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Rarita-Schwinger equation : ウィキペディア英語版
Rarita–Schwinger equation
In theoretical physics, the Rarita–Schwinger equation is the
relativistic field equation of spin-3/2 fermions. It is similar to the Dirac equation for spin-1/2 fermions. This equation was first introduced by William Rarita and Julian Schwinger in 1941.
In modern notation it can be written as:〔S. Weinberg, "The quantum theory of fields", Vol. 3, Cambridge p. 335〕
: \left ( \epsilon^ \gamma_5 \gamma_\kappa \partial_\rho - i m \sigma^ \right)\psi_\nu = 0
where \epsilon^ is the Levi-Civita symbol,
\gamma_5 and \gamma_\nu are Dirac matrices,
m is the mass,
\sigma^ \equiv \frac () ,
and \psi_\nu is a vector-valued spinor with additional components compared to the four component spinor in the Dirac equation. It corresponds to the representation of the Lorentz group, or rather, its part.〔S. Weinberg, "The quantum theory of fields", Vol. 1, Cambridge p. 232〕
This field equation can be derived as the Euler–Lagrange equation corresponding to the Rarita–Schwinger Lagrangian:〔S. Weinberg, "The quantum theory of fields", Vol. 3, Cambridge p. 335〕
:\mathcal=-\tfrac\;\bar_\mu \left ( \epsilon^ \gamma_5 \gamma_\kappa \partial_\rho - i m \sigma^ \right)\psi_\nu
where the bar above denotes the Dirac adjoint.
This equation controls the propagation of the wave function of composite objects such as the delta baryons () or for the conjectural gravitino. So far, no elementary particle with spin 3/2 has been found experimentally.
The massless Rarita–Schwinger equation has a sermonic gauge symmetry: is invariant under the gauge transformation \psi_\mu \rightarrow \psi_\mu + \partial_\mu \epsilon, where \epsilon\equiv \epsilon_\alpha is an arbitrary spinor field.
"Weyl" and "Majorana" versions of the Rarita–Schwinger equation also exist.
==Equations of motion in the massless case==
Consider a massless Rarita-Schwinger field described by the Lagrangian density
: \mathcal L_ = \bar \psi_\mu \gamma^ \partial_\nu \psi_\rho,
where the sum over spin indices is implicit, \psi_\mu are Majorana spinors, and
: \gamma^ \equiv \frac \gamma^\gamma^\nu" TITLE="\mu}\gamma^\nu">\gamma^.
To obtain the equations of motion we vary the Lagrangian with respect to the fields \psi_\mu, obtaining:
: \delta \mathcal L_ =
\delta \bar \psi_\mu \gamma^ \partial_\nu \psi_\rho
+ \bar \psi_\mu \gamma^ \partial_\nu \delta \psi_\rho
= \delta \bar \psi_\mu \gamma^ \partial_\nu \psi_\rho
- \partial_\nu \bar \psi_\mu \gamma^ \delta \psi_\rho
+ \text

using the Majorana flip properties〔Pierre Ramond - Field theory, a Modern Primer - p.40〕
we see that the second and first terms on the RHS are equal, concluding that
: \delta \mathcal L_ = 2 \delta \bar \psi_\mu \gamma^ \partial_\nu \psi_\rho,
plus unimportant boundary terms.
Imposing \delta \mathcal L_ = 0 we thus see that the equation of motion for a massless Majorana Rarita-Schwinger spinor reads:
: \gamma^ \partial_\nu \psi_\rho = 0.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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