Search for jet quenching effects in high-multiplicity pp collisions at $\sqrt{s}$ = 13 TeV via di-jet acoplanarity

The ALICE Collaboration reports a search for jet quenching effects in high-multiplicity (HM) proton$-$proton collisions at $\sqrt{s}$ = 13 TeV, using the semi-inclusive azimuthal-difference distribution $\Delta\varphi$ of charged-particle jets recoiling from a high transverse momentum (high-$p_{\mathrm{T,trig}}$) trigger hadron. Jet quenching may broaden the $\Delta\varphi$ distribution measured in HM events compared to that in minimum bias (MB) events. The measurement employs a $p_{\mathrm{T,trig}}$-differential observable for data-driven suppression of the contribution of multiple partonic interactions, which is the dominant background. While azimuthal broadening is indeed observed in HM compared to MB events, similar broadening for HM events is observed for simulations based on the PYTHIA 8 Monte Carlo generator, which does not incorporate jet quenching. Detailed analysis of these data and simulations show that the azimuthal broadening is due to bias of the HM selection towards events with multiple jets in the final state. The identification of this bias has implications for all jet quenching searches where selection is made on the event activity.


JHEP 05 (2024) 229
HEP Data
e-Print: arXiv:2309.03788 | PDF | inSPIRE
Figure group

Figure 1

Probability distribution of ${\rm V0M/\langle V0M \rangle}$ in MB pp collisions measured at $\sqrt{s}=13$ TeV, and in simulated MB pp events generated by PYTHIA 8 at the particle and detector level. The vertical dashed lines indicate the lower bound for HM selection for data and particle-level simulations.

Figure 2

The Signal (TT{20,30}) and Reference (TT{6,7}) trigger-normalized recoil jet distributions and the corresponding $\Delta_{\rm recoil}$ distribution for MB and HM pp collisions at $\sqrts = 13$ TeV. Left panels: projection onto $p_{\rm T,jet}^{\rm reco,ch}$ for $|\Delta\varphi-\pi|<0.6$. Middle and right panels: projection onto $\Delta\varphi$ in two $p_{\rm T,jet}^{\rm reco,ch}$ intervals.

Figure 3

Fully-corrected $\Delta_{\rm recoil}$ distributions measured in MB and HM-selected events in pp collisions at $\sqrts=13$ TeV. Left panel: $\Delta_{\rm recoil}(p_{\rm T,jet}^{\rm ch})$ in $|\Delta\varphi-\pi|<0.6$; middle and right panels: $\Delta_{\rm recoil}(\Delta\varphi)$ for $20< p_{\rm T,jet}^{\rm ch} < 40$ GeV/$c$ and $40< p_{\rm T,jet}^{\rm ch} <60$ GeV/$c$. Also shown are particle-level simulated distributions calculated with PYTHIA 8 Monash tune, and a pQCD calculation at LO with Sudakov broadening  (MB $\Delta_{\rm recoil}(\Delta\varphi)$ only). The width of the PYTHIA 8 HM band represents the statistical uncertainty. Top row: $\Delta_{\rm recoil}$ distributions; middle row: ratio of $\Delta_{\rm recoil}$ distributions for HM/MB from data; bottom row: Data/PYTHIA 8 separately for MB and HM event selections.

Figure 4

PYTHIA 8 particle-level simulation of the probability distribution of the yield of charged-particle jets ($R= 0.4$) recoiling from a high-$\pT$ hadron (TT{20,30}) as a function of $\eta_{\rm jet}$ for various $p_{\rm T,jet}^{\rm ch}$ intervals, in pp collisions at $\sqrt{s}=13$ TeV. Left: MB events; right: HM events. V0A and V0C acceptances are also shown.

Figure 5

Same as Fig. 4, but for recoil jets with $p_{\rm T,jet}^{\rm ch} >25$ GeV/$c$ for various intervals in ${\rm V0M/\langle V0M \rangle}$. Distributions are normalized per trigger.

Figure 6

Distribution of probability per trigger hadron of number of jets with $R=0.4$ and $p_{\rm T,jet}^{\rm ch}>15$ GeV/$c$ recoiling from a high-$\pT$ hadron (TT{20,30}) at midrapidity ($|\eta_{\rm jet}|<0.5$), for MB and HM pp collisions at $\sqrts=13$ TeV. Left panels: ALICE data; right panels: simulations at the particle level with PYTHIA 8 Monash tune. Lower panels show the ratio HM/MB. The insert in the lower panels has magnified vertical scale to show the ratio of probabilities to observe a single jet.