At the start of July, MIT and Universite Pierre and Marie Curie jointly organized a workshop on Jets which took place in Paris. About twenty high energy and heavy ion theoretical and experimental physicists were in attendance discussing the challenges and new opportunities of doing jet measurements and predictions in heavy ion collisions. A jet is the experimental signature of a quark or gluon that is scattered off at very high momentum perpendicular to the direction of motion of the colliding particles, the beampipe, and into our detector. However a single quark or gluon can not freely propagate through the vacuum to reach our detector but will transform into more quarks and gluons sharing the original momentum, in a process called fragmentation. It will also pull particles from the vacuum to create a collimated spray of color neutral particles that exist for timescales observable by our detectors, in a process called hadronization. This spray of particles is what we call a jet and it carries exactly the energy and momentum of the initial parton, so by reconstructing the jet in our detector we can infer these kinematic properties of the partons that we can’t directly observe.
Jets are also the link between the theory and experiment of high energy quarks and gluons. One challenge is for experimentalists to make sure their measurements of jets are calculable theoretically, as well as theorists making jet predictions that are measurable. For this we have jet algorithms that both theorists and experimentalists agree upon and use, the anti-kt algorithm is most commonly used by our group to reconstruct jets. At this workshop we discussed how to expand the ways in which theorists and experimentalists can compare data and theory in more effective ways.
In the figure on the left are displayed two jets reconstructed with this algorithm in a collision recorded by the CMS detector. One of the jets here has almost three times the energy of the of the second one but is completely back-to-back , pi units apart in azimuth. This means the jets coming from a pair of particles scattering off one another conserve momentum in direction but not in magnitude, which implies the quark gluon plasma seems to slow down high energy quarks and gluons but not significantly change their direction.
However alongside the spray of particles originating from the jet, in heavy ion collisions there is also a large spray of particles everywhere in the detector from the expanding quark gluon plasma. This makes reconstructing jets to accurately reflect the properties of the original parton much more difficult in heavy ion collisions since it becomes ambigous which particles around the jet come from the quark gluon plasma and which come from the parton. At this workshop we discussed various techniques and new ideas to subtract these background effects and understand the nature of jet reconstruction in a dense environment.