Estimation of the flux upper limit for a solar energetic proton beam

Associated with solar flare of 14 July 2000, a muon counting rate enhancement from a narrow sky window was observed by a shallow underground muon detector. This muon counting rate excess was very possibly caused by a directional solar proton beam produced in this flare. Starting from an incident solar proton beam at the top of the atmosphere, following with its interaction in atmosphere and propagating of the produced muons down to the underground detector, a full simulation was carried out in order to estimate the corresponding energies of primary protons and to deduce the upper limit on the flux of solar protons. From this work, for solar protons above 40 GeV, under hypothesis of a power law index of -6, the integral flux upper limit is set to be 5.1 x 10(-3)/(cm(2.)sr(.)s) at 90% confidence level.