by T-A | 2026-02-14 10:07:34+00:00
> so the actual disagreement here seems to be whether adding same-mass right-handed neutrinos counts as a significant modification to the Standard Model

I disagree. That has been the working definition of Standard Model for decades. All quarks and all charged leptons are known to have Dirac masses, which require both left- and right-handed components, so once it became clear that neutrinos have mass too, extending that pattern to them too was the obvious thing to do.

> in the history of physics that actually happened, "no right-handed neutrinos" got codified as the baseline

Again, I disagree. Weinberg introduced what you insist on calling "standard model" in a three-page letter, at a time when there was no evidence for neutrino masses. He correctly designed it as a minimal proof of concept, knowing full well that extending it would be trivial. For the same reason, his "model of leptons" did not even mention quarks; those were also not an established thing in 1967.

I can't imagine anyone seriously claiming that quarks are not part of the standard model. And yet, here I am having to explain for the umpteenth time that neutrinos working like all other standard model particles are part of what everybody competent means by standard model.

>> Plain vanilla standard model fermions have the same mass whether they are left- or right-handed, so quite small for neutrinos
>
> Hm, is that true?

Yes. A Dirac fermion has a left-handed component and a right-handed one. The Dirac mass term is what binds them together and makes them behave like a single particle with one mass. Set that mass to zero and you have two massless Weyl fermions. [1]

> Unless by "beyond the standard model" you just mean that the right-handed mass is different from the left-handed mass

Of course. Different masses for left- and right-handed components of a Dirac fermion is a contradiction in terms.

> I mean you say you're a particle physicist

Do I?

> the popularizations I've seen (generally written by particle physicists) have said it means the former

There is an unfortunate tendency in popularization to blur the lines between established knowledge and speculation (see Feynman's "Cargo cult science", linked elsewhere in this thread), and an understandable desire to make one's own subject look particularly exciting. If you are neutrino physicist (an intrinsically soporific activity which mainly involves staring for years or decades on end at large quantities of a transparent mass hoping to see a rare interesting event [2]) your best bet to achieve that is to push the "window into Beyond the Standard Model (BSM) physics" narrative. So you bring up the fact that neutrino masses are very small, point to the seesaw mechanism [3] as a possible explanation, and emphasize that massive right-handed neutrinos could be cold dark matter [4]. That's fine, although it's getting old and not looking as promising as it once did. What is not fine is stretching the truth to the point of breaking it by claiming that right-handed neutrinos are, by themselves, BSM. That is abject nonsense.

[1] https://en.wikipedia.org/wiki/Dirac_equation#Weyl_and_Majora...

[2] https://en.wikipedia.org/wiki/List_of_neutrino_experiments

[3] https://en.wikipedia.org/wiki/Seesaw_mechanism

[4] https://en.wikipedia.org/wiki/Cold_dark_matter