long lived lsps
DESCRIPTION
Long lived LSPs. SLAC ATLAS forum, 5 December 2007. Ignacio Aracena (SLAC) Keith Bechtol (Stanford) Dan Silverstein (Stanford) Claus Horn (SLAC). Content of this Talk. Motivation Signatures Reach studies Generator level studies Reconstruction Next steps. - PowerPoint PPT PresentationTRANSCRIPT
Long lived LSPs
SLAC ATLAS forum, 5 December 2007
Ignacio Aracena (SLAC)Ignacio Aracena (SLAC)Keith Bechtol (Stanford)Keith Bechtol (Stanford)Dan Silverstein (Stanford)Dan Silverstein (Stanford)Claus HornClaus Horn (SLAC) (SLAC)
SLAC ATLAS forum, 5 Dec 2007
Claus Horn: Long-lived LSPs
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Content of this Talk
Motivation Signatures Reach studies Generator level studies Reconstruction Next steps
Working in collaboration with the ATLAS Hidden Valley group
SLAC ATLAS forum, 5 Dec 2007
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Motivation
L number violating
B number violating
Should not exclude L / B violating effects a priori !
• We have seen L violating effects in neutrino sector. • A HE theory should include B violation to explain baryon asymmetry.
In RP conserving SUSY models these terms are neglected which ensures a stable LSP and, if neutral, a CDM candidate.
The general SUSY Lagrangian contains L and B violating terms:
• There are also RPV scenarios with dark matter candidates.• There are many other motivations for supersymmetry.
However:
Advantage of RPV: Possibility to directly reconstruct sparticle masses.
Striking signature: high pT DVs, no MET
SLAC ATLAS forum, 5 Dec 2007
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Existing limits on RPV couplings
Source Best Limits Comment
Tevatron & HERA ’, ” < 10-2 Mostly squark mass dependent
Low Energy
experiments
’, “ < 10-2 ml/100GeV Proportional to sfermion mass
Proton decay ’” < 10-11 No limit on single couplings!
Cosmology , ’, ” < 5 10-7 Model dependent!
If couplings are small, we will not observe RPV in production, but displaced vertices!
SLAC ATLAS forum, 5 Dec 2007
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Signatures
Other possible LSPs:(gravitino) sneutrino, slepton, squark (disfavoured by cosmology)
LLE
Signature ( LSP)Superpotential
’ LQD
’’ UUD
Search for displaced vertices with 2jets+X or 2 leptons.
‘Underlying SUSY event’ from main IP + Displaced vertex
2 jets + l±/
l+ l- +
3 jets
l*
l
l
l
LSP
l*
l
q
q’
q*
q
q
q”
Focusing on LSP case
SLAC ATLAS forum, 5 Dec 2007
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Reach on RPV couplings
Decays in muon-spectrometerDecays in calorimeter
Decays in inner detectorb-tagging region
d ~ ( ) ml
4
2 m5
Effective coupling
Decay length:
Covering 4 orders of magnitude; Largest region covered by ID
Reach additionally extended due to ml and exp(d/d0) dependence.
’
LQD
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Decay Regions
x-axis : decay length for =1Additional dependencies:
Considering 10K simulated events per .
Classifying displaced vertexposition into 5 diff regions.
• EKin m()-m(q)
• Integrated lifetime distribution
(ID includes b-tag)
Decay probability for a single LSP
Study decay region dependence on lifetime
ID=Decay in Inner detectorMS=Decay in Muon-Spectrometer
SLAC ATLAS forum, 5 Dec 2007
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Combined Probability
Both decaying in ID
Both decaying in calorimeter
Ptot approaches 1 in end region.
Maximal 16% of events have two decays in the calorimeterfor any lifetime.
Calculate combined probability:
P2ID = PID2
PID+CAL = 2*PID*PCAL
5 decay regions (b-tagging, tracker, Calorimeter, MuonSpectrometer,ETmiss)-> In total 15 combinations.
[cm]
[cm]
Two DV requirement may be important background discriminator.
SLAC ATLAS forum, 5 Dec 2007
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Event Generation with Pythia
Setting sparticle masses by hand Suppressing long decay chains Allow long decay length of LSP in
Pythia Switch explicitly off neutrino decays Suppress decays involving third
generation quarks Modified Pythia interface:
introduce new flags to select different RPV couplings
qqqqqR 01
~~
Sparticle mass spectrum
m( g ) = 5137 GeV
m( dL,R ) ≈ 700 GeV
m( uL,R ) ≈ 698 GeV
m( t1,2,b1,2 ) ≈ 5000 GeV
m( lL,R ) ≈ 5000 GeV
m( ) = 800 GeV
m( 01 ) = 494 GeV
Resulting event: Squark pair production+
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Generator level Studies
Neutralino:
Jets:
etapT
Applying simple jet finder to construct jets out of tracks from DV:
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Dependence on E
Average deltaR of decay products
Mean electron pT increases with increasing neutralino energy.
Decay products are well separated even for high neutralino energies.
Electron pT for different E.
SLAC ATLAS forum, 5 Dec 2007
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Sample Production
Simulation in Geant4
Digitization (-> RDO)
Samples
7mm 800 evts100mm 50 evts700mm 50 evts
Reconstruction (->ESD)
Conversion job (->AOD)
Athena analysis (->AANT ntuple)
Generation in Pythia
Change tracking cuts, very slow
Using production transforms, run on the grid
Create Track Selections
ROOT analysis
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Experimental HandlesDecays in the Inner Detector
Decays in the calorimeter
Decays in the muon spectometer
Jets without tracksHigh em/had energy fraction
Clusters of muon ROIs
High mass displaced vertices
GeneralTwo DV decays in each eventCommon DV mass (neutralino)
Missing ET to be studied
Main backgroundsSecondary vertices within jetsMaterial reactions
NoisePi0-jets
Very high pT jets
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Event DisplaysShown are all stable truth particles
Dashed line: electron
Displaced Vertices:Red & blue
(intermediate particles are not shown)
Black:Underlying SUSY from IP
Green:Neutalino
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c = 7mm
x-y view y-z view
Stable truth particles
Leptons and jets well seperated
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c = 700 mm Example
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Reconstruction-Overview
c=7mm
c=1m
Tracking
Jet finding
Vertex finding
Many standard cuts have to be adjusted:
Lepton ReconstructionMatch tracks to DV instead of IPOptimize shower shape variables
Release constraints on vertex positionOptimize for different regions
Optimization for DV tracks
Match calorimeter clusters to DV tracks
SLAC ATLAS forum, 5 Dec 2007
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Reconstruction Strategy
• Standard calorimeter jet finder (jets from IP)
• Track jets, on pre-selected tracks (cut on impact parameter)
Vertexing methods: fitting tracks from one vertex/ topological
High vertex mass requirement
Secondary vertices within jets
Vertices within dead material map
Pre-select all tracks within these jets (match at calo surface)
Reject:
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Impact Parameter
Red: QCD
Blue: RPV(c=7mm)
Many track with d0 of order of cm Tails much wider for RPV sample
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Impact Parameter SignificanceStandard cuts Extended cuts
QCD
SUSY
QCD
RPV
Impact parameter distribution indicates existence of DVs
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Jets in the 700 mm sampleRunning cone (0.4) tower jet finder on calorimeter clusters
Next step: study efficiency as function of decay length
Red: Truth, Black: Reconstructed
Decays beyond the cal will lead to not reconstructed jets
DeltaR
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Conclusions
Exciting new analysis effort started.
ATLAS covers a large region of unexplored RPV coupling space.
Many adjustments are needed in reconstruction
Small RPV couplings lead to striking signatures with disp. vertices.
High pT displaced vertices may enable an early discovery.
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Backup slides
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Triggering
Introduction of specific triggers:
Events may be triggered by jets/leptons from main IP
Triggering on DV properties may extend trigger reach
Decays in calorimeter: low EEM/EHAD
Decays in MuonSpectrometer: large # muon ROIs
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pions
photonsFrom IP
From DV