Systematic Study of Azimuthal Anisotropy in Cu$+$Cu and Au$+$Au Collisions at $\sqrt{s_{{}_{NN}}}=62.4$ and 200 GeV

Abstract

We have studied the dependence of azimuthal anisotropy $v_2$ for inclusive and identified charged hadrons in Au$+$Au and Cu$+$Cu collisions on collision energy, species, and centrality. The values of $v_2$ as a function of transverse momentum $p_T$ and centrality in Au$+$Au collisions at $\sqrt{s_{{}_{NN}}}$=200 GeV and 62.4 GeV are the same within uncertainties. However, in Cu$+$Cu collisions we observe a decrease in $v_2$ values as the collision energy is reduced from 200 to 62.4 GeV. The decrease is larger in the more peripheral collisions. By examining both Au$+$Au and Cu$+$Cu collisions we find that $v_2$ depends both on eccentricity and the number of participants, $N_{\mathrm{part}}$. We observe that $v_2$ divided by eccentricity ($\epsilon$) monotonically increases with $N_{\mathrm{part}}$ and scales as $N_{\mathrm{part}}^{1/3}$. The Cu$+$Cu data at 62.4 GeV falls below the other scaled $v_2$ data. For identified hadrons, $v_2$ divided by the number of constituent quarks $n_q$ is independent of hadron species as a function of transverse kinetic energy $K{E}_T=m_T-m$ between $0.1<K{E}_T/n_q<1$ GeV. Combining all of the above scaling and normalizations, we observe a near-universal scaling, with the exception of the Cu$+$Cu data at 62.4 GeV, of $v_2/(n_q\cdot \epsilon \cdot N_{\mathrm{part}}^{1/3})$ vs $K{E}_T/n_q$ for all measured particles.

424 authors, 22 pages, 22 figures, 6 tables. v2 is the version accepted for publication in Phys. Rev. C. Plain text data tables for the points plotted in figures for this and previous PHENIX publications are (or will be) publicly available at http://www.phenix.bnl.gov/papers.html