# Energy of massless particles before electroweak symmetry breaking

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Ranku
Would it be correct to represent the energy of massless particles before electroweak symmetry breaking as ##E = cp##, just as we do with photons post-symmetry breaking?

Homework Helper
Well, I'm not quite sure, and I'm not an expert, but I would say no. Because this equation is a kinematic equation, therefore it should not care about the dynamic process that gives mass to the particles. In the same way that even after SSB, if a particle acquires a mass ##m_0## is still not correct to write ##E=\sqrt{p^2+m_0^2}##, but rather you need the renormalized mass.

Mentor
Would it be correct to represent the energy of massless particles before electroweak symmetry breaking as ##E = cp##, just as we do with photons post-symmetry breaking?

Massless means ##E = cp##, so yes.

this equation is a kinematic equation, therefore it should not care about the dynamic process that gives mass to the particles

It's a kinematic equation describing massless particles. If a particle gains mass by a dynamic process, it's not massless any more and no longer satisfies a kinematic equation for massless particles. But if that dynamic process hasn't happened yet, and the particle is still massless, it obeys the massless kinematic equation.

even after SSB, if a particle acquires a mass ##m_0## is still not correct to write ##E=\sqrt{p^2+m_0^2}##, but rather you need the renormalized mass

Yes, you would write ##E = \sqrt{p^2 + m_r^2}##, where ##m_r## is the renormalized mass. But this has nothing to do with what kinematic equation you would write before symmetry breaking, when the particle is still massless.

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