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

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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 $\sqrt{{s}_{NN}}=200$ 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 $\sqrt{s}=200$ GeV, integrated over ${p}_{T}$ and in the rapidity range $1.4<y<1.9$, is found to be $d{\ensuremath{\sigma}}_{c\overline{c}}/dy=0.139\ifmmode\pm\else\textpm\fi{}0.029\phantom{\rule{4pt}{0ex}}{(\mathrm{stat})\phantom{\rule{0.16em}{0ex}}}_{\ensuremath{-}0.058}^{+0.051}\phantom{\rule{4pt}{0ex}}(\mathrm{syst})$ 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.

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