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Electron and Photon Interactions in the Regime of Strong LPM Suppression

Most searches for ultra-high energy (UHE) astrophysical neutrinos look for radio emission from the electromagnetic and hadronic showers produced in their interactions. The radio frequency spectrum and angular distribution depend on the shower development, so are sensitive to the interaction cross sections. At energies above about 10^{16} eV (in ice), the Landau-Pomeranchuk-Migdal (LPM) effect significantly reduces the cross sections for the two dominant electromagnetic interactions: bremsstrahlung and pair production. At higher energies, above about 10^{20} eV, the photonuclear cross section becomes larger than that for pair production, and direct pair production and electronuclear interactions become dominant over bremsstrahlung. The electron interaction length reaches a maximum around 10^{21} eV, and then decreases slowly as the electron energy increases further. In this regime, the growth in the photon cross section and electron energy loss moderates the rise in nu_e shower length, which rises from ~10 m at 10^{16} eV to ~50 m at 10^{19} eV and ~100 m at 10^{20} eV, but only to ~1 km at 10^{24} eV. In contrast, without photonuclear and electronuclear interactions, the shower length would be over 10 km at 10^{24} eV.