by Tapan Nayak. Published: 19 November 2012

Tapan Nayak, ALICE scientist

A collage of one of the most spectacular ALICE Pb-Pb events and the nuclear phase diagram made it to the cover page of October 25, 2012 issue of the Current Science journal. This issue features an article on the topic of “Phases of Nuclear Matter”.

Current Science is a popular science journal, published by the Current Science Association in collaboration with Indian Academy of Sciences. Founded in 1932, the journal is intended as a medium for communication and discussion on important issues that concern science and scientific activities. The featured article finds a mention in the “In this Issue” section, followed by the main article. A gist of the article is given in the following paragaphs.

Matter exists in different forms or phases. This had been known to mankind very early on, even to ancient Indian philosophers, who were of the opinion that matter is made up of five great elements, or “Pancha Mahabhuta”, namely, earth, water, wind, fire and aether. Indeed these may be identified with our present knowledge of the states of matter, such as solid (earth), liquid (water), gas (air) and plasma (fire). The classical aether may be thought of as the void or vacuum. By application of external forces, it is possible for matter to change its form from one phase to the other, thereby changing its physical properties. Transition from one phase to another involves an abrupt, discontinuous change in the properties of a system. The study of phase transitions of different forms of matter has attracted the attention of scientists for a long time. The question naturally arises, what to expect in case of nuclear matter, does it also exhibit different forms when exposed to different thermodynamic conditions. The answer is yes, and in fact the transitions between different nuclear phases exhibit some of the most spectacular examples of phase transitions.

ALICE event in the cover of the Current Science magazine

The nucleus in its normal state exhibits liquid-like characteristics. When heated up, the nucleus may transform into a gaseous state, which is an indication of a liquid–gas phase transition. Further increase of heat results in high temperature and high energy density conditions of the nuclear matter, where a system of deconfined quarks and gluons, known as quark–gluon plasma (QGP), is formed. The QGP state is relevant to understanding the origin of the Universe and how matter behaved immediately after the Big Bang. Under circumstances of high baryon densities, matter behave much like the interior of neutron stars, which has been predicted to contain QGP matter. Thus, the phase diagram of nuclear matter spans a vast region of temperature and baryon density. Similar to the existence of critical point in case of phase transitions in water, there have been predictions of critical points corresponding to the phase transitions in nuclear matter. The study of the nature of liquid–gas and the QGP phase transitions and the search for the critical points have been at the forefront of research for last several decades.

The article gently introduces the concept of phase transition and then gives an overview of the nuclear phases, phase diagram and discusses how these phases are probed under laboratory conditions in dedicated facilities around the world. Dedicated experimental facilities at Brookhaven National Laboratory and CERN have been exploring the phase structure in great detail. ALICE at the LHC and the participation of Indian scientists find special mentions in the article. For more details, please follow the link to the article: