New Bounds for Axions and Axion-Like Particles with keV-GeV Masses

TL;DR

The study reevaluates cosmological bounds on axions and axion-like particles with two-photon decays, focusing on decays near BBN that modify $N_{ m eff}$ and $ta$ between BBN and the CMB epoch. By coupling Boltzmann evolution of ALP distributions to Planck CMB measurements and primordial abundances, the authors map excluded regions in mass–lifetime space and identify a previously allowed MeV-ALP window that is largely ruled out unless extra radiation is present. They find that post-neutrino-decoupling decays are tightly constrained by D/H and helium data, but that adding $
elta N_{ m eff}$ can reopen the window with $elta N_{ m eff}\approx 1.1\pm 0.3$. The work also provides forecasts for Stage-IV CMB and SUPER-KEKB, illustrating how future observations could further probe or close the remaining parameter space.

Abstract

We give updated constraints on hypothetical light bosons with a two-photon coupling such as axions or axion-like particles (ALPs). We focus on masses and lifetimes where decays happen near big bang nucleosynthesis (BBN), thus altering the baryon-to-photon ratio and number of relativistic degrees of freedom between the BBN epoch and the cosmic microwave background (CMB) last scattering epoch, in particular such that $N_{\rm eff}^{\rm CMB} < N_{\rm eff}^{\rm BBN}$ and $η^{\rm CMB} < η^{\rm BBN}$. New constraints presented here come from Planck measurements of the CMB power spectrum combined with the latest inferences of primordial $^4$He and D/H abundances. We find that a previously allowed region in parameter space near $m=1\,\rm MeV$ and $τ=100\,\rm ms$, consistent with a QCD axion arising from a symmetry breaking near the electroweak scale, is now ruled out at $>3σ$ by the combination of CMB+D/H measurements if only ALPs and three thermalized neutrino species contribute to $N_{\rm eff}$. The bound relaxes if there are additional light degrees of freedom present which, in this scenario, have their contribution limited to $ΔN_{\rm eff}=1.1\pm0.3$. We give forecasts showing that a number of experiments are expected to reach the sensitivity needed to further test this region, such as Stage-IV CMB and SUPER-KEKB, the latter a direct test insensitive to any extra degrees of freedom.

Figures (6)

• Figure 1: Key regions and contours in the mass-lifetime parameter space according to the analytic approximations in Sec. \ref{['sec:scenario']}. As in other plots in this paper, dashed lines correspond to the temperature at neutrino decoupling, dot-dashed lines the start of BBN, and dotted lines the end of BBN. Blue lines show contours of constant Primakoff freeze-out temperature, $T_{\rm fo}$, and black lines show contours of constant two-photon re-equilibration temperature, $T_{\rm re}$. The line $T_{\rm re}=m_\phi$ divides two regions A and B. Region A is vertically hatched and corresponds to out-of-equilibrium decays. Region B is cross hatched and corresponds to in-equilibrium decays. Constant decay-time contours in region A are $T_{\rm re}=\rm const$ whereas they are $m_\phi=\rm const$ in region B. Region C has no hatching and corresponds to decays before neutrino decoupling, where ALPs leave no cosmologically observable traces. The line $T_{\rm fo}$=QCD leaves a sharp feature on cosmological constraints as $g_*$ changes suddenly during this phase transition.
• Figure 2: The evolution of the energy densities in the various components of the universe for different scenarios which have similar decay time. The temperature of the photons today is held fixed and the y-axis units are such that the final value of the neutrino line is the value of $N_{\rm eff}^{\rm CMB}$. As in other plots in this paper, dashed lines correspond to the scale factor at neutrino decoupling, dot-dashed lines the start of BBN, and dotted lines the end of BBN. The dashed red line is not actually a component, but is shown for illustrative purposes; it is the equilibrium ALP energy density (that is, the energy density APLs would have if they were in chemical and kinetic equilibrium with the photons). As per Eqn. \ref{['eq:boltz']}, interactions serve to always drive the ALP energy density towards equilibrium. The plots labeled A and B correspond to the same regions in Fig. \ref{['fig:mtau_regions']}.
• Figure 3: Exclusion regions in the ALP mass-lifetime parameter space. The dashed and dotted lines labeled "$\nu$ dec" (neutrino decoupling), and "BBN start/end" correspond to particles which decay at these particular times (with decay here arbitrarily defined as when maximum energy injection occurs). The two thick dashed lines are the consistency relations for two particular axion models (see Sec. \ref{['sec:axion']}). The CMB, D/H, and $Y_p$ regions are excluded at 3$\sigma$, the Collider and Beam Dump regions are excluded at 2$\sigma$, and the SN1987a and HB Stars regions are less formal, rough bounds (see Sec. \ref{['sec:astrophysical']}).
• Figure 4: A comparison of exclusion regions from previous works (left panel) and those presented here (right panel). The right panel is identical to Fig. \ref{['fig:mtau_baseline']}.
• Figure 5: The colored contours show the prediction for each of the labeled quantities as a function of different values of ALP mass and lifetime. The dotted/dashed/solid lines give 1/2/3 $\sigma$ contours given the measurements for these quantities discussed in Sec. \ref{['sec:constraints']}. No lines are visible on the lithium plot because the entire parameter space is excluded at $>3\sigma$ (our scenario does not alleviate the lithium problem). We do not give contours for the $N_{\rm eff}^{\rm CMB}$ plot because the CMB constraint is highly degenerate with $Y_p$. For the D/H panel, the colored contours are calculated assuming a best-fit $\eta$ from the CMB, and uncertainties in $\eta$ and nuclear reaction rates are taken into account in producing the $\sigma$ contours (see Sec. \ref{['sec:bbnconstraints']} for discussion).