Tachibana, Yuki; Sirimanna, Chamod; Majumder, Abhijit; Angerami, Aaron; Arora, Raghav; Bass, Steffen A.; Chen, Yang; Datta, Raktim; Du, Lijuan; Ehlers, Raymond; Elfner, Hannah; Fries, Rainer J.; Gale, Charles; He, Yuxin; Jacak, Barbara V.; Jacobs, Peter M.; Jeon, Sangyong; Ji, Yifan; Jonas, Fabian; Kasper, Lukas; Kordell, Michael; Kumar, Ashish; Kunnawalkam-Elayavalli, Rajeev; Latessa, Joseph; Lee, Yen-Jie; Lemmon, Richard; Luzum, Matthew; Mak, Simon; Mankolli, Abhishek; Martin, Clint; Mehryar, Hamidreza; Mengel, Tobias; Nattrass, Christine; Norman, Jennifer; Parker, Christine; Paquet, Jean-Fran莽ois; Putschke, J枚rg H.; Roch, Henri; Roland, Gunther; Schenke, Bj枚rn; Schwiebert, Lyle; Sengupta, Abhijit; Shen, Chun; Singh, Manpreet; Soeder, Daniel; Soltz, Ron A.; Soudi, Iman; Velkovska, Julia; Vujanovic, Gojko; Wang, Xin-Nian; Wu, Xiaojian; Zhao, Wei (2026).听.听Physical Review C, 113(3), 034910.听
This study looks at how high-energy particle 鈥渏ets鈥� (narrow sprays of particles produced in collisions) are altered when they pass through the extremely hot, dense medium created in heavy-ion collisions鈥攕pecifically lead鈥搇ead (Pb鈥揚b) collisions at very high energy. The researchers use detailed computer simulations (Monte Carlo methods, which rely on repeated random sampling) to model how these jets evolve as they travel through this medium. They focus on differences between two types of jets: quark jets and gluon jets (quarks and gluons are the fundamental particles that make up protons and neutrons, and they produce jets with slightly different properties). By studying specific features of the jet鈥檚 internal structure鈥攕uch as the angle between its main branches (called the soft drop prong angle, (r_g)), how momentum is shared between those branches ((k_{T,g})), and the jet鈥檚 effective mass after removing softer components ((m_g))鈥攖hey find that quark jets change in a more complex, non-linear way compared to gluon jets when interacting with the medium.
An especially useful case involves 鈥済amma-tagged鈥� jets, where a high-energy photon (纬) is produced alongside the jet. Because these events are more likely to involve quark jets, they provide a cleaner way to isolate how the medium affects jet structure. Interestingly, the modifications seen in these gamma-tagged jets stand out clearly and are not just due to biases in how the jets are selected for study. The results suggest that much of the observed change comes from the medium鈥檚 response to the jet鈥攅ssentially how the surrounding particles recoil and interact after being disturbed by the jet. Overall, the study shows that examining the fine details of jet structure, especially in gamma-tagged events, can be a powerful way to better understand how jets interact with and are modified by the dense medium created in high-energy nuclear collisions.
Fig 1: Distributions of jet splitting momentum fraction听饾懅饾憯听normalized by the number of all triggered jets (upper panel) and the number of jets passing the soft-drop condition (lower panel) for the leading jets in events generated with听pgun. The jet shower evolution is performed by vacuum听matter听for the parent parton having听饾惛init=140听GeV. Jets are reconstructed with听饾憛=0.4听at midrapidity听| | |饾渹jet鈦�|<2.0. The results are shown for quark jets (solid) and gluon jets (dashed) with different听饾憹jet饾憞听triggers, 112, 84, 56, and 28 GeV. The soft-drop parameters are听饾懅cut=0.2听and听饾浗=0.