Half-life of the Higgs boson

This plot shows how well the Higgs half-life (λ) is known: Less than 20 yoctoseconds is ruled out with 95 percent confidence, but greater than 20 yoctoseconds is still possible. The standard prediction is that the half-life is 100 yoctoseconds.

It is almost two years since physicists from the CMS and ATLAS collaborations announced the discovery of a Higgs-like boson. Today, the evidence has strengthened to the point that they no longer qualify it as “Higgs-like.” The signal is now much clearer, the particle is spinless (as a Higgs boson must be), more decay modes have been observed, and the proportions of decays into those modes are about right (with 15 percent uncertainties). What more could you want?

Perhaps its decay rate: The Higgs boson is an unstable particle, so it decays within a characteristic length of time. Although the time for an individual particle to decay is random, each type of particle has an average lifespan. The time for roughly half of a collection to decay is called its half-life. The half-life of the Higgs boson is not known, but it is predicted to be 100 yoctoseconds (septillionths of a second), which is a rather long time for a particle of its mass.

A measurement of the Higgs boson half-life would tell us a lot. Currently, only a few final states have been observed, which add up to about 2 percent of all predicted decays. For all we know, they might be nonstandard Higgses, and most of them might be decaying into exotic particles. Knowing the total decay rate would put an upper limit on this possibility. It would constrain even the decays that we don’t see.

CMS scientists recently attempted to measure the Higgs boson’s half-life and determined that it is at least 20 yoctoseconds. This analysis established a technique that will be applied to larger data sets, which are needed to fully measure it.

The technique is significant, because direct measurements of the half-life are far too insensitive. If, for instance, you tried to measure a Higgs’ lifespan from the distance it flies between its production and its decay, you’d be trying to measure a distance that is much smaller than an atom, beyond the capabilities of any microscope. Instead, you might take advantage of a fact from quantum mechanics, one that states that the half-life of a particle is inversely proportional to the uncertainty in its mass. Unfortunately, the detector’s mass resolution is a thousand times too insensitive to see that uncertainty. The physicists who performed this study used a clever trick involving the ratio of the real Higgs production rate divided by the virtual Higgs production rate and managed to constrain the half-life within a factor of six of its predicted value. No small feat!

Jim Pivarski

These physicists contributed to this analysis. Top row, from left: Ian Anderson , Ulascan Sarica, Andrei Gritsan, all from Johns Hopkins. Bottom row, from left: Chris Martin (Johns Hopkins) and Roberto Covarelli (Rochester).
Members of the Scientific Computing Division’s Experiments Facilities Department provide support for Linux servers used in the CMS Remote Operations Center at Fermilab.