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

SPIN 2004Vitaliy Ziskin

MIT

Measurement of GnE at

Low Momentum TransferMeasurement of Gn

E at Low Momentum Transfer

Scientific motivation (why Scientific motivation (why GGnnEE at low at low QQ22))

Methodology (how we measure Methodology (how we measure GGnnEE))

BLAST experimentBLAST experiment

Preliminary resultsPreliminary results

Discussion and a look aheadDiscussion and a look ahead


MIT

Why Measure GnE at Low Q2 Why Measure GnE at Low Q2

• GE(Q2=0)=0, rms charge

radius <r2>=-0.11fm2

• GnE at low Q2

is essential for parity violation experiments

Neutron charge density distribution is poorly known as compared to a proton


MIT

Why Measure GnE at Low Q2 (contd.)Why Measure GnE at Low Q2 (contd.)

• Not enough high precision data at low Q2


MIT

How to Measure GnEHow to Measure GnE

• No free neutron targets, 2H or 3He targets are used

• Cross section is dominated

by GnM not sensitive Gn

E

• Asymmetry measurements used instead

)1 Ted

ATd

hPVed

AVd

hPehATd

ATd

PVd

AVd

Pdpded

d


MIT

Neutron Vector Asymmetry, AV

ed(/2,0)Neutron Vector Asymmetry, AV

ed(/2,0)

),( ' CMpq

nM

nE

Ved GGneeA

),( ' CMpq

pM

pE

Ved GGpeeA

n

CMpq

),( q

d

(p)

p (n)

)',( pee )',( nee

200%

100%

0%


MIT

South Hall e- RingSouth Hall e- Ring

• 850 MeV polarized e-

• Stored or extracted modes

• 120 mA average current

• Compton polarimeter


MIT

Polarized Deuterium TargetPolarized Deuterium Target• Cell geometry: circular 15 mm x 600 mm

• Cell coating: Drifilm

• Cell temperature: T = 90K

• Target density: dt=6.0x1013 nucl cm-2 (D)

• Polarization Pz = ~0.72 (D)

• Magnet holding field: B= 500mT (D)

H2 and D2 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


MIT

BLAST DetectorBLAST Detector

• Toroidal spectrometer

• pe/pe = 3%

32o target angle

NeutronCounters

Target

Scintillators

CerenkovDetectors

Wire Chambers


MIT

Neutron DetectorsNeutron 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


MIT

Neutron ReconstructionNeutron Reconstruction

• Time of flight gamma-neutron separation

• Use gamma to measure time resolution of ~2 ns

• Neutron momentum resolution: ~5% (estimated)

nMnP

tnt

counts


MIT

Missing Mass and Momentum SpectrumMissing Mass and Momentum Spectrum

nEd

MmE

npqmP

22mpmEmM

• High signal to noise ratio

• 140k total neutrons after cuts

• <2% empty target contribution

• Some background from hydrogen not yet understood


MIT

Extracting GnE Extracting GnE

• BLASTMC calculation for different GnE from H.

Arenhovel (BONN+MEC+IC+FSI)

• Calculate 2 for each GnE by comparing experimental

AVed(90o,0) to BLASTMC

2(GnE) =

min

GnE + Gn

E) = min+1


MIT

Preliminary ResultsPreliminary Results

Q2(GeV/c)2 GnE Gn

E* 2

min/ndf

0.14 0.039 0.010 1.5

0.20 0.048 0.010 1.3

0.29 0.062 0.012 1.0

*statistical only

• 60 % of data has been analyzed for this conference

• 252 kC

• hPz = 0.475 +/- 0.010

•Assume dipole GnM=GD


MIT

Preliminary GnE World PlotPreliminary GnE World Plot


MIT

ConclusionConclusion Reconstruction is still work in progress, will affect Reconstruction is still work in progress, will affect

experimental asymmetry at large experimental asymmetry at large PPmm

Correction for radiative effects needs to be appliedCorrection for radiative effects needs to be applied

Better value of Better value of GGnnMM will be determined from a will be determined from a

global fit and simultaneous measurement at BLASTglobal fit and simultaneous measurement at BLAST

A study of model dependence is still neededA study of model dependence is still needed

More data still to be analyzedMore data still to be analyzed

Final results to be a available next yearFinal results to be a available next year


MIT

BLAST COLLABORATIONBLAST COLLABORATION

R. Alarcon, E. Geis, J. Prince, B. Tonguc, A. YoungArizona 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. XuDuke 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. ZwartMassachusetts 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. LeeUniversity of New Hampshire, Durham, NH 03824 J. RapaportOhio University, Athens, OH 45701 K. McIlhany, A. MosserUnited States Naval Academy, Annapolis, MD 21402 J. F. J. van den Brand, H. J. Bulten, H. R. PoolmanVrije Universitaet and NIKHEF, Amsterdam, The Netherlands W. Haeberli, T. WiseUniversity of Wisconsin, Madison, WI 53706


MIT

Extracting Gen (Q2=0.14 GeV2/c2) Extracting Gen (Q2=0.14 GeV2/c2)

014.0055.0)14.02( QMnG

EnG

5.1/ ndf


MIT

Extracting Gen (Q2=0.20 GeV2/c2)Extracting Gen (Q2=0.20 GeV2/c2)

016.008.0)20.02( QMnG

EnG

3.1/ ndf


MIT

Extracting Gen (Q2=0.29 GeV2/c2)Extracting Gen (Q2=0.29 GeV2/c2)

025.0124.0)29.02( QMnG

EnG

0.1/ ndf

Documents

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