Sarah Szabo

Independent Study 

Ágnes Mócsy

 

Glamorous Gluons

 

            Scientists interested in the earliest conditions of the universe are conducting research based on “little bangs” created in terrestrial labs. These “little bangs” created by colliding heavy nuclei at nearly the speed of light (more than 99.99%), recreate the superhot, dense conditions that existed a millionth of a second after the big bang. The incredibly high energy of these collisions causes the atoms to smash open, such that their protons and neutrons melt into their component quarks and gluons, temporarily freeing them. This is an extreme state of matter known as "quark-gluon plasma," which scientists believe existed at the birth of the universe [1].  First I will discuss how these collisions create quark-gluon plasma. Next I will explain what happens to the matter when it cools. Then, I will briefly explain how the results from these experiments can inform one about the properties of the quark-gluon plasma and the way matter behaves during and following such conditions. By studying these "little bangs," physicists can learn much about the fundamental nature of matter at the smallest of scales, which directly informs how it has come to exist in the largest of scales, creating the structure of our universe today.

            The Relativistic Heavy Ion Collider (RHIC) in Brookhaven, Long Island, and the Large Hadron Collider (LHC) in Switzerland are both conducting such particle collisions. Both use heavy nuclei (gold at RHIC and lead at LHC) because the more massive a nuclei, the more protons and neutrons, therefore the higher energy will be achieved in the collision. The more energy, the hotter conditions will be reached, because temperature is essentially a measure of the kinetic energy of particles. This is also why the particles are accelerated to relativistic speeds. It is important to mention that, at these speeds, the nuclei experience a Lorentz contraction, a flattening in the direction of motion due to Einstein’s special relativity. So, at the moment of impact, they are compressed into a very tiny volume, with each nucleus carrying 100 times the energy density of its normal state, up to 200 GeV at RHIC [1]. One electron volt is the energy an electron gains from an electric field of one volt. A GeV (giga electron volt) is a billion electron volts. The reason that such energies result in a quark-gluon plasma is due to special characteristics of the strong force.

            The strong force is what binds nuclei together. Like photons carry the electromagnetic force, gluons carry the strong force. The theory of the electromagnetic force is called Quantum Electrodynamics (QED), and similarly, the theory of the strong force, is called Quantum Chromodynamics (QCD).  QCD differs greatly from QED, however, because the gluons themselves have color charge, which causes them to be self interacting, also called self coupling. This characteristic results in the gluon’s limited range, despite being massless (such as the photon, which has infinite range for this reason) and a property referred to as asymptotic freedom. Contrary to the electromagnetic force, which decreases with increasing distances between the charges, the strong force does the opposite—it actually decreases with proximity and higher energies. Therefore, the closer the quarks, the higher the energy, and the less the strong force is felt [2]. It is asymptotic freedom that, under extreme high energy conditions, allows the quarks to be temporarily freed in the quark gluon plasma [3]. Asymptotic freedom is a fundamental property of QCD and Frank Wilczek, David Gross, and H. David Politzer received a Nobel Prize for its discovery in 2004 [4]. Another fundamental property of QCD is confinement; if a quark is knocked out of place, for example, the further away it moves the stronger the strong force will become. The strong interaction mediated by the gluons can be visualized like a rubber band—the harder it is tugged the more the force against the motion is felt. This is called confinement. If the energy is high enough, the “rubber band” snaps, but instead of the quark breaking free, the energy turns into additional quark and antiquark pairs. Thus creating new particles, also bound by the strong force [5]. The reason for this is because energy can be turned into mass, according to Einstein’s famous equation.

         As more and more particles were discovered in the 40’s, 50’s, and 60’s, it was eventually realized that most of the new particles could be understood as arising from the combination of a small number of constituents objects called quarks. The patterns of newly observed particles only made sense if the quarks had three possible sets of charge. In QED, there is only one set: positive and anti-positive (negative). In QCD, the three sets of charges are named after colors (Red, anti-Red, Green, anti-Green, and Blue, anti-Blue). The quarks always combine such that thecolor charge remains neutral or “colorless,” an analogy with how the primary colors of light combine to become white. Quarks always exist in either a triplet, called a baryon, which consists of a red, green, and blue quark, or a meson, a quark-anti quark pair of a color and its anti-color. Protons and neutrons are examples of baryons, and pions, j/psi’s, and upsilons are examples of mesons. The quark gluon plasma quickly expands and begins to cool. Within  of a second (about 50 trillionths of a trillionth of a second), the quarks become "reconfined" into baryons and mesons, which are collectively called hadrons, hence this process being referred to as “hadronisation” [1,6].

 

Although normal matter contains only protons and neutrons, there are over 100 other possible combinations of quarks, consisting of different types, called      flavors, which come in generations of increasing mass. The first generation consists of the lightest up and down flavored quarks, the down about twice as massive as the up. The second generation, called charm and strange, are respectively 100 and 20 times heavier than the up and down. The third generation quarks are referred to as top and bottom. The top is about 100 times heavier than the charm and the bottom about 40 times heavier than the strange. Thus, the top quark is quite heavy compared to the others. Each quark also has an anti-quark, which is also created in the collision, because an equal amount of matter and antimatter must be created in order to maintain neutral charge. So there is an anti-up, anti-down, and so on. Up and down quarks are the only quarks that exist in normal matter, bound inside the protons and neutrons. Protons consist of two ups and a down quark, and neutrons have two downs and an up. Such high energies allows for the creation of these more massive generations of quarks, and, in the transferring of energy, for  quarks to combine into other particles, however, they are unstable and decay (at a variety of rates), or in the case of matter and anti-matter, annihilate each other, creating further new particles [7]. Tracking of these

.decays is central to understanding what happens before the particles hit the detectors.

 

         In fact, this is how all the data must be analyzed. The plasma exists for such a brief time (10−15 seconds) that it cannot be observed directly. The detector’s data consists of measurements from when particles interact with the material in its sensitive volume. Scientists must look for patterns in this data and infer backwards [1]. This has led to some surprising discoveries. First, when nuclei collide, it is either head on or off center, referred to as central and peripheral collisions. The overlap region of off center collisions creates an almond shape (think of a Venn Diagram). The movement of matter from these regions is referred to as “elliptic flow.”  The reason for this is that the medium of the plasma has been found to act more like a liquid than a gas, as was expected based on the property of asymptotic freedom. It has been found that, unlike a gas which would be weakly interacting and spread out evenly, the matter is strongly coupled and flows collectively [3]. Therefore, variations in the pressure, depending on the section of the ‘almond’ from which the matter emerges (higher pressures coming from the short axis) interact directionally together, expanding from the original coordinate into momentum space. As a result, higher momentum and higher mass particles are observed close together (higher mass particles require more energy to be created), in what are called “hotspots” [8]. Much to scientists’ surprise, fluctuations were also observed in central collisions. Therefore, the overlap cross section of the nuclei could not fully account for the anisotropies in the energy distribution reaching the detectors. This discovery meant that these density fluctuations, also referred to as “lumpyness,” must also result from the random distribution of nucleons within nuclei, and that high energy particles must be pulling nearby particles with them. These discoveries bring insight into the expansion of initial density fluctuations from big bang, which are now manifested as the higher gravity areas in which matter has coalesced into superclusters of galaxies [9]. These discoveries have inspired various artworks; those which best illustrate this text are presented with blurbs below. (See References at end of PDF)