Production of Mesons and Baryons at High Rapidity and High $p_T$ in Proton-Proton Collisions at $\sqrt{s}=200\mathrm{GeV}$

We present particle spectra for charged hadrons ${\pi }^{\pm }$, $K^{\pm }$, $p$, and $\overline{p}$ from $pp$ collisions at $\sqrt{s}=200\mathrm{GeV}$ measured for the first time at forward rapidities (2.95 and 3.3). The kinematics of these measurements are skewed in a way that probes the small momentum fraction in one of the protons and large fractions in the other. Large proton to pion ratios are observed at values of transverse momentum that extend up to $4\mathrm{GeV}/c$, where protons have momenta up to 35 GeV. Next-to-leading order perturbative QCD calculations describe the production of pions and kaons well at these rapidities, but fail to account for the large proton yields and small $\overline{p}/p$ ratios.

Figure 1

Invariant cross section distributions for pion, kaons protons, and antiprotons produced in $p+p$ collisions at $\sqrt{s}=200\mathrm{GeV}$ at rapidities $y=2.95$ (left-hand panels) and $y=3.3$ (right-hand panels). In all panels, positive charged particles are shown with solid (red) triangles and negative ones with open (blue) circles. The errors displayed in these plots are statistical.

Figure 2

Particle ratios versus $p_T$ at $y=2.95$ and 3.3. Top: ${\pi }^{-}/{\pi }^{+}$; middle: $\overline{p}/p$; bottom: $p/{\pi }^{+}$ (red triangles) and $\overline{p}/{\pi }^{-}$ (blue squares). The shaded rectangles indicate an overall systematic error (17%) estimated for these ratios. The dashed line shows an upper limit for the $(p+\overline{p})/({\pi }^{+}+{\pi }^{-})$ ratio from $e^{+}{e}^{-}$ collisions.

Figure 3

Top Comparison of invariant cross sections for ${\pi }^{-}$, $K^{+}$, $\overline{p}$, and $p$ at $y=2.95$ and NLO calculations with factorization and renormalization scales set equal to $p_T$. The mKKP set of fragmentation functions (solid red line) produce the best agreement with the ${\pi }^{-}$ and $K^{+}$ data. The $p$ and $\overline{p}$ are compared with the calculation using the AKK set divided by 2 (dashed red line); see text for details. Bottom: Relative differences between data and calculations. The top smooth curves show the effect of setting $\mu =2p_T$ and the bottom curves $\mu =1/2p_T$. For the baryons the (red) filled triangles show $p$ data versus the $\mathrm{AKK}/2$ set.