# Physical electron mass: renormalized VS running VS pole of propagator

This is a follow-up to the question: About the physical, measurable coupling constants, after renormalization

In QFT, we have the bare mass, renormalized mass, physical mass (which corresponds to a pole of the propagator) and the effective mass (which runs with energy/momentum). The bare mass is just a parameter to be fixed in order to remove infinities. Also, as per the answers in the link above, the renormalized mass is just the effective mass that's fixed at a specific energy scale $$s_0$$, $$m_{eff}(s_0)=m_R$$. We can also fix the renormalized mass to be equal to the one that corresponds to the propagator's pole.

In the case of the electron's charge, the numerical value that's usually given is just the effective charge at a specific (implicitly agreed upon) energy level. But the actual charge is the effective charge since it gives measurable corrections to the simple Coulomb potential. In this case, it's clear what the physical value of the electron is: the effective that changes depending on energy/momentum; it's not a constant.

But what's happening with the mass? While I (think I) understand the bare and renormalized mass, I don't know what the physical mass of the electron is. It seems that setting the physical mass as the pole of the propagator is something "more absolute". But, in analogy with the electron's running/effective charge, it also seems plausible that the running/effective mass is the physical mass.

So, what is the physical mass and how does the running mass relate to the mass that's the pole of the propagator?

Note: When I say physical, I don't mean measurable. For the electron's charge, the renormalized charge is measurable (it's defined as the matrix element of a scattering process at a specific energy level) but the actual charge is the running charge. I guess, when I'm saying physical mass, I mean "the" mass. I apologise for (probably) being inaccurate.

• Jan 12 at 21:15
• Well, in general the pole masses will be the momenta squared of all particles in the spectrum. Is that the sort of thing you're looking for? Jan 13 at 4:33
• @RichardMyers I guess my question is: in the case of the couplings, it seems natural to say that the physical coupling is the running one, as it's the one controlling the strength of the interaction (for example, the running electron charge produces observable corrections to the Coulomb potential). So, it would seem natural to assume that any running parameter is the physical one; same for mass. Yet, for mass, we have a condition that the mass that's the pole of the propagator is the physical one. So, is the physical mass the running one or the one corresponding to the propagator's pole? Jan 13 at 8:11

Answers in some of the linked questions do contain the essential information that you need, but I think some conceptual clarifications may be helpful to you. I hope I've understood your question properly, and apologies if I say a lot of things you already know.

First, to set expectations properly, I'll only discuss on-shell renormalization. This is, as the name implies, a nice prescription when you can measure the on-shell particles directly. By the same token, it's not a good prescription for the case of light quarks, for example, which we never detect directly as free particles. Light quarks do of course have mass parameters which are physically important, but one needs to use some more abstract prescription such as $$\overline{MS}$$ to deal with them, and the corresponding renormalized mass parameters don't have a clean physical interpretation in other more "familiar" terms.

But since we are considering on-shell renormalization, note that the physical mass that you measure with your detector is always at the scale $$p^2=m_\mathrm{ph}^2$$ by Lorentz invariance. It doesn't matter what the energy scale of the process you're observing is. When that final state particle emerges from the process, it's on-shell. It always satisfies $$p^2=m_\mathrm{ph}^2$$. You could boost to a frame where that particle is at rest if you want, whatever, it doesn't matter. In the context of an on-shell scheme, it simply makes no sense to ask if the "physical mass would run", or any similar such phrases.

So in on-shell renormalization the procedure goes like this. I compute the 1PI self-energy corrections to the propagator, sum the geometric series, and end up with a propagator like $$\frac{-i}{p^2-m_0^2-M^2(p^2)}$$ where $$m_0$$ is the bare mass parameter in the Lagrangian, and $$M^2(p^2)$$ is what I got from the 1PI diagrams. Then I measure the physical mass with my detector and find it to be $$m_\mathrm{ph}$$. Finally, I demand that $$m_0^2+M^2(m_\mathrm{ph}^2)=m_\mathrm{ph}^2$$, which will involve absorbing some infinity into $$m_0$$ plus an appropriate finite part. That will ensure that the propagator has a pole at the physical mass.

But the propagator is now so much more than just a pole at the physical mass. It's an interesting function of $$p^2$$, and you know that when used as an internal line in a diagram, it will have God knows what value of $$p^2$$ flowing through it. $$p^2$$ might even be negative. At these unholy values of $$p^2$$, $$M^2(p^2)$$ is producing all kinds of interesting effects in your propagator. This, in a sense, is the running mass, if you want to call it that. But again, it doesn't affect what you measure as the physical mass.

In fact, what I said above is not exactly correct, because $$M^2(p^2)$$ is in general complex. What I really meant is that $$m_0^2+\mathrm{Re}~M^2(m_\mathrm{ph}^2)=m_\mathrm{ph}^2$$ should be true. Via the optical theorem, one can show that $$M^2(p^2)$$ will develop an imaginary part for values of $$p^2$$ for which some decay of the particle is possible (which depends on the details of your theory). We see that this imaginary part also affects the structure of the propagator, and in fact, is the reason why particles have a width in $$p^2$$ that is inversely proportional to their lifetime.

• Fantastic, thanks for the answer! Just a question (correct me if I'm wrong): It seems that the running mass only shows up off-shell so we only get it in internal lines (virtual particles); it is thus not observable. This ties well with the fact that it's $m_{ph}$ that's measurable. Now, in the case of the electron's charge, "the" charge is the running charge (since it corrects the Coulomb potential and leads to things like the Lamb shift). So, I guess, "the" mass is not the running mass. So, what's the physical meaning of the running mass and what's its relation to $m_{ph}$? Jan 13 at 11:06
• @TheQuantumMan I'm afraid I'll misinterpret you, so could you give me the precise definition of what you mean by "running mass" or point me to a textbook reference, etc? Jan 14 at 6:15
• When I say running mass, I mean effective mass. It's the one that depends on energy. The renormalized is the effective mass at a specific energy scale (so it's independent of the energy, but definition-dependent). The pole mass is something more absolute, as in it comes from a condition and does not depend on energy (like the effective mass) or an energy scale that we use to define it (like the renormalized mass). Peskin and Schoreder p.600: they say that $\bar{m}(Q)$ is a running mass and then say (same page) that it's the effective mass. Jan 14 at 8:27
• @TheQuantumMan ok got it. I'll try to come back and add more info when I have time. I'll delete this comment when I do that. Jan 14 at 8:37