High Pt Hadron Spectra at High Rapidity

TL;DR

This study probes gluon saturation in d+Au and p+p collisions at RHIC by measuring high-pT charged hadron spectra across a range of pseudorapidities with BRAHMS. Nuclear modification factors R_dAu and central-to-peripheral ratios R_CP reveal a transition from Cronin-like enhancement at mid-rapidity to forward-rapidity suppression, with suppression strengthening for more central collisions. The findings align with Color Glass Condensate expectations of rapidity- and centrality-dependent suppression due to nonlinear QCD evolution, and challenge models predicting persistent enhancements at forward rapidities. Together, these results provide empirical constraints on small-x dynamics and saturation phenomena in heavy-ion collisions. The work advances understanding of parton density effects at high energies and informs theoretical modeling of forward-rapidity particle production.

Abstract

We report the measurement of charged hadron production at different pseudo-rapidity values in deuteron+gold as well as proton+proton collisions at $sqrt{s_{NN}}$ = 200GeV at RHIC. The nuclear modification factors $R_{dAu}$ and $R_{cp}$ are used to investigate new behaviors in the deuteron+gold system as function of rapidity and the centrality of the collisions respectively.

Figures (4)

• Figure 1: Spectra for charged hadrons at different pseudo-rapidities. Panel a shows the spectra obtained from proton-proton collisions and panel b those from d+Au collisions. The top most distributions in both panels correspond to the invariant yields of $\frac{h^+ + h^-}{2}$ measured at 40 degrees with the MRS spectrometer (scaled by 100 for clarity purposes), followed by the yields of negative hadrons measured at 12 (scaled up by 10) and 4 degrees respectively. The distributions obtained at $\eta=2.2$ and 3.2 have variable bin size indicated by horizontal lines, the data points have been located at the mean value calculated within that particular bin. The distribution obtained at $\eta=1$ has fixed bin width of 200 MeV/c and the data points are displayed at the middle of the bin.
• Figure 2: Nuclear modification factor for charged hadrons at pseudorapidities $\eta=0,1.0,2.2,3.2$. Statistical errors are shown with error bars. Systematic errors are shown with shaded boxes with widths set by the bin sizes. The shaded band around unity indicates the estimated error on the normalization to $\langle N_{coll} \rangle$. Dashed lines at $p_T<1$ GeV/c show the normalized charged particle density ratio $\frac{1}{\langle N_{coll}\rangle}\frac{dN/d\eta(d+Au)}{dN/d\eta(pp)}$.
• Figure 3: Central (full points) and semi-central (open points) $R_{cp}$ ratios (see text for details) at pseudorapidities $\eta=0,1.0,2.2,3.2$. Systematic errors ($\sim5\%$) are smaller than the symbols. The ratios at all pseudorapidities are calculated for the average charge $\frac{h^+ + h^-}{2}$.
• Figure 4: Spectra of positive and negative charged particles at $\eta = 3.2$