Quarks, gluons and lo...the Universe

QUANTUM PHYSICS

When the Universe was born 13.7 billion years ago, it was a hot soup of quarks, gluons and other sub-atomic particles. As it cooled, the soup of quarks and gluons eventually congealed into protons, neutrons and the other atomic nuclei which exist today. The description of this soup soon before it congeals is one of the hottest research topics in physics.

Ernest Rutherford’s discovery of the atomic nucleus in 1911 started the study of what later became popular as the strong interaction of matter. From that starting point, physicists gradually established the theory describing how the tiniest building block of matter – known as quarks – and the force carrier called gluons interact with each other to create the world as we see around.

The physics theory which describes this astounding story of matter and force is called Quantum Chromo Dynamics or QCD in brief. Even though the theory – which eventually fetched a few Nobel Prizes by explaining the nature of Strong Interaction – was in place by mid 1970s and was tested in case of one or two particles, it was never tested in the realm of bulk matter.

A team of Indian, Chinese and US scientists for the first time obtained the experimental proof of QCD with bulk matter. “Before us there were no direct observations and no test of the theory in the realm of bulk matter,” lead author Sourendu Gupta at Tata Institute of Fundamental Research told Deccan Herald.

The scientists provided a temperature scale that paints a picture of the energy and thermodynamics involved in the formation of the quark-gluon soup.

The findings – reported in June 24 issue of Science – give physicists a method to better probe the internal structure of atoms, specifically how fundamental particles called quarks are held together to form protons and neutrons. Physicists can use the scale like a kind of thermometer to compare future experiments against.

Changing states of bulk matter

One of the most interesting aspects of bulk matter is that it can change state – from solid to liquid and gas. 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. Even though rough contours are slowly becoming clear, the final map remains 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.

The new finding establishes the first sign-post in the hot part of this phase diagram, said Bedangadas Mohanty, a high-energy physicist at Variable Energy Cyclotron Centre in Kolkata and corresponding author of the paper.

The predictions which are the basis of this work were made by an intensely computational method called Lattice QCD. This was carried out by Gupta and his colleagues in the last few years at TIFR using a 25 teraflop super-computer.

The researchers compared the predictions with results from a high-energy physics experiment carried out by smashing together gold nuclei at energies high enough to approach the big bang birth of the early universe. Millions of such small bangs produced at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven Laboratory, near New York provided the scientists with clues on how to study the conditions prevalent in the early Universe including the matter anit-matter mystery.

The comparison allowed this group 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. Below this temperature, baryonic matter (common matter) can take the form of the nuclei which Rutherford found a hundred years ago.

The QCD cross-over occurs at a temperature of about two trillion Celsius (175 MeV).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 35 million times hotter.

“This is the beginning of a new era of quantitative studies of QCD matter where theory and experiment constantly challenge each other,” Mohanty said.

Another sign-post on the phase diagram of QCD is the critical point where QGP transforms into normal matter (protons and neutrons) with the change of density without any latent heat.

Its position has been predicted by the Lattice QCD group in TIFR. Experiments at RHIC are currently searching for this point, he added.

“The search is happening not only at RHIC but yet another experimental set up called GSI near Frankfurt. Experiments are being planned for the next 15 years involving many young Indian students, which is a proof that Indians are also good at experimental physics and not merely theorists,” Gupta said.

The experiments are aimed at understanding matter, which means studying them after just a few microseconds after the Big Bang. “We cannot go back in time and study the evolution of the Universe - time travel is not possible till now. What we can do by QGP research is produce such a matter in laboratory and then perhaps study how the baby Universe was and perhaps then try to trace how it evolved into present day,” Mohanty said.