Hunting Inflaton at FASER

Abstract

We explore a possibility that an inflaton, which drives the cosmological inflation in the early Universe, can be detected by the recently approved FASER at the High-Luminosity LHC (HL-LHC). We consider nonminimal quartic inflation scenario in the minimal U(1)${}_X$ extension of the Standard Model (SM) with the classical conformal invariance, where the inflaton is identified with the U(1)${}_X$ Higgs field ($\varphi$). By virtue of the classical conformal invariance and the radiative U(1)${}_X$ symmetry breaking via the Coleman-Weinberg mechanism, the inflationary predictions (in particular, the tensor-to-scaler ratio ($r$)), the U(1)${}_X$ coupling ($g_X$) and the U(1)${}_X$ gauge boson mass ($m_{Z^{\prime }}$), are all determined by only two free parameters, the inflaton mass ($m_{\varphi }$) and its mixing angle ($\theta$) with the SM Higgs field. The FASER can search for the inflaton for the parameter ranges of $0.1\lesssim m_{\varphi }[\mathrm{GeV}]\lesssim 4$ and $10^{-5}\lesssim \theta \lesssim 10^{-3}$. Because of the direct connection among $r$, $g_X$ and $m_{Z^{\prime }}$, the $Z^{\prime }$ boson resonance search at the HL-LHC and the future measurement of the primordial gravitational wave are complementary to the inflaton search at the FASER.

arXiv comment8 pages, 4 figures, (updated) version to be published in PRD

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