Quark–gluon plasma
It can be said that QGP emerges to be the new phase of strongly interacting matter which manifests its physical properties in terms of nearly free dynamics of practically massless gluons and quarks.
Both quarks and gluons must be present in conditions near chemical (yield) equilibrium with their colour charge open for a new state of matter to be referred to as QGP.
[3] Discussions around heavy ion experimentation followed suit,[4][5][6][7][8] and the first experiment proposals were put forward at CERN[9][10][11][12][13][14] and BNL[15][16] in the following years.
Quark–gluon plasma is studied to recreate and understand the high energy density conditions prevailing in the Universe when matter formed from elementary degrees of freedom (quarks, gluons) at about 20 μs after the Big Bang.
Experimental groups are probing over a 'large' distance the (de)confining quantum vacuum structure, which determines prevailing form of matter and laws of nature.
The study of the QGP, which has both a high temperature and density, is part of this effort to consolidate the grand theory of particle physics.
It is also of relevance to Grand Unification Theories which seek to unify the three fundamental forces of nature (excluding gravity).
It is possible to reproduce the density and temperature of matter existing of that time in laboratory conditions to study the characteristics of the very early Universe.
The QGP is believed to be a phase of QCD which is completely locally thermalized and thus suitable for an effective fluid dynamic description.
The nuclei are accelerated to ultrarelativistic speeds (contracting their length) and directed towards each other, creating a "fireball", in the rare event of a collision.
[27][28][29][30][31] The important classes of experimental observations are The cross-over temperature from the normal hadronic to the QGP phase is about 156 MeV.
[32] This "crossover" may actually not be only a qualitative feature, but instead one may have to do with a true (second order) phase transition, e.g. of the universality class of the three-dimensional Ising model.
Detailed measurements show that this liquid is a quark–gluon plasma where quarks, antiquarks and gluons flow independently.
[34] In short, a quark–gluon plasma flows like a splat of liquid, and because it is not "transparent" with respect to quarks, it can attenuate jets emitted by collisions.
However, unlike in everyday objects, there is enough energy available so that gluons (particles mediating the strong force) collide and produce an excess of the heavy (i.e., high-energy) strange quarks.
In addition, it is also necessary to take into account the fact that colored quarks and gluons are the elementary objects of the plasma, which differs from the energy loss by a parton in a medium consisting of colorless hadrons.
For a short time, ~1 μs, and in the final volume, quarks and gluons form some ideal liquid.
Energy losses depend on the properties of the quark–gluon medium, on the parton density in the resulting fireball, and on the dynamics of its expansion.
[54] In November 2010, CERN announced the first direct observation of jet quenching, based on experiments with heavy-ion collisions.
[55][56][57][58] Direct photons and dileptons are arguably most penetrating tools to study relativistic heavy ion collisions.
They are hard to decipher and interpret as most of the signal is originating from hadron decays long after the QGP fireball has disintegrated.
Although the experimental high temperatures and densities predicted as producing a quark–gluon plasma have been realized in the laboratory, the resulting matter does not behave as a quasi-ideal state of free quarks and gluons, but, rather, as an almost perfect dense fluid.
[63] Actually, the fact that the quark–gluon plasma will not yet be "free" at temperatures realized at present accelerators was predicted in 1984, as a consequence of the remnant effects of confinement.
[66] A quark–gluon plasma (QGP)[67] or quark soup[68][69] is a state of matter in quantum chromodynamics (QCD) which exists at extremely high temperature and/or density.
This is in analogy with the conventional plasma where nuclei and electrons, confined inside atoms by electrostatic forces at ambient conditions, can move freely.
It is believed that up to a few microseconds (10−12 to 10−6 seconds) after the Big Bang, known as the quark epoch, the Universe was in a quark–gluon plasma state.
[78] QGP differs from a "free" collision event by several features; for example, its particle content is indicative of a temporary chemical equilibrium producing an excess of middle-energy strange quarks vs. a nonequilibrium distribution mixing light and heavy quarks ("strangeness production"), and it does not allow particle jets to pass through ("jet quenching").
[83] Three experiments running on CERN's Large Hadron Collider (LHC), on the spectrometers ALICE,[84] ATLAS and CMS, have continued studying the properties of QGP.
[85] A new record breaking temperature was set by ALICE: A Large Ion Collider Experiment at CERN in August 2012 in the ranges of 5.5 trillion (5.5×1012) kelvin as claimed in their Nature PR.
In experiments carried out at CERN SPS and BNL RHIC more complex situation arose, usually divided into three stages:[92] More and more experimental evidence points to the strength of QGP formation mechanisms—operating even in LHC-energy scale proton-proton collisions.
