1. Neutron Background Simulation: Infrastructure and Validation Vadim Khotilovich, Rick Wilkinson, with help from Piet Verwilligen, Alexei Safonov, Tim Cox Simulation Meeting August 30, 2010

2. 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 2

3. 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

4. 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 log 10 TOF vs log 10 E kin –Plot made in 2001 From Rick’s previous presentation: http://indico.cern.ch/conferenceDisplay.py?confId=55712

5. 5 New Neutron Simulation Infrastructure generator “Pythia6GeneratorFilter” generatorNeutrons “Pythia6GeneratorFilter” g4SimHits “OscarProducer” g4SimHitsNeutrons “OscarProducer” cscNeutronWriter “CSCNeutronWriter” rpcNeutronWriter “RPCNeutronWriter” dtNeutronWriter “DTNeutronWriter” g4SimHits “NeutronHitsCollector” generator “EmptyHepMCProducer” mix “MixingModule” generator Any desired generator g4SimHits “OscarProducer” signal MinBias Pileup Sample Neutron Pileup Sample to digitizers N PU f*N PU 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 into 25ns wide time interval while preserving hits’ time ordering Special utility modules to standardize collections for MixingModule SimMuon/Neutron

6. 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

7. Muon SimHits TOF vs. E kin TDR geometry with YE4 CSC&DT have lots of low energy hits –RPCs are not so sensitive

8. CSC: hits vs. clusters of hits Simple clustering of CSC hits in a chamber layer: 1.Sort by closes wire group, strip, TOF 2.Take first hit 3.Cluster other hits around it requiring  WG ≤ 1 &  S ≤ 3 &  TOF < 2 ns 4.Repeat recursively… Very low energy hits are from secondary interactions

9. 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 9 Fractions of evt. with n-hits: YE4 effect Geometry: current TDR with YE4

10. 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 10 Fractions of evt. with hits: MB vs. neutrons SimHits from: MinBias Neutrons

11. TDR geometry with YE4 11 Fractions of events with neutron hits

12. TDR geometry with YE4 N PU =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 L SimHits from: MinBias Neutrons

13. 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

14. 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 14 Plans

15. Physics Lists Validation 15

16. 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 16 Physics Lists

17. 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 = True g4SimHitsNeutrons.StackingAction.NeutronThreshold = 0. g4SimHitsNeutrons.StackingAction.MaxTrackTime = 1e9 g4SimHitsNeutrons.SteppingAction.MaxTrackTime = 1e9 # the following two enable simulation in the Quad region g4SimHitsNeutrons.StackingAction.MaxTrackTimes[2] = 1e9 g4SimHitsNeutrons.SteppingAction.MaxTrackTimes[2] = 1e9 # no cuts on generator-level particles g4SimHitsNeutrons.Generator.ApplyPCuts = False g4SimHitsNeutrons.Generator.ApplyEtaCuts = False 17 Validation Samples

18. QGSP_BERT_HP17.2 QGSP_BERT_HP_EML16.2 QGSP_BERT_EMLSN1.4 QGSP_BERT_EML1.2 For HP lists Most of time is spent in transport of neutrons from quad regions EMLSN timing: too good to be true?... 18 Timing (min/evt)

19. ~x4 lower for EMLSN HP and HP_EML: within stat errors –Except ME4/2 where quad region simulation is important DT & RPC: –Small statistics –x2-5 lower for EMLSN in DT 19 Fraction of Events with n-hits in CSC

20. _ 20 Validation: TOF vs. E kin in CSC QGSP_BERT_HPQGSP_BERT_HP_EML QGSP_BERT_EMLSN QGSP_BERT_EML Both exhibit TOF cut-off at 10 4 ns

21. P 21 Validation: E kin & TOF in CSC

22. 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 10 4 ns 22

23. BACKUP 23

24. Neutrons leave hits in 1-2 layers most of time By chamber type (no YE4 schielding): 24 CSC # of layers with hits MinBias Neutrons

25. 25 r-z Neutron hits Heatmap Endcap in current geometry TDR geometry with YE4

26. 26 x-y Neutron hits Heatmap in Barrel