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.
Introduction1 What is the status of searches for ultra-high energy neutrinos and photons using air showers? What 2 might be the future prospects, in particular in the next couple of years? What is (are) the physics 3 case(s) for multimessenger observations, and what are the observational experiences and challenges? 4 These questions may summarize the main objective of the Multimessenger Working Group that was 5 formed, together with four other Working Groups, a few months before the UHECR-2012 sympo-6 sium. At this symposium, possible future directions of the field of ultra-high energy cosmic rays were 7 discussed, bringing the major collaborations from air-shower experiments as well as colleagues from 8 theory together.
9Given this objective, one can think of many issues to inspect. In particular, one can compare neutri-10 nos versus photons; neutrinos and photons versus charged cosmic rays; air shower observations versus 11 other techniques; shower observations from ground versus those from space; ground shower tech-12 niques versus each other; current data versus models; various models versus each other; and, above 13 all, the present status versus future directions. Given these many aspects in a highly dynamic field, it 14 is evident this review does not claim to be complete, or even finished. Rather, certain considerations 15 of possible relevance to the aim of the symposium are compiled and highlighted. 16 We start with both neutrinos and photons before each one is examined individually. 17 2 UHE Neutrinos and photons 18 Multimessenger observations are a key ingredient for discovering and for better understanding various 19 phenomena in the Universe. Boosted by the invention of the telescope about 400 years ago, photon 20observations cover now an impressive energy range from radio wavelengths up to about 100 TeV. The 21 discovery of (charged) cosmic rays dates back (at the time of writing these proceedings quite precisely) 22 100 years ago, with cosmic rays being measured now from sub-GeV to more than 100 EeV in energy.
23First non-terrestrial neutrinos were observed about 40 years ago, in the MeV energy range. 24 Various efforts are underway for opening new observational windows to the Universe and for 25 deeper observations of already accessible energy regimes of the different particle types. Examples of 26 significant discoveries in new windows are the cosmic microwave background or gamma-ray bursts 27 arXiv:1306.4199v1 [astro-ph.HE] 18 Jun 2013 EPJ Web of Conferencesfor the case of photon messengers, the muon for the case of cosmic-ray messengers via the air show-28 ers they produce, or the discovery of neutrinos from SN 1987A in the case of neutrino messengers.
29Two things can be seen from this small list. Firstly, some of the discoveries were not expected or 30 predicted beforehand; they gave t...
