Sunday, April 2, 2017

Higgs Boson Decays To Tau Leptons Confirm Standard Model

The Standard Model Higgs boson is predicted to sometimes decay to tau leptons (third generation electrons and positrons), at a frequency that is a function of their mass. The Standard Model Higgs boson is also predicted to have no CP violation at leading order.

Both of these predictions hold true experimentally, although neither exclusion of beyond the Standard Model hypotheses is ultra-precise, even though these are the best exclusions available.

The Latest Results

A March 29, 2017 pre-print combining the ATLAS and CMS data from Run-I at the Large Hadron Collider (LHC) on the topic found that the Higgs boson coupling to the tau lepton (which had a 5.5 sigma signal in the combined data) was consistent with the Standard Model prediction to within the limits of experimental error, and that there was no CP violation observed to within the limits of experimental error. The abstract is vacuous, so I'll quote some highlights from the body text instead:
The search for CP violation in the interactions of the newly discovered Higgs boson with the other Standard Model (SM) particles is motivated by the lack of sources of CP violation to explain the baryon asymmetry observed in the Universe. In the SM, no effect of CP violation is expected at LO in the production or decay of the SM Higgs boson. Hence, an observation of CP violation involving the observed Higgs boson would be a strong sign of physics beyond the SM. The H → ττ final state is very powerful for studies of CP invariance of the Higgs boson couplings. It is one of the most sensitive channels for the Vector Boson Fusion (VBF) Higgs boson production and it is the most sensitive for the Higgs boson decay into fermions. With H → ττ events, it is possible to probe the CP structure of both the HVV couplings to gauge bosons in VBF events and also of the H f f couplings to fermions. . . .
In combination with the ATLAS and CMS results in the other Higgs boson decays, these two analyses lead to the observation of the H → ττ decays at 5.5σ and µ = 1.11+0.24 −0.22. From this combination, the signal strengths for the ggF and VBF productions measured in the H → ττ decays are 1.0 +0.6 −0.6 and 1.3 +0.4 −0.4, respectively. . . .
Investigations of CP-violating Higgs boson couplings in the decays into pairs of massive gauge bosons show no deviation from SM. . . .
The observable is a CP-odd Optimal Observable built from the leading-order matrix element for the VBF production. An effective Lagrangian is used to include CP-violating effects from operators with mass dimension up to six in the HVV couplings. The effective Lagrangian assumes that the same coefficient is multiplying the CP-odd structures for the HW+W−, HZZ and Hγγ vertices and that all other couplings are as predicted in the SM. Under these assumptions, the matrix element M is the sum of the SM CP-even contribution MSM and a CP-odd contribution Mcp−odd from the dimension-six operators parametrised by the parameter d˜: 
M = MSM + d˜ · Mcp−odd. 
The Optimal Observable OO is defined as the ratio of the CP-odd interference term between MSM and Mcp−odd to the SM contribution. . . .
Since the Optimal Observable is CP-odd, a non vanishing mean value (OO) not equal to 0 is an indication of CP violation.  
The observed mean values (OO) in the data selected in the signal regions are 0.3 ± 0.5 in the ll channel and −0.3 ± 0.4 in the lh channel. Both results are consistent with zero within uncertainties and show no hints for CP violation. 
Limits on the CP-odd couplings are set based on a combined maximum-likelihood fit to the Optimal Observable distributions in data both in the ll and lh channels. Fig 2 shows the result of this fit for the SM d˜ = 0 hypothesis and the best-fit µ = 1.55+0.87 −0.76 in the lh signal region. The regions d˜ < −0.11 and d˜ > 0.05 are excluded at 68% CL. These intervals are an order of magnitude better than those obtained by ATLAS using the Higgs boson decays into gauge bosons. The analysed data does not provide enough sensitivity to set 95% CL intervals though. 
Bottom Line

The experimentally observed Higgs boson continues to be consistent with the predictions of the Standard Model for the Higgs boson in the face of all experimental tests to date through the completion of Run-I and preliminary partial data from Run-II of the LHC.

Implications For Beyond The Standard Model Physics

Implications Of The Tau Coupling Measurement For BSM Physics

The measurement of the tau coupling is important not just because it shows the Higgs boson is as predicted, but because the properties of the Higgs boson are globally sensitive to the properties of the entire matter content of the Standard Model. This is true because all of the fundamental particles of the Standard Model derive their masses from interactions with the Higgs boson in the Standard Model. 

If there were other fundamental particles that derived their masses from the Higgs boson, the properties of the Higgs boson in its decays that are observed experimentally would be different. The experimental data would be particularly sensitive to any missing heavy fundamental particles, which are the hardest to exclude by other methods. 

For example, if there were another particle that derived its mass from the Higgs boson with a mass greater than the 1.776 GeV of the tau lepton and less than or equal to half of the Higgs boson mass of about 62.5 GeV, this would cause the tau lepton signal strength to be much different than the observed signal. Yet, any deviation from the Standard Model signal strength of more than 59% greater than the Standard Model signal or more than 33% less than the Standard Model signal strength is ruled out with 95% confidence by the Run-I data.

A new heavy particle with a mass between 1.776 GeV and 62.5 GeV would tend to make the tau lepton signal weaker. A new light particle with a mass under 1.776 GeV would tend to make the tau lepton signal stronger. Both these kinds of deviations from the Standard Model are disfavored pretty much to the point of being ruled out, except for a possible new very light particle that derives its mass from Higgs boson interactions, by this measurement.

Of course, none of this is terribly earth shaking news. Any particle interacting with weak force interactions via W boson or Z boson with a mass of under 45 GeV have been ruled out since the LEP experiment operated at the LHC's location before it was dismantled in 2001 to make room for the LHC. And, other data from experiments including Tevatron and the LHC have ruled out new particles in the 45 GeV to 62.5 GeV range long ago.

New particles with masses more than 62.5 GeV that derive their mass from Higgs boson interactions might also impact the Higgs boson signal strength of the tau lepton, but I don't have the expertise to say that with confidence. Apparently percent level deviations in couplings can arise from multi-TeV scale new physics.

Of course, if beyond the Standard Model particles derived their mass entirely via mechanisms that don't involve the Higgs boson, this limitation would no longer apply. The trouble is that most two Higgs doublet models with five Higgs bosons rather than one, or more arcane models with even more Higgs bosons, don't interact with new particles in a manner entirely independent of the Standard Model Higgs boson. So, the tighter the fit of the experimentally observed Higgs boson to the Standard Model Higgs boson is, the less room there is for beyond the Standard Model theories with additional Higgs bosons as well.

What If There Is A New Physics "Desert"?

A scenario in which any beyond the Standard Model high energy physics manifest only at many orders of magnitude above the "electro-weak scale" of a hundreds of GeVs, after a "desert" of new physics, becomes more likely with each passing month as new LHC results are released.

If this is the case, we will probably never be able to observe beyond the Standard Model high energy physics in man-made particle colliders.

Even most "natural experiments" in the vicinity of high energy events related to stars and black holes would not have high enough energies to display beyond the Standard Model physics.

For example, it is a hundred times too cold in the hottest parts of the Sun (about 15,000,000,000 degrees Kelvin, to reach the temperatures where a high energy phenomena predicted by the Standard Model, called quark-gluon plasma, can occur (1 GeV per cubic femtometer, which is equivalent to a temperature of about 2,000,000,000,000 degrees Kelvin), even though this energy level has been reached and this phenomena has been observed, in Earth bound particle accelerators.

The first time humans were able to artificially create these energy densities (a.k.a. temperatures) was in 2015 at the Large Hadron Collider (although non-definitive hints that we might have done it were seen at other colliders as early as 2005). Nothing in the solar system had ever been that hot previously at any time in the four or five billion years since it came into existence. The temperatures reached in Run-II of the LHC on Earth are already hotter than those found in a supernova.

If there is a "desert" before beyond the Standard Model high energy physics, it could easily take temperatures on the order of 200,000,000,000,000,000,000 degrees Kelvin or more to see even a definitive experimental hint of this high energy new physics. There may not be any place in nature in the last 13.5 billion years (i.e. in any directly observable period of time since the Big Bang) that has been that hot. At best, we might find indirect evidence in gravity waves discernible in the cosmic background radiation of the universe that might suggest new physics at those temperatures.

Indeed, if there is a "desert" of beyond the Standard Model high energy physics, the details of these physics would be relevant only philosophically and for cosmology applications, since the vicinity of the Big Bang is the only time those physics would come into play.

Implications Of The Lack Of CP Violation For Cosmology

While the data aren't inconsistent with zero CP violation, the exclusions aren't terribly strong. But, the exclusions are still strong enough, and robust enough since they confirm finding from a different measurement of CP violation by the Higgs boson, to make the Higgs boson a poor candidate to explain the matter-antimatter asymmetry observed in the universe. This key cosmology question, therefore, remains an open one.

The Standard Model has only one source of CP violation (a phase in the CKM matrix) which is small δ13 = 1.20 ± 0.08 radians, when maximal CP violation would be π, commonly approximated as 3.14159 and is insufficient by itself to explain the matter-antimatter asymmetry in the universe. The observed CP violating phase for the W boson interactions of quarks is about 40% of the maximal CP violating phase.

There is almost certainly a second source of CP violation in the Standard Model extended to accommodate neutrino masses (a phase in the PMNS matrix of neutrino oscillation) which is believed to be much larger and near maximal based upon preliminary measurements with large error margins. But, without a bridge that does not exist in the Standard Model between neutrinos, where the CP violation probably occurs, and quarks, where the matter-antimatter asymmetry of the universe is observed, the cosmology problem remains and initial conditions of the universe in which the matter-antimatter asymmetry already exists are necessary.

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