Unveiling the jet angular broadening with photon-tagged jets in high-energy nuclear collisions

TL;DR

The paper develops a transport-based theoretical framework to study the angular structure of γ+jets and inclusive jets in PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV, achieving good agreement with CMS girth data. By varying the selection cut via $x_{j\gamma}=p_T^{\rm jet}/p_T^{\gamma}$ and employing Jet-by-Jet matching, it demonstrates that γ+jets substantially reduce selection bias and preferentially sample jets that have undergone significant quenching, enabling clearer observation of jet broadening from medium interactions. The analysis isolates the roles of medium-induced gluon radiation and medium response and compares jet substructure observables like $R_g$ and $\Delta R_{\rm axis}$ for γ+jets and inclusive jets, revealing distinct modification patterns and highlighting the importance of bias effects in interpreting heavy-ion jet data. Overall, the work provides a quantitative framework linking jet-medium dynamics, selection effects, and observable angular broadening, with implications for interpreting current and future LHC measurements in heavy-ion collisions.

Abstract

The medium modification of jet substructure in hot and dense nuclear matter has garnered significant interest from the heavy-ion physics community in recent years. Measurements of inclusive jets show an angular narrowing in nucleus-nucleus collisions, while recent CMS results for photon-tagged jets ($γ$+jets) suggest evidence of broadening. In this study, we conduct a theoretical analysis of the angular structure of inclusive jets and $γ$+jets using a transport approach that accounts for jet energy loss and the medium response in the quark-gluon plasma. We examine the girth modification of $γ$+jets in $0-30\%$ PbPb collisions at $\sqrt{s_{NN}} = 5.02$ TeV, achieving satisfactory agreement with recent CMS measurements. We explore the relationship between selection bias and jet kinematics by varying the threshold for $x_{jγ} = p_T^{\rm jet}/p_T^γ$. Notably, we quantitatively demonstrate that $γ$+jets significantly reduce selection bias and can effectively select jets that have been sufficiently quenched in PbPb collisions, which is crucial for capture the jet angular broadening. Additionally, we estimate the contributions of medium-induced gluon radiation and the medium response to the broadening of the jet angular substructure. Lastly, we analyze the modification patterns of jet $R_g$ and $ΔR_{\rm axis}$ in PbPb collisions, which indicate slight broadening for $γ$+jets and noticeable narrowing for inclusive jets compared to pp collisions.

Figures (8)

• Figure 1: (Color online) Normalized girth distributions of $\gamma$+jets in pp $0-30\%$ PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV calculated with $x_{j\gamma}>$ 0.4 (upper panel) and $x_{j\gamma}>$ 0.8 (middle panel) as compared to the recent CMS data CMS:2024zjn. The ratios (PbPb/pp) of girth distribution are also shown in the lower panel.
• Figure 2: (Color online) Normalized $x_{j\gamma}$ distribution of $\gamma$+jets before and after quenching, denoted as unquenched and quenched, in $0-30\%$ PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV when using different $x_{j\gamma}$ cuts by using the Jet-by-Jet matching procedure, $x_{j\gamma} > 0.4$ (upper panel) and $x_{j\gamma} > 0.8$ (lower panel).
• Figure 3: (Color online) Medium modification of $\gamma$+jet girth in $0-30\%$ PbPb collisions relative to pp at $\sqrt{s_{NN}}=5.02$ TeV as compared to the CMS data CMS:2024zjn. In the upper panel, the results without considering the medium-induced gluon radiation and medium response are also presented. In the lower panel, we also compare the girth modification of inclusive jet and $\gamma$+jet with the same $p_T$ cut, as well as the case using the Jet-by-Jet matching (JBJ).
• Figure 4: (Color online) Normalized $p_T$ distribution of unquenched and quenched jets for $\gamma$+jet and inclusive jet in $0-30\%$ PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV by using the Jet-by-Jet matching procedure. The gluon-jet fractions in inclusive jet and $\gamma+$jet samples are also estimated in the lower panel.
• Figure 5: (Color online) Event averaged transverse momentum loss $\langle \Delta p_T \rangle=\langle p_T^{\rm init}-p_T^{\rm fin}\rangle_{\rm evt}$ of $\gamma$+jets (solid line) and inclusive jets (dash line) as a function of initial (left panel) and final (right panel) jet $p_T$ in $0-30\%$ PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV by using the Jet-by-Jet matching procedure.