Revisiting extremely high energy QED bremsstrahlung in matter: large modifications to the LPM effect

Techniqes for understanding energy loss of high-energy partons in quark-gluon plasmas (QGP) owe much to earlier theory of high-energy electromagnetic showers. This talk discusses how new methods, developed for the QGP, shed new light in turn on the behavior of electromagnetic showers at extremely high energies. Very high energy electrons initiate electromagnetic showers in ordinary matter that branch and multiply through bremsstrahlung and pair production. At extremely high energies, the quantum mechanical duration of these processes becomes longer than the mean free time to elastically scatter from the medium, which leads to a very significant suppression of bremsstrahlung (and pair production) known as the Landau-Pomeranchuk-Migdal (LPM) effect. We revisit the LPM effect for bremsstrahlung of energy k_γ from an electron of energy E. We find that there are very large corrections to the LPM bremsstrahlung rate for certain regions of (k_γ, E) due to quantum overlap of bremsstrahlung and subsequent pair production. This possibility was first raised in the 1960s, when it was argued qualitatively that pair production would significantly decrease the bremsstrahlung rate in those regions of (k_γ, E) compared to the already-suppressed LPM bremsstrahlung rate. We find the opposite -- quantum overlap of bremsstrahlung with pair production significantly *increases* the bremsstrahlung rate compared to the LPM calculation -- and we verify our qualitative arguments with an analytic calculation of the effect.