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Neutron Background Simulation: Infrastructure and Validation
Vadim Khotilovich, Rick Wilkinson, with help from
Piet Verwilligen, Alexei Safonov, Tim Cox
Simulation MeetingAugust 30, 2010
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Outline• Intro• Neutron simulation infrastructure• Basic SimHit level results for muon systems
– From high-stat samples produced in 3_6_3 QGSP_BERT_HP physics list
– Comparison to older results– Plans
• Validation of QGSP_BERT_HP_EML and QGSP_BERT_EMLSN physics lists– low stat samples in 3_9_0_pre2
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Intro
• Long-lived neutrons created, diffuse around collision hall
• They get captured by nuclei, emitting a photon
• Compton scattering or photoelectric effect makes MeV electrons, which cause hits in muon chambers
From Rick’s previous presentation:http://indico.cern.ch/conferenceDisplay.py?confId=55712
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Why neutrons are hard to simulate?
• Because neutrons can live up to a second before making a signal
• They can’t be treated like ordinary minimum-bias pileup, because millions of collisions in the past can contribute
• log10TOF vs log10Ekin– Plot made in 2001
From Rick’s previous presentation:http://indico.cern.ch/conferenceDisplay.py?confId=55712
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New Neutron Simulation Infrastructuregenerator
“Pythia6GeneratorFilter”generatorNeutrons
“Pythia6GeneratorFilter”
g4SimHits“OscarProducer”
g4SimHitsNeutrons“OscarProducer”
cscNeutronWriter“CSCNeutronWriter”
rpcNeutronWriter“RPCNeutronWriter”
dtNeutronWriter“DTNeutronWriter”
g4SimHits“NeutronHitsCollector”
generator“EmptyHepMCProducer”
mix“MixingModule”
generatorAny desired generator
g4SimHits“OscarProducer”
signal
MinBiasPileup
Sample
NeutronPileup
Sampleto digitizers
NPU
f*NPU
f = fraction of filled BXs in LHC orbit
MinBias simulated with:- special physics list- tracking time up to 1s- no pt and neutron cuts
Transform TOFs of >250ns SimHits into25ns wide time intervalwhile preservinghits’ time ordering
Special utility modulesto standardize collectionsfor MixingModule S
imM
uo
n/N
eu
tron
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Neutron SIM Samples• CMSSW_3_6_3• QGSP_BERT_HP physics list• MinBias generated @ 14 TeV• ~105K events• Geometries
– Current detector geometry– Most of results: TDR RPC & YE4 shielding
• Along with transformed neutron SimHit collections keep the original ones produced by GEANT
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Muon SimHits TOF vs. Ekin
• TDR geometry with YE4
• CSC&DT have lots of low energy hits– RPCs are
not so sensitive
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CSC: hits vs. clusters of hits• Simple clustering of CSC hits in a
chamber layer:1. Sort by closes wire group, strip, TOF2. Take first hit3. Cluster other hits around it requiring
WG ≤ 1 & S ≤ 3 & TOF < 2 ns4. Repeat recursively…
• Very low energy hits are from secondary interactions
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• Fraction of events with neutron simhits– Can be used for probabilistic
interpretations
• Comparing two geometries:– Current– TDR with YE4
• ME4/2 gets ~4.4 times more neutron hits without YE4 shielding– There is smaller effect in
other ring=2 stations– Should expect similar
magnitude effect in RPC’s RE4/2 & RE4/3
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Fractions of evt. with n-hits: YE4 effect
Geometry:currentTDR with YE4
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• SimHits from regular minbias vs. SimHits from neutrons
• TDR geometry with YE4
• CSC:– Most stations get hits from
neutrons more often then from MinBias (except ME1/1)
– Even with shielding ME4/2 is dominated by neutron hits
• RPCf:– MinBias contribution is higher in
most of stations (except RE 4/3, 4/2, 3/3)
• b/c neutron hits energies are lower
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Fractions of evt. with hits: MB vs. neutrons
SimHits from:MinBiasNeutrons
SimHits from:MinBiasNeutrons
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•T
DR
geo
met
ry w
ith Y
E4
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Fractions of events with neutron hits
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• TDR geometry with YE4
• NPU=25
• Fraction of filled BXs = 0.795
• Only neutron bars have text markers
• DT&CSC n-cluster fluxes ~2 times lower then n-hits fluxes 12
Neutron Hits Fluxes at Nominal LSimHits from:MinBiasNeutrons
SimHits from:MinBiasNeutrons
SimHits from:MinBiasNeutrons
SimHits from:MinBiasNeutrons
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• CMS Trigger TDR results:– Full circles = Neutron hits– Solid lines + open circles =
MinBias
• Current estimates for neutron rates are– ~2-6 times lower in CSC– ~4-30 times lower in DT– Reason not clear
• Current estimates for MinBias rates are– CSC
• Somewhat higher, especially for closer to beamline stations
– DT• Reasonably similar• Slightly higher in outer stations
• Plan to produce somewhat similar plot for easier comparison 13
Neutron Hits Fluxes: Trigger TDR
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• Neutron simulation infrastructure:– Extending the configuration for heavy ion simulation
• With help from Catherine Silvestre– Consolidation of all relevant modules in SimMuon/Neutron– Maybe adding an option to cmsDriver– Adopting new physics lists– Possibly: employ endcaps’ rotational and +-z symmetry to effectively increase
neutron samples statistics x34 for trigger rates studies• Plans for studies:
– Comparison to data• Special runs with 1-layer CSC pretrigger are planned to be taken soon• Will need 7 TeV neutron sample simulated
– DIGI level• Preliminary: fraction of events with CSC digis from neutrons is ~5 times lower than
fraction of events with neutron simhits– Trigger primitives level
• Preliminary: – Without YE4, ME4/2 gets ~40% more of LCTs b/c of neutrons – Much lower effect in other chamber types
– GMT• Combining CSC and RPCf in high pileup environment
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Plans
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Physics Lists Validation
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• Standard one for use in CMS simulation:– QGSP_BERT_EML
• Too crude for neutron modelling
• Special “high precision” lists for modeling of long-lived neutrons:– QGSP_BERT_HP
• Older detailed multiple scattering model– QGSP_BERT_HP_EML
• New simplified/faster multiple scattering model
• Special parameterized neutron cross section table:– QGSP_BERT_EMLSN
• The newest parameterization
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Physics Lists
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• For each physics list:– 2K events sample– Produced with CMSSW_3_9_0_pre2 in the same conditions– Standard geometry– GEANT settings:
g4SimHitsNeutrons.Physics.type = 'SimG4Core/Physics/QGSP_BERT_***‘g4SimHitsNeutrons.Physics.FlagBERT = Trueg4SimHitsNeutrons.StackingAction.NeutronThreshold = 0.g4SimHitsNeutrons.StackingAction.MaxTrackTime = 1e9g4SimHitsNeutrons.SteppingAction.MaxTrackTime = 1e9# the following two enable simulation in the Quad regiong4SimHitsNeutrons.StackingAction.MaxTrackTimes[2] = 1e9g4SimHitsNeutrons.SteppingAction.MaxTrackTimes[2] = 1e9# no cuts on generator-level particlesg4SimHitsNeutrons.Generator.ApplyPCuts = Falseg4SimHitsNeutrons.Generator.ApplyEtaCuts = False
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Validation Samples
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QGSP_BERT_HP 17.2QGSP_BERT_HP_EML 16.2QGSP_BERT_EMLSN 1.4QGSP_BERT_EML 1.2
• For HP lists Most of time is spent in transport of neutrons from quad regions
• EMLSN timing: too good to be true?...
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Timing (min/evt)
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• ~x4 lower for EMLSN
• HP and HP_EML: within stat errors– Except ME4/2 where
quad region simulationis important
• DT & RPC:– Small statistics– x2-5 lower for EMLSN
in DT
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Fraction of Events with n-hits in CSC
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• _
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Validation: TOF vs. Ekin in CSCQGSP_BERT_HP QGSP_BERT_HP_EML
QGSP_BERT_EMLSN QGSP_BERT_EML
Both exhibitTOF cut-offat 104 ns
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• P
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Validation: Ekin & TOF in CSC
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Conclusions• QGSP_BERT_HP and QGSP_BERT_HP_EML show close
results, except for ME4/2 station• QGSP_BERT_EMLSN
– Neutron hit yield lower by x4– It doesn’t track particles longer then 104 ns
2222
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BACKUP
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• Neutrons leave hits in 1-2 layers most of time
• By chamber type (no YE4 schielding):
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CSC # of layers with hitsMinBias
Neutrons
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r-z Neutron hits Heatmap
Endcap in currentgeometry
TDR geometry with YE4
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x-y Neutron hits Heatmap in Barrel