The Relativistic Heavy-Ion Collider (RHIC) started operation in the year 2001 at Brookhaven National Laboratory. For the first time atomic nuclei as heavy as gold were collided in head-on collisions at ultra-relativistic speeds. The commissioning of RHIC fulfilled a long-held dream of the nuclear physics community to finally be able to study in the laboratory a state of matter where the constituent quarks and gluons that are the building blocks of atomic nuclei become freed of their nuclear bounds.  The resulting quark-gluon plasma (QGP) is believed to have similarities to the state of matter in the Universe about a microsecond after the Big Bang.  A QGP may also form the core of some neutron stars.   The KU nuclear physics group is active in studying ultra-relativistic heavy-ion collisions, with participation in the construction, operation and on-going analysis of the BRAHMS experiment, one of the four experiments that was ready to take data with the start of RHIC operations.

At the speeds that particles achieve at RHIC, they become “flattened” in the direction of motion according to Einstein’s special theory of relativity.  When two of these “nuclear pancakes” moving in opposite directions collide, they end up passing through each other.  However, during the instant of overlap the mass of both nuclei occupy the same spatial volume for a fleeting instant.   It would be an understatement to say the energy density at the instant of overlap is astronomical:  the local temperature approaches a trillion degrees, far in excess of the temperature at the center of the Sun or any other star in the Universe. Coming into a typical head-on RHIC collision of two gold nuclei will be 158 protons (79 from each gold nucleus) and 236 neutrons.  Leaving the collision one finds on the order of six-thousand particles carrying electric charge, and thousands more with no charge and therefore difficult to detect.  The vast majority of these particle, charged or uncharged, have no prior existence, they are created in the tumult of the collision.  In addition to protons and neutrons, which are now in the minority, one finds a flood of subatomic particles such as pions and kaons.    Detecting and making sense of the large number of emitted particles was a technological challenge, but one successful met by the RHIC experiments.  An overview of the scientific motivation for the RHIC program, including animations of RHIC collisions, can be found on the RHIC web site.

While there is much yet to learn from the BRHAMS data, the program is starting to change its focus to the Large-Hadron Collider (LHC) located outside of Geneva, Switzerland. This new facility, which will start operations in the latter part of 2007, will increase the beam energy available for heavy-ion collisions by more than a factor of 10. At these energies, heavy quarks will start being produced in abundance, opening up new avenues for investigating the quark-gluon plasma.

The KU Nuclear Physics group has been continuously funded by the Nuclear Science Program of the U.S. Department of energy since 1989. This funding has recently been augmented by a U.S. DOE EPSCoR grant and an NSF Career Award to Professor Michael Murray.

In preparing for RHIC operations it was assumed that the most interesting particles to observe would be those coming off at an angle approximately perpendicular to the directions of the two RHIC beams. At these angles ALL of the particles are likely to have been created in the collision.  The largest of the RHIC experiments, STAR and PHENIX, were consequently developed to focus on this angular range.   BRAHMS was developed to survey the particle production over an extended angular range, covering from 2.7 degrees from the beam direction out to 90 degrees.   With this extended coverage it is possible for us to learn about the extent of the QGP along the beam direction and to learn more about the equation-of-state that describes the matter created in RHIC collisions.  Moreover, we can determine how much of the kinetic energy brought into the collision by the two gold nuclei is left behind to form the QGP and the subsequent production of new particles.

The KU group has had significant hardware and analysis responsibility for BRAHMS.  We have been involved in several hardware project and have had chief responsibility for the design and construction of BRAHMS Multiplicity Array, shown at the top of this page.  This detector system characterizes the “centrality” of collisions, that is, whether the collision occurs head-on or with the two nuclei brushing past each other in a peripheral interaction.  The group also has responsibility for the experiment's Zero-Degree Calorimeters (ZDC’s).  Prof. Murray developed these detectors before joining the KU faculty, but he retains overall responsibility for their operation.  The ZDC’s detect neutrons that leave the interaction region largely unscathed.  These are particle that did not participate in the reaction, and thus also reflect the collision centrality.  In addition, the KU group designed and constructed a threshold Cherenkov detector to help with the identification of created particles at large angles with respect to the beam direction.

The KU group is involved with several ongoing analysis projects.   We have published several papers on charged particle production at RHIC using data from the Multiplicity Array.  Using the threshold Cherenkov detector, we have studied the production of pions, kaons and protons at 40 and 90 degrees with respect to the beam direction. The ratio of protons to pions gives clues as to later stages of the reaction when the quarks recombine to form the observed particles.  We have also studied the angular dependence of the thermal behavior of the QGP medium by looking at different ratios of outgoing particles. 

We are currently focusing our efforts on two different aspects of the heavy-ion collisions.   The first is a study of the dependence of particle production with respect to the angle around the beam axis.  This azimuthal dependence depends sensitively on size and pressure gradients associated with the almond-shaped region of the interaction zone that is created when the ions collide somewhat off-center.  BRAHMS has the unique ability to study these properties based on identified particles emitted to forward angles.   In a separate study we are trying to contrast the behavior of two gold nuclei (with 197 protons and neutrons, each) colliding with that found when two copper nuclei (with 63 protons and neutrons, each) collide.   This comparison will allow us to start disentangling effects that depend on the size of the interaction region from those that may be more sensitive to the shape of this region.  Finally, we are studying the production rates for the phi meson at 40 degrees, exploring how production of this strange meson changes going to more forward angles. The phi meson is sensitive to the difference between left- and right-handed quarks and also measures the strangeness content of the plasma.

In the future we will be shifting our focus to the heavy-ion program now being developed for the CMS experiment at the LHC facility at CERN.  Already, we are actively involved in the development and construction of ZDC detectors for CMS.  By the end of 2007, corresponding to the commissioning of the LHC accelerator facility, we expect to be working primarily on the CMS project.  When the LHC turns on it will become possible to collide two lead nuclei at an energy per nucleon that is almost 27 times greater than the maximum energy achieved at RHIC.   We can try to guess what might change in collisions with that much energy, but the reality is that we can be almost assured of a number of years of discovery and surprises.