New Tests of Low-Scale Quantum Gravity with Cosmic-Ray Collisions

Abstract

Cosmic ray collisions at high center of mass energy could enable graviton and black hole production as expected in theories of low-scale quantum gravity, such as extra-dimensions, many species, or some versions of string theory. Here we propose three novel phenomenological tests of these theories. We first consider the collision of cosmic rays with ambient protons, electrons and photons in Active Galactic Nuclei (AGN), finding that high-energy neutrino data from the blazar TXS 0506+056 places a constraint on the fundamental scale of gravity of $M_f \gtrsim 0.3$ TeV, and future high-energy neutrino data could raise this bound to $M_f \gtrsim 200$ TeV. We then point out that collisions of pairs of cosmic rays could occur at a sizable rate in AGN where the accelerated cosmic rays are not collimated, or on supermassive black hole binaries. This consideration could potentially let us test unprecedented large fundamental scales of $M_f \gtrsim 2$ PeV. We further compute the corresponding thermal neutrino emission arising from the Hawking evaporation of black holes produced in cosmic ray collisions, finding a spectrum that clearly differs from that expected in meson decays. Finally, we speculate with an scenario which would produce high-energy neutrino and gamma-ray emission from regions in the sky where no multi-wavelength counterparts would be expected, via graviton propagation from a different brane, which then decays in our Universe.

Figures (5)

• Figure 1: Timescales for Graviton and Black Hole production from cosmic ray processes in TXS 0506+056, for a new fundamental scale of $M_{f}=1$ TeV and $n=2$. The cut-offs in the timescales corresponds to the limit where the EFT description of graviton processes becomes invalid and black hole production begins to occur, at $\sqrt{s}=M_{f}$. This cut-off is different depending on the process considered. We confront these timescales with the inferred dynamical timescale of cosmic ray protons from TXS 0506+056, and the timescale from inverse compton scattering of cosmic ray electrons. This serves as a benchmark for the leading Standard Model processes cooling cosmic rays in TXS 0506+056. We further show vertical shaded bands indicating the energies at which we have experimental evidence from cosmic ray electron and proton acceleration. We also highlight the region where cosmic rays would be accelerated somewhat beyond the GZK limit, to indicate the maximum cosmic ray energies plausibly achieved in a source of this characteristics. This will be used to set projected constraints (see main text for details).
• Figure 2: Timescales for black hole production via cosmic ray collisions with ambient protons (solid) and off other cosmic rays (dashed), for $n=2$. The different colors correspond to different scales of low-scale gravity $M_{f}$. For low-energy ambient protons, a number density of $n_p=10^{7}$cm$^{-3}$ is assumed. For the cosmic ray-cosmic ray collisions, a magnetic field of $B=100$ kG and acceleration efficiency of $\eta=1$ is taken. For comparison, we show the age of the Universe, which indicates a conservative upper limit on the maximum timescale that could conceivably lead to observable signatures.
• Figure 3: Upper limits on the fundamental scale of gravity $M_f$, or number of species N, versus the parameter $n$, identified with the number of extra-dimensions in some theories. For comparison, we also show upper limits from graviton production in SN1987A Hanhart:2001fx and cosmological bounds from graviton decay contributions to the diffuse gamma-ray flux. It has been discussed that these bounds are relaxed if gravitons can also decay on other branes Hall:1999mk. We also show collider bounds from ATLAS:2021kxvCMS:2017zts.
• Figure 4: High-energy neutrino flux originated from the evaporation of black holes produced in $pp$ collisions at TXS 0506+056, for different values of the fundamental scale of gravity $M_f$, and fixed number of extra-dimensions $n=2$. For comparison, we show as a blue band the TXS 0506+056 high-energy neutrino measurement from the IceCube collaboration IceCube:2018cha.
• Figure 5: Schematic of the possibility that cosmic-ray collisions in an AGN of a different brane produced gravitons that propagate to our brane, potentially yielding high-energy neutrino/photon emission from regions of the sky with no multi-wavelength counterparts.