Do jets lose energy in proton-nucleus and high-multiplicity proton collisions?
In plain words
In large collisions, fast quarks lose energy crossing the plasma, which suppresses high-momentum particles. This suppression has now been seen in collisions of oxygen nuclei, but whether it also occurs when a proton hits a nucleus, or when two protons collide, is not established.
Precise statement
Measure the nuclear modification factor $R_{\mathrm{AA}}(p_T)$ and semi-inclusive recoil-jet yields in p-Pb and high-multiplicity pp collisions at the LHC, and determine whether final-state energy loss is present at 5 or more standard deviations beyond nuclear-PDF and event-selection biases; quantify the O+O energy loss with the p-O reference. Answer: yes or no for p-Pb and pp, and the energy-loss magnitude versus system size.
What would settle it
Semi-inclusive recoil-jet and photon- or Z-boson-tagged jet measurements in p-Pb, p-O and high-multiplicity pp with event-activity biases controlled, compared with the O+O results.
Status in the literature
Unverified note
CMS observed an $R_{AA}$ minimum of $0.69 \pm 0.04$ near $p_{T} = 6\,\mathrm{GeV}$ in $O+O$ at $5.36\,\mathrm{TeV}$ (arXiv 2510.09864, published 2026) and STAR reported about 20 percent jet suppression in $O+O$ at $200\,\mathrm{GeV}$ (arXiv 2604.13935, 2026); no significant final-state suppression had been established in p-Pb or pp as of 2026.