Abstract
A pair-normalized two-particle covariance versus the rapidity of the two particles, called R2, 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 〈amn〉, which can be considered the rapidity analog of the decomposition of azimuthal anisotropies into a set of cosine functions with prefactors vn. The 〈amn〉 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 GeV. The experimental results and comparisons to the UrQMD model are presented.
Highlights
Initial state density fluctuations and the pressure gradient of the strongly interacting medium in transverse plane result in azimuthal anisotropies of the emitted particles in the final state
The density fluctuations can be studied in terms of correlations of particles in the longitudinal direction, i.e. inrapidity [3, 4]
The shape of the longitudinal correlations can be characterized by Chebyshev [3] or Legendre polynomials [4] with magnitudes given by prefactors anm
Summary
Initial state density fluctuations and the pressure gradient of the strongly interacting medium in transverse plane result in azimuthal anisotropies of the emitted particles in the final state. These can be characterized by a Fourier expansion with magnitudes given by prefactors vn [1, 2]. The density fluctuations can be studied in terms of correlations of particles in the longitudinal direction, i.e. in (pseudo)rapidity [3, 4]. The shape of the longitudinal correlations can be characterized by Chebyshev [3] or Legendre polynomials [4] with magnitudes given by prefactors anm. The importance of doing this analysis at RHIC was emphasized in Ref[6]
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