An Improved Limit on the Muon Electric Dipole Moment (0811.1207v2)

Abstract: Three independent searches for an electric dipole moment (EDM) of the positive and negative muons have been performed, using spin precession data from the muon g-2 storage ring at Brookhaven National Laboratory. Details on the experimental apparatus and the three analyses are presented. Since the individual results on the positive and negative muon, as well as the combined result, d=-0.1(0.9)E-19 e-cm, are all consistent with zero, we set a new muon EDM limit, |d| < 1.9E-19 e-cm (95% C.L.). This represents a factor of 5 improvement over the previous best limit on the muon EDM.

An Improved Limit on the Muon Electric Dipole Moment

The paper presents the results of a concerted experimental effort by the Muon $(g-2)$ Collaboration to refine the constraints on the muon's electric dipole moment (EDM). Conducted at the Brookhaven National Laboratory (BNL), this investigation harnesses the resources and infrastructure of the muon $g-2$ storage ring, building on previous experimental findings from CERN to achieve approximately an order of magnitude improvement in these constraints.

Research and Methodology

The paper involved three separate experiments aiming to detect the EDM of both positive ($\mu^+$) and negative muons ($\mu^-$) by leveraging spin precession data obtained from the storage ring. The experimental design sought to uncover deviations in the muons' precession characteristics that would imply a non-zero EDM, specifically through the influence on the angular discrepancy between the spin and momentum vectors, $\theta_{\text{EDM}}$, which would manifest as a distinct oscillation pattern in the decay positron data.

The electric dipole moment emerges as a key observable in probing $CP$ violation beyond the Standard Model (SM), potentially connected to physics constituted by yet-undiscovered sources of such violation. While CP violation has been observed in certain meson decays, its manifestation in the leptonic sector remains elusive. The muon's significant mass relative to the electron renders it an advantageous candidate for EDM searches.

Results and Implications

The experiments culminated in a new combined limit for the muon EDM: $$d_{\mu} = (-0.1 \pm 0.9) \times 10^{-19}~e \cdot \text{cm}$$, tightening the constraint to $|d_{\mu}| < 1.9 \times 10^{-19}~e\cdot\text{cm}$ at the 95% confidence level. This represents a factor of five refinement over previous results, carving out stronger boundaries for theories that might predict larger values for the muon EDM.

The measured results for both $\mu^+$ and $\mu^-$ were consistent and complied with $CPT$ invariance, aligning with the expectation that $d_{\mu^+} = -d_{\mu^-}$. These findings continue to favor the extraordinarily tiny EDM values predicted by the SM, constrained by its minimal coupling constants, suggesting that significant deviations detectable within the current experimental sensitivities remain out of reach.

Future Prospects and Theoretical Implications

Continued improvement in experimental sensitivity to the muon EDM would provide invaluable probes for new physics. The increase in precision achieved here sets a new benchmark, yet the paper importantly highlights systematic effects such as detector misalignments and radial field corrections as critical avenues for further refinement.

Looking ahead, the development of new generation experiments, possibly at the Fermilab Muon $(g-2)$ facility or other advanced platforms, could further constrain or potentially reveal a non-zero muon EDM. Given the close interplay between EDM measurements and the pursuit of new physics processes, enhanced detection capabilities could substantiate or falsify specific extensions to the SM, informing our understanding of $CP$ violation mechanisms in the universe. Such advancements could bring clarity to unresolved conundrums like baryogenesis, indirectly confirming or refuting assumptions regarding the $\bar{\theta}$ parameter within QCD or potential contributions from supersymmetric models.

In conclusion, the results of this paper, driven by meticulous experimental design and analysis, contribute a substantial value to the canon of precision measurements in particle physics, underscoring the quest for a deeper grasp of fundamental forces and symmetries.