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Exclusive B physics

OPAL has now recorded around 4 million hadronic decays of the Z0, of which some 22% involve the creation of a pair of heavy b-quarks. The B hadrons have many decay modes, but the statistics are now sufficient that we can identify significant numbers of events in channels where the B decay is completely (or almost completely) reconstructed. Several analyses at Cambridge are in this area.

B lifetime

The OPAL microvertex detector is of great importance in identifying such decays, and allows us to measure the lifetimes of B hadrons. The relative lifetimes of B+ and B0 mesons are of theoretical interest, since this casts light on the underlying decay mechanism. The B mesons are tagged through decays into a charmed D meson and a lepton. A typical plot of the measured decay lengths of B mesons is shown above.

The tail of negative apparent lifetimes reflects the experimental resolution, while from the positive side we may infer the lifetimes:
1.62+-0.10+-0.10 for B0
1.53+-0.14+-0.11 for B+ .

Simple decays of B mesons to states like pi+pi-, K+K- or K+pi- are very rare, but very interesting. They may receive significant contributions from Penguin diagrams like these:

Penguin diagram

A search for such decays in around 2 million Z decays has been performed, with negative results. A typical candidate is shown here. You can read the paper about this search by clicking here.

We are also investigating B mesons decays into the J/psi (charmonium) system. The J/psi decay can be identified very cleanly through its decay to electron or muon pairs. The J/psi can then be combined with other particles to reconstruct a B hadron decay. An example is the Bs meson containing a b quark and a strange antiquark. This can decay to J/psi + phi via a Feynman diagram like this:

Bs decay

The phi meson decays to K+K-. A candidate event of this type may be seen if you click here. The two magenta tracks are probably K+ and K- produced from the decay of a phi meson, while the yellow tracks are muons from the decay of a J/psi. If we zoom in, we see that these tracks form a displaced vertex.