Friday, April 09, 2021

Bhabha scattering and the anomalous magnetic dipole moment of the muon

Just noting some observations here, likely of no significance at all. 

  •  Bhabha scattering is the electron-positron scattering process. 

  •  Per Wiki
Electron-positron colliders operating in the region of the low-lying hadronic resonances (about 1 GeV to 10 GeV), such as the Beijing Electron Synchrotron (BES) and the Belle and BaBar "B-factory" experiments, use large-angle Bhabha scattering as a luminosity monitor. To achieve the desired precision at the 0.1% level, the experimental measurements must be compared to a theoretical calculation including next-to-leading-order radiative corrections. The high-precision measurement of the total hadronic cross section at these low energies is a crucial input into the theoretical calculation of the anomalous magnetic dipole moment of the muon, which is used to constrain supersymmetry and other models of physics beyond the Standard Model.

  • One would think Bhabha scattering is extremely well understood in terms of theoretical calculations.  So I was surprised to find this paper from 2020

    Patrick Janot, Stanisław Jadach,
    Improved Bhabha cross section at LEP and the number of light neutrino species,
    Physics Letters B, Volume 803, 2020, 135319, ISSN 0370-2693, (

    Abstract: In e+e− collisions, the integrated luminosity is generally measured from the rate of low-angle Bhabha interactions e+e−→e+e−. In the published LEP results, the inferred theoretical uncertainty of ±0.061% on the predicted rate is significantly larger than the reported experimental uncertainties. We present an updated and more accurate prediction of the Bhabha cross section in this letter, which is found to reduce the Bhabha cross section by about 0.048%, and its uncertainty to ±0.037%. When accounted for, these changes modify the number of light neutrino species (and its accuracy), as determined from the LEP measurement of the hadronic cross section at the Z peak, to Nν=2.9963±0.0074. The 20-years-old 2σ tension with the Standard Model is gone. 

  • A discussion of the recent muon result is on Peter Woit's blog.  Some of the comments under that blog post are of interest.
Presumably the large-angle Bhabha scattering used to calibrate the newer experiments is already much more accurate from the get-go.