When two protons collide at enormous speed, a narrow beam of particles (a jet, like the spray from a stone thrown into water) shoots out from the point of impact. In collisions of heavy lead nuclei, things are more complicated: a hot cloud forms inside, which interferes with the jet's straight flight.
The measurement showed: if the jet is weak, its path barely changes. But if the jet is powerful, it noticeably deflects in the dense cloud. It's like a fire hose aimed through thick fog: the stronger the pressure, the more the stream shudders and veers.
Scientists compared the data with three different models. All of them describe how particles lose energy in the hot cloud. The best match is the one where the cloud behaves like a viscous liquid.
Such a jet deflection is a new tool. It allows peering inside the cloud without destroying it and understanding what it is made of.
as written by the authors arXiv:2602.18279
The first measurement of the jet axis decorrelation in events with jets recoiling from an isolated photon is presented for lead-lead (PbPb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The jet axis decorrelation is the angular difference ($$\Delta{j}$$) between two definitions of the jet axis. This quantity is expected to be sensitive to the scattering of jet constituents in the quark-gluon plasma (QGP). Events which have a leading isolated photon with transverse momentum 60 $$\lt$$ $$p_{\mathrm{T}}^{\gamma}$$ $$\lt$$ 200 GeV and recoiling jets with 30 $$\lt$$ $$p_{\mathrm{T}}^{\text{jet}}$$ $$\lt$$ 100 GeV are selected for the analysis. The PbPb result is reported as a function of collision centrality and compared to pp reference data. Jets with $$p_{\mathrm{T}}^{\text{jet}}$$ $$\lt$$ 60 GeV have consistent $$\Delta{j}$$ shapes for pp and PbPb collisions. However, a narrowing is observed for jets with $$p_{\mathrm{T}}^{\text{jet}}$$ $$\gt$$ 60 GeV in central PbPb collisions. The results are compared to predictions from the JEWEL, PYQUEN, and HYBRID theoretical models, which include different descriptions of parton energy loss in the QGP.
The temperature of the hot cloud in such collisions is thousands of times higher than at the center of the Sun.