First measurement of jet axis decorrelation with photon-tagged jets in pp and PbPb collisions at 5.02 TeV

TL;DR

The paper presents the first measurement of jet axis decorrelation Δj between the E-scheme and WTA jet axes in photon-tagged jets from pp and PbPb collisions at √sNN = 5.02 TeV. Using isolated photons (60 < pTγ < 200 GeV) to tag the original parton energy and suppress survivor bias, the study analyzes Δj in two jet pT ranges (30–60 and 60–100 GeV) across four PbPb centralities, with careful background subtraction and unfolding to correct detector effects. The results show central PbPb jets at high pT exhibit a narrowing of the Δj distribution at small Δj, while low-pT jets show little to no modification, and they are contrasted with predictions from JEWEL, HYBRID, and PYQUEN models. The findings indicate that elastic scatterings with the QGP, and to some extent wake effects, play a significant role in jet axis decorrelation, highlighting the observable’s sensitivity to the mechanisms of jet quenching. This photon-tagged approach reduces biases inherent in inclusive jet measurements and provides stringent tests for jet-quenching theories relevant to QGP properties.

Abstract

The first measurement of the jet axis decorrelation in events with jets recoiling from an isolated photon is presented for lead-lead (PbPb) and proton-proton (pp) collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The jet axis decorrelation is the angular difference ($Δ{j}$) between two definitions of the jet axis. This quantity is expected to be sensitive to the scattering of jet constituents in the quark-gluon plasma (QGP). Events which have a leading isolated photon with transverse momentum 60 $\lt$ $p_{\mathrm{T}}^γ$ $\lt$ 200 GeV and recoiling jets with 30 $\lt$ $p_{\mathrm{T}}^{\text{jet}}$ $\lt$ 100 GeV are selected for the analysis. The PbPb result is reported as a function of collision centrality and compared to pp reference data. Jets with $p_{\mathrm{T}}^{\text{jet}}$ $\lt$ 60 GeV have consistent $Δ{j}$ shapes for pp and PbPb collisions. However, a narrowing is observed for jets with $p_{\mathrm{T}}^{\text{jet}}$ $\gt$ 60 GeV in central PbPb collisions. The results are compared to predictions from the JEWEL, PYQUEN, and HYBRID theoretical models, which include different descriptions of parton energy loss in the QGP.

Figures (5)

• Figure 1: Photon-tagged jet axis decorrelation $\Delta{j}$ in ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ collisions (red boxes, open circles) and $\mathrm{PbPb}$ collisions (blue boxes, filled circles), normalized per photon. The columns show different $\mathrm{PbPb}$ centralities, while the top (bottom) row shows low (high) jet $p_{\mathrm{T}}$. The leftmost bins extend down to $\Delta{j}\xspace = 0$. The shaded boxes represent systematic uncertainties, the vertical bars indicate statistical uncertainties, and the horizontal bars indicate the bin width.
• Figure 2: Ratios of photon-tagged jet $\Delta{j}$ spectra in $\mathrm{PbPb}$ and ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ collisions normalized per photon-jet pair. The columns show different $\mathrm{PbPb}$ centralities, while the top (bottom) row shows low (high) jet $p_{\mathrm{T}}$. The leftmost bins extend down to $\Delta{j}\xspace = 0$. The shaded boxes represent total systematic uncertainties, the vertical bars indicate statistical uncertainties, and the horizontal bars indicate the bin width.
• Figure 3: Photon-tagged jet axis decorrelation $\Delta{j}$ in ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ collisions, normalized per photon. The left (right) panel shows low (high) jet $p_{\mathrm{T}}$. The leftmost bins extend down to $\Delta{j}\xspace = 0$. The shaded boxes represent systematic uncertainties, statistical uncertainties are within the marker size, and the horizontal bars indicate the bin width. The results are compared to theoretical predictions from the $\textsc{Hybrid}$, $\textsc{Jewel}$, $\textsc{Pyquen}$, and pythia models. The theory bands represent the statistical uncertainties.
• Figure 4: Photon-tagged jet axis decorrelation $\Delta{j}$ in $\mathrm{PbPb}$ collisions with 0--10% centrality, normalized per photon. The left (right) column shows low (high) jet $p_{\mathrm{T}}$ while the top (bottom) row shows comparisons to predictions from the $\textsc{Hybrid}$ ($\textsc{Jewel}$ and $\textsc{Pyquen}$) models. The leftmost bins extend down to $\Delta{j}\xspace = 0$. The shaded boxes represent systematic uncertainties, the vertical bars indicate statistical uncertainties, and the horizontal bars indicate the bin width. The theory bands represent the statistical uncertainties.
• Figure 5: Ratios of photon-tagged jet axis decorrelation $\Delta{j}$ in $\mathrm{PbPb}$ and ${ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace{ \mathup{{{p}}{} _{ {}} ^{ {}}} }\xspace$ collisions normalized per photon-jet pair, shown for centrality 0--10%. The left (right) column shows low (high) jet $p_{\mathrm{T}}$ while the top (bottom) row shows comparisons to predictions from the $\textsc{Hybrid}$ ($\textsc{Jewel}$ and $\textsc{Pyquen}$) models. The leftmost bins extend down to $\Delta{j}\xspace = 0$. The shaded boxes represent systematic uncertainties, the vertical bars indicate statistical uncertainties, and the horizontal bars indicate the bin width. The theory bands represent the statistical uncertainties.