Cosmic Rays Physics
Important questions in cosmic ray physics
It has been more than one hundred years since the first discovery of cosmic rays (CRs) in 1910’s. Nowadays, the study of CRs has entered another golden era, due mainly to their connection with 1) physics at the highest energy end, 2) indirect search for particle dark matter, and 3) site and mechanism of astrophysical accelerators.
For the analysis of the CR flux, direct measurements carried on space or stratospheric balloons actually give the best performance in terms of both energy resolution and charge identification. However, due to their limited acceptance and the steeply falling CR fluxes, they could hardly reach, up to now, energies of hundreds of TeV and they did not yet give clear information on the steepening of various elements nor on the knee of each of the species or even the all-particle spectrum itself.
Current data suggest a hardening of the spectra above about 0.2 TeV/nucleon and spectral indices γ (above that energy) of about ?2.6 for all considered elements but for protons, the latter show a softer spectrum with γ ≌ 2.7. On the other hand, indirect measurements explore the relevant energy region through the detection of Extensive Air Showers (EAS) in the atmosphere, but with unavoidable large systematics (and worse resolutions) due to detector sampling, shower-to-shower fluctuations and our limited knowledge of hadronic interactions at these energies, mainly in the very forward region. Moreover they cannot reach full efficiency for all nuclear species below few hundred TeV, even at high altitude locations. For light elements their threshold can be lowered to a few TeV, showing interesting hints for a steepening of the proton and helium spectra just below 1 PeV.
It is then mandatory to explore the sub-PeV region with high precision direct measurement in order to study the energy spectra of each of these individual nuclear species, to measure the various spectral indices, to detect any possible hardening and to set mass composition below the knee of the all-particle flux. If possible the detection of the steepening of each of these single species and then the explanation of the all-particle knee would be a crucial result in understanding Galactic CR physics, also serving as fundamental input to the study of the extra-galactic component.
Potential of HERD
The unique design of HERD together with the unprecedented depth of the calorime- ter and the high resolution tracker, will allow the extension of high precision measurements on proton and nuclei spectra up to very high energies, even beyond 1 PeV. Moreover a clear identification of each of the nuclear species will be possible through the plastic scintillator detector, the analog readout of the silicon-tungsten tracker and by the calorimeter itself. Energy resolution for the electromagnetic and hadronic showers will be at the 1% and 30% level, respectively.
HERD will be capable of studying features in the spectra of various nuclei, like single element spectral indices and spectral hardenings/steepenings from hundreds GeV/nucleon up to hundreds TeV/nucleon. In particular the proton and helium component could be measured up to PeV energies, thus giving the possibility for the first direct evidence of the knee of the light component due to the reach of maximum energy provided by the source. This result would be fundamental for understanding Galactic CR acceleration/propagation processe.
