Physics:Quantum Quark–gluon plasma

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Quark–gluon plasma (QGP or quark soup[1]) is a hot, dense state of matter in which quarks and gluons are no longer confined inside hadrons. In ordinary nuclear matter, quarks are bound inside protons and neutrons by quantum chromodynamics (QCD); in quark–gluon plasma they are deconfined and move collectively in a strongly interacting medium.

Quark–gluon plasma occurs at energy densities high enough to melt the protons and neutrons that make up atomic nuclei. It is a very low-viscosity liquid composed of elementary quarks and gluons, a state of matter new to physics when it was discovered.[2][3]

Quark–gluon plasma is studied to understand the characteristics of the Universe at about 20 μs after the Big Bang. Experimental groups use ultrarelativistic beams of ions colliding with other ions or protons to create this plasma in particle accelerators.[4]

QCD phase diagram. Adapted from original made by R. S. Bhalerao.[5]

History

Theories predicting the existence of quark–gluon plasma were developed in the late 1970s and early 1980s.[6] The discovery of color confinement and asymptotic freedom in QCD led to the realization that quark matter would undergo a phase transition at high temperature or density.[7] Using an analogy with electromagnetic plasma, E. V. Shuryak used the term "hadronic plasma" in 1978 for matter much denser than atomic nuclei, in which hadrons merge and quarks act collectively.[8] In his next paper he used "quark-gluon plasma", the name that became standard.[7]Template:Rp[9]

In 2000, CERN issued a press release[10] reporting evidence for a new state of matter based on Pb-Pb heavy-ion collision studies.[11][12] The evidence was consistent with many characteristics of the theoretically predicted quark-gluon plasma.[2]

A competing team at the Relativistic Heavy Ion Collider (RHIC) characterized the CERN results as circumstantial[13] and suggested that the experiments yielded little information about the properties of the new state.[14] A series of Au-Au collision studies from RHIC published in 2005 showed that the collisions produce something more like a liquid than the weakly interacting gas expected in early theoretical models.[15]

Role in the Standard Model

QCD is one part of the modern theory of particle physics called the Standard Model. Other parts of this theory describe electroweak interactions and neutrinos. The theory of electrodynamics has been tested and found correct to a few parts in a billion. The theory of weak interactions has been tested and found correct to a few parts in a thousand. Perturbative forms of QCD have been tested to a few percent.[16] Perturbative models assume relatively small changes from the ground state, simplifying calculations at the cost of generality. In contrast, non-perturbative QCD is much harder to test directly. The study of QGP, which has both high temperature and high density, is part of the effort to understand QCD beyond perturbation theory.

The study of QGP is also a testing ground for finite-temperature field theory, a branch of theoretical physics that studies particle physics under high-temperature conditions. Such studies are important for understanding the early evolution of the Universe, especially the first hundred microseconds after the Big Bang. They are also relevant to grand unification theories, which seek to unify the fundamental forces of nature except gravity.

Occurrence

The generally accepted model of the formation of the Universe states that it happened as the result of the Big Bang. In this model, in the time interval of 10−10–10−6 s after the Big Bang, matter existed in the form of a quark–gluon plasma. It is possible to reproduce the density and temperature of matter from that early time in laboratory conditions. The main method is the collision of two heavy atomic nuclei accelerated to energies of more than a hundred GeV. A head-on collision creates a small fireball with a volume approximately equal to that of an atomic nucleus, allowing physicists to model the density and temperature of the early Universe.

Relation to electromagnetic plasma

A plasma is matter in which charges are screened by the presence of other mobile charges. For example, Coulomb's law can be modified by screening to yield a distance-dependent charge, Q→Qe−r/α, where the charge is reduced exponentially with distance divided by a screening length α. In QGP, the color charge of quarks and gluons is screened. QGP has analogies with an ordinary plasma, but also important differences: color charge is non-abelian, whereas electric charge is abelian. Outside a finite volume of QGP the color-electric field is not screened, so a volume of QGP must still be color-neutral and have integer electric charge.

Because of the extremely high energies involved, quark-antiquark pairs are produced by pair production. A QGP is therefore a roughly equal mixture of quarks and antiquarks of various flavors, with only a slight excess of quarks. This is not a general feature of conventional plasmas, which are often too cool for pair production.

Theory

One consequence of the non-abelian color charge is that the interaction is too strong for perturbative computations in many QGP conditions. The main theoretical tools for exploring QGP include lattice gauge theory.[17][18] The transition temperature, approximately Template:Val in older estimates, was first predicted by lattice gauge theory. Since then, lattice gauge theory has been used to predict many other properties of this kind of matter. The AdS/CFT correspondence may also provide insights into QGP, and the fluid/gravity correspondence has been used as a theoretical framework for understanding strongly coupled fluids. QGP is believed to be a phase of QCD that can become locally thermalized and therefore suitable for an effective fluid-dynamic description.

Production

Production of QGP in the laboratory is achieved by colliding heavy atomic nuclei, or heavy ions, at relativistic energy. Matter is heated well above the Hagedorn temperature TH = 150 MeV per particle, corresponding to a temperature exceeding 1.66 trillion K. Lead and gold nuclei have been used in such collisions at the CERN Super Proton Synchrotron and at the Brookhaven Relativistic Heavy Ion Collider, respectively. The nuclei are accelerated to ultrarelativistic speeds, directed toward each other, and create a hot fireball in the rare event of a collision. Hydrodynamic simulations predict that this fireball expands under its own pressure and cools while expanding. Measurements of spherical and elliptic flow test these models.

Diagnostic tools

Quark–gluon plasma is produced in relativistic heavy-ion collisions.[19][20][21][22]

Important classes of experimental observations include:

Expected properties

Thermodynamics

The cross-over temperature from the normal hadronic phase to the QGP phase is about Template:Val.[23] The phenomena involved correspond to an energy density of a little less than Template:Val. For relativistic matter, pressure and temperature are not independent variables, so the equation of state is a relation between energy density and pressure. This has been found through lattice computations and compared with perturbation theory and string-theory-inspired models. Response functions such as the specific heat and quark-number susceptibilities are active topics of calculation.

Flow

The discovery of QGP as an almost perfect liquid was a turning point in high-energy nuclear physics. Experiments at RHIC revealed a substance with extremely low resistance to flow.[24] Nuclear matter at low temperature is known to behave like a superfluid. When heated, the nuclear fluid evaporates into a dilute gas of nucleons and, on further heating, a gas of baryons and mesons. At the critical temperature, hadrons melt and the gas turns back into a liquid. RHIC experiments showed that this liquid has less resistance to flow than any other substance observed in laboratory experiments at any scale. Detailed measurements indicate that it is a quark–gluon plasma in which quarks, antiquarks and gluons flow collectively.[25]

Schematic representation of the interaction region formed in the first moments after the collision of high-energy heavy ions.[26]

In short, QGP flows like a splat of liquid and, because it is not transparent to quarks, it can attenuate jets emitted in collisions. Once formed, a ball of QGP transfers heat internally by radiation. However, unlike ordinary hot objects, there is enough energy available for gluons to collide and produce an excess of strange quarks. If QGP did not exist and the collision produced only a non-equilibrium mixture, the same energy would instead create different distributions of heavier quarks such as charm or bottom quarks.[26][27]

The equation of state is an important input into flow equations. The speed of sound, meaning the speed of QGP-density oscillations, is studied in lattice computations.[28][29][30] The mean free path of quarks and gluons has been computed using perturbation theory as well as string-theory methods. Lattice computations of transport coefficients indicate that the mean free time of quarks and gluons in QGP may be comparable to the average interparticle spacing, supporting the interpretation of QGP as a liquid.[31][32] The incorporation of dissipative phenomena into hydrodynamics is another active research area.[33][34][35]

Jet quenching effect

Detailed predictions for the production of jets were made in the late 1970s for the CERN Super Proton–Antiproton Synchrotron.[36][37][38][39] The UA2 experiment observed the first evidence for jet production in hadron collisions in 1981,[40] shortly after confirmed by UA1.[41]

The subject was later revived at RHIC. One of the most striking physical effects observed at RHIC energies is jet quenching.[42][43][44] At the first stage of interaction between colliding relativistic nuclei, partons of the colliding nuclei give rise to secondary partons with large transverse momentum. Passing through hot, compressed plasma, these partons lose energy. The magnitude of energy loss depends on QGP properties such as temperature and density. Colored quarks and gluons are the elementary objects of the plasma, so parton energy loss in QGP differs from energy loss in a medium made of colorless hadrons. Under RHIC conditions, parton energy losses are estimated as Template:Tmath. This conclusion is supported by comparing high-transverse-momentum hadron yields in nucleon-nucleon and nucleus-nucleus collisions at the same energy. The reduced yield in nucleus-nucleus collisions suggests that nuclear collisions cannot be regarded as a simple superposition of nucleon-nucleon collisions. For a short time and small volume, quarks and gluons form an almost ideal liquid whose collective properties affect the motion and energy loss of partons.[45]

In November 2010, CERN announced the first direct observation of jet quenching based on heavy-ion collisions.[46][47][48][49]

Direct photons and dileptons

Thermal photons and dileptons are important electromagnetic probes of the QGP formed in relativistic heavy-ion collisions. Unlike hadrons, which predominantly reflect the final stages of the collision, electromagnetic probes are emitted throughout the space–time evolution of the fireball, from the early deconfined phase through the hadronic stage up to kinetic freeze-out, when strong interactions cease. Because photons and leptons interact only electromagnetically, their mean free path is much larger than the size of the collision volume. They can therefore escape the medium with minimal final-state interactions and provide direct information about the temperature and dynamics of the matter created in the collision. Dilepton invariant mass helps separate partonic and hadronic effects, making it useful for studying the average temperature of the plasma and its equilibration time.[50]

Glasma hypothesis

Since 2008, physicists have discussed a hypothetical precursor state of QGP called the glasma, in which dressed particles are condensed into a glassy or amorphous state below the genuine transition between the confined state and the plasma liquid.[51] This would be analogous to the formation of metallic glasses, or amorphous alloys, below the onset of the liquid metallic state.

Although the high temperatures and densities predicted to produce QGP have been realized in the laboratory, the resulting matter does not behave as a quasi-ideal state of free quarks and gluons. Instead, it behaves as an almost perfect dense fluid.[52] The possibility that QGP would not yet be free at accelerator temperatures was predicted in 1984 as a consequence of remnant effects of confinement.[53][54]

Neutron stars

It has been hypothesized that the core of some massive neutron stars may contain deconfined quark matter related to quark–gluon plasma.[55]

Onset of deconfinement

The central issue in the formation of quark–gluon plasma is the search for the onset of deconfinement. From the beginning of QGP research, the question was whether sufficient energy density could be achieved in nucleus-nucleus collisions. This depends on how much energy each nucleon loses. An influential reaction picture was the scaling solution presented by Bjorken.[56] This model applies to ultra-high-energy collisions. In experiments at CERN SPS and BNL RHIC, the situation is usually divided into three stages:[57]

  • Primary parton collisions and baryon stopping at the time of complete overlap of the colliding nuclei.
  • Redistribution of particle energy and creation of new particles in the QGP fireball.
  • Equilibration and expansion of QGP matter before hadronization.

More experimental evidence points to the strength of QGP formation mechanisms, possibly operating even in LHC-energy proton-proton collisions.[20]

See also

Table of contents (72 articles)

Index

Full contents

Further reading

Books

Review articles with a historical perspective

External links

Template:Stellar core collapse Template:Physics-footer Template:Phase of matter Template:Authority control

References

  1. ↑ Heinz, Ulrich (January 20, 2015). [[1](https://www.worldscientific.com/doi/abs/10.1142/S0217751X15300112) "Quark–gluon soup — The perfectly liquid phase of QCD"]. International Journal of Modern Physics A 30 (02): 1530011. doi:10.1142/S0217751X15300112. ISSN 0217-751X. [2](https://www.worldscientific.com/doi/abs/10.1142/S0217751X15300112). 
  2. ↑ 2.0 2.1 Rafelski, Johann (2015). "Melting hadrons, boiling quarks" (in en). The European Physical Journal A 51 (9). doi:10.1140/epja/i2015-15114-0. ISSN 1434-6001. 
  3. ↑ Braun-Munzinger, Peter; Stachel, Johanna (July 2007). [[3](https://www.nature.com/articles/nature06080) "The quest for the quark–gluon plasma"] (in en). Nature 448 (7151): 302–309. doi:10.1038/nature06080. ISSN 0028-0836. PMID 17637661. [4](https://www.nature.com/articles/nature06080). 
  4. ↑ Busza, Wit; Rajagopal, Krishna; Schee, Wilke van der (October 19, 2018). [[5](https://www.annualreviews.org/content/journals/10.1146/annurev-nucl-101917-020852) "Heavy Ion Collisions: The Big Picture and the Big Questions"] (in en). Annual Review of Nuclear and Particle Science 68: 339–376. doi:10.1146/annurev-nucl-101917-020852. ISSN 0163-8998. [6](https://www.annualreviews.org/content/journals/10.1146/annurev-nucl-101917-020852). 
  5. ↑ Template:Cite encyclopedia
  6. ↑ Satz, H. (1981) (in en). [Statistical) Mechanics of Quarks and Hadrons: Proceedings of an International Symposium Held at the University of Bielefeld, F.R.G., August 24–31, 1980. North-Holland. ISBN 978-0-444-86227-3. [https://books.google.com/books?id=OY8uAAAAIAAJ. 
  7. ↑ 7.0 7.1 Gross, Franz; Klempt, Eberhard; Brodsky, Stanley J.; Buras, Andrzej J.; Burkert, Volker D.; Heinrich, Gudrun; Jakobs, Karl; Meyer, Curtis A. et al. (December 12, 2023). "50 Years of quantum chromodynamics" (in en). The European Physical Journal C 83 (12): 1125. doi:10.1140/epjc/s10052-023-11949-2. ISSN 1434-6052. 
  8. ↑ Shuryak, E V. "Theory of hadron plasma." Sov. Phys. - JETP (Engl. Transl.); (United States), vol. 47:2, Jan. 1978.
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  15. ↑ [[9](https://www.bnl.gov/newsroom/news.php?a=110303) "RHIC Scientists Serve Up 'Perfect' Liquid"] (in en). [10](https://www.bnl.gov/newsroom/news.php?a=110303). 
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  19. ↑ Busza, Wit; Rajagopal, Krishna; van der Schee, Wilke (2018). "Heavy ion collisions: The big picture and the big questions" (in en). Annual Review of Nuclear and Particle Science 68 (1): 339–376. doi:10.1146/annurev-nucl-101917-020852. ISSN 0163-8998. 
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  45. ↑ Schukraft, Jürgen (2010). [ALICE—'Little) Bang': The first 3 weeks .... [https://indico.cern.ch/event/114939/contributions/62599/attachments/48842/70225/CERN_HI_Dec_2010.pdf. 
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  51. ↑ Venugopalan, Raju (2008). "From Glasma to Quark Gluon Plasma in heavy ion collisions". Journal of Physics G: Nuclear and Particle Physics 35 (10). doi:10.1088/0954-3899/35/10/104003. Bibcode: 2008JPhG...35j4003V. 
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  54. ↑ Plümer, M.; Raha, S.; Weiner, R. M. (1984). "Effect of confinement on the sound velocity in a quark–gluon plasma". Phys. Lett. B 139 (3): 198–202. doi:10.1016/0370-2693(84)91244-9. 
  55. ↑ Annala, Eemeli; Gorda, Tyler; Hirvonen, Joonas; Komoltsev, Oleg; Kurkela, Aleksi; Nättilä, Joonas; Vuorinen, Aleksi (2023-12-19). "Strongly interacting matter exhibits deconfined behavior in massive neutron stars" (in en). Nature Communications 14 (1): 8451. doi:10.1038/s41467-023-44051-y. ISSN 2041-1723. PMID 38114461. 
  56. ↑ Bjorken, J. D. (1983). "Highly relativistic nucleus-nucleus collisions: The central rapidity region" (in en). Physical Review D 27 (1): 140–151. doi:10.1103/PhysRevD.27.140. ISSN 0556-2821. 
  57. ↑ Letessier, Jean; Rafelski, Johann (2002-05-30) (in en). [Hadrons) and Quark–Gluon Plasma. Cambridge University Press. ISBN 978-1-139-43303-7. [https://books.google.com/books?id=SAlbKkdor1gC. 
Author: Harold Foppele

Source attribution: Quark–gluon plasma

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