Wednesday, February 27, 2013

More on Non-SM Higgs Boson Exclusions

In his most recent blog post, Jester examines some of the mounting evidence that the spin-0, electromagnetically neutral, even parity particle with a mass of about 125 GeV +/- 1 GeV is the Standard Model Higgs boson or something very near to it, rather than being a mere "Higgs-like" particle. 

125 GeV Particle Quantum Number Properties Already Established

The mere existence of diphoton decays (which were diagnostic of the Higgs boson in the first place) established that it was an electrically neutral bosons, as a result of the laws of conservation of electromagnetic charge (which has never been observed to be violated) and the laws regarding conservation of isospin in quatum mechanics (only even spin bosons can decay to two spin-1 particles that combined have an even spin).

Other decays observed in the 125 GeV particle decay established that it was indeed a massive particle and established its approximate mass. 

Large Hadron Collider (LHC) data excluding the possibility that the 125 GeV particle is a spin-2, or a parity odd particle, discussed previously at this blog, can be found here

Both the Standard Model Higgs boson and the supersymmetric Higgs bosons all lack the color charge of quantum chromodynamics which is mediated by color charged gluons.  So far as I know there is not yet any definitive experimental data analysis establishing that the Higgs boson lacks color charge.  But, this seems highly unlikely, as this would surely have some weird impact on the data and this is not well motivated theoretically.  Any color charged interactions would produce wild deviations from the Standard Model Higgs boson branching ratios and decay rates in many different channels that have not bee observed, unless this was highly tuned in unexpected ways or way supressed by some unanticipated mechanism.  No one out there seems to be worrying about the possibility of color charged Higgs bosons at this point.

The W and Z couplings are consistent with a the Standard Model Higgs boson so far.

Jester argues that we may fairly call this particle "the Higgs boson" (meaning the Standard Model Higgs boson or a non-Standard Model Higgs boson almost indistinguishable from it with the same role in the low energy effective theory of particle physics) if its couplings to the W and Z bosons approximates the Standard Model strength of cV=1 and may be called "a Higgs boson" if there is a coupling to the W and Z that is non-zero (which it most definitely is by many, many sigma). 

The combined data to date show that for the 125 GeV particle that has been discovered, at "the 95% confidence level cV is within 15% of the standard model value cV=1" (with a mean of about 1.05).  The LHC data, while less precise than the LEP data so far, has a mean value slightly lower than one of about cV=0.95 while the LEP electroweak precision data which is a bit more precise, implies a value a bit higher than 1 of perhaps cV=1.1.

Thus, the LHC data is consistent with the margin of error with the particle being the Standard Model Higgs boson to the W and Z bosons, and it is inconsistent with the particle being some kind of even-parity, electrically neutral, massive spin-0 particle that does not couple to the massive electroweak gauge boson, which arguably would be outside the definition of the Higgs boson, and hence is fairly described as "a Higgs boson."

One can argue for the theoretical possibility that the Higgs boson might be "fermiophobic" (and hence have suppressed couplings to both quarks and leptons) or "leptophobic" (and hence have suppressed copulings to leptons).  But, none of the data, particularly as it is about to be updated, seem to point strongly to that conclusion. 

These possibilities and the charge SUSY Higgs boson possibilities are discussed here and were motivated mostly by a possible diphoton decay excess that seems to have been nothing more than a statistical blip in the data as more results from the LHC are produced from larger data sets that include more of its run so far.  The diphoton excess is now rumored to be less than two sigma.

Heavy SM-like H' Higgs bosons excluded up to 600 GeV

The exclusion for a particle with the SM Higgs boson properties other than a 125 GeV +/-about 1 Gev extends up to 600 Gev based on LHC (large hadron collider) data released in Feb 2012. These limits exclude, for example, a "second generation" or excited state of the SM Higgs boson, call it "H' aka H prime" (by analogy to hypothetical heavy W and Z bosons called W' and Z') that are hypothesized by QCD physicists like Marco Frasca, in those mass ranges (see also his comments in a related post at Dispatches from Turtle Island).  The LEP and Tevatron data (i.e. data from two prior lower energy collider experiments that have now concluded; LEP focused on leptons, Tevatron was basically a lower powered version of the LHC) exclude ligher Standard Model-like Higgs bosons.

SM4 Higgs bosons excluded up to 600 GeV.

Another non-SUSY, non-SM theory is SM4, i.e. the SM plus a fourth generation of fermions. An SM4 Higgs boson is excluded from 120 GeV to 600 GeV by LHC and at lower masses by the LEP and Tevatron.  This finding together with other LHC data, has effectively killed the SM4 and the SM5+ as viable extensions of the Standard Model.

Limits on SUSY Higgs Bosons

LHC data on the properties of the Higgs boson powerfully constrain supersymmetry theories although there are always ways that the theory can be modified to keep it alive.  A state of the art just around the corner theory can be found, for example, in a December 31, 2012 paper by one of the leading supersymmetry theorists in the world. Even he is aware of the impact of constant fine tuning on the plausibility of the model stating:
[C]ompletely natural supersymmetric theories may still turn out to describe physics at the TeV scale, and there have been no shortage of models of this sort proposed recently in response to null-results for new physics from the LHC. It is however fair to say that these models are rather elaborate. Many of these theories are actually just as ne-tuned as more conventional versions of supersymmetry, but the tuning is more hidden. The more sensible theories of this sort may be "natural" with respect to variations of their Lagrangian parameters, but in an admittedly hard-to-quantify sense, their epicyclic character involves a tuning in "model space."

Easily Excluded SUSY Higgs Bosons.

All supersymmetry (SUSY) theories have at least five spin-0 Higgs bosons, four more than the Standard Model's one Higgs boson, and in general have 1+4n for n equal to a positive integer (Higgs bosons come in "doublets" of four and the first three are "eaten" by the massive Standard Model electroweak gauge bosons, the W+, the W- and the Z).

Two of the SUSY Higgs bosons, commonly labeled "H+" and "H-" are electromagnetically charged and hence inconsistent with the LHC data for the 125 GeV particle.  Some less minimal SUSY models also have doubly charged Higgs bosons (i.e. charges of +2 or -2, rather than the +1 or -1 of the minimal supersymmetric model (the MSSM) and the next to minimal supersymmetric model (the NMSSM) which are also excluded as 125 GeV particle candidates by the LHC data. 

The exclusion range for the H+ and H- SUSY bosons from early LHC data, in general, was at least 80 GeV to 140 GeV, and since most versions of supersymmetry theoretically exclude charged higgs bosons of less than W boson mass, this is really a 0 GeV to 140 GeV exclusion for a SUSY H+ or H-. A later LHC data analysis ruled out a SUSY Higgs boson in the 144 GeV to 207 GeV mass range in a model dependent prediction.  It also isn't obvious to me that there is a meaningful window of possibility that there are charged SUSY Higgs bosons with masses in the 140 GeV to 144 GeV range (even though there is apparently not a full fledged 95% confidence interval exclusion in that mass range).  A few diehards point out that Tevatron and LEP were not sensitive enough to detect and hence exclude SUSY H+ and H- bosons in absolutely every conceivable corner of SUSY parameter space, but acknowledge that that the non-detection of SUSY H+ and H- bosons in these experiments together with the discovery of a neutral, spin-0, partity even Higgs boson with about 125 GeV of mass dramatically narrows the possible range of values for key SUSY parameters like tan beta.

One SUSY Higgs boson, commonly labeled "A" had odd-parity but is electromagnetically neutral, and hence inconsistent with the LHC data regarding the 125 GeV particle.

Less Easily Excluded SUSY Higgs Bosons

The two other SUSY Higgs bosons, commonly labeled "H" for the heavier one and "h" for the lighter one, are even parity and electromagnetically neutral, just like the Standard Model Higgs boson and hence consistent to that degree with the 125 GeV particle. 

The LHC data analysis has not yet definitively ruled out the possibility that both an H and h with almost identical masses at this point.  Each experiment has measured the Higgs boson mass in two different ways.  Both of the measurements at one experiment and one of the measurements at the other are all very close to each other.  Another measurement at one of the experiments is 1-2 GeV different from the other data points, which is on the verge of veing statistically notable given the estimated margins of error at the LHC experiments.

The two almost 125 GeV Higgs boson possibility is disfavored theoretically.  The Standard Model predicts only one Higgs boson.  The SUSY H and h Higgs bosons aren't expected in SUSY theories to have nearly degenerate masses differences, and have other theoretical properties discussed below.

A more plausible possiblity is that the observed Higgs boson mass measurement discrepency at LHC, which is seen at only one of two measurements at one of the two parallel experiments at LHC, is due to some sort of systemic or theoretical calculation error.  This is a better fit to our currently incoomplete knowledge than a genuine bimodal mass distribution, which would be a sign that there are two different Higgs bosons with very similar masses whose data are getting mixed up with each other to make the two particles look like a single Higgs boson.

Distinguishing SUSY H and SUSY h from the SM Higgs boson via particle couplings

In the MSSM at least, and to some extent in SUSY theories generically although to differing degrees, the H and h have different couplings to other particles than the SM Higgs boson.  Given the tight fits of the observed LHC data to the results expected with the 125 GeV had couplings at the SM prediction level from the LHC data so far (and the rumored closer fit after new data that rumor has it will be released in March) are problematic for that model.   As a post at the viXra blog explains, after the conference:
we will have 40% more data for most channels and about 75% more for the diphoton channel. We know that all channels other than diphoton are perfectly in line with the standard model Higgs while the diphoton channel cross-section is a bit too large.
In point of fact, the MSSM has already been moribund for a year or two for a variety of reasons, so this model specific addition to its death of a thousand cuts isn't all that notable in and of itself. 

But, many MSSM predictions are generic to most or all more elaborate SUSY models.  While the specific expected couplings of the MSSM are unlikely to be correct, even if SUSY is the correct description of the universe, the general observation that H and h each have couplings to other particles that are different than those of the SM Higgs bosons is likely to be a generic feature of all SUSY theories. 

This is because one of the key motivations for SUSY theories in the first place (although SUSY has taken a life of its own beyond this purpose) was to solve the hierarchy problem through more transparent sources of particle couplings in the Higgs sector.  Otherwise, the Standard Model higgs boson must have properties which are very finely tuned for no apparent deeper reason.  Generically, SUSY theories accomplish this by making the couplings in the Higgs sector different than those in the Standard Model.  It may be possible to devise a SUSY theory where a single SUSY Higgs bosons nearly matches the Standard Model Higgs bosons and the couplings of all of the other SUSY Higgs bosons are suppressed, but this isn't very well motivated theoretically and destroys the "beauty" and constrained nature of SUSY models that make them attractive in the first place if done too crudely.

Distinguishing SUSY H and h from the SM Higgs boson via resonnance width.

A particle's width is the manifestation of its decay half-life in a way that is natural on a particle resonnance graph plotted from experimental data.

Another way to distinguish a SUSY H or h from a SM Higgs boson in addition to its couplings is with the width of the resonnance of the particle. A SM Higgs has a much narrower width (and hence a shorter half-life) than the SUSY Higgs boson's width in the MSSM and in most SUSY theories.  This is a measurement that should be possible to make soon, possibly from the further analysis of the pre-LHC shutdown data.  A determination of the estimated rest mass of the Higgs boson and its width go hand in hand.

Model specific properties of SUSY Higgs bosons.

Notably, in the MSSM and to a less clear extent in other SUSY models the light Higgs is always lighter than the Z Boson; the Charged Higgs is always heavier than the W Boson.

Thus, in the MSSM, the 125 GeV particle that has been detected must be an "H" and can't be an "h" which must have a mass of less than 90 GeV.  The MSSM also demands an "A" boson on the same order of magnitude in mass to the "h" if the "H" is under 135 GeV or so.

So, the non-detection of an "h" or of an "A" or "H+" or "H-" in a wide range of masses at LHC, LEP and Tevatron is a serious blow, unless the interactions of the other four SUSY Higgs bosons is so slight that they would evade detection, something that requires fine tuning of a SUSY parameter called tan beta and also a number of other SUSY parameters. 

Contrawise, if there is a neutral, parity even SUSY Higgs boson of 150 GeV or more, in which case the 125 GeV particle is an "h" if SUSY theories indeed follow these conclusions, than the "A", "H+", and "H-" must all have masses on the same order of magnitude as the SUSY "H", which could be much, much higher than anything that could be detected at the LHC.

The bottom line is that it takes a lot of fiddling with SUSY model parameters in non-minimal version of SUSY to make it fit the Higgs boson data from the LHC, even before considering other experimental constraints on these models from the non-detection of supersymmetric particles at the LHC or prior colliders, from the weak evidence for SUSY from recent astronomy observations and direct dark matter detection experiments, and from the experimental boundaries on phenomena like proton decay rates and neutrinoless double beta decay.

At some point Occam's razor must favor the far simpler Standard Model that fits all of the experimental evidence to date over the SUSY theories with far more experimentally determined parameters, far more moving parts, vast numbers of particles for which there is absolutely no experimental evidence, an ever shrinking parameter space that is far outside the original expectations of the theory and does not achieve some of the purposes that originally motivated the theory, and so on.  This really matters because SUSY is a part of every Supergravity theory, and all viable versions of M theory aka string theory, have supergravity and SUSY, rather than the Standard Model, as a low energy effective approximation.

Bottom Line: Leading BSM Theories Almost Dead Leaving Room For New Theories

Most of the viable beyond the Standard Model theories that had currency just two or three years ago, including the MSSM are dead.  The Standard Model has been completely verified by experiment.  We are past the beginning of the end of supersymmetry and string theory, although they aren't official dead yet.  We are close to the middle of the end.  But, it is hard for anyone coming to the field fresh in 2013 to see SUSY or String Theory as anything more than mathematical toy models that are unlikely to be accurate explainations of how nature really works.

The theoretical landscape of particle physics is almost empty, even though almost no one thinks that the Standard Model is a complete or final theory of particle physics at all scales and energy levels.  There is a vacuum in the world of fundamental physics right now that is ripe to be filled by someone taking a new approach not burdened by the wasted legacy of decades of dead end string theory and supersymmetry research in the theoretical physics community.  That effort increasingly looks like the worst wrong turn since a heliocentric astronomy theory replaced epicycles.

A Brief Historical Footnote

The emerging consensus is that the particle that has been discovered at the LHC with this mass is the one that Higgs and others building on preliminary electroweak unification theories in 1960, predicted in 1964. The Higgs boson had been more or less fully described theoretically except for its mass in 1972, more than 40 years earlier.

The core of the entire Standard Model that the Higgs boson is a part of, but with just two generations of fermions and zero mass neutrinos, was in place in 1974 when quantum chromodynamics in substantially its current form had finally been described, although it had to be amended later in the 1970s to include a third generation of fermions.  The only significant modification of the Standard Model, other than measurement of its experimentally measured constants, since 1975 when it became clear that it had to have three generations of particles, was that the 1975 version of the theory assumed that neutrinos were massless, which was discovered to be inaccurate in 1998.

Neutrinos were predicted in 1930 and were first observed experimentally in 1956. The muon neutrino was proposed in 1962 and was first observed in 1975. Only by 1981 when all of its particles except the top quark and tau neutrino and Higgs boson had been observed did the Standard Model really achieve full fledged scientific consensus and the existence of the Higgs boson component of the theory remained somewhat controversial until early 2011.

The W and Z bosons were observed experimentally in 1981, each of the third generation fermions were observed experimentally from 1975 to 2000 (tau 1975, bottom quark 1977, top quark 1995, tau neutrino 2000), and the Higgs boson was observed in late 2011 with the discovery becoming definitive in 2012 and the confirmation that the observed particle really is the Higgs boson continues into 2013 although the ranks of the skeptics are getting thinner.

Two competing classes of Standard Model modifications, one called "Dirac mass" and the other called "Majorana mass" have been proposed to explain this and there is not yet sufficient scientific evidence to distinguish the two possibilities. The Majorana mass explanation usually incorporating a see saw mechanism with three undiscovered companion heavy neutrinos for each known flavor, two additional CP violating phases in the PMNS matrix, and an additional kind of intraneutrino interaction, has more support in the theoretical physics community, because it fits well into grand unification theories and uses supersymmetry-like reasoning. But, the Dirac mass proposal is the more conservative of the two proposals as it requires the minimum number of experimentally measured Standard Model constants and no new particles or interactions at the expense of not explaining why neutrino masses are so small (just as the Standard Model doesn't purport to explain the values of its other constants). Both approaches have roots in ideas about the nature of fundamental quantum particles proposed in the 1930s by Majorana and Dirac respectively. The Dirac mass mechanism was adopted for other fermions in the Standard Model when it was originally formulated.

The State of Future Research

It increasingly seems likely that the rest of the LHC run, which will continue for at least another five to ten years, will confirm the Standard Model, refine the measurement of its experimentally measured constants, and rule out any other kind of beyond the Standard Model physics. 

In general, for the foreseeable future, all that the LHC can do by itself is to establish that all but one of the SUSY Higgs particles are either rather heavy, or interact very weakly with ordinary matter, but can't rule out their existence entirely by itself. It may very well be possible, however, in five or ten years, to greatly limit the parameter space of all SUSY theories, to make predictions that flow from the subset of all SUSY theories with experimentally allowed parameters, and then to use different kinds of experiments to falsify that subclass of SUSY theories.

One particularly promising approach along these lines is to set limits in the minimum mass of all particles added to the Standard Model by experimentally allowed SUSY theories and tightly bound parameters like tan beta, and then make generic predictions about what these modesl say about the rates at which they predict phenomena like neutrinoless double beta decay (which happens at a rate that generally increases in SUSY models as the masses of the superpartners in the theory increases), and then to experimentally determine that neutrinoless double beta decay does not occur at such a high rate.

There is fierce debate going on at this time regarding what kind of high energy physics experiments, if any, should be conducted when the capabilities of the LHC have largely been exhausted.  Diehard SUSY believers want bigger colliders to find superpartners at masses that are just around the corner.  SUSY skeptics wonder if scarce fundamental physics research funds aren't better spent elsewhere.

A variety of neutrino physics experiments in progress are likely to resolve that Majorana v. Dirac mass issue for neutrinos within five to fifteen years.  There is still one Standard Model constant that has not been measured at all, the CP violation parameter for neutrino oscillations in the PMNS matrix.  There have been no direct measurements of the absolute value of the neutrino masses although the relative neutrino masses are known and increasingly strict upper bounds on absolute neutrino masses have been accomplished.  The neutrino related constants will all have reasonably meaningful experimentally measured values in the next five to fifteen years as well. 

There are also many experimentally measured Standard Model constants that are not known particularly accurately and will gradually be refined at the LHC and in other experiments over the years, this will be a never ending project unless a deeper theory allowing them to be calculated from first principles is devised.  Likewise, the process of making more and more precise calculations with the Standard Model equations, particularly in QCD, is ongoing.

The process of ruling out all alternatives to the Standard Model is never ending, but I belive that the experimental data will have advanced enough in the next five to fifteen years to develop a consensus in the particle physics community that supersymmetry and string theory are not correct descriptions of the universe.

The Standard Model was fully formulated (except for neutrino mass) and had achieved consensus status, by the time that I was in junior high school.  The project of expanding it to include neutrino mass, and of experimentally validating and measuring all elements of the Standard Model will probably be complete during my lifetime, by the time that my children are old enough to be graduate students.

I also suspect that major progress will be made in formulating and empirically validating dark matter and quantum gravity theories in that time frame, although I don't hold high hopes for grand unification or a theory of everything at that point.  Much of this work will be based on astronomy research and direct dark matter detection experiments (which overlap heavily with neutrino research).  I personally suspect that no real progress will be made in developing a consensus theory that describes why the Standard Model constants have the values that they have, until all of them have been experimentally determined with a fair amount of precision already.

But, in my lifetime, we will have an essentially complete, if somewhat ugly, rulebook for all of physics and a fortiori, all of the laws of nature.

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