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Register a new userAxial form factor of the nucleon at large momentum transfers
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Motivated by the emerging possibilities to study threshold pion electroproduction at large momentum transfers at Jefferson Laboratory following the 12 GeV upgrade, we provide a short theory summary and an estimate of the nucleon axial form factor for large virtualities in the $Q^2 = 1-10~text{GeV}^2$ range using next-to-leading order light-cone sum rules.
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The charge form factor of $^$4He has been extracted in the range 29 fm$^{-2}$ $le Q^2 le 77$ fm$^{-2}$ from elastic electron scattering, detecting $^4$He nuclei and electrons in coincidence with the High Resolution Spectrometers of the Hall A Facility of Jefferson Lab. The results are in qualitative agreement with realistic meson-nucleon theoretical calculations. The data have uncovered a second diffraction minimum, which was predicted in the $Q^2$ range of this experiment, and rule out conclusively long-standing predictions of dimensional scaling of high-energy amplitudes using quark counting.
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J. P. B. C. de Melo
, R^omulo Moreira Moita
, Kazuo Tsushiman (Laboratorio de Fisica Teorica e Computacional
2016
The pion electromagnetic form factor is calculated at lower and higher momentum transfer in order to explore constituent quark models and the differences among those models. In particular, the light-front constituent quark model is utilized here to calculate the pion electromagnetic form factor at lower and higher energies. The matrix elements of the electromagnetic current, are calculated with both plus and minus components of the electromagnetic current in the light-front. Further, the electromagnetic form factor is compared with other models in the literature and experimental data.
It is well established that the nucleon form factors can be related to Generalized Parton Distributions (GPDs) through sum-rules. On the other hand, GPDs can be expressed in terms of Parton Distribution Functions (PDFs) according to Diehls model. In this work, we use this model to calculate polarized GPDs for quarks ($widetilde{H}_q$) using the available polarized PDFs obtained from the experimental data, and then study the axial form factor of nucleon. We determine parameters of the model using standard $chi^2$ analysis of experimental data. It is shown that some parameters should be readjusted, as compared to some previously reported values, to obtain better consistency between the theoretical predictions and experimental data. Moreover, we study in details the uncertainty of nucleon axial form factor due to various sources.
The parity violation programs at MIT-Bates, Jefferson Lab and Mainz are presently focused on developing a better understanding of the sea-quark contributions to the vector matrix elements of nucleon structure. The success of these programs will allow precise semi-leptonic tests of the Standard Model such as that planned by the QWeak collaboration. In order to determine the vector matrix elements, a good understanding of the nucleons axial vector form factor as seen by an electron, G^e_A, is also required. While the vector electroweak form factors provide information about the nucleons charge and magnetism, the axial form factor is related to the nucleons spin. Its Q2=0 value at leading order, g_A, is well known from nucleon and nuclear beta decay, and its precise determination is of interest for tests of CKM unitarity. Most information about its Q2 dependence comes from quasielastic neutrino scattering and from pion electroproduction, and a recent reanalysis of the neutrino data have brought these two types of measurements into excellent agreement. However, these experiments are not sensitive to additional higher order corrections, such as nucleon anapole contributions, that are present in parity-violating electron scattering. In this talk I will attempt to review what is presently known about the axial form factor and its various pieces including the higher order contributions, discuss the the various experimental sectors, and give an update on its determination through PV electron scattering.
A careful reanalysis of both Argonne National Laboratory and Brookhaven National Laboratory data for weak single pion production is done. We consider deuteron nuclear effects and normalization (flux) uncertainties in both experiments. We demonstrate that these two sets of data are in good agreement. For the dipole parametrization of $C_5^A(Q^2)$, we obtain $C_5^A(0)=1.19pm 0.08$, $M_A=0.94pm 0.03$ GeV. As an application we present the discussion of the uncertainty of the neutral current 1$pi^0$ production cross section, important for the T2K neutrino oscillation experiment.
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