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Simulating Neutron Background in Collider Experiments: Challenges and Methodologies

Neutron background simulation in collider environments presents significant challenges due to long-lived neutrons that can create diffuse signals in detection systems. These neutrons can be captured by nuclei, leading to photon emission and subsequent interactions such as Compton scattering, generating MeV electrons that trigger muon chambers. Standard simulation methods often struggle due to the complexity of neutron behavior and the necessity for precise time-of-flight tracking. We propose a refined approach for handling neutron interactions to enhance signal detection and accurately simulate background noise, taking into account recent findings regarding LCTs and CLCT pretriggers.

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Simulating Neutron Background in Collider Experiments: Challenges and Methodologies

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  1. Neutron Background Simulation R. Wilkinson

  2. Neutron Background Simulation • 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

  3. Why is neutron background 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

  4. GEANT Simulation • QGSP_BERT_HP physics tables • Other options needed to give long-lived, low energy particles • No pT cuts or eta cuts • Long tracking time • No neutron threshold

  5. Old Methods • Parametrization (UC Davis) • Database of Chamber Hit Patterns (Wilkinson) • Meant to be added only to chambers with signal

  6. CSC-specific issues • Usually only affect one layer, sometimes two, sometimes more. • At L =1034, we expect at most 3% chance of a neutron per chamber, per BX. • We had thought that the trigger would suppress most of these, but it doesn’t! • We had thought suppression used LCTs, which require four layers • So we thought we should only add neutron hits to chambers that already have signal • In reality, it uses CLCT pretriggers, which only require two layers • Which means we need to simulate all chambers, and implement a more accurate zero suppression in the simulation

  7. Workplan • Treat neutrons like regular pileup • Take SimHits with a high time of flight, change the TOF to 0-25 ns, and save the new hits in a different collection. • SimMuon/Neutron/src/NeutronProducer.cc • Keep relative timing for hits within the same chamber • Drop all data except the high-TOF hits from the events • Let the MixingModule mix in the new events. • Use the average number of interactions per bunch crossing, including gaps • Should get the correct occupancy, assuming steady-state running. • Should just work for DT & RPC

  8. CSC Workplan • Current Simulation: • Saves any layer which has signal • Only simulates and reads out groups of strips (CFEBs) containing or near signals • Doesn’t always make all the noise strips needed • What’s needed: • Create a transient container of unsuppressed digis • Neutron hits come from MixingModule • Run the L1 trigger primitive simulation • Move the module from the L1Trigger sequence to the SimMuon sequence • Code would still live in L1Trigger • Input is unsuppressed digis • Creates a new transient collection of pretrigger digis • Make a zero suppression module which produces the suppressed digis • Generating noise, if needed • Prototype in SimMuon/CSCDigitizer/src/CSCDigiSuppressor.cc

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