Abstract
Axions currently provide the most compelling solution to the strong CP problem. These particles may be copiously produced in the early universe, including via thermal processes. Therefore, relic axions constitute a hot dark matter component and their masses are strongly degenerate with those of the three active neutrinos, as they leave identical signatures in the different cosmological observables. In addition, thermal axions, while still relativistic states, also contribute to the relativistic degrees of freedom, parameterised via $N_{eff}$. We present the cosmological bounds on the relic axion and neutrino masses, exploiting the full Planck mission data, which include polarization measurements. In the mixed hot dark matter scenario explored here, we find the tightest and more robust constraint to date on the sum of the three active neutrino masses, $\sum m_\nu <0.136$ eV at $95\%$ CL, obtained in the well-known linear perturbation regime. The Planck Sunyaev-Zeldovich cluster number count data further tightens this bound, providing a $95\%$ CL upper limit of $\sum m_\nu <0.126$ eV in this very same mixed hot dark matter model, a value which is very close to the expectations in the inverted hierarchical neutrino mass scenario. Using this same combination of data sets we find the most stringent bound to date on the thermal axion mass, $m_a<0.529$ eV at $95\%$ CL.
Highlights
The axion field arises as a solution to solve the strong CP problem in Quantum Chromodynamics [1,2,3]
The bounds on the thermal axion mass are similar to those obtained in the case in which only axion masses are considered, albeit for that case the value of the σ8 parameter is always higher than the one shown here, as only one hot relic suppresses the small-scale clustering
Neutrino oscillation experiments have provided compelling evidence for the existence of neutrino masses and neutrinos must be added as massive particles
Summary
The axion field arises as a solution to solve the strong CP problem in Quantum Chromodynamics [1,2,3]. Axions can be produced via thermal or non thermal processes While in the former the axion contributes as an extra hot thermal relic (together with three active neutrinos), in the latter the axion could be the cold dark matter component. When thermal axions become nonrelativistic particles, they will affect the different cosmological observables in an analogous way to that of massive neutrinos, i.e. by increasing the amount of the (hot) dark matter density in our universe. In light of the recent Planck 2015 temperature and polarization data [12], it is timely to compute the changes on the existing bounds on the thermal axion mass, including the case in which massive neutrinos are present. In the mixed hot dark matter scenario, in which both axion and neutrino masses are allowed to freely vary, we find the tightest and more robust constraint to date on the sum of the three active neutrino masses, mν < 0.156 eV at 95% CL, as it only relies on the (very well-known) linear perturbation regime
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