B-physics anomalies: a guide to combined explanations

Abstract: Motivated by additional experimental hints of Lepton Flavour Universality violation in B decays, both in charged- and in neutral-current processes, we analyse the ingredients necessary to provide a combined description of these phenomena. By means of an Effective Field Theory (EFT) approach, based on the hypothesis of New Physics coupled predominantly to the third generation of left-handed quarks and leptons, we show how this is possible. We demonstrate, in particular, how to solve the problems posed by electroweak precision tests and direct searches with a rather natural choice of model parameters, within the context of a $U(2)q \times U(2)\ell$ flavour symmetry. We further exemplify the general EFT findings by means of simplified models with explicit mediators in the TeV range: coloured scalar or vector leptoquarks and colour-less vectors. Among these, the case of an $SU(2)_L$-singlet vector leptoquark emerges as a particularly simple and successful framework.

• The paper provides a comprehensive EFT analysis identifying NP contributions from third-generation left-handed quarks and leptons to explain B-physics anomalies.
• It derives precise constraints on operator parameters that reconcile LFU deviations in both charged- and neutral-current processes under current experimental limits.
• The study highlights the viability of SU(2)_L-singlet vector leptoquarks as a coherent NP candidate that aligns with electroweak precision tests and collider searches.

Analyzing the Potential for Combined Explanations of $B$-Physics Anomalies

The paper "B-physics anomalies: a guide to combined explanations" by Buttazzo et al. provides a comprehensive examination of the observed deviations from Standard Model (SM) predictions in $B$-physics, with a particular focus on Lepton Flavor Universality (LFU) violations in both charged- and neutral-current processes. The study rigorously investigates these anomalies using an Effective Field Theory (EFT) framework, positing the possible involvement of New Physics (NP) predominantly linked with third-generation left-handed quarks and leptons. This approach, centered on a $U(2)_q \times U(2)_\ell$ flavor symmetry, aims to provide a coherent explanation of these anomalies while aligning with electroweak precision tests and ongoing experimental constraints.

The anomalies primarily discussed include: i) deviations from $\tau/\mu$ and $\tau/e$ universality in charged-current $b\to c \ell \bar\nu$ transitions; ii) deviations from $\mu/e$ universality in neutral-current $b\to s \ell \bar{\ell}$ transitions. The paper underscores the consistency of these deviations with the SM, each remaining below the $3\sigma$ significance individually though collectively hinting at possible NP beyond statistical fluctuations.

The authors employ an EFT methodology to address the possible origins of these anomalies. Within this framework, various NP scenarios involving mediators such as colored or colorless vector leptoquarks and vectors around the TeV scale are considered. Remarkably, the $SU(2)_L$-singlet vector leptoquark is identified as a particularly simple and effective candidate, potentially resolving both sets of anomalies with minimal fine-tuning and a coherence with current constraints.

Key findings include the derivation of precise constraints on the parameters of the NP operators, based on flavor observables significantly impacted by these operators. The study meticulously disentangles these relations, presenting a series of potential EFT descriptions for how anomalies may originate from specific NP models. The constraints from both low-energy observables and high-$p_T$ processes at colliders are juxtaposed to evaluate the viability of different models, emphasizing the role of flavor symmetry in mitigating conflicts with measured processes where deviations are absent.

The results suggest substantial enhancements in $b \to s \tau \bar{\tau}$ transitions, aligning with theoretically expected NP contributions. Moreover, the exploration into the parameter space reveals the potential impact of these mediators on future experimental constraints and their role in ongoing searches at the LHC.

In conclusion, this research thoroughly examines the interplay between possible NP explanations for $B$-physics anomalies and existing experimental constraints. The authors present future outlooks on proposed models, emphasizing the prospects of discovering direct signatures of the outlined NP scenarios. Further exploration in particle physics experiments, characterized by precise flavor and electroweak tests, will be instrumental in verifying or refuting the proposed hypotheses. This study represents a critical step toward refining our understanding of flavor physics and the potential limitations of the SM.