A pair-normalized two-particle covariance versus the rapidity of
the two particles, called R$_2$, was originally studied [1] in
ISR and FNAL data in the 1970's and has recently seen renewed
interest [2] to study the dynamics of heavy-ion collisions in the
longitudinal direction. These rapidity correlations can be
decomposed onto a basis set of Legendre polynomials with
prefactors, $a_{mn}$, which can be considered the rapidity analog
of the decomposition of azimuthal anisotropies into a basis set
of cosine functions with prefactors v$_n$. The $a_{mn}$ values have
been suggested [2] to be sensitive to the number of sources,
baryon stopping, viscosities, and criticality. The rapidity
correlations have been measured by the STAR collaboration as a
function of the centrality and beam energy in the range of 7.7 to
200 GeV. The experimental results and comparisons to those from
the UrQMD model [3] will be presented.
[1] L. Fo`{a}, Phys. Lett. C22, 1 (1975);
H. B\oggild, Ann. Rev. Nucl. Sci. 24, 451 (1974).
[2] T. Lappi and L. McLerran, Nucl. Phys. A 832, 330 (2010);
A. Bzdak and D. Teaney, Phys. Rev. C 87, 024906 (2013);
A. Monnai, B. Schenke, Phys. Lett. B 752, 317 (2016);
A. Bzdak, Volker Koch, and Nils Strodthoff, arXiv:1607.07375;
J. Jia, S. Radhakrishnan, and M. Zhou, Phys. Rev. C 93, 044905 (2016).
[3] S. A. Bass, et al., Prog. Part. Nucl. Phys. 41, 225 (1998).