Thus , the experimental constraints on x s + are provided essentially by the ν andν dimuon data sets . Following Refs . [ 3,9 ] , we determine the uncertainty range of x s + by the 90 % confidence criteria on the dimuon production data sets . This range is 0.018 < x s + < 0.040 . The two sets of PDFs that represent the best fits corresponding to the lower ( upper ) bound value of x s + will be referred to as CTEQ6.5S1 ( CTEQ6.5S2 ) . The variation of x s + 8 The parton number integral is strongly correlated with the normalization of the ν andν dimuon production data sets compared to the theoretically calculated inclusive charm production cross section . There are various sources of uncertainty on this overall factor : experimental ( global and energydependent ) normalization of the total cross sections ( ∼ 2 − 5 % ) , fragmentation function of charm quark to charmed hadrons , branching ratio of charmed hadron decay to muon ( ∼ 10 % ) , . . . , etc . These are taken into account according to our standard uncertainty analysis . The limits of x s + obtained above correspond to ±20 % ovreall variation of the normalization factor , as determined by this analysis procedure . As the magnitude of x s + varies , the shape of s + ( x ) also adjusts to best fit the global data . A plot of s + ( x ) for these PDFs will be shown in the next section .