Neutron Background SimulationNeutron Background Simulation

Long-lived neutrons created, Long-lived neutrons created, diffuse around collision diffuse around collision hallhall

They get captured by nuclei, They get captured by nuclei, emitting a photonemitting a photon

Compton scattering or Compton scattering or photoelectric effect makes photoelectric effect makes MeV electrons, which MeV electrons, which cause hits in muon cause hits in muon chamberschambers

Differences GEANT3 and GEANT4Differences GEANT3 and GEANT4

GEANT3 → no ion simulation

GEANT4 → ion simulation→ perfect elastic scattering of neutron→ additional physics package for

thermal neutrons

CMSIM 121: 1.1 hits per event in ME t>250 ns

OSCAR_3_3_2+QGSP_BERT_HP: 0.46 hits per event in ME t>250 ns

ResultsResults

QGSP_BERT_HP, 2k events, Endcap detectors

Because neutrons can live Because neutrons can live up to a second before up to a second before making a signalmaking a signal

They can’t be treated like They can’t be treated like ordinary minimum-bias ordinary minimum-bias pileup, because millions of pileup, because millions of collisions in the past can collisions in the past can contributecontribute

Why is neutron background hard to Why is neutron background hard to simulate?simulate?

The Way It Was Done Before:The Way It Was Done Before:ParametrizationParametrization

~9 years ago, UC Davis group (Hessian, Fisyak, Breedon)~9 years ago, UC Davis group (Hessian, Fisyak, Breedon)

Based on 2000 simulated min-bias events, simulated down to low Based on 2000 simulated min-bias events, simulated down to low energies and long timesenergies and long times

Start with “mother” hits with some distribution in energy, Start with “mother” hits with some distribution in energy, position, and directionposition, and direction

Add some number additional hits in same layerAdd some number additional hits in same layer

Propagate each hit to next layer and repeatPropagate each hit to next layer and repeat

Disadvantages of ParametrizationDisadvantages of Parametrization

• Hard to maintainHard to maintain Many parametersMany parameters Needs to be done separately for each detector type for Needs to be done separately for each detector type for

CSC, DT, and RPCCSC, DT, and RPC Needs to be updated when geometry or shielding Needs to be updated when geometry or shielding

changeschanges

• Can we use the original events rather than a Can we use the original events rather than a parametrization of them?parametrization of them?

What I’ve Done: What I’ve Done: Database of Neutron Hit PatternsDatabase of Neutron Hit Patterns

Start with a sample of simulated min-bias eventsStart with a sample of simulated min-bias events

Take the events apart. Treat each chamber with hits as an Take the events apart. Treat each chamber with hits as an independent event. Zero out the time.independent event. Zero out the time.

Database of Chamber SimHit PatternsDatabase of Chamber SimHit Patterns

Store these patterns of neutron hits in a ROOT file, grouped by Store these patterns of neutron hits in a ROOT file, grouped by chamber type:chamber type:

ME1/A ME 1/1 ME1/2 ME1/3 ME2/1 ME2/2 ….ME1/A ME 1/1 ME1/2 ME1/3 ME2/1 ME2/2 ….

When I need to add neutron background to the simulation, I just read in When I need to add neutron background to the simulation, I just read in some number of these patterns and superimpose themsome number of these patterns and superimpose them

•Done before electronics simulation, of course, so things pile up Done before electronics simulation, of course, so things pile up correctly.correctly.•Only done for chambers that already have signal!Only done for chambers that already have signal!

How Many Patterns to Superimpose?How Many Patterns to Superimpose?

Say we simulate a window of 24 bunch crossings around the Say we simulate a window of 24 bunch crossings around the event.event.

That’s ~330 min bias events (at 10That’s ~330 min bias events (at 103434) that may create signals in ) that may create signals in future crossings.future crossings.

I think we can assume that the amount of neutron signal in I think we can assume that the amount of neutron signal in our 24-bx window is the total amount that would come from our 24-bx window is the total amount that would come from 330 minimum bias events in the past.330 minimum bias events in the past.

If ME2/1 chambers have a neutron-induced occupancy of If ME2/1 chambers have a neutron-induced occupancy of 0.24% per min-bias event per chamber, we should 0.24% per min-bias event per chamber, we should superimpose a Poisson mean of 330*0.24% = 0.8 hit patterns superimpose a Poisson mean of 330*0.24% = 0.8 hit patterns per chamber.per chamber.

Adding neutron eventsAdding neutron events

Events to add, 10^34, 24 bx

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Shielded G3

G4

Effects of adding neutronsEffects of adding neutrons

Other subdetectors:Other subdetectors: Endcap RPCs may want to use thisEndcap RPCs may want to use this

CSC RecHit occupancy:CSC RecHit occupancy: Not much. A few percent. Not much. A few percent. We’re simulating 24 bx.We’re simulating 24 bx.

Overlapping with existing RecHitsOverlapping with existing RecHits

DAQ rate:DAQ rate: How often will they make fake segments in empty How often will they make fake segments in empty

chambers?chambers? Needs to be implemented separatelyNeeds to be implemented separately

Database of only neutrons which make real segmentsDatabase of only neutrons which make real segments

Other issuesOther issues

RecHit occupancy:RecHit occupancy: Not much. A few percent. Not much. A few percent. We’re simulating 24 bx.We’re simulating 24 bx.

Overlapping with existing RecHitsOverlapping with existing RecHits

DAQ rate:DAQ rate: How often will they make fake segments in empty How often will they make fake segments in empty

chambers?chambers? Needs to be implemented separatelyNeeds to be implemented separately

Database of only neutrons which make real segmentsDatabase of only neutrons which make real segments