Flavor from Consistency: Axion, Anomaly Cancellation, and Emergent Unification

Abstract

We present a framework for flavored grand unification theory (flavored-GUT) in string-derived supergravity based on $G_{\rm SM}\times SL(2,\mathbb{Z})\times U(1)_X\times U(1)_{B-L}$, where gravity is intrinsically incorporated. We show that anomaly cancellation and Standard Model gauge coupling unification act as fundamental consistency conditions that determine the flavor structure, rather than treating flavor as an independent input. Mixed $SL(2,\mathbb{Z})$, $U(1)_{X}$, $U(1)_{B-L}$, and gravitational anomalies are shown to vanish, with the anomalies induced by K{\"a}hler transformations matched by those from chiral rotations of gauginos and the gravitino. For nontrivial $SL(2,\mathbb{Z})$ transformations of SM fermions, the anomaly-free conditions impose strong constraints on the quark and lepton flavor structures while leaving the strong CP phase unchanged. Quark and lepton mass hierarchies, mixing patterns, and the flavored Peccei-Quinn sector emerge from the same underlying structure. The consistency conditions fix the $U(1)_X$ breaking scale, identified with the Froggatt-Nielsen cutoff scale, thereby determining the QCD axion decay constant and predicting the axion mass $m_a=3.35\times10^{-8}$ eV, while simultaneously constraining the seesaw scale and supersymmetry-breaking scale of ${\cal O}(10)$TeV. We further show that the flavored-GUT framework provides a possible resolution of the axion quality problem and that the modulus vacuum expectation value stabilizes near $\langle\tau\rangle\approx i$, where the exact $SL(2,\mathbb{Z})$ ($T$-duality) is spontaneously broken. Our results establish a predictive framework linking flavor physics, anomaly cancellation, gauge coupling unification, neutrino mass generation, and axion physics, without invoking a conventional simple unified gauge group.