Friday, April 6, 2012

Early Higgs data fits Standard Model except for electro-weak boson ratios

It is starting to become possible to compare early probably Higgs boson data to theoretical predictions (which are quite specific in the Standard Model and have margins of error no more than 5%-10% at given Higgs boson masses for many branching ratios). Mostly they fit the Standard Model, but the production of electro-weak bosons isn't quite as predicted. The are more diphoton decays (involving the electromagnetic force boson, the photon) and fewer weak force boson decays (involving the W and Z bosons) than expected.

[O]n average, the production rate is consistent with that predicted by the Standard Model (the green line). Furthermore, one can read off that the the null hypothesis (the red line) is disfavored at the more than 4 sigma level. Thus, black-market combinations confirm that Higgs is practically discovered . . . while the Standard Model is a good fit to the combined data (chi-squared of 16 per 15 degrees of freedom), there are a few glitches here and there. Namely, the inclusive rate in the diphoton channel is somewhat enhanced (in both ATLAS and CMS) while that in the WW* and ZZ* channel is somewhat suppressed (especially ZZ* in CMS and WW* in ATLAS). Moreover, the exclusive final states studied in the diphoton channel (with 2 forward jets in CMS, and with a large diphoton transverse momentum in ATLAS) show even more dramatic enhancements, by more than a factor of 3. It may well be a fluke that will go away with more data, or maybe the simulations underestimate the Higgs event rates in these channels. Or, something interesting is going on, for example the way the Higgs boson is produced in proton collisions not exactly the way predicted by the Standard Model....

From Resonnances. (Update 4/8/12: A scholarly version of essentially the same analysis from Moriond 2012 data, is found here.)

One way to better fit the data would be to assume that fermions and bosons have different coupling constants, with the coupling of fermions to the Higgs boson either being less strong than for bosons, or of opposite sign (although a zero coupling with fermions does not fit the data).

Of course, these are early days, there are many ways one could model the differences between the data and the Standard Model prediction, and deviations from a Standard Model prediction could be due either to errors in the way the calcuation is done or experimental error (all of the results but the diphoton production rates are within the experimental error bars and since diphotons are the most diagnostic of the decays used to identify the existence of the Higgs boson, the sample may be biased, i.e. it may be that but for a diphoton fluke we wouldn't have discovered convincing signs of a Higgs boson at all until many years later).

Still, a slight tweak in the characteristics of the Standard Model Higgs boson, particularly if it were something so elegant as an opposite sign coupling constant for fermions and bosons, would be some interesting beyond the Standard Model physics (interestingly of a kind which had few proponents in advance of experimental indications for it in 2011-2012).

UPDATE 4/8/12: Why are diphoton decays such a big deal diagnostically for the Higgs boson?

Mostly because they are a strong indicator of a spin zero particle, which is what a Standard Model Higgs boson (unlike every other form of fundamental particle yet discovered, although there are pseudo-scalar composite baryons with an overall spin of zero from fundamental particle components that are not spin zero).

Also, perhaps coincidentally, the 125 GeV Higgs boson mass happens to be the one at which predicted branching ratios (i.e. percentage production rates in decays) for diphoton decays is near maximal among all possible Higg boson mass decay rates.

A decaying fundamental fermion (spin 1/2) or spin-1 boson doesn't produce diphoton decays, because spin is preserved in decay processes, and a diphoton decay must have either a spin of zero or a spin of two, neither of which are consistent with fermion or spin-1 boson decays. These particles could decay to produce products including two photons, but then there would be something else produced as well. You can have a decay in which the experiment misses the extra particle, making two photons and something else look like merely two photons, but the more diphoton decays you see, and the better you can establish the rate at which the experiment will miss particles from other decays, the more strongly you can rule out that possibility and establish that you must be seeing a Higgs boson (or other spin zero or spin two particle) decay.

Once one establishes that the particle that you have has (1) a neutral electric charge (something true of a Standard Model Higgs boson, but not of many beyond the Standard Model Higgs bosons, W bosons, hypothetical W' bosons, higher order quarks or electron-muon-taus), (2) a spin of zero or two (which rules out electrically neutral neutrinos, photons, Z bosons and gluons), (3) interacts via the weak force (which implicitly must be the case in anything that decays into something else), and (4) has a mass of around 125 GeV (which rules out gravitons, which would have no mass, and a host of other hypothetical particles which would have to have zero or much smaller masses or much greater masses), all of which have been accomplished, all of the properties that are necessary to distinguish a Standard Model Higgs boson from all other Standard Model particle and a vast variety of beyond the Standard Model particles, are in place.

Coupling constants are among the only moving parts that could even theoretically be different between the conventional Standard Model Higgs boson (which couples equally to fermions and bosons), and something that presents the way that the experiments seem to indicate. And, a difference in Higgs boson coupling constants from the default minimal Standard Model Higgs doesn't necessarily prevent it from solving the theoretical gaps that it was hypothesized to fill in the Standard Model.

This Standard Model extension, if it was present, would be more on a par with the discovery that neutrinos have mass, than the discovery of a new kind of fundamental particle or fundamental force (although, describing the interactions mediated by the Higgs boson, basicallly inertia, as a fundamental force in its own right, wouldn't be profoundly wrong minded, as the term "force" has come to mean in particle physics a "type of interaction mediated by a particular kind of boson").

A variety of alternatives to the Standard Model Higgs boson that might still be consistent with the data are explored in a March 30, 2012 preprint. A March 26, 2012 preprint suggests strategies for experimentally distinguishing CP odd and CP even Higgs bosons, something akin to the distinction between left handed and right handed fermions.

Given the deep connections between the weak force and the Higgs boson in the Standard Model (three of the four Goldstone bosons predicted by electro-weak theory are "eaten" by the three weak force bosons, the W+, W- and Z), a conjecture comes to mind. It might very well be the case that a unpredicted discrepency between the Higgs boson interactions with bosons and fermions, if real, could flow from the possibility that the Higgs boson in only coupling to left handed fermions (the way that the other weak force bosons do), cutting its effective strength with fermions in half.

A theory favored by the kind of anomolous Higgs couplings seen would be a light composite Higgs boson model. Higgs couplings with invisible dark sector particles are disafavorex.

1 comment:

andrew said...

A bit more than three years later, with more data collected, the fit of the data to the Standard Model Higgs boson is significantly improved.