## 1 Introduction
Protons will cutoff first, followed by other nuclei according to the relation  
\[E_{\rm max}(Z)=Z\times E_{\rm max}(Z=1) \tag{1}\]  
If the dominant primary mass of the knee is light (protons and helium), then, according to this scheme, the Galactic CR spectrum is expected to end around 10\({}^{17}\) eV with iron. The sum of the fluxes of all elements, with their individual knees at energies proportional to the  
Figure 1: All–particle energy spectrum of primary cosmic rays, the flux is multiplied by E\({}^{3}\). Results from direct and indirect experiments updated to the year 2021 are shown. nuclear charge, makes up the CR all-particle spectrum shown in Figure 1. With increasing energies, not only does the spectrum become steeper due to such cutoffs, but also heavier. In this scenario, the knee would represent the end of the spectrum of CR accelerated by SNRs in the galaxy. Indeed, it is widely believed that the bulk of CRs up to about \(10^{17}\) eV are galactic, produced and accelerated by the shock waves of SuperNova Remnants (SNR) expanding shells [9], and that the transition to extra-galactic CRs occurs somewhere between \(10^{17}\) and \(10^{19}\) eV. The experimental results, however, do not demonstrate the capability of SNRs to produce the power needed to sustain the population of galactic CRs and to accelerate particles up to the knee, and beyond. Indeed, to accelerate protons up to the PeV energy domain, a significant amplification of the magnetic field at the shock is required, but this process is problematic [10]. Unlike neutrinos that are produced only in hadronic interactions of CRs, the question whether the observed \(\gamma\)-rays are produced by the decay of \(\pi^{0}\) from CR interactions (_'hadronic'_ mechanism), or by a population of relativistic electrons via Inverse Compton scattering or bremsstrahlung (_'leptonic'_ mechanism), still needs a conclusive answer. In a hadronic interaction, the secondary photons have, on average, an energy factor of 10, lower than the primary proton. Therefore, the quest for CR sources to be able to accelerate particles up to the PeV range in a multi-messenger approach requires the observation of the \(\gamma\)-ray sky above 100 TeV. However, the first results reported by the LHAASO experiment [11; 12], that is, the observation of a number of gamma sources emitting photons beyond 500 TeV, show that SNRs are likely not the main sources of PeV CRs in our galaxy. In fact, none of the 12 observed ultra-high energy gamma sources can be clearly described with hadronic mechanisms operating in SNRs. We note that the highest photon emission at 1.4 PeV comes from a system of massive stars in the Cygnus Region, the so-called _'Cygnus Cocoon'_, a possible factory of fresh CRs, as suggested by other experiments [13; 14]. In this note, we will focus on galactic CRs in the PeV energy range detected from ground with air shower arrays. This is not a place for a complete review of CR physics and models (for which we recommend, for instance, [2; 15; 16; 17; 18; 19] and the references therein), but only to provide elements useful to understand the main techniques used in reconstructing primary particle characteristics from the ground with particle arrays, and to show why indirect measurements are difficult and the results are still conflicting. In the next section, we will introduce the detection techniques. In Section 3 we will describe the main characteristics of Extensive Air Showers to understand how different observables measured by arrays are related to the properties of the primary CRs. In Section 4, we will discuss the general scheme of the air shower array analysis. In Section 5 the experimental results in the \(10^{14}\)-\(10^{18}\) eV energy range are summarized. The prospects for new measurements in the knee region are introduced in Section 6.