LHC To Announce Its Lepton Universality Violation Results On Tuesday (20-Dec)

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In summary, the LHCb collaboration has announced that they have found evidence of lepton universality violations (i.e. a different probability for decays to tau leptons, muons, and electron-positron pairs respectively, mass-energy conservation permitting). This is the most notable experimental anomaly from Standard Model predictions outstanding right now in high energy physics. However, the statistical significance is merely a tension that may fade with a major new data point like the one to be announced on Tuesday by the LHC. There isn't a good explanation for why it isn't seen in other phenomena that should involve the same intermediate W boson decay driven processes.
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ohwilleke

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TL;DR Summary
On Tuesday, the LHC will announce the most definitive data point to date on whether B meson decays to leptons are identical by lepton flavor except for particle mass effects. Earlier data has shown moderate tensions with this SM prediction called "lepton universality."
Experimental results tending to show lepton universality violations (i.e. a different probability for decays to tau leptons, muons, and electron-positron pairs respectively, mass-energy conservation permitting) are the most notable experimental anomalies from Standard Model predictions outstanding right now in high energy physics.

But the statistical significance is merely a tension that may fade with a major new data point like the one to be announced on Tuesday by the LHC. There isn't a good explanation for why it isn't seen in other phenomena that should involve the same intermediate W boson decay driven processes.

Measurements of 𝑅(𝐾) and 𝑅(𝐾∗) with the full LHCb Run 1 and 2 databy Renato Quagliani (EPFL - Ecole Polytechnique Federale Lausanne (CH))

In this seminar we present the first simultaneous test of muon-electron universality in 𝐵+→𝐾+ℓ+ℓ− and 𝐵0→𝐾∗0ℓ+ℓ− decays, known as 𝑅(𝐾) and 𝑅(𝐾∗), in two regions of di-lepton invariant mass squared.

The analysis operates at a higher signal purity compared with previous analyses and implements a data-driven treatment of residual hadronic backgrounds. The analysis uses the full LHCb Run 1 and 2 data recorded in 2011-2012 and 2015-2018, corresponding to an integrated luminosity of 9 fb−1. This analysis is the most sensitive lepton universality test in rare b-decays and the results obtained supersede the previous LHCb measurements of 𝑅(𝐾) and 𝑅(𝐾∗0).
To seminar is on Tuesday 20 December at 11am, CERN time and sign up information is available at the link.
 
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Just in time for Christmas! 😍

-Dan
 
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Thanks it seems interesting. However, I have to give a lecture at that time :/
 
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FYI for those of you in the Continental U.S., 11 a.m. CERN Time is:

5 a.m. Eastern Time
4 a.m. Central Time
3 a.m. Mountain Time
2 a.m. Pacific Time.
 
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The most notable anomaly in HEP appears to have disappeared with more LHC data of improved quality.

Preprints:

Screenshot 2022-12-20 at 12.26.20 AM.png

arXiv:2212.09152 [pdf, other]
Test of lepton universality in b→sℓ+ℓ− decays
LHCb collaboration
Comments: All figures and tables, along with any supplementary material and additional information, are available at this https URL (LHCb public pages)
Subjects: High Energy Physics - Experiment (hep-ex)
The first simultaneous test of muon-electron universality using B+→K+ℓ+ℓ− and B0→K∗0ℓ+ℓ− decays is performed, in two ranges of the dilepton invariant-mass squared, q2. The analysis uses beauty mesons produced in proton-proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of 9 fb−1. Each of the four lepton universality measurements reported is either the first in the given q2 interval or supersedes previous LHCb measurements. The results are compatible with the predictions of the Standard Model.

arXiv:2212.09153 [pdf, other]
Measurement of lepton universality parameters in B+→K+ℓ+ℓ− and B0→K∗0ℓ+ℓ− decays
LHCb collaboration
Comments: All figures and tables, along with any supplementary material and additional information, are available at this https URL (LHCb public pages)
Subjects: High Energy Physics - Experiment (hep-ex)
A simultaneous analysis of the B+→K+ℓ+ℓ− and B0→K∗0ℓ+ℓ− decays is performed to test muon-electron universality in two ranges of the square of the dilepton invariant mass, q2. The measurement uses a sample of beauty meson decays produced in proton-proton collisions collected with the LHCb detector between 2011 and 2018, corresponding to an integrated luminosity of 9 fb−1. A sequence of multivariate selections and strict particle identification requirements produce a higher signal purity and a better statistical sensitivity per unit luminosity than previous LHCb lepton universality tests using the same decay modes. Residual backgrounds due to misidentified hadronic decays are studied using data and included in the fit model. Each of the four lepton universality measurements reported is either the first in the given q2 interval or supersedes previous LHCb measurements. The results are compatible with the predictions of the Standard Model.
 
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ohwilleke said:
The most notable anomaly in HEP appears to have disappeared with more LHC data of improved quality.

The results are compatible with the predictions of the Standard Model.
I have tried to follow what has being happening with this from previous threads and non-technical explanations on line (Sabine H)

In layman’s speak, those anomalies seen at Fermilab and Brookhaven both blips?

https://news.fnal.gov/2021/04/first...xperiment-strengthen-evidence-of-new-physics/

The announcement is @10am my time so I will check in after lunch and see what you guys have to say.
 
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The g-2 discrepancy could simply be a problem with the theory prediction (and I think that's the most likely explanation). There are two different methods for the calculation, one is in agreement with the experimental values.

Here are the two older LHCb papers:
https://arxiv.org/abs/1406.6482 $$0.745^{+0.090}_{-0.074}\mathrm{\,(stat)}\,\pm0.036\mathrm{\,(syst)}$$
https://arxiv.org/abs/1705.05802
$$\begin{eqnarray*} R_{K^{*0}} =
\begin{cases} 0.66~^{+~0.11}_{-~0.07}\mathrm{\,(stat)} \pm
0.03\mathrm{\,(syst)} & \textrm{for } 0.045 < q^{2} < 1.1~\mathrm{\,GeV^2}c^4
\, , \\ 0.69~^{+~0.11}_{-~0.07}\mathrm{\,(stat)} \pm 0.05\mathrm{\,(syst)} &
\textrm{for } 1.1\phantom{00} < q^{2} < 6.0~\mathrm{\,GeV^2}c^4 \, .
\end{cases} \end{eqnarray*}$$
That certainly looked more interesting. There are still related b-physics measurements with an anomaly, but the shift in this paper suggests we might find similar shifts in others.

Edit: Reading a bit deeper into the material, a large part of the shift is statistical, but there is also a big systematic effect from a different treatment of background from misidentified events (larger than the original systematic uncertainty). That's something that will likely affect other measurements, too. We might get more re-analyses.
 
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pinball1970 said:
I have tried to follow what has being happening with this from previous threads and non-technical explanations on line (Sabine H)

In layman’s speak, those anomalies seen at Fermilab and Brookhaven both blips?

https://news.fnal.gov/2021/04/first...xperiment-strengthen-evidence-of-new-physics/

The announcement is @10am my time so I will check in after lunch and see what you guys have to say.
Please keep me informed we can bang out heads together.
 
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Sorry our heads
 
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Astranut said:
Sorry our heads
LHC : Large Head Collider
 
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  • #11
a digression re muon g-2
pinball1970 said:
In layman’s speak, those anomalies seen at Fermilab and Brookhaven both blips?

https://news.fnal.gov/2021/04/first...xperiment-strengthen-evidence-of-new-physics/
I concur 100% with mfb on muon g-2 who said:
The g-2 discrepancy could simply be a problem with the theory prediction (and I think that's the most likely explanation). There are two different methods for the calculation, one is in agreement with the experimental values.
muon g-2 is the sum of an EM contribution (QED) (the lion's share with negligible uncertainty), a weak force contribution (a modest contribution with a very small uncertainty), and a strong force (QCD) contribution which is small but not insignificant and has a comparatively huge uncertainty.

The consensus value of the QED plus weak force contribution to SM prediction for muon g-2 in units of 10-11 is:

116,584,872.53 ± 1.01 (with most of the uncertainty coming from the weak force contribution)

In the Theory Initiative analysis the QCD amount is 6937(44) which is broken out into two parts: hadronic vacuum polarization (HVP) = 6845(40), which is a 0.6% relative error, and hadronic light by light (HLbL) = 98(18) which is about a 20% relative error.

The calculation from the Theory Initiative of the SM prediction (which mixed experimental data for parts of the HVP calculation and lattice computations for other parts of the HVP calculations) is in tension with the experimental measurement.

But, the first principles lattice calculation of the HVP part of the SM prediction for muon g-2 by the BMW group is consistent with the experimental results (see below for the actual values times 10-11):

Fermilab (2021): 116,592,040(54)
Brookhaven's E821 (2006): 116,592,089(63)
Combined measurement: 116,592,061(41)
Theory Initiative calculation: 116,591,810(43)
BMW calculation: 116,591,954(55)
Combined measurement - Theory Initiative: 251(59)
Combined measurement - BMW: 107(69)

Essentially all of the subsequent work has confirmed the BMW calculation in parts that have been replicated, especially the HVP "window", which is a key subcomponent of the overall HVP contribution that is somewhat easier to calculate. Some of these papers have even narrowed down to some extent where the discrepancy between the two SM predictions is coming from. See, e.g., https://arxiv.org/abs/2212.09340 and https://arxiv.org/abs/2212.10490

In addition, on the day that the new muon g-2 experimental results was released a new calculation of the hadronic light by light contribution to the muon g-2 calculation was also released on arXiv which doesn't seem to be part of the BMW calculation. This increases the contribution from that component from 92(18) x 10-11 to 106.8(14.7) x 10-11. This boost of 14.8 in the overall QCD component isn't as big as the BMW HVP calculation's impact on it, but the two combined narrow the gap even more.

With these two SM calculation refinements the discrepancy between the combined measurement and the BMW plus new HLbL prediction is about 85(68) x 10-11, so barely more than a 1 sigma difference.

Why care?

Because muon g-2 is an indirect global measure of the consistency of the SM with experiment that is sensitive to new or different particles and/or forces at scales into the TeV to tens of TeVs (or more if the deviation from the SM is really strong) scale, because all three SM forces and all SM particle parameters contribute to it to some extent.

If the SM in consistent with experiment at the parts per billion or ten billion level, then there is basically no room for BSM physics that don't cancel out in the muon g-2 calculation at the energy scales of the next generation collider.

For example, it is basically impossible to have SUSY with 1-10 TeV scale sparticles without tweaking muon g-2. Likewise, adding leptoquarks to the SM, which have been a popular BSM physics explanation for hints of lepton universality violations (which now seem to be basically ruled out) should also tweak muon g-2.

The Really Big Picture

If there are no lepton universality violations, as the new LHCb results would tend to show, AND there is no muon g-2 anomaly, as new lattice computations of the SM prediction for muon g-2 are increasingly showing, AND there are no non-standard neutrino interactions or sterile neutrinos (which experiments are tending to show), AND the Higgs boson is just the SM Higgs boson (which is confirmed ever more tightly every few months by new LHC data) - then there is less and less room for BSM physics at energy scales that can be tested experimentally at current colliders or next generation colliders (i.e. up to the hundreds of TeV energy scale).
 
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ohwilleke said:
AND the Higgs boson is just the SM Higgs boson (which is confirmed ever more tightly every few months by new LHC data)
Not until they can measure things like H-H couplings etc.
 
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"The Really Big Picture

If there are no lepton universality violation, as the new LHCb results would tend to show, AND there is no muon g-2 anomaly, as new lattice computations of the SM prediction for muon g-2 are increasingly showing, AND there are no non-standard neutrino interactions or sterile neutrinos (which experiments are tending to show), AND the Higgs boson is just the SM Higgs boson (which is confirmed ever more tightly every few months by new LHC data) - then there is less and less room for BSM physics at energy scales that can be tested experimentally at current colliders or next generation colliders (i.e. up to the hundreds of TeV energy scale)."

Thanks for summing up.
 
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malawi_glenn said:
Not until they can measure things like H-H couplings etc.
The statement is even more nonsensical than that, What is known is that if there are two (or more, I suppose) Higgs bosobs, the one we have found is the more SM-like of the pair (or group).

It actually makes more gymnastics to make the 2nd Higgs visible with the data taken than not,

Confirmed? Pfui!
 
  • #15
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.
 
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  • #16
It is very SM-like indeed, we all know that. But to say it is "confirmed to be the SM Higgs boson" implies that SM is all that there is to particle physics.
 
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  • #17
malawi_glenn said:
It is very SM-like indeed, we all know that. But to say it is "confirmed to be the SM Higgs boson" implies that SM is all that there is to particle physics.
I'm not sure I understand what you are saying.
 
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  • #18
ohwilleke said:
I'm not sure I understand what you are saying.
Ok.
 
  • #19
I'm sorry, but that wall of text is a load of nonsense. Rather than debunk it point by point (and it's tempting to point out that the Higgs decay to gluons has never been observed) let me point out that the vast majority of Higgs properties have never been measured.

  • We don't know its coupling to the first and second generations at all.
  • We don't know its coupling to neutrinoss at all.
  • We only know the couplings to the third generation charged fermions and the electroweak bososns to around 10%, maybe a little better, but this is a statement that depends on what you hold constant and what you allow to vary.
  • We don't know the Higgs self-coupling, and precise measurements of the mass are unhelpful without the self-coupling to compare it to,.
It is simply untrue that this is "confirmed" that the Higgs sector is SM. It is a trivial exercise to put together a model that is very much BSM and yet the measurements we have today are all consistent. (Example: two Higgs fields - one coupes to the 3rd generation fermions at full strength, to the electroweak bosons at 95% strength, and nowhere else. The second coupes to the 1st and 2nd generation fermions at full strength, to the electroweak bosons at 5% strength, and nowhere else.)
 
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  • #20
Vanadium 50 said:
I'm sorry, but that wall of text is a load of nonsense. Rather than debunk it point by point (and it's tempting to point out that the Higgs decay to gluons has never been observed) let me point out that the vast majority of Higgs properties have never been measured.

  • We don't know its coupling to the first and second generations at all.
  • We don't know its coupling to neutrinoss at all.
  • We only know the couplings to the third generation charged fermions and the electroweak bososns to around 10%, maybe a little better, but this is a statement that depends on what you hold constant and what you allow to vary.
  • We don't know the Higgs self-coupling, and precise measurements of the mass are unhelpful without the self-coupling to compare it to,.
It is simply untrue that this is "confirmed" that the Higgs sector is SM. It is a trivial exercise to put together a model that is very much BSM and yet the measurements we have today are all consistent. (Example: two Higgs fields - one coupes to the 3rd generation fermions at full strength, to the electroweak bosons at 95% strength, and nowhere else. The second coupes to the 1st and 2nd generation fermions at full strength, to the electroweak bosons at 5% strength, and nowhere else.)
Just to be clear, what I said was that: "If . . . the Higgs boson is just the SM Higgs boson (which is confirmed ever more tightly every few months by new LHC data)".

So I'm not saying that it is definitely the SM Higgs boson yet, I'm saying that the evidence pointing to that conclusion is getting steadily stronger and that it is already a "reasonably good fit."

Likewise, I specifically stated (emphasis added) that: "All predicted Higgs boson decay channels, except gluon pairs, with a branching fraction of one part per 5000 or more have been detected."

We've measured the coupling to charm quarks and muons which are in the second generation. The muon branching fraction is measured to be relative to a SM prediction of 1.00 equal to 1.19±0.34. The charm quark pair branching fraction, as I state, is only not more than 14x the SM value.

The Higgs self-coupling is constrained to be not less than 10x the SM expected value (I've since seen another paper constraining it to be not less than 6.6x at 95% CI). Yet another paper bounds it to 0.1 to 2.3 times the SM predicted value at plus or minus one sigma.

The SM doesn't really say one way or the other if the Higgs couples to neutrinos (since the source of their mass is undetermined) and since the coupling is expected to be so tiny, we haven't even made enough Higgs bosons at colliders to expect to be able to see that coupling if there is one. The expected branching fraction for each of the neutrinos in Higgs boson decays is on the order of one decay per 1022 Higgs bosons produced, and you'd need a sample a thousand times that large to bring down the statistical error to a workable level.

The couplings of the top quark, bottom quark, tau, the W and the Z are all within 20% and all of the couplings measured are consistent at two sigma.

"It is a trivial exercise to put together a model that is very much BSM and yet the measurements we have today are all consistent."

I don't claim that you can't propose a BSM alternative, I just state that:

"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."

Anyone can just make something up to fit the data after the fact, but nobody was proposing an alternative set of properties for the Higgs boson that fit this data but was different from the SM before it was discovered in 2012 and really there haven't been many papers proposing something else after the fact.

No continuous quantity can ever be proven to be an a perfect match to a prediction with experimental evidence. But if there it lots of evidence to corroborate a match, even if it isn't ultra-precise, and there aren't well motivated hypotheses for alternatives, one can reasonably be quite confident that you've got the real deal.

For example, a BSM model that couples to different generations at different strengths looks a lot less well motivated after Tuesday's announcements that were the start of this thread, when the only data point out of dozens of tests that have been done of lepton universality that previously showed a lepton universality violation has been superseded and is not consistent with lepton universality.

The point is not that we have at this moment absolutely ruled out every possible BSM possibility, the point is that each of the currently or recently live anomalies in the data relative to the SM have either been ruled out or disfavored with new developments.

A few years ago, this wasn't the case. There was no solution on the horizon for example, for muon g-2, for lepton universality violation hints, for the muonic proton radius, or for the reactor neutrino anomaly. All of these first glance breaks with the Standard Model have clear paths to resolution now.

The only new one recently, the new CDF W boson mass measurement which is out of step with multiple other recent precise measurements of the W boson mass was met with skepticism on day one and at most would have nudged up a world average measurement of the quantity.

It is hard to point to a time that I can recall when there has been less positive evidence pointing to the need for BSM physics than now. One can hypothesize alternatives, but it is less necessary to do so now, when there are no big breaks between SM theory and the data that don't have very plausible and visible resolutions in sight, than it has been for a long time.
 
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  • #21
ohwilleke said:
We've measured the coupling to charm quarks...the charm quark pair branching fraction, as I state, is only not more than 14x the SM value.
That is not a measurement. It is a limit. There is not even a clear signal there and the data is statistically compatible with zero.

The data does not say what you c;laijm it says.
 
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  • #22
Vanadium 50 said:
that wall of text
A common technique used by the OP. Also the technique of paragraph-long sentences was used.
 
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  • #23
malawi_glenn said:
It is very SM-like indeed, we all know that. But to say it is "confirmed to be the SM Higgs boson" implies that SM is all that there is to particle physics.
How does neutrinos get mass if the current SM is all there is? Thought it needed an extension to explain that if it's not multiple Higgs bosons.
 
  • #24
Lord Crc said:
How does neutrinos get mass if the current SM is all there is?
Dirac mass via a Higgs Yukawa. Same old Higgs,
 
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  • #25
Lord Crc said:
How does neutrinos get mass if the current SM is all there is? Thought it needed an extension to explain that if it's not multiple Higgs bosons.
There are debates, really more about style than substance, over whether the mechanism by which neutrinos acquire mass is part of the SM or not.

Prior to the discovery of neutrino oscillation, the SM assumed that neutrinos did not have mass.

After this discovery it was recognized that neutrinos had mass and that they oscillate according to the PMNS matrix parameters and differences in mass between three neutrino mass eigenstates. This limited "black box" understanding of neutrino mass is usually considered to be an addition to the SM, although pedantic "purists" disagree and would consider everything about neutrino mass to be technically beyond the Standard Model. But, this addition to the SM doesn't really supply the full story of where neutrino mass comes from in the way that the Higgs mechanism definitively does in the SM for other SM fundamental fermions.

There is some doubt about whether the Higgs mechanism really applies to neutrino mass or not, which relates to the fact that neutrinos do not have both left and right parity matter variations, and left and right parity anti-matter variations, the way that other SM particles do. Instead, neutrinos only come in left parity matter versions and right parity anti-matter versions.

Another possibility is that neutrinos have Majorana mass, which arises when a particle is its own anti-particle, or a mix of Majorana mass and "Dirac" mass.

But many theorists suggest that whatever Dirac mass that neutrinos have arises from a source other than the Higgs mechanism, such as a "see saw mechanism" with much more massive right-handed neutral leptons.
 
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  • #26
ohwilleke said:
Prior to the discovery of neutrino oscillation, the SM assumed that neutrinos did not have mass.
False,

I tire of the game where you post nonsense that takes more time to debunk than it took to write down.
 
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  • #27
Vanadium 50 said:
I tire of the game where you post nonsense that takes more time to debunk than it took to write down.
Brandolini's Law? :cry:
 
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  • #28
Vanadium 50 said:
Dirac mass via a Higgs Yukawa. Same old Higgs,
That requires a right-handed neutrino, which in turn allows a Majorana mass term. You then need to explain why this term is zero.
 
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  • #30
pinball1970 said:
From the coda to that story:

The result is likely to disappoint many theorists who have spent time trying to come up with models that could explain the anomalies. “I’m sure people would have liked us to find a crack in the standard model,” says Parkes, but in the end, “you do the best analysis with the data you have, and you see what nature gives you”, he says. “It’s really how science works.”

Although the latest result had been rumoured for months, its confirmation comes as a surprise, says Gino Isidori, a theoretical physicist at the University of Zurich who was at the CERN talk, because a coherent picture seemed to be emerging from related anomalies. This could have pointed to the existence of previously unseen elementary particles that affect the decay of B mesons. Isidori gives the LHCb collaboration credit for being “honest” in admitting that its previous analyses had problems, but he regrets that it took so long for the collaboration to find the issues. . . .

The remaining hopeful hints of new physics include a measurement that found the mass of a particle called the W boson to be greater than expected, announced in April. But a separate anomaly, also involving muons, could be going away. The muon’s magnetic moment had seemed to be stronger than predicted by the standard model, but the latest theoretical calculations suggest that it is not, after all. Instead, the discrepancy could have originated in miscalculations of the standard model’s predictions.
 

Suggested for: LHC To Announce Its Lepton Universality Violation Results On Tuesday (20-Dec)

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