Letter: Cosmic Rays and Large Extra Dimensions

Abstract

We have proposed that the cosmic ray spectrum “knee”, the steepening of the cosmic ray spectrum at energy E ≳ 10^15.5 eV, is due to “new physics”, namely new interactions at TeV cm energies which produce particles undetected by the experimental apparatus. In this letter we examine specifically the possibility that this interaction is low scale gravity. We consider that the graviton propagates, besides the usual four dimensions, into an additional δ, compactified, large dimensions and we estimate the graviton production in pp collisions in the high energy approximation where graviton emission is factorized. We find that the cross section for graviton production rises as fast as ($\sqrt s$/M _f)^2+δ, where M _f is the fundamental scale of gravity in 4 + δ dimensions, and that the distribution of radiating a fraction y of the initial particle's energy into gravitational energy (which goes undetected) behaves as δ y ^{δ − 1}. The missing energy leads to an underestimate of the true energy and generates a break in the inferred cosmic ray spectrum (the “knee”). By fitting the cosmic ray spectrum data we deduce that the favorite values for the parameters of the theory are M _f ∼ 8 TeV and δ = 4.