# Input to the European Particle Physics Strategy Update 2018-2020

1 November 2018 to 19 December 2018
Europe/Zurich timezone

## GRAND: Giant Radio Array for Neutrino Detection input for the European Particle Physics Strategy Update 2020

Not scheduled
1m
Neutrino physics (accelerator and non-accelerator)

### Description

The Giant Radio Array for Neutrino Detection (GRAND) is a planned large-scale observatory of ultra-high-energy (UHE) cosmic particles — cosmic rays, gamma rays, and neutrinos with energies exceeding $10^8$ GeV. Its ultimate goal is to solve the long-standing mystery of the origin of UHE cosmic rays. It will do so by detecting an unprecedented number of UHECRs and by looking with unmatched sensitivity for the undiscovered UHE neutrinos and gamma rays associated to them. Three key features of GRAND will make this possible: its large exposure at ultra-high energies, sub-degree angular resolution, and sensitivity to the unique signals made by UHE neutrinos. The Science Case and the Design of GRAND have recently been documented in a White Paper (J. Àlvarez-Muñiz et al., Giant Radio Array for Neutrino Detection: Science and Design, arXiv:1810.09994) that is publicly available and attached to this submission.
The strategy of GRAND is to detect the radio emission coming from large particle showers that develop in the terrestrial atmosphere — extensive air showers — as a result of the interaction of UHE cosmic rays, gamma, rays, and neutrinos. To achieve this, GRAND will be the largest array of radio antennas ever built. The relative affordability of radio antennas makes the scale of construction possible. GRAND will build on years of progress in the field of radio-detection and apply the large body of technological, theoretical, and numerical advances, for the first time, to the radio-detection of air showers initiated by UHE neutrinos.
The design of GRAND will be modular, consisting of several independent sub-arrays, each of 10,000 radio antennas deployed over 10,000 km$^2$ in radio-quiet locations. A staged construction plan ensures that key techniques are progressively validated, while simultaneously achieving important science goals in UHECR physics, radio astronomy, and cosmology early during construction.
Already by 2025, using the first sub-array of 10,000 antennas, GRAND could discover the long-sought cosmogenic neutrinos — produced by interactions of ultra-high-energy cosmic-rays with cosmic photon fields — if their flux is as high as presently allowed, by reaching a sensitivity comparable to planned upgraded versions of existing experiments. By the 2030s, in its final configuration of 20 sub-arrays, GRAND will reach an unparalleled sensitivity to cosmogenic neutrino fluxes of $4\times 10^{–10}$ GeV cm$^{–2}$ s$^{–1}$ sr$^{–1}$ within 3 years of operation, which will guarantee their detection even if their flux is tiny. Because of its sub-degree angular resolution, GRAND will also search for point sources of UHE neutrinos, steady and transient, potentially starting UHE neutrino astronomy. Because of its access to ultra-high energies, GRAND will chart fundamental neutrino physics at these energies for the first time.
GRAND will also be the largest detector of UHE cosmic rays and gamma rays. It will improve UHECR statistics at the highest energies ten-fold within a few years. Many thousands of hadronic interactions with a center-of-mass energy ranging between 15 and 60 times the LHC center-of-mass energy will be collected. GRAND will either discover UHE gamma rays or improve their limits ten-fold. In the case of discovery photon-nucleon interactions at much higher energy than that of the LHC will be measured.
Further, GRAND will be a valuable tool in radio astronomy and cosmology, allowing for the discovery and follow-up of large numbers of radio transients — fast radio bursts, giant radio pulses — and for precise studies of the epoch of reionization.
Following the discovery of high-energy astrophysical neutrinos, gravitational waves, and the multi-wavelength, multi-messenger detection of neutron-star mergers, we stand today at the threshold of a new era in astroparticle physics. Several exciting high-energy astroparticle experiments are planned, both extensions of existing cosmic-ray and neutrino experiments — AugerPrime, TA×4, IceCube-Gen2 — and new experiments — LSST, CTA, LISA. At the ultra-high-energy front, GRAND completes the picture.

The realisation of GRAND will be greatly facilitated by help from the European Particle Physics Community in the following domains:
1. Recognition by the community that ultra-high-energy cosmic ray physics is one of the essential building blocks to understand our universe in terms of particle physics would strongly support our own arguments.
2. GRAND will be a large collaboration and advice of the Particle Physics Community in setting up its governance will be highly appreciated.
3. GRAND will be faced with numerous technical issues that overlap with competences in the Particle Physics community, such as cheap (low power) electronics, cheap strong low mass mechanical constructions, reconstruction and analysis software and computing resources. Help with and advice on resolving these issues will be highly appreciated.
4. If GRAND would become a recognised experiment at CERN it would have access to central repositories for documents and design documentation. It would also get access to a meeting venue that is central in Europe. And it would also facilitate access to the help and advice mentioned in the two previous points.
5. The interpretation of hadronic interactions in the early stages, at the TeV scale, of particle showers from ultra-high-energy tau hadronic decay products and cosmic rays would be greatly helped by the study of intermediate mass (e.g. nitrogen or oxygen) nucleus scattering off each other (N-N) or off protons (p-N) at LHC energy and we encourage realising these experiments at the LHC in its next run after the LS2 shutdown.

### Primary authors

Olivier Martineau-Huynh ( Laboratoire de Physique Nucleaire et de Hautes Energies (LPNHE)) Kumiko Kotera (Sorbonne University, UMR 7095, Institut d'Astrophysique de Paris)