I am just back from STFC’s media event covering what did, in the end, turn out to be the discovery of a particle that appears to be the long-predicted Higgs boson, the last component in the Standard Model of Particle Physics to be discovered, and in many ways its linchpin.

Via a mechanism known as spontaneous symmetry breaking (first applied to so-called gauge theories of particle physics in a set of 1964 papers by Higgs himself, Imperial’s Tom Kibble with Guralnik and Hagen, as well as Brout and Englert) the Higgs couples to all of the other particles in the standard model, and gives them mass.

Suffice to say that the results so far, from the LHC experiments, ATLAS and CMS, are consistent with a Higgs with a mass of 125 GeV, interacting with other particles more or less as predicted (although only the couplings to photons and the Z boson have been strongly confirmed by the current measurements). The details have implications for any physics beyond the standard model, in particular for supersymmetry — the measurements already put tight constraints on the simplest such models. (The New York Times has a comprehensive overview, including a mention of the cosmological implications of the result; Sean Carroll has an expectedly excellent discussion of Higgs physics; Peter Coles quotes and comments on the CERN press release, and like him I will avoid mentioning the confusing frequentist statistics underlying today’s results. I’m sure a search will turn up dozens of more blogs and news articles.)

But for cosmologists, one of the most exciting things about the Higgs is that it seems to exist at all. The Higgs is a boson, which means that you can pack many of them into a single state, and therefore can be thought of as a field pervading all of space — photons, which make up the electromagnetic field, are also bosons. (This is in contrast to fermions, which cannot be brought into the same state and are thus more usefully thought of as individual particles of matter.) An even more precise categorisation of particles is via their spin: bosons can take on integer values (0, 1, 2, …) , and fermions half-integer values (1/2, 3/2, …). The known bosons, like the photons, have spin 1 and are known as *vector* particles. The Higgs, however, has spin 0, and is called a *scalar*.

Scalar fields are ubiquitous in cosmology: if they are roughly constant across space they act like a vacuum energy or cosmological constant, their (negative) pressure making the Universe accelerate in its expansion. They are therefore thought to be responsible for an early period of inflation, and possibly also for the recent domination of the Universe by dark energy. However, there have been longstanding questions about whether fundamental scalar particles can exist in quantum theories. The data are still consistent with alternative models (for example, the Higgs could be a tightly-bound pair of fermions). But if confirmed, the existence of the Higgs as a scalar particle encourages us cosmologists to continue our occasionally wild speculations on the properties of scalar fields, making the Universe accelerate at early times and again today.

## Comments