New Measurement of the Flux of Atmospheric Muons

M. Boezio, P. Carlson, T. Francke, N. Weber, M. Suffert, M. Hof, W. Menn, M. Simon, S. A. Stephens, R. Bellotti, F. Cafagna, M. Castellano (+23 others)
1999 Physical Review Letters  
We report a new measurement of the momentum spectra of both positive and negative muons as a function of atmospheric depth in the momentum range 0.3 -2 and 0.3 40 GeV͞c, respectively. The measured flux values have been compared with the spectra obtained from simulations, which were carried out to interpret the atmospheric neutrino data. We find that our data disagree with the results from the simulations. The ratio of the flux of muons derived from simulations to that measured is at largest 1.8
more » ... d is at largest 1.8 and varies with atmospheric depth and muon momentum. [S0031-9007(99)09347-3] PACS numbers: 96.40.Tv, 14.60.Pq, 14.60.Ef Recently the Super-Kamiokande collaboration has reported evidence for neutrino oscillations from the study of atmospherically produced neutrinos in the 50 kt underground water detector [1]. The fully contained neutrino interactions in this experiment are in the energy range 0.1-10 GeV with most events having energies around 1 GeV. The reported value of the ratio R ͑n m ͞n e ͒ data ͑͞n m ͞n e ͒ MC is 0.63 6 0.05, in agreement with earlier results of less statistical significance [2] . This result indicates that either too few muon neutrino induced events or too many electron neutrino induced events were observed in this experiment. Several calculations have been made of the flux and interaction rates of atmospherically produced neutrinos [3] [4] [5] . Recently Gaisser et al. [6] have compared different calculations and concluded that, although the neutrino fluxes may differ by as much as 30% between the different calculations, the ratio of the flux of muon neutrinos to that of electron neutrinos is consistent within 5%. The differences in the calculated flux values have been attributed to (i) the different parametrization of the particle production in strong interactions of the cosmic ray protons and helium nuclei with atmospheric nitrogen and oxygen nuclei; and (ii) the absolute energy spectra of primary cosmic ray proton and helium nuclei, which differed between experiments by as much as 40%. More recent measurements of the primary cosmic ray spectra are in agreement within 610% on the proton and helium spectra between 10 and 50 GeV [7, 8] . On the other hand, the available accelerator data on the production of pions in proton-nucleus and helium-nucleus collisions are unfortunately limited, and they do not cover relevant ranges of transverse momentum and Feynman variable x. Furthermore, Perkins [9] has pointed out that the Feynman x distribution of produced charged pions that is used in the simulations does not agree with the existing data for low values of x. The Super-Kamiokande data have been compared with the calculation by Honda et al. [3] . For neutrino data below about 1.3 GeV, where the majority of the events are [1], the simulations give 15% less electron events and 36% more muon events than observed. A neutrino energy of 1 GeV on the average corresponds to a muon energy of about 3 GeV. It is essential at this stage to check the other predictions of the simulations by observations. A direct measurement of the flux of muons in the atmosphere, together with the simultaneous measurements of the primary proton and helium spectra [8, 10] , is a powerful method, which provides a direct test of these simulations. We report in this Letter a new measurement on the flux of atmospheric muons by the CAPRICE94 experiment. As a function of atmospheric height, we have measured the spectrum of positive muons in the range 0.3 2 GeV͞c and of negative muons in the range 0031-9007͞99͞82(24)͞4757(4)$15.00
doi:10.1103/physrevlett.82.4757 fatcat:srlsahdi6beh7mpglhu5hupqsi