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ATLAS SUSY SEARCHES

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Part of Muon drift tubes and half cathode strip layers ... Red: signal. Black: bgd. Top Background estimate. The Top mass reasonably uncorrelated with ETMISS ... – PowerPoint PPT presentation

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Title: ATLAS SUSY SEARCHES


1
ATLAS SUSY SEARCHES
  • Gianluca Comune
  • Michigan State University
  • On Behalf of the ATLAS Collaboration
  • PANIC 2005, Santa Fe 27/10/2005

2
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

ATLAS
3
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
  • M0, M1/2, A0, tan(ß), sgn(µ), mtop
  • Assuming R parity conservation gt
    escaping LSP gt large ETMISS 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)
4
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 ETMISS
  • Powerful handle for Standard Model background
    removal
  • SUSY events have generally large jet multiplicity
    and large jet pT
  • Depending on mass hierarchy multi lepton
    signatures as well

5
Inclusive Searches
ATLAS Physics TDR
0 lept.
10 fb-1
  • Discovery
  • Assuming luminosity 1033 cm2 s-1
  • 1300 GeV gt 1 week
  • 1800 GeV gt 1 month
  • 2200 Gev gt 1 year
  • Backgrounds
  • Real missing energy from SM processes with hard
    neutrino (tt, Wjets, Zjets)
  • Fake missing energy from detector
  • Jet energy resolution (expecially non-gaussian
    tails) critical

1 TeV SUSY
SM (PYTHIA)
  • 1 jet with pT gt100 GeV, 4 jets (pTgt50 GeV)
  • ETMISS gt max(100 GeV ,0.2Meff)
  • Transverse sfericity STgt0.2
  • No isolated muon or electron (pTgt20 GeV)

(Fast parametric detector response)
6
Realistic Bgd Estimation
Previous analysis uses Parton Shower for SM
processes gt badly underestimates hard jet
emission
(pT of hardest jet)
ATLAS
GeV
Parton shower is a good model in
collinear region, but fails to describe hard jet
emission
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
ATLAS
SM (ALPGENPYTHIA)
7
Inclusive Searches (2)
0 leptons (preliminary)
  • High pT jets are produced also in background
    processes
  • gt bad separation!!
  • ETMISS excess can be
  • ETMISS gt 800 GeV
  • Need to optimize the selection
  • Meff still a good discovery signal (requiring 1
    lepton)

ATLAS
ATLAS
Focus Point 4.2 fb-1
ATLAS
Red signal Black bgd
1 lepton
1 lepton
SUSY production dominated by ??
  • 0 lepton mode
  • No leptons, xEtgt100GeV, gt 1 jet with pTgt100GeV,
    gt4 jets with pTgt50GeV, Transv. Sphericity gt0.2
  • 1 lepton mode
  • e,µ Pt gt10 GeV, xEtgt100 GeV, gt 1 jet with
    pTgt100GeV, gt4 jets with pTgt50GeV, Transv.
    Sphericity gt0.2, Transverse mass between lepton
    and xEt gt100GeV (to suppress WN jets Background)

8
Top Background estimate
  • The Top mass reasonably uncorrelated with ETMISS
  • 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 ETMiss region
  • SUSY contribution is small
  • Procedure gives estimate consistent with Top
    distribution also when SUSY is present
  • Zjets big contribution from Z ? ? ?
  • Can use Z ?ee, apply same cuts as analysis,
    substitute ET(ee) with ETmiss and rescale by BRs.

9
SUSY Spectroscopy
  • After SUSY is discovered it needs to be
    characterized
  • particle masses, spin
  • 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/0007009
10
Leptonic Signatures
Coannhilation P.. 5.6 fb-1
Point 5a 4.37 fb-1
Mod. Point 5 5.0 fb-1
ATLAS
ATLAS
ATLAS
Blackt-tbar bgd
Mll (GeV)
Mll (GeV)
p
p
Coannhilation point 5.6 fb-1
ql(max)
Larger of M(llq)
ql(min)
ATLAS
ATLAS
ATLAS
  • SM background negligible (could be a discovery
    signal)
  • Opposite-Flavour/Opposite Sign is subtracted
    (removes SUSY bgd)

11
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?10 (very
    small)

Coannihilation point 20 fb-1
Point 5A 4.4 fb-1
ATLAS
ATLAS
(1 tau pT gt 40 GeV, 1 Track pTgt6 Gev No other
track pT gt 1 GeV in R lt 0.4)
  • Currently investigating a track seeded tau
    reconstruction algorithm

12
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

13
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.

14
Backup
Matrix Element and double counting (MLM)
  • Jet should be matched to the parton generated
    with ME (R0.7) except for the soft and collinear
    regions.
  • Blue show perfect matching between ME parton and
    jet.
  • Soft jet was emitted collinearly gt Matched
    (Accepted)
  • One parton divided into 2 jets. (outside ME cone
    0.7) gt Not Matched
  • Event should be covered with 5jet ME (double
    counting) gt Reject event



M. Mangano http//mlm.home.cern.ch/mlm
15
Other Background Sources
  • At startup calibration data will be limited
  • Miscalibrated detector is a source of ETMiss
  • QCD jets can add non gaussian tails to ETMiss
  • Very important given the CS

16
Other Endpoints
Coannhilation Point 5.6 fb-1
ATLAS
With t-tbar bgd
Without t-tbar bgd
(using a mixed event technique for the SUSY bgd
reduction)
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