“ATLAS SUSY SEARCHES” Gianluca Comune Michigan State University On Behalf of the ATLAS Collaboration PANIC 2005, Santa Fe’ 27/10/2005

LHC and ATLAS LHC –14 TeV CoM p-p collisions –Start of operations 04/2007 –Total integ. luminosity 300 fb -1 ATLAS –(A Toroidal LHC ApparatuS) –General purpose detector –Vast physics program Higgs, SUSY, Exotics, top, B physics... Staged ATLAS components: One Pixel layer Transition Radiation Tracker outer end-caps Cryostat gap scintillators Part of Muon drift tubes and half cathode strip layers Part of forward shielding Part of LAr read-out Large part of trigger/DAQ CPUs

SUSY and mSUGRA Every particle has a super-partner –“Heaven” for particle physicists MSSM Lagrangian depends on 105 parameters (!!) –Need to make some assumption to reduce the degree of freedom mSUGRA depends on 5 (+1) parameters M 0, M 1/2, A 0, tan(β), sgn(μ), m top –Assuming R parity conservation => escaping LSP => large E T MISS and scalar particles produced in pairs Event cannot be fully reconstructed SUSY is a bgd to itself –Various regions in the par. space Coannihilation, Focus Point, Funnel, Bulk region (Ellis et al., Phys. B565 (2003) 176) M 0 (GeV)M 1/2 (GeV) A0A0 tanβsgn(μ)m top (GeV) Coannihilation Focus point

SUSY Production at LHC Production cross sections vary widely –From few to several hundreds pb -1 Actual kinematics and CS depend heavily on the chosen model –Long and complex decay chains If R parity is conserved large E T MISS –Powerful handle for Standard Model background removal SUSY events have generally large jet multiplicity and large jet p T Depending on mass hierarchy multi lepton signatures as well p p (stau Coannihilation point)

Inclusive Searches 0 lept. ATLAS Physics TDR SM (PYTHIA) 10 fb -1 Discovery –Assuming luminosity cm 2 s GeV => “1 week” 1800 GeV => “1 month” 2200 Gev => “1 year” Backgrounds: –Real missing energy from SM processes with hard neutrino (tt, W+jets, Z+jets) –Fake missing energy from detector –Jet energy resolution (expecially non- gaussian tails) critical (Fast parametric detector response) 1 jet with p T >100 GeV, 4 jets (p T >50 GeV) E T MISS > max(100 GeV,0.2M eff ) Transverse sfericity S T >0.2 No isolated muon or electron (p T >20 GeV) 1 TeV SUSY

Realistic Bgd Estimation Previous analysis uses Parton Shower for SM processes: => badly underestimates hard jet emission SM (ALPGEN+PYTHIA) Recent ATLAS background studies: - hard process with exact ME computation -Alpgen, Sherpa (collinear and soft region through PS) - hadronization -HERWIG,PYTHIA -Solve double counting problems - MLM matching Parton shower is a good model in collinear region, but fails to describe hard jet emission GeV (p T of hardest jet)

Inclusive Searches (2) High p T jets are produced also in background processes => bad separation!! E T MISS excess can be –E T MISS > 800 GeV –Need to optimize the selection Meff still a good discovery signal (requiring 1 lepton) 0 leptons (preliminary) 1 lepton 0 lepton mode –No leptons, xEt ＞ 100GeV, >= 1 jet with pT>100GeV, >=4 jets with pT>50GeV, Transv. Sphericity >0.2 1 lepton mode –e,μ Pt >10 GeV, xEt ＞ 100 GeV, >= 1 jet with pT>100GeV, >=4 jets with pT>50GeV, Transv. Sphericity >0.2, Transverse mass between lepton and xEt >100GeV (to suppress W+N jets Background) Focus Point 4.2 fb-1 1 lepton SUSY production dominated by  Red: signal Black: bgd

Top Background estimate The Top mass reasonably uncorrelated with E T MISS Select events with m(lj) in top window –apply W mass constraint –no b-tag used – Estimate combinatorial background with sideband subtraction. Normalize to low E T Miss region –SUSY contribution is small Procedure gives estimate consistent with Top distribution also when SUSY is present Z+jets: big contribution from Z →  –Can use Z →ee, apply same cuts as analysis, substitute E T (ee) with E T miss and rescale by BRs. Blue: tt Green: SUSY Dots: top estimate Preliminary Full Simulation 0.5 fb -1

SUSY Spectroscopy After SUSY is discovered it needs to be characterized –particle masses, spin … p p In every sequential double two body decay of the form The maximum of the invariant Mass distribution is related to the initial particle masses through: Use it on a “typical” SUSY decay chain Formulas in Allanach et al., hep-ph/

Leptonic Signatures ql(max) Larger of M(llq) Coannhilation point 5.6 fb -1 ql(min) p p SM background negligible (could be a discovery signal) Opposite-Flavour/Opposite Sign is subtracted (removes SUSY bgd) Coannhilation P fb -1 M ll (GeV) Point 5a 4.37 fb -1 Mod. Point fb -1 Black:t-tbar bgd

Coannihilation point 20 fb-1 Tau Signatures Tau signatures play a very important role –Tau BR relevant over a large portion of SUSY parameter space –In stau coannihilation ( ) region is critical to reconstruct the stau mass (one tau is very soft) The relic dark matter density of the universe depends from the mass difference M  1 -M  1 0 (very small) Point 5A 4.4 fb-1 m (1 tau pT > 40 GeV, 1 Track pT>6 Gev No other track pT > 1 GeV in R < 0.4) Currently investigating a track seeded tau reconstruction algorithm

SUSY Particle Masses Once the edge values (and the errors) are known one can determine the SUSY particle masses –It is critical to understand how to fit all edges Work in progress –Difficult to develope a true model independent approach More than one decay scenario (i.e. SUSY model) can lead to the same signature Need an independent measure of one of the SUSY particle to set the absolute scale m  1 0 (GeV) m  2 0 (GeV) (GeV)

Conclusions Few fb -1 of data should allow ATLAS to measure a clear excess over the SM contribution and reconstruct several mass relations. –this can be achieve in the first year of data taking depending on how quickly the detector and the SM backgrounds will be understood Large scale productions of Geant4 realistic detector simulated data –To understand detector systematics and prepare for real data analysis. –Scan of parameter space to understand different problems Recent ATLAS (and CMS) collaboration efforts are focused on understanding of Standard Model backgrounds with the use of the latest Montecarlo tools Developing strategies to validate the Montecarlo predictions with data.

Backup Jet should be matched to the parton generated with ME (R=0.7) except for the soft and collinear regions. –Blue show perfect matching between ME parton and jet. –Soft jet was emitted collinearly => Matched (Accepted) –One parton divided into 2 jets. (outside ME cone 0.7) => Not Matched Event should be covered with 5jet ME (double counting) => Reject event Jet should be matched to the parton generated with ME (R=0.7) except for the soft and collinear regions. –Blue show perfect matching between ME parton and jet. –Soft jet was emitted collinearly => Matched (Accepted) –One parton divided into 2 jets. (outside ME cone 0.7) => Not Matched Event should be covered with 5jet ME (double counting) => Reject event Matrix Element and double counting (MLM) M. Mangano

Other Background Sources At startup calibration data will be limited Miscalibrated detector is a source of E T Miss QCD jets can add non gaussian tails to E T Miss –Very important given the CS

Coannhilation Point 5.6 fb -1 Other Endpoints (using a mixed event technique for the SUSY bgd reduction) Without t-tbar bgd With t-tbar bgd