Path length dependence of jet quenching measured with ALICE at the LHC

Jets are used to probe the quark-gluon plasma (QGP) that is created in heavy-ion collisions, by using the fact that medium-induced parton energy loss from elastic and radiative interactions between partons and the QGP lead to a modification of the measured jet spectrum. The dependence of the energy loss on the in-medium path-length provides insight into the energy-loss mechanisms and can be studied by measuring jet production relative to the orientation of the second-order symmetry plane. The azimuthal asymmetry in the jet production is quantified as `jet flow’, defined as the second-order coefficient of the Fourier expansion of the azimuthal distribution of jets relative the orientation of the symmetry axes of the initial nucleon distribution of the collision overlap region.
In this dissertation, measurements of jet flow of R = 0.2 charged jets, reconstructed with the anti-kt jet finder algorithm in Pb-Pb collisions with 0-5% and 30-50% collision centrality are presented. Jets are reconstructed at mid-rapidity using charged constituent tracks with transverse momenta between 0.15 and 100 GeV/c, and are required to contain a charged hadron with a transverse momentum larger than 3 GeV/c. The underlying event energy is subtracted jet-by-jet using a description which takes into account dominant second and third hydrodynamic flow harmonics. Jet flow is obtained from jet yields measured as function of transverse momentum, with respect to the experimentally accessible event plane, which is reconstructed at forward rapidities.
The reported jet flow has been corrected back to the azimuthal anisotropy with respect to the underlying symmetry plane by applying an event plane resolution correction. The jet spectra are corrected for fluctuations in the background transverse momentum density and detector effects through an unfolding procedure which is applied for different azimuthal orientations independently. The detector corrections correct back to particle level jets consisting of only primary charged particles from the collision.
Significant non-zero jet flow is observed in peripheral collisions. The observed jet flow in central collisions is of similar magnitude, but the uncertainties are larger and therefore the results are not significantly different from zero. The azimuthal dependence of the jet production is similar to suppression observed in measurements of flow of single charged particles at high transverse momenta and flow of jets comprising both charged and neutral fragments. Good agreement between the data and predictions from JEWEL, an energy-loss model simulating parton shower evolution in the presence of a dense QCD medium, is found in peripheral collisions.