Determination of the ratio r= d u of the valence quark distributions in the proton from neutrino and antineutrino reactions on hydrogen and deuterium.

Based on a QCD analysis of the parton momentum distributions in the proton, the ratio r= d/ u of the d and u valence quark distributions is determined as function of x in the range 0.01< x<0.7. The analysis uses data from neutrino and antineutrino charged current interactions on hydrogen and deuterium, obtained with BEBC in the (anti)neutrino wideband beam of the CERN SPS. Since mainly depends on the deuterium/hydrogen ratios of the normalised x-y-Q-distributions many systematic effects cancel. It is found that r decreases with increasing x, and drops below the naive SU(6) expectation of 0.5 for x≳0.3. An extrapolation of r to x=1 is consistent with the hypothesis r(1)=0. [ABSTRACT FROM AUTHOR]