EXPERIMENTAL STUDIES OF PARTON PROPAGATION IN STRONGLY INTERACTING MATTER
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One important area of forefront research in strong interaction physics involves understanding how thefundamental particles of Quantum ChromoDynamics (QCD), the quarks and gluons, interact in spatiallyextended systems. Well-known research topics in this area include the study of gluon saturationat high parton densities, and jet quenching in ultra-relativistic collisions of large atomic nuclei. Whileperturbative treatments of QCD (pQCD) in proton-proton collisions produce excellent descriptions ofthe experimental data, the QCD-based description of processes in-medium is at a much more primitivestage, and there are many outstanding questions. One example is in the area of quark energyloss in-medium. Quark energy loss has long been conceived as occurring from two processes: partonicelastic scattering, and radiation of gluons, in analogy with the well-known processes in QuantumElectrodynamics (QED). Elastic scattering is expected to play a minor role for light quarks, while it isexpected to play a larger role for heavy quarks; gluon radiation is expected to be the dominant effectfor light quarks, and to play a smaller role for heavy quarks. These intuitive expectations have beenvalidated by explicit pQCD calculations in numerous studies. However, a comparison of light quarkobservables to heavy quark observables does not currently appear to conform to these expectations atall. As an example, b-quark jets appear to demonstrate the same suppression in heavy ion collisionsas light-quark jets in studies at the Large Hadron Collider (LHC). We propose to study heavy quarksuppression using data from the ATLAS detector at the LHC. The experimental technique of choice isto measure J= mesons in heavy ion collisions, comparing their production characteristics with thoseseen in proton-proton collisions, where no medium is present. To isolate the b-quark component, wefocus on J= mesons with a measured detached vertex relative to the primary collision vertex. Theb quarks predominantly decay into J= mesons long after they are produced, and thus these mesonsserve as a proxy for the primordial heavy quarks passing through the medium. Since any suppressionseen in heavy ion collisions is the result of attenuation of b-quarks in the hot, dense medium formedin those collisions, it is also of interest to compare these results to the same process as observed in coldmatter, such as the system produced in p+Pb collisions at the LHC. Further comparisons to cold mattercan be performed in electron-nucleus interactions, where the interaction of virtual photons provides aprobe that is free of initial-state interactions. Studies of this kind can be carried out with light-quarksignals using data from the EMC, HERMES, and CLAS experiments. Inter-comparison of the resultsfound from these different types of systems will provide new insights into the emerging field of partonicinteractions within systems of strongly interacting particles.