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Seminars/Colloquia
"Transverse Momentum Broadening: weak vs. strong coupling", by Francesco D'Eramo (Berkeley)
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Europe/Berlin
PH HS 3 (Physik-Department, TUM)
PH HS 3
Physik-Department, TUM
James-Franck-Str.
Garching
Description
Transverse momentum broadening of an energetic parton propagating
through a quark-gluon plasma is described P(k_\perp), defined as the
probability that after propagating through the medium for a distance L
the parton has acquired transverse momentum k_\perp. In this talk I will
first review how to formulate the problem within the Effective Field
Theory framework, and I will show how P(k_\perp) can be expressed in
terms of the expectation value of two transversely separated light-like
path-ordered Wilson lines of gluon fields from the medium. The nature of
the medium does not enter this calculation, and any of its property
becomes relevant only when one seeks to evaluate the expectation value.
I will then present the calculation of P(k_\perp) for a weakly-coupled
quark gluon plasma, where this expectation value can be evaluated by
using perturbation theory. I will compare the result at weak coupling to
expectations from holographic calculations that assume a plasma that is
strongly coupled at all length scales. The shape of P(k_\perp) at modest
k_\perp may not be very different in weakly coupled and strongly coupled
plasmas, but at large enough k_\perp it must be parametrically larger in
a weakly coupled plasma than in a strongly coupled plasma. This means
that by looking for rare (but not exponentially rare) large-angle
deflections of the jet resulting from a parton produced initially
back-to-back with a hard photon, experimentalists can find the weakly
coupled short-distance quark and gluon quasiparticles within the
strongly coupled liquid quark-gluon plasma produced in heavy ion
collisions, much as Rutherford found nuclei within atoms or Friedman,
Kendall and Taylor found quarks within nucleons.
through a quark-gluon plasma is described P(k_\perp), defined as the
probability that after propagating through the medium for a distance L
the parton has acquired transverse momentum k_\perp. In this talk I will
first review how to formulate the problem within the Effective Field
Theory framework, and I will show how P(k_\perp) can be expressed in
terms of the expectation value of two transversely separated light-like
path-ordered Wilson lines of gluon fields from the medium. The nature of
the medium does not enter this calculation, and any of its property
becomes relevant only when one seeks to evaluate the expectation value.
I will then present the calculation of P(k_\perp) for a weakly-coupled
quark gluon plasma, where this expectation value can be evaluated by
using perturbation theory. I will compare the result at weak coupling to
expectations from holographic calculations that assume a plasma that is
strongly coupled at all length scales. The shape of P(k_\perp) at modest
k_\perp may not be very different in weakly coupled and strongly coupled
plasmas, but at large enough k_\perp it must be parametrically larger in
a weakly coupled plasma than in a strongly coupled plasma. This means
that by looking for rare (but not exponentially rare) large-angle
deflections of the jet resulting from a parton produced initially
back-to-back with a hard photon, experimentalists can find the weakly
coupled short-distance quark and gluon quasiparticles within the
strongly coupled liquid quark-gluon plasma produced in heavy ion
collisions, much as Rutherford found nuclei within atoms or Friedman,
Kendall and Taylor found quarks within nucleons.