The large centre of mass energy and luminosity provided by the Large Hadron Collider will offer a unique opportunity to discover new particles in the TeV range. The potential to discover and measure supersymmetric particles with the ATLAS detector using the missing transverse energy signature is discussed

Duxfield, R

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SUSY Physics and Missing    Energy in ATLAS  Robert DuxfieldUniversity of SheffieldThe ATLAS Detector  * See A. Parker's talk, mondaySupersymmetryz Most well motivated extension to the Standard Model.z Usually consider R-Parity conserving models (gives stable proton) :− Multiplicative quantum number: R(SM) = +1, R(SUSY) = -1.− Sparticles are produced in pairs and lightest SUSY particle (LSP) is stable.z LSP must be only weakly interacting on cosmological grounds.Î final state characterised by observed imbalance in momentum.z Dominated by production of strongly interacting sparticles → lots of jets.p pqg~ q~l~χ02~χ01~lq lSUSY at the LHC   z The Large Hadron Collider Provides pp collisions with √s = 14TeV.z With MSUSY ~ 1TeV and well controlled systematics can achieve ,   5 discovery with first 100pb-1 of data using the Missing Transverse     Energy signature.− Limiting factor is control of systematics rather than statistics− Need to quickly understand detector and SM backgrounds.   The Missing Transverse Energy Variablez Can only apply momentum conservation in the plane transverse to the beam Typical SUSY cuts:     .z Measure apparent imbalance in final state i l i t (+ )z NJets >= 4z pT(j1) > 100GeV, pT(j4) > 50GeVE miss > 100GeVus ng ca or me ry  muonsÎ 'ETmiss'.z T   z Transverse Sphericity ST > 0.2z 0 leptonsz SUSY selection: high-pT jets, large ETmiss and (possibly) isolated leptons. ATLAS preliminary(ATLFAST)z ETmiss gives excellent discrimination against most SM processes.z Remaining background from events with neutrinos – W/Z + jets, tt, bb.z QCD: from  in bb and cc events. Also huge event rate means rare effects due to imperfections in detector can be significant.Reconstructing ETmiss z ETmiss calculated from vector sum over calorimeter cells plus contribution from muons corrected for energy loss in calorimeter.Noise cells removed with topological clustering algorithmz       .z Cells calibrated with H1 style weights (low energy density cells up-weighted to compensate for invisible processes in hadronic showers).z Resolution scales with square t f l Eroo  o  sca ar sum T :(ETmiss) ~ 0.5 √ETsumATLAS preliminary Dealing With BackgroundsIn addition to jet and E missz      T  cuts, apply additional cuts to reduce certain backgrounds1 lepton  − number of isolated leptons− Meff = ETmiss + ∑pTjetsAbilit t li bl ti t b k d i it l t d t tMeff (GeV) z y o re a y es ma e ac groun s s v a  o emons ra e excess in signal region.z Systematics from Monte Carlo likely to be large, so try to estimate from data wherever possible− particularly important with early data.W/Z + jets  z Significantly reduce Z →  and W backgrounds with 1 lepton requirement and mT(l, ETmiss) >0 leptons     mW cut at expense of statistics.B k d d i d bz ac groun  now om nate  y tops.z Estimate Z t  in 0 leptons case Meff (GeV) ATLAS(ZÎll)by using Z → ll data,  replacing lepton pT with ETmiss.Preliminaryz Can use same channel to obtain ti t f W les ma e or  → .topz For tt → bblqq, can reduce background with transverse mass cut. Then tt→ bbllqq becomes the dominant background in the 1 lepton          channel.z To obtain estimate: select semi-leptonic top t f i d d th t V) ATLASeven s rom a mass w n ow aroun  e op mass (mt = 140 – 200GeV).Mass (GeVPreliminarysidebandi lz Subtract combinatorial background using sideband (mt = 200 – 260GeV).Top MT1s gnaMissing ET (GeV) ATLASEstimateSUSY selectionz Get estimate for semi-leptonic top ETmiss distribution.PreliminaryQCD (1) z Two main sources:− fake ETmiss (gaps in acceptance, dead/hot cells, non-gaussian tails etc.) real E miss (neutrinos from b/c quark−  T     decays) z Hard to estimate with Monte Carlo     − depends on details of detector response− need large statistics to get into tailsPythia dijetsz 1 lepton requirement minimises contribution− may be best until detector is well understood (real/fake?) C d t ib ti b ttiz an re uce con r u on y cu ng on correlations in M between the leading jets and ETmiss, jets pointing at poorly SUSY SU3instrumented regions of the detector etc...QCD (2) z To obtain estimate :z Step 1: Measure jet smearing function from dataATLAS preliminary jets  − Select events: ETmiss > 60 GeV, Δφ(ETmiss, jet) < 0.1 MET− Estimate pT of jet closest to ETmiss  as  t t j t ifluctuatingpT rue-es = pT e + ETm ss jetATLAS preliminaryz Step 2: Smear low ETmiss multijet QCDest (stat errors only) SUSY  events with measured smearing function. 22pb-1z Technique does not work in low ETmiss region (gaussian jet response), but gives good agreement in tails      (SUSY signal region!) Summaryz Potential for discovery of 1TeV scale SUSY with        first data at ATLAS, but...z Must understand detector and backgrounds.z Data driven approaches most likely to give robust background estimates with first data ongoing     –  work!

SUSY physics and missing energy at ATLAS

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