Standard Model Predictions for the Muon

Abstract

The current status of the Standard Model predictions for the muon anomalous magnetic moment is described. Various contributions expected in the Standard Model are discussed. After the reevaluation of the leading-order hadronic term based on the new e + e − data, the theoretical prediction is more than three standard deviations lower than the experimental value.

A global treatment of VMD physics up to the φ: II. τ decay and hadronic contributions to g−2

“…This turns also out to readdress the long-standing problem of the discrepancy between the BNL measurement [7] of g − 2 and the predictions based on e + e − annihilations and τ spectra as reported in the literature [8][9][10][11][12][13]. A quite recent study [14] tends to lessen the disagreement between these two kinds of predictions, but not to resorb it.…”

“…Comprehensive discussions of this issue can be found in [8][9][10] and more recently in [13]. Summaries can also be found in [11,12], for instance. A quite recent reanalysis of this discrepancy [14] concludes to a smaller disagreement between τ and e + e − based approaches (about 2 σ ); consequently, the newly proposed τ based estimate moves farther from the BNL measurement.…”

“…Another method, is simply to minimize the χ 2 as defined by (12), but amputated this from the (λ/η) 2 term which constrains the scale in accordance with the claimed experimental uncertainty. Indeed, in this way, the scale factor is allowed to vary freely within the global fit procedure.…”

“…Instead, in order to treat properly the correlated uncertainty, one should consider the data sets in [19][20][21]48] as a single (merged) data set and use as χ 2 an expression like (12) to introduce the common scale to be fitted. Here also, if λ/0.4% ≤ 1 ÷ 2, one could remove this scale while performing the change V ij ⇒ V ij + (0.4%) 2 m i m j .…”

“…We also let the scale factors vary for both the e + e − and τ data sets. 12 Some numerical results obtained with various data set configurations are reported in Table 2. One notes that all scale factors introduced for the e + e − → π + π − and e + e − → π 0 π + π − cross sections are in good correspondence with the expectations reminded in the first data column.…”