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Register a new userRe-examining the Solar Axion Explanation for the XENON1T Excess
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The XENON1T collaboration has observed an excess in electronic recoil events below $5~mathrm{keV}$ over the known background, which could originate from beyond-the-Standard-Model physics. The solar axion is a well-motivated model that has been proposed to explain the excess, though it has tension with astrophysical observations. The axions traveled from the Sun can be absorbed by the electrons in the xenon atoms via the axion-electron coupling. Meanwhile, they can also scatter with the atoms through the inverse Primakoff process via the axion-photon coupling, which emits a photon and mimics the electronic recoil signals. We found that the latter process cannot be neglected. After including the $rm{keV}$ photon produced via inverse Primakoff in the detection, the tension with the astrophysical constraints can be significantly reduced. We also explore scenarios involving additional new physics to further alleviate the tension with the astrophysical bounds.
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We argue that the interpretation in terms of solar axions of the recent XENON1T excess is not tenable when confronted with astrophysical observations of stellar evolution. We discuss the reasons why the emission of a flux of solar axions sufficiently intense to explain the anomalous data would radically alter the distribution of certain type of stars in the color-magnitude diagram in first place, and would also clash with a certain number of other astrophysical observables. Quantitatively, the significance of the discrepancy ranges from $3.3sigma$ for the rate of period change of pulsating White Dwarfs, and exceedes $19sigma$ for the $R$-parameter and for $M_{I,{rm TRGB}}$.
This work is a study of some possible background sources in the XENON1T environment which might affect the energy spectrum of electronic recoil events in the lower side and might contribute to the observed excess. We have identified some additional possible backgrounds, like $^{41}$Ca, $^{49}$V, $^{63}$Ni, $^{106}$Ru and $^{125}$Sb coming from cosmogenic production, where the former two emit monoenergetic $X$-rays and the latter three have $beta$ decays, or isotopes, like $^{210}$Pb, from the decay chain of $^{222}$Rn emanated in liquid xenon from the materials, or isotopes, like $^{137}$Cs, produced due to neutron capture. We perform a $chi^2$ fitting of the ER spectrum from these backgrounds along with tritium to the observed excess events by varying their individual rates to understand whether they can be present to contribute to the low energy excess or their presence is constrained from the data. We also study the possibility of simultaneous presence of more than one such backgrounds, and how this affects the rates required by individual backgrounds to explain the excess.
Solar interpretations of the recent XENON1T excess events, such as axion or dark photon emissions from the sun, are thought to be at odds with stellar cooling bounds from the horizontal branch stars and red giants. We propose a simple effective field theory of a dark photon in which a $Z_2$ symmetry forbids a single dark photon emission in the dense stars, thereby evading the cooling bounds, while the $Z_2$ is spontaneously broken in the vacuum and sun, thereby explaining the XENON1T excess. The scalar responsible for the $Z_2$ breaking has an extremely flat potential, but the flatness can be maintained under quantum corrections. The UV completion of the EFT generally requires the existence of new electrically charged particles with sub-TeV masses with $O(1)$ couplings to the dark photon, offering the opportunity to test the scenario further and opening a new window into the dark sector in laboratory experiments.
We consider a renormalizable theory, which successfully explains the number of Standard Model (SM) fermion families and whose non-SM scalar sector includes an axion dark matter as well as a field responsible for cosmological inflation. In such theory, the axion gets its mass via radiative corrections at one-loop level mediated by virtual top quark, right handed Majorana neutrinos and SM gauge bosons. Its mass is obtained in the range $4$ keV$div$ $40$ keV, consistent with the one predicted by XENON1T experiment, when the right handed Majorana neutrino mass is varied from $100$ GeV up to $350$ GeV, thus implying that the light active neutrino masses are generated from a low scale type I seesaw mechanism. Furthermore, the theory under consideration can also successfully accommodates the XENON1T excess provided that the PQ symmetry is spontaneously broken at the $10^{10}$ GeV scale.
Recently XENON1T Collaboration announced that they observed some excess in the electron recoil energy around a 2-3 keV. We show that this excess can be interpreted as exothermic scattering of excited dark matter (XDM), $XDM + e_{atomic} rightarrow DM + e_{free}$ on atomic electron through dark photon exchange. We consider DM models with local dark $U(1)$ gauge symmetry that is spontaneously broken into its $Z_2$ subgroup by Krauss-Wilczek mechanism. In order to explain the XENON1T excess with the correct DM thermal relic density within freeze-out scenario, all the particles in the dark sector should be light enough, namely $sim O(100)$ MeV for scalar DM and $sim O(1-10)$ MeV for fermion DM cases. And even lighter dark Higgs $phi$ plays an important role in the DM relic density calculation: $X X^dagger rightarrow Z phi$ for scalar DM ($X$) and $chi bar{chi} rightarrow phi phi$for fermion DM ($chi$) assuming $m_{Z} > m_chi$. Both of them are in the $p$-wave annihilation, and one can easily evade stringent bounds from Planck data on CMB on the $s$-wave annihilations, assuming other dangerous $s$-wave annihilations are kinematically forbidden.
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