Tight New Bounds On The Higgs Boson Mean Lifetime

The ATLAS experiment at the Large Hadron Collider (LHC) has experimentally limited the "width" of the Standard Model Higgs boson with a rest mass of about 125 GeV to 4.5 + 3.3 - 2.5 MeV, with a maximum value of 10.5 MeV at a 95% confidence level. Existing physics instruments aren't powerful enough to directly measure the Higgs boson's width, although they can indirectly bound it, as the linked latest experimental measurement did.

In the Standard Model, the theoretically calculated width of its sole 125 GeV mass Higgs boson is 4.1 MeV (which implies a mean lifetime of 1.56 * 10^-22 seconds). The result is consistent with the Standard Model expectation and very tightly observationally constrains deviations from the Standard Model width of the Higgs boson.

The width of a particle in this sense is the mean lifetime of a particle expressed in terms of electron volts rather than seconds. Width is inversely related to mean lifetime. The larger the width, the shorter the mean lifetime. The smaller the width, the longer the mean lifetime. One divided by width equal mean lifetime subject to a unit conversion constant from electron volts to seconds.

For comparison purposes, the width of the top quark is about 1,320 MeV, the width of the W boson is 2,085 ± 42 MeV, and the width of the Z boson is 2,495.2 ± 2.3 MeV. These imply mean lifetimes on the order of 10^-25 seconds. A particle that can't decay and is stable, like an electron or a proton, has a width of zero.

The width of a particle, in this sense, can be calculated theoretically in the Standard Model for any particle in the Standard Model fundamental or composite, from its Standard Model properties. 

This is done by identifying every possible way that the Standard Model fundamental or composite particle is allowed to decay in the Standard Model, calculating the likelihood that this will happen in a given time period given the Standard Model experimentally determined parameters, converting all of these probabilities into width units, and then adding up all of the widths for particular decay paths to get a total width of the particle. If a particle has an experimentally measured width greater than the Standard Model prediction, then that means that you missed a possible decay channel of the particle (possibly via a non-Standard Model particle, and possibly because you just screwed up and missed a possibility).

The measured value of the Higgs boson width in consistent at a 0.16 sigma level with the Standard Model predicted value. The 95% confidence interval boundary implies that overlooked beyond the Standard Model decays of the Higgs boson not considered in determining the Standard Model predicted value can't have combined widths of more than 6.4 MeV without being in tension with this observation. So, this measurement significantly limits the extent to which there can be beyond the Standard Model particles that get their rest mass via the Higgs mechanism.

Since all fundamental particles in the Standard Model (with the possible exception of neutrinos) get their rest mass via the Higgs mechanism, the width of the Higgs boson, like the anomalous magnetic moment of the muon (muon g-2), the decays of the W and Z bosons, and the relative masses of the W boson and the Z boson, are significant precision global constraints on possible undiscovered fundamental particles (e.g. particles that could give rise to dark matter particles, or fifth forces). They place tight limits on the masses and properties of any beyond the Standard Model particles.