Astroparticle Physics

7.3 Cosmic Rays Underground 155Fig. 7.23Muon shower in the ALEPHexperiment. Muon tracks are seenin the central time-projectionchamber and in the surroundinghadron calorimeter. Even thoughthere is a strong 1.5 Teslamagnetic field perpendicular to theprojection shown, the muon tracksare almost straight indicating theirhigh momenta. Only a knock-onelectron produced in thetime-projection chamber by amuon is bent on a circle {23}at shallow depths underground of typically several metersexclusively caused by transferred transverse momenta areobtained. Typical multiple-scattering angles for energeticmuons (≈ 100 GeV) in thick layers of rock (50–100 m) areon the order of a few mrad.The multiplicity of produced secondary particles increaseswith energy of the initiating particle (for a 1 TeVproton the charged multiplicity of particles for proton–protoninteractions is about 15). Since the secondaries producedin these interactions decay predominantly into muons, oneobserves bundles of nearly parallel muons underground inthe cores of extensive air showers. Figure 7.23 shows sucha shower with more than 50 parallel muons observed by theALEPH experiment at a depth of 320 m w.e.High-energy muons are produced by high-energy primariesand, in particular, muon showers correlate with evenhigher primary energies. Therefore, one is tempted to localizeextraterrestrial sources of high-energy cosmic rays viathe arrival directions of single or multiple muons. SinceCygnus X3 has been claimed to emit photons with energiesup to 10 16 eV, this astrophysical source also representsan excellent candidate for the acceleration of high-energycharged primary cosmic rays. Cygnus X3 at a distance of approximately33 000 light-years is an X-ray binary consistingof a superdense pulsar and a stellar companion. The materialflowing from the companion into the direction of the pulsarforms an accretion disk around the pulsar. If apparentlyphotons of very high energy can be produced, one wouldexpect them to originate from the π 0 decay (π 0 → γγ).muon bundlesALEPHas cosmic-ray detectorCygnus X3