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Developing Model Independent sparticle mass measurements at ATLAS

Developing Model Independent sparticle mass measurements at ATLAS. Cambridge SUSY Working Group B.C. Allanach, C.G. Lester, M.A. Parker, B.R. Webber. See also: authors paper: hep-ph/0007009 , and principal reference: ATL-PHYS-2000-010 (Bachacou, Hinchliffe & Paige). Motivation.

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Developing Model Independent sparticle mass measurements at ATLAS

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  1. Developing Model Independent sparticle mass measurements at ATLAS Cambridge SUSY Working Group B.C. Allanach, C.G. Lester, M.A. Parker, B.R. Webber • See also: • authors paper: hep-ph/0007009, and • principal reference: ATL-PHYS-2000-010 (Bachacou, Hinchliffe & Paige)

  2. Motivation • Supersymmetry: • Large spectrum of new particles. • What are their masses? • Consider R-parity conserving models. Pair of massive LSPs go unobserved, so: • model is easy to spot from missing energy signature, but ... • the incomplete decay chains make it hard to measure masses. Cambridge SUSY Working Group

  3. Decay chains used Sequential Branched Cambridge SUSY Working Group

  4. Example Cambridge SUSY Working Group

  5. The dilepton edge • Right: The dilepton invariant mass distribution clearly displays the kinematic endpoint coming from the sequential decay chain. • 109.10±0.13 GeV Cambridge SUSY Working Group

  6. Overall technique MonteCarlo the distributions with kinematic edges Estimate STATISTICAL contribution to error on edge position, by fitting edges in simplistic manner. (assume calibration can be done) Simulate an ensemble of statistics limited ATLAS experiments, using the errors estimated above. “Reconstruct” sparticle masses in each experiment, and thus obtain estimate of statistical contribution to sparticle mass resolution. Cambridge SUSY Working Group

  7. The Models S5: “LHC SUGRA Point 5” ... well known ... O1: “Optimised string model”. Attempts to remove dangerous CCB/UFB problems. A three parameter, weak coupling model. It is non-universal, but in a family independent way. Modular weights: Cambridge SUSY Working Group

  8. Cross sections Cambridge SUSY Working Group

  9. Typical cuts • Exactly 2 OSSF leptons > 10 GeV • 2 jets > 150 GeV (A) • 4 jets > 100,50,50,50 GeV (B) • missing PT > max(100 GeV, 0.2 Meff) (A) • missing PT > 300 GeV (B) • mllmax/2<mll<mllmax (llq-threshold only) • “OSSF-OSDF” flavour subtraction • mll < mllmax +1 GeV (both lq-edges) (A={ll-edge, llq-threshold}, B={others}) Cambridge SUSY Working Group

  10. Fitted distributions ll llq ll llq S5 lq high lq low lq high lq low O1 llq Xq llq Xq

  11. Fit results • Fitted edge positions (GeV) • “Fit error” is statistical contribution only • No jet energy calibration performed Cambridge SUSY Working Group

  12. ΔM = MT2(χ) - χ Given: • the lepton momenta • the missing transverse momentum • an estimate “Χ” of the neutralino mass Deduce: • lower bound MT2(Χ) on slepton mass • slepton-neutralino mass difference ΔM Cambridge SUSY Working Group

  13. Dislepton event cuts Hard cuts: • Exactly 2 OSSF leptons > 50, 30 GeV • T = |pTl1,pTl2+pTmissing| < 20 GeV • No jets above 40 GeV (50 GeV) - expect to lose 10% (1%) of signal to minimum bias events in same bunch crossing. • mll not within 5 GeV of mZ. • mll,pTmissing>80 GeV Soft cuts: • pTmissing requirement lowered to 50 GeV • T < 90 GeV • No mll cuts at all. Cambridge SUSY Working Group

  14. ΔM - 50 GeV Jet Cut (ΔM = 92 GeV) Hard cuts Soft cuts Cambridge SUSY Working Group

  15. ΔM - Dependence on Chi Cambridge SUSY Working Group

  16. ΔM - Backgrounds from SM • Slepton ΔM edge is visible when slepton-neutralino mass difference is greater than about 60 GeV. Cambridge SUSY Working Group

  17. Reconstructing smasses Choose model to “represent nature” (S1/O5) Calculate “ideal” edge positions. Stop at best fit Calculate chi-squared between these edges and “smeared” edge positions. “Simulate” single ATLAS experiment by smearing edge positions according to estimated statistical error. Improve upon mass guess. Predict edge positions based upon sparticle mass guess. Make a random guess at sparticle masses. Cambridge SUSY Working Group

  18. Fitted masses (S5) • Fitted masses for an ensemble of ATLAS experiments • Arrows indicate masses from model • 17 GeV resolution on non-squarks • 22 GeV resolution for squark scale Cambridge SUSY Working Group

  19. Fitted masses (O1) • Fitted masses for an ensemble of ATLAS experiments • Arrows indicate masses from model • 20 GeV resolution on non-squarks • 29 GeV resolution for squark scale Cambridge SUSY Working Group

  20. Reconstruction Widths • 14-18 % for neutralino1 • 9-11 % for slepton • 7-8 % for neutralino2 • 3-5 % for squark scale Cambridge SUSY Working Group

  21. Separation • Left: • Reconstructed neutralino and slepton masses for an ensemble of ATLAS experiments • Good separation • Correlation evident Cambridge SUSY Working Group

  22. Developments • Removal of theoretical dependencies on mass relationships • Addition of two new measurements: , {Low/High lq-edge} • Simultaneous testing against second model with same set of cuts • Reduce subjectivity in estimation of edge fitting resolutions Cambridge SUSY Working Group

  23. Edge positions

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