High Precision Measurement of the Pion Form Factor at BESIII - Improving the Hadronic Vacuum Polarization Correction to the muon g-2

• Applicant: Professor Dr. Achim Denig
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 455635585

The goal of the current project is a measurement of the cross section sigma(e+e- -->pi+pi-), i.e. of the timelike pion form factor. The measurement will be performed over a very wide energy range from the two-pion threshold up to energies above 3 GeV by exploiting the initial state radiation (ISR) method at the BESIII experiment in Beijing. We aim for world-class accuracy in the full energy range. Two analysis methods will be pursued, which differ by the accessible energy ranges and the systematic effects to be considered. The first analysis intends a selection of ISR photons emitted at large polar angles (LA), while the second analysis focuses on small-angle ISR photons (SA). From the technological point of view, high-precision analysis methods based on machine learning algorithms need to be developed and calibrated in order to allow for an efficient separation of pion and muon tracks in the BESIII detector.1) The LA analysis will measure the pion form factor in the energy range below 1 GeV, which is completely dominated by the rho resonance. We aim for a reduction of the systematic uncertainty compared to an already existing BESIII measurement in the peak region of the rho resonance. A systematic uncertainty of 0.6% (or better) is possible. Two independent normalization methods will allow for important cross checks of the cross section measurement.2) The SA analysis will measure the pion form factor for energies above 0.8 GeV. A determination of the pion form factor with unprecedented accuracy is planned with important consequences for our understanding of the hadron spectrum in the mass range above 1 GeV. The project described in this proposal will allow for an improved determination of the hadronic vacuum polarization (HVP) contribution to the muon anomaly. It will furthermore be possible to clarify a discrepancy observed between the two leading experiments in the field so far, KLOE and BABAR. This will directly impact the Standard Model (SM) prediction of (g - 2) as this discrepancy currently limits the overall SM prediction.

• DFG Programme: Research Grants