Supersymmetry Searches with the ATLAS Experiment at the LHC Sven Vahsen, LBNL for the ATLAS collaboration DPF 2006 Meeting, Honolulu, Hawaii October 31, 2006

The ATLAS detector at the LHC 2006 2007 2008 Full physics run (14TeV) First beams and collisions (0.9TeV) LHC (Large Hadron Collider) Under construction at CERN 14-TeV proton-on-proton in 2008  7 x Tevatron ATLAS (A Toroidal LHC apparatus) One of the two general purpose experiments at LHC, to be completed in time for first beam Supersymmetry (SUSY) Exciting possibility for physics beyond the standard model ATLAS sensitive to previously un-probed SUSY-particle masses with very little 14-TeV data Honolulu, October 31 2006 Sven Vahsen, LBNL

Standard Model at 14 TeV Expect most events written to tape from high cross-section SM processes Use to calibrate and measure detector performance Constitute main backgrounds to SUSY searches The SUSY-discovery challenge Use trigger and event selection to reject SM by factor of ~109 (for S/B ~ 1/10) Demonstrate that we understand SM events that survive SUSY selection Channel # of Events, 1pb-1 (17 minutes at 1033cm-2 s-1 !) W -> µ 7000 Z -> µµ 1100 t tbar -> µ + X 80 QCD jets PT>150 GeV 1000(for 10% of trigger bandwith) Minimum bias Trigger limited gluino-gluino, M~ 1TeV 1-10 SUSY? Honolulu, October 31 2006 Sven Vahsen, LBNL

Beyond the SM: Supersymmetry? Supersymmetry (SUSY) is one of the more well-motivated extensions to the standard model Keeps corrections to higgs mass finite. Requires wino and stop masses ~ few hundred GeV Unifies gauge couplings at large Q2 Provides cold dark matter candidate via LSP if neutral, weakly interacting, stable. Cosmological arguments prefer mass ~ a few hundred GeV MSSM: Minimal Supersymmetric extension of the Standard Model. particles sparticles =“Neutralino” =“Chargino” Honolulu, October 31 2006 Sven Vahsen, LBNL

Sparticle production Cross-section for producing SUSY particles in proton-proton collisions calculated at NLO Production of the strongly interacting sparticles dominates: pp gg,qq,qq,gq √s=14TeV, mSUSY ~ 0.5-1.0 TeV  σppSUSY ~ 1-100 pb  Potential for copious sparticle production at the LHC! _ ~~ ~~ ~~ ~~ Honolulu, October 31 2006 Sven Vahsen, LBNL

Sparticle Decays ~ c01 ~ qR ~ g q p p ~ _ c01 ~ ~ ~ qL ~ q c02 g l q q Event selection guided by typical decay chain of SUSY particles c01 ~ qR ~ g ~ q p p c01 ~ q _ ~ qL ~ ~ ~ c02 g l q q l l Two strongly interacting sparticles produced If “R-Parity” conserved Each sparticle decays into lighter sparticle + SM particle ( jets + leptons) cascade decays down to stable, undetected LSP (usually neutralino in mSUGRA)  large ETmiss  Canonical SUSY signature: ETmiss, high-pT jets, often leptons Honolulu, October 31 2006 Sven Vahsen, LBNL

Missing ET + jets Event selection (0-lepton case) Jets 1,2 with pT > 100 GeV Jets 3,4 with pT > 50 GeV ETMISS > max(100 GeV,0.2Meff) Transverse sphericity ST > 0.2 No isolated muon or electron with pT > 20 GeV Plot Effective Mass variable: Meff = Σ|pTi| + ETmiss SUSY events typically have higher Meff than SM backgrounds Z(νν) + jet W(μν)+ jet QCD Initial SUSY search strategy tbd. Counting experiment after cut on Meff? Jets + ETmiss + 0 leptons ATLAS TDR, 1999 Honolulu, October 31 2006 Sven Vahsen, LBNL

SM background: Monte Carlo predictions Isajet (1999) MC prediction of SM events surviving SUSY selection uncertain by a factor 2-5 Need to measure normalization of these backgrounds! Strategy to do so early (10-100 pb-1) is the topic of much current work in in ATLAS SUSY group Different SUSY signals! ALPGEN (2006) Pythia (2006) Honolulu, October 31 2006 Sven Vahsen, LBNL

Optimizing the event selection 0-leptons If more Standard Model background than expected  Re-optimize event selection lower signal efficiency higher S/B ratio Two options tighten cuts in ETMISS + jets + 0 leptons require extra particles in final state (next slide) 0-leptons Honolulu, October 31 2006 Sven Vahsen, LBNL

Missing ET + jets + 1 lepton Require an additional lepton  higher S/B, and different BG composition than 0-lepton mode (0 leptons) (1 leptons) ATLAS preliminary ATLAS preliminary Semi-leptonic top decays now dominate BG. May be possible to directly measure normalization and shape (see later slide) Typical SUSY cut NJet>=4 (PT1st>100GeV, pT4th>50GeV) MET>100GeV and MET>0.2xMeff ST>0.2 Honolulu, October 31 2006 Sven Vahsen, LBNL

Missing ET + jets + 2 leptons Example: Opposite sign, same flavor di-leptons from single neutralino decay Less general search, but edges in di-lepton mass distribution smoking gun Low Background Estimate BG using opposite sign, opposite flavor di-leptons  Robust, i.e. less prone to fake discovery in early searches? Position of mass-edge sensitive to SUSY masses First step in larger program beyond discovery: constraining sparticle masses in decay chain using partial reconstruction l+ l- e+e-, μ+μ- SUSY e+μ-, e-μ+ SUSY e+μ-, e-μ+ top-pair ATLAS preliminary Honolulu, October 31 2006 Sven Vahsen, LBNL

Early SUSY discovery possible! Reach gluino / squark masses of 1.0 TeV with 0.1 fb-1 1.5 TeV with 1.0 fb-1 2.0 TeV with 10 fb-1 To be repeated with more realism: full simulation, non-perfect detector, systematic from background estimation Different final states may “win” at different times, depending on tradeoff between systematic and statistical uncertainties (i.e. understanding of detector, SM) Honolulu, October 31 2006 Sven Vahsen, LBNL

Only the finest ingredients! Main ingredients to SUSY search: jets, ETmiss,leptons  Bad ingredient  spoil the dish! (= false discovery…) Example of ETmiss can spoil ETMISS resolution with large large tails due to Miss-measured jets in cracks Colorimeter noise & inefficiencies CDF Honolulu, October 31 2006 Sven Vahsen, LBNL

Estimating background from W+jet, Z+jet Example of directly measuring SM background: Z(nn)+jets, especially in region with high ETMISS W+jet ~ 10x higher cross-section than Z+jet, similar kinematics Select W(mn)+jets and “forget the lepton” to estimate shape of Z(nn)+jets background Looks promising, and might be feasible already at 100 pb-1 Need to demonstrate that it still works with other SM backgrounds present Missing ET [GeV] Z(nn)+njets Estimated [W(mn)] Events/50GeV/1fb-1 normalization ATLAS preliminary signal region Result (Missing ET>300GeV, 1fb-1) Z(nn)+njets : 157+/-13 Estimated : 134+/-10 SUSY cuts NJet>=4 (PT1st>100GeV, pT4th>50GeV) MET>100GeV and MET>0.2xMeff Nm>0 with Pt(m)>10GeV Honolulu, October 31 2006 Sven Vahsen, LBNL

Conclusion LHC projected to produce 14-TeV proton collisions in 2008 With 0.1–1 fb-1 of data, expected by end of that year, ATLAS sensitive to sparticle masses in the 1.0 – 1.5 TeV range However, convincing SUSY search requires good understanding of Detector Jets, ETMISS, leptons SM events passing SUSY selection Lots to do, including defining the full strategy If SUSY is actually found, there will be even more work! 2006 2007 2008 Full physics run (14TeV) First beams and collisions (0.9TeV) Honolulu, October 31 2006 Sven Vahsen, LBNL

BACKUP SLIDES Honolulu, October 31 2006 Sven Vahsen, LBNL

Estimating background from top-pairs Top mass is largely uncorrelated with ETMISS (calibration variable). Select semi-leptonic top candidates in a mass window 140-200 GeV Using early data: no b-tagging availablecombinatorial BG to top mass  estimate this from the sideband (Mtop=200-260GeV) and subtract Normalize ETMISS distribution in low ETMISS region, where SUSY contribution is small Extrapolate to high ETMISS region to estimate the background after SUSY selection Needs to be demonstrated that this still works when both SUSY and rest of SM (especially W+jets) present in data sample How to apply this in 0-lepton mode? Top Mass (GeV) ttbar sideband ttbar signal T1 Missing ET (GeV) ATLAS preliminary Missing ET (GeV) Honolulu, October 31 2006 Sven Vahsen, LBNL

Only the finest ingredients! Main ingredients to SUSY search: jets, ETmiss,leptons  control their quality, so as to not compromise the final product (= avoid fake discoveries!) Example of ETmiss Ideally only from undetected particles (neutrino, out of acceptance, LSP) But “fake” contribution can spoil ETMISS resolution and give large tails Miss-measured jets in cracks Colorimeter noise & inefficiencies Monitor resolution using control sample, e.g. W(mn)+ jet Clean up by rejecting bad runs (e.g. large average ETMISS) events with ETMISS in direction of jets events with high-pT jets pointing at cracks Critical issues for SUSY analysis Jets: resolution, jet energy scale ETMISS: resolution and scale Leptons: purity, efficiency, isolation ATLAS wide challenge to get the above under control with early data samples Special challenge for SUSY group pT higher than SM processes High multiplicity typical of SUSY events Tails of resolutions CDF Honolulu, October 31 2006 Sven Vahsen, LBNL

Optimizing the event selection BG level in data similar or worse than high-background MC prediction  not show-stopper for discovery Can re-optimize selection for lower signal efficiency and higher S/B  discovery will take only “a bit” longer Two options tighten cuts in ETMISS + jets require extra particles in final state, e.g. ETMISS + jets+ 1 lepton ETMISS + jets+ 2 leptons (SS, OS) Suitability of particular final states for discovery also depends on feasibility of measuring the SM BG in that channel, so as to achieve to smallest possible σBG Note that optimum selection cuts depend on SUSY model as well 0-leptons 0-leptons 1-lepton Honolulu, October 31 2006 Sven Vahsen, LBNL