CODATA Physical Constants Updated

CODATA is one of the global standards for measurements of physical constants (fundamental and otherwise). The 2022 update is now available at arXiv with commentary on how the values were established.

The 2026 CODATA adjustment of the fundamental constants is the next regularly scheduled adjustment. Data being used in this adjustment is required to have been discussed in a publication preprint or a publication prior to 31 December 2026.

The muon g-2 discussion in the preprint is already outdated (in part by design, as it is only considering papers before December 31, 2022).

The Muon g-2 Issue In A Nutshell

The introduction of a new paper by authors who describe themselves by the first initials of their surnames (KNTW) nicely sums of the state of the efforts to compare experimental measurements of muon g-2 with predictions of its value using the Standard Model of Particle Physics.

The anomalous magnetic moment of the muon, aµ, and its potential for discovering new physics stand at a crossroads. The accuracy and precision of the Standard Model (SM) prediction, a(SM)µ, relies on resolving significant tensions in evaluations of the hadronic vacuum polarization (HVP) contributions, a(HVP)µ . Data-driven evaluations of the HVP using e+e− → hadrons cross section data as input result in a value for a(SM)µ that is ∼ 5σ below the most recent experimental measurement from the Muon g−2 Experiment at Fermilab, a(exp)µ. With an unprecedented 200 parts-per-billion (ppb) precision, confirmation of previous measurements, and final results (expected in 2025) projected to improve the experimental precision by another factor of two, the measurements of aµ appear to be on solid ground.<1> However, high-precision lattice QCD calculations (incorporating QED corrections) and the most recent experimental measurement of the dominant e+e− → π+π− cross section from the CMD-3 experiment result in independent, but consistent values for aHVP that are >4σ larger than previous data-driven evaluations. They therefore generate values for a(SM)µ that are consistent with a(exp)µ and support a no-new-physics scenario in the muon g−2, whilst leaving an unexplained discrepancy with the vast catalogue of previously measured hadronic cross section data. 

The KNT (now KNTW) data-driven determinations of a(HVP)µ are crucial inputs to previous and future community-approved predictions for a(SM)µ from the Muon g−2 Theory Initiative. With multiple, independent lattice QCD evaluations of a(HVP)µ becoming significantly competitive only in recent years, it was one of only a few data-driven HVP evaluations which exclusively formed the value for a(HVP) lattice QCD and updated data-driven evaluations, with KNTW being a key input to the latter. An alternative approach to determine a(HVP)µ by experimentally measuring the spacelike vacuum polarization is under preparation at the MUonE Experiment. 

The KNTW procedure for evaluating the total hadronic cross section and a(HVP)µ (plus other precision observables which depend on hadronic effects) is undergoing a major overhaul and modernization of the analysis framework. The aim of this revamp is to make use of sophisticated analysis tools, perform new evaluations of various contributions, incorporate handles in the analysis structure that result in flexible and robust ways to test various systematic effects, improve determinations of corresponding systematic uncertainties and ultimately produce a new state-of-the-art in the determination of these quantities. These changes will be described in detail in the next full KNTW update. 

Such future data-driven evaluations of a(HVP)µ depend largely on new experimentally measured hadronic cross section data, particularly for the π+π− final state. These require increased precision and a more robust understanding of higher-order radiative corrections, which are currently being studied in detail within the STRONG2020 program and The RadioMonteCarlow 2 Effort. Whilst a discussion of these improvements is outside the scope of this letter, such future results have been announced from the BaBar, Belle II, BESIII, CMD-3, KLOE and SND experiments within the next few years. These new measurements could either fundamentally adjust the previous data-driven evaluations of a(HVP)µ used in the SM prediction that exhibits the ∼ 5σ discrepancy with a(exp)µ. Future SM predictions are expected to incorporate both to bring them more in line with e.g. the recent CMD-3 π+π− measurement or make the current tensions even worse if new measurements confirm lower cross section values with increased precision. 

Importantly, and as will be discussed in the next section, analysis choices in how to use these data can produce significantly different results. With this being the case, the future of a(HVP)µ and a(SM)µ being so uncertain, and the crossroads in the current tensions ultimately suggesting either a discovery of new physics or a multi-method confirmation of the SM, analysis blinding for data-driven determinations of the HVP is now paramount. 

<1> Alternative future measurements of aµ are also planned at JPARC and PSI.

Unification In Physics Doesn't Work

This figure was the classic illustration of force unification, although it turns out that unification doesn't happen, even under SUSY, with the constraints of recent high precision coupling constant measurements.

Woit at his "Not Even Wrong" blog shares some slides he did for a podcast on grand unified theories of physics, which try to combine the three forces of the Standard Model into a facets of a single force, typically within a single Lie group, rather than the SU(3) x SU(2) x U(1) of the Standard Model. He explains that:

The main goal of the slides is to explain the failure of the general paradigm of unification that we have now lived with for 50 years, which involves adding a large number of extra degrees of freedom to the Standard Model. All examples of this paradigm fail due to two factors: 

• The lack of any experimental evidence for these new degrees of freedom.  

• Whatever you get from new symmetries carried by the extra degrees of freedom is lost by the fact that you have to introduce new ad hoc structure to explain why you don’t see them.