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August 27, 2003

Belle de Jour

There’s recent data out from BELLE (the B-meson factory at KEK) indicating new (beyond the Standard Model) CP-violating physics.

Asymmetry data in B to phi K_S from BELLE
Asymmetry data from BELLE, from which the value of sin(2ϕ 1)\sin(2\phi_1) is determined, plotted as a function of the decay time. The smooth dotted curve indicates what one would expect from the Standard Model, whereas the smooth solid curve is a fit to the observed data.

CP violation in the Standard Model is due to a single CP-violating phase in the Kobayashi-Maskawa matrix. CP violation has been known for a long time in the K 0K^0 system. But, in the past couple of years, BELLE and BABAR (at SLAC) have been measuring CP violation in the B d 0B^0_d system. As in the K 0K^0 system the dominant CP-violating effect comes from B 0B¯ 0B^0\overline{B}^0 mixing, and manifest itself in a time-dependence of the asymmetry A(t)=Γ(B¯ 0f)Γ(B 0f)Γ(B¯ 0f)+Γ(B 0f)=sin(2ϕ 1)sin(Δmt) A(t) = \frac{\Gamma(\overline{B}^0\to f) - \Gamma(B^0\to f)}{\Gamma(\overline{B}^0\to f) + \Gamma(B^0\to f)}= \mp \sin(2\phi_1) \sin (\Delta{m} t) where the final state, ff, is assumed to be a CP eigenstate with CP=±1\mathrm{CP} = \pm 1 and ϕ 1\phi_1 is the relative phase between V tdV tb *V_{td}V_{tb}^* and V cdV cb *V_{cd}V_{cb}^*.

B0 to J/Psi+K_S: direct and via B0-B0bar mixing
Diagrams contributing to B 0J/Ψ+K SB^0\to J/\Psi + K_S

The asymmetry in B 0J/Ψ+K SB^0 \to J/\Psi + K_S (never mind that K SK_S is almost, but not quite a CP-eigenstate; that’s a negligible effect here) has been measured quite well by both BELLE and BABAR, and they agree sin(2ϕ 1)=+0.731±0.056 sin(2\phi_1)= +0.731\pm 0.056 which is consistent with other measurements of CP violation and the assumption that it is all due to single CP-violating phase in the KM matrix.

But in B 0ϕ+K SB^0\to \phi+ K_S, BELLE finds a 3.5σ3.5\sigma deviation from the Standard Model prediction. The decay is given, in the Standard Model, by a penguin diagram, of the form at right. To fit this to the BELLE data, would require sin(2ϕ 1)=0.96±0.50±0.11 \sin(2\phi_1)= - 0.96\pm 0.50\pm 0.11 which, as I said, is 3.5σ3.5\sigma away from the above value. If this holds up, it’s clear indication there must be further sources of CP violation.

B0 to phi +K_S via a penguin diagram
Penguin diagram contributing to B 0ϕ+K SB^0\to \phi + K_S

BABAR also has data on this process. Their result is more or less consistent with the Standard Model, sin(2ϕ 1)=+0.45±0.43±0.07 \sin(2\phi_1)=+0.45\pm0.43\pm 0.07 That’s a 2.1σ2.1\sigma discrepancy between the two experiments. If you average the two experiments (something I’m always suspicious of, but people do it all the time), you get sin(2ϕ 1)=0.15±0.33\sin(2\phi_1)= - 0.15\pm 0.33, which is still 2.7σ2.7\sigma from the Standard Model.

This could all still go away, but these penguin-diagram type processes are the ones most sensitive to new physics and there’s plenty of reason to expect new sources of CP violation in any extension (like supersymmetry) to the Standard Model. It would not be surprising if this were the first place where new physics were to reveal itself.

Or … this could all go away …

Posted by distler at August 27, 2003 12:22 AM

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