First Year of BABAR/PEP-II - Harvard University Laboratory for ...

B 0 → J/ψ K S Reconstruction
Combine J/ψ and K S into B 0 .
◆ J/ψ mass cut: 2.5 < m < 3.28 GeV/c 2 for e + e − ; ±3σ for µ + µ − .
◆ Looser “lepton ID” using only the EMC.
◆ K S mass cut: ±3σ.
◆ Looser K S flight direction cut: cosα > 0.9.
Constrained fit using m(J/ψ) and m(K S ) from the PDG.
Helicity angle cut:
◆ θ h = angle between K S and l + momenta
in the CMS of J/ψ.
➤Signal distribution ∝ sin 2 θ h .
➤Background peaks near cosθ h = ±1.
◆ Cut at cosθ h
< 0.95.
K S
θ h
l +
J/ψ
l −
First Year of BaBar/PEP-II Masahiro Morii SLAC