GeV) T. Lari SUSY searches at ATLAS

  14. Other mass measurements ~ ~ qLgc02q → c01hq →c01bbq qRgc01q ~ co2  tt c01 tt ATLAS 30 fb-1 ATLAS Point 5 100 fb-1 Tau edge Higgs-jet invariant mass ATLAS 30 fb-1 Right squark M(tt) (GeV) M(bbq) (GeV) MT2 (GeV) Two body decay of c02 to higgs and c01. Reconstruct higgs mass (2 b-jets) and combine with hard jet. Get additional mass constraint. Tau decay dominates neutralino BR at large tanb. No sharp edge because of n,but end-point can still be measured. 2 hard jets and lots of ETmiss. Reconstruct with Also works for sleptons. ~ m(qR)-m(c01) = (424.2 ± 10.9) GeV T. Lari SUSY searches at ATLAS

  15. Parameter Expected precision (300 fb-1) m0 2% m1/2 0.6% tan(b)  9% A0 16% From masses to model parameters From a given set of measurements one scans the parameter space and finds the points compatible with data. These points are fed to relic density calculators to get constraints on relic density. Micromegas 1.1 (Belanger et al.)+ ISASUGRA 7.69 ATLAS measurements Wch2 = 0.1921  0.0053 log10(scp/pb) = -8.170.04 Wch2 300 fb-1 ATLAS T. Lari SUSY searches at ATLAS

  16. Full simulation studies • Goals: test software for data reconstruction and analysis, computing grid production. Study detector-related systematic. Validate fast simulation results. • 10M events produced in 2005. • Five mSUGRA models studied. Focus on cosmologically interesting regions. Coannihilation SU1 Focus Point SU2 Bulk SU3 No EWSB LEP excluded T. Lari SUSY searches at ATLAS

  17. p p ~ c01 ~ ~ ~ q ~ c02 l g q q l l Full simulation: bulk model Larger of M(llq) ATLAS Preliminary 4.20 fb-1 Already with a few fb-1 of data several edges are visible. All results preliminary. 272 GeV (e+e-) + β2(η) (μ+μ-) – β(η) (e+μ-) ATLAS Preliminary 4.37 fb-1 Smaller of M(llq) ATLAS Preliminary 4.20 fb-1 501 GeV Edge at99.8 ± 1.2 GeV T. Lari SUSY searches at ATLAS

  18. Coannihilation ATLAS Preliminary Full Sim. 20 fb-1 ql(min) edge ATLAS Preliminary Full Sim. 20 fb-1 qR edge ql(max) edge ~ ATLAS Preliminary Full Sim. 20 fb-1 ll edge qll threshold qll edge T. Lari SUSY searches at ATLAS

  19. Focus-Point SU2 SUSY production is: •  (direct) (4.5 pb) Do not pass cuts to reject SM (little jets & ETmiss) • gg →+jets(0.5 pb) Can be separated efficiently from SM • SUSY Scalars heavy (3 TeV) - only fermions (gluino, chargino, neutralino) accessible to LHC • Fast discovery, kinematical edges require larger statistics. ~ ~ ATLAS Preliminary Fast Sim. 300 fb-1 2.6s excess ATLAS Preliminary Full Simulation 6.9 fb-1 Mll (GeV) Mll (GeV) T. Lari SUSY searches at ATLAS

  20. Conclusions • Supersymmetry isone of the priorities of ATLAS studies. • Large scale productions on the grid of full simulation data are used • to study detector systematic and prepare for real data analysis. • Knowledge of the Standard Model background is crucial for • discovery. Lot of work is being done on SM background computation • and measurement from data. • Still on-going studies of new models and new techniques to measure • the properties of SUSY particles. • In most models, a few fb-1 of data will allow ATLAS to measure • a clear excess over the SM contribution and reconstruct several • mass relations. • Looking eagerly forward to the first data! T. Lari SUSY searches at ATLAS

  21. Backup slides T. Lari SUSY searches at ATLAS

  22. Mass Relation Method • Hot off the press: new idea for reconstructing SUSY masses! • ‘Impossible to measure mass of each sparticle using one channel alone’ (Page 8). • Should have added caveat: Only if done event-by-event! • Remove ambiguities by combining different events analytically g ‘mass relation method’ (Nojiri et al.). • Also allows all events to be used, not just those passing hard cuts (useful if background small, buts stats limited – e.g. high scale SUSY). Preliminary ATLAS ATLAS SPS1a T. Lari SUSY searches at ATLAS

  23. Split SUSY • Ignore hierachy problem (also there for cosmological constants, one may invoke huge number of vacua and antropic principle) • Keep SUSY (unification of coupling constants, dark matter…) • Scalar particles are (VERY) heavy • Gluino is long-lived (decays to gaugini through virtual squarks) – from a narrow resonance to cosmological lifetimes • If gluino prompt decay: like mSUGRA with heavy scalars (focus-point) • If gluino lifetime in ps – us range: secondary vertices • If quasi-stable gluino: neutral and charged R-hadrons produced • Charge-exchange reaction every nuclear int. length: charge state changes in calorimeter • EM+nuclear interaction: no shower, but more energy loss than heavy muon • Energy profile in calorimeter, time-of-flight in muon chambers, … : very typical signature (almost no background expected) • LHC sensible up to 1.7 TeV mass T. Lari SUSY searches at ATLAS

  24. SUSY Higgs sector 2 doublets, 5 physical states: h0,H0,A0,H (mix if CPV) h light, SM-like. m  133 GeV Lots of free parameters in MSSM • Often assume heavy SUSY states (no Higgs decay into SUSY nor Higgs production in SUSY decays) • Define banchmark scenarios. Example (Carena et al. , Eur.Phys.J.C26,601): • MASSH – maximum h mass allowed by theory • Nomixing – small h mass (difficult for LHC) • gluophobic – reduces hg coupling (and LHC production xSection) • Small a - reduces hbb and htt couplings (harms some discovery channels) • Parameter scans performed on two free parameters (mA, tanb) • SM xSection + MSSM correction factors • Higgs decays (FeynHiggs) • Efficiency and background from MC studies of different channels • Corrections from Higgs width and overlap of states T. Lari SUSY searches at ATLAS

  25. SUSY Higgs scans ATLAS ATLAS h discovery curves H/A discovery curves LEP limit depends on top mass (here mtop = 175 GeV). No tanb limit for mt > 183 GeV Statistic is 30 fb-1 or 300 fb-1 depending on channels. Stat. errors only. Always at least one Higgs is seen (also for the other scenarios). Over a large parameter space, only h is observable and discrimination from SM Higgs is very difficult. T. Lari SUSY searches at ATLAS

  26. polarization of induces asymmetry seems feasible, with 150 fb1 parton level detector level A± Supersymmetry – Spin Measurement A.J. Barr, hep-ph/0405052 • Evidence for supersymmetry (vs extra dimensions, for example) T. Lari SUSY searches at ATLAS

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