Ultra-high energy cosmic rays and new physics
Subir Sarkar2002
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Abstract
Cosmic rays with energies beyond the Greisen-Zatsepin-Kuzmin 'cutoff' at ∼ 4 × 10 10 GeV pose a conundrum, the solution of which requires either drastic revision of our astrophysical understanding, or new physics beyond the Standard Model. Nucleons of such energies must originate within the local supercluster in order to avoid excessive energy losses through photopion production on the cosmic microwave background. However they do not point back towards possible nearby sources, e.g. the active galaxy Cen A or M87 in the Virgo cluster, so such an astrophysical origin requires intergalactic magnetic fields to be a hundred times stronger than previously believed, in order to isotropise their arrival directions. Alternatively the primaries may be high energy neutrinos, say from distant gamma-ray bursts, which annihilate on the local relic background neutrinos to create "Z-bursts". A related possibility is that the primary neutinos may initiate the observed air showers directly if their interaction cross-sections are boosted to hadronic strength through nonperturbative physics such as TeV-scale quantum gravity. Or the primaries may instead be new strongly interacting neutral particles with a longer mean free path than nucleons, coming perhaps from distant BL-Lac objects or FR-II radio galaxies. Yet another possibility is that Lorentz invariance is violated at high energies thus suppressing the energy loss processes altogether. The idea that has perhaps been studied in most detail is that such cosmic rays originate from the decays of massive relic particles ("wimpzillas") clustered as dark matter in the galactic halo. All these hypotheses will soon be critically tested by the Pierre Auger Observatory, presently under construction in Argentina, and by proposed satellite experiments such as EUSO.
Key takeaways
- As late as 1993 it was stated that the GZK cutoff is indeed present in the data, confirming the expectation that the sources of UHECRs are distant FR-II radio galaxies [12].
- Finally, with regard to the composition of UHECRs, new data has muddled the picture suggested earlier by Fly's Eye and HiRes [15,16] of a change from heavy nucleus ("iron") domination at 3 × 10 8 GeV to nucleon domination at 10 10 GeV.
- The lack of a GZK cutoff, which requires the sources to be relatively nearby, coupled with the isotropy, which implies a cosmologically distant population, pose severe problems for any astrophysical expalanation of UHECRs [37].
- (In principle, high energy neutrinos may be produced through the decays of super-massive relic particles [49] but such decays ought to also produce UHECRs directly as we discuss below, making this mechanism of less interest.)
- As mentioned earlier, the possible existence of relic metastable massive particles whose decays can create high energy cosmic rays and neutrinos had been discussed [18,20,21,22] before the famous Fly's Eye event [13] which focussed attention on the enigma of UHECRs.
