Ultra-high energy cosmic rays and new physics

GeV pose a conundrum, the solution of which requires eitherdrastic revision of our astrophysical understanding, or new physics beyondthe Standard Model. Nucleons of such energies must originate within the lo-cal supercluster in order to avoid excessive energy losses through photopionproduction on the cosmic microwave background. However they do not pointback towards possible nearby sources, e.g. the active galaxy Cen A or M87 inthe Virgo cluster, so such an astrophysical origin requires intergalactic mag-netic fields to be a hundred times stronger than previously believed, in orderto isotropise their arrival directions. Alternatively the primaries may be highenergy neutrinos, say from distant gamma-ray bursts, which annihilate on thelocal relic background neutrinos to create “Z-bursts”. A related possibilityis that the primary neutinos may initiate the observed air showers directly iftheir interaction cross-sections are boosted to hadronic strength through non-perturbative physics such as TeV-scale quantum gravity. Or the primariesmay instead be new strongly interacting neutral particles with a longer meanfree path than nucleons, coming perhaps from distant BL-Lac objects or FR-IIradio galaxies. Yet another possibility is that Lorentz invariance is violatedat high energies thus suppressing the energy loss processes altogether. Theidea that has perhaps been studied in most detail is that such cosmic raysoriginate from the decays of massive relic particles (“wimpzillas”) clusteredas dark matter in the galactic halo. All these hypotheses will soon be criti-cally tested by the Pierre Auger Observatory, presently under construction inArgentina, and by proposed satellite experiments such as EUSO.PRESENTED AT