When is a Higgs not a Higgs?

Last Thursday, along with most of the STFC Technology and Accelerators Advisory Board (TAAB) I had a tour of RAL Space and RAL Technology Division. Lots of very cool stuff (and in some cases we are talking milli-Kelvin-and-below cool).

Since I intend to be analysing data from it for the next decade or more, it was satisfying to see a stave of the ATLAS Inner Tracking Detector upgrade, and peer through the window at the clean room where the detector is being assembled. Even though this is “my experiment” in one sense, this is the first time I’d seen this. The stave is an array of silicon detectors delicately bonded to their readout electronics. You can find more information here.

Meanwhile, you may have seen some headlines (although I’m pleased to see my old colleagues at the Guardian don’t seen to have fallen for it) claiming a new Higgs has been discovered. This appears to be some fascinating condensed matter physics dressed up as particle physics for kicks. I wish the scientists and university press officers concerned didn’t feel the need to do that. There’s a harsh-but-fair summary at Peter Woit’s blog.

The distinction between particles thought to be fundamental, and particles known to be composite, is pretty important. When we say “fundamental” we essentially mean that a particle has no intrinsic size, or internal structure. It is just itself, not made of anything else. This is always a provisional statement of course, since if you increase your resolution (for example by colliding particles with higher energy), a previously unobservable inner structure may be revealed. But in the Standard Model (itself a provisional theory) particles like the electron, the quarks, or the Higgs boson discovered nearly ten years ago at CERN, are fundamental. The axial Higgs in the articles linked above is not.

Condensed matter physicists aren’t the only ones to blur this line. Some colleagues from the LHC do enjoy talking about how many new particles they have discovered. They aren’t wrong, but apart from the Higgs boson these are all composite particles, made up of quarks bound together by the strong force. It’s fun to blur the line sometimes, but it has caused genuine confusion on occasion; I pity the science journalists trying to make sense of it sometimes.

Which is not to say these new particles are boring, and nor is the condensed matter “Higgs on a table-top” version. Observing and understanding the ways in which new phenomena emerge from the fundamental constituents and forces of nature is important, interesting and often extremely useful. But it is not the same thing as discovering a new fundamental object. Maybe think of it as the difference between discovering a new tactic in a chess game, and discovering a new rule of the game.

Back to the TAAB, and the tour I started with: One of the best things about being on that board is getting to see great science and technology across very broad range of areas (including particle physics and condensed matter physics, but much, much more besides), and meeting and discussing with the people doing it. There are so many ways in which these disparate areas enhance each other, deliberately or serendipitously. It’s all rather exciting, and there’s no need to dress up in each others clothes.

Update: Here’s another commentary on this from Matt Strassler, and see also his comment below.

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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3 Responses to When is a Higgs not a Higgs?

  1. Peter Hobson says:

    I am surprised that no one managed to work graphene into this somewhere 🙂 Thank you for the link to Peter Woit.

  2. Hi Jon — just wanted to point out another subtlety of language and concept here. It’s not just an issue between “elementary” and “composite”. After all, protons are composite, but they share a feature with elementary electrons: they can travel throughout the vacuum, and cross the whole universe. The analogue particles such as this new “axial Higgs mode” are trapped within their materials; they’re not just “composite”, but “emergent from a material”, and so they’re stuck inside it and can’t go anywhere else. [For those who might ask: the vacuum is not itself like an ordinary material, because no ordinary material can have such a strong degree of Galilean/Einsteinian relativity as we see in the vacuum.] This is true, of course, for all the analogues for particles that arise in condensed matter physics… and that’s why high-energy particle physics, which is exploring the cosmos itself, differs from exploring the quasi-particle zoo of condensed matter systems.

    • Good point Matt. And I guess you could add that for unstable-but-fundamental particles like the SM Higgs, which decays before it crosses the universe, the quantum field that supports it propagates in the vacuum and doesn’t require a material substrate (and even has a non-zero expectation value in the vacuum in that case, of course…)

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