Friday, December 14, 2012

LHC: "Higgs Boson Mass Estimates Fuzzy"

The big news in physics in 2012 was the official discovery of the Higgs boson, which was actually all but certain a year ago. Now that the dust is settled, the details are being examined and an interesting nuance has come up.

There are two experiments at the Large Hadron Collider (LHC) that have looked for and found the Higgs boson. One is called CMS, and the other is called ATLAS.

The various means of detecting a Higgs boson at CMS has produced mass estimates for the Higgs boson that are consistent with each other.

But, at ATLAS, the mass determined by some methods is a bit more than two standard deviations different than the mass determined by other methods used at the experiment. The high number at ATLAS (from diphoton measurements) is 126.6 GeV. The low number from ZZ decays is 123.5 GeV. A combined number that is the best fit to the combined data is about 125 GeV. The measured value of the signal strength of the Higgs boson evidence at ATLAS is also about 80% higher (about two standard deviations) than the expected value, although this may be due in part to systemic measurement errors and biases in the mass fitting formula used.

By comparison at CMS the mass estimate based on ZZ decays are at 126.2 +/- .6 GeV, and the mass estimate based on gamma gamma decays is around 125 GeV. So the CMS masses are compatible with and in the middle between the two extreme ATLAS values, and a best fit to the combined CMS data is between 125 GeV and 126 GeV. In all likelihood, the discrepency seen at ATLAS is simply a matter of measurement error and, in fact, there is just one Higgs boson with a mass of something like 125 GeV (a crude average of the four measurements would be 125.3 GeV, and where there are good reasons that a more sophisticated combination of the four measurements are more technically correct, this isn't far from the mark of what makes sense). This would be consistent with both of the ATLAS measurements and the CMS measurement at about the two standard deviation level. (For comparison, estimates from four and a half months ago are summarized here.)

But, there is another possibility. There are well motivated beyond the standard model theories in which there is more than one neutral charge, spin zero Higgs boson, and if there were, there could be two such Higgs bosons similar in mass to each other and that would also produce a greater than otherwise expected Higgs boson signal. This is the case in almost all SUSY models.

While signal strengths after new data are mostly migrating towards the Standard Model expected strength, the diphoton data remain stronger than expected at both ATLAS and CMS even as new data come in. So this is looking more like the stronger than expected signal in the diphoton channel could have real physical meaning, rather than simply being a fluke.

I don't think that the LHC is really seeing two different Higgs bosons, and neither do lots of other people who nevertheless have duly noted the possibilty. But, it is the most interesting story from the LHC results at the moment, so it deserves a mention. The existence of two neutral Higgs bosons, rather than just one, would revolutionize physics, would be the only beyond the standard model experimental result other than neutrino oscillation in the last half a century, and would dramatically tip the balance in the SUSY v. no SUSY determination.

Another interesting new little tidbit is that further analysis of the data has determined that the Higgs boson has even parity and spin-zero, rather than spin-2 or odd parity, at a 90% confidence level, as expected.


Andrew Oh-Willeke said...

More discussion here including this update on spin and parity:

"Spin (from photons): spin 2 disfavored at the 91% level; compatible with spin 0.

Spin (from leptons): spin 2 disfavored only at the 85% level; compatible with spin 0.

Parity (from leptons): Exclusion of odd-parity spin-0 particle at 99%."

and on the possibility of two Higgs bosons of similar mass here.

Andrew Oh-Willeke said...

As I noted in the post on the previous ATLAS and CMS data releases linked in the post above, the "Higgs mass suggested by the equation 2H=2W+Z implies a mass of 125.988 +/- 0.015 GeV at current Particle Data Group estimates for the W and Z masses[.]"

The current CMS mass estimate, the ATLAS estimate before the data released last week, and one of the two ATLAS estimates in last week's data, are all consistent with the 2H=2W+Z value at a less than one sigma level.

In this reading of the data, the ZZ value of 123.5 GeV would be an outlier from the other values by this measure, perhaps due to some combination of a statistical fluke (the 123.5 GeV ZZ number is based on only about a dozen four lepton events according to Matt Strassler) and perhaps a modest calibration error.

It is quite exciting that just two experimentally measured parameters can describe the W, Z and Higgs boson masses, particularly since the relative values of the W and Z masses are themselves already related by the weak mixing angle. So, one experimentally measured mass value and one experimentally measured mixing angle produces all of the boson masses.

The relationship is also exciting because it suggests that the Higgs boson mass and spin are equal to a linear combination of the masses and spins of the four electroweak bosons: the W+, the W-, the Z and photon according to an ansatz suggested by the QCD mesons that are considered to be linear combinations such as the kaon.

Of course, this still leaves some mysteries, such as why the vacuum expection value of the Higgs field of about 246 GeV differs from the 252 GeV value that would be two times the Higgs boson mass. Query if the 3% discrepency could be accounted for by the running of the weak boson masses since the 246 GeV vev is as far as I know, determined at a W boson mass energy level rather than a Higgs boson mass energy level. (The Higgs boson rest mass of 126 GeV of whatever it turns out to be is implicity determined at the energy level of the Higgs boson mass, just as the W and Z boson rest masses, respectively, at their usually quoted values, are determined at their own respective masses.)