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spaghetti3451
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Homework Statement
A simple classical example that demonstrates spontaneous symmetry breaking is described by the Lagrangian for a scalar with a negative mass term:
##\mathcal{L}=-\frac{1}{2}\phi\Box\phi + \frac{1}{2}m^{2}\phi^{2}-\frac{\lambda}{4!}\phi^{4}##.
(a) How many constants ##c## can you find for which ##\phi(x)=c## is a solution to the equations of motion? Which solution has the lowest energy (the ground state)?
(b) The Lagrangian has a symmetry under ##\phi \rightarrow -\phi##. Show that this symmetry is not respected by the ground state. We say the vacuum expectation value of ##\phi## is ##c##, and write ##\langle\phi\rangle=c##. In this vacuum, the ##\mathbb{Z}_2## symmetry ##\phi \rightarrow -\phi## is spontaneously broken.
(c) Write ##\phi(x)=c+\pi(x)## and substitute back into the Lagrangian. Show that now ##\pi = 0## is a solution to the equations of motion. How does ##\pi## transform under the ##\mathbb{Z}_2## symmetry ##\phi \rightarrow -\phi## ? Show that this is a symmetry of ##\pi##'s Lagrangian.
Homework Equations
The Attempt at a Solution
How does the Lagrangian ##\mathcal{L}=-\frac{1}{2}\phi\Box\phi + \frac{1}{2}m^{2}\phi^{2}-\frac{\lambda}{4!}\phi^{4}## for the scalar field ##\phi(x)## have a negative mass term?
As far as I can see, the mass term ##\frac{1}{2}m^{2}\phi^{2}## is positive!
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