High precision calculation of the hadronic vacuum polarisation contribution to the muon anomaly (2407.10913v1)

Abstract: We present a new lattice QCD calculation of the leading order hadronic vacuum polarization contribution to the muon anomalous magnetic moment $a_\mu$. We reduce uncertainties compared to our earlier computation by $40\%$, arXiv:2002.12347. We perform simulations on finer lattices allowing for an even more accurate continuum extrapolation. We also include a small, long-distance contribution obtained using input from experiments in a low-energy regime where they all agree. Combined with other standard model contributions our result leads to a prediction that differs from the measurement of $a_\mu$ by only 0.9 standard deviations. This provides a remarkable validation of the standard model to 0.37ppm.

• The paper achieves a new high-precision lattice QCD calculation for the LO-HVP contribution to the muon anomaly, reducing uncertainty by 40%.
• It employs finer lattice spacing and computational windows, refining continuum extrapolations and minimizing statistical errors.
• The findings narrow the gap between theory and experiment to within 0.9 standard deviations, reinforcing confidence in the Standard Model.

Contribution to the Muon Anomaly

The paper focuses on a detailed paper of the anomalous magnetic moment of the muon, denoted as $a_\mu$, within the context of the Standard Model (SM) of particle physics. The paper addresses the well-documented discrepancy between the experimental measurements of $a_\mu$ and its predictions based on the SM, especially concerning the contributions from quantum chromodynamics (QCD).

Methodology and Improvements

The authors provide a new, first-principles calculation of the most uncertain contribution to $a_\mu$ — the hadronic vacuum polarization (LO-HVP) at leading order. Utilizing large-scale lattice QCD simulations, the paper achieves unprecedented precision, which purportedly reduces the uncertainties from previous lattice calculations by 40%. Key advancements include the use of finer lattice spacing, which aids more accurate continuum extrapolations. Additionally, long-distance contributions from low-energy regimes based on experimental inputs further refine the calculations.

Strong Numerical Results

A significant result from their work reveals that the predicted $a_\mu$ now differs from experimental measurements by only 0.9 standard deviations, providing a marked improvement from prior calculations. Compared to earlier computations, the uncertainty in their LO-HVP contribution has been significantly curtailed, reaching a prediction agreement with experiment to within 0.37 parts per million (ppm).

Analytical Techniques

The paper meticulously explores the LO-HVP contribution using lattice quantum field theory, which allows predictions in the strongly interacting non-linear regime of QCD. Their approach involves splitting the evaluation into computationally manageable regions or "windows," facilitating better control over statistical uncertainties and lattice artifacts.

Implications and Future Directions

The results bolster confidence in the accuracy of the SM to describe particle interactions with incredible precision, highlighting particular areas such as the role of muons due to their heavier mass and thus greater sensitivity to unknown physics. The implications of this paper are significant in that they potentially resolve discrepancies that could indicate new physics beyond the SM.

The ongoing pursuit involves refining the understanding of QCD contributions at even finer levels. Moreover, incorporating upcoming data-driven evaluations and alternative methods (e.g., MUonE’s exploration of spacelike regions) could further reduce uncertainties surrounding $a_\mu$. As the landscape of particle physics research evolves, comprehensive studies like this could critically align theoretical predictions with experimental realities, thereby shaping future investigations in high-energy physics.

Conclusion

This paper underscores significant progress in resolving long-standing issues with the SM’s prediction of the muon's magnetic moment anomaly. While affirming the robustness of the SM under stringent examination, it invites deeper inquiry into any underlying phenomena that might be concealed within the measurement discrepancies or emerge from future high-precision experiments.