- #1
ergospherical
- 1,072
- 1,365
It is given that a theory is invariant under the length scaling:\begin{align*}
x &\rightarrow \lambda x \\
\phi(x) &\rightarrow \lambda^{-D} \phi(\lambda^{-1} x)
\end{align*}for some ##D## to be determined. The action of a real scalar field is here:\begin{align*}
S = \int d^4 x \dfrac{1}{2}\partial_{\mu} \phi \partial^{\mu} \phi - \dfrac{1}{2}m^2 \phi^2 -g\phi^p
\end{align*}Since ##\partial_{\mu} = \frac{\partial x'^{\nu}}{\partial x^{\mu}} \partial'_{\nu} = {(\Lambda^{-1})^{\nu}}_{\mu} \partial'_{\nu}## then would I be correct in thinking that the derivative of the field transforms as:\begin{align*}
\partial_{\mu} \phi(x) \rightarrow \partial_{\mu} \phi'(x) &= \lambda^{-D} \partial_{\mu} \phi(\lambda^{-1} x) \\
&= \lambda^{-D} {(\Lambda^{-1})^{\nu}}_{\mu} \partial'_{\nu} \phi(x')
\end{align*}so the derivative term in the action transforms as \begin{align*}
(\partial_{\mu} \phi)^2 &\rightarrow \lambda^{-2D} {(\Lambda^{-1})^{\nu}}_{\mu} {\Lambda^{\mu}}_{\rho} (\partial'_{\nu} \phi(x'))( \partial'^{\rho} \phi(x')) \\
&= \lambda^{-2D} (\partial'_{\mu} \phi(x'))^2
\end{align*}Meanwhile ##d^4 x = \lambda^{-4} d^4 x'##, and this would imply scale invariance when ##D=-2##? That feels wrong and I worry that I have transformed the wrong things.
x &\rightarrow \lambda x \\
\phi(x) &\rightarrow \lambda^{-D} \phi(\lambda^{-1} x)
\end{align*}for some ##D## to be determined. The action of a real scalar field is here:\begin{align*}
S = \int d^4 x \dfrac{1}{2}\partial_{\mu} \phi \partial^{\mu} \phi - \dfrac{1}{2}m^2 \phi^2 -g\phi^p
\end{align*}Since ##\partial_{\mu} = \frac{\partial x'^{\nu}}{\partial x^{\mu}} \partial'_{\nu} = {(\Lambda^{-1})^{\nu}}_{\mu} \partial'_{\nu}## then would I be correct in thinking that the derivative of the field transforms as:\begin{align*}
\partial_{\mu} \phi(x) \rightarrow \partial_{\mu} \phi'(x) &= \lambda^{-D} \partial_{\mu} \phi(\lambda^{-1} x) \\
&= \lambda^{-D} {(\Lambda^{-1})^{\nu}}_{\mu} \partial'_{\nu} \phi(x')
\end{align*}so the derivative term in the action transforms as \begin{align*}
(\partial_{\mu} \phi)^2 &\rightarrow \lambda^{-2D} {(\Lambda^{-1})^{\nu}}_{\mu} {\Lambda^{\mu}}_{\rho} (\partial'_{\nu} \phi(x'))( \partial'^{\rho} \phi(x')) \\
&= \lambda^{-2D} (\partial'_{\mu} \phi(x'))^2
\end{align*}Meanwhile ##d^4 x = \lambda^{-4} d^4 x'##, and this would imply scale invariance when ##D=-2##? That feels wrong and I worry that I have transformed the wrong things.