Phenomenology of High-Energy Neutrinos in Low-Scale Quantum-Gravity Models

Abstract

We show that neutrino telescopes, optimized for detecting neutrinos of TeV to PeV energy, can reveal threshold effects associated with TeV-scale gravity. The signature is an increase with energy of the cross section beyond what is predicted by the Standard Model. The advantage of the method is that the neutrino cross section is measured in an energy region where i) the models are characteristically distinguishable and ii) the Standard Model neutrino cross section can be reliably calculated so that any deviation can be conclusively identified. 04.50.+h, 99.55.Vj, 95.85.Ry, 98.54.Cm, 98.70.Rz Motivated by the absence of a self-consistent theory of quantum gravity and the unresolved hierarchy problem between the electroweak scale (10 2 GeV) and the Planck scale (10 19 GeV), a great deal of attention has been given to theories of low-scale quantum gravity which envision significant quantum gravity effects at an energy scale of the order of M s ∼ 1 TeV [1,2]. In these scenarios, potentially large effects on high energy processes may occur due to the contributions from, e.g., Kaluza-Klein excitations of gravitons (KK) or other stringy states near M s . An interesting motivation for models in which cross sections at TeV scale become enhanced is the ultra-high energy cosmic ray problem. Protons above the GZK cutoff (∼ 10 19 eV) interact with the cosmic microwave background cataclysmically by the ∆-resonance [3,4]. Thus, the cosmic ray events observed above this energy must be produced by local sources, or involve new physics. Local sources of particles of such energy being unlikely, many exotic solutions have been proposed [5]. A solution which has received a great deal of attention in recent literature proposes that neutrinos with enhanced cross sections at GZK energies constitute the highest energy cosmic rays [6][7][8]. This solution requires neutrino-nucleon cross sections on the scale of 10's of mbarnes. Unfortunately, most scenarios of low-scale quantum gravity as low-energy effective theories are valid only up to the order of < ∼ M s . Above this scale, the naive calculations typically violate unitarity [6,7]. One has to introduce some ad hoc unitarization scheme, since the fundamental theory, such as a realistic string theory, is yet unavailable. It is also very difficult to reliably predict the parton distribution functions needed at GZK energies in neutrino-nucleon interactions. For these reasons, studies of ultra-high energy (∼ 10 20 eV) quantum gravity enhancements to neutrinonucleon interactions are extremely speculative.These problems are far more manageable at energies below or near M s . Unitarity may not be violated at this scale, calculations are generally perturbative and the relevant parton distributions are known at these energies [9]. The characteristics for different theoretical models can be also qualitatively distinguishable near and slightly above the threshold. Therefore, the TeV regime provides a natural scale for probing the features of low-scale quantum gravity models. These tests includ...

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“…Recent numerical simulations of radio pulses from both electromagnetic and hadronic showers [14,16,17] are based on this expression. For energies below 10 PeV full simulations are possible [14].…”

Status of the program for microwave detection of cosmic rays at the Pierre Auger observatory

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TeV string state excitation via high energy cosmic neutrinos

“…Enhanced neutrinonucleon cross sections may be an observable signature for theories with a low string scale [10,11,13,14,21]. Very high energy neutrinos are predicted to be generated in a variety of astrophysical sources.…”

“…Due to the large gluon luminosity at high energies, including the gluons in the total cross section increases the final result by about a factor of 5 − 10 for E ν ∼ 10 5 − 10 10 TeV. As commented in the introduction, graviton exchange in the large extra dimension scenario is the leading effect below M S [4,14]. However, black hole production far above M S may be dominant [6,7].…”