New Physics In B Decays

Description

The Standard Model (SM) of particle physics has withstood thus far every attempt by experimentalists to show that it does not describe data. We discuss the SM in some detail, focusing on the mechanism of fermion mixing, which represents one of its most intriguing aspect. We discuss how this mechanism can be tested in b-quark decays, and how b decays can be used to extract information on physics beyond the SM. We review experimental techniques in b physics, focusing on recent results and highlighting future prospects. Particular attention is devoted to recent results from b decays into a hadron, a lepton and an anti-lepton, that show discrepancies with the SM predictions — the so-called B-physics anomalies — whose statistical significance has been increasing steadily. We discuss these experiments in a detailed manner, and also provide theoretical interpretation of these results in terms of physics beyond the SM. Contents Acknowledgments Preface 1 Introduction 1.1 The Standard Model and its open problems 1.1.1 The flavor structure of the SM and the CKM matrix 1.2 A first overview of b physics 1.2.1 Searching for NP in b decays 1.3 Experimental aspects 1.4 Theoretical tools 1.4.1 Some properties of the CKM matrix 1.4.2 Low-energy effective Lagrangians 1.4.3 Hadronic matrix elements 1.4.4 Effective Lagrangians for physics beyond the SM 1.4.5 Accidental and approximate symmetries 2 Traditional New Physics searches 2.1 Determining the size of the CKM elements 2.1.1 Measurements of |Vcb| 2.1.2 Measurements of |Vub| 2.1.3 The composition of the inclusive semileptonics bottom to charm branching fractions 2.2 The measurement of γ 2.3 B–B mixing and CP violation 2.3.1 Time evolution of neutral B mesons 2.3.2 Measurements of ΔM 2.3.3 CP violation in mixing 2.3.4 CP violation in the interference between mixing and decay amplitudes 2.3.5 B0 and B0s mixing within the SM 2.3.6 Summary of neutral B mixing and CP violation observables 2.4 The global CKM fit and B–B mixing beyond the SM 2.4.1 Fits to experimental CKM constraints 2.4.2 NP bounds from Bs,d mixing and the flavor problem 3 Current anomalies: Experimental evidence 3.1 Introduction 3.2 The b → sℓ+ℓ− anomalies 3.2.1 Generalities 3.2.2 Branching fraction measurements of exclusive b → hsμ+μ− decays 3.2.3 The LFU ratios in B → K(∗)ℓ+ℓ− decays 3.2.3.1 RK 3.2.3.2 RK∗ 3.2.4 Angular distributions in B → K∗μ+μ− 3.2.5 Summary of B → K(∗)ℓ+ℓ− decays 3.2.6 Angular analysis of B0s→ ϕμ+μ− decay 3.3 The decays B0s→ μ+μ− and B0 → μ+μ− 3.4 Anomalies in the ratio of tauonic to muonic semileptonic decays 3.4.1 Introduction 3.4.2 Measurements of RD(∗) using fully reconstructed B meson tags at B-factories 3.4.3 Measurements of RD(∗) using semileptonic decay tags at B factories 3.4.4 Measurements of RD∗ and RJ/ψ at LHCb 3.4.5 Summary of RD and RD∗ 4 Theoretical models addressing the current anomalies 4.1 Introduction 4.2 The b→ sℓ+ℓ− anomalies 4.2.1 The effective b→ sℓ+ℓ− Lagrangian 4.2.2 Decay amplitudes and clean observables 4.2.3 New-physics hypotheses and fit to data 4.2.4 Summary 4.3 The b → cℓν anomalies 4.3.1 Effective Lagrangian and NP hypothesis 4.3.2 Data analysis 4.3.3 Summary 4.4 Combined analysis of the two anomalies 4.4.1 Historical remarks and main strategy 4.4.2 Operator basis 4.4.3 Combined EFT analysis 4.4.4 Summary 4.5 High-scale mediators: general considerations 4.5.1 Leptoquarks 4.5.2 Colorless mediators 4.6 A closer look to the U1 leptoquark 4.6.1 Fit of low-energy observables 4.6.2 Leptoquark signals at high energies 4.6.3 Implications for low-energy observables 4.7 Ultraviolet-complete models 4.7.1 The U1 field and quark-lepton unification 4.7.2 Other options: scalar leptoquarks and Z′ bosons 4.7.3 A multi-scale picture at the origin of the flavor hierarchies 5 Searches for lepton flavor violation and lepton number violation 5.1 Lepton flavor violation 5.1.1 Theoretical considerations 5.1.2 Experimental searches 5.2 Lepton number violation 6 Conclusions 6.1 Summary about the evidence of New Physics and theoretical insights 6.2 Future prospects A The SM Lagrangian A.1 Dirac versus Majorana masses B Flavor symmetries B.1 U(3)5 and minimal flavor violation B.2 The U(2)5 symmetry and its minimal breaking C Effective b → sℓ+ℓ− operators D Simplified expressions for selected observables Bibliography