Measurement of the ratio of the proton form factors, GE/GM, at high momentum transfers and the question of scaling

Litt, J.; Buschhorn, G.; Coward, D. H.; DeStaebler, H.; Mo, Luke W.; Taylor, Richard E.; Barish, B. C.; Loken, S. C.; Pine, J.; Friedman, J.; Hartmann, G. C.; Kendall, Henry W.

doi:10.1016/0370-2693(70)90015-8

Cited by 159 publications

(90 citation statements)

References 18 publications

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“…Furthermore, referring to the open data points in the left panel of Fig. 1, one can see that the various Rosenbluth separation data sets [4][5][6][7][8][9] for the ratio µ p G Ep /G Mp , where µ p = 2.79 is the magnetic moment of the proton, are not consistent with one another for…”

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confidence: 98%

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New empirical fits to the proton electromagnetic form factors

et al. 2002

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Recent measurements of the ratio of the elastic electromagnetic form factors of the proton, GEp/GMp, using the polarization transfer technique at Jefferson Lab show that this ratio decreases dramatically with increasing Q 2 , in contradiction to previous measurements using the Rosenbluth separation technique. Using this new high quality data as a constraint, we have reanalyzed most of the world ep elastic cross section data. In this paper, we present a new empirical fit to the reanalyzed data for the proton elastic magnetic form factor in the region 0 < Q 2 < 30 GeV 2 . As well, we present an empirical fit to the proton electromagnetic form factor ratio, GEp/GMp, which is valid in the region 0.1 < Q 2 < 6 GeV 2 .The elastic electromagnetic form factors are crucial to our understanding of the proton's internal structure. Indeed, the differential cross section for elastic ep → ep scattering is described completely in terms of the Dirac and Pauli form factors, F 1 and F 2 , respectively, based solely on fundamental symmetry arguments. Further, the Sachs form factors, G Ep and G Mp , which are simply derived from F 1 and F 2 , reflect the distributions of charge and magnetization current within the proton.Until recently, the form factors of the proton have been determined experimentally using the Rosenbluth separation method [3], in which one measures elastic ep cross sections at constant Q 2 , and varies both the beam energy and scattering angle to separate the electric and magnetic contributions. In terms of the Sachs form factors, the differential cross section for elastic ep scattering has traditionally been written as, θ e is the in-plane electron scattering angle. For elastic ep scattering, the so-called nonstructure cross section, σ ns is given bywhere α em is the electromagnetic coupling constant, and E ′ (E) is the energy of the scattered (incident) electron. From the measured differential cross section, one typically derives a "reduced cross section", defined according towhere ǫ = {1 + 2(1 + τ ) tan 2 (θ e /2)} −1 is a measure of the virtual photon polarization. Equation 3 is known as the Rosenbluth formula, and shows that fits to reduced cross section measurements made at constant Q 2 but varying ǫ values may be used to extract both form factors independently.With increasing Q 2 , the reduced cross sections are increasingly dominated by the magnetic term G Mp ; at Q 2 ≈ 3 GeV 2 , the electric term contributes only a few percent of the cross section. Furthermore, referring to the open data points in the left panel of Fig. 1, one can see that the various Rosenbluth separation data sets [4][5][6][7][8][9] for the ratio µ p G Ep /G Mp , where µ p = 2.79 is the magnetic moment of the proton, are not consistent with one another for Q 2 > 1 GeV 2 . It is clear that a tremendous effort has gone into the analysis of these difficult experiments, however, one is forced to speculate that some of the experiments have underestimated the systematic errors. For example, the Rosenbluth experiments apply radiative correcti...

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confidence: 98%

“…[4][5][6][7]9,10]), we have reanalyzed these data using the aforementioned form factor ratio prescription. The net effect of this procedure is that the form factor ratio constraint fixes the ratio of the slope to the intercept of the graph of reduced cross section vs. ǫ.…”

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confidence: 99%

New empirical fits to the proton electromagnetic form factors

et al. 2002

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“…Earlier results generally indicated that the form factor ratio stays around 1 but with poor quality. World data of G M p from unpolarized measurements [1,3,14,15,5,7,9,10,11,12,13], using the Rosenbluth method, normalized to the dipole parameterization.…”

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confidence: 99%

High Precision Measurement of the Proton Elastic Form Factor Ratio at Low Q<sup>2</sup>

Zhan

2010

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Experiment E08-007 measured the proton elastic form factor ratio µ p G E /G M in the range of Q 2 = 0.3−0.7(GeV/c) 2 by recoil polarimetry. Data were taken in 2008 at the Thomas Jefferson National Accelerator Facility in Virginia, USA. A 1.2 GeV polarized electron beam was scattered off a cryogenic hydrogen target. The recoil proton was detected in the left HRS in coincidence with the elasticly scattered electrons tagged by the BigBite spectrometer. The proton polarization was measured by the focal plane polarimeter (FPP).In this low Q 2 region, previous measurement from Jefferson Lab Hall A (LEDEX) along with various fits and calculations indicate substantial deviations of the ratio from unity. For this new measurement, the proposed statistical uncertainty (< 1%) was achieved. These new results are a few percent lower than expected from previous world data and fits, which indicate a smaller G Ep at this region. Beyond the intrinsic interest in nucleon structure, the new results also have implications in determining the proton Zemach radius and the strangeness form factors from parity violation experiments.

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“…The use of analytic trajectories 05. The data for the proton magnetic form factor G p M are from [46] (open squares), [47] (open circles), [48] (solid stars), [49] (open stars), [50] (solid circles), [51] (solid squares), and those for the ratio G p E /G p M are from [52] (solid circles), [53] (open triangles) and [54] (solid triangles). Right panel: Ratio of the neutron magnetic form factor to the dipole form factor (right upper panel) and neutron electric form factor (right lower panel) with curve conventions as on the left panel.…”

Section: Numerical Estimatesmentioning

confidence: 99%

Duality, analyticity, andtdependence of generalized parton distributions

Jenkovszky

2006

Phys. Rev. D

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Basing upon dual analytic models, we present arguments in favor of the parametrization of generalized parton distributions (GPD) in the form ∼ (x/g 0 ) (1−x)α(t) , whereα(t) = α(t) − α(0) is the nonlinear part of the Regge trajectory and g 0 is a parameter, g 0 > 1. For linear trajectories it reduces to earlier proposals. We compare the calculated moments of these GPD with the experimental data on form factors and find that the effects from the nonlinearity of Regge trajectories are large. By Fourier-transforming the obtained GPD, we access the spatial distribution of protons in the transverse plane. The relation between dual amplitudes with Mandelstam analyticity and composite models in the infinite momentum frame is discussed, the integration variable in dual models being associated with the quark longitudinal momentum fraction x in the nucleon.⋆ e-mail address: jenk@bitp.kiev.ua IntroductionGeneralized parton distributions (GPD) [1,2,3] combine our knowledge about the onedimensional parton distribution in the longitudinal momentum with the impact-parameter, or transverse distribution of matter in a hadron or nucleus. It is an ambitious program to access the spatial distribution of partons in the transverse plane and thus to provide a 3-dimensional picture of the nucleon (nucleus) [4,5,6,7,8]. This program involves various approaches, including perturbative QCD, Regge poles, lattice calculations etc. (see Ref.[9] for reviews). The main problem is that, while the partonic subprocess can be calculated perturbatively, the calculation of GPDs require non-perturbative methods. GPDs enter in hard exclusive processes, such as deeply virtual Compton scattering (DVCS); however, they cannot be measured directly but instead appear in convolution integrals, that cannot be easily converted. Hence the strategy is to guess the GPD, based on various theoretical constraints, and then compare it with the data. In the first approximation, the GPD is proportional to the imaginary part of a DVCS amplitude, therefore, as discussed in [10], the knowledge (or experimental reconstruction) of the DVCS amplitude may partly resolve the problem, provided the phase of the DVCS amplitude is also known. In other words, a GPD can be viewed as the imaginary part of an antiquark-nucleon scattering amplitude, or a quark-nucleon amplitude in the u channel.Alternatively, one can extract [11,12], still in a model-dependent way, the nontrivial interplay between the x and t dependence of GPD from light-cone wave functionswhere ψ(x, k ⊥ ) is a 2-particle wave function (see, e.g., [13]) and t ≡ q ⊥ .In two recent papers [14,15] various forms of GPD for ξ = 0 were tested against the experimental data on the related form factors. The agreement with the data in Ref. [14] is impressive; in Ref.[15] the spatial distribution of partons in the transverse plane was also calculated. We pursue the approach of Refs. [14,15] by bringing more arguments coming from duality in favor of the parametrization for H(x, t) used in [14] and exploring how analyticity...

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Measurement of the ratio of the proton form factors, GE/GM, at high momentum transfers and the question of scaling

Cited by 159 publications

References 18 publications

New empirical fits to the proton electromagnetic form factors

New empirical fits to the proton electromagnetic form factors

High Precision Measurement of the Proton Elastic Form Factor Ratio at Low Q<sup>2</sup>

Duality, analyticity, andtdependence of generalized parton distributions

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