Hunting inflatons at FASER
Nobuchika Okada, Digesh Raut
Abstract
We consider a nonminimal quartic inflation scenario in the minimal ${\mathrm{U}(1)}_{X}$ extension of the Standard Model (SM) with the classical conformal invariance, where the inflaton is identified with the ${\mathrm{U}(1)}_{X}$ Higgs field ($\ensuremath{\phi}$). By virtue of the classically conformal invariance and the radiative ${\mathrm{U}(1)}_{X}$ symmetry breaking via the Coleman-Weinberg mechanism, the inflationary predictions (in particular, the tensor-to-scaler ratio $r$), the ${\mathrm{U}(1)}_{X}$ coupling ${g}_{X}$, and the ${\mathrm{U}(1)}_{X}$ gauge boson mass ${m}_{{Z}^{\ensuremath{'}}}$ are all determined by only two free parameters: the inflaton mass ${m}_{\ensuremath{\phi}}$ and its mixing angle $\ensuremath{\theta}$ with the SM Higgs field. FASER can search for a long-lived scalar, which is the inflaton in our scenario, for the parameter ranges $0.1\ensuremath{\lesssim}{m}_{\ensuremath{\phi}}[\mathrm{GeV}]\ensuremath{\lesssim}4$ and ${10}^{\ensuremath{-}5}\ensuremath{\lesssim}\ensuremath{\theta}\ensuremath{\lesssim}{10}^{\ensuremath{-}3}$. Therefore, if such a scalar is discovered at FASER, both ${m}_{\ensuremath{\phi}}$ and $\ensuremath{\theta}$ would be fixed, leading to the predictions for $r$, ${g}_{X}$, and ${m}_{{Z}^{\ensuremath{'}}}$ in our model. These predictions can be tested by future cosmological observations and LHC searches for the ${Z}^{\ensuremath{'}}$ boson resonance.