The Quark-Gluon Plasma (QGP) is a state of matter at very high temperature in which quarks and gluons roam freely rather than being confined in protons and neutrons. Experiments using collisions of energetic heavy nuclei at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and the Large Hadron Collider (LHC) at CERN reveal that the QGP has remarkable properties, flowing like a perfect liquid with the least internal friction allowed by Nature. The understanding of how the interactions of elementary quarks and gluons generate complex phenomena, such as perfect fluidity, is an important open question.

The study of matter often utilizes the scattering of an external beam, much like Ernest Rutherford uncovering the atomic nucleus by scattering alpha particles from a gold foil. The QGP generated in the laboratory is too small and short-lived for the scattering of an external beam to be practicable; instead, physicists at RHIC and the LHC use internally generated “jets”, hard-scattered quarks and gluons which are generated in the same collision as the QGP, to scatter off it and probe its properties (“jet quenching”). The STAR Collaboration at RHIC and the ALICE Collaboration at the LHC recently carried out several related jet quenching analyses, led by members of the RNC Program in the NSD, which are reported in six publications [1]-[6]. (Refs. [3],[4] were recently highlighted in a recent article.)

Figure 1: Artist’s rendering of QGP formation in a nuclear collision at RHIC, in which a high-energy photon (purple line) and jet of correlated particles (cone) propagate back-to-back in the QGP. Jet scattering in the QGP broadens the distribution of their azimuthal opening angle ∆φ .

Jets are mainly produced in azimuthally back-to-back pairs (i.e. in the plane perpendicular to the colliding beams). In Refs. [1] and [5], the ALICE and STAR collaborations utilize this feature to characterize jet scattering in the QGP, by measuring the distribution in opening angle ∆φ between the pair (Fig. [1]): jet scattering will broaden the distribution of ∆φ.

Figure 2: Results from Refs. [1] and [5], showing the distribution opening angle ∆φ between a “trigger” photon or pion and the recoiling jet. Strictly back-to-back pairs correspond to ∆φ = π radians (180 degrees). The quantity “IAA” is the ratio of recoil jet rate in collisions where a QGP is generated, compared to the same measurement in collisions in which no QGP is formed. The parameter R is the jet cone opening angle, in radians.

Fig. 2 shows the results of these measurements. Jets manifest in experiments as a spray of tightly-correlated particles, and jet measurements cluster all detector particles within a cone opening angle R.. The figure shows marked azimuthal broadening for jets in nuclear collisions (QGP) relative to those in p+p collisions (no QGP), but only for large cone radius R. The observed  increase in jet yield at large angular deviation from back-to-back is up to a factor 20, which is by far the largest jet quenching signal ever observed. However, the marked dependence of this effect on R is a strong indication that this broadening does not arise from jet-QGP scattering, which should result in similar effects for all values of R, but rather from the shock wave or wake of the QGP liquid excited by the passage of the jet, similar to the wake of a power boat on a still lake. While this finding is of great interest, evidently, uncovering the specific effects of jet-QGP scattering might require a different approach.

Ref. [6] takes one alternative approach, using the distribution of particles within the jet cone itself to search for evidence of the scattering of individual jet components with the QGP. It utilizes recently developed theoretical approaches to the study of “jet substructure,” applying them for the first time to study jet quenching. The measurement is challenging, requiring careful treatment of large background effects that limits the range of jet momentum that can be explored with the ALICE Run 2 dataset. No evidence of jet-QGP scattering was found within this range.

The experimental study of jet quenching is complex, with other physical mechanisms competing with direct signatures of jet-QGP scattering. The measurements in these six papers provide new insights into the physics underlying jet quenching, and point the way to more precise measurements with the full RHIC dataset and LHC Run 3 data, both of which are in progress, which may finally reveal the microscopic details of jet-QGP scattering. More generally, incisive interpretation of these findings requires  a systematic multi-observable comparison to theoretical models, such as that being carried out by the JETSCAPE Collaboration. 

Refs. [1]-[5] were led by Peter Jacobs, RNC Senior Staff, together with STAR and ALICE colleagues. Ref. [6] was led by Raymond Ehlers, UC Berkeley postdoc and an RNC affiliate, together with Jacobs.

References
[1] Observation of Medium-Induced Yield Enhancement and Acoplanarity Broadening of Low-pT Jets from Measurements in pp and Central Pb-Pb Collisions at sNN=5.02  TeV; ALICE Collaboration (S. Acharya et al.); Phys. Rev. Lett. 133, 022301;
[2] Measurements of jet quenching using semi-inclusive hadron+jet distributions in pp and central Pb-Pb collisions at sNN=5.02 TeV; ALICE Collaboration (S. Acharya et al.); Phys. Rev. C 110, 014906; 
[3] Measurement of In-Medium Jet Modification Using Direct Photon+Jet and π0+Jet Correlations in p+p and Central Au+Au Collisions at sNN=200  GeV; STAR Collaboration (B. E. Boona et al); Phys. Rev. Lett. 134, 232301;
[4] Semi-inclusive direct photon + jet and π0+jet correlations measured in p+p and central Au+Au collisions at sNN=200GeV; STAR Collaboration (B. E. Boona et al); Phys. Rev. C 111, 064907; 
[5] Measurement of medium-induced acoplanarity in central Au-Au and pp collisions at √s_NN=200 GeV using direct-photon+jet and π0+jet correlations; STAR Collaboration (B. E. Boona et al);
[6] Search for quasi-particle scattering in the quark-gluon plasma with jet splittings in pp and Pb−Pb collisions at √s_NN = 5.02 TeV; ALICE Collaboration (S. Acharya et al.); Phys. Rev. Lett. 135, 031901;