The QCD axion is a compelling mechanism for solving the strong CP problem. Most studies have focused on axion models with a large decay constant, $f_a \gtrsim 10^9,\mathrm{GeV}$. However, recent work has pointed out that viable axion models may also exist in the low-scale regime, $f_a \sim \mathcal{O}(1),\mathrm{GeV}$, corresponding to an axion mass in the MeV range.
In this mass range, the...
"We explore the theoretical consequences of residual symmetries that persist at the modular self-dual point $\tau = i$ within modular-invariant frameworks. Assuming the three generations of lepton doublets form an irreducible representation of a finite modular group $\Gamma_N$, and that the light neutrino masses arise from the Weinberg operator constructed from modular forms, we demonstrate...
Oscillation phenomena occur in both active and sterile neutrinos, where the oscillation phases are triggered by vacuum mass difference. Before the electroweak gauge symmetry breaking, leptons are massless, and cannot oscillate in the conventional way. However, they can still oscillate in the background plasma. Using the flavor-covariant nonequilibrium quantum field theory, I will discuss how...
