  The PHENIX Collaboration at the Relativistic Heavy Ion Collider has measured
open heavy flavor production in Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}$=200
GeV through the measurement of electrons at midrapidity that originate from
semileptonic decays of charm and bottom hadrons. In peripheral Cu$+$Cu
collisions an enhanced production of electrons is observed relative to
$p$$+$$p$ collisions scaled by the number of binary collisions. In the
transverse momentum range from 1 to 5 GeV/$c$ the nuclear modification factor
is $R_{AA}$$\sim$1.4. As the system size increases to more central Cu$+$Cu
collisions, the enhancement gradually disappears and turns into a suppression.
For $p_T>3$ GeV/$c$, the suppression reaches $R_{AA}$$\sim$0.8 in the most
central collisions. The $p_T$ and centrality dependence of $R_{AA}$ in Cu$+$Cu
collisions agree quantitatively with $R_{AA}$ in $d+$Au and Au$+$Au collisions,
if compared at similar number of participating nucleons $\langle N_{\rm part}
\rangle$.
