Science Case

Ultra-high-energy cosmic rays (UHECRs) — extraterrestrial charged particles with the highest energies detected — have been observed for more than fifty years, yet their origin is unknown. They are purportedly made in powerful cosmic accelerators, though none has been identified. Thus, discovering the sources of UHECRs is fundamental to understanding the high-energy Universe.

 

Neutrino physics is booming with 2 recent Nobel prizes and the first detection of neutrinos above 10¹² eV with the IceCube experiment. On the other hand, ultra-high-energy neutrinos remain unchartered territory. Their existence is guaranteed as they are bound to be produced by the interactions of UHECR with the cosmic backgrounds, on their way from their sources to the Earth. Neutrinos should also be produced directly at the sources. Because they are produced with 5% of their parent cosmic-ray energy and travel undeflected by cosmic magnetic fields, neutrinos with E > 10¹⁷ eV are unique messengers to identify the sources of UHECRs.

 

The recent observation of neutron-star merger GW170814 has brilliantly shown that the challenges of high-energy astronomy will be solved by combining data from a large number of multi-messenger experiments. Ultra-high-energy neutrino astronomy will be central to the quest of understanding the violent Universe in this new multi-messenger era.

 

 

 

Ultra-High-Energy Neutrino Astronomy

• GRAND will have unrivaled sensitivity to diffuse ultra-high-energy neutrino fluxes, down to ∼10⁻¹⁰ GeV cm⁻² s⁻¹ sr⁻¹ at energy 10¹⁸ eV. This will likely ensure the detection of "cosmogenic" neutrinos that are produced during the propagation of ultra-high-energy cosmic rays in the most common scenarios.

• For a diffuse neutrino flux of 10⁻⁸ GeV cm⁻² s⁻¹ sr⁻¹, as is predicted by diverse astrophysical source models, GRAND could collect > 100s of events in 3 years.
• With 200,000 km² at one location in Western China, the integred exposure over 3 years is ~10¹⁶ cm² s in the energy range ~10^17-20 eV. GRAND cover 80% of the sky every 24 hours.
• The ideal way to identify high-energy neutrino sources would be to observe point sources. GRAND opens this possibility with its excellent angular resolution and sky coverage. GRAND would kick-start ultra-high-energy neutrino astronomy.

Ultra-High-Energy Cosmic Rays and Photons

• GRAND will observe ultra-high-energy cosmic rays with an effective area > 10x larger than the Auger Observatory. The high statistics and reconstruction performances will resolve small-scale anisotropies, the chemical composition and features near the end of the cosmic ray spectrum.
• GRAND will also reach an UHE photon sensitivity exceeding that of current experiments.
• By the 2020s, GRANDproto300 can already probe the transition between the Galactic and extra-galactic cosmic-ray components, by measuring accurately the chemical composition in the energy range 10^17-18 eV.

Fundamental Neutrino Physics

By detecting neutrinos above 10¹⁸ eV, GRAND will probe fundamental particle physics at energies that are orders of magnitude larger than in particle accelerators, allowing for stringent tests of the Standard Model and potential discovery of new physics. Astrophysical and cosmogenic neutrinos provide a chance to test fundamental physics in new regimes, on account of their being unparalleled in two key aspects:

• The highest energies: PeV–EeV neutrinos can test particle interactions at energies far beyond the reach of man-made neutrinos. Many new‑physics effects are expected to grow with energy, so PeV–EeV neutrinos could probe new physics at these scales.
• The longest baselines: with baselines between megaparsecs and a few gigaparsecs — the size of the observable Universe — even tiny new‑physics effects could accumulate during propagation and reach detectable levels.

Numerous new-physics models have effects whose intensities are proportional to some power of the neutrino energy E and to the source‑detector baseline L, i.e., ∼ κ_n Eⁿ L, where the energy dependence n and the proportionality constant κ_n are model-dependent. For instance, for neutrino decay, n = −1; for CPT-odd Lorentz violation or coupling to a torsion field, n = 0; and for CPT-even Lorentz violation or violation of the equivalence principle, n = 1. If GRAND were to detect neutrinos of energy E coming from sources located at a distance L then, nominally, it could probe new physics with exquisite sensitivities of κ_n ∼ 4·10⁻⁵⁰(E/EeV)⁻ⁿ(L/Gpc)⁻¹ EeV¹⁻ⁿ. This is an enormous improvement over current limits of κ₀ ≲10⁻³² EeV and κ₁ ≲ 10⁻³³, obtained with atmospheric and solar neutrinos. This holds even if the diffuse neutrino flux is used instead, since most of the contributing sources should anyway lie at distances of Gpc.

References:
[1] J. S. Diaz and A. Kostelecky, Phys. Rev. D 85, 016013 (2012), arXiv:1108.1799 [hep-ph].
[2] A. Kostelecky and M. Mewes, Phys. Rev. D 85, 096005 (2012), arXiv:1112.6395 [hep-ph].
[3] R. Abbasi et al. (IceCube), Phys. Rev. D 82, 112003 (2010), arXiv:1010.4096 [astro-ph.HE].
[4] K. Abe et al. (Super-Kamiokande), Phys. Rev. D 91, 052003 (2015), arXiv:1410.4267 [hep-ex].
[5] P. Baerwald, M. Bustamante, and W. Winter, JCAP 1210, 020 (2012), arXiv:1208.4600 [astro-ph.CO].

Bursting Radio Astronomy

GRAND could detect Fast Radio Bursts (FRBs) by incoherently adding the signals from individual antennas. This method allows to infer the dispersion measure, though it does not allow to locate the FRBs. This approach could unveil different categories of events, in nearby galaxies or at cosmological distances, unique, repeating, chaotic, or regular events, all making FRB-like signatures. With potential orders of magnitude more FRBs detected than the currently available sample, GRAND could bring important clues in answering a number of fundamental questions including:

• what is the space density of FRBs in the local Universe?
• how do their radio spectra evolve in the low-frequency range?
• whether the spectra at low frequencies is as dispersed as at high frequencies?
• whether there is a low-frequency cut-off in the spectrum?
• and, how common FRB repeaters are?

Cosmic Dawn and Epoch of Reionization

GRAND will also strongly contribute to other science topics such as the measurement of the global signature of the Cosmic Dawn and of the Epoch of Reionization.