star focus: A new approach to jet quenching measurements

The STAR Collaboration has recently published a paper, Phys. Rev. C 96, 024905, presenting a novel approach to measurements of jet quenching, one of the most important ways to study the Quark-Gluon Plasma (QGP) generated in nuclear collisions at RHIC and the LHC.

High energy collisions generate "jets", which are correlated sprays of particles arising from the decay of energetic quarks and gluons. In heavy ion physics, jets provide self-generated tomographic probes of the QGP; they are produced in the collisions itself and interact with the surrounding matter before flying off to be observed in the detectors. This interaction between a jet and the QGP modifies jet properties dramatically relative to those in vacuum (“jet quenching”), and has produced some of the most striking measurements of the QGP. Such jet measurements are challenging, however. Jet quenching was initially discovered by STAR and PHENIX, in Au+Au collisions at RHIC, by studying distributions of single high-momentum particles and their correlations, which are indirect jet messengers.

The experimentalist’s task for accurate measurements of complete jets is to cluster together the particles of the jet spray (“jet reconstruction”) without including the particles generated by other, background processes in the same collision. This task is especially complex in nuclear collisions, where background processes generate thousands of particles in the detector. Reconstructed jet measurements have recently been carried out for heavy ion collisions at the LHC, focusing on very energetic jets to minimize the large background effects. The RHIC jet energy range is more limited, however, and accurate jet measurements in RHIC heavy ion collisions require new approaches to tackle the jet background problem head-on.

Left Figure: Measured jet rate (signal plus background) in head-on Au+Au collisions (red points) and the "mixed event" background (grey distribution). Right Figure: Azimuthal deflection of jets recoiling from a trigger particle (data points), and a calculation including QCD effects but not scattering in the QGP (red curve).

The new STAR paper utilizes the distribution of charged-particle jets recoiling from a high momentum single-hadron trigger to study jet quenching. The huge backgrounds underlying the reconstructed jet signal in Au+Au collisions are measured using a sophisticated event-mixing technique. The contribution of uncorrelated background is then corrected "statistically", i.e. on the measured jet spectrum averaged over the entire ensemble of events, rather than attempting to correct for background on an event-by-event basis. This statistical-correction method enables jet measurements at RHIC over the complete range of jet momenta - including very low momentum - in all collision systems, for large jet-cone radius. This approach enables qualitatively new ways of studying jet quenching.

In left Figure, the red points show the distribution of reconstructed jet momentum from all jets measured in the recoil region, both signal and background. The shaded region shows the mixed-event background distribution. It matches the red points precisely on the left side of the distribution, which is strongly dominated by background, thereby validating the method. The difference of the two distributions gives the jet signal, which measures jet quenching effects. Comparison of these results to similar heavy ion jet measurements by the ALICE Collaboration at the LHC shows that jet quenching at RHIC is a factor two smaller, providing the first direct comparison of jet quenching at the two colliders.

Right Figure shows another observable in the analysis, the azimuthal angular separation between a reconstructed jet and a high-momentum trigger hadron. This distribution is sensitive to the possible deflection of very low momentum jets due to scattering in the QGP, an important process that was highlighted as a key measurement of the RHIC heavy ion program in the 2015 NSAC Long Range Plan. The grey points in Fig. 2 are measured data, while the red band is a reference distribution due to all other processes. There is a hint in the data of angular broadening due to scattering in the QGP, but with insufficient significance to claim a discovery. This analysis establishes the methodology to carry out these unique measurements with much higher statistics STAR data, using both hadrons and direct photons as trigger particles. Such analyses are now in progress.

Posted Sep. 12, 2017

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