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Anatomy of New Physics in B-Bbar mixing

Abstract : We analyse three different New Physics scenarios for Delta F=2 flavour-changing neutral currents in the quark sector in the light of recent data on neutral-meson mixing. We parametrise generic New Physics contributions to Bq - Bq-bar mixing (q=d,s), in terms of one complex quantity Delta_q, while three parameters Delta_K^{tt}, Delta_K^{ct} and Delta_K^{cc} are needed to describe K - Kbar mixing. In Scenario I, we consider uncorrelated New Physics contributions in the Bd, Bs, and K sectors. In this scenario, it is only possible to constrain the parameters Delta_d and Delta_s whereas there are no non-trivial constraints on the kaon parameters. In Scenario II, we study the case of Minimal Flavour Violation (MFV) and small bottom Yukawa coupling, where Delta=Delta_d=Delta_s=Delta_K^{tt}. We show that Delta must then be real, so that no new CP phases can be accomodated, and express the remaining parameters Delta_K^{cc} and Delta_K^{ct} in terms of Delta in this scenario. Scenario III is the generic MFV case with large bottom Yukawa couplings. In this case, the Kaon sector is uncorrelated to the B_d and B_s sectors. As in the second scenario one has Delta_d=Delta_s=Delta, however, now with a complex parameter Delta. Our quantitative analyses consist of global CKM fits within the Rfit frequentist statistical approach, determining the Standard Model parameters and the new physics parameters of the studied scenarios simultaneously. We find that the recent measurements indicating discrepancies with the Standard Model are well accomodated in Scenarios I and III with new mixing phases, with a slight preference for Scenario Ithat permits different new CP phases in the Bd and Bs systems. Within our statistical framework, we find that the Standard-Model hypothesis Delta_d=Delta_s=1 is disfavoured with p-values of 3.4 sigma and 3.1 sigma in Scenarios I and III respectively.
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A. Lenz, U. Nierste, Jérôme Charles, S. Descotes-Genon, A. Jantsch, et al.. Anatomy of New Physics in B-Bbar mixing. Physical Review D, American Physical Society, 2011, 83, pp.036004. ⟨10.1103/PhysRevD.83.036004⟩. ⟨hal-00515524⟩

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