2. HIGH ENERGY ASTROPHYSICAL NEUTRINOS

2.1. Production

The purely hadronic (such as pp or pn) and photo hadronic (such as p or n) interactions taking place in cosmos currently represent the main source interactions for the production of high energy astrophysical neutrinos. Examples of the astrophysical sites where these interactions (may) take place include our galaxy, the AGNs and the GRBs. In some model calculations for high energy astrophysical neutrino flux, the proton acceleration mechanism is considered to be the same as for electron acceleration at the astrophysical sites.

The accelerated protons in the above interactions in these sites produce unstable hadrons such as ^± and D_s^± that decay into neutrinos of all three flavors. The same interactions also produce ⁰ that can contribute dominantly towards the observed high energy photons, whereas the escaping accelerated protons may (or may not) dominantly constitute the observed ultra high energy cosmic rays depending upon the finer details of the relevant astrophysical site. The absolute normalization of the high energy astrophysical neutrino flux is obtained by assuming that a certain fraction of the observed high energy photon flux has (purely) hadronic origin and (or) that the observed ultra high energy cosmic ray flux can dominantly originate from that class of astrophysical sites. Typically, the muon neutrino flux is twice the electron neutrino flux with essentially negligible tau neutrino flux at the production site. For a recent review article, see Halzen & Hooper (2002). High energy astrophysical neutrino production is in principle also conceivable in purely electromagnetic (such as ) interactions taking place in cosmos, see Athar & Lin (2002).

The dedicated high energy neutrino detectors provide us a clue as well as a check for the absolute normalization of the high energy neutrino flux. For instance, the present upper bound from Antarctic Muon and Neutrino Detector Array (AMANDA) give value of 9.8 × 10^-6 cm^-2 s^-1 sr^-1 GeV for absolute flux of diffuse high energy neutrinos for the energy range between 5 × 10³ GeV to 3 × 10⁵ GeV, see Ahrens et al. (2002). The AMANDA (B10) is at the south pole and its current upper bound is based on non observation. Its present cylindrical configuration searches for upward going high energy (muon) neutrinos covering the northern hemisphere with an effective area of ~ 0.01 km² for (muon) neutrino energy ~ 10⁴ GeV.

2.2. Propagation

With three light stable neutrinos, as suggested by standard model of particle physics, neutrino flavor mixing is a dominant effect during high energy astrophysical neutrino propagation, once they are produced, see Athar, Jezabek, & Yasuda (2000). Since the average interstellar matter density is rather low, therefore the neutrino nucleon deep inelastic scattering (DIS) effects are usually negligible. These neutrinos thus restore the arrival direction and energy information starting from the production site. On the other hand, because of rather large unobstructed distances traversed by these neutrinos, typically greater than 1 pc, where 1 pc 3 × 10¹⁸ cm, the neutrino flavor mixing equally distribute the three neutrino flavors in the mixed high energy astrophysical neutrino flux. The present neutrino oscillation data implies that the deviations from these symmetric distributions are not more than a few percent. The absolute level of (downward going) high energy astrophysical electron neutrino flux, arriving at the detector, is essentially independent of the relative ratio at the production, as it is least effected by neutrino oscillations, see Athar & Lin (2001).

2.3. Prospects for observations

Typical high energy astrophysical neutrino observation can be achieved by attempting to observe the Cherenkov radiation from the associated charged leptons and/or showers produced in DIS occurring near or inside the detector. For simplicity, here I ignore a possible observational difference between neutrinos and anti neutrinos.

The mixed high energy astrophysical neutrino flux arrives at an earth based detector in three general directions. The downward going neutrinos do not cross any significant earth cord while reaching the detector. In Fig. 1, the downward going event rate for the three neutrino flavors is displayed under the assumption of neutrino flavor mixing in units of yr^-1 sr^-13 volume ice or water detector such as the proposed extension of AMANDA (B10), although as mentioned in the last subsection, the three neutrino flavors arrive at the detector in almost equal proportion. The contained e-like event rate is obtained by rescaling the µ-like event rate for illustration. A diffuse AGN neutrino flux model is used here as an example where pp interactions are considered to play an important role, see Szabo & Protheroe (1994). The event topology for each flavor can possibly be identified separately in km³ volume water or ice detector within the energy range shown in Fig. 1. For details, see Athar, Parente, & Zas (2000).

Figure 1. Expected downward going e-like, µ-like and -like event rate produced by AGN neutrinos as a function of minimum energy of the corresponding charged lepton in a large km3 volume ice or water neutrino detector. Three flavor neutrino mixing is assumed.

The near horizontal neutrino flux crosses a small earth cord before reaching the detector. Several proposals are under study to construct a specific detector for such type of neutrinos, see Hou & Huang (2002). The upward going neutrino flux crosses a significant earth cord before reaching the detector, and is therefore absorbed by the earth to a large extent for energy typically greater than 5 × 10⁴ GeV, see, for instance, Hettlage & Mannheim (2001). Above this energy, the earth diameter exceeds the charged current DIS length. In addition to attempting to measure the Cherenkov radiation, several other alternatives are also presently being explored. See, for instance, Chiba et al. (2001).