Quark Gluon Plasma

Quark-Gluon Plasma

Rahul Soni4th Year BS-MS (Dual)Department of Physics

IISER Bhopal

April 24, 2013

1 Introduction:

In the twentieth century many remarkable discoveries were made with the discovery of many particles(like proton, neutron, electron, muon etc). These particles were grouped in two categories as leptons andhadrons (baryons and mesons) based upon their physical properties (like spin, mass, charge, strangenessetc.). But there is need for a good theoretical explanation for these particles. Murray Gell-Mann in1961, proposed the Eight Fold way diagrams to explain and fit these particles theoretically. The eightfold way successfully classified the hadrons but wasn’t able to explain why do the hadrons follow thiskind of pattern? There was no explanation till 1964, when Gellman and George Zweig independentlyproposed that the properties of hadrons can be explained if they are formed by ‘Quarks’ (u-up, d-downand s-strange quark). The quark theory was of great success on theoretical grounds but suffered fromits existence experimentally. No independent quark had been discovered. Many hypotheses were made,some said that the quarks are confined; Greenberg proposed that quarks not only occur in 3 flavors butalso in 3 colors. But these hypotheses were in sufficient to rescue the quark model. In 1974, a newparticle called the J/Ψ was discovered. It was quickly explained that this was a meson made from afourth quark, the charm quark (c) whose existence had been theoretically inferred earlier. The charmquark was followed by the discovery of bottom quark and top quark. The quark model got establishedtill 1995. But “What is the matter made of?” The standard model says that matter is made up of threetypes of elementary particles namely: quarks, leptons and mediators. Mediators are the force carriers inall kinds of interactions . In nature there are 4 kinds of fundamental interactions (weak, electromagnetic,strong and gravity). The mediators for strong interactions are ‘Gluons’, there are eight of them. Fromthe understanding of the forces that bind quarks, scientists proposed that there will exist a state ofmatter named-Quark-Gluon Plasma (QGP) at extremely high temperature and density, in which thequarks and gluons are completely free from their strong interaction force. It is called as plasma becauseit contains ionized particles (quarks).

For studying QGP we need to know about QCD (Quantum Chromo Dynamics) since it falls underthe strong interaction regime. I am briefly describing some of the basics of QCD which one shouldknow to study QGP. QCD is one of the three interaction described in the Standard Model (the theorythat describes the interaction of the subatomic particles excluding gravitational interaction). Like every

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interaction in Standard Model QCD also possess mediating particles called gluons. The strong interactionis mediated among the subatomic particles called quarks. The complications of QCD arise due to the factthat gluons can interact among themselves. In QCD there are two aspects which need to be understoodbefore studying QGP. They are ‘quark confinement’ and ‘asymptotic freedom’. Quarks are actuallyconfined within hadrons due to the QCD interaction and no free quark is observed. At extremely hightemperatures (energies) the strength of the QCD interaction start lowering until the quarks no longerfeel the interaction which we call as asymptotic freedom.

2 Theory behind QGP:

Since QGP is also one of the states of matter it can be transform from one phase to another like thephase transition of water. QGP is actually a part of the phase diagram of the QCD matter. Quarkmatter or QCD matter refers to any of a number of theorized phases of matter whose degrees of freedominclude quarks and gluons. These theoretical phases would occur at different ranges of temperatures anddensities [1]. The phase diagram of the QCD matter of a neutron star is shown below:

We will not go into the detail of deriving this phase diagram since it requires knowledge of advancedstatistical mechanics. But we can infer most of the things from the phase diagrams. The phase diagramis plotted in µ versus T, where µ is the chemical potential of the quark, basically it is a measure ofimbalance between quark and antiquark, higher the value of µ stronger bias of quarks over antiquarksis favoured and T is the Temperature [1]. Ordinary atomic matter is a phase consisting of a droplet ofnuclear matter surrounded by vacuum at µ=310 MeV and T close to zero If we increase the chemicalpotential (i.e. increasing quark density) keeping the temperature low, we move into a phase of moreand more compressed nuclear matter. At a certain value of µ there is transition to quark matter andat ultra high densities we can expect the CFL (Color Flavor Locked) phase of super conducting quarkmatter. CFL phase is a phase in which the quarks form Cooper pairs within themselves. At intermediatedensities we expect some other phases (labelled ”non-CFL quark liquid” in the figure) whose nature ispresently unknown [1].Now as the temperature is increased keeping the chemical potential fixed the nuclear matter disintegratesinto the gas of hadrons with quarks confined in them. At a particular value of T=175 MeV (as predictedby Lattice Gauge Theory) there is a transition from the hadron phase to the QGP, a state in whichquarks, antiquarks and gluons as well as other lighter particles like photons, electrons, positrons etc.exist. This is how QCD brings us to the state of QGP. There also exist some other features of this phasediagram but it requires deep insight of the subject.

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3 Laboratory formation of QGP:

The QGP can be created in the laboratory by heating matter up to a temperature of 2×1012 K or we cansay with a particle having ∼ 175MeV of energy. We can get this much amount of temperature if we cancollide two large nuclei at very high energy. This can be done by ultrarelativistic heavy ion collisions. Inthis process collisions of heavy nuclei takes place at very speed almost equal to that of light at this highspeed they get Lorentz contracted and form a smear of nucleons which graze through each other allowingsome part of nucleus to make head on collision. The heavy ions are used because they contain largenumber of nucleons and the probability of colliding two heavy nuclei is more as compared to the lighternuclei or individual nucleons The proton and neutron which get collided break up into their constituentquarks and gluons. The hot volume which formed after head on collision is called as the “fireball”. Oncethis fireball is created it is expected to expand under its own pressure and gets cooled while expanding [3].

The above image shows the collision of two heavy nuclei forming QGP [8]. The ultrarelativistic heavy ioncollisions have been done from last two decades at particle accelerators. SPS (Super Proton Synchrotron)was the first relativistic heavy ion collider at CERN. After that RHIC (Relativistic Heavy Ion Collider)and LHC (Large Hadron Collider) were built. Lead and Gold nuclei have been used respectively atSPS and RHIC for testing the QGP. At LHC lead-lead collisions and lead-proton collisions were done atenergy of 1400 GeV.

3.1 The RHIC Experiment:

The RHIC is located in the state of New York at Brookhaven National Laboratory. It is now thesecond highest energy heavy ion collider in the world after LHC. In 2010, RHIC scientists publishedthat temperature in excess of 345 MeV has been achieved in Gold ion collisions and that these collisiontemperatures leads to the breakdown of normal matter and the formation of liquid like QGP.There are two major accelerators works at RHIC: STAR and PHENIX. STAR is aimed at the detection ofhadrons whereas PHENIX is specialized in detecting rare and electromagnetic particles. There were alsotwo more detectors named PHOBOS and BRAHMS. aimed at bulk particle multiplicity measurementand for momentum spectroscopy, respectively.

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The experimental set up of RHIC is shown below:

3.2 The Large Hadron Collider:

It is the worlds largest and highest energy particle accelerator, built by CERN from 1998 to 2008 inorder to test the different theories of particle physics and high energy physics, and to particularly proveor disprove the existence of the theorized Higgs boson. The experimental set up of LHC is shown below:

The ATLAS (A Toroidal LHC Apparatus) experiment and the CMS (Compact Muon Solenoid) arethe two general purpose particle detectors aimed to look for the signs of new physics, including theorigin of mass and extra dimensions, hunt for Higgs Boson and to look for the clues of origin of Darkmatter. ALICE (A Large Ion Collider Experiment) is used for the study of Quark-Gluon Plasma.Apartfrom these detectors we have LHCb which looks for the missing antimatter, TOTEM and LHCf are verysmall and used for very specialized purposes. At ALICE Pb-Pb (lead-lead) collisions were taken placeat energy of 2.76 TeV per nucleus, more than RHIC.

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4 QGP and the Early Universe:

The presence of QGP in the early arises from two facts. First, it forms at extremely high temperatureand with very high density which was indeed what scientists predicted about the early universe. Second,they are the basic building block for the hadronic matter. So it is believed that shortly after the bigbang all matter of the universe was in a state of QGP.It was hypothesized that free quarks probably existed in the extreme conditions of the very early universeuntil about 30 microseconds after the Big Bang, in a very hot gas of free quarks, antiquarks and gluons.This gas is called a quark–gluon plasma (QGP), since the quark-interaction charge (color charge) ismobile and quarks and gluons move around. This is possible because at a high temperature the earlyuniverse is in a different vacuum state, in which normal matter can not exist but quarks and gluons can,they are deconfined [5]. After the big bang the universe expanded and cooled and after 10−6 seconds theuniverse consisted of a soup of quarks, gluons, electrons and neutrinos and their respective antiparticles(reason for this belief is that they are actually the most fundamental particles among their respectivefamilies). As the universe cooled during the subsequent expansion, at about 1012 K the soup coalescedto form baryonic matter (protons and neutrons). After that within 3 min at about 109 K lower massesnuclei were formed. These low mass nuclei combined with electrons formed neutral atoms which underthe action of gravity combined to form stars. Thus the Big Bang theory also predicts the possibleexistence of QGP.The image showing the existence of QGP in the early universe [9].

5 Discussion:

Much exploration is indeed required for QGP. The study of QGP involves a central problem of theconfinement of quarks and gluons, they are not directly observable. If we would be able to find freequark and gluons it would be obvious to verify the QCD predictions of the QGP state. The study ofQGP is important in order to understand the early evolution of our universe. It is also important forthe study of GUTS (Grand Unified Theories).

References

[1] QCD, http://en.wikipedia.org

[2] David J Griffiths, Introduction to particle physics

[3] Quark Gluon Plasma, http://en.wikipedia.org

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[4] Chapter 2: QGP and the Early Universe, www.physics.umd.edu

[5] Strangeness production, http://en.wikipedia.org.

[6] Laboratory formation, http://lmanceau.web.cern.ch

[7] Quantum Diaries,http://www.quantumdiaries.org

[8] www.physik.uni-frankfurt.de

[9] www.phy.duke.edu

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