Charge ratio of ultra-high-energy cosmic-ray muons

Abstract

We describe the data analysis and report the results of the final charge-ratio experiment using the Utah cosmic-ray muon detector. Values for the charge ratio were obtained from least-squares fits to the observed bending-angle distributions. The fitting function was the result of folding the noise scattering distribution into the ideal local-spectrum bending distribution, with the charge ratio as a parameter of the fit. This method permitted a greater precision while circumventing the main causes of charge-ratio dilution. We obtain the average value 1.378±0.015 for muon momentum at production between 1 and 8 TeV/c, corresponding to a median primary cosmic-ray momentum range 8-61 TeV/c. With the results of this experiment the charge ratio is precisely known over about three decades of muon energy beginning around 10 GeV. Thus, we believe the charge ration can serve as a touchstone in determining the hadronic interaction dynamics and composition of the primary cosmic rays. Already approximate agreement has been obtained with experimental results from predictions utilizing the scaling-limiting-fragmentation hypothesis and constant composition of the primary cosmic rays.