Exploring the “Higgs Portal”

The Higgs boson is unique. Does it open a door to Dark Matter?

All known fundamental particles acquire mass by interacting with the Higgs boson. Actually, more correctly, they interact with a quantum field which is present even in “empty” space, and the Higgs boson itself is an “excitation” – a quantum ripple – in that field. Either way, what this means is that if you create a Higgs boson, it can and will decay, producing other particles, via those same interactions.

If Dark Matter is some new, currently unknown, fundamental particle, then it would make sense if it also got its mass in the same way. If so, when we create Higgs bosons at the LHC perhaps they sometimes decay to Dark Matter? Models like this are called “Higgs portal” models.

Unfortunately, Dark Matter is, well, dark. Difficult to see. The only way to know whether a particle collision created a Higgs boson which then decayed to some invisible dark particle is to look for imbalances in the momentum of the particles you can see, and maybe some clues from other features which are often produced together with a Higgs.

This is one of the things the ATLAS and CMS experiments have been doing, and today ATLAS have combined their results so far. The bottom line is that the Higgs may be decaying into Dark Matter, but not more than about 25% of the time. If the Higgs portal exists, only one in four at most of the bosons go through it.

You can see this result on the plot above for a couple of possible Dark Matter models (red and blue curves), along with Dark Matter limits from some other experiments. Note that once the Dark Matter mass (mWIMP) is greater than about half the Higgs mass of 125 GeV, there’s no information, since the Higgs decay would not release enough energy to produce two Dark Matter particles.

About Jon Butterworth

UCL Physics prof, works on LHC, writes (books, Cosmic Shambles and elsewhere). Citizen of England, UK, Europe & Nowhere, apparently.
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