MUPAGE: a fast atmospheric MUon GEnerator for neutrino telescopes based on PArametric formulas

📝 Abstract

Neutrino telescopes are opening new opportunities in observational high energy astrophysics. In these detectors, atmospheric muons from primary cosmic ray interactions in the atmosphere play an important role. They provide the most abundant source of events for calibration and for testing the reconstruction algorithms. On the other hand, they represent the major background source. The simulation of a statistically significant number of muons in large volume neutrino telescopes requires a big effort in terms of computing time. Some parameterizations are currently available, but they do not explicitly take into account the arrival of muons in bundles. A fast Monte Carlo generator (MUPAGE) was developed to generate single and multiple atmospheric muon events in underwater/ice neutrino telescopes. The code reduces the computing time for the simulation of atmospheric muons significantly. The event kinematics is produced on the surface of a user-defined cylinder, virtually surrounding the detector volume. The flux of muon bundles at different depths and zenith angles, the lateral spread and the energy spectrum of the muons in the bundles are based on parametric formulas. These formulas were obtained according to a specific primary cosmic ray flux model and constrained by the measurements of the muon flux in the MACRO underground experiment. Some MUPAGE applications are presented.

💡 Analysis

Neutrino telescopes are opening new opportunities in observational high energy astrophysics. In these detectors, atmospheric muons from primary cosmic ray interactions in the atmosphere play an important role. They provide the most abundant source of events for calibration and for testing the reconstruction algorithms. On the other hand, they represent the major background source. The simulation of a statistically significant number of muons in large volume neutrino telescopes requires a big effort in terms of computing time. Some parameterizations are currently available, but they do not explicitly take into account the arrival of muons in bundles. A fast Monte Carlo generator (MUPAGE) was developed to generate single and multiple atmospheric muon events in underwater/ice neutrino telescopes. The code reduces the computing time for the simulation of atmospheric muons significantly. The event kinematics is produced on the surface of a user-defined cylinder, virtually surrounding the detector volume. The flux of muon bundles at different depths and zenith angles, the lateral spread and the energy spectrum of the muons in the bundles are based on parametric formulas. These formulas were obtained according to a specific primary cosmic ray flux model and constrained by the measurements of the muon flux in the MACRO underground experiment. Some MUPAGE applications are presented.

📄 Content

PROCEEDINGS OF THE 31st ICRC, Ł ´OD´Z 2009 1 Atmospheric MUons from PArametric formulas: a fast GEnerator for neutrino telescopes (MUPAGE) M. Bazzotti∗, S. Biagi∗†, G. Carminati∗†, S. Cecchini∗‡, T. Chiarusi†, A. Margiotta∗†, M. Sioli∗† and M. Spurio∗† ∗Dipartimento di Fisica dell’Universit`a di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy † INFN, Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy ‡INAF-IASF, Via Gobetti 101, 40129 Bologna, Italy Abstract. Neutrino telescopes are opening new op- portunities in observational high energy astrophysics. In these detectors, atmospheric muons from primary cosmic ray interactions in the atmosphere play an important role. They provide the most abundant source of events for calibration and for testing the reconstruction algorithms. On the other hand, they represent the major background source. The simulation of a statistically significant number of muons in large volume neutrino telescopes requires a big effort in terms of computing time. Some parameterizations are currently available, but they do not explicitly take into account the arrival of muons in bundles. A fast Monte Carlo generator (MUPAGE) was developed to generate single and multiple atmospheric muon events in underwater/ice neutrino telescopes. The code reduces the comput- ing time for the simulation of atmospheric muons significantly. The event kinematics is produced on the surface of a user-defined cylinder, virtually surrounding the detector volume. The flux of muon bundles at different depths and zenith angles, the lateral spread and the energy spectrum of the muons in the bundles are based on parametric formulas [1], [2]. These formulas were obtained according to a specific primary cosmic ray flux model and constrained by the measurements of the muon flux in the MACRO underground experiment [3]. Some MUPAGE applications are presented. Keywords: Simulation of atmospheric muons, neu- trino telescopes, Monte Carlo generator I. INTRODUCTION The most abundant signals for a neutrino telescope are due to high energy muons resulting from the extensive air showers produced by interactions between primary cosmic rays (CRs) and atmospheric nuclei. Although neutrino telescopes are located at large depths under water or in ice, taking advantage of the shielding effect offered by these detector media, a large flux of high energy atmospheric muons can reach the active volume of the detectors. The atmospheric muons represent an insidious back- ground for track reconstruction as their Cherenkov light can mimic fake upward going tracks. This kind of signatures can be confused with the cosmic neutrino signals that neutrino telescopes are searching for. On the other hand, atmospheric muons are a useful tool to test offline analysis software, to check the understanding of the detector and to estimate systematic uncertainties. Moreover, the pointing capability of the telescopes can be studied using atmospheric muons through the mea- surement of the moon shadow. Atmospheric muon bundles can be accurately repro- duced by a full Monte Carlo (MC) simulation (e.g. CORSIKA [4]), starting from primary CR interactions with atmospheric nuclei, generating the resulting large number of air showers and propagating the muons through sea water or ice. A full MC requires a large amount of computing time and therefore the production of a large statistical event sample cannot be easily obtained for a detector of a cubic kilometer size. In a recent search for a diffuse flux of neutrinos [5], the use of a full MC simulation limited the number of generated atmospheric muons at 63 equivalent detector days to represent a background for 807 active days of data. In order to save the big effort of computing time, a fast Monte Carlo generator is therefore essential. Some parameterizations for the underwater/ice flux and energy spectrum are available in literature [6], [7], [8]. None of them, however, gives the possibility to simulate two or more muons produced in the same CR interaction (muon bundle). In this paper the event generator MUPAGE is presented. It is based on parametric formulas and takes into account the multiplicity of muon bundles. II. SIMULATION OF ATMOSPHERIC MUON BUNDLES Parametric formulas are derived in [1], [2] and de- scribe the flux, the angular distribution and the energy spectrum of underwater/ice muon bundles. The muon flux and energy spectrum are parameterized, for the first time, in terms of the bundle multiplicity m. From these parametric formulas an event generator called MUPAGE (MUon GEnerator from PArametric formulas) [9] was developed. The program output is a formatted ASCII table containing the kinematics of atmospheric muon bundles on the surface of a can. The can (Fig. 1) is an imaginary cylinder surrounding the active volume of any proposed detector between arXiv:0907.5563v1 [astro-ph.IM] 31 Jul 2009 2 G. CARMINATI et al. MUPAGE GENERATOR 1.5 and 5.0 km of water equivalent depth. The output table wit

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