Absence of Jet Quenching in Peripheral Nucleus-Nucleus Collisions
Summary
The interaction of a fast parton (quark or gluon) with QGP matter results in energy loss of the parton. This Jet quenching leads to a dramatic suppression of high-transverse momentum (fast) particles. The suppression can be quantified by the nuclear modification or attenuation factor R, which relates the measured yields in nucleus-nucleus (AA) collisions to that measured in proton-proton (pp) collisions assuming AA collisions to be a mere superposition of elementary pp collisions. In absence of nuclear modification, R is hence expected to be at unity. Surprisingly, since the first measurements at RHIC back in the early 2000's, R measured in peripheral AA collisions, where only a few nucleons from each nucleus participate, turned out to be smaller than unity, about R=0.8. The measured suppression was ascribed to small but non-zero jet quenching, and hence it was concluded that the QGP is still present in peripheral AA collisions. Scientists at ORNL have recently demonstrated with a particle production model inspired from pp collisions without jet quenching, that the event selection necessary for the determination of collision centrality in AA can cause the apparent nuclear modification in peripheral collisions. Their rigorous measurement of R, including even the most peripheral collisions (beyond 90% in the above figure), for the first time unambiguously demonstrated the effect of the event selection on the measurement, and confirmed predictions of the simple pp-based model without jet quenching. Hence, the interpretation of R below unity in peripheral collisions does not require jet quenching, but rather is consistent with measurements in pp and proton-lead collisions.
The Impact
The energy loss of a fast parton as it traverses a volume of QGP, also known as jet quenching, was a key piece of evidence in the discovery of the QGP at RHIC. In early analyses of this effect, the energy loss was observed to be strong in central (head on) and weaker – but still significant - in peripheral (grazing) nucleus-nucleus collisions. This observation creates the puzzling question of how to account for the apparent absence of jet quenching in small collision systems like proton-proton or proton-lead collisions. The new analysis shown in the above figure resolves the puzzle, by demonstrating the absence of measurable jet quenching effects in peripheral nucleus-nucleus collisions, aligning the interpretation with small collision systems, where no jet quenching effects have been seen.
S.Acharya et al. (ALICE Collaboration), “Analysis of the apparent nuclear modification in peripheral Pb-Pb collisions”, Phys. Letters B793 (2019), https://doi.org/10.1016/j.physletb.2019.04.047
Related Links
Further reading at http://cerncourier.com/alice-sheds-new-light-on-high-pt-suppression/
More information at http://alice-publications.web.cern.ch/node/4437