Zoccoli, A (reprint author), Univ Bologna, Dipartimento Fis, I-40126 Bologna, Italy.
The cross section ratio R-J/psi = Br(Y -> l(+)l(-)) center dot d sigma(Y)/dyl(y=0)/sigma(J/psi) 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 root s = 41.6 GeV. The combined results for the decay channels Y -> e(+)e(-) and Y -> mu(+)mu(-) yield a ratio R-J/psi = (9.0 +/- 2.1) x 10(-6). The corresponding Y production cross section per nucleon at mid-rapidity (y = 0) has been determined to be Br(Y -> l(+)l(-)) center dot d sigma(Y)/dy vertical bar(y=0) = 4.5 +/- 1.1 pb/nucleon.