High Precision Measurement of the Proton Elastic Form Factor Ratio at Low Q 2 Xiaohui Zhan Argonne National Lab

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1 High Precision Measurement of the Proton Elastic Form Factor Ratio at Low Q Xiaohui Zhan Argonne National Lab A B C 1 th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon Introduction E Analysis Results & Discussions Future Outlook & Summary 1

2 Electron Elastic Scattering Formalism Pioneered by Hofstadter et. al at Stanford in 1950s, first proton form factor measurement reported in As theory for Strong force, QCD has been tested well in the asymptotic region, understanding hadron structure in confinement region still challenging. Dirac and Pauli form factors: F 1, F J hadronic Q q eu( p)[ F i q M 1 ( Q ) F ( Q )] u( p) single photon exchange (Born approximation) d d Mott 1 1 ( Q ) [ F ( Q ) ( F1 ( Q ) F ( Q )) tan { F 1 e ]}

3 Linear combination of F 1 and F, Fourier transform of the charge (magnetization) densities in the Breit frame at non relativistic limit. d Mott [ GE G d 1 Electric: GE F1 F Magnetic: G F F 1 M Early experiments found ~ dipole form (Q < GeV ), naively corresponds to an exponential shape in space. G D ( Q G E P GM ) (1 1 M 1 Q 0.71GeV Sachs Form Factors ) ] 3

4 Direct measurement of form factor ratios by measuring the ratio of the transferred polarization P t and P l. Recoil Polarimetry l l 0 0 P G G E l M t P Ee E M Pt ( E P l (1 ) G G e' (1 ) G E M e e' E M e tan M ) e tan tan e Advantages: Only one measurement is needed for each Q. Much better precision than a cross section measurement. Complementary to XS measurements. Famous discrepancy between Rosenbluth and polarized measurement, mostly explained by -γ exchange. (J. Arrington, et al., Phys. Rev. C (007)) 4

5 FFs at Low Q Small Q larger length scale, closely related to the proton size. J. Friedrich and Th. Walcher, Eur. Phys. J. A 17, 607 (003) 003 Fit by Friedrich & Walcher Eur. Phys. J. A17, 607 (003): Smooth dipole form + bump & dip All four FFs exhibit similar structure at small momentum transfer (Q ~ 0.5 GeV ). Proposed interpretation: effect of pion cloud. Improved EMFFs: Strange form factors through PV Proton Zemach radius and hydrogen hyperfine splitting Proton charge RMS radius. r E, M G 6 (0) E, M d dq G E, M ( Q ) Q 0 5

6 World Data Bates BLAST result consistent with 1. Crawford et al., Phys. Rev. Lett (007) Substantial deviation from unity is observed in LEDEX (Ron et al.). Both data inconsistent with F&W fit. New dedicated experiment E Complementary to the high precision XS measurement at Mainz (Q ~ GeV ). 6

7 E08-007: Low Q GEp LHRS E e : 1.19GeV P b : ~83% Δp/p0: ± 4.5%, out-of-plane: ± 60 mrad in-plane: ± 30 mrad ΔΩ: 6.7msr QQDQ Dipole bending angle 45 o VDC+FPP P p : 0.55 ~ 0.93 GeV/c BigBite A high precision (<1%) survey of the proton FF ratio. 8 Q data points: 0.3 ~ 0.7 (GeV/c). Non-focusing Dipole Big acceptance. Δp: MeV ΔΩ: 96msr PS + Scint. + SH 7

8 Detect scattered electrons. BigBite Spectrometer Scintillator Only elastic-peak blocks were in the trigger. Background minimized with tight elastic cut. e Pre-shower Shower 8

9 HRS acceptance cut: out of plane: +/- 60 mr in plane: +/-30 mr momentum: +/ (dp/p 0 ) reaction vertex cut Elastic Events Selection proton dpkin FPP cuts: scattering angle θ fpp 5 o ~ 5 o reaction vertex (carbon door) conetest cut Other cuts: Coin. Timing cut Coin. event type (trigger) single track event dpkin (proton angle vs. momentum) 9

10 Focal Plane Polarimeter (FPP) Carbon doors Left-right asymmetry gives the vertical component while the updown asymmetry gives the horizontal component. Need well determined scattering azimuthal angle fpp, chamber alignment checked with straight through data. 10

11 Detection probability at focal plane with azimuthally angle fpp f 1 [1 A y ( fpp )( P Helicity difference: f diff f 1 C Ay P tan P f fpp y fpp x ( P 1 [ Ay ( P fpp x ) ( P fpp x fpp x fpp y ) sin( sin( Focal Plane Asymmetry fpp fpp ) P fpp y ) P fpp y cos( fpp cos( ))] fpp ))] C cos( ) By dipole approximation: R G p G E M P sin P fpp x fpp y K (K: kinematic factor) 11

12 Spin Transport in HRS (COSY) 1

13 Systematic Budget Spin transport: OPTICS and COSY---major uncertainty (0.7 ~ 1. %) Others negligible: FPP alignment, Al end cap contamination, VDC reconstruction, spectrometer settings, beam energy, charge asymmetry, pion contamination, etc. 13

14 E Final Results Agreement with independent analysis of Paolone et al. at 0.8 GeV. Slow decrease with Q. A few percent below typical expectations. No obvious indication of Structure, inconsistent with F&W fit. No obvious trend to rise quickly to unity at the lowest Q point. 14

15 Comparison with Models 15

16 Results with World Polarization Data 16

17 Global Fits Combined global fits (John Arrington). AMT fit (black) : include all previous data with TPE correction. New fit (red) : same procedure, include new data. Preliminary fits suggest lower G E (~%). 17

18 Impacts I Strangeness form factor by PV: asymmetry arises from the interference between EM and neutral weak current. Q ΔA ΔA/σ ΔA/A Exp % G0 FWD % G0 FWD % G0 FWD Rely on knowledge of EMFFs. With New FF parameterization, HAPPEX III results shift ~ 0.5σ % HAPPEX III % G0 BCK % G0 BCK Table: Difference in the extracted asymmetries. 18

19 Impacts II Proton Zemach radius: E ( 1 ) E hfs QED p hvp p vp p weak S p F r S Z Z p R pol, Z memp Z m m 4 dq [ G ( ) ( )/(1 ) 1] 0 E Q GM Q p Q e p r Z FFs at Low Q (<1 GeV ) accounts for >70% of r Z, and also dominate the uncertainty. Carlson, Nazaryan, and Griffioen, arxiv: v1 (009) FFs r z (fm) Δz year Dipole FW Kelly AS AMT New fit

20 Future Outlook E08007 analysis finalized. Publication in preparation. Updated paper for LEDEX (G. Ron et al.) in preparation. Second half of the experiment (DSA) is tentatively scheduled in early 01 v zcosθ'g + v sinθ 'cosφ'g EG Aphys = M x εg + τg / ε1 + τ Ep Mp M Opportunity to see the FFR behavior at even lower Q ( GeV ) region. Third independent measurement, direct comparison with BLAST, examine any unknown systematic errors for previous measurements. Challenges: Solid polarized proton target & effect of target field to septum magnets. 0

21 Summary Nucleon FFs are fundamental quantities describing the nucleon internal structure, and has been a longstanding subject of interest in nuclear and particle physics. pqcd not applicable at low momentum transfer region, precision FF measurements are needed for all the experimental accessible region to test various models. A new high precision measurement was conducted in Jefferson Lab Hall A at low Q, new results strongly deviate from unity, systematically lower than previous world data. While adding further constraints on various models, high precision data also have impacts to other physics quantities: proton Zemach radius, strange form factor through PV, etc. Future experiments accessing extremely lower Q are necessary, more unexpected results? 1

22 Acknowledgements J. Arrington, D. Higinbotham, J. Glister, R. Gilman, S. Gilad, E. Piasetzky, M. Paolone, G. Ron, A. Sarty, S. Strauch and the entire E collaboration & Jefferson Lab Hall A Collaboration

23 E Collaboration Argonne National lab Jefferson Lab Rutgers University St. Mary s University Tel Aviv University UVa CEN Saclay Christopher Newport University College of William & Mary Duke University Florida International University Institut de Physique Nuclaire d Orsay Kent State University MIT Norfolk State University Nuclear Research Center Negev Old Dominion University Pacific Northwest National Lab Randolph-Macon College Seoul National University Temple University Universite Blaise Pascal University of Glasgow University of Maryland University of New Hampshire University of Regina University of South Carolina 3

24 Thank you! 4

25 Back up slides 5

26 Spin Transport in HRS Binning test for graphical cut. A rough check for existence of any possible background under elastic peak. No obvious indication of dependence on such variable. 6

27 Optics matrix COSY Spin Precession Matrix target variables Set #0 Polarization at FPP transport variables COSY spin map SP matrix S ij COSY reverse map target variables Set #1 Polarization at Target Different SP matrix were generated by changing the default settings in COSY: dipole radius, drift distances, quadrupoles alignment central bending angle: 5.5 mrad use COSY transport map to reconstruct target variables Uncertainties on target variables (OPTICS): dp: y_tg: m ph_tg: 0.7~1. mrad th_tg: 1 mrad 7

28 Individual Form Factors With the extract ratio constraint, refit the world reduced cross section data. 8

29 Extraction of Polarization Full spin precession by COSY: differential algebra-based. defines the geometry and related setup of magnets. S ij C klmnp ij k, l, m, n, p k l x y m n p focal plane target frame Weighted-sum: efficiency cancels with different beam helicity 9

30 Impacts III Isoscalar & Isovector FFs (important for Lattice QCD): F s i 1 p n v p F ), n i Fi Fi ( Fi 1 ( Fi Plots show fractional change in IS and IV FFs by using the new parameterization vs. the old parameterization. ) 30

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