The effect of recoils on soft-drop-groomed observables in $γ$-tagged jets in a multistage approach

Abstract

We investigate medium-induced modifications to jet substructure observables that characterize hard components in central Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$~TeV. Using a multistage Monte Carlo simulation of in-medium jet shower evolution, we explore flavor-dependent medium effects through simulations of inclusive and $γ$-tagged jets. The results show that quark jets undergo a nonmonotonic modification compared with gluon jets in observables such as the Pb-Pb to $p$-$p$ ratio of the soft drop prong angle $r_g$, the relative prong transverse momentum $k_{T,g}$, and the groomed mass $m_g$ distributions. Due to this nonmonotonic modification, $γ$-tagged jets, enriched in quark jets, provide surprisingly clear signals of medium-induced structural modifications, distinct from effects dominated by selection bias. Further systematic studies demonstrate that these effects are dominated by recoil medium response. This work highlights the potential of hard substructures in $γ$-tagged jets as powerful tools for probing the jet-medium interactions in high-energy heavy-ion collisions, in particular by enabling detailed investigations of jet-medium parton scatterings via their associated medium response. All simulations for $γ$-tagged jet analyses carried out in this paper used triggered events containing at least one hard photon, which highlights the utility of these observables for future Bayesian analysis.

Figures (35)

• Figure 1: Distributions of jet splitting momentum fraction $z_{g}$ normalized by the number of all triggered jets (upper panel) and the number of jets passing the soft-drop condition (lower panel) for the leading jets in events generated with pgun. The jet shower evolution is performed by vacuum matter for the parent parton having $E_{\mathrm{init}} = 140$ GeV. Jets are reconstructed with $R=0.4$ at midrapidity $|\eta_{\mathrm{jet}}|<2.0$. The results are shown for quark jets (solid) and gluon jets (dashed) with different $p^{\mathrm{jet}}_{T}$ triggers, 112, 84, 56, and 28 GeV. The soft-drop parameters are $z_{\mathrm{cut}}=0.2$ and $\beta = 0$.
• Figure 2: Ratios of $z_{g}$ distributions for the leading jets in events with the parent parton having a fixed initial energy $E_{\mathrm{init}} = 140$ GeV generated by pgun. The jet shower evolution in the QGP medium produced in 0%--10% Pb-Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV is performed by matter$+$lbt. The results are shown for quark jets (solid) and gluon jets (dashed) with different $p^{\mathrm{jet}}_{T}$ triggers, 112, 84, 56, and 28 GeV. The soft-drop parameters are $z_{\mathrm{cut}}=0.2$ and $\beta = 0$.
• Figure 3: Same as Fig. \ref{['fig:pgun_zg_in_medium']} for matter$+$lbt simulations in which the medium effects from recoils and holes are artificially removed.
• Figure 4: Same as Fig. \ref{['fig:pgun_zg_in_medium']} for matter alone simulations, where the medium effect is turned off for jet partons with virtuality $Q<2$ GeV.
• Figure 5: Same as Fig. \ref{['fig:pgun_zg_vacuum']} for jets from hard scatterings in $p$-$p$ collisions at $\sqrt{s}=5.02$ TeV, generated by pythiagun with ISR and MPI. The results are shown for $\gamma$-tagged jets (solid) from prompt photon-generating hard processes (HardQCD:all=off+PromptPhoton:all=on) and inclusive jets (dashed) from inclusive hard processes (HardQCD:all=on+PromptPhoton:all=on) generated at leading order by pythia 8 and with different $p^{\mathrm{jet}}_{\mathrm{T}}$ triggers. For $\gamma$-tagged jets, the isolation requirement, the relative azimuth angle cut, and the additional cut of $p^{\mathrm{jet}}_{\mathrm{T}} < p^{\gamma}_{\mathrm{T}}$ are imposed.