What are the essential requirements for the formation of a quark–gluon plasma (QGP)? Do only the most violent, head-on lead–lead (Pb–Pb) interactions at the LHC provide such conditions? The answer to such questions will provide key insights into the mechanisms driving the QGP towards equilibration, converting kinetic collision energy into a hot and strongly interacting medium.

Recent measurements of proton–lead (p–Pb) and proton–proton (pp) collisions at the LHC have shown intriguing hints of QGP-like behaviour in such systems, which were initially thought to be too small for QGP formation. Experimentalists classify p–Pb collisions by a parameter called the event activity (EA), which is characterised by particle or energy production in the forward Pb-going direction; the most violent p–Pb collisions, with the largest EA, exhibit correlations that are characteristic of the collective flow of the QGP. Verification of this picture requires measurements of other QGP signals, notably the “quenching” of energetic quark and gluon jets as they propagate through the dense QCD medium.

Jets arise from the scattering of quarks and gluons in the incoming projectiles, and are produced predominantly in azimuthally back-to-back pairs. The first jet-quenching measurements in p–Pb collisions looked for suppression of the inclusive production rate of high momentum hadrons and jets by counting all such objects and comparing them to a reference rate from proton–proton (pp) collisions. Some inclusive suppression measurements indicate significant jet suppression in the highest-EA p–Pb collisions. Quantitative comparison to the pp collision reference spectrum requires the assumption that high-EA is correlated with central p–Pb collisions, in which the proton ploughs through the centre of the Pb-nucleus. However, the relation between forward particle and energy production used to measure EA with the geometry of a p–Pb collision may be modified in events containing jets, complicating its interpretation. An approach to jet quenching in p–Pb that does not invoke this assumption is therefore needed.

For this purpose, the ALICE collaboration has reported measurements of the semi-inclusive distribution of jets recoiling from a high-momentum hadron trigger (h+jet) in p–Pb collisions, as a function of EA. The h+jet distribution is self-normalising, due to the back-to-back nature of jet-pair production: jet quenching is observed as a reduction in jet rate per trigger, without comparison to a pp reference spectrum or the assumption that high-EA corresponds to central p–Pb collisions. The analysis applies a data-driven statistical approach to correct the complex uncorrelated background, enabling the accurate measurement of recoil jets over a broad phase space in the complex LHC environment.

The upper panel of the figure shows distributions of this observable, Δrecoil, for p–Pb collisions with high and low EA. Jet quenching corresponds to the transport of energy out of the jet cone, thereby suppressing Δrecoil for high EA. The ratio is however consistent with unity at all jet energies, indicating negligible jet quenching effects within the uncertainties.

These data provide a limit on the magnitude of medium-induced energy transport to large angles due to jet quenching: for events with high EA, medium-induced charged energy transport out of the jet cone is less than 0.4 GeV/c (90% confidence level). This limit is a factor 20 smaller than the magnitude of jet quenching measured using this observable in Pb–Pb collisions, in contrast to some of the current inclusive jet suppression measurements in p–Pb collisions. This result challenges theoretical models that predicted strong jet quenching in p–Pb collisions. Comparison of these data with the surviving models promises new insight into QGP formation in small systems, and the fundamental processes of equilibration in QCD.

Further reading

ALICE Collaboration 2017 arXiv:1712.05603.