Science Case

Introduction
Neutrino Astronomy
Cosmic Rays and Photons
Neutrino Physics
Radio Astronomy
Cosmic Dawn

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.
Sensitivity of GRAND and its first 10k hotspot, for 3 neutrino flavors, over 3 years. Horizontal line: 90% confidence level detection limit for 10 years. Yellow: IceCube astrophysical flux. Blue: projected sensitivity of ARA-37. The gray band spans possible cosmogenic neutrino fluxes, and possible neutrino fluxes produced at the source.
  • 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 ~1017-20 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.
GRAND field of view for 3 years.

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 1017-18 eV.
Predicted cosmogenic UHE photon flux by primary proton and iron UHECRs, as estimated in Sarkar, Kampert & Kulbartz, ICRC 2011. For comparison, we include the existing upper limits from Auger and the Telescope Array (TA), the projected reach of Auger by 2025, the projected reach of GRAND after 3 years of operation.

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., ∼ κn En L, where the energy dependence n and the proportionality constant κn 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 κn ∼ 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.


Left: neutrino flavors at source. Right: flavors after neutrino oscillation during propagation. fτ,⊕ outside [0.30,0.35] could imply new physics.

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?
Simulated FRB with a flat spectrum of 100 Jy, intrinsic duration of 5 ms, and dispersion measure of 500 pc cm-3, after being dispersed and scattered by propagation and detected with a resolution of 10 ms and 25 kHz. After propagation and instrumental effects, the flux density varies from 0 to 3.33 Jy. The Galactic background noise is not shown since its power would largely dominate the signal. The FRB dispersive drift lasts for ~ 185 s (370 s for DM = 1000 pc cm-3). It occurs at an arbitraty time t = 23 s in our simulations.
Result of a blind search for FRBs.For each trial DM value, the dynamic spectrum is de-dispersed and integrated in frequency, and the resulting intensity profile is normalized by its standard deviation after subtracting its mean, i.e., it is displayed as a signal-to-noise ratio (SNR). For any trial DM, the SNR is small, except near t = 23 s, where it rapidly increases to reach a maximum value of ~ 46 for the a DM of 500 pc cm-3. GRAND would detect that FRB with that SNR.

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.