An analysis of muon showers at large zenith angles

Abstract

A theoretical model has been constructed in order to explain the known features of near vertical extensive air showers; the intention being to use this model to predict the features of showers at large zenith angles. The adopted model explains satisfactorily the longitudinal development of both the hard and soft components of the shower, but two discrepancies are found when comparison is made with the muon lateral distribution. The first is at small distances from the shower core where the model overestimates the muon density; a solution to this is found to be a reduction in the probability of transverse momentum transfers below 0.1 GeV/c, At large distances from the core and high threshold energies there is a considerable underestimate in the predicted muon densities. Many of the model parameters have been investigated in an effort to find a solution; apart from a drastic change in the model a fit between theory and experiment may be obtained by postulating an increase in the mean transverse momentum. The necessary values are <Pt> = 0.6 ± 0.2 GeV/c for pion interactions of mean energy ~200 GeV and <Pt> = 1.0 ± 0.3 GeV/c for interactions of mean energy ~4000 GeV. The Durham Horizontal Extensive Air Shower Array has been used to measure the zenith angle distribution of muon showers at large zenith angles (0 > 45º), and the muon number spectrum for 57.5º < 0 < 90º has also been determined. The former is satisfied by the adopted model with <Pt> ~ 0.8 Gev/c; there is however a discrepancy between the expected frequency of >2 muon events and the observed rate. In order to find a solution to this problem changes are made in the primary flux; the best fit is obtained when the composition is such that protons are predominant for primary energies up to a few times 10(^15) eV, above this the effective mass of the primary flux increases with energy until 10(^17) eV is reached.