Post on 20-Jan-2018
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“ATLAS SUSY SEARCHES”
Gianluca ComuneMichigan 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
M0, M1/2, A0, tan(β), sgn(μ), mtop– Assuming R parity conservation
=> escaping LSP => 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)
M0 (GeV) M1/2(GeV) A0 tanβ sgn(μ) mtop (GeV)
Coannihilation 70 350 0 10 + 175Focus point 3550 300 0 10 + 175
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
pg~
Lq~qq
l~0
2χ~01χ~
l l
p
(stau Coannihilation point)
Inclusive Searches
0 lept.
ATLAS Physics TDR
SM (PYTHIA)
10 fb-1
• Discovery– Assuming luminosity 1033 cm2 s-1
• 1300 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 pT >100 GeV, 4 jets (pT>50 GeV) • ET
MISS > max(100 GeV ,0.2Meff)• Transverse sfericity ST>0.2• No isolated muon or electron (pT>20 GeV)
1 TeV SUSY
Realistic Bgd EstimationPrevious 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 collinearregion, but fails to describe hard jet emission
GeV
(pT of hardest jet)
Inclusive Searches (2)• High pT jets are produced also
in background processes=> bad separation!!
• ETMISS excess can be
– ETMISS > 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 Point4.2 fb-1
1 lepton
SUSY production dominated by
Red: signalBlack: bgd
Top Background estimate
• The Top mass reasonably uncorrelated with ET
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 ET
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 ET(ee) with ETmiss and rescale by
BRs.
Blue: tt (MC@NLO)Green: SUSYDots: top estimate
Preliminary
Full Simulation 0.5 fb-1
SUSY Spectroscopy• After SUSY is discovered it needs to be characterized
– particle masses, spin …
pg~
Lq~qq
l~02χ~
01χ~
l l
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/0007009
Leptonic Signatures
ql(max) Larger of M(llq)Coannhilation point 5.6 fb-1
ql(min)
minllqllq
maxllq mmm
p g~
Lq~qq
l~02χ~
01χ~
l l
p
• SM background negligible (could be a discovery signal)• Opposite-Flavour/Opposite Sign is subtracted (removes SUSY bgd)
Coannhilation P..5.6 fb-1
Mll (GeV) Mll (GeV)
Point 5a 4.37 fb-1
Mod. Point 55.0 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 M1-M1
0 (very small)
γττ~χ~ 101
Point 5A4.4 fb-1
m
(1 tau pT > 40 GeV, 1 Track pT>6 GevNo 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
m10 (GeV)
m 2
0 (G
eV)
(GeV)
(GeV)
~Lq
~ 01
~R
~L ~ 0
2
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
Matrix Element and double counting (MLM)
M. Mangano http://mlm.home.cern.ch/mlm
Other Background Sources • At startup calibration
data will be limited• Miscalibrated
detector is a source of ET
Miss
• QCD jets can add non gaussian tails to ET
Miss
– Very important given the CS
Coannhilation Point 5.6 fb-1
Other Endpoints
qqqqL
~~ 01
~~1
~ (using a mixed event technique for the SUSY bgd reduction)
Without t-tbar bgdWith t-tbar bgd
20.6 fb-1No cuts
2.6 excess
01
~03,2
~
Focus Point 4.2 fb-1