GRAND End-to-End simulation chain

The development of a simulation chain and of several reconstruction tools dedicated to inclined EAS have enabled to assess the performances of GRAND for UHE neutrino, cosmic ray, and gamma-ray detection. The simulation chain comprises a 3-D Monte-Carlo sampler of tau leptons generated by ντ interactions underground (DANTON, Niess and Martineau-Huynh 2018), a semi-analytical radio- signal fast computation tool (Radio-Morphing, Zilles et al. 2018), and an antenna response module (NEC4). The final step is the detector trigger simulation. Our trigger condition requires ≥ 5 units in one antenna square cell, the peak-to-peak amplitude of the voltage signal at the output of the antennas to be ≥ 30[75] μV (twice the expected stationary background noise in the 50 − 200 MHz frequency range) in the aggressive [conservative] scenario.

Expected neutrino sensitivity

This simulation chain was run over a 10, 000 km 2 area, with 10, 000 antennas deployed along a square grid of 1 km step size in an basin surrounded by high peaks of the TianShan mountain range in China. The 3- and 10-year 90% C.L. GRAND sensitivity limits are scaled from the simulated region. The integrated limits correspond to the Feldman-Cousins upper limit per decade in energy at 90% C.L., assuming a power-law neutrino spectrum ∝ E -2, for no candidate events and ν null background. The 10-year integrated sensitivity limit is of ∼ 10 -10 GeV cm -2 s -1 sr -1 above 5 × 10 17 eV.

The figure on the right-hand side shows the differential and integrated neutrino sensitivity limits calculated from the 10, 000 antennas simulation (“GRAND10k”, pink area) and the extrapolation for the 20-times larger GRAND array (“GRAND200k”, maroon line). The gray region represents the all-flavor cosmogenic neutrinos flux expectations derived from the results of the Pierre Auger Observatory. Adapted from the GRAND White Paper (2018).

Exquisite angular resolution of 0.1 degrees can be obtained with GRAND. From Decoene et al. (2020).

Exquisite angular resolutions and excellent Xmax reconstruction

Novel reconstruction methods performing fits to the strength of the radio signal as a function of the angle from the shower axis (angular distribution function) have demonstrated that angular resolutions of ∼ 0.1◦ could be achieved on the particle arrival direction, rendering neutrino and gamma-ray astronomy possible with GRAND.

Preliminary results obtained on the energy resolution are encouraging, as expected generally for energy reconstruction with radio measurements, that do not depend on hadronic uncertainties. Simulations with a single EAS geometry (zenith angle 80◦ and azimuth 0◦), simple layout and no detector response implemented lead to a 4% energy resolution. Finally, resolutions on Xmax better than 40 g cm 2 were achieved in preliminary studies based on. More refined and optimized methods are being developed to improve the reconstruction of all EAS parameters.

Full sky coverage

For a given sub-array location, the instantaneous field of view of GRAND is a band between zenith angles 85◦≤θz≤95◦, corresponding to <5% of the sky. Assuming that all azimuth angles are observed at any instant, approximately 80% of the sky is observed every day by each sub-array. With 10 − 20 locations spread around the globe, GRAND will offer a full-sky coverage at all times.

GRAND instantaneous FoV (45% sky) 10 random site locations uniformly spaced between 60N and 40S.

Ultra-high-energy cosmic ray detection

For UHECR detection, GRAND will be fully efficient above 1019 eV and sensitive to cosmic rays in a zenith-angle range of 65◦ −85◦, hence it will have a geometrical aperture of 107,000 km 2 sr. Including uncontained events (with shower cores falling outside the instrumented area) and taking trigger conditions into account, ZHAiReS UHECR air-shower simulations indicate that GRAND would have an exposure of 4 − 5 × 107, 000 km 2 sr per year. The Figure on the left-hand-side presents an example of the GRAND exposure to UHECR detection, assuming 10 random locations of sub-arrays of 20, 000 antennas uniformly spaced between geographical latitudes 60N and 40S. An uniform acceptance was assumed over zenith angles of 65◦ − 85◦. A full-sky coverage is obtained with such a configuration.

Ultra-high-energy gamma-ray detection

The aperture of GRAND to UHE gamma rays should be similar to UHECRs. The figure on the right-hand side shows that the sensitivity of GRAND200k to UHE gamma rays is sufficient to detect them even in the pessimistic case where UHECRs are heavy. To compute the preliminary sensitivity of GRAND200k to UHE gamma rays, we assumed that the detector is fully efficient to gamma ray- initiated air showers with energies above 1010 GeV in the zenith range 60◦–85◦. The sensitivity is the Feldman-Cousins upper limit at the 95% C.L., assuming no candidate events, null background, and a UHE gamma-ray spectrum ∝ E -2. The assumption of a background-free search is reasonable in the 10 10–10 10.5 GeV range, even for the conservative hypothesis that GRAND reaches a resolution in Xmax of only 40 g cm 2 resolution.

White papers and presentation proceedings of

Giant Radio Array for Neutrino Detection
science & design