Baryon Number and Lepton Universality Violation in Leptoquark and Diquark Models (1708.06350v2)

Abstract: We perform a systematic study of models involving leptoquarks and diquarks with masses well below the grand unification scale and demonstrate that a large class of them is excluded due to rapid proton decay. After singling out the few phenomenologically viable color triplet and sextet scenarios, we show that there exist only two leptoquark models which do not suffer from tree-level proton decay and which have the potential for explaining the recently discovered anomalies in B meson decays. Both of those models, however, contain dimension five operators contributing to proton decay and require a new symmetry forbidding them to emerge at a higher scale. This has a particularly nice realization for the model with the vector leptoquark $(3,1)_{2/3}$, which points to a specific extension of the Standard Model, namely the Pati-Salam unification model, where this leptoquark naturally arises as the new gauge boson. We explore this possibility in light of recent B physics measurements. Finally, we analyze also a vector diquark model, discussing its LHC phenomenology and showing that it has nontrivial predictions for neutron-antineutron oscillation experiments.

• The paper presents a systematic evaluation of leptoquark and diquark models that predict baryon number and lepton universality violation while addressing proton decay constraints.
• It identifies two viable vector leptoquark scenarios that avoid tree-level proton decay but induce dimension five operators affecting proton stability.
• The analysis also explores diquark-induced neutron-antineutron oscillations, offering novel experimental signatures for physics beyond the Standard Model.

Overview of "Baryon Number and Lepton Universality Violation in Leptoquark and Diquark Models"

The paper, "Baryon Number and Lepton Universality Violation in Leptoquark and Diquark Models," authored by Nima Assad, Bartosz Fornal, and Benjamín Grinstein, presents a detailed analysis of theoretical models involving leptoquarks and diquarks. The primary focus is to evaluate these models under the constraints of experimental observations, especially concerning rapid proton decay, a significant issue in grand unification theories (GUTs).

Proton Decay Constraints in Leptoquark Models

Proton decay remains one of the most stringent tests for any model predicting physics beyond the Standard Model (SM). The paper conducts a systematic evaluation of leptoquark and diquark models with masses significantly below the grand unification scale. The paper finds that a substantial class of these models is strongly disfavored due to their prediction of rapid proton decay. A crucial outcome of the analysis is the identification of only two viable leptoquark scenarios, the vector leptoquarks $(3,1)_{2/3}$ and $(3,3)_{2/3}$, that do not produce tree-level proton decay and have the capacity to explain the observed anomalies in $B$ meson decays. However, both models result in dimension five operators contributing to proton decay, necessitating the introduction of a new symmetry to suppress their impact.

Pati-Salam Unification and $B$ Meson Decay Anomalies

The vector leptoquark $(3,1)_{2/3}$ finds a natural place within the Pati-Salam unification model, which represents an extension of the SM. This leptoquark emerges as a gauge boson in the Pati-Salam framework, suggesting an elegant UV completion that inherently avoids dimension five operators leading to proton decay. The paper further probes the implications of these models in the context of observed anomalies in $B$ meson decay processes, such as the deviations in the decay rates in $R_{K^{(*)}}$ measurements. The leptoquarks provide potential explanations for these deviations, aligning with data favorably when certain conditions are met.

Neutron-Antineutron Oscillations in Diquark Models

Moving beyond leptoquarks, the paper also explores a vector diquark model, particularly involving the color sextet vector diquark $(6,2)_{-1/6}$. Its implications for neutron-antineutron ($n-\bar{n}$) oscillations are analyzed in depth. The paper highlights that although leptoquarks have been extensively studied, diquark phenomenology, especially concerning $n-\bar{n}$ oscillations, remains underexplored. This model predicts baryon number violation through $n-\bar{n}$ oscillations without invoking proton decay, providing a testable phenomenon in ongoing and future experiments.

Implications and Future Directions

The paper's findings have significant implications for high-energy phenomenology and the search for new physics. The constraints imposed by proton decay present a rigorous test for new particle models, and any extension of the SM must account for these limitations. The discussed leptoquark models offer pathways to address current anomalies in particle physics, and their presence within larger unification theories like Pati-Salam presents a coherent picture that connects various physical observations under a unified framework.

The exploration of diquark models and their connection to neutron-antineutron oscillations opens new avenues for experimental tests that can probe the presence of such heavy vector particles. As experiments progress, especially with increased sensitivity toward baryon number violation processes, the predictions offered by these models will be critical for guiding the search for physics beyond the SM. Future theoretical work could build upon these foundations to explore additional symmetry-breaking mechanisms or to refine the predictions of leptoquark and diquark signatures at current and next-generation colliders.