Measurement of the electric form factor of the neutron at low momentum transfers using a vector polarized deuterium gas target at BLAST

Abstract

Elastic form factors are fundamental quantities that characterize the electromagnetic structure of the nucleon. High precision measurements of these quantities are essential in understanding the structure of hadronic matter. Although the proton elastic form factors are well known, knowledge of the neutron form factors has been limited due to the lack of pure neutron targets. Few nucleon targets, deuterium in particular, are typically used to study the electromagnetic structure of the neutron. Cross section measurements are not sufficient for high precision determination of the electric form factor of the neutron, ..., due to its small value. Recently, experiments using polarization observables which are proportional to the product of the electric and magnetic form factors of the neutron have been used instead. Such measurements require highly polarized electron beams and either a vector polarized neutron target (typically ²H) or else a neutron final state polarimeter. The Bates Large Acceptance Spectrometer Toroid (BLAST) provides a unique opportunity to measure! the shape of the neutron electric form factor at low momentum transfers. BLAST combines a high duty-factor polarized electron beam in the South Hall Ring (SHR), an Atomic Beam Source (ABS) target of highly polarized deuterium atoms and a large acceptance detector. This work reports the results of measurements of the neutron electric form factor using the ... reaction at five 4-momentum transfer squared, Q², points of 0.14, 0.20, 0.29, 0.38 and 0.50 (GeV/c)² using data taken in 2004. The experimental setup is discussed in detail and the results for ... are presented and discussed in the context of various theoretical predictions.

(cont.) A fit to the world's data including new BLAST data determines ... to ... over 0 < Q² < 1 ((GeV/c)². The best fit includes contributions from a low Q² bump and a smooth dipole term.