  Three-particle azimuthal correlation measurements with a high transverse
momentum trigger particle are reported for pp, d+Au, and Au+Au collisions at
200 GeV by the STAR experiment. Dijet structures are observed in pp, d+Au and
peripheral Au+Au collisions. An additional structure is observed in central
Au+Au data, signaling conical emission of correlated charged hadrons. The
conical emission angle is found to be 1.37 +- 0.02(stat) +0.06-0.07(syst),
independent of pt.
