NEUTRINO-INDUCED AND ATMOSPHERIC SINGLE-MUON FLUXES MEASURED OVER 5 DECADES OF INTENSITY BY LVD AT GRAN-SASSO-LABORATORY

We report data taken by the LVD Experiment during a live-time period of 11 556 h. We have measured the muon intensity at slant depths of standard rock from about 3000 hg/cm(2) to about 20 000 hg/cm(2). This is an exclusive study, namely our data include only events containing single muons. This interval of slant depth extends into the region where the dominant source of underground muons seen by LVD is the interaction of atmospheric neutrinos with the rock surrounding LVD. The interesting result is that this flux is independent of slant depth beyond a slant depth of about 14 000 hg/cm(2) of standard rock. Due to the unique topology of the Gran Sasso Laboratory the muons beyond about 14 000 hg/cm(2) of standard rock are al a zenithal angle near 90 degrees. Hence we have, for this fixed angle, a muon flux which is independent of slant depth. This is direct evidence that this flux is due to atmospheric neutrinos interacting in the rock surrounding LVD. The value of this flux near 90 degrees is (8.3 +/- 2.6) x 10(-13) cm(-2) s(-1) sr(-1), which is the first reported measurement at a zenithal angle near 90 degrees and for slant depths between 14 000 and 20 000 hg/cm(2). Our data cover over five decades of vertical intensity, and can be fit with just three parameters over the full range of our experiment. This is the first time a single experiment reports the parameters of a fit made to the vertical intensity over such a large range of standard rock slant depth. The results are compared with a Monte Carlo simulation which has as one of the two free parameters gamma(pi k) the power index of the differential energy spectrum of the pions and kaons in the atmosphere. This comparison yields a value of 2.75 +/- 0.03 for gamma(pi k), where the error includes the systematic uncertainties. Our data are compared to other measurements made in our slant depth interval. We also report the value of the muon flux in Gran Sasso at theta = 90 degrees as a function of the azimuthal angle.