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SUSY squark flavour studies w i th ATLAS

SUSY squark flavour studies w i th ATLAS. Tommaso Lari Università and INFN Milano On Behalf of the ATLAS Collaborat ion. Outline. Introduction SUSY mass scale from inclusive searches Left-handed squark Sbottom and gluino Right-handed squark Stop Conclusions.

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SUSY squark flavour studies w i th ATLAS

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  1. SUSY squark flavour studies with ATLAS Tommaso Lari Università and INFN Milano On Behalf of the ATLAS Collaboration

  2. Outline Introduction • SUSY mass scale from inclusive searches • Left-handed squark • Sbottom and gluino • Right-handed squark • Stop Conclusions • Most of the ATLAS work since Physics TDR (1999) done on mSUGRA models. • A particularly extensive study is available for SPS1a point (fast simulation) – it will be used here to illustrate techniques to reconstruct the squark mass spectrum. • When available for a given signature, recent full simulation results will also be shown (obtained for other mSUGRA points) Flavour Workshop CERN, 08/11/2005

  3. Introduction SPS1a mass spectrum (colored states) In mSUGRA: • At the Unification scale, all scalars have the same mass. • At the EW scale: • First two generation almost mass degenerate, but m(qL) ≠ m(qR) • Large L-R mixing in 3° generation: b1,b2,t1,t2 mass eigenstates • Light b1 and t1 ~ ~ SPS1a: m0= 100 GeV, m1/2= 250 GeV, A0= -100 GeV, tan(β)=10 , μ>0 ~ ~ ~ ~ ~ ~ mSUGRA free parameters: m0, m1/2, A0, tan b, sgn(m) Flavour Workshop CERN, 08/11/2005

  4. p p ~ c01 ~ ~ ~ q ~ c02 l g q q l l mSUGRA events topology A typical decay chain: • Strongly interacting sparticles (squarks, gluinos) dominate LHC production. • 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, t-jets (depending on model parameters) If sbottom or stop quarks in the decay chain: b-jets Charm tagging impossible? Flavour Workshop CERN, 08/11/2005

  5. Inclusive signatures • First hint of the existence of non-SM physics will probably be an excess of events with large missing energy and hard jets • The peak of the distribution of the effective mass: if visible above the background, is strongly correlated with the mass of the SUSY particle produced (gluino or squark): first estimate of SUSY mass scale ATLAS fast simulation SM background (ALPGEN+PYTHIA) 10 fb-1 SUSY (1 TeV) Effective mass (GeV) 10 fb-1 ATLAS SUSY mass scale (GeV) Effective mass (GeV) Flavour Workshop CERN, 08/11/2005

  6. Inclusive searches (Full simulation) ATLAS Preliminary Full Simulation ATLAS Preliminary Full Simulation • Full simulation data, produced for seven different mSUGRA points, confirms the good correlation between the Effective Mass peak and the SUSY mass scale coannihilation point SM background (fast simulation) Stau coannihilation point: m0 =70 GeV m1/2 = 350 GeV A=0 tanb=10 m>0 Flavour Workshop CERN, 08/11/2005

  7. SPS1a A detailed study for SPS1a(bulk region of parameter space)was done in fast simulation Gjelsten, Lytken, Miller, Osland, Polesello, ATL-PHYS-2004-007 ~ • qL mass from • qL → qc02 → q l l → q l l c01 • qR mass from • qR → q c01 • b, g mass from • g → bb → bb c02 → bb ll → bb ll c01 ~ ~ ~ SPS1a ~ ~ ~ ~ ~ ~ Flavour Workshop CERN, 08/11/2005

  8. Left squark cascade decay The decay chain (note the two isolated leptons) ll edge Allows to measure several kinematical endpoints, each providing one mass relation between the SUSY particles. 2 SFOS lep., pT>20, 10 GeV ≥ 4 jets, pT>150,100,50,50 GeV Meff > 600 GeV ETmiss >max(100, 0.2 Meff) SPS1a 100 fb-1 Fast simulation 0 40 80 qlmin edge qll threshold qll edge qlmax edge 600 0 0 600 0 600 600 0 Flavour Workshop CERN, 08/11/2005 Invariant mass (GeV) Gjelsten, Lytken, Miller, Osland, Polesello,ATL-PHYS-2004-007

  9. Left squark mass fit Fit results (L = 100 fb-1) 5 relations for 4 masses! Can compare qll and bll thresholds, and measure m(b)-m(uL) ~ ~ Precision limited by systematics: error on jet energy scale (1% expected) ~ m(lR ) = 143 GeV m(χ10) = 96 GeV Mass reconstruction ~ m(qL) = 540 GeV m(χ20) = 177 GeV Δm(χ10)= 4.8 GeV, Δm(χ20)= 4.7 GeV, Δm(lR) = 4.8 GeV, Δm(qL)= 8.7 GeV ~ ~ Flavour Workshop CERN, 08/11/2005 Gjelsten, Lytken, Miller, Osland, Polesello, ATL-PHYS-2004-007

  10. Going up the decay chain ~ • Once the mass of the c01 is known, it is possible to get the momentum of the c02 using the approximate relation p(c02) = ( 1-m(c01)/m(ll) ) pll valid for lepton pairs with invariant mass near the edge. • The c02 can be combined with b-jets to reconstruct the gluino and sbottom mass peaks: g → bb → bbc02 ~ ATLAS fast sim. ATL-PHYS-2004-007 ~ ~ ~ ~ ~ ~ ~ ~ b-tagging used to separate light and bottom squark decay chains Dilepton invariant mass Flavour Workshop CERN, 08/11/2005

  11. Gluino and sbottom reconstruction Good combinations. m(g) and m(b) correlated (Dominant error from c02 Momentum affects both) ~ ~ ATLAS fast simulation ATL-PHYS-2004-007 300 fb-1 ~ ~ Bad c02 b combinations (b-jet is from gluino decay) Flavour Workshop CERN, 08/11/2005

  12. Gluino mass reconstruction ATLAS fast simulation ATL-PHYS-2004-007 300 fb-1 ATLAS fast simulation ATL-PHYS-2004-007 300 fb-1 M(cbb) (GeV) M(cbb) (GeV) M(LSP) (GeV) ~ ~ ~ m(g)-0.99m(c01) = (500.0 ± 6.4) GeV with 300 fb-1 ~ ~ Error is statistical plus (dominant) 1% uncertainty on jet energy scale Central value 10 GeV lower than nominal because no dedicated calibration for b-jet energy scale used in this (fast sim) study Flavour Workshop CERN, 08/11/2005

  13. sbottom mass ~ ~ m(c02 bb) and m(c02 b) strongly correlated : use the difference ATLAS fast simulation ATL-PHYS-2004-007 300 fb-1 ATLAS fast simulation ATL-PHYS-2004-007 300 fb-1 ~ b2 ~ b1 m(cbb)-m(cb) (GeV) m(cbb)-m(cb) (GeV) ~ ~ ~ ~ With 300 fb-1 it should be possible to separate the b1 and b2 peaks ~ ~ ~ ~ m(g)-m(b1) = (103.3 ± 1.8) GeV m(g)-m(b2) = (70.6 ± 2.6) GeV With lower statistics only measure the average Flavour Workshop CERN, 08/11/2005

  14. Right-handed squark ATLAS fast simulation ATL-PHYS-2004-007 SPS1a 30 fb-1 ~ ~ qR does not couple to Wino c01 is nearly a Bino c02 is nearly a Wino ~ ~ qR → q c01 ~ ~ ~ ~ ~ qR qR production: two high pT jets and missing energy The different decay chains allow separation from qL (veto additional jets and b-tagged jets) Combine the transverse momentum of two leading jets with missing transverse momentum as follows: ~ ~ ~ ~ m(qR)-m(c01) = (424.2 ± 10.9) GeV ~ ~ ATLAS full simulation stau coannihilation point Flavour Workshop CERN, 08/11/2005

  15. SPS1a: summary The following masses would be measured by ATLAS for SPS1a: ~ m(qR)-m(c01) = (424.2 ± 10.9) GeV 30 fb-1 ~ m(qL) = (444.0 ± 4.9) GeV 300 fb-1 ~ m(g)-m(c01) = (500.0 ± 6.4) GeV 300 fb-1 ~ ~ m(g)-m(b1) = (103.3 ± 1.8) GeV 300 fb-1 ~ ~ m(g)-m(b2) = (70.6 ± 2.6) GeV 300 fb-1 After a few years at LHC design luminosity, precision would be limited by systematic error on jet energy scale The analysis works in a large fraction of parameter space (when relevant decay chains exist) but results depend on specific point Flavour Workshop CERN, 08/11/2005

  16. Stop edges ~ ~ ~ ~ ~ ~ g → tt1 → tbc±1 or g → bb → bt c±1 ~ ~ ~ ~ ~ ~ tb invariant mass has a maximum function of the masses of g, b(or t) andc±1 Two closely spaced edges from the two decays: can measure a weighed average. • Selections: • - total jet energy and missing energy • 2 b-jets • lepton veto • 4 to 6 non-b jets ATLAS Reconstruction: - m(jj) close to m(W) - m(jjb) close to m(t) - W-sidebands to estimate and subtract combinatorial background See J. Hisano et al., Phys. Rev. D68, 035007 for details Flavour Workshop CERN, 08/11/2005

  17. Conclusions On a significant fraction of mSUGRA parameter space, the LHC would be able to measure the mass of qR,qL,b1,b2 Many measurements studied also with Full Simulation data for a variety of points – they confirm that fast simulation results are realistic. A model-independent measurement of the stop mass is more challenging (but kinematical decays related to stop decays would be observed for most parameter values) Measurement of mixing angles needs more study CPV, FV, mixing and non-degenerate masses in first two generations also poorly covered • - availability in HERWIG/PYTHIA/ISAJET ? • - criteria to choose parameters? flavour benchmarks? ~ ~ ~ ~ Flavour Workshop CERN, 08/11/2005

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