M. Germinario

We report on a study of the transverse momentum dependence of nuclear modification factors ${R}_{d\mathrm{A}\mathrm{u}}$ for charged hadrons produced in $\mathrm{\text{deuteron}}\text{ }\text{ }+\text{ }\text{ }\mathrm{\text{gold}}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$, as a function of collision centrality and of the pseudorapidity ($\ensuremath{\eta}=0$, 1, 2.2, 3.2) of the produced hadrons. We find a significant and systematic decrease of ${R}_{d\mathrm{A}\mathrm{u}}$ with increasing rapidity. The midrapidity enhancement and the forward rapidity suppression are more pronounced in central collisions relative to peripheral collisions. These results are relevant to the study of the possible onset of gluon saturation at energies reached at BNL RHIC.

We present spectra of charged hadrons from $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ and $d+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$ measured with the BRAHMS experiment at RHIC. The spectra for different collision centralities are compared to spectra from $p+\overline{p}$ collisions at the same energy scaled by the number of binary collisions. The resulting ratios (nuclear modification factors) for central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\ensuremath{\eta}=0$ and $\ensuremath{\eta}=2.2$ evidence a strong suppression in the high ${p}_{T}$ region ($>2\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}/c$). In contrast, the $d+\mathrm{A}\mathrm{u}$ nuclear modification factor (at $\ensuremath{\eta}=0$) exhibits an enhancement of the high ${p}_{T}$ yields. These measurements indicate a high energy loss of the high ${p}_{T}$ particles in the medium created in the central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions. The lack of suppression in $d+\mathrm{A}\mathrm{u}$ collisions makes it unlikely that initial state effects can explain the suppression in the central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions.

We have measured rapidity densities $dN/dy$ of ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}$ and ${K}^{\ifmmode\pm\else\textpm\fi{}}$ over a broad rapidity range ($\ensuremath{-}0.1<y<3.5$) for central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$. These data have significant implications for the chemistry and dynamics of the dense system that is initially created in the collisions. The full phase-space yields are $1660\ifmmode\pm\else\textpm\fi{}15\ifmmode\pm\else\textpm\fi{}133$ (${\ensuremath{\pi}}^{+}$), $1683\ifmmode\pm\else\textpm\fi{}16\ifmmode\pm\else\textpm\fi{}135$ (${\ensuremath{\pi}}^{\ensuremath{-}}$), $286\ifmmode\pm\else\textpm\fi{}5\ifmmode\pm\else\textpm\fi{}23$ (${K}^{+}$), and $242\ifmmode\pm\else\textpm\fi{}4\ifmmode\pm\else\textpm\fi{}19$ (${K}^{\ensuremath{-}}$). The systematics of the strange to nonstrange meson ratios are found to track the variation of the baryochemical potential with rapidity and energy. Landau-Carruthers hydrodynamics is found to describe the bulk transport of the pions in the longitudinal direction.

Transverse momentum spectra and rapidity densities, $dN/dy$, of protons, antiprotons, and net protons ($p\ensuremath{-}\overline{p}$) from central (0%--5%) $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$ were measured with the BRAHMS experiment within the rapidity range $0\ensuremath{\le}y\ensuremath{\le}3$. The proton and antiproton $dN/dy$ decrease from midrapidity to $y=3$. The net-proton yield is roughly constant for $y<1$ at $dN/dy\ensuremath{\sim}7$, and increases to $dN/dy\ensuremath{\sim}12$ at $y\ensuremath{\sim}3$. The data show that collisions at this energy exhibit a high degree of transparency and that the linear scaling of rapidity loss with rapidity observed at lower energies is broken. The energy loss per participant nucleon is estimated to be $73\ifmmode\pm\else\textpm\fi{}6\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$.