Nucleon Form Factors and the BLAST Experiment at MIT-Bates Probing Nucleon Structure with Spin-Dependent Electron Scattering at Low Q2 Introduction/Motivation BLAST Experiment Results and Discussion May 18, 2006 R. Alarcon @ PAV06

Spin 1/2: Elastic electron scattering from nucleons t = Q2/4M2 e = [1+2(1+t)tan2(q /2)]-1 Sach’s ff’s: charge GEp(0) = 1, GEn(0) = 0 magnetism GMp(0) = 2.79 GMn(0) = –1.91

Polarized e-N scattering Polarized beam + polarized target: Donnelly + Raskin, Ann. Phys. 169 (1986)247 p n p  neutron charge ff n  neutron magnetic ff Polarized beam + polarization of recoil nucleon: neutron charge ff proton GE/GM Akhiezer+Rekalo, Sov.JPN 3 (1974) 277 Arnold,Carlson+Gross, PRC 21 (1980) 1426

Why study hadron form factors? They give us the ground state properties of (all visible) matter: size and shape charge and magnetism distributions spin and angular momentum They are required for knowledge of many other things: structure of nuclei at short distances Proton charge radius and Lamb shift precision tests of Weak interaction at low Q2 They should give clues on how to connect QCD to the NN force cPT pQCD form factors quarks/gluons p cloud lattice QCD They can be measured NOW with high precision!

Scientific Motivation for BLAST Comprehensive study of spin observables from few-body nuclei at low Q2. Nucleon form factors provide basic information on nucleon structure. Low-Q2 region is a testing ground for QCD and pion-cloud inspired and other effective nucleon models. GnE is the least known among the nucleon form factors, with errors of typically 15-20%. GnE related to neutron charge distribution. Precise knowledge of GnE is essential for parity violation experiments.

MIT-Bates Linear Accelerator Center Linac: 2500 MeV Beam: 850 MeV / Imax = 225 mA / Pe = 65 % SHR: Siberian Snake + Compton Polarimeter Target: Internal Target = Atomic Beam Source

Internal Target Physics at MIT-Bates Ee  1 GeV, Pe= 40-80 % Im= 200 mA,   10 min pure species thin high polarization thin cell low holding field South Hall Ring L = 1031-1033 atoms cm-2 s-1 Novosibirsk, AmPS, HERMES, IUCF, COSY

Atomic Beam Source Isotopically pure H or D Vector Polarized H Vector and Tensor D Target Thickness/Luminosity Flow 2.2 x 1016 atoms/s Density 6 x 1013 atoms/cm2 Luminosity 6 x 1031 cm-2s-1 Target Polarization typically 70-80%

Atomic Beam Source

Atomic Beam Source Quasielastic Beam-Target Asymmetry Aved(exp) = h·Pz ·Aved(th) <hPz> = 0.567 ± 0.006 <Pz> = 0.85 ± 0.04 <h> = 0.67 ± 0.04

Bates Large Acceptance Spectrometer Toroid Left-Right symmetry Large Acceptance 0.1 < Q2/(GeV/c)2 < 1.0 Coils: B = 3.8 kG Drift Chambers PID,tracking  ≈ 0.5º, Cerenkov Counters e,  separation Scintillators TOF, PID, trigger Neutron Counters Neutron ToF

The BLAST Collaboration

Experimental Program High quality data for nucleon and deuteron structure by means of spin-dependent electron scattering Pol. H Inclusive GpE/GpM N-∆: C2/M1 Vect-Pol. D GnM D-state GnE Te11 Tens-Pol. D T20

Event Selection

Neutron: Invariant Mass and Time of Flight Very clean quasi-elastic 2H(e,e’n)p spectrum Highly efficient proton veto (Wire Chambers)

Neutron: Invariant Mass and Time of Flight Very clean quasi-elastic 2H(e,e’n)p spectrum Highly efficient proton veto (Wire Chambers)

Target Spin Orientation

Target Spin Orientation nucleons Electron - Right Sector “Parallel” Kinematics * ≈ 0 electrons

Target Spin Orientation Neutrons electrons Electron - left sector “Perpendicular” Kinematics * ≈ 90 nucleons electrons

Kinematics and Observables Electrodisintegration of the Deuteron Quasi-elastic 2H(e,e’n) Beam + Target Polarized

BLAST Data Collection

BLAST Data Collection

BLAST Data Collection

Nucleon Form Factors Parameterizations However, the slope at Q^{2} predicted by the neutron charge radius is under predicted. The BLAST fit uses the same form with the slope at Q^{2}=0 fixed to the charge radius as such.

Proton Form Factors

Neutron Magnetic Form Factor GMn

Neutron Magnetic Form Factor GMn

Neutron Magnetic Form Factor GMn 1. New measurement technique. 2. Includes full deuteron structure. 3. Consistent with recent polarization and other data. 4. Provides a tighter fit to form factor in the low Q2 region.

Preliminary BLAST GMn Data N. Meitanis

Extraction of GnE Quasielastic Full Monte Carlo Simulation of the BLAST experiment Deuteron Electrodisintegration cross section calculations by H. Arenhövel Accounted for FSI, MEC, IC, RC Spin-perpendicular beam-target asymmetry AedV(90º,0º) shows high sensitivity to GnE

Extraction of GnE Quasielastic Full Monte Carlo Simulation of the BLAST experiment Deuteron Electrodisintegration cross section calculations by H. Arenhövel Accounted for FSI, MEC, IC, RC Spin-perpendicular beam-target asymmetry AedV(90º,0º) shows high sensitivity to GnE Compare measured AedV with BLASTMC, varying GnE

Systematic Errors Uncertainty of target spin angle 5% 12% per degree Beam-target polarization product 2.5% Radiative effects <1.0% Small helicity dependency Uncertainty of GnM 1.5% Model dependency <3% Effect of potential negligible Final state interaction reliable Total: 6.6%

PRELIMINARY GnE World Plot This shows the calculated Gen result obtained from measuring the vector asymmetry of quasi-elastic neutron scattering. Only 60% of the existing data are included in this calculation so the uncertainties should still come down. Also many systematics still have to be checked. The error bars include statistical and systematic errors. To extract Gen we have used Arenhovel’s calculations to calculate GMn and also to determine the effect of varying contributions of Gen. Will eventually use BLAST measurements of GMn and other results in a global fit. Hopefully we will be able to provide some useful results at low Q**2 to check the bump and for details of the pion cloud.

PRELIMINARY GnE World Plot V. Ziskin and E. Geis Preliminary result Only 50% of data, final data should reach 0.5 (GeV/c)2 Use Arenhovel’s calculations for GnM and contribution of GnE Need to combine with other BLAST measurements for global fit Provide low Q2 data Check bump Pion cloud PRELIMINARY This shows the calculated Gen result obtained from measuring the vector asymmetry of quasi-elastic neutron scattering. Only 60% of the existing data are included in this calculation so the uncertainties should still come down. Also many systematics still have to be checked. The error bars include statistical and systematic errors. To extract Gen we have used Arenhovel’s calculations to calculate GMn and also to determine the effect of varying contributions of Gen. Will eventually use BLAST measurements of GMn and other results in a global fit. Hopefully we will be able to provide some useful results at low Q**2 to check the bump and for details of the pion cloud. V. Ziskin and E. Geis

BLAST Fit to World Polarization Data Remarkable consistency of all modern polarization experiments! Here, I plot the contribution to the total value of GeN from the smooth dipole part in green and the bump part in blue. At low momentum transfer the parameterization is dominated by the bump part. At show distances the dipole part dominates, thus preserving pQCD scaling predictions. The gray bend around the total value of one sigma uncertainty of the fit, plotted here explicitly in percentage of the value of GeN. The uncertainty at Q^{2} is the uncertainty in charge radius measurement. Global fit determines GEn to better than ±7%

Nucleon Models Dispersion theory gives the best description Here, I plot the contribution to the total value of GeN from the smooth dipole part in green and the bump part in blue. At low momentum transfer the parameterization is dominated by the bump part. At show distances the dipole part dominates, thus preserving pQCD scaling predictions. The gray bend around the total value of one sigma uncertainty of the fit, plotted here explicitly in percentage of the value of GeN. The uncertainty at Q^{2} is the uncertainty in charge radius measurement. Dispersion theory gives the best description No theory describes GnE at low and high Q2 simultaneously

Conclusions & Outlook BLAST at MIT-Bates has measured at low Q2 the nucleon form factors using polarized electrons from vector-polarized hydrogen/deuterium. Very small systematic errors GnE overall known to ≈ 5% at Q2 < 1 (GeV/c)2 Dispersion theory gives the best description No theory describes GnE at low and high Q2 simultaneously Evidence for enhancement at low Q2 - role of pion cloud? With new precision data of T20 from BLAST and with improved A(Q2) new attempt to GnE determine from GQ ed elastic analysis: Mainz-Saclay discrepancy 8% in A  factor of 2 in GnE New measurements of A(Q2) at JLab (E-05-004) and MAMI Extension of GnE at high Q2 < 3.5 (GeV/c)2 (E-02-13) Proposal of BLAST@ELSA/Bonn Measure GnE for Q2 = 0.04-1.5 (GeV/c)2 with both vector-polarized 2H and polarized 3He