Astroparticle Physics

7.2 Cosmic Rays at Sea Level 1477.2 Cosmic Rays at Sea Level“The joy of discovery is certainly theliveliest that the mind of man can everfeel.”Claude BernardA measurement of charged particles at sea level clearlyshows that, apart from some protons, muons are the dominantcomponent (Fig. 7.12).Approximately 80% of the charged component of secondarycosmic rays at sea level are muons. Their flux througha horizontal area amounts to roughly one particle per cm 2and minute. These muons originate predominantly frompion decays, since pions as lightest mesons are producedin large numbers in hadron cascades. The muon spectrumat sea level is therefore a direct consequence of the pionsource spectrum. There are, however, several modifications.Figure 7.13 shows the parent pion spectrum at the locationof production in comparison to the observed sea-level muonspectrum. The shape of the muon spectrum agrees relativelywell with the pion spectrum for momenta between 10 and100 GeV/c. For energies below 10 GeV and above 100 GeVthe muon intensity, however, is reduced compared to thepion source spectrum. For low energies the muon decayprobability is increased. A muon of 1 GeV with a Lorentzfactor of γ = E/m µ c 2 = 9.4 has a mean decay length ofFig. 7.12Measurement and identification ofcharged particles at sea level {21}s µ ≈ γτ µ c = 6.2km. (7.1)Since pions are typically produced at altitudes of 15 kmand decay relatively fast (for γ = 10 the decay length is onlys π ≈ γτ π c = 78 m), the decay muons do not reach sea levelbut rather decay themselves or get absorbed in the atmosphere.At high energies the situation is changed. For pionsof 100 GeV (s π = 5.6 km, corresponding to a column densityof 160 g/cm 2 measured from the production altitude)the interaction probability dominates (s π > λ). Pions ofthese energies will therefore produce further, tertiary pionsin subsequent interactions, which will also decay eventuallyinto muons, but providing muons of lower energy. Therefore,the muon spectrum at high energies is always steepercompared to the parent pion spectrum.If muons from inclined horizontal directions are considered,a further aspect has to be taken into account. For largezenith angles the parent particles of muons travel relativelyFig. 7.13Sea-level muon spectrum incomparison to the pion parentsource spectrum at production