
We present a high statistics, quenched lattice calculation of the Bparameters BBd and BBs, computed at lowest order in the HQET. The results were obtained using a sample of 600 quenched gauge field configurations, generated by Monte Carlo simulation at beta = 6.0 on a 24(3) x 40 lattice. For the light quarks the SWClover action was used; the propagator of the lattice HQET was also treelevel improved. Our best estimate of the renormalization scale independent Bparameter is (B) over cap(Bd) = 1.08 +/ 0.06 +/ 0.08. (B) over cap(Bd) has been obtained by using ''boosted'' perturbation theory to calculate the renormalization constants which relate the matrix elements of the lattice operators to the corresponding amplitudes in the continuum. Due to the large statistics, the errors in the extraction of the matrix elements of the relevant bare operators are rather small. The main systematic error, corresponding to +/0.08 in the above result, comes from the uncertainty in the evaluation of the renormalization constants, for which the oneloop corrections are rather large, The nonperturbative evaluation of these constants will help to reduce the final error. We also obtain (B) over cap(Bs)/(B) over cap(Bd) = 1.01 +/ 0.01 and f(Bs)(2) (B) over cap(Bs)/f(Bd)(2) (B) over cap(Bd) = 1.38 +/ 0.07.
