D$^{0}$-meson-tagged jet axes difference in proton-proton collisions at $\mathbf{\sqrt{\textit{s}} = 5.02}$ TeV

TL;DR

This work presents the first measurement of D$^{0}$-meson-tagged jet axis differences in pp collisions at $\sqrt{s}=5.02$ TeV, probing how heavy-quark mass and color factors shape jet fragmentation and soft radiation. Using three jet-axis definitions—Standard, Soft Drop groomed, and Winner-Takes-All—the study correlates the D$^{0}$ direction with jet axes via observables $\Delta R_{\rm axis}$ and $\Delta R_{\rm axis-D^{0}}$, across jets with $10\le p_T^{\rm ch~jet}<50$ GeV/$c$ and $p_T^{\rm D^{0}}\ge 5$ GeV/$c$. The results show that the D$^{0}$ direction closely aligns with the WTA axis, confirming the dead-cone effect and that the D$^{0}$ is the leading particle in charm jets; grooming reduces sensitivity to soft radiation, with broader implications for hadronization models. Comparisons to PYTHIA8, SHERPA, and HERWIG illustrate how string- and cluster-based fragmentation reproduce or fail to capture the observed grooming and flavor-dependent jet-shape features, informing future refinements of QCD event generators and providing a baseline for heavy-flavor studies in heavy-ion environments.

Abstract

Heavy-flavor quarks produced in proton--proton (pp) collisions provide a unique opportunity to investigate the evolution of quark-initiated parton showers from initial hard scatterings to final-state hadrons. By examining jets that contain heavy-flavor hadrons, this study explores the effects of both perturbative and non-perturbative QCD on jet formation and structure. The angular differences between various jet axes, $ΔR_{\rm axis}$, offer insight into the radiation patterns and fragmentation of charm quarks. The first measurement of D$^{0}$-tagged jet axes differences in pp collisions at $\sqrt{s}=5.02$ TeV by the ALICE experiment at the LHC is presented for jets with transverse momentum $p_{\rm T}^{\rm ch~jet} \geq 10$ ${\rm GeV}/c$ and D$^0$ mesons with $p_{\rm T}^{\rm D^{0}} \geq 5$ ${\rm GeV}/c$. In this D$^0$-meson-tagged jet measurement, three jet axis definitions, each with different sensitivities to soft, wide-angle radiation, are used: the Standard axis, Soft Drop groomed axis, and Winner-Takes-All axis. Measurements of the radial distributions of D$^0$ mesons with respect to the jet axes, $ΔR_{\mathrm{axis-D^0}}$, are reported, along with the angle, $ΔR_{\mathrm{axis}}$, between the three jet axes. The D$^{0}$ meson emerges as the leading particle in these jets, closely aligning with the Winner-Takes-All axis and diverging from the Standard jet axis. The results also examine how varying the sensitivity to soft radiation with grooming influences the orientation of the Soft Drop jet axis, and uncover that charm-jet structure is more likely to survive grooming when the Soft Drop axis is further from the D$^{0}$ direction, providing further evidence of the dead-cone effect recently measured by ALICE.

Figures (12)

• Figure 1: A representation of the different jet axes; Standard (STD), Soft Drop (SD), and Winner-Takes-All (WTA). The axis of the jet containing the ${\rm D}^{0}$ meson (shown in solid black) and all gluonic radiation is referred to as the Standard axis. The Winner-Takes-All axis is also determined from this initial jet sample but aligns with the hardest subjet at each clustering step. Grooming away softer radiation leaves a jet with the Soft Drop axis, defined by the remaining higher-momentum particles. Representations for the six $\Delta R_{\rm axis}$ observables are shown in the right of the figure.
• Figure 2: Left: the ${\rm D}^{0}$-decay candidate invariant-mass distribution for 10 $\leq$${\it p}_{\rm T}^{\rm ch~jet}$$<$ 20 ${\rm GeV/}c$ and $8 < p_{\rm T}^{\rm D^0} < 12$${\rm GeV/}c$. The total fit function of the signal and background is represented by the blue line. The background fit function is represented by the red line. Right: the raw yields of the ${\rm D}^{0}$-tagged jets as a function of $\Delta R_{\rm STD - {\rm D}^{0}\xspace}$ in the signal region and sideband region.
• Figure 3: Ratio of the simulated non-prompt ${\rm D}^{0}$ distribution in the detector (detector-level) over the efficiency-corrected data, which includes contributions from the prompt and feed-down ${\rm D}^{0}$. The non-prompt ${\rm D}^{0}$ fraction is shown for $\Delta R_{\rm STD - {\rm D}^{0}\xspace}$ and $\Delta R_{\rm SD - {\rm D}^{0}\xspace}$ ($z_{\rm cut}\xspace=0.1,0.2$ and $\beta=0$). Systematic and statistical uncertainties are represented by the corresponding color boxes and error bars, respectively.
• Figure 4: Unfolded jet axes difference distribution for $\Delta R_{\rm WTA - {\rm D}^{0}\xspace}$ and $\Delta R_{\rm STD - {\rm D}^{0}\xspace}$ in 10 $\leq$${\it p}_{\rm T}^{\rm ch~jet}$$<$ 20 ${\rm GeV/}c$. Includes systematic and statistical uncertainties represented by color boxes and error bars, respectively, and a comparison to predictions from PYTHIA 8.
• Figure 5: Fully unfolded jet axes difference distribution for $\Delta R_{\rm STD - {\rm D}^{0}\xspace}$ (left) and $\Delta R_{\rm STD - WTA}$ (middle) for 10 $\leq$${\it p}_{\rm T}^{\rm ch~jet}$$<$ 20 ${\rm GeV/}c$. Systematic and statistical uncertainties are represented by color boxes and error bars, respectively, and comparisons to MC event generators PYTHIA 8, HERWIG 7, and SHERPA 2 (Ahadic and Lund) are shown in the respective bottom panels. The bottom right panel shows a ratio of the two data distributions.