A three-site gauge model for flavor hierarchies and flavor anomalies (1712.01368v2)

Abstract: We present a three-site Pati-Salam gauge model able to explain the Standard Model flavor hierarchies while, at the same time, accommodating the recent experimental hints of lepton-flavor non-universality in $B$ decays. The model is consistent with low- and high-energy bounds, and predicts a rich spectrum of new states at TeV scale that could be probed in the near future by the high-$p_T$ experiments at the LHC.

• The paper introduces a three-site Pati-Salam gauge model that unifies SM flavor hierarchies with observed lepton-flavor universality anomalies in B decays.
• It employs a hierarchical symmetry breaking approach and a TeV-scale vector leptoquark to accurately reconcile theoretical predictions with experimental data.
• The findings predict new vector leptoquark states potentially observable at the LHC, offering clear targets for future collider searches.

A Three-Site Gauge Model for Flavor Hierarchies and Flavor Anomalies

The paper presents a three-site Pati-Salam (PS) gauge model as an attempt to address the flavor hierarchies observed in the Standard Model (SM) and the recently noted anomalies in lepton-flavor universality in $B$ decays. Researchers Marzia Bordone, Claudia Cornella, Javier Fuentes-Martín, and Gino Isidori have constructed this gauge model to simultaneously explain these phenomena while maintaining consistency with existing experimental data.

Overview of the Model

The proposed model extends the original Pati-Salam gauge group into three distinct sites, each associated with one fermion family. The gauge group is thus extended to $$\mathrm{PS^3} = \mathrm{PS_1} \times \mathrm{PS_2} \times \mathrm{PS_3}$$, where each $\mathrm{PS_i}$ is $$\mathrm{SU(4)}_i \times \mathrm{[SU(2)_L]}_i \times \mathrm{[SU(2)_R]}_i$$. This structure leads to hierarchical breaking of the gauge symmetries across different sites characterized by various energy scales, starting from a high energy scale (>10^3 TeV) where the symmetry is like $\mathrm{PS_1} \to \mathrm{SM}_1$ and eventually descending to the electroweak scale, reproducing SM-like interactions.

Resolving Flavor Hierarchies and Anomalies

The model primarily focuses on addressing two categories of experimental anomalies:

1. Deviations from $\tau/\mu$ and $\tau/e$ universality in charged currents ($b\to c \ell \bar\nu$).
2. Deviations from $\mu/e$ universality in neutral currents ($b\to s \ell \bar{\ell}$).

The gauge group is reduced in stages until it reaches a residual symmetry containing TeV-scale dynamics where a leptoquark (LQ) field is primarily coupled to the third generation. By utilizing a localized Higgs field and controlling the Yukawa couplings, the model naturally conceives the flavor hierarchies observed in SM. This structure allows for an effective field theory (EFT) realization that accounts for LFU anomalies without conflicting with other experimental data.

Implications and Predictions

The model predicts a spectrum of new states at the TeV scale, potentially observable by higher-energy experiments at the LHC. The vector LQ field emerges as a centerpiece, leading to observable effects in both charged and neutral current anomalies. A fit to existing experimental data suggests that the model can successfully reconcile these anomalies under certain parameter conditions, with significant implications for future experiments.

Theoretical and Experimental Considerations

From a theoretical standpoint, the model links flavor anomalies with the SM’s Yukawa hierarchies, particularly through the lens of UV completions. The $\mathrm{U(2)_Q}\times \mathrm{U(2)_L}$ global flavor symmetry evident in the gauge sector plays a critical role in ensuring phenomenological consistency, especially as concerns low-energy observables and LHC measurements.

Experimentally, the presence of a vector leptoquark offers a tangible target for collider searches. The model's constraints from high-energy physics experiments at the LHC, especially concerning dijet and $Z'$ resonance searches, ground the theoretical framework within realistic bounds.

Future Directions

The implications of this model suggest a fertile ground for both theoretical developments and experimental tests. As collider technologies advance, improved measurement techniques and higher precision in particle physics experiments at the LHC and beyond will either further validate or constrain the model. The paper sets the agenda for pursuing TeV-scale new physics, with the proposed model serving as a comprehensive framework to address current unsolved issues in the field of flavor physics.

In conclusion, by introducing a three-site gauge symmetry model, the paper meticulously bridges the gap between observed flavor anomalies and established SM hierarchies, pointing towards possible next steps in probing new physics at higher energy scales.