Precision Measurement of the Proton Elastic Form Factor ...

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Precision Measurement of the Proton Elastic Form Factor Ratio at Low Q 2 Nucleons Form Factors Recoil polarimetry World data New results Impact Outlook and summary Shalev Gilad, MIT

Transcript of Precision Measurement of the Proton Elastic Form Factor ...

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Precision Measurement of the Proton Elastic Form Factor Ratio at Low Q2 !

•  Nucleons Form Factors!

•  Recoil polarimetry!

•  World data!

•  New results!

•  Impact!

•  Outlook and summary!

Shalev Gilad, MIT!

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•  Dirac and Pauli form factors: F1 , F2

single photon exchange (Born approximation)

Jhadronicµ = eu ( ′ p )[γ µF1(Q

2) +iσ µνqν2M

F2(Q2)]u(p)

Q2 = −q2

• As theory for Strong force, QCD has been tested well in the asymptotic region, understanding hadron structure in confinement region still challenging!

• Pioneered by Hofstadter et. al at Stanford in 1950s, first proton form factor measurement reported in 1955!

Dirac and Pauli Form Factors!

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Sachs Form Factors�

•  Early experiments found ~ dipole form (Q2 < 2 GeV2), naively corresponds to an exponential shape in space

GD (Q2) = (1+

Q2

0.71GeV 2 )−2

µPGE

GM

=1

•  Linear combination of F1 and F2!

•  Fourier transform of the charge (magnetization) densities in the Breit frame at the non-relativistic limit!

Electric: Magnetic:�

•  Rosenbluth separation!Hard to determine GE at large Q2, GM at low Q2 (except at θe’≈90°)!

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Rosenbluth vs. Polarimetry

P.A.M.Guichon and M.Vanderhaeghen,  PRL 91, 142303 (2003) 

M.K.Jones, et al., PRL 84, 1398 (2000) O.Gayou, et al., PRL 88, 092301 (2003) I.A.QaHan, et al., PRL 94, 142301 (2005) 

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Recoil Polarimetry�

l0Pt = −2 τ (1+ τ )GEGM tanθe2

l0Pl =Ee + Ee '

Mτ(1+ τ)GM

2 tan2 θe2

GE

GM

= −PtPl(Ee + Ee ' )2M

tanθe2

•  Direct measurement of form factor ratios by measuring the ratio of the transferred polarization Pt and Pl !

Advantages: !•  Only one measurement is needed

for each Q2!

•  Much better precision than a cross section measurement!

•  Complementary to cross section measurements!

•  Discrepancy between Rosenbluth and polarized measurement, mostly explained by 2-γ exchange!(J. Arrington, et al., Phys. Rev. C 76 035205 (2007)) �

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Form Factors at Low Q2 �

rE ,M2 =

−6GE ,M (0)

ddQ2 GE ,M (Q

2)

Q 2= 0

J. Friedrich and Th. Walcher, Eur. Phys. J. A 17, 607 (2003) �

•  2003 – Fit by Friedrich & Walcher !•  Smooth dipole form + “bump & dip”!•  All four FFs exhibit similar structure at small momentum transfer (Q2 ~ 0.25 GeV2)!•  Interpretation: effect of pion cloud!

• Small Q2 → larger length scale, closely related to proton size!

•  Improved EMFFs:!• Strange form factors through PV!• Proton Zemach radius and hydrogen hyperfine splitting!• Isoscalar and isovector form factors for Lattice QCD !• Proton RMS radius!

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World Data at Low Q2 �

• Bates BLAST result consistent with 1!

Crawford et al., Phys. Rev. Lett 98 052301 (2007)!

• Substantial deviation from unity observed in LEDEX – smaller GE!

G. Ron et al., Phys. Rev. Lett 99 202002 (2007)!

• Both data inconsistent with F&W fit!

• Complementary to high precision XS measurement at Mainz (Q2~ 0.003 – 1 GeV2)!

• New dedicated experiment JLAB E08-007!

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Ee: 1.192GeV Pb: ~83% 

BigBite 

•  Δp/p0: ± 4.5% , •  out-of-plane: ± 60 mrad •  in-plane: ± 30 mrad •  ΔΩ: 6.7msr •  QQDQ •  Dipole bending angle 45o •  VDC+FPP •  Pp : 0.55 ~ 0.93 GeV/c

LHRS�

E08-007: Low Q2 GEp/GMp!

• A high precision (<1%) survey of the proton FF ratio • 8 Q2 data points: 0.3 ~ 0.7 (GeV/c)2

•  Non-focusing Dipole • Big acceptance.

• Δp: 200-900MeV •  ΔΩ: 96msr

•  PS + Scint. + SH

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Focal Plane Polarimeter (FPP)!

•  Left-right asymmetry gives the vertical component while the up-down asymmetry gives the horizontal component�

• Need well determined scattering azimuthal angle, , chamber alignment checked with straight through data�

φ fpp

Carbon doors

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Systematic Budget�

•  Spin transport: OPTICS and COSY---major uncertainty (0.7 ~ 1.2 %)!

•  Others negligible: FPP alignment, Al end cap contamination, VDC reconstruction, spectrometer settings, beam energy, charge asymmetry, pion contamination, etc.!

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E08-007 Final Results�

•  Slow decrease with Q2. A few percent below previous data, fits!

•  No obvious indication of “Structure”, inconsistent with F&W fit �

•  Agreement with independent analysis of Paolone et al. at 0.8 GeV2 �

•  Suggests that R = μpGEp/GMp < 1 even at low Q2 

X-H. Zhan, MIT Ph.D. thesis (2010), to be submitted 

AMT - W. Melnitchouk J. Arrington and J.A. Tjon, Phys. Rev. C 76,035205 (2007)!J. Arrington and I. Sick. Phys. Rev. C, 76, 035201 (2007)!

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Comparison to Previous Data�

•  LEDEX re-analysis consistent with new data!

•  No current plans to re-examine BLAST data�

•  Agreement with independent analysis of Paolone et al. at 0.8 GeV2 �

•  Indictions of cut dependence in GEp-I point !

X-H. Zhan, MIT Ph.D. thesis (2010), to be submitted 

•  Disagreement with GEp-I, BLAST, LEDEX!

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Comparison with Models�

Boffi – Chiral consDtuent quark model Faessler – Lorentz covariant chiral quark model Belushkin, Lomon – VMD Miller – Light front cloudy bag model 

 No model consistent with data!!

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World Data with Polarization�

Note: LEDEX re-analysis!## Examination of Gep-I, Gep-II!

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Global Fits�

•  Combined global fits (John Arrington).!•  AMT fit (black) : include all previous

data with TPE correction!

•  New fit (red) : same procedure, includes new data, remove lowest point of GEp-I, (and highest point of LEDEX)!

•  Preliminary fits suggest lower GE (~2%), higher GM (0.5%)!

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Impact I – Strangeness Form Factors by PV�

•  Asymmetry arises from the interference between EM and neutral weak currents!

•  Rely on knowledge of EMFFs

•  With New FF parameterization, HAPPEX III results shift ~ 0.5σ �

Q2� ΔA� ΔA/σ � ΔA/A� Exp.�

0.38� ‐0.178� 0.42� 1.6%� G0 FWD �

0.56� ‐0.347� 0.50� 1.6%� G0 FWD �

1.0 � ‐0.414� 0.30� 0.8%� G0 FWD �

0.50� ‐0.299� 0.50� 1.7%� HAPPEX III�

0.231� +0.038� 0.12� 0.2%� G0 BCK�

0.65� 0.142� 0.14� 0.3%� G0 BCK�

Table: Difference in the extracted asymmetries. �

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Impact II – Proton Zemach Radius�

Carlson, Nazaryan, and Griffioen, arXiv:0805.2603v1 (2009)

FFs� rz (fm) � Δz � year�

Dipole � 1.025� ‐39.29� ‐ �

FW � 1.049� ‐40.22� 2003�

Kelly� 1.069� ‐40.99� 2004�

AS � 1.091� ‐41.85� 2007�

AMT� 1.080� ‐41.43� 2007�

New fit� 1.075� ‐41.21� 2009�

•  FFs at Low Q2 (<1 GeV2) accounts for >70% of rZ, and also dominate the uncertainty�

Hyperfine splitting of hydrogen ground state!

•  Leading theoretical uncertainty!

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Impact III – Isoscalar & Isovector FFs!

•  Isoscalar & Isovector FFs (important for Lattice QCD):!

•  Change in FFs by using new parameterization vs. old parameterization�

FJS =

12(FJ

p + FJn )

FJV =

12(FJ

p − FJn )

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Impact IV - e-d Elastic Cross Section!

dσdΩ

=σMott ∗′ E

E∗[A(Q2) + B(Q2)tan2(θ /2)]

A(Q2) =GC2 (Q2) +

89η2GQ

2 (Q2) +23ηGM

2 (Q2)

B(Q2) =43η(1+η)GM

2 (Q2)

Where!

and!

η =Q2 /4md2

At low Q2, elastic e-d cross section depends mainly on GEp !New results change model cross sections by a few percent! !

Small except at θ≈180°!

Very small at low Q

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Impact V – Proton RMS Radius!

New Global fit with TPE (Arrington):!

<rp2> = 0.873(14) fm!

3% below previous value (Sick): 0.897(18) fm!CODATA(2006) value: 0.877(7) fm  

If R=1 as Q2 approaches zero, yields 0.015 fm change in charge radius!

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Near Future Outlook

 Phase-II (pol. Target - 2012)!– Extract R down to Q2=0.015!– Good overlap with Phase-I, BLAST!– First precise extraction of magnetic radius!

– Linear approach to Q2=0 ?!•  ∼3% smaller magnetic radius!•  No region where magnetization,

charge are simply sum of quarks!•  Disagreement with muonic

hydrogen radius?!

LEDEX re-analysis almost complete; consistent with X-H. Zhan et al.!

  New GEn measurement   One data point was measured parasitically in Hall A during experiment E05-102.

  Data in analysis

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Summary�

•  Nucleon FFs are fundamental quantities describing the nucleon internal structure!

•  A new high precision measurement was conducted in Jefferson Lab Hall A at low Q2, new data strongly deviate from unity, systematically lower than previous world data!

•  While adding further constraints on theoretical models, the new high precision data also impact determination of other physics quantities: proton Zemach radius, strange form factor through PV, proton RMS radius etc.!

•  Near-future experiment with polarized target accessing very low Q2:!

 Precisely determine magnetic radius for the first time!

 Important for precisely determining proton RMS radius, check possible disagreement with determination from muonic hydrogen data!

 Help to settle disagreement between blast (pol. target) and JLAB (recoil polarization) data�

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E08-007 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�

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Thank you!

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Acknowledgements �

J. Arrington, D. Higinbotham, J. Glister, R. Gilman, S. Gilad, E. Piasetzky, M. Paolone, G. Ron, A. Sarty, S.

Strauch and the entire E08-007 collaboration &

Jefferson Lab Hall A Collaboration�

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Back up slides 

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Future Outlook�

•  Second half of the experiment (DSA) is tentatively scheduled in early 2012�

•  Opportunity to see the FFR behavior at even lower Q2 (0.015-0.4 GeV2) 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.

•  E08007 analysis finalized. •  Publication in preparation. •  Updated paper for LEDEX (G. Ron et al.) in preparation.

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Elastic Events Selection�

•  HRS acceptance cut: •  out of plane: +/- 60 mr •  in plane: +/-30 mr •  momentum: +/- 0.04 (dp/p0) •  reaction vertex cut�

•  Other cuts: •  Coin. Timing cut •  Coin. event type (trigger) •  single track event •  dpkin (proton angle vs. momentum)

•  FPP cuts: •  scattering angle θfpp 5o ~ 25o •  reaction vertex (carbon door) •  conetest cut

proton dpkin�

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Focal Plane Asymmetry�

•  Detection probability at focal plane with azimuthally angle �

•  Helicity difference: �

•  By dipole approximation:

(K: kinemaDc factor)�

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Spin Transport in HRS (COSY)�

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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.�

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COSY Spin Precession Matrix�

•  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: 0.001 •  y_tg: 0.001 m •  ph_tg: 0.7~1.2 mrad •  th_tg: 1 mrad

transport variables�

target variables Set #0�

OpDcs matrix�

COSY reverse map�

COSY spin map�

target variables Set #1�

SP matrix Sij 

PolarizaDon at FPP�

PolarizaDon at Target�

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Individual Form Factors�

•  With the extract ratio constraint, refit the world reduced cross section data.�

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Extraction of Polarization�

•  Full spin precession by COSY: •  differential algebra-based. •  defines the geometry and related setup of magnets.

target frame�focal plane�

•  Weighted-sum:�

•  efficiency cancels with different beam helicity�

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Impacts III �

•  Isoscalar & Isovector FFs (important for Lattice QCD):

•  Plots show fractional change in IS and IV FFs by using the new parameterization vs. the old parameterization.�

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