# What is the difference between pole and running mass?

For example, when we meassure Higgs boson mass to be 125 GeV, do we think about renormalized or pole mass? Should the mass of the Higgs change if it is produced at higher energies?

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Measured masses are the renormalized masses, though one should usually take into account the scale at which collisions occur. At sufficiently large energies the electroweak symmetry will be restored. –  genneth Jul 4 '12 at 15:00
Do you have a specific renormalization scheme in mind? –  Ron Maimon Jul 5 '12 at 4:09
yes in example MS bar –  Newman Jul 5 '12 at 9:39

The pole mass is closer to the intuitive physical mass of a particle, and is typically what is reported by experimentalists. The jargon comes from the well-known fact that resonances (and stable particles) show up as simple poles in the scattering amplitude continued to complex kinematic variables. This mass does not change with energy.

The 'running mass' refers to a parameter in the Lagrangian, of mass dimension = 1. This parameter is to be treated as if it were just another coupling constant; and just like any coupling constant in QFT, it changes with (the renormalization) scale.

A calculation can be done to relate the two, typically done perturbatively by computing the self energy. Thus, when an experimentalist quotes a mass (which is almost always the pole mass), this fixes the value of the running mass (in some renormalization scheme) by the said relation.

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For example, when we meassure Higgs boson mass to be 125 GeV, do we think about renormalized or pole mass?

Pole mass is the physical mass and independent of any renormalization scheme we use to subtract any infinite parts of the loop corrections. It is what we observe.

Should the mass of the Higgs change if it is produced at higher energies?

So far we know 125 Gev is the physical mass/pole mass. Does it change with energy? (Here I use the energy as a center of mass energy of the accelerator.) Renormalized Green function is $$iG^R(\not{p})= \frac{i}{\not{p}-m_R+\Sigma_R(\not{p})}$$ Set $\not{p}=m_p$ and then, $$m_p-m_R+\Sigma_R(m_P)=0$$ Here $m_R$ depends on a arbitrary scale $\mu$ which leads one to construct renormalization group equation using $\mu$ independence of $m_p$ $$\mu\frac{\partial( Z_mm_R)}{\partial\mu}=0$$ It means $m_R$, coupling constant in Lagrangian, is a running coupling. So your answer is, observed Higgs mass does not depend on energy scale. To make the observable invariant, $m_R$ coupling constant runs with the energy.

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