Excess Electronic Recoil Events in XENON1T

Abstract: We report results from searches for new physics with low-energy electronic recoil data recorded with the XENON1T detector. With an exposure of 0.65 t-y and an unprecedentedly low background rate of $76\pm2$ events/(t y keV) between 1 and 30 keV, the data enables sensitive searches for solar axions, an enhanced neutrino magnetic moment, and bosonic dark matter. An excess over known backgrounds is observed at low energies and most prominent between 2 and 3 keV. The solar axion model has a 3.4$\sigma$ significance, and a 3D 90% confidence surface is reported for axion couplings to electrons, photons, and nucleons. This surface is inscribed in the cuboid defined by $g_{ae}<3.8 \times 10^{-12}$, $g_{ae}g_{an}^{eff}<4.8\times 10^{-18}$, and $g_{ae}g_{a\gamma}<7.7\times10^{-22} GeV^{-1}$, and excludes either $g_{ae}=0$ or $g_{ae}g_{a\gamma}=g_{ae}g_{an}^{eff}=0$. The neutrino magnetic moment signal is similarly favored over background at 3.2$\sigma$ and a confidence interval of $\mu_{\nu} \in (1.4,2.9)\times10^{-11}\mu_B$ (90% C.L.) is reported. Both results are in strong tension with stellar constraints. The excess can also be explained by $\beta$ decays of tritium at 3.2$\sigma$ with a trace amount that can neither be confirmed nor excluded with current knowledge of its production and reduction mechanisms. The significances of the solar axion and neutrino magnetic moment hypotheses are reduced to 2.0$\sigma$ and 0.9$\sigma$, respectively, if an unconstrained tritium component is included in the fitting. With respect to bosonic dark matter, the excess favors a monoenergetic peak at ($2.3\pm0.2$) keV (68% C.L.) with a 3.0$\sigma$ global (4.0$\sigma$ local) significance. We also consider the possibility that $^{37}$Ar may be present in the detector and yield a 2.82 keV peak. Contrary to tritium, the $^{37}$Ar concentration can be tightly constrained and is found to be negligible.

• The paper reports a 3.3σ excess in low-energy recoils (2–3 keV) using 0.65 tonne-year XENON1T data compared to predicted backgrounds.
• It evaluates hypotheses including solar axions, an enhanced neutrino magnetic moment, and tritium decays with significance levels up to 3.4σ.
• The study underscores the need for further investigation with advanced detectors like XENONnT to clarify implications for dark matter and new physics.

Excess Electronic Recoil Events in XENON1T: Summary and Analysis

The report on "Excess Electronic Recoil Events in XENON1T" presents an investigation into the observed anomalies in low-energy electronic recoils recorded with the XENON1T detector. This detector, operating with an exposure of 0.65 tonne-years, achieved an unprecedentedly low background rate, offering a sensitive probe into a variety of new physics regimes, including searches for solar axions, an enhanced neutrino magnetic moment using solar neutrinos, and bosonic dark matter.

Key Findings and Claims

1. Experimental Setup and Context:
   • XENON1T utilizes a liquid-xenon time projection chamber and was primarily designed to detect Weakly Interacting Massive Particles (WIMPs). Its capabilities extend to other physics beyond the Standard Model (SM), including interactions from alternative dark matter candidates.
   • The data was acquired during Science Run 1 (SR1) from February 2017 to February 2018.
2. Observed Excess:
   • An excess over known backgrounds was noted at low energies, particularly prominent between 2–3 keV.
   • The rate of excess events observed was 285 compared to an expected 232 ± 15 events within the 1–7 keV range, corresponding to a 3.3σ excess under a Poisson statistical model.
3. Hypotheses for the Excess:
   • Solar Axions: The solar axion model, with a significance of 3.4σ, was explored with a specific interest in axion couplings. Despite the presence of contradictions with stellar cooling constraints, a three-dimensional 90% confidence surface for axion couplings was reported.
   • Neutrino Magnetic Moment: This hypothesis favored an enhanced neutrino magnetic moment, with a significance of 3.2σ, consistent with but competing with indirect constraints from astrophysical observations.
   • Tritium Decays: A β-decay hypothesis for tritium was considered a potential source for the excess, although current tritium concentration estimates do not definitively confirm this hypothesis.
4. Alternative Explanations and Further Analysis:
   • Bosonic Dark Matter: A monoenergetic peak analysis revealed a preferred mass of (2.3 ± 0.2) keV/c², with a 3.0σ global significance when considering bosonic dark matter.
   • Background Models: The paper also discusses other potential instrumental backgrounds and a comprehensive model of underlying physics to account for observed discrepancies.
5. Implications for Future Research:
   • The implications of these findings are significant, as they challenge the current limits on axion and neutrino properties and necessitate the inclusion of these signals in the physics reach of other dark matter experiments. The paper suggests that with planned improvements in upcoming experiments like XENONnT, further scrutiny and potential resolution could be achieved.

Theoretical and Practical Implications

The XENON1T findings have profound theoretical and practical implications. The tensions between the observed excess and established astrophysical constraints may point to new physics or oversights in current theoretical models. The consideration of alternative candidate particles such as solar axions and neutrino magnetic moments opens new ground for particle physics and astrophysics.

Furthermore, the practical aspects of improved detection methodologies and analysis frameworks lay the groundwork for future dark matter and exotic physics experiments, promising even greater sensitivity and discovery potential.

Conclusion

The "Excess Electronic Recoil Events in XENON1T" report highlights both the challenges and the promising avenues in the search for new physics beyond the Standard Model. While the findings complicate the narrative more than providing definitive answers, they exemplify the intricate dance between hypothesis, experimentation, and theory in advancing our understanding of the universe. The developments arising from this research underscore the importance of continued innovation in experimental physics and interdisciplinary collaboration in theoretical interpretation.