Higgs Inflation: Particle Factory

TL;DR

This work analyzes cosmological gravitational particle production in Higgs inflation with a nonminimal coupling $\xi$, revealing a new two-peak CGPP spectrum dominated by a peak at $k_{peak} \sim 2\xi^{2/3} a_e H_e$ and a second at $2k_{peak}$, with peak occupation $n_{k_{peak}}$ scaling linearly with $\xi$. By solving the spectator-field mode equations in the evolving Higgs-inflation background, the authors show a universal scaling collapse when rescaling $k$ by $\xi^{2/3}$ and $n_k$ by $\xi$, and demonstrate that backreaction is negligible. The enhanced particle production acts as a robust dark-matter generator, either directly yielding DM or via decay into DM, with viable parameter windows for $m_\chi/H_e$ and $\xi$ that could be probed by future cosmological, collider, or gravitational-wave observations. Overall, Higgs inflation functions as a particle factory, linking early-universe dynamics to present-day relics and offering new avenues for beyond-Standard-Model phenomenology.

Abstract

We study cosmological gravitational particle production (CGPP) in Higgs inflation, wherein the inflaton is a scalar field with quartic self-coupling $λ$ and a nonminimal coupling to gravity $ξ$, and which may, but need not be, the Higgs boson of the Standard Model (SM). We find an explosive particle production peaked on a characteristic comoving wavenumber $k\sim ξ^{2/3} a H$ with a peak occupation number that scales with $ξ$. This new peak in production can easily dominate over the conventional (minimally coupled inflation) CGPP even for modest values of $ξ$. The results apply for a wide range of $ξ$, e.g., as low as $ξ=10$, which can be realized for the Standard Model Higgs given suitable RG flow of the quartic coupling. We discuss implications for late time relics such as dark matter.

Figures (14)

• Figure 1: Evolution of the inflaton $\phi$ for three fiducial values of $\xi$ with respect to time in the Jordan frame is shown in the left panel. The inset shows the cohesion of frequencies for large values of $\xi$ at early times. At late times, the inflaton begins to oscillate at different rates regardless of the value of $\xi$ and the frequencies begin to slow down with time. In all cases $\lambda$ is rescaled to match to the CMB $A_s$ constraint. The right panel shows the evolution in $\dot{\phi}$, featuring sharp spikes.
• Figure 2: Scaling of the oscillation frequencies of the inflation at late times. We define the cycle-averaged frequency as the inverse period of oscillations. At late times the frequency approaches a universal scaling solution which scales as $\xi^{2/3}$.
• Figure 3: Evolution of the comoving scale factor $a/a_e$ with time shows cuspy oscillations. In the bottom panel, the evolution of the Hubble scale is shown with respect to $a/a_e$. Dashed lines show the evolution of a radiation-dominated universe. The time at which $H$ evolves like radiation is distinguished with vertical dotted lines, for three different values of $\xi$.
• Figure 4: Evolution of the slow-roll parameter $\varepsilon$ (left panel), scaled by $\xi$, and of the Ricci scalar $R$ (right panel), scaled to $\xi H^2$. At the end of inflation $\varepsilon/\xi=1/\xi$, i.e., $\varepsilon=1$, which rapidly grows to $\varepsilon/\xi \sim 6$, i.e., $\varepsilon\sim 6\xi$ when $\phi$ passes through zero. This differs from conventional inflation models, where $\varepsilon(\phi=0)=3$. Similarly, the Ricci scalar is $R \sim 36 \xi H^2$ at $\phi=0$, in comparison with conventional inflation models where $R(\phi=0)=6 H^2$. We note that, relative to $H_e^2$, the peak value of $R$ is ${\cal O}(1)\xi$, i.e., max$(R)={\cal O}(1)\xi H_e^2$ (see Fig. \ref{['fig:minimal-ricci']}).
• Figure 5: Time evolution of the Ricci scalar in Higgs inflation, scaled to $\xi H_e ^2$.