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Muon simulation codes MUSIC and MUSUN for underground physics
V. A. Kudryavtsev
TL;DR
Underground and underwater physics requires accurate, efficient simulation of muon transport and distributions. The paper presents MUSIC for three-dimensional transport through rock or water and MUSUN for generating underground muons from those transport results. Tests show good agreement with experimental data and other packages, within stated code-scope limitations.
Problem
Underground and underwater experiments require accurate muon transport, spectra, and angular distributions for detector interpretation and background studies.
Method
The paper presents MUSIC for muon transport through large rock or water thicknesses and MUSUN for generating underground muons using MUSIC results.
Results
Various tests showed good agreement of MUSIC and MUSUN results with experimental data and other packages.
Takeaways & Limitations
MUSIC and MUSUN provide transport and underground-muon generation capabilities for underground, underwater, and neutrino-detector applications.
Takeaways & Limitations
Specific code versions are not all submitted to the code library and must be obtained by request; MUSIC also omits muon-pair-production energy loss because of its small value.
Abstract
from arXiv · showhide
The paper describes two Monte Carlo codes dedicated to muon simulations: MUSIC (MUon SImulation Code) and MUSUN (MUon Simulations UNderground). MUSIC is a package for muon transport through matter. It is particularly useful for propagating muons through large thickness of rock or water, for instance from the surface down to underground/underwater laboratory. MUSUN is designed to use the results of muon transport through rock/water to generate muons in or around underground laboratory taking into account their energy spectrum and angular distribution.
1 Introduction
Muon transport simulations support particle and astroparticle physics, but underground applications require accurate, efficient propagation through large matter thicknesses. The paper introduces MUSIC for three-dimensional transport and MUSUN for underground muon generation.
- Motivation: Accurate muon spectra, energies, and angular distributions are essential for interpreting underground, underwater, and neutrino-detector measurements.These simulations also address cosmic-ray backgrounds that can mimic neutrino, dark-matter, and neutrinoless double-beta-decay signals.
- Motivation: Large-thickness transport must reduce CPU time without compromising calculation accuracy.
- Related work: Muon transport codes include multipurpose tools such as GEANT4 and FLUKA and specialized codes such as PROPMU, MUSIC, MUM, and MMC.
- Related work: Specialized muon codes can focus on muon interactions and energy losses when secondary-particle tracking is unnecessary.This targets tasks such as muon transport through homogeneous material without requiring multipurpose-code expertise.
- Contribution: The paper describes MUSIC for three-dimensional muon propagation and MUSUN for sampling underground or underwater muons using MUSIC transport results.
2 Muon transport through large thickness of matter: MUSIC
MUSIC transports muons through large material thicknesses using modeled energy-loss and scattering processes, with standard and thin-slab versions. Its spectra, survival probabilities, and intensities agree with measurements and other transport packages across tested conditions.
- Transport method: MUSIC models ionisation, bremsstrahlung, electron-positron pair production, and muon-nucleus inelastic scattering, while accounting for multiple-scattering deflection and other interaction-induced deviations.
- Code versions: MUSIC provides standard large-thickness and thin-slab versions, with the latter supplying segment-level energy, position, direction, and energy-loss information.
- Transport method: The standard version treats sufficiently large fractional energy losses stochastically, with recommended vcut = 10^-3 balancing accuracy and speed.
- Scope and limitation: The code omits muon-pair-production energy loss because it is small relative to electron-positron pair production, while allowing muon-pair generation along the path.
- Results: Transport results show material-dependent survival probabilities and underground spectra for standard rock and water, with differences explained by their composition and energy-loss processes.At large depths, pair production and bremsstrahlung dominate, and water survival probabilities exceed those in standard rock.
- Validation: Measured and simulated muon intensities agree across broad zenith-angle and slant-depth ranges, supporting MUSIC’s transport validity.A comprehensive comparison found data points distributed symmetrically around the calculated depth–intensity curve.
- Validation: MUSIC energy distributions agree well with GEANT4 and FLUKA for high-energy muons transported through small and large material slabs.For 2 TeV muons through 3 km of water, survival probabilities were 0.779 for MUSIC, 0.793 for GEANT4, and 0.756 for FLUKA.
3 Simulations of muons in underground laboratories using MUSUN
MUSUN generates underground muons by combining MUSIC transport results with surface spectra and angular distributions. It supports configurable sampling and reproduces measured Gran Sasso distributions and mean energy.
- MUSUN samples underground muons by combining MUSIC transport through matter with the surface energy spectrum and angular distribution.
- The workflow propagates varied initial energies through specified matter and stores underground energy distributions for later MUSUN processing.Initial energies span 100 GeV to 10^7 GeV, with transport distances from 100 to 15000 m w. e.
- MUSUN computes differential intensities from the transported energy probability and surface spectrum, then samples energies and directions from stored distributions.Azimuth is sampled uniformly under the flat-surface assumption, and muon charge uses the measured high-energy ratio µ+/µ−≈1.3.
- Users can select the surface spectrum, prompt-muon fraction, laboratory depth, angular ranges, and energy ranges without additional propagation for each option.Different rock compositions require separate muon transport.
- MUSUN can generate muons on user-sized rectangular parallelepipeds and export their parameters for multipurpose event generators.
- At Gran Sasso, MUSUN agrees well with measured azimuthal intensities and gives a mean muon energy of 273 GeV versus 270 ± 3 (stat.) ±18 (syst.) GeV measured.
- Versions exist for Gran Sasso, Modane, Boulby, and Soudan, and have been used to study muon-induced neutron backgrounds in rare-event experiments.
4 Conclusions
The paper describes MUSIC for large-thickness muon transport and MUSUN for underground muon generation. Tests report agreement with experimental data and other packages, while versions are distributed by request.
- MUSIC transports muons through large thicknesses of rock or water and can be integrated into neutrino-detector event generators.
- MUSUN uses rock/water transport results to generate underground muons with their energy spectrum and angular distribution.
- Tests found good agreement between the codes’ results, experimental data, and other packages.
- Specific code versions are available by request because submitting all versions to the code library was considered impractical.
Muon survival probability
Figure 1 shows muon survival probability versus surface energy across depths from 0.5 to 10 km w. e. in standard rock and water.
- Survival-probability curves compare standard rock and water for depths from 0.5 to 10 km w. e. as functions of surface muon energy.
Vertical muon intensity, cm-2s-1sr-1GeV-1
Figure 2 compares vertical muon energy spectra across depths in standard rock and water.
- The spectra are shown at vertical for multiple depths in standard rock and water.
- Standard-rock spectra use black solid curves, while water spectra use red dashed curves.
- Depths for standard-rock curves are labeled in kilometres of water equivalent, with water curves vertically shifted for comparison.
Vertical muon intensity, cm-2s-1sr-1
The figures cover depth-dependent muon intensity, transport distributions, and underground angular and energy distributions generated or evaluated with MUSIC and MUSUN.
- Figure 3 relates vertical muon intensity to depth for standard rock and water using experimental data points from rock and underwater experiments.
- Figure 4 compares energy distributions after transporting 2 TeV muons through 3 km of water with MUSIC, GEANT4, and FLUKA.
- Figures 5 and 6 compare MUSIC and GEANT4 distributions for angular deviation and lateral displacement after the same 2 TeV, 3 km water transport.
- Figure 7 compares measured and MUSUN-generated azimuthal single-muon intensities in Gran Sasso for zenith angles up to 60°.
- Figures 8 and 9 show the MUSUN-generated Gran Sasso muon energy spectrum and the number of generated muons across zenith and azimuthal angles.