RHIC Resumes Operation With First Deuteron-Gold Collisions

Complementary data sought in quest for quark-gluon plasma

UPTON, NY — Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory are continuing their quest for an elusive form of matter, but this time with a twist — instead of colliding gold ions at nearly the speed of light in the Relativistic Heavy Ion Collider (RHIC), they are colliding gold ions with deuterium ions in an attempt to help unravel the mystery.

End view of collision between deuterons and gold ions enlarge

An end view of collision between deuterons and gold ions captured by the STAR detector at Brookhaven

Last night marked the start of this year’s physics run at the 2.4-mile-circumference, two-ringed particle accelerator, the world’s biggest for studies in nuclear physics (see the RHIC website for more info). These new collisions of very heavy gold nuclei with relatively light deuterons (nuclei consisting of one proton and one neutron), scheduled to continue through late March, will provide a complementary view of these complex interactions and give physicists deeper insights into the forces that bind all nuclei together.

“If the protons and neutrons that make up all atomic nuclei were indestructible little spheres made up of tightly bound-together quarks and gluons, the properties of the deuteron-gold collisions could be related in a simple way, called ‘scaling,’ to the gold-gold collisions,” said Thomas Kirk, Associate Laboratory Director for High-Energy and Nuclear Physics at Brookhaven. “We believe, however, that the protons and neutrons in gold-gold collisions at RHIC energies are instead melting into a very hot gas of quarks and gluons.”

By comparing these two clearly distinguishable collision behaviors (hard spheres vs. plasma), RHIC experimenters will be able to seek additional evidence for the existence of quark-gluon plasma, a prime research goal of the RHIC physics program.

Analysis of data from collisions of gold nuclei obtained in previous RHIC physics runs has shown a peculiar effect, called "jet quenching," in which quarks emerging from the collision's center appear to be slowed or even suppressed by their interactions with the hot nuclear material. This effect is one possible signature of quark-gluon plasma formation, but other possible interpretations need to be examined.

In deuterium-gold collisions, while the two nuclei collide with the same high velocity (nearly the speed of light), the total energy of the collision is much lower, and the interaction takes place within the cold nuclear material of the gold nucleus. The scientists hope the differences they see in jet-quenching behaviors between hot matter and cold nuclei will provide additional evidence of quark-gluon plasma formation.

Collision between deuterons and gold ions

Collision between deuterons and gold ions captured by the STAR detector at Brookhaven.

Another effect that could help distinguish gold-gold from deuteron-gold jet quenching behavior is the phenomenon that theorists call the “color glass condensate.” One manifestation of this effect in gold-gold collisions would be to strongly enhance the number of gluons in the quantum state of the incoming ions right before each collision takes place. Sharing the quarks’ kinetic energy with extra gluons would then decrease the average energy of the quarks in the incoming gold ions, decreasing the number of jets observed.

This effect could partially offset or mask the quark-gluon plasma jet-quenching effect, but a full set of measurements, likely including a later RHIC run using intermediate size ions (such as silicon) could help sort out combined results arising from quark-gluon plasma and the color glass condensate.

“Even though the RHIC experimenters don't yet know how the story of quark-gluon plasma and color glass condensate will play out,” said Kirk, “they are well on their way to making the measurements that will tell the tale.”

Machine Upgrades

New RHIC instrumentation systems installed for this run include an accelerator tune measurement and tune feedback system, and injection oscillation dampers, important for preserving beam stability and small beam sizes, which produce high numbers of collision events. These new instruments will also play important roles during another RHIC run, using polarized protons, scheduled to begin later this year. These experiments seek to explain where protons get their spin, a property of elementary particles as basic as mass and electrical charge.

This work was funded by the U.S. Department of Energy, which supports basic research in a variety of scientific fields.

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