Nonthermally trapped inflation by tachyonic dark photon production

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Nonthermally trapped inflation by tachyonic dark photon production

Naoya Kitajima, Shota Nakagawa, and Fuminobu Takahashi

Phys. Rev. D 105, 103011 – Published 11 May 2022

Abstract

We show that a dark Higgs field charged under ${U (1)}_{H}$ gauge symmetry is trapped at the origin for a long time, if dark photons are produced by an axion condensate via tachyonic preheating. The trapped dark Higgs can drive late-time inflation, producing a large amount of entropy. Unlike thermal inflation, the dark Higgs potential does not have to be very flat because the effective mass for the dark Higgs is enhanced by large field values of dark photons with extremely low momentum. After inflation, the dark Higgs decays into massive dark photons, which further decay into the SM particles through a kinetic mixing. We show that a large portion of the viable parameter space is within the future experimental searches for the dark photon because the kinetic mixing is bounded below for successful reheating. We also comment on the Schwinger effect, which can hamper the tachyonic production of dark photons, when the mass of the dark photon is not the Stückelberg mass but is generated by the Higgs mechanism. Such nonthermal trapped inflation could be applied to other cosmological scenarios such as the early dark energy, which is known as one of the solutions to the Hubble tension.

Received 22 November 2021
Accepted 25 April 2022

DOI:https://doi.org/10.1103/PhysRevD.105.103011

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Inflation Particle astrophysics

Gravitation, Cosmology & Astrophysics

Authors & Affiliations

Naoya Kitajima^1,2, Shota Nakagawa ¹, and Fuminobu Takahashi¹

¹Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan
²Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Miyagi 980-8578 Japan

Article Text

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Issue

Vol. 105, Iss. 10 — 15 May 2022

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Images

Figure 1
Left: time evolution of the spatial averaged field values of the axion (red) and dark photon (green), both of which are normalized by $f_{ϕ}$ , as well as the dark Higgs field (blue) normalized by $v$ . The horizontal axis is the conformal time normalized by the axion mass. The dotted line corresponds to the case without dark photon production, i.e., $β = 0$ . The horizontal dashed line corresponds to $m_{Ψ} / (e f_{ϕ})$ , so the intersection between the green line and the horizontal dashed line corresponds to $e | A | = m_{Ψ}$ . Right: time evolution of the effective dark Higgs mass squared. The red line shows the total, and the contributions from the dark photon field, the Hubble-induced mass, and the bare mass are shown by the red, green, and blue lines, respectively. The solid and dashed lines correspond, respectively, to positive and negative values. For $m_{ϕ} τ ≳ 3$ , the effective mass at the origin is dominated by the (tachyonic) bare mass. We set the parameters as follows: $f_{ϕ} = 5 \times 10^{17} GeV$ , $m_{ϕ} = 5 \times 10^{8} GeV$ , $v = f_{ϕ}$ , $β = 30$ , $e = 2 m_{ϕ} / f_{ϕ}$ , and $λ = 10^{- 4} m_{ϕ}^{2} / v^{2}$ .
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Figure 2
Left: time evolution of the energy density of the axion (red), dark photon field (green), Higgs (blue), and background matter (magenta) normalized by $m_{ϕ}^{2} f_{ϕ}^{2}$ . Right: time evolution of the equation-of-state parameter. The dotted line represents the case without dark photon production. Note that $w$ approaches $- 1$ when the dark Higgs is trapped at the origin.
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Figure 3
Spectrum of the energy density of the dark photon field, $ρ_{A}$ , normalized by $m_{ϕ}^{2} f_{ϕ}^{2}$ , evaluated at $m_{ϕ} τ = 1$ (red), 2 (green), 3 (blue), 4 (magenta), and 5 (cyan). The comoving wave number corresponding to the peak stays almost constant before the Higgs destabilization.
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Figure 4
Contours of the e-folding number $N$ are shown by the blue solid lines in the $(m_{ϕ}, m_{Ψ})$ plane, where we take $β = 30$ , $e = 0.1$ , $λ = 1$ , and $f_{ϕ} = 10^{17} GeV$ . The gray shaded region denotes the BBN bound on the reheating temperature, $T_{reh} ≳ 4 MeV$ , and no trapped inflation in the green shaded region. The orange dotted lines represent the required values of $ξ$ to trap the Higgs before the onset of the axion oscillations, determined by $ξ R_{osc} = m_{Ψ}^{2}$ .
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Figure 5
Viable parameter region for the dark photon mass and the kinetic mixing, and the current constraints as well as projected sensitivities. The shaded regions are excluded, and the colored dashed curves indicate the expected sensitivity reach of future experiments. The dotted contours show the reheating temperatures $T_{reh} = 10 MeV, 100 MeV$ , and 1 GeV. The vertical dot-dashed line is an upper bound on $m_{γ^{'}}$ required for trapping the dark Higgs before the axion oscillation when $ξ = 1$ . See the text for details.
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