Constraining color-charge effects of partonic energy loss with jet axis-based inclusive jet substructure measurement

TL;DR

The paper addresses how color charge affects parton energy loss in the QGP by leveraging the jet-axis decorrelation observable $\Delta j$ in CMS PbPb data at 5.02 TeV. It builds a phenomenological framework using PYTHIA8-based quark/gluon templates and two approaches: (i) template fits to bound the gluon-jet fraction in the inclusive jet sample across centrality and $p_T$, and (ii) $p_T$-shift analyses to infer energy-loss magnitudes for inclusive, quark-like, and gluon-like jets under universal and color-dependent scenarios. The results indicate a centrality- and flavor-dependent picture consistent with larger energy loss for gluons, with gluon-fraction limits decreasing in more central events and jet $R_{AA}$ estimates from shifted $p_T$ aligning with measured values from CMS/ATLAS. Overall, the study demonstrates that jet substructure observables can provide meaningful constraints on the color-charge dependence of jet quenching, informing and refining jet-quenching models.

Abstract

This study investigates the color-charge dependence of parton energy loss in the quark-gluon plasma (QGP) medium and the associated relative modifications of quark and gluon jet fractions compared to vacuum, using jet axis decorrelation observables. Recent CMS jet axis decorrelation measurements in PbPb collisions at 5.02 TeV are interpreted using Pythia simulations with varied quark/gluon jet compositions and emulated color-charge dependent energy loss. A template-fit procedure is employed to estimate the limits on gluon jet fractions in the published CMS data and average shift in jet momentum due to quenching for quark- and gluon-initiated jets traversing the QGP. The extracted gluon jet fractions and the estimated quark and gluon energy losses based on this study of jet axis decorrelations are found to be consistent with other model calculations based on inclusive observables. This work illustrates the use of jet substructure measurements for providing constraints on the color-charge dependence of parton energy loss and offers valuable insights for jet quenching models.

Figures (5)

• Figure 1: Left: $\Delta R_{\text{axis}}$ distributions for quark and gluon jets in pythia8, illustrating flavor dependence. Right: $p_{\text{T}}\xspace$ dependence of the distribution for inclusive jets. The bottom panel shows the ratio of the distributions.
• Figure 2: Template fits to the inclusive jet $\Delta\text{j}$ distributions (black circles) in $\text{PbPb}$ collisions for 50--80% (top left), 30--50% (top right), 10--30% (bottom left), and 0--10% (bottom right) centrality intervals, measured by CMS across different $p_{\text{T}}$ intervals. Quark and gluon jet templates from pythia are shown as red and blue shaded areas, respectively. The lower panels show the data-to-fit ratios. Systematic uncertainties in CMS data are represented by rectangles (correlated) and shaded areas (uncorrelated), while vertical solid lines denote statistical uncertainties. In the lower panels, vertical lines represent total data uncertainties.
• Figure 3: The normalized $\Delta\text{j}$ distributions from pythia-based simulations using shifted inclusive $p_{\text{T}}$ (blue bands) and shifted quark/gluon $p_{\text{T}}$ (red bands) are compared with CMS unfolded measurements in $\text{PbPb}$ collisions at 0--10% centrality (black circles). The results are also compared with unquenched pythia simulations (yellow dashed-line bands), a two-component fit (green bands), and the medium q/g model (hatched lines) across various $p_{\text{T}}$ intervals. Only statistical uncertainties are shown for the simulation distributions. For the CMS data points, systematic uncertainties are indicated by rectangles (correlated uncertainties) and shaded areas (uncorrelated uncertainties), while vertical solid lines depict statistical uncertainties. In the lower panels, the grey bands correspond to the total uncertainties in the data.
• Figure 4: Left: Limits on gluon jet fraction obtained from template fits to the CMS $\text{PbPb}$ inclusive $\Delta\text{j}$ measurement using a pythia-based templates for the 0--10% (black circles), 10--30% (red squares), 30--50% (blue triangles), and 50--80% (green triangles) centrality intervals. The purple and black bands represent the gluon fraction expectations from the unquenched pythia and medium q/g models, respectively. Right: The ratio of the extracted gluon fraction limits for each centrality interval to those measured in the 50--80% centrality interval is shown. The black band represents the ratio between the expectations from the pythia and medium q/g models. Vertical lines represent statistical uncertainties, while systematic uncertainties are shown as rectangles (correlated uncertainties) and shaded areas (uncorrelated uncertainties). Points in each jet $p_{\text{T}}$ bin are shifted horizontally for better visibility.
• Figure 5: Left: The jet $R_{\text{AA}}$ estimates based on inclusive jet $p_{\text{T}}$ shifts along with the quark and gluon jet $p_{\text{T}}$ shift values extracted from template fits in this analysis, are shown alongside other measurements and predictions (see text), for 0--10% centrality. Right: The average fractional transverse momentum loss of quark and gluon jets, determined from the $p_{\text{T}}$ shift values extracted for 0--10% $\text{PbPb}$ collisions.