The Muon g-2

TL;DR

The paper discusses the precise measurement of the muon anomalous magnetic moment, highlighting the persistent discrepancy with Standard Model predictions and recent experimental and theoretical advancements that could indicate new physics.

Contribution

It reviews recent experimental improvements and theoretical efforts that have reduced uncertainties and explored the potential for new physics beyond the Standard Model.

Findings

Persistent 3-sigma discrepancy between theory and experiment

Significant reduction in theoretical uncertainties due to new data

Enhanced precision in electron g-2 measurements

Abstract

The muon anomalous magnetic moment is one of the most precisely measured quantities in particle physics. In a recent experiment at Brookhaven it has been measured with a remarkable 14-fold improvement of the previous CERN experiment reaching a precision of 0.54ppm. Since the first results were published, a persisting "discrepancy" between theory and experiment of about 3 standard deviations is observed. It is the largest "established" deviation from the Standard Model seen in a "clean" electroweak observable and thus could be a hint for New Physics to be around the corner. This deviation triggered numerous speculations about the possible origin of the "missing piece" and the increased experimental precision animated a multitude of new theoretical efforts which lead to a substantial improvement of the prediction of the muon anomaly a_mu=(g_mu-2)/2. The dominating uncertainty of the prediction, caused by strong interaction effects, could be reduced substantially, due to new hadronic cross section measurements in electron-positron annihilation at low energies. Also the recent electron g-2 measurement at Harvard contributes substantially to the progress in this field, as it allows for a much more precise determination of the fine structure constant alpha as well as a cross check of the status of our theoretical understanding.