In the conjectured phase diagram of QCD the hadronic state exists at low temperature and density and changes into the quark gluon plasma at the cross over temperature of 175 MeV, or about 2,000,000,000,000 Celsius. A critical point is also predicted.
The universe was born as a hot soup of quarks, gluons and other particles. As it cooled, the soup eventually congealed into protons, neutrons and the other atomic nuclei which exist today. The description of this soup at temperatures just above where it congeals is one of the hottest research topics in physics.
Rutherford's discovery of the atomic nucleus in 1911 started the study of what came to be called the strong interactions of matter. From that starting point to building the theory of quarks and their interactions with the force carriers, called gluons, has occupied nuclear and particle physicists for the last one hundred years. The final theory is called Quantum ChromoDynamics, QCD in brief. It has been tested in part earlier, and indeed the long development of this theory gave rise to a string of Nobel-prize winning work. However, until now, it was not tested in the realm of bulk matter.
One of the most interesting aspects of bulk matter is that it can change state. Water, for example, can change from solid (ice) to liquid or a gas (steam). A map of the states of quark matter as its temperature and the excess density of matter over antimatter changes is called the phase diagram of QCD. This map remains to be drawn. Rough contours are slowly becoming clear, but remain the subject of intense theoretical debate and experimental scrutiny. Measuring distances and putting sign-posts on this map is one of the hardest challenges in theoretical physics. Today the first sign-post in the hot part of this phase diagram has been posted.
The predictions which are the basis of this work were made by an intensely computational method called Lattice QCD. This was carried out by theorists in TIFR Mumbai using a method that they developed on the super-computers of the Indian Lattice Gauge Theory Initiative (ILGTI).
These predictions were compared to the results of experiments which were performed by smashing together gold nuclei at energies high enough to approach the big bang birth of the early universe. In millions of these small bangs at the Relativistic Heavy-Ion Collider (RHIC) in Brookhaven Laboratory, near New York, an experimental collaboration called STAR tracked fluctuations in the excess of matter over antimatter.
Each small bang creates a fireball of very energetic matter seething with quarks and gluons. As the fireball expands and cools, this matter comes into an equilibrium which depends on the initial energy pumped into it by the collider. If the collider energy is changed, then this equilibrium freeze out condition changes. That is the theory. This theory was verified by comparing the lattice QCD predictions of the fluctuations of the net amount of baryonic matter with experimental results.
The comparison also allowed an unique collaboration of experimental and theoretical particle physicists to put the first sign-post in the hot regions of this map. They marked a point called the QCD cross-over temperature (denoted Tc in the figure). Below this temperature baryonic matter can take the form of protons and neutrons (called hadronic phase in the figure). Above it one has the primordial quark soup (called the quark gluon plasma in the figure). This single temperature sets the scale of the phase diagram; that is, it says how distances are represented on this map.
This QCD cross-over occurs at a temperature of about 2 trillion Celsius (175 MeV). This is the value of the points on the vertical and horizontal scales which are marked as 1 in the figure. This is by far the largest temperature at which laboratory experiments have probed the states of matter. The surface temperature of the sun is about 5600 Celsius, so the QCD soup is about 350 million times hotter.
This is the beginning of a new era of quantitative studies of QCD matter where theory and experiment can improve by constantly challenging each other. Major lattice QCD facilities exist in the USA and Japan. India has made a start on building such facilities through the ILGTI, which plans to expand its computational ability. Experimental facilities include RHIC with the STAR and PHENIX experiments, the Large Hadron Collider (LHC) in Geneva with the ALICE, CMS and ATLAS experiments. In the near future more experiments are planned to start in Germany (Facility for Antiproton and Ion Research, FAIR) and Russia (Nuclotron-based Ion Collider Facility, NICA). India is a participant in several of these experiments. Older experiments such as CERN's Super Proton Synchroton (SPS) and Brookhaven's Alternating Gradient Synchrotron (AGS) also gave valuable insights into this field.
The first search in this new era is for another sign-post in the phase diagram of QCD. This is the critical point, that is to say, the point where one begins to see a latent heat of transition between the hadron and quark gluon plasma phases of matter. Its position has been predicted by the Lattice QCD group in TIFR. Experiments at RHIC are currently searching for this point.