Nuclear-modification factor for open-heavy-flavor production at forward rapidity in Cu+Cu collisions at √s_NN=200 GeV

Background: Heavy-flavor production in p + p collisions is a good test of perturbative-quantum-chromodynamics (pQCD) calculations. Modification of heavy-flavor production in heavy-ion collisions relative to binary-collision scaling from p + p results, quantified with the nuclear-modification factor (R_AA), provides information on both cold- and hot-nuclear-matter effects. Midrapidity heavy-flavor R_AA measurements at the Relativistic Heavy Ion Collider have challenged parton-energy-loss models and resulted in upper limits on the viscosity-entropy ratio that are near the quantum lower bound. Such measurements have not been made in the forward-rapidity region. Purpose: Determine transverse-momentum (p_T) spectra and the corresponding R_AA for muons from heavy-flavor meson decay in p + p and Cu + Cu collisions at √s_NN=200 GeV and y=1.65. Method: Results are obtained using the semileptonic decay of heavy-flavor mesons into negative muons. The PHENIX muon-arm spectrometers measure the p_T spectra of inclusive muon candidates. Backgrounds, primarily due to light hadrons, are determined with a Monte Carlo calculation using a set of input hadron distributions tuned to match measured-hadron distributions in the same detector and statistically subtracted. Results: The charm-production cross section in p + p collisions at √s=200 GeV, integrated over p_T and in the rapidity range 1.4<y<1.9, is found to be dσ_cc̄/dy=0.139±0.029(stat)-0.058+0. 051(syst) mb. This result is consistent with a perturbative fixed-order-plus- next-to-leading-log calculation within scale uncertainties and is also consistent with expectations based on the corresponding midrapidity charm-production cross section measured by PHENIX. The R_AA for heavy-flavor muons in Cu + Cu collisions is measured in three centrality bins for 1<p_T<4 GeV/c. Suppression relative to binary-collision scaling (R_AA<1) increases with centrality. Conclusions: Within experimental and theoretical uncertainties, the measured charm yield in p + p collisions is consistent with state-of-the-art pQCD calculations. Suppression in central Cu + Cu collisions suggests the presence of significant cold-nuclear-matter effects and final-state energy loss.