  The experimental observation of hadrons correlated back-to-back with a
(semi-)hard trigger in heavy ion collisions has revealed a splitting of the
away side correlation structure in a low to intermediate transverse momentum
(P_T) regime. This is consistent with the assumption that energy deposited by
the away side parton into the bulk medium produced in the collision excites a
sonic shockwave (a Mach cone) which leads to away side correlation strength at
large angles. A prediction following from assuming such a hydrodynamical origin
of the correlation structure is that there is a sizeable elongation of the
shockwave in rapidity due to the longitudinal expansion of the bulk medium.
Using a single hadron trigger, this cannot be observed due to the unconstrained
rapidity of the away side parton. Using a dihadron trigger, the rapidity of the
away side parton can be substantially constrained and the longitudinal
structure of the away side correlation becomes accessible. However, in such
events several effects occur which change the correlation structure
substantially: There is not only a sizeable contribution due to the
fragmentation of the emerging away side parton, but also a systematic bias
towards small energy deposition into the medium and hence a weak shockwave. In
this paper, both effects are addressed.
