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/ , and principal reference: ATL-PHYS (Bachacou, Hinchliffe & Paige)

Christopher Lester, October 2000Cambridge SUSY Working Group Motivation zSupersymmetry: yLarge spectrum of new particles. yWhat are their masses? zConsider R-parity conserving models. Pair of massive LSPs go unobserved, so: ymodel is easy to spot from missing energy signature, but... ythe incomplete decay chains make it hard to measure masses.

Christopher Lester, October 2000Cambridge SUSY Working Group Decay chains used Sequential Branched

Christopher Lester, October 2000Cambridge SUSY Working Group Example

Christopher Lester, October 2000Cambridge SUSY Working Group The dilepton edge zRight: The dilepton invariant mass distribution clearly displays the kinematic endpoint coming from the sequential decay chain. z109.10±0.13 GeV

Christopher Lester, October 2000Cambridge SUSY Working Group 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) “Reconstruct” sparticle masses in each experiment, and thus obtain estimate of statistical contribution to sparticle mass resolution. Simulate an ensemble of statistics limited ATLAS experiments, using the errors estimated above.

Christopher Lester, October 2000Cambridge SUSY Working Group 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:

Christopher Lester, October 2000Cambridge SUSY Working Group Cross sections

Christopher Lester, October 2000Cambridge SUSY Working Group Typical cuts zExactly 2 OSSF leptons > 10 GeV z2 jets > 150 GeV (A) z4 jets > 100,50,50,50 GeV (B) zmissing P T > max(100 GeV, 0.2 M eff ) (A) zmissing P T > 300 GeV (B) zm ll max /  2<m ll <m ll max (llq-threshold only) z“OSSF-OSDF” flavour subtraction zm ll < m ll max +1 GeV (both lq-edges) (A={ll-edge, llq-threshold}, B={others})

Fitted distributions ll lq high llqXq lq low llq Xqllq lq lowlq high llqll

Christopher Lester, October 2000Cambridge SUSY Working Group Fit results zFitted edge positions (GeV) z“Fit error” is statistical contribution only zNo jet energy calibration performed

Christopher Lester, October 2000Cambridge SUSY Working Group ΔM = M T2 (χ) - χ Given: zthe lepton momenta zthe missing transverse momentum zan estimate “Χ” of the neutralino mass Deduce: zlower bound M T2 (Χ) on slepton mass zslepton-neutralino mass difference ΔM

Christopher Lester, October 2000Cambridge SUSY Working Group Dislepton event cuts Hard cuts: zExactly 2 OSSF leptons > 50, 30 GeV z  T = |p T l1,p T l2 +p T missing | < 20 GeV zNo jets above 40 GeV (50 GeV) - expect to lose 10% (1%) of signal to minimum bias events in same bunch crossing. zm ll not within 5 GeV of m Z. zm ll,p T missing >80 GeV Soft cuts: zp T missing requirement lowered to 50 GeV z  T < 90 GeV zNo m ll cuts at all.

Christopher Lester, October 2000Cambridge SUSY Working Group ΔM - 50 GeV Jet Cut Hard cutsSoft cuts (ΔM = 92 GeV)

Christopher Lester, October 2000Cambridge SUSY Working Group ΔM - Dependence on Chi

Christopher Lester, October 2000Cambridge SUSY Working Group ΔM - Backgrounds from SM zSlepton ΔM edge is visible when slepton-neutralino mass difference is greater than about 60 GeV.

Christopher Lester, October 2000Cambridge SUSY Working Group Reconstructing smasses Choose model to “represent nature” (S1/O5) Calculate “ideal” edge positions. “Simulate” single ATLAS experiment by smearing edge positions according to estimated statistical error. Make a random guess at sparticle masses. Calculate chi-squared between these edges and “smeared” edge positions. Predict edge positions based upon sparticle mass guess. Improve upon mass guess. Stop at best fit

Christopher Lester, October 2000Cambridge SUSY Working Group Fitted masses (S5) zFitted masses for an ensemble of ATLAS experiments zArrows indicate masses from model z17 GeV resolution on non-squarks z22 GeV resolution for squark scale

Christopher Lester, October 2000Cambridge SUSY Working Group Fitted masses (O1) zFitted masses for an ensemble of ATLAS experiments zArrows indicate masses from model z20 GeV resolution on non-squarks z29 GeV resolution for squark scale

Christopher Lester, October 2000Cambridge SUSY Working Group Reconstruction Widths z14-18 % for neutralino1 z9-11 % for slepton z7-8 % for neutralino2 z3-5 % for squark scale

Christopher Lester, October 2000Cambridge SUSY Working Group Separation zLeft: yReconstructed neutralino and slepton masses for an ensemble of ATLAS experiments yGood separation yCorrelation evident

Christopher Lester, October 2000Cambridge SUSY Working Group Developments zRemoval of theoretical dependencies on mass relationships zAddition of two new measurements: , {Low/High lq-edge} zSimultaneous testing against second model with same set of cuts zReduce subjectivity in estimation of edge fitting resolutions

Edge positions