Update on the positron beam dark photon search proposal

• Bogdan Wojtsekhowski (Jefferson Lab)

This experiment has the potential to discover a dark matter particle and has a unique feature : the search sensitivity has no impact from the uncertainty in the boson decay mode and branching value. The proposed search for a narrow peak in the missing mass spectrum will allow us to find or put an upper limit on the new particle coupling with normal matter (electron/positron). The projected statistical sensitivity for the reduced coupling constant f2/e2 reaches 2$\times$10$^{−8}$ with 55 days of run at a positron beam current of 50 nA. The result will be very important in dark matter parameter analysis.

NTPE+ : measurement of the Rosenbluth slope in quasi-elastic positron neutron scattering

• Eric Fuchey (College of William and Mary)

The first measurements of elastic electron nucleon scattering with polarization transfer, allowing to separate the electric and magnetic nucleon form factors, showed a significant discrepancy with Rosenbluth measurement of the cross section of the same process. The two-photon exchange, suspected to be the cause of this observation, got a major focus since then. The two-photon exchange is expected to depend on the charge of the lepton, meaning that a Rosenbluth measurement on positron-nucleon scattering should clarify the role of the two photon exchange in the aforementioned discrepancy. We propose a measurement of the Rosenbluth slope in positron neutron scattering at $Q^2$ = 3.0 (GeV/$c$)$^2$ and $Q^2$ = 4.5 (GeV/$c$)$^2$. This measurement purports to complete and extend the measurement of the two-photon exchange in electron-neutron scattering submitted to and approved by PAC48 in 2020, and recorded in 2022 (experiment E12-20-010, currently under analysis). It would be performed with the Super BigBite Spectrometer installed in Hall C and will measure simultaneous positron-proton and positron-neutron scattering off deuterium. I will present the motivations and details of the proposed measurement, and provide a few aspects of the ongoing analysis of experiment E12-20-010 relevant to this new measurement.