University of Illinios at Chicago, UIC
European Center for Nuclear Research, CERN

Why Collide Heavy Ions? (for the layman)

One microsecond after the big bang, the incredible amount of energy that was initially compressed into the singularity at the center of our universe began to “hadronize”, thus forming the stuff that would one day make up everything and anything we know. Our current understanding is that after this initial microsecond, protons and neutrons were formed, eventually making atoms, then gases, galaxies, stars, and planets. In light of all that has happened in the 15-billion-year history of our universe, one particular microsecond can seem rather insignificant and uninteresting. However, if one begins to ponder the question of what this state of the universe during the unusual period of time after the big bang, but before the formation of matter actually was like, that particular microsecond can seem quite special.

Not since this first microsecond had this state of matter existed in our universe (that we know), until physicists had the zealous idea to try to recreate these conditions in the laboratory. In the field of high-energy heavy-ion physics, heavy atoms are first stripped of their electrons, then slammed together at near the speed of light, The purpose is to recreate and study this very special moment in history.

To understand how one studies the first microsecond of the universe by colliding lead ions at near the speed of light, one has to first understand some things about matter. Most of the matter in the universe is made up of protons and neutrons, which are specific examples of particles called hadrons. In the critical first microsecond of our universe, the energy density was so high, due to enormous pressures and temperatures, that hadrons simply could not exist, much the way ice cannot exist in a stable way inside your 350-degree oven. However, when hadrons melt, so to speak, something more profound happens than in the case of ice and water. Their mass gets primarily converted to energy, and what exists instead is a quark-gluon plasma governed by the strong nuclear force. The familiar phase boundary between condensed water (ice) and liquid water has a temperature associated with it (0 degrees Celsius). Similarly, there is a critical phase boundary temperature where hadrons or the chiral condensate melts. Theorists, utilizing the techniques of lattice gauge theory, have made a prediction of around 2x1012 degrees Celsius for the temperature of this phase boundary. To put this in perspective, that is about 100,000,000 times the temperature of the surface of the sun. Our estimations of the energy density created in relativistic heavy ion collisions tell us that we were above this temperature by a factor of 5 or so at the Relativistic Heavy ion Collider, an accellerator on Long Island, and we expect to be much higher at the Large Hadron Collider. We believe we are melting protons and neutrons into a quark-gluon plasma similar to the one that existed right after the big bang.

The quark-gluon plasma cannot be directly observed. This is due in part to the fact that quarks and gluons are fundamentally very different from the particles we are used to. They can never exist in isolation like hadrons. In order to form observable particles, they must bind with each other in very specific ways governed by the laws of quantum chromodynamics. In the quark-gluon plasma, hadronic matter has melted and conditions are too hot for quarks to bind with each other. In order to study the plasma, we must wait the small fraction of a second it takes to cool and freeze into new particles that radiate out into our detector. It is from these particles that we try to reconstruct the properties of the plasma itself.

To study the plasma, we exploit the fact that proton collisions of the same beam energy are not expected to create the quarkgluon plasma found in heavy ion collisions. Many of the measurements in heavy ion physics are actually comparisons of features between proton collisions and heavy ion collisions. This thesis is concerned with the yields and shapes of dijets.

When a hard scattering occurs in a proton collision, two scattered quarks will fly apart in opposite directions constantly pulling other quarks out of the vacuum that recombine to form showers of observable particles known as jets. Collisions that produce jets in opposite directions are known as dijet events, Figure 1 (left).

Figure 1. A drawing of a dijet event from a proton collision (left) and a immersed in a quark-gluon plasma (right)

Dijets make an excellent probe of the plasma in the following sense. When we look at dijet events where one jet has passed through the plasma, and the other jet has not, we can learn a great deal about the properties of the plasma from the modified shape of the jet that has traversed the plasma. To understand what we are looking at, these dijets produced in heavy ion collisions must be compared to dijets produced in proton collisions where there is no plasma. For example, one expectation is that a jet that travels through the plasma comes out the other side more spread out and with lower energy compared to a jet that doesn't traverse the plasma, ( Figure 1).

Among the things that could be learned is the nature of the energy loss mechanism in the plasma. Simply understanding how much energy is lost to the plasma as a function of how fast the quark is traveling through it would be extremely useful to theorists in better understanding the fundamentals of matter. Another measurement that could be made is the sound speed of the plasma, which could be deduced by measuring the angle of a machcone shock wave left by the quark. This phenomenon is similar to how a passing speed boat leaves a triangle shaped wake on water. By knowing the speed of the boat, and the angle of the wake, one can deduce the speed of the water waves, which tells us about the density and surface tension of water. This was the original motivation behind many correlation studies at the Relativistic Heavy Ion Collider in the mid 2000s. My research continues the work done at RHIC, but at higher energy density and in finer detail.