Critical Analysis of Solar Axion Hypothesis for XENON1T Excess
In the paper titled "Solar axions cannot explain the XENON1T excess," the authors, Di Luzio et al., investigate the hypothesis that solar axions might be responsible for the observed low-energy electronic recoil excess reported by the XENON1T collaboration. They rigorously assess the viability of this hypothesis in light of astrophysical observations, particularly focusing on its implications for stellar evolution.
Overview of the XENON1T Excess and Solar Axion Hypothesis
The XENON1T collaboration reported excess events in their low-energy electronic recoil data, peaking around 2-3 keV. Although there is speculation about the origin of this excess, the collaboration considers solar axions as a potential explanation, providing a best fit to the observed data with significant preference over background-only hypotheses. The solar axion hypothesis posits three main production mechanisms: atomic recombination, bremsstrahlung, Compton scattering (collectively referred to as ABC processes), Primakoff conversion, and nuclear transitions. However, Di Luzio et al. contend that the solar axion interpretation is not tenable due to constraints posed by astrophysical benchmarks.
Astrophysical Constraints
The authors detail several astrophysical observables that challenge the solar axion hypothesis. Significant among these is the potential alteration of the color-magnitude distribution of stars, particularly on the Red Giants Branch (RGB) and Horizontal Branch (HB). If solar axions with the suggested coupling strength were indeed emanated from the Sun as required to explain the XENON1T excess, such processes would dramatically affect stellar cooling rates, leading to deviations in observable properties:
- Tip of RGB Stars in Globular Clusters: The luminosity at the tip of the RGB would be inconsistent with observed values, as axion-induced cooling delays helium ignition, influencing stellar evolution lines.
- R-parameter in HB Stars: Anomalies would manifest in the HB stars’ measurements, as axion cooling significantly reduces their population in the color-magnitude diagram.
- White Dwarf Luminosity Function (WDLF): The cooling rate of white dwarfs would be notably accelerated, creating disparity between predicted and observed values.
Quantitatively, discrepancies are highlighted, with tensions exceeding 19σ for both the RGB luminosity tip and the R-parameter, indicating an untenable hypothesis under current astrophysical understanding.
Implications and Future Exploration
The theoretical implications are profound, as axion-like particles remain an attractive candidate for new physics beyond the Standard Model, potentially linked to dark matter and the strong CP problem. However, the results presented make a compelling case against solar axions as an explanation for the XENON1T excess when considering current astrophysical constraints. Di Luzio et al. suggest that either systematic errors in the XENON1T data must be resolved, or alternative non-solar particle origins should be explored further.
These findings push the boundaries of existing theoretical models and demand robustness in experimental interpretations. Future developments may involve alternative particle physics scenarios or novel astrophysical models to accommodate observed data without conflicting with established stellar phenomena.
In concluding, the authors deliver a critical examination of the solar axion hypothesis within the technological landscape of modern astroparticle physics, urging reconsideration of other new physics avenues. Such discourse will undoubtedly drive further theoretical innovation and observational scrutiny in the paper of fundamental particles and astrophysical processes.