Search for jet quenching with dijets from high-multiplicity pPb collisions at $\sqrt{s_\mathrm{NN}}$ = 8.16 TeV

The CMS collaboration
Chekhovsky, Vladimir , Hayrapetyan, Aram , Makarenko, Vladimir , Tumasyan, Armen , Adam, Wolfgang , Andrejkovic, Janik Walter , Benato, Lisa , Bergauer, Thomas , Damanakis, Konstantinos , Dragicevic, Marko
JHEP 07 (2025) 118, 2025.

Abstract (data abstract)
The first measurement of the dijet transverse momentum balance ($x_{j}$) in proton-lead (pPb) collisions at nucleon-nucleon center-of–mass energy $\sqrt{s_{\text{\tiny NN}}} = $8.16$~\text{TeV}$ is presented. The analysis uses data collected with the CMS detector in 2016, corresponding to an integrated luminosity of 174.6$~\text{nb}^\text{$-$1}$. The average values and distributions of $x_{j}$ are measured as functions of the event charged-particle multiplicity for various combinations of the pseudorapidity regions in which the two jets are found. These different configurations enable measurements of distinct nucleon momentum fractions x in both proton- and lead-going directions. Thanks to high-multiplicity triggers, designed for probing potential jet-quenching effects, we reached a charged-particle multiplicity as high as 400. Collective behavior of the outgoing particles was previously observed in very high multiplicity pPb events, possibly indicating the formation of small quark-gluon plasma (QGP) droplets. The ratio of $x_{j}$ for high multiplicity to low-multiplicity events is used to quantify possible medium effects. The results are found to be consistent with simulations of the hard-scattering process that do not include (QGP) production. These measurements provide a limit on medium-induced energy loss of 1.26% at the 90% confidence level in terms of subleading jet's transverse momentum in high multiplicity pPb events.

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