In the meantime, a few very energetic particles with energy thousands of times larger than the rest of the system are also produced sometimes amid the plasma. These jets of energetic particles scatter and lose energy as they propagate through the plasma, leading to a phenomenon called jet quenching. Traveling at the speed of light, jets are also expected to generate Mach-cone waves inside the QGP, as illustrated in Figure 1. In this hydrodynamic simulation, a jet accompanied by a photon that does not interact with the plasma created a Mach-cone wave (an animation of the simulations can be viewed here). However, the experimental detection of these jet-induced Mach waves remains elusive because of the enormous number of background hadrons from the rest of the plasma. Experimental observables of the Mach waves are further obscured by the variation of the path length and direction of the jet propagation.

Figure 2. Excess hadron yield correlated with a gluon-tagged jet as function of their relative azimuthal angle Dfhjet in Pb+Pb collisions relative to p+p collisions with and without restriction on the transverse asymmetry Ax and energy asymmetry pTjet/pTgluon. The negative yield in the direction of the photon is caused by the diffusion wake. The grey band is estimated background from multiple parton interaction.

In the latest study, NSDs Xin-Nian Wang and his collaborators propose to utilize a previously-developed 2-dimensional jet tomography event-selection algorithm to localize jet-production positions for different path lengths, based on both the transverse and energy asymmetry [1]. They find that the signal of the Mach waves and the diffusion wake is much enhanced over the background in these selected events, as shown in Figure 2. Wang and his collaborators propose that this event-selection technique can be applied to the current experimental analyses at LHC and future experiments at RHIC to identify jet-induced Mach waves. If confirmed, this will be the first time such Mach waves are detected on the subatomic scale. This development will open a new window to examine many properties of the quark-gluon plasma.

Reference

[1] W. Chen et al. Search for the Elusive Jet-Induced Diffusion Wake in Z/γ-Jets with 2D Jet Tomography in High-Energy Heavy-Ion Collisions, Phys. Rev. Lett. 127, 082301 (2021).