Advances in High Energy Physics

Volume 2014 (2014), Article ID 217389, 8 pages

http://dx.doi.org/10.1155/2014/217389

Research Article

Influence of Neutral Currents on Electron and Gamma Polarizations in the Process

N. V. Samsonenko,¹ Adamou Ousmane Manga,² Almoustapha Aboubacar,² and Aboubacar Moussa³

¹Department of Theoretical Physics, Faculty of Sciences, Russian Friendship University, 3 Ordjonikidze, Moscow 117923, Russia

²Department of Physics, Faculty of Sciences and Techniques, Abdou Moumouni University, 10662 Niamey, Niger

³Department of Mathematics and Computer Science, Faculty of Sciences and Techniques, Abdou Moumouni University, 10662 Niamey, Niger

Received 25 January 2014; Accepted 29 March 2014; Published 5 June 2014

Academic Editor: Duncan L. Carlsmith

Copyright © 2014 N. V. Samsonenko et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP³.

Abstract.

The differential cross section of electron inelastic scattering by nuclei followed by γ radiation is calculated using the multipole decomposition of the hadronic currents and by taking into account the longitudinal polarization of the initial electron and the circular polarization of the γ radiation. We performed the analysis of the angular and energy dependence of the degree of electron and photon polarization which can yield information on values of weak neutral currents parameters.

1. Introduction

For many years lepton scattering by nucleon and nuclei has been playing a key role in the determination of the nuclear electromagnetic form factors and in testing the standard model of electroweak interactions [1–4]. During the last decade there has been a considerable effort to measure parity-violating asymmetries in electron scattering [5–7]. These asymmetries can improve the precision of the determination of the mixing angle [8–11]. Electron scattering has also been used for the determination of the vector strange-quark matrix element [11–16].

In this work we study the influence of weak neutral currents parameters on the spin asymmetry coefficient of electron and photon polarization in the electron inelastic scattering by nuclei followed by gamma radiation. This process is illustrated by the following equation:

(1)

2. Differential Cross Section

In the first order perturbation theory the square of matrix element of the process (1) can be expressed as [17]

where and are the matrix elements of and transitions. , , and are, respectively, the spins of the initial, intermediate (excited), and the final states of the nucleus. Let us consider

the interaction Hamiltonian for the emission of gamma rays with polarization in the direction . Here . The magnetic and electric multipole operators and are defined in a system of coordinates where -axis is oriented along the momentum of the photon (Figure 3).

In order to define all the multipole operators in a system of coordinates where -axis is oriented along the transferred momentum we perform a rotation of the system of coordinates by angles and . So we obtain the relation [18]:

By using (3) and (4) and applying the Wigner-Eckart theorem the matrix element of the Hamiltonian can take the following form:

So the matrix element for the nuclear transition from the initial state to the intermediate state is given by

where

are the leptonic and hadronic currents. and are related, respectively, to the electromagnetic and weak interactions. is the transferred 4-momentum, is the fine structure constant, and is the Fermi coupling constant for weak interaction. The constants and take specific values depending on the process. In the Weinberg-Salam model, for electron or muon scattering processes we have according to [19] and .

The density of hadronic current is composed by vector and axial-vector currents with isoscalar () and isovector () components [18]:

where and are the linking constants whose values depend on the process considered. For the electromagnetic interactions , , and and the hadronic current is given by

In the case of processes with weak neutral currents, so the hadronic current is given by

In the Weinberg-Salam model, for scattering processes according to [19] we have

where is the Weinberg angle.

The square of the matrix element (7) for electron electroweak scattering by nuclei takes the following form:

where .

In (13) we neglect the term proportional to . In the ultrarelativistic longitudinally polarized electrons case we obtain the relation

where the tensor is given by

Here and ( and ) are the energy and the 4-momentum of the initial (final) electron; is the helicity of the initial electron. The factor takes the following values:

The differential cross section, calculated by multipole decomposition of matrix elements [19, 20] and by taking into account the circular polarization of the emitted photon, is given by the following formula:

where and indicate, respectively, the relative and total disintegration widths of the excited nucleus state and is Legendre polynomial. The angles and determine the photon direction, is photon helicity, and is the differential cross section of the electron scattering by unpolarized nuclei given by

where and are, respectively, the Mott cross section and the nuclear correction factor given by

where is the mass of the nucleus and is the atomic number.

The functions and are given by the following formulae:

Here are the leptonic functions and are the hadronic functions. The expressions of the hadronic and the leptonic functions by taking into account the longitudinal polarization of electrons are given in the Appendix. The quantum number is defined by the relation . The functions are defined as follows:

where

Hence is the gamma transition energy.

Quantum numbers and are given by

3. Study of the Transitions

As an example of transitions let us consider the following process:

(24)

So the quantum numbers , , and are defined as

The differential cross section of the process (24) which takes into account the longitudinal polarization of the electron and the circular polarization of the photon is given by the formula

where

The matrix elements , , , and , calculated in the shell model with a harmonic oscillator potential, are given by [21]:

where [19], , is the energy of the transition, and , where is the parameter of the harmonic oscillator.

Writing explicitly the electron and photon polarization, the differential cross section (26) takes the following form:

where

Let us consider now two coefficients of asymmetry which can be experimentally determined, that is, the electron polarization ratio and the photon circular polarization degree:

Assuming that the photon is emitted in the direction of the incident electron we have

and after averaging on the photon polarization, the asymmetry coefficient for the electron defined by (31) is given by

Averaging the differential cross section (29) on the electron polarization, the photon polarization degree defined by (32) takes the form

Figures 1(a) and 1(b) show the dependence on the diffusion angle of the asymmetry coefficient for electron and photon polarization degrees for three values of the energy for incident electron. Calculation is done by using from [8] and formulas (12) relative to the coupling constants of standard model.

(a)

(b)

Figure 1: (a) Asymmetry coefficient for the electron. (b) Photon polarization degree.

Figure 1(a) shows for  MeV and  MeV variation of asymmetry coefficient. This variation is greater for electron scattering around value of angle °. However, the photon degree of polarization undergoes greater variation for large values of scattering angle.

Figures 2(a)–2(f) represent the angular dependence of asymmetry coefficient for electron with energy values and  MeV and for different values of parameters and . Electron asymmetry coefficient is very sensitive to change of these parameters, but it could be sensitive to the change of which depend on these parameters in the standard model.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2: Asymmetry coefficient for the electron.

Figure 3: System of coordinates.

4. Conclusion

The differential cross section of the inelastic electron scattering by nuclei is calculated and the expressions of the asymmetries coefficients are obtained; their angular-energy dependence is analyzed and some results are carried out. The experimental study of the electron scattering processes accompanied by gamma radiation can play an important role in research on the parameters of the weak neutral currents.

Appendix

By taking into account the longitudinal polarization of electrons we obtain for the leptonic functions the following expressions:

The hadronic functions are given by the following formulae:

The coefficients and are given by the following formulae:

The functions , , , and , , , and ) are the matrix elements of the multipole vector (axial-vector) Coulomb, longitudinal, transverse magnetic, and transverse electric operators defined as follows:

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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