At the LHC, lead ions are smashed together at high speeds, creating quark-gluon plasma (QGP), a unique state of matter where quarks and gluons are free from confinement in protons and neutrons, similar to ice melting into water in QED. The QGP lasts only 10-22 seconds, making it difficult to probe directly. However, high-energy quarks and gluons from collisions form jets, which are narrow cones of particles. Scientists study how jets are modified when passing through QGP, such as energy loss and deflection.
Jets also form in proton-proton collisions, so comparing them with lead-lead collisions helps understand how QGP affects jets. As jets interact with QGP, they lose energy and change direction, akin to how a pellet slows down in water. This study focused on jets recoiling from colorless photons, which allow for a less biased energy comparison.
Graduate student Molly Park at MITHIG, under the guidance of Dr. Christopher McGinn, analyzed the angular difference (Δj) between two jet axes in photon-tagged jets. The Δj distributions in proton-proton and lead-lead collisions were similar, likely due to competing effects: jets surviving QGP tend to be narrow while scattering in QGP widens the jet axes.
The results are compared with the HYBRID theoretical model developed, which can variably include the quark-gluon plasma wake and deflections from medium particles. The HYBRID model, developed by Prof. Krishna Rajagopal’s group at MIT CTP, shows that the jet axis difference is relatively insensitive to medium wake effects, but is very sensitive to potential scatterings from the quark gluon plasma particles. The inclusion of scattering effects is found to significantly improve agreement with data.
* CMS News: https://cms.cern/news/journey-through-quark-gluon-plasma