Measurement of ${\rm Y}$ Production for $p+p$ and $p+d$ Interactions at $800\mathrm{GeV}/c$

We report a high statistics measurement of ${\rm Y}$ production with an $800\mathrm{GeV}/c$ proton beam on hydrogen and deuterium targets. The dominance of the gluon-gluon fusion process for ${\rm Y}$ production at this energy implies that the cross section ratio, $\sigma (p+d\to {\rm Y})/2\sigma (p+p\to {\rm Y})$, 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 $\sigma (p+d{)}_{\mathrm{DY}}/2\sigma (p+p{)}_{\mathrm{DY}}$. This result shows that the gluon distributions in the proton and neutron are very similar. The ${\rm Y}$ production cross sections are also compared with the $p+d$ and $p+\mathrm{Cu}$ cross sections from earlier measurements.

Figure 1

Dimuon mass spectrum for $p+d$ interactions at $800\mathrm{GeV}/c$. Fit to the spectrum is shown as the solid curve. Contributions from the Drell-Yan continuum and ${\rm Y}$ resonances are shown as dashed curves. The luminosity weighted spectrum from the empty-target measurements is also shown.

Figure 2

Upper panel: $Bd\sigma /x_F$ (per target nucleon) for ${\rm Y}(1S)$, ${\rm Y}(2S)$, and ${\rm Y}(3S)$ production cross sections for $p+d$ at $800\mathrm{GeV}/c$. The E605 data [10] for $p+\mathrm{Cu}$ are also shown as open symbols. Lower panel: $Bd\sigma /x_F$ for ${\rm Y}(1S)$, ${\rm Y}(2S)$, and ${\rm Y}(3S)$ production for $p+p$ at $800\mathrm{GeV}/c$.

Figure 3

$Bd\sigma /d{p}_T^2$ (per target nucleon) for ${\rm Y}$ production cross sections for $p+d$ and $p+p$ from the E866 measurement. The curves correspond to fits described in the text.

Figure 4

Comparison of the ${\rm Y}(1S)$ production cross sections between E866, E772 [8], and E605 [10]. 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

The E866 $\sigma (p+d)/2\sigma (p+p)$ cross section ratios for ${\rm Y}$ resonances as a function of $x_2$. The corresponding ratios for the E866 Drell-Yan cross sections [4] are also shown. The error bars are statistical only.