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Register a new userStellar cooling anomalies and variant axion models
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A number of observations of stellar systems show a mild preference for anomalously fast cooling compared with what predicted in the standard theory, which leads to a speculation that there exists an additional energy loss mechanism originated from the emission of axions in stars. We revisit the global analysis of the stellar cooling anomalies by adopting conservative assessments on several systematic uncertainties and find that the significance of the cooling hints becomes weaker but still indicates a non-vanishing axion-electron coupling at around 2.4$,sigma$. With the revised analysis results, we explore the possibility that such excessive energy losses are interpreted in the framework of variant axion models, which require two Higgs doublets and flavor-dependent Peccei-Quinn charge assignments. These models resolve two fundamental issues faced in the traditional KSVZ/DFSZ models by predicting a sizable axion coupling to electrons required to explain the cooling anomalies and at the same time providing a solution to the cosmological domain wall problem. We also find that a specific structure of the axion couplings to electrons and nucleons slightly relaxes the constraint from supernova 1987A and enlarges viable parameter regions compared with the DFSZ models. It is shown that good global fits to the observational data are obtained for axion mass ranges of $0.45,mathrm{meV} lesssim m_a lesssim 30,mathrm{meV}$, and that the predicted parameter regions can be probed in the forthcoming helioscope searches.
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We study a class of DFSZ-like models for the QCD axion that can address observed anomalies in stellar cooling. Stringent constraints from SN1987A and neutron stars are avoided by suppressed couplings to nucleons, while axion couplings to electrons and photons are sizable. All axion couplings depend on few parameters that also control the extended Higgs sector, in particular lepton flavor-violating couplings of the Standard Model-like Higgs boson $h$. This allows us to correlate axion and Higgs phenomenology, and we find that that ${rm BR}(h to tau e)$ can be as large as the current experimental bound of 0.22%, while ${rm BR} (h to mu mu)$ can be larger than in the Standard Model by up to 70%. Large parts of the parameter space will be tested by the next generation of axion helioscopes such as the IAXO experiment.
We consider a novel scenario of dark photon-mediated inelastic dark matter to explain the white dwarf cooling excess suggested by its luminosity function, and the excess in electron recoil events at XENON1T. In the Sun, the dark photon $A$ is produced mainly via thermal processes, and the heavier dark matter $chi_2$ is produced by the scattering of halo dark matter $chi_1$ with electrons. The XENON1T signal arises primarily by solar $A$ scattering, and $A$ emission by white dwarfs accommodates the extra cooling while maintaining consistency with other stellar cooling observations. A tritium component in the XENON1T detector is also required. We show for parameters that explain the XENON1T data, but not the white dwarf cooling anomaly, that a second signal peak may be buried in the XENON1T data and revealable at XENONnT. However, the parameters that give the double peak in the spectrum are incompatible with constraints from horizontal branch stars.
There are a number of observational hints from astrophysics which point to the existence of stellar energy losses beyond the ones accounted for by neutrino emission. These excessive energy losses may be explained by the existence of a new sub-keV mass pseudoscalar Nambu--Goldstone boson with tiny couplings to photons, electrons, and nucleons. An attractive possibility is to identify this particle with the axion -- the hypothetical pseudo Nambu--Goldstone boson predicted by the Peccei--Quinn solution to the strong CP problem. We explore this possibility in terms of a DFSZ-type axion and of a KSVZ-type axion/majoron, respectively. Both models allow a good global fit to the data, prefering an axion mass around 10 meV. We show that future axion experiments -- the fifth force experiment ARIADNE and the helioscope IAXO -- can attack the preferred mass range from the lower and higher end, respectively. An axion in this mass range can also be the main constituent of dark matter.
Helioscopes, such as the proposed International Axion Observatory (IAXO), have significant discovery potential for axions and axion-like particles. In this note, we argue that beyond discovery they can resolve details of the model. In particular, in the region suggested by stellar cooling anomalies, there is a good chance to measure the mass of the particle and separately its couplings to electrons and photons. This can give crucial information on the nature of the underlying model. To achieve this, energy resolved detectors and a setup with low energy threshold are needed.
The physics case for axions and axion-like particles is reviewed and an overview of ongoing and near-future laboratory searches is presented.
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