Third Family Quark-Lepton Unification at the TeV Scale (1802.04274v2)

Abstract: We construct a model of quark-lepton unification at the TeV scale based on an $SU(4)$ gauge symmetry, while still having acceptable neutrino masses and enough suppression in flavor changing neutral currents. An approximate $U(2)$ flavor symmetry is an artifact of family-dependent gauge charges leading to a natural realization of the CKM mixing matrix. The model predicts sizeable violation of PMNS unitarity as well as a gauge vector leptoquark $U_1^\mu = ({\bf 3}, {\bf 1}, 2/3)$ which can be produced at the LHC -- both effects within the reach of future measurements. In addition, recently reported experimental anomalies in semi-leptonic $B$-meson decays, both in charged $b \to c \tau \nu$ and neutral $b \to s \mu \mu$ currents, can be accommodated.

Third Family Quark-Lepton Unification at the TeV Scale: A Summary

The paper, "Third Family Quark-Lepton Unification at the TeV Scale," presents a detailed theoretical framework for quark-lepton unification operating at the TeV scale. This model explores the unification based on an $SU(4)$ gauge symmetry, specifically engineered to meet constraints on neutrino masses and flavor changing neutral currents (FCNCs).

Model Overview

The authors propose a model where quark-lepton unification is realized at the TeV scale through family-dependent gauge interactions. The third family of fermions is unified under the $SU(4)$ group, while the lighter families are charged under the standard model’s $SU(3)_c \times SU(2)_L \times U(1)_Y$. This partition effectuates an approximate $U(2)$ flavor symmetry, an artifact that naturally emerges from the chosen gauge representations, which adeptly explains the CKM matrix structure.

Key Features and Predictions

1. Gauge Symmetry and Breaking: The model considers the extended gauge group "4321", defined as $SU(4) \times SU(3)' \times SU(2)_L \times U(1)'$, and details a symmetry breaking mechanism down to the Standard Model (SM) through specific scalar field arrangements. This symmetry breaking process introduces gauge bosons such as $U_1^\mu$, $g'$, and $Z'$, all possessing masses tied to the symmetry-breaking scales. The vector leptoquark $U_1^\mu$ is predicted to be within the observable range at the LHC, which is significant for high-energy physics experiments.
2. Fermion Mass and Mixing: The third family quark-lepton unification facilitates mass patterns suggesting the tau lepton and bottom quark similarity, with predicted corrections to neutrino masses through inverse seesaw mechanisms to rectify discrepancies in the up-family sector. The model predicts potential non-unitarity in the PMNS matrix, linking it to quark-lepton unification scales, which might be detectable in future neutrino oscillation experiments.
3. Flavor Physics and Constraints: With the novel gauge group architecture, flavors are organized to suppress FCNC, fitting the observed CKM texture through accidental $U(2)$ symmetries. This suppression is crucial as it allows the model to adhere to experimental limits and propose solutions to observed $B$-meson decay anomalies. The vector leptoquark interactions are poised to reconcile deviations in semi-leptonic $B$-meson decay parameters such as $R(D^{(*)})$ and $R(K^{(*)})$.

Implications and Future Directions

The authors emphasize that their framework provides a coherent construct for third family quark-lepton unification without venturing into the dilemmas associated with high-scale models regarding electroweak stability. The paper suggests that indirect effects in flavor and neutrino sectors arising from this model could be significant, prompting further exploration in ongoing and future collider experiments.

Furthermore, the paper opens avenues for exploring gauge-mediated unification consequences on various low-energy phenomena, potentially advancing insights into flavor physics anomalies, and guiding inquiries into non-standard neutrino oscillation patterns.

Concluding Remarks

The contributions made in this paper provide substantive intelligence towards understanding quark-lepton unification at low scales. The articulations of mixing mechanisms, gauge symmetries, and the discussions on various particle implementations at the TeV scale mark this research as a cornerstone for models targeting gaps in the Standard Model. By integrating flavor dynamics with gauge theories, the model outlines a forward path for both theoretical explorations and empirical validations within high-energy physics contexts. As experimental capabilities expand, the insights derived from this work may prove invaluable for enhancing our grasp on particle unification and interactions.