Of Particular Significance

1. Is it a Higgs?

Matt Strassler [January 17, 2013]

Is the Newly Discovered Particle a Higgs Particle of Some Type?

The first set of questions to address is whether this new particle at all resembles a Higgs particle. What properties must a Higgs particle have? And does the new particle have those properties? The short answer is that using the November and December measurements, which supplement the measurements made for the July discovery, we can see that everything indicates the new particle has all of the properties of a Higgs particle, though we still do not have enough data for this conclusion to be convincing. Nothing so far has shown up to make us doubt this is a Higgs particle of some type.

Now here are some of the details, reviewed at the Higgs Symposium by the two experimentalist speakers, Joe Incandela and Eilam Gross, and by theorist John Ellis. Some of this section is a bit technical, so if you like, you can skip to the next section.

  • A Higgs particle must be a boson, and must not be impacted directly by the strong nuclear force or electromagnetic force (i.e. it must not have electric charge). This is true of the new particle; we know this because it can decay to two photons (which are also bosons with no electric charge and with no direct effects from the strong nuclear force.)  [Experts: what I mean is that the new particle has neither electric charge nor `color’ (strong-interaction) charge.  I of course do not mean that it cannot have any electromagnetic interactions or strong nuclear interactions of any sort; if it is composite (see below), it may, though these effects will be inaccessible in current experiments.]
  • A Higgs particle should interact most strongly with those particles which get a large mass from the Higgs field, such as W and Z particles. As a result of these interactions, a Higgs particle should decay sometimes to W and Z particles. The new particle has indeed been observed to decay to W and to Z particles (more precisely, to one real W or Z particle and one virtual W or Z particle) with interaction strengths that appears to be proportional to their masses.
  • Furthermore, a Higgs should not be observed to decay directly to very lightweight particles, such as to an electron and an anti-electron (“positron”) — and indeed, the new particle is not observed to decay in this way.
  • An exception is that a Higgs particle should decay to massless photons, through an indirect pathway. This effect also is observed.
  • Each type of particle in nature comes with a “spin” and a “CP value”. These are not very intuitive properties, but suffice it to say that they determine how a particle can behave. A classic Higgs particle should have spin 0 and positive CP; and there is increasingly strong evidence that the new particle has these properties.

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A decay of a Higgs boson, as reconstructed by the CMS experiment at the LHC