Measurement of Upsilon Production for p+p and p+d Interactions at 800 GeV

TL;DR

A high statistics measurement of Upsilon production with an 800 GeV/c proton beam on hydrogen and deuterium targets shows that the gluon distributions in the proton and neutron are very similar, in striking contrast to the behavior of the Drell-Yan cross section ratio.

Abstract

We report a high statistics measurement of Upsilon production with an 800 GeV/c proton beam on hydrogen and deuterium targets. The dominance of the gluon-gluon fusion process for Upsilon production at this energy implies that the cross section ratio, $σ(p + d \to Υ) / 2σ(p + p\to Υ)$, is sensitive to the gluon content in the neutron relative to that in the proton. Over the kinematic region 0 < x_F < 0.6, this ratio is found to be consistent with unity, in striking contrast to the behavior of the Drell-Yan cross section ratio $σ(p+d)_{DY}/2σ(p+p)_{DY}$. This result shows that the gluon distributions in the proton and neutron are very similar. The Upsilon production cross sections are also compared with the p+d and p+Cu cross sections from earlier measurements.

Figures (5)

• Figure 1: (color online). Dimuon mass spectrum for $p+d$ interactions at 800 GeV/c. Fit to the spectrum is shown as the solid curve. Contributions from the Drell-Yan continuum and $\Upsilon$ resonances are shown as dashed curves. The luminosity weighted spectrum from the empty-target measurements is also shown.
• Figure 2: (color online). Upper panel: $B\cdot d\sigma/x_F$ (per target nucleon) for $\Upsilon(1S)$, $\Upsilon(2S)$, and $\Upsilon(3S)$ production cross sections for $p+d$ at 800 GeV/c. The E605 data e605 for $p~+$ Cu are also shown as open symbols. Lower panel: $B\cdot d\sigma/x_F$ for $\Upsilon(1S)$, $\Upsilon(2S)$, and $\Upsilon(3S)$ production for $p+p$ at 800 GeV/c.
• Figure 3: (color online). $B\cdot d\sigma/dp_T^2$ (per target nucleon) for $\Upsilon$ production cross sections for $p+d$ and $p+p$ from the E866 measurement. The curves correspond to fits described in the text.
• Figure 4: (color online). Comparison of the $\Upsilon (1S)$ prodution cross sections between E866, E772 pat, and E605 e605. The solid and dashed curves correspond to the color-evaporation model calculations using the CTEQ4M and CTEQ5L parton distribution functions, respectively. The absolute normalizations of the calculations are adjusted to fit the E866 data.
• Figure 5: (color online). The E866 $\sigma(p + d) /2 \sigma(p + p)$ cross section ratios for $\Upsilon$ resonances as a function of $x_2$. The corresponding ratios for the E866 Drell-Yan cross sections towell are also shown. The error bars are statistical only.