The KU Nuclear Physics Group is an active member of the Compact Muon Spectrometer (CMS) experiment at the Large Hadron Collider (LHC) and works in the area of relativistic heavy-ion physics. During the November/December periods of both 2010 and 2011, the LHC collided lead nuclei at a sufficiently high energy to form a quark-gluon plasma. Here the neutrons and protons that make up the lead nuclei lose their identities and form a plasma state of the constituent quarks and gluons. These conditions are similar to what might have been present in the Universe shortly after the Big Bang. The goal of our group is to understand the properties of this very high-energy medium.
Although somewhat counterintuitive, at some level the study of high-energy heavy-ion collisions can be viewed as exploring the vacuum. At the quantum level the vacuum is not empty space but a sea of quarks, antiquarks and gluons. The regular protons and neutrons of which ordinary matter is made can be thought of as bubbles in this vacuum. They only exist because at this time the vacuum is very cold. In the early universe the vacuum was too hot for protons and neutrons to exist and the universe was a plasma of quarks and gluons. It is this plasma state that we recreate when colliding heavy ions together at CERN.
As one area of research, we are exploring the state of the colliding lead system before a plasma is formed. Because of the interaction of quantum mechanics and very strong time dilation the nucleus may resemble a solid, glass like sheet of gluons. The CMS is ideally suited to study this state because of its almost complete angular coverage for detecting particle emerging from the collisions.
We are also exploring how the geometric shape of the plasma formed in the collisions influences the azimuthal distribution of emitted particles. When two heavy nuclei, such as lead, collide it is likely that their centers will be offset somewhat. This leads to an almond-shaped region of maximum overlap. A remarkable discovery that occured at the Relativistic Heavy Ion Collider—the machine that held the record for high-energy heavy-ion collisions before the LHC came online—was that the plasma state formed in these ultrarelativistic collisions behaves as an almost perfect fluid. As such, the plasma can develop presure gradients and "flow" as it expands. This results in a very characteristic anisotropy of particles coming out of the collision that can then be used to explore how, for example, particle moving in the direction of the almond's waist might lose a different amount of energy than those moving in the direction of the long axis. These studies give us information about the detailed nature of the plasma state.
The KU group is also involved with detector development and, in particular, has been responsible for the CMS zero-degree calorimeters (ZDCs). These detectors, located along the beam axis on either side of the main CMS system, detect neutrons and electromagnetic radiation emitted at zero degrees in the colliding system. The geometry is made possible because the LHC beams circulate in two separate beam pipes other than the points where the beams collide. The ZDC detectors are sandwiched between the beam pipes.