Physicists from the four experimental collaborations collecting data at the Relativistic Heavy Ion Collider (RHIC) -- the world's largest facility for nuclear physics research, located at the U.S. Department of Energy's Brookhaven National Laboratory -- presented their latest results and analyses at the Quark Matter 2004 meeting held in Oakland, California, January 11 – 17. The Quark Matter meeting is an international affair, drawing upwards of 650 physicists to discuss the latest findings on heavy ion physics from facilities around the world.
The RHIC presentations were marked by various bits of corroborative evidence that collisions of gold ions at the Brookhaven accelerator are producing an extremely dense, "sticky" form of matter, quite possibly the postulated quark-gluon plasma, which scientists believe last existed a few microseconds after the Big Bang. There was also animated discussion about other intriguing physics results, including the possibility that RHIC experiments have detected the presence of another dense form of matter, known as color glass condensate, in RHIC's gold ions before collisions take place, and possibly, also, an exotic type of particle containing five quarks.
"We have quite a lot of intriguing results, but it may take some time to sort out their significance in relation to the search for quark gluon plasma or other new discoveries," said Sam Aronson, chairman of Brookhaven's Physics Department and a collaborator on RHIC.
Others were not quite so conservative. Scientists quoted in stories appearing after the first day of the conference in the Oakland Tribune and the New York Times, respectively, all but declared that discoveries of quark gluon plasma and color glass condensate had been made. But the general tenor of the physicists at the conference was less focused on answering such yes or no questions, and more concerned with probing the observed phenomena and understanding the detailed properties of what is being created at RHIC, which all agree behaves as a new form of matter.
RHIC results that generated the most "buzz" in the halls of the Oakland Convention Center:
* More evidence that some jets of particles produced in RHIC's gold-gold collisions are getting stuck in some sort of "goo" created by the collisions. This phenomenon, known as jet quenching, was first hinted at after RHIC's first run in 2000, and presented at the Quark Matter 2001 meeting held at Stony Brook University and BNL. But the findings from more recent runs, including control experiments colliding protons with protons and deuterons with gold ions, now confirm that the quenching in gold-gold collisions is a result of the environment created by the collisions, and not some preexisting condition of the gold ions. The more central the collisions of gold ions, the more likely that some jets get stuck in the goo.
* A new geometrical analysis. Physicists have now been able to map out the geometry of the collision zone, showing that it is indeed almond shaped, as anticipated. This confirmation gives them yet another way to test the idea that of jet quenching: Jets that have to travel along the long axis of this almond should be more likely to get stuck than those traveling the shorter distance across the almond's width. That's exactly what the physicists found.
* The first measurements of direct photons coming from the collision zone at RHIC. Because such photons are unaffected by the collision environment, they can serve as a thermometer. Just as the color of a glowing hot object gives an indication of the object's temperature, the number and wavelength distribution of these photons will tell scientists with good precision the temperature in the first stage of the collision.
* Preliminary results on J/psi production in deuteron-gold collisions. The production of J/psi particles (jointly discovered at Brookhaven and the Stanford Linear Accelerator Center in 1974) is expected to be suppressed by the presence of quark-gluon plasma. This is because the charm and anti-charm quarks that make up the J/psi have a hard time finding one another in the goo. But in order to see whether that is the case in the gold-gold collisions, the physicists need a control experiment – a measurement of J/psi production in collisions where no plasma is created. This measurement from deuteron-gold collisions will serve as that baseline for comparison with results from the gold-gold run now under way at RHIC.
* A confirmation that mesons and baryons emerge differently from RHIC collisions. In a certain range of momentum, mesons (particles made of two quarks, such as pions and kaons) have less of a certain type of collective motion known as "elliptic flow" than baryons (particles with three quarks, such as protons and neutrons). This could be a function of their different quark number, or of their different masses (the most abundant mesons are generally lighter than baryons). RHIC scientists now have data comparing the elliptic flow of the phi meson and the proton, which are very close in mass. The phi meson appears to emerge with reduced flow similar to the other mesons, indicating that this effect is indeed due to the quark structure, not particle mass. If this result holds up, it could provide crucial evidence that these two- and three-quark particles all came from a hot soup of strongly interacting quarks and gluons in the first moments of the collision.
* The ability to measure particles containing a heavy charm or bottom quark. These quarks are among the heaviest in the quark family. They are interesting because the extent to which they mimic the behavior of lighter quarks may be an indication of the extreme conditions, such as high temperature, present in the very first moments of collision, when the quark-gluon plasma is expected to be formed.
* A new type of particle suppression most evident close to the direction of the beam in deuteron-gold collisions. Some theorists have postulated that as gold ions are accelerated to the very high energies at RHIC, gluons that flit into and out of existence within the gold nucleus would appear to be longer lived, resulting in an extreme dense collection of gluons known as color glass condensate. A small particle colliding with such a gluon "pudding" would be much less likely to interact with any individual gluon in the nucleus to produce the kinds of particle jets observed in other collisions. Three of RHIC's experiments have now detected a deficit of these particle jets in deuteron-gold collisions. Some physicists have interpreted this finding as evidence for the discovery of the color glass condensate within the gold nuclei. Others would like to see further evidence before drawing such a bold conclusion.
* Possible detection of a particle composed of five quarks. All the known particles that are composed of quarks contain either two or three. But a number of institutions around the world recently presented results indicating that other multiquark states were possible. Heavy ion physicists, being the experts in "scrambling" quarks, figured they should take a look to see if anything similar was emerging from collisions at RHIC and also at CERN, the European particle physics laboratory. Early results indicate that a new form of pentaquark has been seen at CERN, and an anti-pentaquark has been produced at RHIC, but further analysis is needed to confirm these results.
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