Several air shower observatories have established that the number of muons produced in UHE air showers is significantly larger than that predicted by models. We argue that the only solution to this muon deficit, compatible with the observed Xmax distributions, is to reduce the transfer of energy from the hadronic shower into the EM shower, by reducing the production or decay of pi0s. We present four different models of new physics, each with a theoretical rationale, which can accomplish this. One has a pure proton composition and three have mixed composition. Two entail new particle physics and suppress pi0 production or decay above LHC energies. The other two are less radical but nonetheless require significant modifications to existing hadron production models -- in one the changes are only above LHC energies and in the other the changes extend to much lower energies. We show that the models have distinctively different predictions for the correlation between the number of muons at ground and Xmax in hybrid events, so that with future hybrid data it should be possible to discriminate between models of new physics and disentangle the particle physics from composition.
Observational evidence from Auger and earlier experiments shows a deficit of signal in a surface detector compared to predictions, which increases as a function of zenith angle, when the energy of the event is fixed by fluorescence measurements. We explore three potential explanations for this: the Cronin effect (growth of high-transverse momentum cross sections with nuclear size), the need for more particles at high transverse momentum in p- p collisions than currently predicted by high energy hadronic models used for air shower simulations, and the possibility that secondary interactions in the target air nucleus produces additional soft pions not included in simulations. We report here on the differences between Pythia and QGSJet II, especially for high Pt particles. The possible impact of these effects on the predicted surface array signal and attenuation with zenith angle are also reported.
SENECA is a hybrid air shower simulation written by H. Drescher that utilizes both Monte Carlo simulation and cascade equations. By using the cascade equations only in the high energy portion of the shower, where the shower is inherently one-dimensional, SENECA is able to utilize the advantages in speed from the cascade equations yet still produce complete, three dimensional particle distributions at ground level which capture the shower to shower variations coming from the early interactions. We present a comparison, on an event by event basis, of SENECA and CORSIKA, a well trusted MC simulation code. By using the same first interaction in both SENECA and CORSIKA, the effect of the cascade equations can be studied within a single shower, rather than averaged over many showers. Our study shows that for showers produced in this manner, SENECA agrees with CORSIKA to a very high accuracy with respect to densities, energies, and timing information for individual species of ground-level particles from both iron and proton primaries with energies between 1 EeV and 100 EeV. Used properly, SENECA produces ground particle distributions virtually indistinguishable from those of CORSIKA in a fraction of the time. For example, for a shower induced by a 10 EeV proton, SENECA is 10 times faster than CORSIKA, with comparable accuracy.
