1/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Potential to Discover Supersymmetry in Events with Same-sign Dimuon, Jets and Missing Energy at LHC Yuriy Pakhotin for CMS and ATLAS Collaboration SUSY08, COEX, Seoul, Korea June 19 th, 2008 2/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 LHC: Large Hadron Collider LHC is pp-collider nominal energy: E=14 TeV design luminosity: L=10 34 cm -2 s -1 first physics run: (Fall 2008) E = 10 TeV, L=210 33 cm -2 s -1 3/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 CMS: Compact Muon Solenoid Muons: muon system acceptance: || 10 GeV to avoid problems with low-pT range where efficiency is not very good Other cut variables: 1.muon trigger (di-muon High-Level Trigger) 2.transverse momentum of muons 3.combined (calorimeter + tracker) muons isolation 4.muons track parameters 5.jet multiplicity 6.pT of 3 leading jets 7.large missing transverse energy 14/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 LEP Tevatron A0=0, tan()=10, sign()=+1 Optimized cuts for 10 fb -1 luminosity mSUGRA 5 reach contours (Monte Carlo simulation) of the same-sign dimuon analysis, including systematic uncertainties, for different integrated luminosities and assuming no re-optimization of the selection cuts CMS: Same-sign Dimuon Reach Contour Sample QCD0.01 TTbar0.3 W/Z+jets0.05 mSUGRA LM117 (S 5.5) CMS Preliminary: Number of expected events after selection cuts applied for L=1 fb -1 15/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Atlas: In-situ Background Estimation 2D Side Band (SB) is chosen: pTjet2 [40 GeV ~ 80 GeV] MET [50 GeV ~ 80 GeV] A SR =A SB *B SR /B SB True background events in 1 fb -1 =14.8 Side Band ContentEstimated BG in 1 fb -1 All BG15.2 4.0 BG+SU115.4 4.1 BG+SU215.2 4.0 BG+SU315.3 4.1 BG+SU421.4 4.8 BG+SU615.3 4.1 Transverse mass (MT) of leading lepton and MET is a good candidate as a variable for Side Band to Signal Region (SR) normalization: Transverse mass distributions for SB and SR are in a very good agreement. 16/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008Conclusion 1.The preparation for topological search of new physics (excess over SM background) with same-sign dimuon in LHC experiments is presented 2.Assuming unknown signal, mSUGRA model for cut optimization is used 3.It is shown that SM background can be suppressed with optimized cuts almost to zero with luminosity less than 1 fb -1 (early running of LHC). Significant number of signal events are survived 4.Small number of expected SM background events leads to less dependency on the statistical and systematical uncertainties, which is crucial for initial experiment data. Then even relatively small excess (a few events) over expected SM background may be safely interpreted as a discovery of physics beyond SM 5.The same set of cuts may be applied to search for different theoretical predictions beyond SM 6.Data driven methods to estimate SM backgrounds for low luminosity (1 fb -1 ) are currently elaborating 17/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Back-up slides 18/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Early running of LHC it was decided to push for collisions at an energy of 10 TeV this year, as quickly as possible, with full commissioning to 14 TeV to follow over the winter shutdown. Robert Aymar, CERN Director-general CERN Bulletin, Issue No / Monday 31 March 2008 Reduction in all cross sections is expected Consider two particular parton combinations, q qbar (e.g. for Z, Z', etc) and gg (for Higgs, ttbar, etc). Thus, the reduction in cross section for a 200 > GeV Higgs boson is almost exactly a factor of 2 for W,Z the > reduction factor is less (70%) 19/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 SUSY: Supersymmetry Avoids fine-tuning of SM, can lead to GUTs Generally assume LSP is stable (R-parity conservation) possible dark matter candidate SUSY breaking mechanism is unknown many parameters mSUGRA: supergravity inspired model 5 free parameters: m0, m, A0, tan() and sign() A possible symmetry between fermions and bosons |S=0 or S=1 |S= 20/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008mSUGRA mSUGRA stands for minimal supergravity. The construction of a realistic model of interactions within N = 1 supergravity framework where supersymmetry breaking (Gravity Mediated Supersymmetry Breaking ) is communicated through the supergravity interactions. Parameters of mSUGRA: 1.m 0 the universal scalar mass 2.m 1/2 the universal gaugino mass 3.tan() the ration of the vacuum expectation values of the two Higgs fields 4.A 0 the Higgs-squark-squark trilinear coupling constant 5.sign() where is the unmixed Higgsino mass or the SUSY-conserving Higgs mass parameter 21/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Physics motivation: other theories Another model is one with a Majorana particle that decays through SM-like bosons into leptons: pp -> M l X Heavy Majorana neutrinos ( M ) can be produced in pp collisions in association with a lepton through a virtual W boson [T. Han and B. Zhang, Phys. Rev. Lett. 97, (2006)]. This new particle can subsequently decay to a W and another lepton: M -> W l more than half of such events will contain like-sign dileptons in the final state Given the Majorana nature of this neutrino, i.e., that it is its own antiparticle, more than half of such events will contain like-sign dileptons in the final state 22/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008Muons Two different collections of muons are stored STA - Standalone (muon system) GMR - Global reconstructed (muon system + tracker) Kinematics distributions for GMR: Eta distribution of leading muon (Pt>5 GeV) LM1 Eta distribution of leading muon (Pt>5 GeV) LM1 23/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008Muons Signal (LM1) muons have muon Pt spectrum: harder than QCD similar to W/Z+jets and TTbar Pre-selection cut PT > 10 GeV to avoid problems with low-PT range where efficiency is not very good Pt of leading muons QCD Pt of leading muons LM1 24/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Muons Multiplicity SUSY events have a more of muons, hence we can require 2 same sign muons. This cut efficiently kills QCD, W+jets and Z+jets, because these backgrounds typically have less than 2 muons in most of events Nmuons = 2 same sign muons Number of muons LM1 Number of muons W/Z + jets TTbar Number of muons QCD 25/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, SS muons from hadron decay 2SS prompt SUSY muons Efficiency Muons isolation In order to distinguish SUSY diagrams combined isolation was used in the dimuon analysis: Combined Isolation = Tracker_Iso * Calorimeter_Iso < 10 GeV 65% efficient at identifying SUSY diagrams, 90% pure w.r.t. SUSY hadrons decay and SM backgrounds Iso < 10 GeV prompt SUSY muons muons from hadrons decay 26/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, Same Sign Muons No remarkable difference in the PT distribution for muons between signal and background (see plots below) Pt of leading muon in 2SS LM1 Pt of leading muon in 2SS LM1 Pt of second muon in 2SS W/Z+jets TTbar Pt of second muon in 2SS W/Z+jets TTbar 27/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008Jets Because of large squark masses we expect high energy jets in SUSY events, hence we can cut on their ET Transverse energy of jets: ET1st > 175 GeV ET2nd > 130 GeV ET3rd > 55 GeV Et of leading jet W/Z+jets TTbar Et of leading jet QCD Et of leading jet LM1 28/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Multiplicity of jets Number of jets W/Z+jets TTbar Number of jets LM1 Number of jets QCD SUSY events have a lot of jets, hence we can cut that number at 3 This cut efficiently kills QCD, W+jets and Z+jets, because these backgrounds typically have 2 jets or less in most of events Njets >= 3 29/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 MET QCDMET MET W/Z+jets TTbar MET LM1 Missing energy is one of the most important cuts, because large value of missing energy is an inherent SUSY signature due to LSP which escape detection (R-Parity is conserved in mSUGRA). MET > 200 GeV 30/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 m0m0 m 1/2 m0m0 no ewsb Gluino and squark isomass 31/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 no ewsb ( p + + p + ~q + ~q + X) ( p + + p + ~q + ~g + X) ( p + + p + ~g + ~g + X) Cross section iso-contours 32/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 no ewsb m(~ 2 0 ) = m(~l L )m(~ 2 0 ) = m(~l R ) reach contour for 10fb -1 x-section isolines Same-Sign di-muons: Contour behavior 33/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Significance as a measure of merit If for a particular set of cuts we observe n s (signal) and n b (background) MC events, is it better of worse than a different set of event numbers for a different set of cuts? Common approach: expected number of signal events in an experiment is s = w s n s actually should be = w s (n s +1) expected number of bkgd events in an experiment is b = w b n b actually should be = w b (n b +1) estimate significance S of observing (b+s) events, when ones expect b events for background Significance estimators implemented in Garon? popular choices, but very poor estimators work only in the limit of very large b>>1 the bias is even greater when n b is not large Yes, but NOT RECOMMENDDED!!! correct significance for any b, as long as n b >>1, but would be an overestimate for a smaller n b Yes, see S c below very close approximation for S cP Yes, OK when n b >>1 correct significance, takes into account statistical uncertainties arising from limited MC statistics; identical to S cP for n b >>1 Yes, Strongly recommended 34/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Significance: S c (one bkgd) number of MC events before cuts N event weight w number of MC events after cuts n 1) pdf (probability distribution function) for the cut efficiency 2) probability to observe in experiment exactly k events 3) calculate probability to observe k 0 events and convert it to significance S c we use this approximation (easy to integrate) very good for large N>>1 on a conservative side for smaller N side note: 35/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Significance: S c vs. S cL Blue = S cL ~ S cP b=w(n+1) Red = S c w=0.1 n=0 w=0.5 n=0 w=0.1 n=10 w=0.5 n=10 Observed events k 0 Significance Observed events k 0 Significance Observed events k 0 Significance Observed events k 0 Significance 36/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Systematic uncertainties SystematicUncertainty dN B /N B Single muon analysis Same sign dimuon analysis Theory 13% Luminosity 5% Jet energy scale 10%15% Jet energy resolution 5%10% Muon efficiency/resolution negligible Fast vs. full simulation 2% Total systematic uncertainty 18%23% 36 Summary table: 37/16 Yuriy Pakhotin SUSY08, Seoul, Korea, June 19th, 2008 Results for fully simulated points 37 Summary table: total number of background and signal events which pass the optimized selection cuts for 10 fb -1, together with the corresponding significance (with and without systematic uncertainties) to discover different signal benchmark points.