The current status of searches for ultra-high energy neutrinos and photons using air showers is reviewed. Regarding both physics and observational aspects, possible future research directions are indicated.
The experimental technique of fluorescence light observation is used in current and planned air shower experiments that aim at understanding the origin of ultra-high energy cosmic rays. In the fluorescence technique, the geometry of the shower is rec
onstructed from the correlation between arrival time and incident angle of the signals detected by the telescope. The calculation of the expected light arrival time used so far in shower reconstruction codes is based on several assumptions. Particularly, it is assumed that fluorescence photons are produced instantaneously during the passage of the shower front and that the fluorescence photons propagate on a straight line with vacuum speed of light towards the telescope. We investigate the validity of these assumptions, how to correct them, and the impact on reconstruction parameters when adopting realistic conditions. Depending on the relative orientation of the shower to the telescope, corrections can reach 100 ns in expected light arrival time, 0.1 deg in arrival direction and 5 g/cm^2 in depth of shower maximum. The findings are relevant also for the case of hybrid observations where the shower is registered simultaneously by fluorescence and surface detectors.
The experimental technique of fluorescence light observation is used in current and planned air shower experiments that aim at understanding the origin of ultra-high energy cosmic rays. In the fluorescence technique, the geometry of the shower is rec
onstructed based on the correlation between viewing angle and arrival time of the signals detected by the telescope. The signals are compared to those expected for different shower geometries and the best-fit geometry is determined. The calculation of the expected signals is usually based on a relatively simple function which is motivated by basic geometrical considerations. This function is based on certain assumptions on the processes of light emission and propagation through the atmosphere. For instance, the fluorescence light is assumed to propagate with vacuum speed of light. We investigate the validity of these assumptions and provide corrections that can be used in the geometry reconstruction. The impact on reconstruction parameters is studied. The results are also relevant for hybrid observations where the shower is registered simultaneously by fluorescence and surface detectors.
