  The observation of earth skimming neutrinos has been proposed as a rather
sensitive method to detect ultra-high energy (UHE) cosmic neutrinos. Energetic
cosmic neutrinos can interact inside the rock and produce leptons via a charged
current interaction. In the case of an incoming electron neutrino undergoing a
charged current interaction, the produced UHE electron will induce an
underground electromagnetic shower. At high energy (above 7.7 TeV in standard
rock), such showers are subject to LPM (Landau, Pomeranchuk and Migdal)
suppression of the radiative processes cross sections (bremsstrahlung and pair
production). The consequence of this suppression is that showers are elongated.
This effect will increase the detection probability of such events allowing
deeper showers to emerge with detectable energies. On the other hand, the
photonuclear processes which are usually neglected in electromagnetic showers
with respect to radiative processes, turn out to become dominant in the LPM
regime and will reduce the shower length. In this work, we have performed a
complete Monte Carlo study of an underground shower induced by UHE electrons by
taking into account both the LPM suppression and the photonuclear interaction.
We will discuss the effects of both of these processes on the shower length and
on the detectability of such events by ground arrays or fluorescence
telescopes. We show that limits on neutrino fluxes that were obtained using
simulations that were obviously neglecting photonuclear processes are
overoptimistic and should be corrected.
