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The observation of photons with energies above 10¹⁸ eV would open a new window in cosmic-ray research, with possible impact on astrophysics, particle physics, cosmology and fundamental physics. Current and planned air shower experiments, particularly the Pierre Auger Observatory, offer an unprecedented opportunity to search for such photons and to complement efforts of multimessenger observations of the universe. We summarize motivation, achievements, and prospects of the search for ultra-high energy photons.

The energies of cosmic rays, fully ionized charged nuclei, extend over a wide range up to 10²⁰ eV. A particularly interesting energy region spans from 10¹⁴ to 10¹⁸ eV, where the all-particle energy spectrum exhibits two interesting structures, the "knee" and the "second knee". An explanation of these features is thought to be an important step in understanding the origin of the high-energy particles. Recent results of air shower experiments in this region are discussed. Special attention is drawn to explain the principle of air shower measurements — a simple Heitler model of (hadronic) air showers is developed.

The origin of the highest energy cosmic ray showers is a long standing puzzle. In models where the neutrino–proton cross-section rises to typical hadronic values at energies above 10²⁰ eV, the neutrino becomes a candidate for the primary that initiates these showers. Large extra dimension, low scale gravity models suggest just such a strong neutrino interaction possibility. This raises a further question: are the characteristics of observed showers consistent with a neutrino origin? We simulate neutrino induced showers and compare them to proton-induced showers and to data. The comparison includes the study of starting depth, profile with depth, lateral particle distribution at ground and muon lateral distribution at ground level. Our comparison applies to any model of a strongly interacting, neutral particle with an energy transfer per collision of order 10% similar to the neutrino model studied here. We find that for cross-sections above 20 mb there are regions of parameter space where the two types of showers are nearly indistinguishable. We conclude that the neutrino candidate hypothesis cannot presently be ruled out on the basis of shower characteristics.

The ultrahigh energy neutrino cross section is well understood in the standard model for neutrino energies up to 10¹² GeV, Tests of neutrino oscillations (ν_μ ↔ ν_τ) from extragalactic sources of neutrinos are possible with large underground detectors. Measurements of horizontal air shower event rates at neutrino energies above 10¹⁰ GeV will be able to constrain nonstandard model contributions to the neutrino-nucleon cross section, e.g., from mini-black hole production.

The ultrahigh energy neutrino cross section is well understood in the standard model for neutrino energies up to 10¹² GeV. Tests of neutrino oscillations (v_µ ↔ ν_τ) from extragalactic sources of neutrinos are possible with large underground detectors. Measurements of horizontal air shower event rates at neutrino energies above 10¹⁰ GeV will be able to constrain nonstandard model contributions to the neutrino-nucleon cross section, e.g., from mini-black hole production.

A brief description of the research topics addressed by the Hanoi cosmic ray laboratory, VATLY, is given in the form of a progress report with respect to the situation in 2002 which had been presented at the ISPUN02 conference in Halong Bay.

The simulation of air showers in the atmosphere is indispensable for experiments aiming at cosmic rays and gamma-rays well above 100 GeV. Simulations are equally important for the data interpretation, the optimization of reconstruction algorithms and the design of new experiments. Over the last 15 years the quality of simulations has greatly improved, mainly due to better hadronic interaction models and the vast increase in computing power. This article reviews the current status.

The energies of cosmic rays, fully ionized charged nuclei, extend over a wide range up to 10²⁰ eV. A particularly interesting energy region spans from 10¹⁴ to 10¹⁸ eV, where the all-particle energy spectrum exhibits two interesting structures, the ‘knee’ and the ‘second knee’. An explanation of these features is thought to be an important step in understanding of the origin of the high-energy particles. Recent results of air shower experiments in this region are discussed. Special attention is drawn to explain the principle of air shower measurements — a simple Heitler model of (hadronic) air showers is developed.

A new promising development in astroparticle physics is to measure the radio emission from extensive air showers. The particles in the cascade emit synchrotron radiation (30 - 90 MHz) which is detected with arrays of dipole antennas. Recent experimental efforts are discussed.