Muon simulation codes MUSIC and MUSUN for underground physics

arXiv:0810.4635v1 [physics.comp-ph] 25 Oct 2008 Muon simulation codes MUSIC and MUSUN for underground physics V. A. Kudryavtsev 1, Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK Abstract 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. Keywords: Muons; Muon interactions; Muon transport; Muons underground; Muon- induced background PACS: 14.60.Ef, 25.30.Mr, 24.10.Lx, 95.85.Ry 1Corresponding author; address: Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK, e-mail: v.kudryavtsev@sheffield.ac.uk 1 1 Introduction Muon transport through matter plays an important role in many areas of particle and astroparticle physics. Cosmic-ray muons are detected at large depths underground and underwater (here and hereafter we use the term underwater that includes also under-ice experiments). They are used to study the energy spectrum and composition of primary cosmic rays and calculations of their fluxes, energy and angular distributions are the key element of this research (see, for instance, Refs. [1, 2, 3, 4, 5]. Experiments with high-energy muon neutrino beams from accelerators require accurate simulations of muon transport from the point of neutrino interaction to the detector. Similarly, neutrino telescopes are detecting (or expecting to detect) muons from atmo- spheric and astrophysical neutrinos, and three-dimensional propagation of muons from their production point to the detector is crucial for the interpretation of experimental data [6, 7, 8]. Cosmic-ray muons are also a background in experiments looking for rare events at low and high energies deep underground or underwater. Atmospheric down-going muons can be erroneously reconstructed as upward-going muons that mimic neutrino-induced events in a search for astrophysical neutrinos at GeV-TeV energies or in an atmospheric neu- trino detection for neutrino oscillation studies. Cosmic-ray muons also produce secondary neutrons (with MeV-GeV energies) by interacting with rock. These neutrons can mimic low-energy (keV-MeV) events in detectors looking for WIMP (Weakly Interacting Massive Particle) dark matter, neutrinoless double-beta decay and neutrinos (solar, geophysical, supernova neutrinos, etc.) (see Ref. [9] for a review and Refs. [10, 11, 12, 13] for example calculations of muon-induced neutron fluxes underground) . High-energy (GeV) neutrons from muons can produce events with a signature similar to proton decay. There are a few more applications from different areas of science. A morphological re- construction of mountains and natural caves using atmospheric muons was suggested in Ref. [14]. A search for hidden chambers in pyramids was discussed in Ref. [15]. A ‘muon radiography’ using multiple scattering of cosmic-ray muons was proposed recently [16] to discriminate between low-A and high-A materials in cargo. All applications mentioned above require accurate calculations of muon spectra and scat- tering beyond a slab of material. Most of them involve muon transport through large thickness of matter. Hence the CPU time should be reduced to a minimum without compromising the accuracy of calculations. Several Monte Carlo codes are able to transport muons through matter with high accuracy. The codes can be split in two categories: (i) multipurpose particle transport codes, such as GEANT4 [17] and FLUKA [18], and (ii) codes developed specifically for muon propagation through large thickness of material, such as PROPMU [19], MUSIC [20, 21], MUM [22] and MMC [23]. Significant progress has recently been achieved in the development of the multipurpose transport codes for particle physics applications. The codes have become faster, more ro- bust, flexible and accurate. However, their flexibility requires a good knowledge of physics and programming skills from a user. GEANT4, for instance, is designed as a powerful toolkit but a good knowledge of the code including models and programming language is 2 needed to use it properly. Significant efforts and time are required to become familiar with such a toolkit. These codes are absolutely necessary when simulating events consisting of many particles that should be produced, transported and detected practically at the same time. Meanwhile, some tasks, for instance muon transport through a homogeneous material, may be accomplished without using multipurpose codes. If a user is interested in transporting muons without following secondary particles produced by them, it is enough

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