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15 pages, 3695 KiB

Open AccessArticle

Lepton Flavour Universality in Rare B Decays

by Paula Álvarez Cartelle and Richard Morgan Williams

Symmetry 2024, 16(7), 822; https://doi.org/10.3390/sym16070822 - 30 Jun 2024

Viewed by 1530

Abstract

Tests of lepton flavour universality in rare decays of b hadrons mediated by flavour-changing neutral-current transitions constitute sensitive probes for physics beyond the standard model. In recent years, such tests have become increasingly precise and have attracted significant theoretical and experimental attention. In [...] Read more.

Tests of lepton flavour universality in rare decays of b hadrons mediated by flavour-changing neutral-current transitions constitute sensitive probes for physics beyond the standard model. In recent years, such tests have become increasingly precise and have attracted significant theoretical and experimental attention. In this article, we review the status of searches for lepton flavour universality violations in these processes and discuss prospects for future measurements at various facilities. Full article

(This article belongs to the Special Issue Symmetries and Anomalies in Flavour Physics)

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15 pages, 2655 KiB

Open AccessArticle

Strange Things in Bottom-to-Strange Decays: The Standard Model Turned Upside Down?

by Martin Andersson, Alexander Mclean Marshall, Konstantinos A. Petridis and Eluned Smith

Symmetry 2024, 16(6), 638; https://doi.org/10.3390/sym16060638 - 21 May 2024

Cited by 1 | Viewed by 1348

Abstract

The flavour anomalies are a set of experimental deviations from the Standard Model (SM) predictions in several observables involving decays of bottom quarks. In particular, tensions between theory and experiment in measurements involving a bottom quark decaying into a strange quark and a [...] Read more.

The flavour anomalies are a set of experimental deviations from the Standard Model (SM) predictions in several observables involving decays of bottom quarks. In particular, tensions between theory and experiment in measurements involving a bottom quark decaying into a strange quark and a pair of muons have motivated much theoretical work to explore possible new physics explanations. This review summarises the tumultuous evolution of these tensions, focusing on the most recent experimental results and their implications for physics beyond the SM. We also discuss the prospects for future measurements and tests of the flavour anomalies at the LHC and other facilities. Full article

(This article belongs to the Special Issue Symmetries and Anomalies in Flavour Physics)

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Review

Jump to: Research

23 pages, 940 KiB

Open AccessReview

Overview of B→K^(∗)ℓℓ Theoretical Calculations and Uncertainties

by Farvah Mahmoudi and Yann Monceaux

Symmetry 2024, 16(8), 1006; https://doi.org/10.3390/sym16081006 - 7 Aug 2024

Cited by 6 | Viewed by 1061

Abstract

The search for New Physics (NP) beyond the Standard Model (SM) has been a central focus of particle physics, including in the context of B-meson decays involving

b \to s ℓ ℓ

transitions. These transitions, mediated by flavour-changing neutral currents, are highly [...] Read more.

The search for New Physics (NP) beyond the Standard Model (SM) has been a central focus of particle physics, including in the context of B-meson decays involving

b \to s ℓ ℓ

transitions. These transitions, mediated by flavour-changing neutral currents, are highly sensitive to small NP effects due to their suppression in the SM. While direct searches at colliders have not yet led to NP discoveries, indirect probes through semi-leptonic decays have revealed anomalies in observables such as the branching fraction

B (B \to K μ μ)

and the angular observable

P_{5}^{'} (B \to K^{*} μ μ)

. In order to assess the observed tensions, it is essential to ensure an accurate SM prediction. In this review, we examine the theoretical basis of the

B \to K^{(*)} ℓ ℓ

decays, addressing in particular key uncertainties arising from local and non-local form factors. We also discuss the impact of QED corrections to the Wilson coefficients, as well as the effect of CKM matrix elements on the predictions and the tension with the experimental measurements. We discuss the most recent results, highlighting ongoing efforts to refine predictions and to constrain potential signs of NP in these critical decay processes. Full article

(This article belongs to the Special Issue Symmetries and Anomalies in Flavour Physics)

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26 pages, 1071 KiB

Open AccessReview

Lepton Flavor Universality Tests in Semileptonic b→c Decays

by Suzanne Klaver and Marcello Rotondo

Symmetry 2024, 16(8), 964; https://doi.org/10.3390/sym16080964 - 29 Jul 2024

Viewed by 1313

Abstract

Semileptonic decays of b- to c-hadrons provide an exciting environment to probe new physics and currently present some of the most compelling anomalies in the field of flavor physics. Measurements of the lepton flavor universality ratios

R (D^{*})

, comparing [...] Read more.

Semileptonic decays of b- to c-hadrons provide an exciting environment to probe new physics and currently present some of the most compelling anomalies in the field of flavor physics. Measurements of the lepton flavor universality ratios

R (D^{*})

, comparing branching fractions with

τ

and

μ

leptons, show a discrepancy of over 3σ with respect to the Standard Model, and suggest that the coupling to

τ

leptons is stronger than predicted. Measurements of angular distributions as well as polarization in b- to c-hadron decays provide additional sensitivity to new physics. This review article offers an overview of the theory of semileptonic b- to c-hadron decays, presents the experiments and experimental techniques used to perform measurements of these decays, and summarizes the latest experimental results with their implications. Full article

(This article belongs to the Special Issue Symmetries and Anomalies in Flavour Physics)

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13 pages, 1812 KiB

Open AccessReview

Short-Distance Physics with Rare Kaon Decays

by Siavash Neshatpour

Symmetry 2024, 16(8), 946; https://doi.org/10.3390/sym16080946 - 24 Jul 2024

Cited by 1 | Viewed by 1456

Abstract

In this write-up, we provide an overview of the existing theoretical framework concerning rare kaon decays, with a particular emphasis on flavour-changing neutral current processes. These decays offer crucial indirect pathways for investigating short-distance new physics. Our discussion will encompass standard model predictions [...] Read more.

In this write-up, we provide an overview of the existing theoretical framework concerning rare kaon decays, with a particular emphasis on flavour-changing neutral current processes. These decays offer crucial indirect pathways for investigating short-distance new physics. Our discussion will encompass standard model predictions for relevant observables, alongside an assessment of their capacity to probe new physics through a comparison with experimental data. Full article

(This article belongs to the Special Issue Symmetries and Anomalies in Flavour Physics)

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16 pages, 532 KiB

Open AccessReview

Current Status of the Standard Model Prediction for the B_s → μ⁺μ⁻ Branching Ratio

by Mateusz Czaja and Mikołaj Misiak

Symmetry 2024, 16(7), 917; https://doi.org/10.3390/sym16070917 - 18 Jul 2024

Cited by 6 | Viewed by 1344

Abstract

The rare decay

B_{s} \to μ^{+} μ^{-}

provides an important constraint on possible deviations from the Standard Model in b-s-ℓ-ℓ interactions. The present weighted average of its branching ratio measurements amounts to [...] Read more.

The rare decay

B_{s} \to μ^{+} μ^{-}

provides an important constraint on possible deviations from the Standard Model in b-s-ℓ-ℓ interactions. The present weighted average of its branching ratio measurements amounts to

(3.34 \pm 0.27) \times 10^{- 9}

, which remains in good agreement with the theoretical prediction of

(3.64 \pm 0.12) \times 10^{- 9}

within the Standard Model. In the present paper, we review calculations that have contributed to this prediction and discuss the associated uncertainties. Full article

(This article belongs to the Special Issue Symmetries and Anomalies in Flavour Physics)

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19 pages, 3756 KiB

Open AccessReview

Charged Lepton-Flavour Violation

by Giulia Frau and Christoph Langenbruch

Symmetry 2024, 16(3), 359; https://doi.org/10.3390/sym16030359 - 16 Mar 2024

Cited by 2 | Viewed by 3098

Abstract

We review the experimental status of searches for lepton-flavour violation in the charged sector. We give an overview of searches for lepton-flavour violation in purely leptonic decays, hadron decays, and decays of heavy bosons. We focus on the most stringent constraints on lepton-flavour [...] Read more.

We review the experimental status of searches for lepton-flavour violation in the charged sector. We give an overview of searches for lepton-flavour violation in purely leptonic decays, hadron decays, and decays of heavy bosons. We focus on the most stringent constraints on lepton-flavour violating processes in these areas and give prospects for ongoing and future experiments. Full article

(This article belongs to the Special Issue Symmetries and Anomalies in Flavour Physics)

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21 pages, 895 KiB

Open AccessEditor’s ChoiceReview

Analysis of

B_{(s)}^{0}

→ μ⁺μ⁻ Decays at the Large Hadron Collider

by Kai-Feng Chen, Titus Mombächer and Umberto De Sanctis

Symmetry 2024, 16(2), 251; https://doi.org/10.3390/sym16020251 - 18 Feb 2024

Cited by 3 | Viewed by 1768

Abstract

This article reviews the most recent measurements of

B_{(s)}^{0}

→ μ⁺μ⁻ decay properties at the Large Hadron Collider (LHC) which are the most precise to date. The measurements of the branching fraction and effective lifetime of [...] Read more.

This article reviews the most recent measurements of

B_{(s)}^{0}

→ μ⁺μ⁻ decay properties at the Large Hadron Collider (LHC) which are the most precise to date. The measurements of the branching fraction and effective lifetime of the

B_{s}^{0}

→ μ⁺μ⁻ decay by the ATLAS, CMS, and LHCb collaborations, as well as the search for

B^{0}

→ μ⁺μ⁻ decays, are summarized with a focus on the experimental challenges. Furthermore, prospects are given for these measurements and new observables that become accessible with the foreseen amounts of data by the end of the LHC. Full article

(This article belongs to the Special Issue Symmetries and Anomalies in Flavour Physics)

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