Jets are primarily experimental signatures of gluons and quarks produced in high-energy processes like head-on proton-proton collisions at the Large Hadron Collider (LHC). The collimated sprays of particles generated by a high-momentum gluon or quark have been a key observable at subatomic scales in high-energy particle and heavy ion physics.
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Gluons and quarks are not directly observed in nature as they have a net color charge and cannot exist freely due to color-confinement. Instead, they undergo hadronization to form color-neutral hadrons. This hadronization results in a collimated spray of hadrons called a jet.1,2
The Mystery behind Jet Formation
During the powerful proton-proton collisions, the colliding protons’ individual building blocks/quarks and gluons scatter off one another and get knocked free with enormous energy. However, quarks cannot stay free for long. Thus, they, along with the gluons that hold them together, immediately undergo fragmentation. Fragmentation is a rapid branching process where quarks and gluons split and recombine to form new composite particles made of triplicates/pairs of quarks (hadrons), which spray out of the collision in a coordinated way.1-3
These jets, though born from chaotic collisions, display surprisingly structured directional patterns. Thus, a mystery remains: how do these resulting particles "remember" the properties of their origin? Scientists have been investigating whether the distribution of hadrons within a jet could be influenced by quantum entanglement among the gluons and quarks at the time the jet first formed —an area of ongoing research at the frontiers of quantum chromodynamics (QCD).1-3
Role of Quantum Entanglement
Recent studies suggest that quantum entanglement plays a crucial role in how particles retain their origin information in jets. In groundbreaking research conducted by physicists at Stony Brook University and Brookhaven National Laboratory, evidence shows that particles produced within jets in proton-proton collisions at the LHC preserve information about their origins. This discovery reveals a direct connection between entanglement entropy (a measure of quantum correlation) at the earliest jet formation stage and the emerging particle distribution/distribution of hadrons like kaons, pions, and protons in the resulting jets.3-5
Using data from the ATLAS experiment at CERN’s LHC, the study showed that the entropy of hadrons, which form after the fragmentation of quarks and gluons during collisions, matches the fragmentation process’s entanglement entropy. This means the apparent “messiness” or disorder in the resulting hadron distribution/final hadron spray correlates with the earliest jet formation stage’s entanglement entropy. Entanglement entropy measures the quantum entanglement between particles in a system. Here, maximal entanglement implied a high degree of disorder among the constituent hadrons of the jet.3-5
The findings displayed that observed particle distributions align with the maximal entanglement-based predictions during fragmentation. Hence, this connection provides new insights into the influence of quantum effects on hadron and jet formation. This work builds on earlier findings linking quark-gluon entanglement within protons to the particle distributions emerging from both proton-proton and electron-proton collisions. By extending this approach to jet production, researchers have uncovered a fundamental quantum mechanism underlying one of high-energy physics’ most striking phenomena: the structured yet complex behavior of jets.3-6
Entanglement as an Information Bridge
In QCD, the asymptotic freedom means that gluons and quarks (partons) interact weakly at short distances, while parton interactions become strong and lead to color confinement that binds partons within hadrons at large distances. Particles within a high-energy hadron are described using parton distribution functions (PDFs). These functions represent the probability of finding a parton with a given resolution scale and momentum. PDFs are probabilistic and do not contain full information regarding the proton wave function that represents coherent quantum superposition of Fock states with different phases and with various numbers of partons.4
Due to QCD evolution, the maximal entanglement within a hadron has reached at small x owing to equal probabilities of several configurations with different parton numbers. The maximal entanglement at small x has been observed in diverse QCD processes, including inclusive electron-proton deep inelastic scattering and minimum-bias proton-proton collisions at the LHC. Recent studies have determined and validated the QCD evolution of entanglement entropy against hadron entropy, establishing quantum entanglement as a complementary approach to understanding parton distributions within hadrons.4
A paper recently published in Physical Review Letters extended the study of maximal entanglement in QCD to the hadronization of high-momentum jets. At high momentum, the QCD renormalization group flow transforms a jet into a complex multiparton system, expected to exhibit maximal entanglement. Since PDFs and fragmentation functions (FFs) are related by crossing symmetry, reciprocal relationships between them suggest that jet states also become maximally entangled.4
The study found that the entanglement entropy of jets is closely linked to FFs, much like its connection with PDFs. Moreover, the initial entanglement entropy of partons has a lower bound defined by the number of color degrees of freedom, consistent with asymptotic freedom at weak coupling. These findings show that fragmentation processes at high transverse momentum exhibit maximal entanglement, mirroring the behavior of proton wave functions at high energies.4
Theoretical and Experimental Advances
Recent advances in quantum information theory are shedding light on the internal structure of jets in high-energy physics. Both simulations and experimental data reveal that quantum entanglement plays a key role in shaping jet dynamics. For instance, a paper published in Nature reported the first observation of quantum entanglement in a pair of quarks, specifically top–antitop quark pairs, using data from proton–proton collisions at the LHC with a center-of-mass energy of √s = 13 TeV and an integrated luminosity of 140 fb⁻¹ recorded by the ATLAS experiment.7
Spin entanglement was inferred through the measurement of a single observable D, derived from the angle between charged leptons in the rest frames of the parent top and antitop quarks. The entanglement marker was measured to be D = −0.537 ± 0.002 (stat.) ± 0.019 (syst.) for 340 GeV < mtt< 380 GeV. The observed result deviated from the no-entanglement scenario by more than five standard deviations. This constitutes the first direct evidence of entanglement between quarks and marks the highest-energy observation of quantum entanglement to date.7
Implications for Fundamental Physics
Recent studies provide a novel quantum-level perspective on jet fragmentation, offering insights into how quantum entanglement influences hadron formation. This framework, first applied experimentally to the hadronization process, sheds new light on the transition from perturbative to nonperturbative QCD. These findings have profound implications for understanding the nature of confinement and hadronization, potentially guiding future discoveries in QCD. The upcoming Electron-Ion Collider promises to further explore these quantum effects, advancing our knowledge of the quantum nature of hadronization.3-7
Looking Ahead
Ongoing studies are focused on mapping the "entanglement footprint" in collider events, seeking to trace how quantum entanglement influences particle creation. As quantum effects are increasingly understood, this intersection of quantum physics and high-energy experiments could reshape the interpretation of jet formation and hadronization. These investigations are expected to deepen the understanding of confinement and fragmentation. The future of this research holds promise for revealing new insights into the quantum nature of high-energy particle interactions. Ultimately, this work could open the door to groundbreaking discoveries in QCD.
References and Further Reading
- Jets at CMS and the determination of their energy scale [Online] Available at https://cms.cern/news/jets-cms-and-determination-their-energy-scale (Accessed on 21 April 2025)
- The Definition of Jets in a Large Background [Online] Available at https://www.bnl.gov/jets18/ (Accessed on 21 April 2025)
- Study Sheds New Light on Particle Creation [Online] Available at https://news.stonybrook.edu/university/study-sheds-new-light-on-particle-creation/ (Accessed on 21 April 2025)
- Datta, J., Deshpande, A., Kharzeev, D. E., Naïm, C. J., & Tu, Z. (2025). Entanglement as a Probe of Hadronization. Physical Review Letters, 134(11), 111902. DOI: 10.1103/PhysRevLett.134.111902, https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.134.111902
- Maximal entanglement sheds new light on particle creation [Online] Available at https://phys.org/news/2025-04-maximal-entanglement-particle-creation.html (Accessed on 21 April 2025)
- 'Spooky action' at a very short distance: Scientists map out quantum entanglement in protons [Online] Available at https://phys.org/news/2024-12-spooky-action-short-distance-scientists.html (Accessed on 21 April 2025)
- The ATLAS Collaboration (2024). Observation of quantum entanglement with top quarks at the ATLAS detector. Nature, 633(8030), 542-547. DOI: 10.1038/s41586-024-07824-z, https://www.nature.com/articles/s41586-024-07824-z
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