In high-energy physics, jet quenching is a
phenomenon that can occur in the collision
of ultra-high-energy particles.
In general, the collision of high-energy particles
can produce jets of elementary particles that
emerge from these collisions.
Collisions of ultra-relativistic heavy-ion
particle beams create a hot and dense medium
comparable to the conditions in the early
universe, and then these jets interact strongly
with the medium, leading to a marked reduction
of their energy.
This energy reduction is called "jet quenching".
== Physics background ==
In the context of high-energy hadron collisions,
quarks and gluons are collectively called
partons.
The jets emerging from the collisions originally
consist of partons, which quickly combine
to form hadrons, a process called hadronization.
Only the resulting hadrons can be directly
observed.
The hot, dense medium produced in the collisions
is also composed of partons; it is known as
a quark–gluon plasma (QGP).
In this realm, the laws of physics that apply
are those of quantum chromodynamics (QCD).
High-energy nucleus-nucleus collisions make
it possible to study the properties of the
QGP medium through the observed changes in
the jet fragmentation functions as compared
to the unquenched case.
According to QCD, high-momentum partons produced
in the initial stage of a nucleus-nucleus
collision will undergo multiple interactions
inside the collision region prior to hadronization.
In these interactions, the energy of the partons
is reduced through collisional energy loss
and medium-induced gluon radiation, the latter
being the dominant mechanism in a QGP.
The effect of jet quenching in QGP is the
main motivation for studying jets as well
as high-momentum particle spectra and particle
correlations in heavy-ion collisions.
Accurate jet reconstruction will allow measurements
of the jet fragmentation functions and consequently
the degree of quenching and therefore provide
insight on the properties of the hot dense
QGP medium created in the collisions.
== Experimental evidence of jet quenching
==
First evidence of parton energy loss has been
observed at the Relativistic Heavy Ion Collider
(RHIC) from the suppression of high-pt particles
studying the nuclear modification factor and
the suppression of back-to-back correlations.In
ultra-relativistic heavy-ion collisions at
center-of-mass energy of 2.76 and 5.02 TeV
at the Large Hadron Collider (LHC), interactions
between the high-momentum parton and the hot,
dense medium produced in the collisions, are
expected to lead to jet quenching.
Indeed, in November 2010 CERN announced the
first direct observation of jet quenching,
based on experiments with heavy-ion collisions,
which involved ATLAS, CMS and ALICE.
== See also ==
Parity (physics
