Astroparticle Physics

13.2 Motivation for Dark Matter 275If light neutrinos would fill up the galactic halo, onewould expect a narrow absorption line in the spectrum ofhigh-energy neutrinos arriving at Earth. The observation ofsuch an absorption line would be a direct proof of the existenceof neutrino halos. Furthermore, one could directly inferthe neutrino mass from the energetic position of this line.For a neutrino mass of 10 eV the position of the absorptionline can be calculated from, see (3.16), ν +¯ν → Z 0 → tohadrons or leptons,to be2m ν E ν = M 2 Z (13.19)E ν = M2 Z= 4.2 × 10 20 eV . (13.20)2m νThe verification of such an absorption line, which wouldresult in a burst of hadrons or leptons from Z decay (‘Zbursts’) represents a substantial experimental challenge. Thefact that recent fits to cosmological data indicate that thecontribution of neutrino masses to the total matter density inthe universe is rather small (Ω ν ≤ 1.5%), makes the observationof such an absorption line rather unlikely.neutrino absorption lineneutrino halosZ bursts13.2.3 Weakly Interacting Massive Particles (WIMPs)“In questions like this, truth is only to behad by laying together many variationsof error.”Virginia WolfBaryonic matter and neutrino masses are insufficient to closethe universe. A search for further candidates of dark mattermust concentrate on particles, which are only subjectto weak interactions apart from gravitation, otherwise onewould have found them already. There are various scenarioswhich allow the existence of weakly interacting massiveparticles (WIMPs). In principle, one could consider afourth generation of leptons with heavy neutrinos. However,the LEP 5 measurements of the Z width have shown that thenumber of light neutrinos is exactly three (see Chap. 2, Fig.2.1, and Sect. 10.7) so that for a possible fourth generationthe mass limitm νx ≥ m(Z)/2 ≈ 46 GeV (13.21)fourth generation?heavy neutrinos5 LEP – Large Electron–Positron Collider at CERN in Geneva