High-energy nuclear physics

High-energy nuclear physicsstudies the behavior of nuclear matter in energy regimes typical ofhigh-energy physics.The primary focus of this field is the study of heavy-ion collisions, as compared to lighter atoms in otherparticle accelerators.At sufficient collision energies, these types of collisions are theorized to produce thequark–gluon plasma.In peripheral nuclear collisions at high energies one expects to obtain information on the electromagnetic production of leptons and mesons that are not accessible in electron–positron colliders due to their much smaller luminosities.

Previous high-energy nuclearacceleratorexperiments have studied heavy-ion collisions using projectile energies of 1 GeV/nucleon atJINRandLBNL-Bevalacup to 158 GeV/nucleon atCERN-SPS.Experiments of this type, called "fixed-target" experiments, primarily accelerate a "bunch" of ions (typically around 10⁶to 10⁸ions per bunch) to speeds approaching thespeed of light(0.999c) and smash them into a target of similar heavy ions. While all collision systems are interesting, great focus was applied in the late 1990s to symmetric collision systems ofgoldbeams on gold targets atBrookhaven National Laboratory'sAlternating Gradient Synchrotron(AGS) anduraniumbeams on uranium targets atCERN'sSuper Proton Synchrotron.

High-energy nuclear physics experiments are continued at theBrookhaven National Laboratory'sRelativistic Heavy Ion Collider(RHIC) and at theCERNLarge Hadron Collider.At RHIC the programme began with four experiments— PHENIX, STAR, PHOBOS, and BRAHMS—all dedicated to study collisions of highly relativistic nuclei. Unlike fixed-target experiments, collider experiments steer two accelerated beams of ions toward each other at (in the case of RHIC) six interaction regions. At RHIC, ions can be accelerated (depending on the ion size) from 100 GeV/nucleon to 250 GeV/nucleon. Since each colliding ion possesses this energy moving in opposite directions, the maximal energy of the collisions can achieve acenter-of-masscollision energy of 200 GeV/nucleon for gold and 500 GeV/nucleon for protons.

TheALICE(A Large Ion Collider Experiment) detector at the LHC at CERN is specialized in studying Pb–Pb nuclei collisions at a center-of-mass energy of 2.76 TeV per nucleon pair. All major LHC detectors—ALICE,ATLAS,CMSandLHCb—participate in the heavy-ion programme.

History

The exploration of hot hadron matter and ofmultiparticle productionhas a long history initiated by theoretical work on multiparticle production byEnrico Fermiin the US andLev Landauin the USSR. These efforts paved the way to the development in the early 1960s of the thermal description of multiparticle production and thestatistical bootstrapmodel byRolf Hagedorn.These developments led to search for and discovery ofquark-gluon plasma.Onset of the productionof this new form of matter remains under active investigation.

First collisions

The first heavy-ion collisions at modestly relativistic conditions were undertaken at theLawrence Berkeley National Laboratory(LBNL, formerly LBL) atBerkeley,California, U.S.A., and at theJoint Institute for Nuclear Research(JINR) inDubna,Moscow Oblast, USSR. At the LBL, a transport line was built to carry heavy ions from the heavy-ion accelerator HILAC to theBevatron.The energy scale at the level of 1–2 GeV per nucleon attained initially yields compressed nuclear matter at few times normal nuclear density. The demonstration of the possibility of studying the properties of compressed and excited nuclear matter motivated research programs at much higher energies in accelerators available atBNLandCERNwith relativist beams targeting laboratory fixed targets. The first collider experiments started in 1999 at RHIC, and LHC begun colliding heavy ions at one order of magnitude higher energy in 2010.

CERN operation

TheLHCcollider atCERNoperates one month a year in the nuclear-collision mode, withPbnuclei colliding at 2.76 TeV per nucleon pair, about 1500 times the energy equivalent of the rest mass. Overall 1250 valence quarks collide, generating a hot quark–gluon soup. Heavyatomic nucleistripped of their electron cloud are called heavy ions, and one speaks of (ultra)relativistic heavy ions when thekinetic energyexceeds significantly therest energy,as it is the case at LHC. The outcome of such collisions is production of very manystrongly interacting particles.

In August 2012 ALICE scientists announced that their experiments producedquark–gluon plasmawith temperature at around 5.5 trillionkelvins,the highest temperature achieved in any physical experiments thus far.This temperature is about 38% higher than the previous record of about 4 trillion kelvins, achieved in the 2010 experiments at theBrookhaven National Laboratory.The ALICE results were announced at the August 13Quark Matter 2012conference inWashington, D.C.The quark–gluon plasma produced by these experiments approximates the conditions in the universe that existed microseconds after theBig Bang,before the matter coalesced intoatoms.

Objectives

There are several scientific objectives of this international research program:

• The formation and investigation of a new state of matter made of quarks and gluons, thequark–gluon plasmaQGP, which prevailed inearly universe in first 30 microseconds.
• The study ofcolor confinementand the transformation of color confining = quark confining vacuum state to the excited state physicists call perturbative vacuum, in which quarks and gluons can roam free, which occurs atHagedorn temperature;
• The study the origins ofhadron(proton,neutronetc.) matter mass believed to be related to the phenomenon of quark confinement and vacuum structure.

Experimental program

This experimental program follows on a decade of research at theRHICcollider atBNLand almost two decades of studies using fixed targets atSPSat CERN andAGSat BNL. This experimental program has already confirmed that the extreme conditions of matter necessary to reach QGP phase can be reached. A typical temperature range achieved in the QGP created

$T=300~{\text{MeV}}/k_{\text{B}}=3.3\times 10^{12}~{\text{K}}$

is more than100000times greater than in the center of theSun.This corresponds to an energy density

$\epsilon =10~{\text{GeV/fm}}^{3}=1.8\times 10^{16}~{\text{g/cm}}^{3}$.

The corresponding relativistic-matterpressureis

$P\simeq {\frac {1}{3}}\epsilon =0.52\times 10^{31}~{\text{bar}}.$

More information

• Rutgers University Nuclear Physics Home Page
• Publications - High Energy Nuclear Physics (HENP)
• https://web.archive.org/web/20101212105542/http:// er.doe.gov/np/