Searching for New Physics in Rare K and B Decays without |V_cb| and |V_ub| Uncertainties

2021/09/22 by Andrzej J. Buras, Buras, Andrzej J., Elena Venturini +1 · 2 citations
Computer Science · Physics and Astronomy · #Computational Physics and Python Applications #FOS: Physical sciences #High Energy Physics - Experiment (hep-ex) #High Energy Physics - Lattice (hep-lat) #High Energy Physics - Phenomenology (hep-ph) #High-Energy Particle Collisions Research #Particle physics theoretical and experimental studies

paper · pdf · doi:10.48550/arxiv.2109.11032

openalex publication_date 2021/09/22 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28

Abstract

We reemphasize the strong dependence of the branching ratios B(K⁺→π⁺νν) and B(K_L→π⁰νν) on |V_cb| that is stronger than in rare B decays, in particular for K_L→π⁰νν. Thereby the persistent tension between inclusive and exclusive determinations of |V_cb| weakens the power of these theoretically clean decays in the search for new physics (NP). We demonstrate how this uncertainty can be practically removed by considering within the SM suitable ratios of the two branching ratios between each other and with other observables like the branching ratios for K_S→μ⁺μ^-, B_s,d→μ⁺μ^- and B→ K(K^*)νν. We use as basic CKM parameters V_us, |V_cb| and the angles β and γ in the unitarity triangle (UT). This avoids the use of the problematic |V_ub|. A ratio involving B(K⁺→π⁺νν) and B(B_s→μ⁺μ^-) while being |V_cb|-independent exhibits sizable dependence on the angle γ. It should be of interest for several experimental groups in the coming years. We point out that the |V_cb|-independent ratio of B(B⁺→ K⁺νν) and B(B_s→μ⁺μ^-) from Belle II and LHCb signals a 1.8σ tension with its SM value. As a complementary test of the Standard Model, we propose to extract |V_cb| from different observables as a function of β and γ. We illustrate this with ε_K, ΔM_d and ΔM_s finding tensions between these three determinations of |V_cb| within the SM. From ΔM_s and S_ψKS alone we find |V_cb|=41.8(6)× 10^-3 and |V_ub|=3.65(12)× 10^-3. We stress the importance of a precise measurement of γ. We obtain most precise SM predictions for considered branching ratios of rare K and B decays to date.

Cited by

• Challenging B_(s)→ D_(s)^(*) Form Factors with the Heavy Quark Expansion
• Correlating lepton flavour violating b → s and leptonic decay modes in a minimal abelian extension of the Standard Model

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