Background: Heavy-flavor modification in relativistic $p\left(d\right)+A$ collisions are sensitive to different kinds of strong-interaction physics ranging from modifications of the nuclear wave function to initial- and final-state energy loss. Modifications to single heavy-flavor particles and their decay leptons at midrapidity and forward rapidity are well established at the Relativistic Heavy Ion Collider (RHIC).

Purpose: This paper presents measurements of azimuthal correlations of electron-muon pairs produced from heavy-flavor decays, primarily $c\overline{c}$, in $\sqrt{s_{{}_{NN}}}=200$ GeV $p+p$ and $d+\mathrm{Au}$ collision using the PHENIX detector at RHIC. The electrons are measured at midrapidity while the muons in the pair are measured at forward rapidity, defined as the direction of the deuteron beam, in order to utilize the deuteron to probe low-$x$ partons in the gold nucleus.

Methods: This analysis uses the central spectrometer arms for electron identification and forward spectrometer arms for muon identification. Azimuthal correlations are built in all sign combinations for $e\text{−}\mu$ pairs. Subtracting the like-sign yield from the unlike-sign yield removes the correlations from light flavor decays and conversions.

Results: Comparing the $p+p$ results with several different Monte Carlo event generators, we find the results are consistent with a total charm cross section ${\sigma }_{c\overline{c}}=538\pm 46$ (stat) $\pm$ 197 (data syst) $\pm$ 174 (model syst) $\mu$b. These generators also indicate that the back-to-back peak at $\Delta \varphi =\pi$ is dominantly from the leading-order contributions (gluon fusion), while higher-order processes (flavor excitation and gluon splitting) contribute to the yield at all $\Delta \varphi$. We observe a suppression in the pair yield per collision in $d+\mathrm{Au}$. We find the pair yield suppression factor for $2.7<\Delta \varphi <3.2$ rad is $J_{dA}=0.433\pm 0.087$ (stat) $\pm$ 0.135 (syst).

Conclusions: The $e\text{−}\mu$ pairs result from partons at $x_{\mathrm{Au}}\sim {10}^{-2}$ at $Q^2=10$ GeV/$c^2$ at the edge of the shadowing region. The pair suppression indicates modification to $c\overline{c}$ pairs for these kinematics in the cold nuclear medium at RHIC.

• Received 6 November 2013

DOI:https://doi.org/10.1103/PhysRevC.89.034915