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A Mathematica based Version of the CKMfitter Package

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8 Chapter 2. Theory<br />

2.4 Neutral Meson Oscillation<br />

The phenomenon <strong>of</strong> quark flavor oscillation is predicted in <strong>the</strong> four neutral meson<br />

systems:<br />

K 0 (¯sd) ↔ ¯ K 0 (s ¯ d) , D 0 (cū) ↔ ¯ D 0 (¯cu) , B 0 d (¯ bd) ↔ ¯ B 0 d (b ¯ d) , B 0 s ( ¯ bs) ↔ ¯ B 0 s (b¯s) .<br />

Since <strong>the</strong> mean life time <strong>of</strong> <strong>the</strong> neutral D mesons is very small compared to <strong>the</strong>ir<br />

oscillation frequency, neutral meson mixing has been only observed in <strong>the</strong> Kaon and<br />

B-meson systems, yet.<br />

The time evolution <strong>of</strong> <strong>the</strong>se transitions is given by <strong>the</strong> Schrödinger Equation for<br />

two-state systems <strong>of</strong> instable particles:<br />

i ˙ ψ(t) = ˆ H ψ(t) , (2.20)<br />

where ψ(t) is <strong>the</strong> two-state system <strong>of</strong> <strong>the</strong> neutral mesons, e. g. for <strong>the</strong> Bd system:<br />

�<br />

|B0 (t)〉<br />

ψ(t) =<br />

| ¯ B0 �<br />

. (2.21)<br />

(t)〉<br />

The Hamilton Operator ˆ H contains <strong>the</strong> two hermitian 2 × 2 mass (Mij) and decay<br />

width (Γij) matrices:<br />

⎛<br />

ˆH = ˆ M − i<br />

2 ˆ ⎜<br />

Γ = ⎝<br />

M11 − i<br />

2 Γ11 M12 − i<br />

2 Γ12<br />

M21 − i<br />

2 Γ21 M22 − i<br />

2 Γ22<br />

⎞<br />

⎟<br />

⎠ . (2.22)<br />

Since CPT symmetry requires equal masses and decay rates for a particle and its<br />

anti-particle, <strong>the</strong> diagonal elements must be equal:<br />

Γ11 = Γ22 = Γ (2.23)<br />

M11 = M22 = m (2.24)<br />

and <strong>the</strong> eight real parameters can be reduced to six independent parameters. The<br />

hermiticity <strong>of</strong> ˆ Γ and ˆ M leads to:<br />

M21 = M ∗ 12 and Γ21 = Γ ∗ 12 . (2.25)<br />

Because <strong>of</strong> an arbitrary global phase, only five observables can be defined<br />

� � � �<br />

Γ12<br />

Γ12<br />

m , Γ , |M12| , Re and Im . (2.26)<br />

M12<br />

The Schrödinger Equation (2.20) has well defined solutions ψ(t) for any ψ(0), but<br />

only two mass eigenstates with time-independent flavor composition for each neutral<br />

meson system.<br />

M12

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