Progress in Particle and

Nuclear Physics PERGAMON Progress in Particle and Nuclear Physics 50 (2003) 483-485

http://www.elsevier.comflocate/npe

Measurement of the Neutron Electric Form Factor

U. MaLER and M. SEIMETZ for the A 1 Collaboration*

Institutjfir Kemphysik, Universifdf Mairu, Bechenvcg 45, 55099 Maim, Germany

The ueutron electric form factor GE,~ has been measured at three different, four-

momentum transfers in a D(Z, e’ii)p experiment. Neutrons were detected in a recoil

polarirneter, using a spin precession method. We present the experiment and tho

status of data analysis.

1 Introduction

The capability of the nucleon t,o accept a momentum transfer in an elastic scattering reaction, i.e.

without excitation or particlc emission, is parametcrised by its electromagnetic form factors. ‘I%e

Sachs form factors, Gk; and (2~~ can hc interprctcd in the Rreit, system as the Fourier transform

of the nucleon’s charge and magnctisation densities, respectively, and are therefore of fundamental

importance for understanding the nucleon’s internal structure.

Measurement of the electric neutron form factor, GE,,,, provides several challenges to the exper-

iment: Since: no free neutron target exists, elastic or quasi-elastic electron scattering off light nuclei

(D, “He) has to he used, implying model-dependent corrections due to nuclear binding. Experiments

based on Roscnhluth separation arc: rather insensitive to the electric form fact,or due to the smallness

of G2E.n compared to G&,n and give zero-compatible results. Making USC of the development of high-

quality polarised electron beams during the last decade, a new class of double polarisation experiments

sensitive to GE,JGM,~ became feasible, leading to a significantly improved precision in the knowledge

of the neutron electric form factor.

2 Experimental setup

The method of our experiment is based on the spin transfer in (quasi-)elastic scattering of polariscd

clcctrons on unpolarised nucleons [I]. The components I’, and I’, of the outgoing nucleon polarisation

‘Supported by the Doutsche Ibschungsgctrueinschaft, SFB 443

0146~6410/03/$ - see front matter 0 2003 Elsevier Science BV. All rights reserved. PII: SOl46-6410(03)00041-3

484 U. Miiller Ed al. / Prog. Part. Nucl. Phys. 50 (2003) 483-485

are proportional to the form factor products GEG~, and GyU, respectively, where the z and z coordi-

nates lie in t,he scattering plauc and 2 coincides with the direction of momentum transfer. ‘I’hc ratio

P,./Pz is then given by GE/G’*, times factors dctermincd by reaction kinematics.

The: D(e’, “In’)p cxperirncnt is pcrformcd at thr three-spectromett:r facility at MAMI 12, 3). A

polarised electron beam (I’, zz 80 %, monitored by a Mtiller polarimc:t,cr) wit,11 up to 15 /LA current

hits a 5 cm liquid deutcrium target. ‘L’hc momentum of the: scattcrcd electrons is nlc>lsured wit,h

high resolution iu magnetic spectrometer A. Neutrons are dctccted at, forward angles in a neut,ron

polarimeter. Their direction and time of flight is measured in a scgmentcd first wall of scintillator bars.

Since the n-p scattering reaction in tho organic scintillator niatcrial carries analysing power,

a transverse neutron polarisation will lead to it11 up-down asymmetry that, can bc analyscd by another

neutron detection in a second scint.illator wall. l3y preccssing the neutron spin in a magnct,ic field b)

an angle x around the y axis, both polarisation components P, and Pz can bc measured [4]. At a

cc&in angle ~1) t,he projections of both components will cancel, leading to zero asymmetry.

The polarimcter is optimiscd to withstand high backgrourld rates. The large-arca scintillators

arc surrounded by massive concrete shielding. Low-energy background is rcduccti by a load shield

installed between target and detector. In order to suppress charged background particlcs, individual

veto detectors are placed in front, of the scintillator bars.

3 Data analysis

Analysis starts by reconstructing the kinematics of each event, which is determined by the momentum

of the scattered electron, measured in spectrometer A: and by the lab-system angle of the neutron,

measured in the first scint,illator wall. Neutrons can be discriminated against protons and other charged

particles by various conditions based on time of flight. mcasurcmcnt and 011 the part,icles’ energy

deposition in different layers of the detector. Additional information is provided by the correlation of

t,he (quasi-)c:lastic p(Z:, p7b) neutron scattering an&: and the energy signal in the first, scintillator wall.

For the final cvcnt sample, the azimuthal angle &, of the analysing scattering of the neutron

and the beam helicity can bc combined to an ~asymmctry double ratio, depending 011 sin &,, with an

amplitude proportional to the transverse polarisation component. Data have been taken at difl’erent

precession angles x, permitting a determination of the zero crossing xc1 and thus of the ratio GE,,,/G~,,~.

Data taking was completed in August 2002 and analysis is in progress. Our aim is a relative

error on Gk;.n of about 10 %.

References

[l] R. G. Arnold, C. E. Carlsol! and F. Gross: Phys. R.w. C 23 (1981) 363.

U. Miiller et al. / Prog. Part. Nucl. Phys. 50 (2003) 483-485

(21 K. I. Rlomqvist, ct al., Nucl. Instr. and Meth. A 403 (1998) 263.

485

[3] P. Bartsch et, al., Proposal Al-2/99, Inst. fiir Kernphysik, Univ. Mainx, and references therein.

(41 M. Ostrick ct al., Phys. Rev. Lctt. 83 (1999) 276.