Astroparticle Physics

206 9 The Early Universebaryogenesisleptogenesisbetween matter and antimatter. The universe known to us,however, seems to consist of matter only. The relative abundanceof baryons will now be looked at in detail and thereforethis topic is called the baryon asymmetry of the universe.One has to examine how this asymmetry could evolvefrom a state which initially contained equal amounts of matterand antimatter, a process called baryogenesis. Foreveryproton there is an electron, so the universe also seems tohave a non-zero lepton number. This is a bit more difficultto pin down, however, since the lepton number could in principlebe compensated by unseen antineutrinos. In any case,models of baryogenesis generally incorporate in some waylepton production (leptogenesis) as well.Baryogenesis provides a nice example of the interplaybetween particle physics and cosmology. In the final analysisit will be seen that the Standard Model as it stands cannotexplain the observed baryon asymmetry of the universe.This is a compelling indication that the Standard Model isincomplete.9.5.1 Experimental Evidence of Baryon AsymmetryIf antiparticles were to exist in significant numbers locally,one would see evidence of this from proton–antiprotonor electron–positron annihilation. p ¯p annihilation producestypically several mesons including neutral pions, which de-cay into two photons. So, one would see γ rays in an energyrange up to around 100 MeV. No such gamma rays resultingfrom asteroid impacts on other planets are seen, man-madespace probes landing on Mars survived, and Neil Armstronglanding on the Moon did not annihilate, so one can concludethat the entire solar system is made of matter.One actually finds some antiprotons bombarding theEarth as cosmic rays at a level of around 10 −4 comparedto cosmic-ray protons (see Sect. 6.1), so one may want toleave open the possibility that more distant regions are madeof antimatter. But the observed antiproton rate is compatiblewith production in collisions of ordinary high-energy protonswith interstellar gas or dust through reactions of thetypep ¯p annihilationp + p → 3p +¯p. (9.46)antinuclei in cosmic rays?There is currently no evidence of antinuclei in cosmic rays,although the Alpha Magnetic Spectrometer based on the