Monday, June 25, 2012

The Higgs: To be, or not to be?

[Note from Shaun: The following is a guest post from Higgs Hunter, Mikko Voutilainen. Mikko is a colleague of mine here in Helsinki. He is a postdoc working on the CMS experiment at the LHC in CERN. Below, he rhetorically asks the Higgs boson whether it exists or not. The irony of this is that Mikko asks this question, non-rhetorically, for a living and it is quite possible that he has already received an answer. He cannot (unfortunately) tell us this answer, yet. You should consider the following a teaser for what will follow next Wednesday when CERN unveils its latest results to the world. On that date, Mikko has promised to give us another guest post where he will reveal everything he knows about, The Higgs... (I've even heard rumours that this follow-up post has already been written)]

To be, or not to be?

That's a question for the Higgs boson to answer, and we might know soon enough. CERN just (well, three days ago really, but everybody here was out in the countryside celebrating Midsummer) published a press release about having a seminar on the new results on Wednesday 4th of July.

Coincidence that it's also Independence Day for the folks in the US? Probably yes, although my collaboration, the Compact Muon Solenoid (CMS) experiment at CERN, does have a strong representation from the States, including our spokesperson Joe Incandela.

The real reason, though, is that the 4th of July is also the eve of a major particle physics conference, ICHEP, starting in Melbourne. The ATLAS and CMS experiments will deliver the preliminary results of their 2012 data analysis there, and the seminar will be a kickoff for these presentations (you can see the live broadcast at webcast.cern.ch).

The experiments at the Large Hadron Collider stopped collecting data only on the 18th of June, and everybody is now busily analysing this dataset. We actually collected quite a nice bunch of data, just over 6/fb, which is a bit better than last year. The collision energy was also raised from 7 TeV to 8 TeV, which should increase the production rate of possible Higgs bosons by 20--30%.

The amount of data collected in 2010, 2011 and 2012. One fb-1 amounts to almost 100 trillion proton-proton collisions.

People are really eager to see the new results, and for a reason. The data collected in 2011 showed some hints of a Higgs boson in the 124-126 GeV range. The amount of data collected this year is nearly equal to that collected last year so the results are directly comparable. We should be able to see whether the earlier trends are still there, or whether they've gone away. Either way, it should be pretty exciting.

The predictions made earlier indicate that a combination of the 2011 and 2012 datasets should get pretty close to five sigma, the traditional standard for a discovery in the field. Or, we should be able to rule the existence of the Higgs boson out at a 95% confidence level from the whole remaining mass window.

Predictions for the significance of a Higgs signal as a function of the boson mass. The combination of 2011 (5 fb-1, 7 TeV) and 2012 (5 fb-1, 8 TeV) data will correspond to roughly the average of the two red lines.

What happens in a week depends both on the hard work of the physicists, who are improving the sensitivity of their analysis, and, due to statistical fluctuations, pure luck. If we're unlucky, the existence of the Higgs boson may still remain a mystery, but if we're lucky, we might end the quest earlier than expected.

So, what if we find the Higgs or not? Is it the answer to Life, the Universe, and Everything? Or a piece in the puzzle of the origin of mass for the elementary particles? The latter, more likely.

If we find that the Higgs boson lacks existence, much of the theoretical work done in particle physics for the past few decades will end up in the dustbin. It's not all that bad, really, because it will allow the theorists to start from a clean slate, and that's often been a very fruitful thing. The experimentalists will continue to hunt for other particles that could replace the Higgs boson.

If the Higgs boson is found, it's properties will have to be scrutinized carefully. There are many theories out there besides the Standard Model of particle physics that predict the Higgs boson (or bosons) so determining it's precise identity might take a while. Many of the alternative theories also predict other particles, leaving plenty of work to be done for the experimentalists.

[Note: Mikko writes for a Finnish language blog, Higgs Hunters. This post is an English translation of his latest post at Higgs Hunters.]

3 comments:

  1. “The Higgs field is a triumph of quantum field theory, using the most advanced theories of our time and building on the successes of decades of research. But then the Higgs field has similar properties to the aether, it’s a perfect, isotropic field filling the universe. It interacts with everything, but it’s surprisingly difficult to see, no matter how hard we look. If we don’t see the Higgs boson, but instead see something else then we could be in for another glorious revolution in physics, and the quantum field theories could seem like quaint approximations of a bygone era.” — Aidan Randle-Conde
    http://www.quantumdiaries.org/2011/09/08/higgs-skeptic Higgs Skeptic – Quantum Diaries, Sept. 8, 2011
    Is there any conceivable way to prove that nature does not contain a Higgs field?

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    Replies
    1. Hi David, thanks for the comment.

      The answer to your question depends on what you mean by a "Higgs field". If you simply mean a scalar field that is "a perfect isotropic field filling the universe" then no, there is no way of proving that no such field exists. If such a field existed but didn't couple (or only coupled extremely weakly) to anything we can observe then there would be no way of ever observing it.

      If instead you mean you are wondering if there is a way of showing that that nature does not contain the standard model Higgs field, then yes there is a way. That is, by looking for it and failing to find it. If the LHC doesn't find excitations of the standard model Higgs field, then its existence will be ruled out at *all* masses. Its effects would have shown up.

      Of course, something must break electroweak symmetry. If it isn't the standard model Higgs field it must be something else. However, whether you give that something else the label "Higgs" or not is an issue of semantics, not physics. It needn't be a perfectly isotropic scalar field.

      I actually disagree with the sentiment expressed in your quote. If the standard model Higgs field doesn't exist, it is only the death-knell for one particular quantum field theory, not all quantum field theories. I would be much more inclined to expect that it was a different quantum field theory that was responsible for electroweak symmetry breaking than a deviation from quantum field theory entirely.

      I apologise if I haven't answered your question. If so, could you be more specific about what you mean by "Higgs field"?

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  2. Hi David,

    Excluding Higgs boson(s) from the currently studied mass window below 1 TeV would be as close to disproving the existence of a Higgs field as it gets. There are theoretical considerations forbidding the boson to have a mass much above 1 TeV, and any odd tweaks would make the theory look unnatural. From an experimentalist's point of view, if the boson is not where we're looking right now, the theory should go the way of the aether, i.e. it may not be fully disproven, but becomes very uninteresting and eventually withers away (and is replaced by something more interesting).

    Did I mention Michelson-Morley experiment is one of my all-time favorites?

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