Study of Heavy Flavor Dynamics and Energy Loss using b jet Shapes Measured with CMS

Quantum Chromodynamics (QCD), the theory describing strong interactions, have achieved remarkable success in describing many aspects of experimental data from particle physics experiments, especially in perturbative regime. However, the higher-order contributions and non-perturbative effects in particle production present remaining challenges for the theory. These contributions are essential for modeling the production and subsequent fragmentation of b quarks; experimental inputs on the subject are essential for further theoretical developments. Experimental studies of jets initiated by the heavy b quarks (known as "b jets") provide an excellent opportunity to improve our understanding of QCD. The details of b-jet fragmentation in the vacuum and modification of b-jet shower properties due to interactions with quark-gluon plasms (QGP) are novel aspects of this work. This study presents the first measurements of b jet shapes representing the transverse momentum distribution of jet constituents about the jet axis, measured in proton-proton (pp) and lead-lead (PbPb) collisions at a center-of-mass energy per nucleon pair of 5.02 TeV using the data collected by the CMS detector.Measured b-jet distributions are compared to inclusive jets, elucidating fragmentation differences between the light and the heavy flavor partons. A deficit of transverse momenta at small radial distances from the jet axis is observed in charged jet constituents distributions around b jets relative to inclusive jets. Model predictions for pp data provided by the PYTHIA and HERWIG++ event generators indicate that the gluon splitting contribution plays an important role in b quark production. A comparison of b jet shapes between pp and PbPb collisions shows this depletion is not substantially affected by the presence of QGP. On the other hand, this comparison indicates that the energy flow around the b jet axis is redistributed towards larger radii when QGP presents. This effect is more pronounced for b jets.