Dilepton Correlations from Heavy Flavor Decays

Abstract

Background: Azimuthal correlations between heavy flavor hadrons have been previously studied in $p+p$ collisions (Phys. Rev. C {\bf 98}, 034907 (2018), {\bf 101}, 034910 (2020)). These studies found good agreement with the data and provide a baseline for further studies in $p+A$ and $A+A$ collisions. Purpose: This work extends those studies to heavy flavor hadron decays to low mass lepton pairs. The low mass dilepton region is important for heavy ion collisions because of the interest in thermal dilepton production as a signature of the early time dynamics of the medium. Methods: Building on previous work, azimuthal correlations between leptons from semileptonic decays of heavy flavor hadrons are examined. The exclusive \textsc{HVQMNR} code is used in the calculations, made in the PHENIX acceptance at $\sqrt{s} = 200$~GeV and for dielectrons in ALICE at $\sqrt{s} = 13$~TeV. The $b \overline b$ decay contributions subtract like-sign lepton pairs from the opposite-sign signal. Results: The next-to-leading order calculations reproduce the trends of the PHENIX data. The calculations at 13~TeV show a change in signal as electron pair $p_T$ is increased, with the peak of the $Δφ$ distribution moving from $Δφ\sim π$ to $\sim 0$ and the contribution from bottom decays becoming dominant. The sensitivity of the signal to $k_T$ broadening is also studied and found to be small. Conclusions: It is found that the dependence on $k_T$ broadening previously observed is significantly reduced by the decay process. Despite this, the correlations between decay leptons retains some memory of the correlations between the parent hadrons in $p+p$ collisions. However, to study these correlations in heavy-ion collisions, it is necessary to separate them from thermal dilepton production in the same mass region.

Figures (6)

• Figure 1: The azimuthal correlations (a) and mass distributions (b) for heavy flavor decays to $\mu^+ \mu^-$ pairs in the PHENIX acceptance are shown for $\Delta = 1$ in Eq. (\ref{['eq:avekt']}). The uncertainty bands for $c \overline c$ decays are given in red; those for $b \overline b$ decays in blue; while the sum of the two is given by the black histograms. The central value is denoted by the solid histograms while the dashed histograms are the upper and lower limits of the uncertainty bands.
• Figure 2: The azimuthal correlations (a) and mass distributions (b) for heavy flavor decays to $e^\pm \mu^\mp$ pairs in the PHENIX acceptance are shown for $\Delta = 1$ in Eq. (\ref{['eq:avekt']}). The uncertainty bands for $c \overline c$ decays are given in red; those for $b \overline b$ decays in blue; while the sum of the two is given by the black histograms. The central value is denoted by the solid histograms while the dashed histograms are the upper and lower limits of the uncertainty bands.
• Figure 3: The azimuthal correlations (a-c) and mass distributions (d) for heavy flavor decays to $e^+ e^-$ pairs in the PHENIX acceptance are shown for $\Delta = 1$ in Eq. (\ref{['eq:avekt']}). (a) The uncertainty bands for $c \overline c$ decays are given in red; those for $b \overline b$ decays in blue; while the sum of the two is given by the black histograms. The central value is denoted by the solid histograms while the dashed histograms are the upper and lower limits of the uncertainty bands. In (b) and (c) the charm and bottom contributions respectively are broken down according to which central spectrometer arms the dielectron was detected with pairs appearing in the opposite east and west arms (E+W) in the same color as the total and the other pairs appearing in the same arm (E+E and W+W) in different colors.
• Figure 4: The azimuthal correlation between heavy flavor decays to $e^+ e^-$ pairs in mass region $1.1<M_{ee}<2.8$ GeV at $\sqrt{s} = 13$ TeV in the ALICE acceptance are shown for $\Delta = 1$ in Eq. (\ref{['eq:avekt']}). The uncertainty bands for $c \overline c$ decays are given in red; those for $b \overline b$ decays in blue; while the sum of the two is given by the black histograms. The central value is denoted by the solid histograms while the dashed histograms are the upper and lower limits of the uncertainty bands. Results are given for different $p_T$ bins: (a) $p_T < 1$ GeV, (b) $1 < p_T < 2$ GeV, (c) $2 < p_T < 3$ GeV, (d) $3 < p_T < 4$ GeV, (e) $4 < p_T < 5$ GeV, and (f) $5 < p_T < 6$ GeV.
• Figure 5: The mass distributions of heavy flavor decays to $e^+ e^-$ pairs at $\sqrt{s} = 13$ TeV in the ALICE acceptance are shown for $\Delta = 1$ in Eq. (\ref{['eq:avekt']}). The uncertainty bands for $c \overline c$ decays are given in red; those for $b \overline b$ decays in blue; while the sum of the two is given by the black histograms. The central value is denoted by the solid histograms while the dashed are the upper and lower limits of the uncertainty bands. Results are given for different $p_T$ bins: (a) all $p_T$, (b) $p_T < 1$ GeV, (c) $1 < p_T < 2$ GeV, (d) $2 < p_T < 3$ GeV, (e) $3 < p_T < 4$ GeV, and (f) $4 < p_T < 5$ GeV.