Studies of Charm Quark Diffusion inside Jets Using Pb-Pb and $pp$ Collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$

The first study of charm quark diffusion with respect to the jet axis in heavy ion collisions is presented. The measurement is performed using jets with $p_T^{\text{jet}}>60\mathrm{GeV}/c$ and $D^0$ mesons with $p_T^D>4\mathrm{GeV}/c$ in lead-lead (Pb-Pb) and proton-proton ($pp$) collisions at a nucleon-nucleon center-of-mass energy of $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$, recorded by the CMS detector at the LHC. The radial distribution of $D^0$ mesons with respect to the jet axis is sensitive to the production mechanisms of the meson, as well as to the energy loss and diffusion processes undergone by its parent parton inside the strongly interacting medium produced in Pb-Pb collisions. When compared to Monte Carlo event generators, the radial distribution in $pp$ collisions is found to be well described by pythia, while the slope of the distribution predicted by sherpa is steeper than that of the data. In Pb-Pb collisions, compared to the $pp$ results, the $D^0$ meson distribution for $4<p_T^D<20\mathrm{GeV}/c$ hints at a larger distance on average with respect to the jet axis, reflecting a diffusion of charm quarks in the medium created in heavy ion collisions. At higher $p_T^D$, the Pb-Pb and $pp$ radial distributions are found to be similar.

Figure 1

Distributions of $D^0$ mesons in jets, as a function of the distance from the jet axis ($r$) for jets of $p_T^{\text{jet}}>60\mathrm{GeV}/c$ and $|{\eta }^{\text{jet}}|<1.6$ measured in $pp$ and Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. The measurement is performed in the $p_T^D$ range $4–20\mathrm{GeV}/c$ (left) and $p_T^D>20\mathrm{GeV}/c$ (right). Each spectrum is normalized to its integral in the region $0<r<0.3$. The vertical bars (boxes) correspond to statistical (systematic) uncertainties. The Pb-Pb spectra are compared to the CCNU energy loss model [21], while the $pp$ spectra are compared with predictions from the pythia and sherpa $pp$ MC event generators. The ratios of the $D^0$ meson radial distributions in Pb-Pb and $pp$ data are shown in the middle panels. In the bottom panels, the ratios of the $D^0$ meson radial distributions of $pp$ over the two MC event generators are presented.