Jet quenching
Encyclopedia
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 jet
s of elementary particle
s 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".
collisions, quark
s and gluon
s are collectively called parton
s. 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.
(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 5.5 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.
Collision
A collision is an isolated event which two or more moving bodies exert forces on each other for a relatively short time.Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies...
of ultra-high-energy particles. In general, the collision of high-energy particles can produce jet
Jet (particle physics)
A jet is a narrow cone of hadrons and other particles produced by the hadronization of a quark or gluon in a particle physics or heavy ion experiment. Because of QCD confinement, particles carrying a color charge, such as quarks, cannot exist in free form. Therefore they fragment into hadrons...
s of elementary particle
Elementary particle
In particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s 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 hadronHadron
In particle physics, a hadron is a composite particle made of quarks held together by the strong force...
collisions, quark
Quark
A quark is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. Due to a phenomenon known as color confinement, quarks are never directly...
s and gluon
Gluon
Gluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....
s are collectively called parton
Parton (particle physics)
In particle physics, the parton model was proposed by Richard Feynman in 1969 as a way to analyze high-energy hadron collisions. It was later recognized that partons describe the same objects now more commonly referred to as quarks and gluons...
s. The jets emerging from the collisions originally consist of partons, which quickly combine to form hadrons, a process called hadronization
Hadronization
In particle physics, hadronization is the process of the formation of hadrons out of quarks and gluons. This occurs after high-energy collisions in a particle collider in which free quarks or gluons are created. Due to postulated colour confinement, these cannot exist individually...
. 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
Quark-gluon plasma
A quark–gluon plasma or quark soup is a phase of quantum chromodynamics which exists at extremely high temperature and/or density. This phase consists of asymptotically free quarks and gluons, which are several of the basic building blocks of matter...
(QGP). In this realm, the laws of physics that apply are those of quantum chromodynamics
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
(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 ColliderRelativistic Heavy Ion Collider
The Relativistic Heavy Ion Collider is one of two existing heavy-ion colliders, and the only spin-polarized proton collider in the world. It is located at Brookhaven National Laboratory in Upton, New York and operated by an international team of researchers...
(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 5.5 TeV at the Large Hadron Collider
Large Hadron Collider
The Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
(LHC), interactions between the high-momentum Parton
Parton (particle physics)
In particle physics, the parton model was proposed by Richard Feynman in 1969 as a way to analyze high-energy hadron collisions. It was later recognized that partons describe the same objects now more commonly referred to as quarks and gluons...
and the hot, dense medium produced in the collisions, are expected to lead to jet quenching. Indeed, in November 2010 CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
announced the first direct observation of jet quenching.