  Measurements by the STAR and PHENIX collaborations indicate that a
quark-gluon plasma, a hot and dense state of matter in which quarks and gluons
are not confined inside hadrons, is formed in heavy-ion collisions at the
Relativistic Heavy Ion Collider. Charm and bottom quarks have been predicted to
interact with the medium differently than the light quarks; a study of heavy
quark interactions with the medium provides an important test of theoretical
models of the quark-gluon plasma.
  The spectrum of non-photonic electrons (and positrons) is dominated by
electrons from the semileptonic decays of D and B mesons. Therefore,
non-photonic electrons serve as proxies for heavy quarks. A measurement of the
modification of the non-photonic electron spectrum in nucleus-nucleus
collisions relative to p + p collisions allows the interactions of heavy quarks
with the medium to be studied. Previous measurements indicate that
high-transverse-momentum non-photonic electrons are suppressed in Au + Au
collisions at 200 GeV (center-of-mass energy per nucleon pair) relative to p +
p collisions at the same energy. The magnitude of that suppression is larger
than was anticipated and it has been a challenge for theoretical models to
predict the in-medium energy loss of light and heavy quarks simultaneously.
  This dissertation presents the first measurement of the yield of non-photonic
electrons from open heavy-flavor decays in Cu + Cu collisions at 200 GeV and
the suppression of that yield relative to p + p collisions. A comparison of
this result to similar results for Au + Au collisions provides some indication
that the geometry of a heavy-ion collision affects the average amount of energy
loss by heavy quarks passing through the quark-gluon plasma.
