Institut für Kernphysik, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany and Institut für Theoretische Teilchenphysik, Karlsruhe Institute of Technology, Engesserstraße 7, D-76128 Karlsruhe, Germany.
Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland.
Phys Rev Lett. 2018 Jul 6;121(1):011801. doi: 10.1103/PhysRevLett.121.011801.
Lepton number as a fourth color is the intriguing theoretical idea of the famous Pati-Salam (PS) model. While in conventional PS models, the symmetry breaking scale and the mass of the resulting vector leptoquark are stringently constrained by K_{L}→μe and K→πμe, the scale can be lowered to a few TeV by adding vectorlike fermions. Furthermore, in this case, the intriguing hints for lepton flavor universality violation in b→sμ^{+}μ^{-} and b→cτν processes can be addressed. Such a setup is naturally achieved by implementing the PS gauge group in the five-dimensional Randall-Sundrum background. The PS symmetry is broken by boundary conditions on the fifth dimension, and the resulting massive vector leptoquark automatically has the same mass scale as the vectorlike fermions and all other resonances. We consider the phenomenology of this model in the context of the hints for lepton flavor universality violation in semileptonic B decays. Assuming flavor alignment in the down sector, we find that in b→sℓ^{+}ℓ^{-} transitions, the observed deviations from the standard model predictions [including R(K) and R(K^{})] can be explained with natural values for the free parameters of the model. Even though we find sizable effects in R(D), R(D^{}), and R(J/Ψ), one cannot account for the current central values in the constrained setup of our minimal model due to the stringent constraints from D-D[over ¯] mixing and τ→3μ.
轻子数是第四种颜色，这是著名的 Pati-Salam（PS）模型的一个有趣的理论构想。在传统的 PS 模型中，对称破缺尺度和产生的矢量轻夸克的质量受到 KL→μe 和 K→πμe 的严格限制，而通过添加矢量类费米子，这个尺度可以降低到几 TeV。此外，在这种情况下，可以解决 b→sμ+μ-和 b→cτν过程中轻子味普适性破坏的有趣线索。通过在五维 Randall-Sundrum 背景下实现 PS 规范群，可以自然地实现这种设置。PS 对称性通过第五维的边界条件被打破，产生的大量矢量轻夸克自动具有与矢量类费米子和所有其他共振相同的质量尺度。我们考虑了这种模型在半轻 B 衰变中轻子味普适性破坏线索方面的现象学。假设在下方部门的味道对齐，我们发现，在 b→sℓ+ℓ-跃迁中，对于标准模型预测的观察到的偏离[包括 R(K)和 R(K*)]，可以用模型的自由参数的自然值来解释。尽管我们在 R(D)、R(D*)和 R(J/Ψ)中发现了相当大的影响，但由于 D-D[over ¯]混合和 τ→3μ的严格限制，在我们的最小模型的受限设置中，无法解释当前的中心值。
