## 5 Elemental Composition in the \(10^{14}\) to \(10^{18}\) eV Region  
Several experimental results associate the knee with the bending of the light component (p and He), and are compatible with a rigidity-dependent cut-off [22; 23; 24; 53; 54; 55]. However, the flux of the different components vary significantly depending on the interaction model used to interpret the data [22; 23; 24]. On the contrary, other results (in particular those obtained by arrays located at high altitudes) seem to indicate that the knee of the all-particle energy spectrum is due to heavier nuclei and that the light component cuts off well below 1 PeV [21; 26; 31; 52; 53; 56]. In this section the measurements of the light component energy spectrum in the \(10^{14}\) to \(10^{18}\) eV region will be presented by using different plots to point out the conflicting results between the experiments. In Figure 5 the energy spectra of the light component as measured by Tibet AS\(\gamma\)[25; 31] and ARGO-YBJ [52] are shown. Both experiments are located in the YangBajing Cosmic Ray Laboratory in Tibet (China) at 4300 m a.s.l. and did not exploit a measurement of the muon component to determine the elemental composition of the primary CR flux. The Tibet AS\(\gamma\) Collaboration reconstructed the energy spectrum studying the shower core region with a burst detector as well as with emulsion chambers. The ARGO-YBJ experiment measured the CR energy spectra exploiting completely different and independent approaches [52]:  
* _'Digital-Bayes' analysis_, based on the strip multiplicity, that is, the picture of the EAS provided by the RPC strip/pad system, in the few TeV-300 TeV energy range. The selection of light elements is based on the characteristics of the charged particle lateral distribution [30; 59]. * _'Analog-Bayes' analysis_, based on the RPC charge readout [60], covers the 30 TeV-10 PeV range. The energy is reconstructed (as in the previous analysis), on a statistical basis, by using a bayesian approach. * _'Hybrid measurement'_, carried out by ARGO-YBJ and a wide field of view Cherenkov telescope, a prototype of the LHAASO telescopes, in the 100 TeV-3 PeV region. The selection of (p+He)-originated showers is based on two observables, the shape of the Cherenkov image and the particle density in the core region measured by the ARGO-YBJ central carpet. The energy is reconstructed by the telescope with a resolution better than 20% [56; 61]. All the results are in excellent agreement. In the ARGO-YBJ experiment the selection of (p+He)-originated showers is performed not by means of an unfolding procedure after the measurement of electronic and muonic sizes, but on an event-by-event basis exploiting showers topology, that is, the lateral distribution of charged secondary particles. This approach is made possible by the full coverage of the central carpet, the high segmentation of the read-out and the high altitude location of the experiment that retains the characteristics of showers lateral distribution in the core region. The contamination of nuclei heavier than helium is estimated smaller than 15% at 1 PeV in all analyses. In Figure 5 the direct measurements reported by CREAM [57] and NUCLEON [58] are also shown. ARGO-YBJ is the only experiment that traced the (p+He) spectrum across the knee starting from an energy so low (\(\approx\)TeV) to overlap with direct measurements and to cross-calibrate the fluxes on a wide energy range (5-250 TeV). These results show that, when indirect measurements are capable of selecting almost pure beams, their findings are in fair agreement with direct ones and confirm that current simulation models provide a satisfactory description of the EAS development in the atmosphere.