Local Muon Density Spectra at Various Zenith Angles Measured with NEVOD-DECOR
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ELEMENTARY PARTICLES AND FIELDS Experiment
Local Muon Density Spectra at Various Zenith Angles Measured with NEVOD-DECOR R. P. Kokoulin1)* , A. G. Bogdanov1) , G. Mannocchi2), A. A. Petrukhin1) , V. V. Shutenko1) , G. Trinchero2) , I. I. Yashin1) , and E. A. Yurina1) Received July 12, 2019; revised July 12, 2019; accepted July 12, 2019
Abstract—Data on cosmic ray muon bundles accumulated at the NEVOD-DECOR complex over the period from May 2012 to December 2018 have been analyzed. Local muon density spectra at various zenith angles have been reconstructed and compared with CORSIKA-based simulations. At large zenith angles and high muon multiplicities corresponding to primary particle energies more than about 3 × 1017 eV an excess of multi-muon events compared to simulations is clearly seen. Present data are compatible with the expectation for recent LHC-adjusted hadron interaction models only under assumption of extremely heavy (iron group nuclei) primary composition. The assumption of a heavy composition is however in contradiction with other EAS observables, such as maximum depth and its fluctuations. DOI: 10.1134/S1063778819660293
1. INTRODUCTION Investigations of the muon component of extensive air showers (EAS) allow one to obtain information about the primary energy spectrum, mass composition, and the features of hadronic interaction at ultra-high energies. Though the experimental situation with muon content in EAS is somewhat controversial, a recent comparative analysis [1] has shown that most of the experiments indicate for an increasing excess of multi-muon events in comparison with expectation at energies around and above 1018 eV. One of the approaches to study EAS muon component in a wide range of primary energies is the measurement of local muon density spectra (LMDS) at various zenith angles [2, 3]. In the present paper, we describe results of new measurements of LMDS at large zenith angles with the coordinate-tracking detector DECOR. 2. FEATURES OF THE LOCAL MUON DENSITY SPECTRUM PHENOMENOLOGY At large zenith angles, extensive air showers arrive at the ground level as practically pure muon showers. Due to muon lateral spread caused by transverse momenta at production and decay of parent mesons and deflection in the Earth magnetic field, the lateral 1)
National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Moscow, Russia. 2) Osservatorio Astrofisico di Torino—INAF, Torino, Italy. * E-mail: [email protected]
extension of the EAS in the muon component reaches several kilometers. In such conditions, a muon detector with sizes of tens of meters may be considered as a point-like probe. In an individual event with a muon bundle, the muon density D at the observation point is measured, which in the first approximation may be estimated as the ratio of the number of muons and the detector area, D ∼ m/Sdet . Distribution of the events in muon density D forms the local muondensity spectrum. The expected LMDS for a certain direction may be evaluated as [2] F (D) = N (E(r, D))dS, (1)
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