A Precision Measurement of GEp/GMp with BLAST Chris Crawford MIT Laboratory for Nuclear Science June 7, 2005 I’m reporting on our measurement of the proton form factor ratio which was recently completed at MIT/Bates.

Outline Introduction Experimental Setup Analysis Conclusion Formalism World Data Experiment overview Experimental Setup LDS polarized target BLAST detector Calibrations Analysis Cuts & Yields Asymmetry Extraction of mGE/GM Systematic errors Conclusion Results: mGE/GM Separation of GE, GM

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.

Form Factors of the Nucleon Nucleon current

Rosenbluth Separation Breit frame Nucleon Current Elastic Cross Section

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 1970. 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.

Polarization Transfer Recoil proton polarization Focal Plane Polarimeter recoil proton scatters off secondary 12C target Pt, Pl measured from φ distribution Pb, and analyzing power cancel out in ratio

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.

Theory and Models Direct QCD calculations Meson Degrees of Freedom pQCD scaling at high Q2 Lattice QCD Meson Degrees of Freedom Dispersion analysis, Höhler et al. 1976 Soliton Model, Holzwarth 1996 VMD + Chiral Perturbation Theory, Kubis et al. 2000 Vector Meson Dominance (VMD), Lomon 2002 QCD based constituent quark models (CQM) LF quark-diquark spectator, Ma 2002 LFCQM + CBM, Miller 2002 †Nucleon Electromagnetic Form Factors, Haiyan Gao, Int. J. of Mod. Phys. E, 12, No. 1, 1-40(Review) (2003)

Models Consistent with Polarized Data

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 – 0.9 (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.

Elastic Cross Section b = target spin angle w/r to the beam line

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.

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: 98 pb-1 D2: 126 pb-1+2005 run

Atomic Beam Source (ABS) Standard technology Dissociator & nozzle 2 sextupole systems 3 RF transitions 1 3 2 4 nozzle 6-pole MFT (2->3) Spin State Selection:

Laser Driven Source (LDS) Optical pumping & Spin Exchange Spincell design Target and Polarimeter Results

Spin-Exchange Optical pumping Spin Temperature Equilibrium (STE)

LDS Experimental Setup

Pictures of the LDS

Comparison of Polarized Targets

BLAST Detector Package Detector Requirements Definition of q e  2, e  .°, z  1 cm e/p/n/ separation PID: t  1, Čerenkov Optimize statistics Large Acceptance Asymmetry Super-ratios Symmetric Detector Polarized targets 1 m diameter in target region Zero field at target B-gradients  50 mG/cm

TOF Scintillators timing resolution: σ=350 ps velocity resolution: σ= 1% coplanarity cuts ADC spectrum

Cosmics TOF Calibration channels L 9 L 6 L 3 L 0 R 0 R 3 R 6 channels

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

Čerenkov Detectors 1 cm thick aerogel tiles Refractive index 1.02-1.03 White reflective paint 80-90 % efficiency 5" PMTs, sensitive to 0.5 Gauss Initial problems with B field Required additional shielding 50% efficiency without shielding

Drift Chambers 2 sectors × 3 chambers 954 sense wires 200μm wire resolution signal to noise ratio 20:1

NSED (Online Display)

Reconstruction Scintillators Wire chamber PID, DST timing, calibration hits, stubs, segments link, track fit PID, DST

Newton-Rhapson Track Fitter

Hyperbolic timedist function TDC

Linear T2D Calibration 72 33 28 MeV 12 MeV ~ 1mm resolution c2 D p (GeV/c)

WC Offsets/Resolution/Cuts pe qe fe ze pp qp fp zp

Comparison of Yields with MC

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

Single-asymmetry Method i = left,right sector j = Q2 bin (1..n) b = spin angle Single-asymmetry Method measure P first, use to calculate R model-dependent Super-ratio Method 2 equations in P, R in each Q2 bin j independent measure of polarization in each bin! 2n parameters Pj, Rj Global Fit Method fit for P, R1, R2, … from all Aij together model independent better statistics n+1 parameters can also fit for b

Extractions of m GE/GM

Single Asymmetry Extraction

Systematic Errors DQ2 (1.8%) Db (0.8%) comparison of qe and qp difference between left and right sectors most problematic appeal to TOF timing ! Db (0.8%) fieldmap: 47.1° ± 1° Hohler: 47.5° ± 0.8° Fit Method: 42° ± 3° (1st 7 bins) 48° ± 4° T20 analysis: 46.5° ± 3°

GE/GM Results

Extraction of GE and GM

GE and GM Results BLAST + World Data

Q2 Corrections from TOF

Conclusion 1st measurement of mGE/GM using double spin asymmetry 2 – 3.5× improvement in precision of mGE/GM at Q2 = 0.1– 0.5 GeV2 sensitive to the pion cloud is dip in GE around Q2=0.3 GeV2 real? systematic errors are being reduced