Abstract
Full text: Relativistic heavy ion collisions make it possible to study the properties of strongly interacting matter at energy densities far above those of normal nuclear matter. General arguments based on the property of asymptotic freedom suggest, and quantitative calculations of lattice quantum chromodynamics confirm that QCD matter undergoes a transition from a hadronic gas to a quark-gluon plasma at a temperature T{sub c} {approx} 160 MeV. The quark- gluon plasma phase is characterized by a much reduced condensate of light quarks, reflecting the approximate restoration of chiral symmetry, and by screening of the chromo-electric force between heavy quarks, implying the absence of quark confinement. A large body of experimental and theoretical research conducted over the past decade at RHIC has led to a standard model of heavy ion collisions. The energy deposited in the mid- rapidity kinematic range is controlled by the density of gluons contained in the colliding nuclei, resulting in the formation of a dense QCD plasma with highly occupied gauge field modes, often called a glasma, which rapidly thermalizes and forms a nearly thermal quark-gluon plasma, whose evolution can be described accurately by viscous hydrodynamics because of its very small kinematic shear viscosity. The plasma cools
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Muller, Berndt
[1]
- Duke University, Durham, NC (United States). Dept. of Physics
Citation Formats
Muller, Berndt.
What have we learned from heavy ion collisions at the LHC?.
Brazil: N. p.,
2012.
Web.
Muller, Berndt.
What have we learned from heavy ion collisions at the LHC?.
Brazil.
Muller, Berndt.
2012.
"What have we learned from heavy ion collisions at the LHC?"
Brazil.
@misc{etde_22125540,
title = {What have we learned from heavy ion collisions at the LHC?}
author = {Muller, Berndt}
abstractNote = {Full text: Relativistic heavy ion collisions make it possible to study the properties of strongly interacting matter at energy densities far above those of normal nuclear matter. General arguments based on the property of asymptotic freedom suggest, and quantitative calculations of lattice quantum chromodynamics confirm that QCD matter undergoes a transition from a hadronic gas to a quark-gluon plasma at a temperature T{sub c} {approx} 160 MeV. The quark- gluon plasma phase is characterized by a much reduced condensate of light quarks, reflecting the approximate restoration of chiral symmetry, and by screening of the chromo-electric force between heavy quarks, implying the absence of quark confinement. A large body of experimental and theoretical research conducted over the past decade at RHIC has led to a standard model of heavy ion collisions. The energy deposited in the mid- rapidity kinematic range is controlled by the density of gluons contained in the colliding nuclei, resulting in the formation of a dense QCD plasma with highly occupied gauge field modes, often called a glasma, which rapidly thermalizes and forms a nearly thermal quark-gluon plasma, whose evolution can be described accurately by viscous hydrodynamics because of its very small kinematic shear viscosity. The plasma cools by mainly longitudinal expansion until it converts to a gas of hadron resonances when its temperature falls below T{sub c}. At that temperature the chemical composition of the produced hadrons gets frozen, but the spectral distribution of the hadrons is still modified by final-state interactions. The very small ratio {eta}/s of the shear viscosity to the entropy density of the QCD plasma produced in heavy ion collisions was one of the main discoveries of the RHIC experiments. Such a small ratio requires that the interactions among the constituents are extremely strong and the cross sections among them approach the unitarity limit. The strongly coupled nature of the quark-gluon plasma is confirmed by its ability to quench jets, observed at RHIC via the suppression of single hadron yields at high transverse momentum relative to p+p reactions and the additional suppression of back-to-back emission of energetic hadrons. The first round of data from the LHC have brilliantly confirmed this picture, while opening up new observables and extending the range those studied by the experiments at RHIC. The quark-gluon plasma produced in Pb+Pb collisions at LHC is three times as dense as that at RHIC, implying a 30% higher initial temperature and a volume at freeze-out that is twice as large. The collective flow measured at LHC is well described by the same small ratio of {eta}/s at at RHIC. The angular flow pattern measured in individual events provides insight into the density fluctuations on the colliding nuclei. The most exciting new results from the LHC come from jet measurements. The much higher jet energies permit identification and reconstruction of complete jets, as well as the study of jet quenching over a very large kinematic range. Models that treat the jet as weakly interacting with a strongly coupled plasma extrapolate well from RHIC to LHC, but the opacity of the QCD matter seems to be increasing slightly less than the entropy density. The large increase of the asymmetry of back-to-back jets indicates that the energy lost by the leading parton is quickly thermalized. The second round of data from the LHC, with its much higher number of events, promises to convert many of the qualitative conclusions from the first Pb+Pb run into quantitative insights: How are jet fragmentation functions and jet shapes modified by the medium? Are heavy quarks affected by the quark-gluon plasma just like light quarks? Are bound states of b-quarks dissolved by the hot matter similarly to charmonium states at RHIC? Are bound charmonium states formed at hadronization by recombination of c-quarks? My lecture will describe the latest state of insights gained from the LHC heavy ion data that will be reported at the major conferences this summer. (author)}
place = {Brazil}
year = {2012}
month = {Jul}
}
title = {What have we learned from heavy ion collisions at the LHC?}
author = {Muller, Berndt}
abstractNote = {Full text: Relativistic heavy ion collisions make it possible to study the properties of strongly interacting matter at energy densities far above those of normal nuclear matter. General arguments based on the property of asymptotic freedom suggest, and quantitative calculations of lattice quantum chromodynamics confirm that QCD matter undergoes a transition from a hadronic gas to a quark-gluon plasma at a temperature T{sub c} {approx} 160 MeV. The quark- gluon plasma phase is characterized by a much reduced condensate of light quarks, reflecting the approximate restoration of chiral symmetry, and by screening of the chromo-electric force between heavy quarks, implying the absence of quark confinement. A large body of experimental and theoretical research conducted over the past decade at RHIC has led to a standard model of heavy ion collisions. The energy deposited in the mid- rapidity kinematic range is controlled by the density of gluons contained in the colliding nuclei, resulting in the formation of a dense QCD plasma with highly occupied gauge field modes, often called a glasma, which rapidly thermalizes and forms a nearly thermal quark-gluon plasma, whose evolution can be described accurately by viscous hydrodynamics because of its very small kinematic shear viscosity. The plasma cools by mainly longitudinal expansion until it converts to a gas of hadron resonances when its temperature falls below T{sub c}. At that temperature the chemical composition of the produced hadrons gets frozen, but the spectral distribution of the hadrons is still modified by final-state interactions. The very small ratio {eta}/s of the shear viscosity to the entropy density of the QCD plasma produced in heavy ion collisions was one of the main discoveries of the RHIC experiments. Such a small ratio requires that the interactions among the constituents are extremely strong and the cross sections among them approach the unitarity limit. The strongly coupled nature of the quark-gluon plasma is confirmed by its ability to quench jets, observed at RHIC via the suppression of single hadron yields at high transverse momentum relative to p+p reactions and the additional suppression of back-to-back emission of energetic hadrons. The first round of data from the LHC have brilliantly confirmed this picture, while opening up new observables and extending the range those studied by the experiments at RHIC. The quark-gluon plasma produced in Pb+Pb collisions at LHC is three times as dense as that at RHIC, implying a 30% higher initial temperature and a volume at freeze-out that is twice as large. The collective flow measured at LHC is well described by the same small ratio of {eta}/s at at RHIC. The angular flow pattern measured in individual events provides insight into the density fluctuations on the colliding nuclei. The most exciting new results from the LHC come from jet measurements. The much higher jet energies permit identification and reconstruction of complete jets, as well as the study of jet quenching over a very large kinematic range. Models that treat the jet as weakly interacting with a strongly coupled plasma extrapolate well from RHIC to LHC, but the opacity of the QCD matter seems to be increasing slightly less than the entropy density. The large increase of the asymmetry of back-to-back jets indicates that the energy lost by the leading parton is quickly thermalized. The second round of data from the LHC, with its much higher number of events, promises to convert many of the qualitative conclusions from the first Pb+Pb run into quantitative insights: How are jet fragmentation functions and jet shapes modified by the medium? Are heavy quarks affected by the quark-gluon plasma just like light quarks? Are bound states of b-quarks dissolved by the hot matter similarly to charmonium states at RHIC? Are bound charmonium states formed at hadronization by recombination of c-quarks? My lecture will describe the latest state of insights gained from the LHC heavy ion data that will be reported at the major conferences this summer. (author)}
place = {Brazil}
year = {2012}
month = {Jul}
}