Physicists found hidden order in violent proton collisions
Inside high-energy proton collisions, quarks and gluons briefly form a dense, boiling state before cooling into ordinary particles. Researchers expected this transition to change how disordered the system is, but LHC data tell a different story. A newly improved collision model matches experiments better than older ones and reveals that the “entropy” remains unchanged throughout the process. This unexpected result turns out to be a direct fingerprint of quantum mechanics at work.
High energy proton collisions can be pictured as a roiling sea of quarks and gluons, including short lived virtual particles. At first glance, this extreme environment seems far more complex than the later stage, when fewer and more stable particles fly outward from the collision point. One might expect particles in this early phase to behave very differently. But data from the Large Hadron Collider (LHC) show that this intuition is misleading. The results are better explained by a refined model that captures how proton collisions truly unfold.
When two protons collide at very high energies, an enormous amount happens in an instant. Protons are hadrons, meaning they are made of partons, which include quarks and the gluons that hold them together. During a collision, these quarks and gluons, including virtual ones that appear only briefly, interact in complicated ways. As the system cools, quarks combine to form new hadrons that scatter outward and are detected by experiments. Based on this picture, it seems reasonable to assume that the disorder of the system, known as entropy, should change between the early parton phase and the later hadron phase. The parton stage looks especially chaotic, with many particles interacting at once.
New Research on Entropy in Proton Collisions
The latest findings on this question were published in Physical Review D by Prof. Krzysztof Kutak and Dr. Sandor Lokos from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow. Their work focuses on comparing entropy in the early quark gluon phase with the entropy of the particles eventually produced and measured.
"In high-energy physics, so-called dipole models have been used for some time to describe the evolution of dense gluon systems. These models assume that each gluon can be represented by a quark-antiquark pair that forms a dipole of two colors -- here we are not talking about ordinary colours, but the colour charge that is a quantum property of gluons. Dipole models based on the average number of hadrons produced in a collision allow us to estimate the entropy of partons," explains Prof. Kutak, who has studied the entropy of quark gluon systems for more than ten years.
Improving Dipole Models With New Ideas
Two years ago, Prof. Kutak and Dr. Pawel Caputa of Stockholm University introduced an updated version of the dipole model. They started with an established model that describes how gluon systems evolve and treated it as the dominant contribution. They then added additional effects that become important at lower collision energies, where fewer hadrons are produced. This advance was possible because the researchers identified links between the equations used in dipole models and those found in complexity theory.
To test this generalized dipole model, Dr. Lokos suggested comparing it with real experimental data from the LHC. Measurements from the ALICE, ATLAS, CMS and LHCb experiments were included. Together, these data span a broad range of collision energies, from 0.2 teraelectronvolts up to 13 TeV, which is the highest energy currently achievable at the LHC.