The study of collisions between atomic nuclei at ultrarelativistic energies makes it possible to create nuclear matter in the laboratory under conditions that have not existed since the universe was about a millionth of a second old.

In the collisions, it is possible to create energy densities that exceed the energy density of the proton by a factor of about 30. At such high energy densities, the relevant degrees of freedom are subnuclear and are those of quarks and gluons, which in our ordinary world are confined (and thus hidden) inside protons and neutrons.

“This type of fundamental research aims at understanding how nature works.”

The high energy state that is formed in the accelerator experiments makes it possible to study the strong interaction and its theory, Quantum Chromo-Dynamics, in the so-called non-perturbative regime. It bears on the understanding of the fundamental phenomenon of quark and gluon confinement, particularly in regard to the strong interaction.

The paper describes the results obtained by the BRAHMS experiment in its first three years of operation at the first nuclear collider, the Relativistic Heavy Ion Collider (RHIC).

It summarizes several important new discoveries demonstrating the formation of a new state of matter—the so called "strongly interacting Quark-Gluon Plasma" (sQGP) in head-on collisions between two nuclei of gold, each moving with energies of 19.7 TeV. This state represents a fluid-like state of quarks and gluons.

Among the discoveries are the phenomena of jet-quenching by the high density nuclear medium and indications for a possible new type of Bose-Einstein condensate consisting of low-momentum gluons inside the colliding nuclei.

The paper contributes to substantiating the picture that the early universe consisted of particles (quarks, electrons, neutrinos) and force carriers (gluons, photons) in an extremely dense and hot state (temperatures in excess of 10° to 13° Kelvin).

As the temperature of the expanding universe decreased, the quarks and gluons became trapped into the nuclear particles that constitute over 99% of the mass of all atomic nuclei in the universe. The temperature and energy density at which this transition from the none-trapped to the trapped phase occurred has been a theoretical conjecture.

The experiments at RHIC are now revealing the fluid-like properties of the matter and its properties before the confinement occurred. In this sense, the experiments constitute a controlled "Little Big Bang" in the laboratory.

I became interested in ultra-relativistic heavy ion collisions when the decision to build RHIC became a reality. I then decided to move from other types of nuclear physics research at lower energies to this new energy domain.

This type of research involves the design, construction, and operation of a very large, costly, and complex apparatus. It is the result of a collaboration between many physicists, computer experts, and technicians—in the case of the BRAHMS experiment, a group consisting of about 70 individuals.

To mount such an enterprise requires convincing funding agencies of the necessity of large grants, assembling a research team of adequate size, managing the necessarily complex infrastructure, and all the while keeping an eye out for any new phenomena that hide within the experimental data.

Problems have been abundant, but none have proved to be insurmountable.

This type of fundamental research aims at understanding how nature works. The type of experiments described here can be thought of as extending our understanding of our "history" back by about 15 billion years until the universe was only a millionth of a second old.

Any child who has access to modern quality education today knows about the Big Bang theory. The perspective that follows is of high cultural importance and places civilization in a grander context. It helps make individuals better understand their placement in the scheme of nature. In this sense it cannot but affect all of cultural, philosophical, and religious thinking.

Experience has, however, also repeatedly shown that an increased comprehension of basic rules can lead to applications and inventions that will contribute to daily life. A deeper understanding of the strong interaction will no doubt one day lead to practical uses, just as a deeper understanding of electromagnetic interaction has shaped our present daily life.