malawi_glenn said:
Not until they can measure things like H-H couplings etc.
I agree that it isn't established beyond all doubt yet, but every few months since it has been discovered the constraints on differences from the SM Higgs have gotten smaller and more restricted.
What do we know about how strong the fit of what we observe to the SM Higgs is?
There is basically no data that is contrary to the predictions of the SM Higgs hypothesis made about 50 years ago (subject to determining its mass), and for a given Higgs boson mass the properties of the SM Higgs boson are completely predetermined with no wiggle room at all down to parts per ten million or better.
The global average value for the mass of the Higgs boson is currently 125.25±0.17 GeV, a relative accuracy of about 1.4 parts per thousand.
There is also basically no data strongly suggesting one or more additional BSM Higgs bosons (although there is
a bit of an anomaly at 96 GeV), even though BSM Higgs bosons aren't directly ruled out yet above the hundreds of GeVs. BSM Higgs bosons are also allowed in pockets of allowed parameter spaces at lower masses if the properties of the hypothetical particles are just right. For example, new Higgs bosons with a charge of ± 2 are ruled out at masses
up to about 900 GeV, and so are
many other heavy Higgs boson hypotheses. Indirect constraints also
greatly limit the parameter space of BSM Higgs bosons unless they have precisely the right properties (which turn out to be not intuitively plausible or well-motivated theoretically).
The data strongly favor the characterization of the observed Higgs boson as a spin-0 particle, just like the SM Higgs boson, and strongly disfavors any other value of spin for it.
The data is fully consistent at the 0.6 sigma level with an even parity SM Higgs boson, see
here, while the pure CP-odd Higgs boson hypothesis is disfavored at a level of 3.4 standard deviations. In other words, the likelihood that the Higgs boson is not pure CP-odd is about 99.9663%.
A mix of a CP-odd Higgs boson and a CP-even Higgs boson of the same mass is (of course) harder to rule out as strongly, particularly if the mix is not equal somehow and the actual mix is more CP-even than CP-odd. There isn't a lot of precedent for those kinds of uneven mixings, however, in hadron physics (i.e., the physics of composite QCD bound particles), for example.
Eight of the nine Higgs boson decay channels theoretically predicted to be most common in a SM Higgs of about 125 GeV have been detected. Those channels, ranked by branching fraction are:
b-quark pairs, 57.7% (observed)
W boson pairs, 21.5% (observed)
gluon pairs, 8.57%
tau-lepton pairs, 6.27% (observed)
c-quark pairs, 2.89% (observed May 2022)
Z boson pairs, 2.62% (observed)
photon pairs, 0.227% (observed)
Z boson and a photon, 0.153% (observed April 2022)
muon pairs, 0.021 8% (observed)
electron-positron pairs, 0.000 000 5%
All predicted Higgs boson decay channels, except gluon pairs, with a branching fraction of one part per 5000 or more have been detected.
Decays to gluon pairs are much harder to discern because the hadrons they form as they "decay" are hard to distinguish from other background processes that give rise to similar hadrons to those from gluon pairs at high frequencies. Even figuring out what the gluon pair decays should look like theoretically due to QCD physics, so that the observations from colliders can be compared to this prediction, is
very challenging.
The total adds 99.9518005% rather than to 100% due to rounding errors, and due to omitted low probability decays including strange quark pairs (a bit less likely than muon pairs), down quark pairs (slightly more likely than electron-positron pairs), up quark pairs (slightly more likely than electron positron pairs), and asymmetric boson pairs other than Z-photon decays (also more rare than muon pairs).
The Higgs boson doesn't decay to top quarks, but the measured top quark coupling is within 10% of the SM predicted value in a measurement with an 18% uncertainty at one sigma in one kind of measurement, and within 1.5 sigma of the predicted value using another less precise kind of measurement.
The Particle Data Group summarizes the strength of some of the measured Higgs boson couplings relative to the predicted values for the measured Higgs boson mass, and each of these channels is a reasonably good fit relative to the measured uncertainty in its branching fraction.
Combined Final States = 1.13±0.06
W W∗= 1.19±0.12
Z Z∗= 1.01±0.07
γγ= 1.10±0.07
bb= 0.98±0.12
μ+μ−= 1.19±0.34
τ+τ−= 1.15+0.16−0.15
ttH0Production = 1.10±0.18
tH0production = 6±4
The
PDG data cited above predates the cc decay and Zγ channel discovery made this past spring, so I've omitted those from the list above in favor of the data from the papers discovering the new channels.
One of these papers shows that the branching fraction in the Zγ channel relative to the SM expectation is μ=2.4±0.9. The ratio of branching fractions
B(H→Zγ)/
B(H→γγ) is measured to be 1.5+0.7−0.6, which agrees with the standard model prediction of 0.69 ± 0.04 at the 1.5 standard deviation level. The branching fraction of the cc channel isn't very precisely known yet, but
isn't more than 14 times the SM prediction at the 95% confidence level.
The Higgs boson self-coupling is observationally constrained to be
not more than about ten times stronger than the SM expected value, although it could be weaker than the SM predicted value. But the crude observations of its self-coupling are entirely consistent with the SM expected value so far. This isn't a very tight constraint, but it does rule out wild deviations from the SM paradigm.
The width of the Higgs boson (equivalently, its mean lifetime) is consistent to the best possible measurements with the theoretical SM prediction for the measured mass. The full width Higgs boson width Γ is 3.2+2.8−2.2MeV, assuming equal on-shell and off-shell effective couplings (which is a quite weak assumption). The predicted value for a 125 GeV Higgs boson is about 4 MeV.
There are really no well motivated hypotheses for a Higgs boson with properties different from the SM Higgs boson that could fit the observations to date this well.
For a particle that has only been confirmed to exist for ten and a half years, that's a pretty good set of fits. And, the constraints on deviations from the SM Higgs boson's properties have grown at least a little tighter every year since its discovery announced on July 4, 2012.
Higgs, W, and Z boson properties as constraints on BSM physics
This reasonably good fit of the observed properties of the Higgs boson to the properties it is predicted to have in SM at its measured mass is especially notable because the decay properties and couplings of the Higgs boson, like muon g-2, are good global tests of the SM, although not as comprehensive muon g-2, and not extending to BSM phenomena in excess of about 62.5 GeV (half the Higgs boson mass), which is a much lower threshold than the muon g-2 indirect exclusion which is in the TeVs.
Any BSM particle that couples to the Higgs boson in proportion to its rest mass, as the SM Higgs boson is predicted to do, with a mass between about 1 GeV and 62.5 GeV would have thrown off the branching fractions of the Higgs boson that have been observed to date dramatically. On the other hand, a new BSM massive fundamental particle that coupled to the Higgs boson in proportion to its rest mass with a mass of less than 20 MeV would not discernibly change the properties of the Higgs boson observed to date at all.
All quarks, charged leptons, and massive fundamental bosons in the Standard Model get their mass from the Higgs mechanism and couple to the Higgs boson (the source of the neutrino masses is unknown at this time), so it would be surprising to see some new massive fundamental particle that got its mass in some other manner.
In the same way, W and Z boson decays are sufficiently close to the SM predicted values that we can be confident that there are no particles that couple to the weak force with the strength that SM particle that do so, at any rest mass whatsoever from 0 to 45 GeV.
Incidentally, all known massive fundamental particles in the SM (quarks, charged leptons, neutrinos, W bosons, Z bosons, and Higgs bosons) couple to the weak force with the same "weak force charge" strength, and none of the zero rest mass fundamental particles in the SM (i.e. photons and gluons) couple directly to the weak force in the SM.
The number of SM "left handed" neutrinos that exist, for example, must be exactly three in the mass range from 0 to 45,000,000,000 eV. We know that none of the SM neutrinos can have an absolute mass of more than about 1 eV from direct measurements of lightest neutrino mass together with neutrino oscillation data (ten orders of magnitude smaller than the next possible least massive Standard Model neutrino). Indirect cosmology limits combined with neutrino oscillation based mass differences put the upper limit on the mass of the most massive neutrino eigenstate closer to 0.07 eV at 95% confidence (twelve orders of magnitude smaller the 45 GeV).
There are no good theoretical motivations for a hypothetical fourth generation Standard Model neutrino to be so profoundly more massive than neutrinos in the three known generations of Standard Model fermions. This is why searches for BSM neutrinos almost entirely focuses on new "sterile" a.k.a. "right handed" neutrinos.
And, since mathematical consistency in the SM calls for generations of new fermions to always include an up-type quark, a down-type quark, a charged lepton, and a neutrino, the non-existence of a SM left-handed neutrino at masses up to 45 GeV pretty much rules out the possibility that any fourth generation SM fermions exist.
