Quarks and gluons are elementary particles in the universe, and they cannot be divided into smaller components. Under the action of a strong force (one of the four forces in nature), quarks and gluons are tightly bound together to form composite particles, such as the familiar protons and neutrons.
The only way to separate these composite particles is to create some of the hottest states of matter in the lab, the quark gluon plasma (QGP). In this plasma, the density and temperature are so high that both protons and neutrons melt.
After the Big Bang, this "soup of particles" of quarks and gluons permeated the entire universe, but in less than a second, the universe had cooled enough for quarks to combine into protons and neutrons. Today, we can only study QGP through special devices, such as the Relativistic Heavy Ion Collider (RHIC). Recently, RHIC published its latest findings, which will help scientists further understand the properties of QGP.
Let quarks and gluons be free
RHIC is an "atomic shredder" with a circumference of about 4 kilometers that can create and study QGP at extremely high energies by accelerating and colliding two beams of gold ions (nuclei stripped of electrons). These high-energy collisions can melt the boundaries of protons and neutrons in atoms, releasing quarks and gluons in them.
But the question is, how do scientists determine whether a collision produced QGP? One way is to look for evidence of free quarks and gluons interacting with other particles. A particle called (upsilon) is the ideal particle for this task.
A particle is a short-lived particle that is bound by heavy quark-antiquark (bottom-anti-bottom) pairs and is very difficult to melt. But when the particle is placed in QGP, there are a large number of quarks and gluons around the quark-antiquark, and all of these interactions compete with the quark-antiquark interaction of the particle itself.
This "shielding" interaction breaks down the particles, allowing them to effectively melt and suppress the number of particles. But if quarks and gluons are still confined to a single proton and neutron, they cannot participate in the competitive interaction of decomposing quark-antiquark pairs.
In a quark-gluon plasma (background in figure), free quarks and gluons interact with the bottom and anti-bottom quarks that make up the particles. This quark-antiquark shielding interaction causes the particles to melt. (Photo/STAR)
In the past, scientists have observed the suppression of other quark-antiquark particles, that is, J/ψ particles composed of millet-antiquark pairs, but the particles are not the same as J/ψ particles. There are two main reasons for this: first, particles cannot be reformed in QGP; Secondly, there are actually three types of them.
Advantages of particles
We can first understand how these particles form. Corn quarks and bottom quarks and their antiquarks were produced very early in collisions, even before QGP. At the moment of collision, when the kinetic energy of the colliding gold ions is deposited in a tiny space, it triggers the production of many matter and antimatter particles because the energy is converted into mass. Quarks and antiquarks pair up to form particles with J/ψ particles, which can then interact with the newly formed QGP.
But since it takes more energy to make heavier particles, in this bowl of particle soup, there are much more light quarks and antiquarks than heavier bottom quarks and anti-bottom quarks. This means that even after some J/ψ particles are melted in QGP, other particles can continue to form, because slug quarks and reverse quarks find each other in plasma.
Because heavier bottom quarks and anti-bottom quarks are relatively rare, particle re-formation is rare. Suffice it to say, once a particle melts, it disappears. This means that, for scientists, particle counts are so clear that they don't get messed up like J/ψ counts.
Schematic diagram of three types of particles, from left to right, are the tightly bound ground state, the intermediate bound state, and the loosest bound state. (Photo/STAR)
Another advantage of particles is that, unlike J/ψ particles, they have three classes, including a tightly bound ground state, and two different excited states, where quark-antiquark pairs are more loosely combined. It's not hard to understand that the tightest version is the most difficult to pull apart and needs to be melted at a higher temperature. In this way, it is possible to rely on the differences between these three types of particles to establish a range of QGP temperatures.
First measurements were made at RHIC
Another group of scientists had previously measured the sequential melting of particles using the Large Hadron Collider's (LHC) CMS detector, and this is the first time the researchers have measured the inhibition of three particles on RHIC.
In the new study, scientists don't measure particles directly because they are fleeting. Instead, they measured the decay products of the particles. The team studied two decay "channels," one that would lead to electron-positron pairs, which would be received by RHIC's STAR's electromagnetic energy, and a decay path with positive and negative μ, which would be tracked by STAR's μ telescope detector.
In both cases, by reconstructing the momentum and mass of the decay products, it is possible to determine whether they came from a single particle. And, because different types of particles have different masses, scientists can also distinguish between the three.
In the absence of quark gluon plasma (yellow histogram) and in plasma (orange histogram with QGP background), the ground state (1s) and two different excited states (2s and 3s) vary in relative abundance and particle yield. There is no yield of particles in the 3s state in QGP, which means that all 3s particles may have melted. (Photo/STAR)
They found the expected pattern. The most tightly bound ground state has the lowest degree of inhibition or melting degree, the middle bound state has a higher degree of inhibition, and the loosest bound state has basically no particles, which means that all particles in the last group may have melted. But scientists also say that the level of uncertainty measured for this most highly excited loosely bound state is also considerable.
Exciting results
This is the most anticipated result of the μ telescope detector. The module, which was specifically proposed for tracking particles, was planned as early as 2005, began construction in 2010, and was installed before the RHIC ran in 2014.
According to the researchers, this is an extremely challenging measurement, but it also proves that STAR's μ telescope detector project is successful. The team will continue to use this detector component in the coming years, collecting more data and reducing the uncertainty of the results.
By measuring the level of particle inhibition or melting, researchers can infer the properties of QGP. Although the researchers did not yet know exactly what the average temperature of QGP would be based on this measurement, this measurement is gradually narrowing down the target range to get a clearer picture of this unique form of matter.
Reference source:
https://www.bnl.gov/newsroom/news.php?a=121097
Cover image & first image: STAR