QCD Lunch

High-energy showering inside the QGP: weak vs. strong coupling, the LPM effect, and overlapping sequential bremsstrahlung

by Peter Arnold (University of Virginia)




Consider a (nearly-on-shell) high-energy parton (E >> T) traveling
through a quark-gluon plasma (QGP), losing energy by showering inside
the medium like a cosmic ray showers in the atmosphere.  There are two
extremes for picturing such a shower: If we imagine that the running
strong coupling is at least moderately small at the relevant energy
scale, we can calculate the development of the shower by picturing the
shower as a collection of individual high-energy partons that keep
splitting via bremsstrahlung and pair production.  If we instead
imagine that the running strong coupling is very strong, we cannot
treat the high-energy partons individually, and we might, for example,
turn to AdS/CFT descriptions of the process for qualitative insight.
As one increases coupling, when does the (weak-coupling) particle
picture of the shower irretrievably break down?  In this talk, I will
discuss the current status of field theory calculations meant to
address that question (in simple theoretical situations) by
calculating the size of corrections to the weak-coupling picture of
high-energy in-medium parton showers.  The discussion involves
investigating what happens when the quantum durations of consecutive
splittings of a parton in the shower overlap each other.  The
calculation requires a significant generalization of the
Landau-Pomeranchuk-Migdal (LPM) effect, which plays a roll in the
theory of both cosmic ray showers and jets in quark-gluon plasmas.
The calculation of the main elements needed for this analysis in
(large-N) QCD has recently been completed.


on-line via ZOOM