Figure 2

Measuring B-meson decays At a B-factory, electrons and positrons collide with just enough energy to produce an Y(4S) particle, which immediately decays to a B⁰- and a B-meson. As the electrons are given more energy than the positrons, the B-mesons move away in the direction of the electron beam before decaying within a few picoseconds. A particularly interesting type of decay for measuring CP violation is when a B-meson decays to a K_s and J/ψ meson, which decay in turn to a pair of pions and a pair of muons, respectively. However, since either a B⁰ or a B⁰ can produce these products (a B⁰ in the figure), researchers must examine the decay of the other B-meson in the event in order to establish which is which. The position at which the two B-mesons decay must then be determined in order to establish how long the particles lived (t₁ and t₂). The Standard Model predicts that B⁰-mesons will on average decay slightly later than B⁰-mesons, by an amount that depends on the angle β of the unitarity triangle.

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