Nuclear physicists have found a way to compare differently shaped particle collision events to study the influence of geometry on the patterns of particles picked up by detectors at the Relativistic Heavy Ion Collider (RHIC). Along the way, they uncovered evidence that led to a new understanding of how particles are produced in these collisions. Instead of the number of particles produced being dependent on the number of times any given nucleon (proton or neutron) collides with another nucleon, the results indicate that particle production depends only on whether a quark has a collision or not. These data rule out earlier models neglecting the colliding particles' internal structure, but they are consistent with at least two models that consider the substructure of nucleons—including one model that invokes super-dense fields of gluons, the glue-like particles that bind the building blocks of ordinary matter.
Being able to sort particle collision data into sets that depend on the colliding particles' orientations will allow RHIC physicists to explore the effect of collision geometry on other research questions, including patterns of particle flow and the separation of positive and negative charges, while disentangling the effects of magnetic fields created in some collisions. Scientists can also better study particles' tendency to get stuck, or "quenched," while traveling through the quark-gluon plasma created in these collisions to explore its properties.
RHIC was set up to collide spherical gold ions (nuclei of gold atoms stripped of their electrons) to create a quark-gluon plasma—a hot, dense soup of the building blocks of matter as they existed very early in the universe. These collisions have revealed many intriguing features of this early universe form of matter, including patterns of particle flow that indicate it behaves like a nearly perfect, friction-free liquid. Some of those patterns appear to be influenced by the "football" shape of the overlap region formed when the spherical particles collide off center. But those off-center collisions also create powerful magnetic fields as the charged particles surrounding the football-shaped interaction region swirl. To study the effects of shape without the magnetic field, RHIC physicists collided oblong uranium ions. In this study, physicists from RHIC's STAR collaboration explored a way to sort out collision data from events where the ions collided tip to tip, creating a spherical overlap, with ones where they collided "body to body" to create the football shape without the magnetic field. Their method looked for differing patterns of particle flow that varied with the total number of particles produced.
While this method worked to separate the tip-tip from body-body collisions, the data didn't fit with the model the physicists were using to describe the initial density of the colliding ions. Instead, they discovered that two different models fit the data. One of these models describes an initial state that scientists believe emerges as gluons multiply and linger, reaching a state of saturation, in ions accelerated close to the speed of light (as they are at RHIC). When the scientists used those calculations to model the particle production that should be expected from the differently oriented uranium collisions, the calculations agreed with the data.
There is, however, another model that also fits the data, so the evidence in support of gluon saturation is not definitive. But both models indicate that the initial state of the colliding ions is granular at a scale smaller than that of the individual protons and neutrons that make up the colliding nuclei. Additional experiments may help to discern whether this granularity arises from a dense state of gluons, or something else.
STAR collaboration, RHIC, Brookhaven National Laboratory
Research at RHIC is supported by the DOE Office of Science (NP) and these agencies and organizations.
L. Adamczyk et al. (STAR Collaboration), "Azimuthal anisotropy in U+U and Au+Au collisions at RHIC." Physical Review Letters 115, 222301 (2015). [DOI: 10.1103/PhysRevLett.115.222301].