WIDG Seminar: Andrew Tamis, Yale, “Measurement of Two-Point Energy Correlators within Jets in p+p Collisions at STAR”

Hard-scattered partons that are ejected from high-energy collisions at both the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) undergo fragmentation as described by quantum chromodynamics (QCD) and hadronize into final state particles that are measured by the detector. The behavior of these showers can be studied using jets, clusters of final state particles used as a proxy for the initial parton. The substructure of these jets contains information about the time evolution of the parton shower. One observable that utilizes this substructure is the 2-point Energy Correlator (EEC), which uses the distribution of angular distance of all combinations of two final state particles within a jet weighted by their energy to cleanly reveal the separation and transition between two regimes of fragmentation: partonic (at large opening angles) and hadronic (at small opening angles).
In this talk, the first measurement of EECs at RHIC is presented, using the data taken at a center of mass energy of 200 GeV in p+p collisions by the STAR experiment. The EEC will be shown for several jet transverse momentum and radius selections and compared to predictions from PYTHIA, a Monte-Carlo event generator, as well as theoretical predictions for the calculable partonic region. This work will be useful as a baseline for comparisons to future studies using data from collisions of heavy ions. In these collisions, enough energy density is deposited to create a state of matter which is opaque to colored probes such as jets. Signatures of jet modification due to this interaction should be imprinted at a characteristic angular scale, which will be accessible using EECs.