Researchers have discovered that entropy remains constant during the transition from a chaotic quark-gluon state to stable particles in proton collisions at the Large Hadron Collider. This unexpected stability serves as a direct signature of quantum mechanics' unitarity principle. The finding, based on refined models and LHC data, challenges initial intuitions about the process's disorder.
High-energy proton collisions at the Large Hadron Collider (LHC) create a brief, dense state of quarks and gluons, resembling a boiling sea of particles, before cooling into detectable hadrons. Intuitively, this shift from a seemingly chaotic early phase to a more ordered later one should alter the system's entropy, a measure of disorder. However, data from LHC experiments reveal that entropy stays unchanged throughout, defying expectations.
Prof. Krzysztof Kutak and Dr. Sandor Lokos from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Cracow published their analysis in Physical Review D. They refined dipole models, which represent gluons as quark-antiquark pairs with color charges, to better describe gluon system evolution. "Dipole models based on the average number of hadrons produced in a collision allow us to estimate the entropy of partons," Prof. Kutak explained.
Two years prior, Kutak and Dr. Pawel Caputa of Stockholm University enhanced the model by integrating effects relevant at lower energies and drawing from complexity theory. Testing against data from ALICE, ATLAS, CMS, and LHCb experiments across energies from 0.2 to 13 teraelectronvolts, the generalized model outperformed predecessors. "We show that the generalized dipole model describes the existing data more accurately than previous dipole models and works well in a wider range of proton collision energies," Prof. Kutak stated.
This constancy aligns with the Kharzeev-Levin formula and stems from quantum mechanics' unitarity, which preserves probability and allows reversible processes. "The unitarity of quantum mechanics is something that physics students learn about... it is one thing to deal with a theory that exhibits a certain feature at the level of quarks and gluons... and quite another to observe it in real data," Prof. Kutak noted.
Future validations will come from the LHC upgrade enhancing the ALICE detector for denser gluon studies and the Electron-Ion Collider under construction at Brookhaven National Laboratory, where electron-proton collisions will probe gluon systems more directly.