Observability of mixing in partially reconstructed Bd and Bs at the SFT

Observation of mixing (Bd → bar Bd and Bs → bar Bs) is of great physical interest. Mixing is also experimentally challenging since it requires that the beauty particle be detected, that its particle-antiparticle nature be tagged both at production and decay, and that its lifetime in the rest frame be measured with high resolution. The authors propose to study mixing at the SSC Fixed Target experiment (SFT). In one year of running 1.6 x 1010 B bar B events will be produced. The branching ratio B → lX, where l is a charged lepton, is 21% (for the average of Bd and Bu), and they assume Bs is the same. Assuming Bu/Bd/Bs fractions 0.38/0.38/0.14, they get 2.0 x 108 Bu → lX, Bd → lX events and 7.5 x 107 Bu → lX, Bs → lX events. Their trigger efficiency for double semi-leptonic B events is 51%. They can afford to cut very hard on the data. Because of the high vertex resolution and long flight paths of the B, the distance the B travels in the lab is well measured (L/σL ∼ 380). The difficulty is to reconstruct the Lorentz Γ = EB/MB since much of the energy of the B is in undetected neutral particles. They define the MvisB and EvisB to be the invariant mass and energy of all charged daughters of the B which fall inside the spectrometer angular acceptance of 2-75 mrad. They find that Γvis = EvisB/MvisB gives a useful estimate of the true Γ since much of the effect of missing neutrals cancels in the ratio