mitchell porter said:
This thread was launched by the idea of a
"waterfall" of Koide-like relations that relate the masses of all the quarks as well as the charged leptons. An esoteric idea buried in that paper (in part 3), is that the more fundamental version of this "waterfall" starts with a massless up quark, but that instantons add a finite correction to the up quark mass, a correction which then propagates through the waterfall and gives rise to the observed values of the masses.
The idea that the up quark is fundamentally massless was proposed as a solution to the strong CP problem (why the theta angle of QCD is zero), but lattice QCD calculations imply that there must be a nonzero fundamental mass, in addition to any mass coming from QCD instantons. However, this just means that the up yukawa must be nonzero at the QCD scale. It is still possible that the up mass comes from instantons of a larger gauge group for which SU(3) color is just a subgroup.
"Non-Invertible Peccei-Quinn Symmetry and the Massless Quark Solution to the Strong CP Problem" illustrates this for the example of SU(9) color-flavor unification. Actually they talk about a massless down quark, but they state it could work for the up quark as well, and they cite some 2017 papers (references 86-87) which feature a massless up quark in the context of SU(3)^3. Also see their reference 2, which posits a similar origin for neutrino masses, and illustrates that these instantons can be thought of as arising from virtual flavored monopoles.
Thanks for the heads up.
The Many Experimentally Determined Constants Of The SM Belong In The Electroweak Sector - The QCD Sector Isn't The Right Place To Look For Answers To Koide-Like Questions
The idea that the quark masses are related to the SU(3) QCD interactions, at all, as opposed to being basically an electroweak phenomena, however, doesn't seem right.
QCD has nothing to do with the CKM matrix, the PMNS matrix, the charged lepton masses that follow the original Koide's rule, W boson and Z boson decays, or the SM Higgs mechanism which has been demonstrated well at the LHC.
QCD related hadron mass doesn't even have the same origin (or even a similar origin) to the masses of the charged fundamental fermions and massive fundamental bosons that arise from the Higgs mechanism. QCD is doing its part of the mass generation thing in composite particles, dynamically, in a quite feasible to calculate with lattice QCD way already.
Rather than QCD instantons, starting with a zero mass or small self-interaction origin mass for the up quark (indeed, the up quark, down quark, electron, and lightest neutrino mass eigenstate are all reasonably close to what they should be due to self-interactions), and then modifying it with loop level modifications (much like the electroweak part of the muon g-2 calculation), seems so much more in tune with the way all of the other relevant parts of the Standard Model work.
Similarly, the seductive LC & P relationship (i.e. that the sum of the squares of the masses of the fundamental particles is equal to the square of the Higgs vev), is still true to within about 2 sigma or a hair more for a very slight statistical tension (almost all of the uncertainty arising from uncertainty in the top quark pole mass and the Higgs boson mass, which combined, are a little light at current best fit values). But this relationship really only makes sense in the context of the electroweak part of the Standard Model model ignoring QCD.
Even if the simple LC & P relationship comes into a greater tension with the best fit fundamental particle masses with new data, it doesn't take much of a BSM fix to solve that in a situation where there are a lot more potential moving parts.
A single BSM 3 GeV gauge boson (perhaps serving a similar role for neutrino mass to the role that the Higgs boson serves for all of the other fundamental particle masses), for example, would be enough to bring the current 2 sigma deviation from best fit values to a perfect LC & P fit.
The two sigma range for the top quark pole mass
according to the last paper with combined LHC data sets is 171.86-173.18 GeV with a best fit value of 172.52 GeV. This result is
essentially the same as the Particle Data Group value, but cuts the uncertainty in half. If the true value is on the high end of this range, and the true value of the Higgs boson is on the high end of the range it is experimentally permitted to have, then either the need for a BSM particle vanishes entirely or the mass needed to make it balance gets much smaller than 3 GeV.
In contrast, there's no way to fix any LC & P (or other) discrepancies between theory and experiment in the QCD part of the SM because it doesn't have enough moving parts.
The strong force coupling constant is really the only experimentally measured physical constant in the SU(3) QCD sector of the SM. What is it?
α(s)(n(f)=5)(M(Z))=
0.1171(7) at the renormalization group summed perturbation theory (RGSPT) value;
0.1179(9) at the
Particle Data Group (PDG) value,
0.1184(8) at the
2021 Flavor Lattice Averaging Group (FLAG) value.
These values are consistent with each other at the usual two sigma level and are each precise to a bit under the one percent level. There are lots of deep intrinsic barriers to making that value more precise because strong force propagator loops converge so much more slowly and with so much more computational effort than electroweak propagator loops do (and reach peak precision at a much lower level before they start to diverge in perturbative QCD). And, I'm not aware of any real strong theoretical hint at what value it should have form any theory.
So, there just isn't much to work with there and perhaps unsurprisingly as a result of this simplicity there isn't even any significant amount of BSM variant theorizing about the QCD sector of the SM model. It isn't fruitful because the experimental and lattice data isn't precise enough to confirm or deny any reasonable variant on it.
The beta function of the strong force coupling constant is deterministically set by renormalization theory without any experimental input, and the color charges possibilities and relative values are similar fixed in the theory at small integer or ratio of small integer values, that are confirmed by experiment to high precision and by the need for theoretical consistency.
So why turn to QCD to explain the unexplained values of the fundamental constants or to reduce the degrees of freedom in the model?
In contrast, eight of the SM constants are CKM/PMNS matrix parameters in the electroweak sector of the model that basically describe W boson interactions, twelve are Higgs boson Yukawas in the electroweak sector, and two are electroweak coupling constants. The Higgs vev doesn't have any measurable QCD contributions either. The three neutrino mass eigenstates may not be Higgs Yukawas, but they certainly have nothing to do with QCD with which neutrinos don't even interact at the tree level.
What's one more experimentally measured physical constant in the electroweak sector if you need it to balance the books and make a credible prediction of new physics, in a sector where you already have 25 experimentally measured physical constants (less one or two degrees of freedom because they aren't fully independent of each other)?
The genius of Koide sum rules, if you can make them work, is that it can, in principle, greatly reduce the number of independent degrees of freedom associated with those 25 experimentally measured physical constants in the electroweak sector, eliminating seven or more of them, in addition to the one or two that we can already trim in the existing SM electroweak sector due to related electroweak quantities like the EM and weak force coupling constants, and W and Z boson masses.
The Strong CP Problem Is A Non-Problem
I also continue to be unimpressed with the notion that the Strong CP problem is really a problem at all.
Nature can set its physical constants to anything it wants to in the SM. It is sheer arrogance to impose our expectations on those values and the quest for "naturalness" driving this "problem" has been perhaps the most fruitless and most scientific effort consuming scientific program since we tried to explain planetary motions with epicycles.
Also, the fact that gluons are massless in SM QCD alone, for example, by symmetry and the fact that massless particles don't experience time in their own reference frame because they travel at exactly the speed of light, alone, makes any possibility other than zero CP violation in QCD very hard to justify or consider to be "natural".
We don't theoretically need a zero mass up quark to get that result. So, finding a cheat by which we can get a zero mass up quark isn't very impressive either.