Paper detail

Constraining parton energy loss via angular and momentum based differential jet measurements at STAR

Parton energy loss has been established as an essential signature of the Quark-Gluon Plasma (QGP) in heavy ion collisions since the earliest measurements at RHIC indicating suppression of hadron spectra at high $p_{\rm{T}}$ and coincidence yields. Understanding this phenomenon of jet quenching is a requirement for extracting the microscopic properties of the QGP via jet-tomography. STAR has recently introduced a technique called Jet Geometry Engineering (JGE) wherein we enforce particular selection criteria imposed on the jet collection, such as recoiling off a high $p_{\rm{T}}$ hadron trigger along with an additional transverse momentum threshold for jet constituents in events with back-to-back di-jets. With JGE, we are able to control the extent of energy loss ranging from quenched/imbalanced to recovered/balanced di-jets. Since jet quenching is also expected to be dependent on the resolution/transverse-length scales with which the jet probes the medium, it is necessary to perform differential measurements with a handle on both momentum and angular scales. To quantify the angular scale within jets, we present the first measurement of the jet's inherent angular structure in Au$+$Au collisions at $\sqrt{s_{\mathrm{NN}}} = $ 200 GeV via the opening angle between the two leading sub-jets ($θ_{\rm{SJ}}$). We also measure the di-jet asymmetry $A_{\rm{J}}$ differentially as a function of the $θ_{\rm{SJ}}$ observable for these di-jets and find no significant dependence of the energy loss on the opening angle of the recoil jet.