Measurement of the Upsilon Production Cross Section in 920 GeV Fixed-Target Proton-Nucleus Collisions

Abstract

The proton-nucleon cross section ratio $R=Br(Υ\to l^+l^-) dσ(Υ)/dy|_{y=0} / {σ(J/ψ)}$ has been measured with the HERA-B spectrometer in fixed-target proton-nucleus collisions at 920 GeV proton beam energy corresponding to a proton-nucleon cms energy of sqrt{s}=41.6 GeV. The combined results for the Upsilon decay channels Upsilon $\to e^+e^-$ and Upsilon $\toμ^+μ^-$ yield a ratio $R=(9.0 \pm 2.1) 10^{-6}$. The corresponding Upsilon production cross section per nucleon at mid-rapidity (y=0) has been determined to be $Br(Υ\to{}l^+l^-) {dσ(Υ)/dy}|_{y=0}= 4.5 \pm 1.1 $ pb/nucleon.