New evidence supporting the existence of the hypothetic X17 particle (1910.10459v1)

Abstract: We observed electron-positron pairs from the electro-magnetically forbidden M0 transition depopulating the 21.01 MeV 0$^-$ state in $^4$He. A peak was observed in their $e^+e^-$ angular correlations at 115$^\circ$ with 7.2$\sigma$ significance, and could be described by assuming the creation and subsequent decay of a light particle with mass of $m_\mathrm{X}c^2$=16.84$\pm0.16 (stat) \pm 0.20 (syst)$ MeV and $\Gamma_\mathrm{X}$= $3.9\times 10^{-5}$ eV. According to the mass, it is likely the same X17 particle, which we recently suggested [Phys. Rev. Lett. 116, 052501 (2016)] for describing the anomaly observed in $^8$Be.

• The paper presents a refined experimental setup using the 3H(p,γ)4He reaction to detect a significant 7.2σ e⁺e⁻ correlation peak at 115°, indicating the existence of X17.
• It confirms an X17 mass of 16.84 ± 0.16 MeV, aligning with previous 8Be experiments and strengthening the argument for its presence.
• The findings suggest that if verified, X17 could mediate a new fundamental force and offer insights into anomalies such as the (g-2)μ discrepancy.

New Evidence Supporting the Existence of the Hypothetical X17 Particle

The paper, authored by A.J. Krasznahorkay and colleagues, presents experimental findings potentially supporting the existence of a hypothetical particle, designated as X17. This research builds upon previous observations in $^8$Be nuclei and extends the investigation to $^4$He, utilizing an advanced experimental setup to mitigate background noise and improve detection accuracy.

Main Findings

1. Experimental Setup and Observations:
   • The paper explores the $^{3}$H(p,$\gamma$)$^{4}$He reaction at $E_p$=900 keV, which populates the 21.01 MeV $0^-$ state in $^4$He.
   • A distinct peak in the electron-positron ($e^+e^-$) angular correlation was detected at 115$^\circ$ with a significance of 7.2$\sigma$. This observation suggests the existence of an intermediate particle, resembling the proposed X17, with a calculated mass of $16.84 \pm 0.16$ MeV.
2. Comparison with Previous Observations:
   • The inferred mass is consistent with the previously suggested X17 particle observed during the $^8$Be experiments. The angular correlation anomaly has similarities in both experiments, potentially indicating a recurring phenomenon attributable to the same particle.
3. Particle Physics and Theoretical Implications:
   • If validated, the X17 particle could have substantial implications for particle physics, including the potential mediation of a new fundamental force. This particle's characteristics, as a vector gauge boson, imply a coupling to standard model particles, which has implications for addressing existing anomalies, such as the (g-2)$_\mu$ discrepancy.
4. Potential Explanations and Constraints:
   • Theoretical efforts by Feng et al. and others suggest scenarios involving vector gauge bosons and couplings to known forces within particle physics frameworks, like the Standard Model. Various constraints, including those on the hypothetical $Z'$ particles from other experiments, require these new particles to interact weakly with protons and neutrons to avoid contradiction with present observational data.
5. Signal Analysis Techniques:
   • The experimental methodology involved precise measurements of $e^+e^-$ pairs, enabling robust background subtraction and correlation analysis via advanced detector systems. This thorough approach supports the robustness of the observed effect, achieved by simulating potential backgrounds and isolating genuine signals.

Future Directions

The finding of the X17 particle invites further experimental verification through independent avenues, notably those initiatives currently underway at high-energy physics facilities worldwide. Efforts like NA64 at CERN and future experiments at the LHC's FASER project aim to refine these initial observations. Intriguingly, these projects could explore not only the existence of such a particle but also its broader theoretical implications across the domain of particle physics, including potential roles in dark matter models.

Conclusion

The reported data in $^4$He complements and extends prior findings in $^8$Be, suggesting the presence of a light particle that challenges traditional models of particle physics. The confirmation of X17's characteristics might open new avenues for addressing compelling questions in the field such as the nature of dark matter and additional forces beyond the established theoretical landscape. Continued inquiries by the broader scientific community will be critical in assessing the credibility and integrating such potential discoveries in the framework of modern physics.