1. A Precision Measurement of GEp/GMp with BLAST
Chris Crawford
MIT Laboratory for Nuclear Science
for the BLAST collaboration
I’m reporting on our measurement of the proton form factor ratio which was recently completed at MIT/Bates.

2. Introduction
GE,GM fundamental quantities describing charge/magnetization in the nucleon
Test of QCD based calculations and models
Provide basis for understanding more complex systems in terms of quarks and gluons
Probe the pion cloud
QED Lamb shift
The form factors are fundamental descriptions of the charge and magnetic distributions in the nucleon. They form a stringent test of QCD in the low energy regime, where calculations are extremely difficult due the non-perturbative nature. For example color anti-screening and confinement. But by studying the proton we are in a better position to understand more complex systems in terms of their underlying quark and gluon degrees of freedom. Also I’d like to point out that one of the largest sources of error in the QED Lamb Shift calculation is the radius of the proton, which can be determined by measuring G_e at very low Q^2.

3. World Data World Unpolarized Data
The spin ½ nucleon’s electromagnetic current has only 2 terms which conserve general symmetries, and Rosenbluth figured out in the 1950’s how two separate both of these from the single unpolarized cross section by varying the beam energy at fixed Q^2 point. Since then there has been a rich history of Rosenbluth extractions of G_e and G_m. I show here the unpolarized world data since But at cross section is dominated by G_m and high Q^2, which makes Rosenbluth separations above Q^2=1 very difficult. Plus, variation of the beam energy introduces extra uncertainties.

4. m GE/GM — World Data
The technology of polarized beam and polarized target or recoil polarimetry has make possible a new generation of high precision measurements of the form factor. Now one can vary spin degrees of freedom instead of the beam energy, and rely on the interference term between G_e and G_m in the polarized cross section. The first such measurement was done at Bates using a polarized beam and a focal plane polarimeter for the recoil proton. These measurements were repeated at JLab to higher Q^2 and better precision.

5. Form Factor Ratio @ BATES
Exploits unique features of BLAST
internal target: low dilution, fast spin reversal
large acceptance: simultaneously measure all Q2 points
symmetric detector: ratio measurement
Different systematics
also insensitive to Pb and Pt
no spin transport
Q2 = 0.1 – 0.8 (GeV/c) 2
input for P.V. experiments
structure of pion cloud
This experiment exploits many unique features of the BLAST spectrometer.
It uses an internal gas target, with low dilution and fast spin reversal. With the large acceptance, we can measure all Q^2 points simultaneously, and the symmetry of the detector allows for super-ratio measurements, which I will describe next.
This experiment has different systematics than recoil polarimetry, and is also insensitive to beam and target polarizations. We do not have to worry about spin transport effects of the proton in flight to the polarimeter.
Our highest Q^2 points overlap with the JLab data, but our results are to low in Q^2 to make meaningful comparisons with JLab. However, the low Q^2 data is important in the extraction of the proton charge radius, which is critical input into Lamb shift measurements. Also Lattice QCD is improving to the level where it can be compared with high precision data.

6. Asymmetry Super-ratio Method
Beam-Target Double Spin Asymmetry
Super-ratio
b = 45
I will now describe our technique. The experimental asymmetry depends on the beam and target asymmetries and the ratio of the polarized cross section over the unpolarized Rosenbluth part. There are two terms, one for longitudinal polarization to the q vector, and the other transverse term has G_E, not G_E^2.
If we measure both components of the polarized cross section at the same time and Q^2, then we can form a super-ratio where the polarization and Rosenbluth term cancel out. This is done by orienting the spin at 45 degrees, so that the right sector asymmetry is predominantly transverse while the left asymmetry is longitudinal.

7. Polarized Beam and Target
Storage Ring
E = 850 MeV
Imax=225 mA
Pb = 0.65
Internal ABS Target
60 cm storage cell
t = 4.91013 cm-2
Pt = 0.80
The experiment is being carried out in the South Hall Ring at MIT-Bates.
The ring stores a very intense, highly polarized beam at 850 MeV, with a snake to preserve the polarization, a Compton polarimeter, and spin-flipping capability.
There is an Atomic Beam Source embedded in the BLAST spectrometer, which provides a pure atomic Hydrogen target without entrance and exit windows for the beam. The ABS can alternate quickly between polarization states to further reduce systematics.
The luminosity is rather low, therefore our detector must have large acceptance.
isotopically pure internal target
high polarization, fast spin reversal
L = 3.1  1031 cm-2s-1
H2: 94 pb D2: 170 pb run

8. BLAST Here are some pictures of the various components. wire chambers
Cerenkov (TOF’s are behind)
ABS, target storage cell

9. TOF Scintillator Cuts TOF paddle, proton TOF paddle, electron
The time-of-flight scintillators already do a clean separation of elastic events. This is a plot of coincidence events of an electron in one of the 16 right sector paddles with a proton in the left. Along the elastic ridge the proton and electron have roughly 90^\deg separation all the way from forward scattering to the back.
TOF paddle, electron

10. Resolution and Yields
preliminary
TOF paddle #

11. Experimental Spin Asymmetry
context of high precision measurement:
bulk of data has been deuterium to date, hydrogen run planned for end of year.

12. Preliminary Results: m GE/GM

13. Extraction of GE and GM
BLAST + World Data

14. Conclusion 1st measurement of mGE/GM using double spin asymmetry
2 – 3.5£ improvement in precision of mGE/GM at Q2 = 0.1—0.8 GeV2
sensitive to the pion cloud
narrow “dip” structure in GE around Q2=0.3 GeV2
systematic errors are under investigation