It is the first paper in the refereed literature demonstrating that the "away-side jet" is strongly modified when propagating through a quark-gluon plasma, the state of matter formed in the collision of two heavy nuclei at very high energies. The measurement was performed by the PHENIX experiment at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC).

“The redistribution of energy to angles far from the 180-degree peak of the away-side jet is a new phenomenon. This has forced theorists to consider various hydrodynamic scenarios such as wakes, shock waves, and Mach cones to describe the response of the fluid.”

Previous work by PHENIX and three other experiments at RHIC had established that the matter formed in these collisions is nothing like a diffuse plasma, but is better described as a very dense "perfect liquid" which strongly absorbs high-momentum particles formed in the initial nuclear collision. The fluid is so opaque that the characteristic back-to-back orientation of high-momentum collision products ("jets") seen in proton+proton collisions is destroyed—the high-momentum away-side distribution is observed to disappear completely. The PHENIX paper demonstrates that the energy and momentum is transferred to the medium, that is, to the much lower energy particles that form the fluid.

It is a clear discovery. The redistribution of energy to angles far from the 180-degree peak of the away-side jet is a new phenomenon. This has forced theorists to consider various hydrodynamic scenarios such as wakes, shock waves, and Mach cones to describe the response of the fluid.

The rapid expansion of the system and particularly the "strong coupling" of the medium make these calculations very difficult, leading some theorists to work by analogy, computing the response of a similar plasma that, according to string theory, has a parallel description as a black hole in five dimensions.

The two major initial discoveries at RHIC were the fluid properties of the produced plasma and its strong absorption of high-momentum particles. This paper unifies those two observations. It shows that even high-momentum particles efficiently transfer their energy to the fluid, just as a bullet’s energy is absorbed by water and transferred to a splash or a shock wave.

For nine years I was privileged to serve as scientific spokesperson for PHENIX. The PHENIX experiment is a collaboration consisting of over 500 scientists drawn from more than 70 institutions in 14 nations. Planning for the experiment began in 1991.

From the outset, PHENIX was designed to combine measurements of both the low-momentum "bulk" medium properties and the high-momentum "jet" properties. Finding this beautiful interplay between those phenomena has been a wonderful confirmation of the wisdom of the PHENIX design concept.

The particular signal that is the subject of this paper is a subtle one that requires careful subtraction of a large background from the bulk flow of the medium. A great deal of work was spent in developing the proper subtraction techniques and cross-checking them. An even greater amount of work then went into convincing one of the referees of the validity of this procedure.

There is much more to be done on this particular topic. We need to determine if the away-side response actually has the geometric shape of a cone (analogous to a sonic boom). New and larger data sets will use high-momentum particles to perform detailed "tomography" on the plasma. Future upgrades to the experiment will allow us to extend these measurements to jets initiated by heavy quarks, which may be more amenable to theoretical treatment.

Not that I am aware of, but the converse is a very topical subject, that is, the implications of politics for this research. Both PHENIX and RHIC benefited during their construction phase from substantial contributions from our foreign collaborators. The Office of Science of the U.S. Department of Energy supplied a large fraction of the funds to build PHENIX, and since then essentially all of the money to operate RHIC each year.

Therefore, it has been very heartening to see broad bipartisan support for the President’s plan to double the budget of the DOE’s Office of Science. However, it has been extraordinarily disheartening to see those increases again threatened by the current budget agreement.

Apparently there is even broader bipartisan Congressional support for earmarking, which drains money from carefully planned and peer-reviewed programs in the Office of Science. The omnibus spending bill will mean another drastically shortened running period for RHIC this year. No research program, even one as exciting and fruitful as RHIC’s, can survive repeated years of such budgetary brinksmanship.