The third Heavy Ion Jet Workshop was held recently in Lisbon, sponsored by MIT and Instituto Superior Tecnico. Over three days, more than twenty five theoretical and experimental physicists gathered and discussed recent results in jet physics and the modifications observed when jets propagate through the strongly coupled medium created in the high energy collisions of nuclear matter, or Quark Gluon Plasma (QGP). Of particular interest at this workshop were new experimental results recently presented at the Quark Matter conference in Darmstadt, new background subtraction techniques for a many particle environment, and the introduction of several Monte Carlo generators modeling heavy ion collisions.
Jets are of particular interest for study of the QGP, as in dijet and gamma-jet events they make excellent probes of medium properties. Jets correspond to high momentum partons scattered perpendicular to the beam pipeline, producing a highly collimated spray of particles which can be reconstructed with our detector and then clustered into the final object. As opposed to the vacuum case studied by our colleagues in particle physics, a loss of jet energy in the QGP has been observed. This energy loss is dependent on the size of the produced medium, as an increasing number of nuclear participants in the collision corresponds to greater energy loss. New experimental results and Monte Carlo generators seek to characterize this effect, and thus develop our understanding of the produced medium.
Doga Gulhan, Yen-Jie Lee, and Chris McGinn presented recent results on behalf of the MIT Heavy Ions Group, showing respectively results from dijet events in pPb collisions, an exploration of nPDF sensitive observables at the CMS detector, and a missing momentum study of dijet events in PbPb collisions. Additionally, Doga Gulhan presented a generator level study of dijet system eta distributions in pPb events, showing that the effect can be reproduced under the assumption that the proton is largely driving the shift. These experimental results in heavy ion jet physics can constrain the space of current explanations for observed jet quenching, and perhaps even inspire new ideas for further measurement.
To study jets and the impact of the QGP, a rigorous background subtraction method specific to the high particle multiplicity environment of heavy ion collisions is needed. The large numbers of particles produced in collisions of nuclear matter can obscure a signal from a jet, or enhance said signal if found on an upward fluctuation of the background. Thus, a jet must be corrected to more accurately reflect the true momentum and energy. To this end MIT postdoc Yue-Shi Lai presented the new HF/Voronoi subtraction technique. The panel here from his slides shows a map of momentum in eta-phi space before and after applying HF/Voronoi background subtraction.
The workshop concluded with some discussion of standardizing Monte Carlo output such that these new generators can be more readily incorporated into existing software frameworks. With standardization in place, cross comparisons between different generators and data will hopefully teach us more about the properties of jets in the QGP.