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ATLAS Su per sy mmetry WG Journée de réflexion – Sept. 14 th 2007

ATLAS Su per sy mmetry WG Journée de réflexion – Sept. 14 th 2007. Till Eifert DPNC – ATLAS group. What’s going on there ?. Till. ?. Andree, Tuan Clemencia, Moritz. * Computing System Commissioning. Currently, people concentrate on the so-called CSC * notes …

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ATLAS Su per sy mmetry WG Journée de réflexion – Sept. 14 th 2007

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  1. ATLAS Supersymmetry WGJournée de réflexion – Sept. 14th 2007 Till Eifert DPNC – ATLAS group

  2. What’s going on there ? Till ? Andree, Tuan Clemencia, Moritz * Computing System Commissioning • Currently, people concentrate on the so-called CSC* notes … • 1 & 2: Data-driven estimations of Z/W & top backgrounds • Generator & detector uncertainties • Many analyses, most data-driven • 3: Data-driven estimations of QCD backgrounds • Fake MET rejection • MC, data-driven estimates • 4: Estimation of Heavy Flavor backgrounds and associated systematic • 5: Searches and inclusive SUSY studies • RPC, no GMSB, no split SUSY • Study signatures; scan parameter space • 6: Exclusive measurements for SUSY events • DiLepton edge, lepton+jet edge • Mass reconstruction • Extract susy parameters • 7: Gaugino direct productions • 8: ‘Studies for Gauge mediated SUSY’ -> ‘Photonic and long-lived SUSY signatures’ ATLAS SUSY WG

  3. Supersymmetry (SUSY) Fermion loop Boson loop • The light scalar Higgs boson is unprotected at GUT/ Planck scales • On the contrary, all the other light particles of the SM are protected against large scales: • Due to chiral symmetry, their mass corrections are logarithmic in E (and not quadratic) • Gauge symmetry protects the bosons (no correction to photon or gluon masses) • Fermion and boson loops contribute with different signs to the Higgs radiative corrections:if there existed a symmetry relating these two, this could protect the masses of the scalar ! • Supersymmetry realises this by transforming bosonsfermions • SUSY transforms for example a scalar boson into a spin-½ fermion, whose mass is protected • Hence, the scalar mass is also protected • This solves the naturalness and the hierarchy problems of the SM • Local gauge invariance of SUSY requires existence of spin-3/2 and spin-2 particles • This naturally introduces the spin-2 graviton, assumed to mediate the gravitational force ATLAS SUSY WG

  4. Minimal Supersymmetric Standard Model (MSSM) • To create supermultiplets, we need to add one superpartner to each SM particle • Need to introduce an additional Higgs doublet to the non-SUSY side • Mutual superpartners have equal masses and couplings SM SUSY ATLAS SUSY WG

  5. Minimal SuperGravity (mSUGRA) RG evolution of unified mSUGRA mass parameters • Reduce the ~ 105 parameters of MSSM to 5 ! • mSUGRA assumes that at the GUT scale • all scalars (squarks, sleptons, and Higgs bosons) have a common mass m0, • all gauginos and Higgsinoshava a common mass m1/2, • and all the trilinear Higgs-sfermion-sfermion couplings have a common value A0 • Remaining two parameters (at GUT scale): • SUSY conserving Higgs mass m => sign m • Ratio of Higgs vacuum expectation values tan b = n1/n2 • Renormalisation group equations (RGEs) govern the running to the EW scale • Lightest neutralino is LSP • R-parity is conserved R = (-1)( 3(B-L) + 2S) where B, L, and S are the baryon number, lepton number, and spin respectively.=> R=+1 for SM particles R=-1 for SUSY particles ATLAS SUSY WG

  6. Characteristic SUSY “Cascades” at the LHC • Conserved R-parity requires existence of a lightest stable SUSY particle = “LSP”. Since no exotic strong or EM bound states (isotopes) have been observed, the LSP should be neutral and colourless  WIMP ! • The experimental signature of the LSP would be just as the one of a heavy neutrino ! • The LSP is typically found to be a spin-½ “neutralino”, a linear combination of gauginos (in much of the SUSY parameter space the neutralino is a mixture of photino and zino) escapes detection  missing ET “Typical” SUSY decay chain at the LHC ATLAS SUSY WG

  7. Run II V. Shary @ CALOR04 Inclusive SUSY Searches • The precise signatures of the SUSY “cascades” are driven by the masses of the SUSY particles Measuring missing energy is a tough task ! • To good generality we can expect: • High-pT jets from squark & gluino decays • Leptons from gaugino & slepton decays • Missing energy from LSPs This lays out an inclusive search strategy • Detector requirements: • Excellent jet-energy measurement • Excellent lepton identification • Hermeticity of the detector (good acceptance) ATLAS SUSY WG

  8.  1 Lepton 10 fb1 Inclusive SUSY Searches … continued • A sensitive variable to detect SUSY decays is the “effective mass”: Events fully inclusive Meff • Requiring at least one lepton reduces QCD background by factor of 20–30, with signal loss of only factor of ~3  better signal-to-background ratio than fully inclusive analysis ATLAS SUSY WG

  9. “focus point” “funnel region” “low mass point” “bulk region” “coannihilation point” Inclusive SUSY Searches … continued Most SUSY searches are prepared by studying few “characteristic” points: • At the limit of experimental exclusion (SU4) • “Typical” point (SU3) • Special-feature points (SU1, SU2, SU6) SU2 SU2 m0 (GeV) SU6 SU6 Idea of this study: • Simulate MC signals for a grid in the m0, m1/2 space • Require ≥ 1 lepton (inclusive 1 lepton) • Find 1 optimal set of cuts for the whole grid SU4 SU4 SU3 SU3 no neutral LSP SU1 SU1 m½ (GeV) ATLAS SUSY WG

  10. TDR SUSY analysis • ATLAS TDR vol. II, page 820 • Reach for S/sqrt(S+B) > 5 for various SUSY signatures in the mSugra parameter space • TDR Selection • Transverse mass (l, MET) • ≥ 100 GeV • “..reduce W+jet bkg..” • Jet cut • ≥ 2 Jets • pT ≥ 100 GeV optimize pT cut for each point • MET • ≥ 100 GeV optimize cut for each point • transverse sphericity • > 0.2 • “ .. To reduce dijet background .. “ • Lepton • pT > 20 GeV • Eta < 2.5 • Integrated lumi = 10 fb-1

  11. All opt result After preSelection • Each point is separately optimized to yield the min p-value (max sigma) • As in TDR analysis, except for the missing ST cut .. • .. different datasets • .. different detector simulations … • .. different isajet version -> different susy spectra • Can we do better • Other/more variables ? • Other methods ? ATLAS SUSY WG

  12. New analysis 1 lep channel 2 lep channel • Start from pre-selection (as before) • Choice of variables for NN • MET • TransverseMass (l, MET) • JetLepPt = ΣEl_pT+ΣMu_pT+ΣJet_pT … less correlated to MET as allMeff • Jet_C4_N … total number of jets • TopInvMass … ttbar-veto analysis • t -> jet + W -> jet + lepton + nu (MET) • 1 lep case: assume lep is boosted -> η(lep) = η(nu) • 2 lep case: share MET b/w 2 nu, η, φ from lep • Future: • use kinematic fit (HITFIT) • Split analysis into 1, 2 lepton channel ATLAS SUSY WG

  13. All points optimized Sign-plot from TDR (box-cuts on JetPt 1,2, MET) Sign-plot (NN on MET, TM, Jet_N, JetLepPt, TopMass) L=10fb-1 Each point is optimized! Opt. against T1, W bkgs ATLAS SUSY WG

  14. Conclusions • Contribution to CSC 5 note • Lep (electron) ID in SUSY environment • mSugra study (presented here) • SM background validation with first data • common tools developments • Need to find out best (most sensitive) cut approach (single cut, cut as function of integrated lumi, multiple cut regions) including systematics • Also follow non-box-cut approaches ATLAS SUSY WG

  15. Backup slides ATLAS SUSY WG

  16. Data samples • mSugra signal • Grid in parameter space • A0 = 0 • tan b = 10 • sign m = + • scalar mass m0 = 0 .. 3TeV • Gauginos mass m1/2 = 0 .. 1.5 TeV • 5k events on each par. Point • All AtlFast 12.0.6 • SM Backgrounds • Consider various SM bkg samples, see next slide • All AtlFast 12.0.6+ • Software • Isajet 7.75 (for the mSugra spectra) + HERWIG/Jimmy • AtlFast (Athena) 12.0.6 • HighPtView • Production • LCG grid • Private productions ATLAS SUSY WG

  17. SM Background Samples ? ATLAS SUSY WG

  18. PreSelection Background efficiencies out of statistics out of statistics • Put samples on an equal basis & reduce #evts • Lepton cut • ≥ 1 lepton (El / Mu) • pT ≥ 20GeV • Jet cut • ≥ 2 Jets • pT ≥ 80, 40 GeV • MET ≥ 100 GeV • Add some variables • AllMeff = MET+ΣJet_pT • TransverseMass of hardest lepton + MET ATLAS SUSY WG

  19. Optimizing each point ? • Optimizing each point separately effectively means having one analysis per point… • decreases rate of the statistical type-II error (missing a true signal)  • increases the rate of the statistical type-I error (finding a wrong signal)  • One needs to find a balance • Divide parameter region into regions with different signatures => optimize on as few points as possible… ? ATLAS SUSY WG

  20. A single optimization point • Apply set of optimized cuts of signal @ • m0=300, m1/2=150 • 5-sigma region smaller, see sigma plot • High-sigma points stay • Low-sigma points gone Ratio of significance w.r.t. “all optimized points” plots ATLAS SUSY WG

  21. A single optimization point .. II • Try lower-sigma point: • Apply set of optimized cuts of signal@ • m0=1500 m1/2=450 • High-sigma points go down, but … • Keep some more low-sigma points Ratio of significance w.r.t. “all optimized points” plots ATLAS SUSY WG

  22. Details @ m0=300 m1/2=150 Signal & Bkg variable dists Input vars not strongly correlated NN output variable => we run out of stats! ATLAS SUSY WG

  23. Details @ m0=1500 m1/2=750 Signal & Bkg variable dists Jet_N & JetLepPt 75% corr. NN output variable =>better seperation power ATLAS SUSY WG

  24. A single optimization point • Apply set of optimized cuts of signal @ • m0=300, m1/2=150 (left) • m0=1500, m1/2=750 (right) • Net result: quite good coverage with 2 optimized NN (w.r.t. all points opt.) Study of systematics -> need more stats ATLAS SUSY WG

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