Measurement of the Neutron Electromagnetic Form Factor Ratio at High Momentum Transfer

The inner structure of the nucleon (proton and neutron) remains a topic of great interest in nuclear and particle physics, after many decades of study. For example, understanding the quark-gluon dynamics inside the nucleon would shed light on how 99% of the nucleon mass is created. The neutron electromagnetic form factors, GEn and GMn , give important insights into the neutron structure. The Super BigBite Spectrometer (SBS) program at Jefferson Lab (JLab) seeks to extend the form factor measurements for both the proton and the neutron. The neutron electric form factor, GEn, has been historically difficult to measure due to the short lifetime of the free neutron and the small value of GEn. The GEn-II experiment is part of the SBS program and seeks to measure GEn, significantly increasing the high momentum transfer coverage. A newly designed polarized 3He target increased the figure of merit by three times compared to previous measurements. The analysis of this data is especially challenging due to the unprecedented high-rate environment caused by the open nature of the spectrometer with a direct line of sight to the target. This required developing new Gas Electron Multiplier (GEM) particle trackers which can cover large areas demanded by this setup and handle particle rates up to 500 kHz/cm^2. Rates this high over a large area is unprecedented in particle tracking systems and came with a number of challenges. Data taken in the SBS program was critical to understanding hardware and software solutions that improved the track reconstruction efficiency to be >97% with a position resolution of 70 µm. In previous experiments the proton electromagnetic form factors, GEp and GMp, were measured up to Q^2 = 8.5 GeV^2 and Q^2 = 30 GeV^2, respectively, while GEn has only been measured up to Q^2 = 3.4 GeV^2. The GEn-II experiment has measured the neutron form factor ratio, GEn/GMn, at Q^2 values of 2.90, 6.50, and 9.47 GeV^2 by scattering a polarized electron beam with a polarized 3He target, used here as an effective polarized neutron target, and measuring the double spin asymmetry of the cross section. Previous GEn measurements do not extend above Q^2 = 3.4 GeV^2, and therefore this analysis has extended the world data by almost three times. The background correction is especially difficult at the higher Q^2 settings leading to large systematic errors. As very exploratory results from this early analysis of the data, we find for Q^2 = 2.90 GeV^2, GEn = 0.0157 ± 0.0016 ± 0.0011, for Q^2 = 6.50 GeV^2, GEn = 0.0067 ± 0.0019 ± 0.0005, and for Q^2 = 9.46 GeV^2, GEn = 0.0046 ± 0.0023 ± 0.0005. These results are compared to predictions from the Dyson-Schwinger Equations (DSE) model and a Relativistic Constituent Quark Model (RCQM).