Measurement of the Υ production cross section in 920 GeV fixed-target proton–nucleus collisions

Abstract The cross section ratio R J / ψ = Br ( ϒ → l + l − ) ⋅ d σ ( ϒ ) / d y | 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 c.m.s. energy of s = 41.6 GeV . The combined results for the decay channels ϒ → e + e − and ϒ → μ + μ − yield a ratio R J / ψ = ( 9.0 ± 2.1 ) × 10 −6 . The corresponding ϒ production cross section per nucleon at mid-rapidity ( y = 0 ) has been determined to be Br ( ϒ → l + l − ) ⋅ d σ ( ϒ ) / d y | y = 0 = 4.5 ± 1.1 pb / nucleon .