Heavy Quarks in Heavy-Ion Collisions
by Manuel Calderón de la Barca Sánchez
One of the most talked about topics in the physics of Heavy-Ion collisions is studying the production of heavy quarkonium states. This is one of the studies that aims to obtain information about possible formation of a Quark-Gluon Plasma in the laboratory.
The notes below are an attempt at clarifying what I mean in this two sentence summary.
QCD & QGP
RHIC has as one of its research goals the study of the theory that is at the heart of particles like protons and neutrons that make up atomic nuclei: QCD (Quantum Chromo-Dynamics). This is the theory of the strong nuclear interactions whose main actors are quarks and gluons. The quarks and gluons are the building blocks of protons, neturons and all atomic nuclei in the universe. There are many ways to study QCD and in particular in relativistic heavy ion collisions we are interested in trying to do it at a high energy density. The interesting aspect is that in our everyday world, we never see quarks and gluons, we only see the protons and neutrons formed by the complicated interaction of quarks and gluons at low energies. We say that the quarks and gluons are "confined", so that loosely speaking one can think of protons and neutrons as a bag of quarks and gluons, but this is a very special bag which never lets its contents out. The understanding of the nature of the confinement property of QCD is the subject of the 2004 Nobel Prize in Physics. At high energies, many theoretical calculations of QCD on large dedicated computers have indicated that a very different behaviour should emerge. The expectation is that at high energies, putting many protons and neutrons together will result in a new state where the quarks and gluons will be the main players, able to exist by themselves in a region of space larger than a proton. The type of matter that should be formed at these high energy densities can be thought of another phase of QCD matter, and it has been dubbed the "Quak-Gluon Plasma". This is only one of the interesting aspects of QCD, as the sketch below illustrates.
QGP & Heavy Ions
Experimentally, we hope to reach the high-energy domain of QCD by doing collisions of heavy ions. For our purposes, heavy ions are the largest chunks of matter made out of protons and neutrons that we can conveniently obtain and manipulate in the laboratory (our technology is far away from being able to produce a collision of neutron stars at high energies!). For more than 25 years, physicists have been doing experiments colliding heavy ions to look for clues of behaviour indicative with formation of a QGP. The experiments have been gathering evidence not from doing a single measurement, but rather from studying how several different observables behave.
At RHIC, the larges nuclei used are of gold (Au). In a Au+Au collision at the highest energies (200 GeV per nucleon pair), the normally spherical nuclei are Lorentz-contracted into a pancake-shaped object moving at about 99.995% of the speed of light. The contraction is due to special relativity, if you squeeze a sphere 100 times along its direction of motion it will look like a pancake. Each collision produces hundreds of particles. It is by studying the outgoing particles that we hope to learn if we create matter in which the quarks and gluons dominate the picture, even if they do so very briefly.
Heavy Quarks and "Quarkonium"
Quarks are one of the building blocks of the Standard Model of particles and forces. As we mentioned, they make up protons and neutrons, but also of many other particles (that's why they are sometimes called “building blocks” of matter). The Standard Model as we currently understand it, can be seen in a poster prepared by Berkeley Lab, shown here.
For more information on quarks and other particles, you can also visit a web page at CERN with lots of information on the fundamental particles and forces: The Particle Adventure.
Finding Bottomonium in Experiments at RHIC
There are 4 experimental collaborations at RHIC: BRAHMS, PHENIX, PHOBOS and STAR. Both PHENIX and STAR have capabilities to measure the charmonium and bottomonium states through their decay products. I will focus on the STAR experiment because that is the one I am more familiar with. The heart of the STAR experiment is a large acceptance tracker. You can get an idea of the size of the detector by looking at the platforms on the left of the picture, the detector is as big as a house with three floors.
Of interest to us lately is the addition of Electromagnetic Calorimetry to the STAR setup. These detectors are very important in the goal of measuring quarkonium states through their decay into an electron-positron pair. We are interested in reconstructing quarkonia states, such as the J/ψ (a bound state of a charm quark and its antiparticle) and the Υ (Upsilon, a bound state of a bottom quark and its antiparticle), through their di-electron decays in STAR to study their possible suppression in the QGP. The heaviest quark, the top quark, is much too heavy to be produced in the collisions at the energy available at RHIC. The bottom and the charm quarks are the next heaviest quarks, but are light enough to be produced at RHIC.
Heavy Ion collisions in STAR
The hundreds of particles produced in the collisions seen in the STAR detectors are shown in the Figure on the right. This is the information obtained from the large tracker, a detector components in STAR which is like a giant 3-D digital camera which records the trajectories of the particles. With it, we can obtain information on the number of particles in each collision, as well as the momentum of each particle. The detectors also help to discern the identity of the particles we measure: protons, electrons, as well as many other charged particles.
Finding Υ (Upsilons) in STAR
The picture at right shows an event with the response of the electro-magnetic calorimeter detector given by the more than 1000 detector boxes (shown mainly in green) making up a cylinder. When an electron or a photon strikes the calorimeter, it will deposit all its energy in the detector. We can measure the energy, and we can visualize the response of the detector by drawing a taller box with a different color when the energy is large. In the collision event shown at right, there are two detector modules that show a high energy deposition, shown as the red boxes sticking out near the 6 o'clock and the 10 o'clock position. These are consistent with a signal from high energy electron and its anti-paricle. The Υ (and the J/ψ) both decay leaving one electron and one positron, each with high energy (which comes from the large mass of the parent Υ or J/ψ thanks to E=mc2). Therefore, whenever we see an event like the one on the right, the detector system is programmed to record it for further study. By recording millions of such events, we can measure the properties of these particles and see if they get modified in the hot environment left in the wake of a heavy ion collision. There is already evidence that some of the states melt, which has been exciting news. The next questions will be to use these particles to tell how hot does the matter get, and get a quantitative estimate of the temperature.