B-decay Anomalies in a Composite Leptoquark Model (1611.04930v1)

Abstract: The collection of a few anomalies in semileptonic $B$-decays, especially in $b\rightarrow c \tau \bar{\nu}$, invites to speculate about the emergence of some striking new phenomena, perhaps interpretable in terms of a weakly broken $U(2)^n$ flavor symmetry and of leptoquark mediators. Here we aim at a partial UV completion of this interpretation by generalizing the minimal composite Higgs model to include a composite vector leptoquark as well.

• The paper presents a composite leptoquark framework that explains B-decay anomalies by extending conventional composite Higgs models.
• It employs detailed numerical analysis to fit experimental deviations in semileptonic B-decay processes and constrains mixing angles.
• It identifies promising LHC signatures such as pp → b¯b → τ⁺τ⁻, providing actionable insights for probing physics beyond the Standard Model.

Analysis of B-decay Anomalies within a Composite Leptoquark Framework

The paper "B-decay Anomalies in a Composite Leptoquark Model" explores a theoretical framework that addresses persistent anomalies observed in the decays of B mesons. Central to this theoretical exploration is the idea that these anomalies, particularly in processes like $b \rightarrow c \tau \bar{\nu}$, might be indicative of new physics beyond the Standard Model (SM), possibly associated with composite leptoquark states. Leveraging the framework of Composite Higgs Models (CHMs), this paper extends the conventional approach by incorporating a composite vector leptoquark to address these anomalous decay processes.

Overview and Context

This research is motivated by anomalies in semileptonic B-decays that have been detected across multiple experiments, including BaBar, Belle, and LHCb. Particularly noteworthy is the combined $4.0\sigma$ excess in the charged current process $R_{D^{(*)}}$, and a $2.6\sigma$ deviation observed in the $R_K$ neutral current process. These deviations from SM expectations suggest potential lepton flavor universality (LFU) violations and are challenging to reconcile within existing theoretical models.

Theoretical Framework

The authors propose a model grounded in a generalization of minimal composite Higgs models by extending the symmetry group from $SU(3) \times SO(5) \times U(1)$ to $SU(4) \times SO(5) \times U(1)$. Within this setup, $SU(4)$ acts as the Pati-Salam group, which naturally includes leptoquark interactions. This extension aims to provide a satisfactory ultraviolet (UV) completion for models that incorporate massive vector fields like leptoquarks.

Key to this model is the interplay between different actors: composite fermions, including leptoquarks, and new gauge bosons that arise due to the extended symmetry. This enables the authors to propose that the anomalies are driven by strong interactions in a newly extended flavor sector, which separates third-generation fermions from the first two based on a $U(2)^n$ symmetry structure.

Numerical and Phenomenological Analysis

The paper presents a detailed numerical exploration of the parameter space, aiming to find regions that fit the experimental anomalies while maintaining consistency with other observations, particularly those from electroweak precision data and LHC searches.

The paper indicates that mixing angles, particularly of third-generation fermions with the composite sector, play a significant role. For instance, addressing the $R_{D^{(*)}}$ anomaly requires a significant mixing angle, quantified as $s_{Lu3}^2 s_{L\nu3}^2 = (0.49 \div 0.77)$ for certain parameter choices like scaling factor $\xi$ and CKM parameters. Moreover, it is crucial to address constraints from $\Delta B_s = 2$ processes, which the model manages through careful choice of mixing matrices and couplings.

Additionally, the authors explore potential signatures at the LHC. They identify $pp \to b\bar{b} \to \tau^+\tau^-$ as a promising channel, given that composite vectors could contribute significantly to this process through s-channel exchange.

Implications and Future Directions

This work contributes to the theoretical understanding of B-decay anomalies, suggesting a framework where composite dynamics with leptoquarks offer a coherent explanation. The implications stretch to both particle phenomenology and the broader comprehension of flavor physics, hinting at a future where leptoquark searches might play a central role in unearthing physics beyond the SM.

Future research should refine these predictions with more detailed lattice QCD calculations to better constrain the hadronic uncertainties inherent in these processes. Moreover, expanding the theoretical analysis to include one-loop corrections and higher precision LHC data could further validate or challenge the predictions of this composite model.

In conclusion, the composite leptoquark model offers an intriguing, tangible path forward in explaining B-decay anomalies and sets the stage for further experimental and theoretical explorations in the quest for new physics.