The QGP, which has been studied at colliders for two decades, is a “perfect liquid,” with exotic properties. Among the most important experimental tools to study the QGP are jets, which arise from rare hard scatterings of quarks and gluons from the colliding nuclei, and are seen in the detectors as correlated sprays of particles. Jets generated in head-on (“central”) nuclear collisions plough through the QGP and interact with it before flying off to the detectors. This interaction causes the jets to lose energy (“jet quenching”), suppressing their production rate relative to that observed in proton-proton collisions and other simple systems, where a large QGP is not expected to be formed.

Over its continuing mission, the STAR detector at RHIC played a leading role in the initial discovery and elucidation of jet quenching two decades ago, and it continues to be a pioneer in this area. Figure 1 shows a STAR event display of a central Au+Au collision which includes a pair of energetic jets that are back-to-back in azimuthal and at 90 degrees to the beam direction, as expected from the hard scattering of incoming quarks or gluons. While such jets are easy to see when highlighted in color, finding and measuring them accurately in the complex environment of Au+Au collisions is very challenging. The key to an accurate measurement was a novel approach to background suppression. Members of NSD’s RNC program were leaders in this development.

Figure 2. New STAR measurement of the yield suppression of jets in head-on Au+Au collisions (filled blue points). No suppression corresponds to the value of unity. Also shown are similar measurements for jets at the LHC and single charged particles.

Using these novel techniques, STAR recently reported the first measurement of jet yield suppression in central Au+Au collisions at RHIC [1]. This paper, which was written by members of the RNC and their collaborators in Prague, opens a new chapter in the study of jet quenching. Figure 2 shows the strong yield suppression of jets in central Au+Au collisions compared to that in glancing (“peripheral”) collisions (filled blue points). The figure also shows a similar jet measurement by ALICE for jets at the LHC (filled red) and for single charged particles at both RHIC and LHC (faded blue and red); such comparisons provide crucial constraints on theoretical models. These new data are a significant step towards meeting the goal of the 2015 NSAC Long Range Plan to explore the inner workings of the QGP using jet probes.

Reference
[1] J. Adam et al., arXiv:2006.00582, to appear in Physical Review C.