Press release of the Pierre Auger Collaboration:
In a paper to be published in Science on 22 September, the Pierre Auger Collaboration reports observational evidence demonstrating that cosmic rays with energies a million times greater than that of the protons accelerated in the Large Hadron Collider come from much further away than from our own Galaxy. Ever since the existence of cosmic rays with individual energies of several Joules1 was established in the 1960s, speculation has raged as to whether such particles are created there or in distant extragalactic objects. The 50 year-old mystery has been solved using cosmic particles of mean energy of 2 Joules recorded with the largest cosmic-ray observatory ever built, the Pierre Auger Observatory in Argentina. It is found that at these energies the rate of arrival of cosmic rays is ~6% greater from one half of the sky than from the opposite one, with the excess lying 120˚ away from the Galactic centre.
In the view of Professor Karl-Heinz Kampert (University of Wuppertal), spokesperson for the Auger Collaboration, which involves over 400 scientists from 18 countries, "We are now considerably closer to solving the mystery of where and how these extraordinary particles are created, a question of great interest to astrophysicists. Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way". Professor Alan Watson (University of Leeds), emeritus spokesperson, considers this result to be "one of the most exciting that we have obtained and one which solves a problem targeted when the Observatory was conceived by Jim Cronin and myself over 25 years ago".
Cosmic rays are the nuclei of elements from hydrogen (the proton) to iron. Above 2 Joules the rate of their arrival at the top of the atmosphere is only about 1 per sq km per year, equivalent to one hitting the area of a football pitch about once per century. Such rare particles are detectable because they create showers of electrons, photons and muons through successive interactions with the nuclei in the atmosphere.
More here. [Thanks to Sean Carroll.]