Number 392 (Story #1), September 23, 1998 by Phillip F. Schewe and Ben Stein|
DO COSMIC RAYS COME FROM QUASARS? Cosmic ray particles, which crash into Earth's atmosphere setting up huge showers of particles detected on the ground, have mysterious origins. Looking at the five most energetic events ever recorded (energies above 1020 eV), Glennys Farrar of NYU (email@example.com) and Peter Bierman of the Max Planck Institute for Radio Astronomy in Bonn have found that all the events are consistent with the cosmic rays having originated in radio-loud quasars with redshifts in the range 0.3-2.2, and propagating undeflected and unattenuated in energy through the intervening thousands of Mpc (Farrar et al., Physical Review Letters, 19 October 1998). If the particles (conventionally assumed to be protons) are indeed coming from a great distance then how do they evade the Greisen-Zatsepin-Kuzmin (GZK) cutoff, according to which cosmic rays with energies above about 1019.5 eV would be sapped of their energy through interactions with cosmic microwave background photons if they traveled much more than 20 Mpc (roughly 60 million light years)? Such interactions would typically produce pions and electron-positron pairs. Some speculate that the high-energy particles are not protons at all but some exotic new particle. One explanation is that energetic neutrinos make the long cosmic journey and then annihilate relatively near the Earth with massive dark-matter neutrinos to create the cosmic ray primary particle. Farrar herself is partial to the notion that the primary is the neutral S particle, an amalgam of three quarks and a gluino (one of the shadow particles associated with supersymmetry theory; see Updates 86 and 265). With a mass two or three times that of the proton, the S would not as readily produce pions in interactions with microwave photons, thus ensuring for itself a more robust passage through the cosmos. Thus the highest energy cosmic rays could be produced at cosmological distances but still survive the trip to Earth. (Physics Today, October 1998.) Future measurements determining whether the primary is a photon or hadron will help decide the question of whether the correlation between cosmic rays and quasars holds up.