  A pair-normalized two-particle covariance versus the rapidity of the two
particles, called R$_2$, was originally studied in ISR and FNAL data in the
1970's. This variable has recently seen renewed interest for the study of the
dynamics of heavy-ion collisions in the longitudinal direction. These rapidity
correlations can be decomposed into a basis set of Legendre polynomials with
prefactors $\langle a_{mn}\rangle$, which can be considered the rapidity analog
of the decomposition of azimuthal anisotropies into a set of cosine functions
with prefactors v$_{\rm n}$. The $\langle a_{mn}\rangle$ values have been
suggested to be sensitive to the number of particle emitting sources, baryon
stopping, viscosities, and critical behavior. The rapidity correlations have
been measured by the STAR collaboration as a function of the beam energy for
0-5% central Au$+$Au collisions with beam energies ranging from 7.7 to 200
$\mathrm{GeV}$. The experimental results and comparisons to the UrQMD model are
presented.
