1. SPIN 2004 Vitaliy Ziskin MIT Measurement of G n E at Low Momentum Transfer  Scientific motivation (why G n E at low Q 2 )  Methodology (how we measure G n E )  BLAST experiment  Preliminary results  Discussion and a look ahead

2. SPIN 2004 Vitaliy Ziskin MIT Why Measure G n E at Low Q 2 G E ( Q 2 =0)=0, rms charge radius =-0.11fm 2 G n E at low Q 2 is essential for parity violation experiments Neutron charge density distribution is poorly known as compared to a proton

3. SPIN 2004 Vitaliy Ziskin MIT Why Measure G n E at Low Q 2 (contd.) Not enough high precision data at low Q 2

4. SPIN 2004 Vitaliy Ziskin MIT How to Measure G n E No free neutron targets, 2 H or 3 He targets are used Cross section is dominated by G n M not sensitive G n E Asymmetry measurements used instead

5. SPIN 2004 Vitaliy Ziskin MIT Neutron Vector Asymmetry, A V ed (  /2,0) ),( '  CM pq n M n E V ed GG n eeA  ),( '  CM pq p M p E V ed GG p eeA  n CM pq  ),(q  d (p) p (n) )',(pee )',(nee 200% 100% 0%

6. SPIN 2004 Vitaliy Ziskin MIT South Hall e - Ring 850 MeV polarized e - Stored or extracted modes 120 mA average current Compton polarimeter

7. SPIN 2004 Vitaliy Ziskin MIT Polarized Deuterium Target Cell geometry: circular 15 mm x 600 mm Cell coating: Drifilm Cell temperature: T = 90K Target density: d t =6.0x10 13 nucl cm -2 (D) Polarization P z = ~0.72 (D) Magnet holding field: B= 500mT (D)  H 2 and D 2 target gases  Beam sees no foils  Low target density  Low-energy recoils  High beam currents  High polarization  Rapid reversal  Easy to orient I n t e r n a l T a r g e t T e c h n i q u e

8. SPIN 2004 Vitaliy Ziskin MIT BLAST Detector Toroidal spectrometer  p e /p e = 3%  32 o target angle Neutron Counters Target Scintillators Cerenkov Detectors Wire Chambers

9. SPIN 2004 Vitaliy Ziskin MIT Neutron Detectors Six walls of plastic scintillator bars (Bicron-408) Each scintillator is equipped with a flasher for timing Total estimated neutron efficiency from effective material thickness: ~30% (right), ~10% (left) Distance from target is 3.5 to 5.5 m Double charged particle veto: TOF + Wire Chamber

10. SPIN 2004 Vitaliy Ziskin MIT Neutron Reconstruction Time of flight gamma-neutron separation Use gamma to measure time resolution of ~2 ns Neutron momentum resolution: ~5% (estimated) counts

11. SPIN 2004 Vitaliy Ziskin MIT Missing Mass and Momentum Spectrum High signal to noise ratio 140k total neutrons after cuts <2% empty target contribution Some background from hydrogen not yet understood

12. SPIN 2004 Vitaliy Ziskin MIT Extracting G n E BLASTMC calculation for different G n E from H. Arenhovel (BONN+MEC+IC+FSI) Calculate  2 for each G n E by comparing experimental A V ed (90 o,0) to BLASTMC   2 (G n E ) =   min     G n E +  G n E ) =   min +1

13. SPIN 2004 Vitaliy Ziskin MIT Preliminary Results Q 2 (GeV/c) 2 GnEGnE  G n E *  2 min /ndf 0.140.0390.0101.5 0.200.0480.0101.3 0.290.0620.0121.0 * statistical only 60 % of data has been analyzed for this conference 252 kC hP z = 0.475 +/- 0.010 Assume dipole G n M = G D

14. SPIN 2004 Vitaliy Ziskin MIT Preliminary G n E World Plot

15. SPIN 2004 Vitaliy Ziskin MIT Conclusion  Reconstruction is still work in progress, will affect experimental asymmetry at large P m  Correction for radiative effects needs to be applied  Better value of G n M will be determined from a global fit and simultaneous measurement at BLAST  A study of model dependence is still needed  More data still to be analyzed  Final results to be a available next year

16. SPIN 2004 Vitaliy Ziskin MIT BLAST COLLABORATION R. Alarcon, E. Geis, J. Prince, B. Tonguc, A. Young Arizona State University, Tempe, AZ 85287 J. Althouse, C. D’Andrea, A. Goodhue, J. Pavel, T. Smith, Dartmouth College, Dartmouth, NH D. Dutta, H. Gao, W. Xu Duke University Durham, NC 27708-0305 H. Arenhövel, Johannes Gutenberg-Universität, Mainz, Germany T. Akdogan, W. Bertozzi, T. Botto, M. Chtangeev, B. Clasie, C. Crawford, A. Degrush, K. Dow, M. Farkhondeh, W. Franklin, S. Gilad, D. Hasell, E. Ilhoff, J. Kelsey, H. Kolster, M.Kohl, A. Maschinot, J. Matthews, N. Meitanis, R. Milner, R. Redwine, J. Seely, S. Sobczynski, C. Tschalaer, E. Tsentalovich, W. Turchinetz, Y. Xiao, C. Zhang, V. Ziskin, T. Zwart Massachusetts Institute of Technology, Cambridge, MA 02139 and Bates Linear Accelerator Center, Middleton, MA 01949 J. Calarco, W. Hersman, M. Holtrop, O. Filoti, P. Karpius, A. Sindile, T. Lee University of New Hampshire, Durham, NH 03824 J. Rapaport Ohio University, Athens, OH 45701 K. McIlhany, A. Mosser United States Naval Academy, Annapolis, MD 21402 J. F. J. van den Brand, H. J. Bulten, H. R. Poolman Vrije Universitaet and NIKHEF, Amsterdam, The Netherlands W. Haeberli, T. Wise University of Wisconsin, Madison, WI 53706

17. SPIN 2004 Vitaliy Ziskin MIT Extracting G e n (Q2=0.14 GeV 2 /c 2 )

18. SPIN 2004 Vitaliy Ziskin MIT Extracting G e n (Q2=0.20 GeV 2 /c 2 )

19. SPIN 2004 Vitaliy Ziskin MIT Extracting G e n (Q2=0.29 GeV 2 /c 2 )