Higgs: What’s In A Name?

UPDATE: The event has taken place. CMS observes a Higgs candidate. Adding up all channels except for tau decays, they see 5.1 sigma. Adding in the tau channel (where there has been a lack of the rare event) drops it to 4.9 sigma. The big news is from ATLAS, which is seeing a Higgs-like signal at 5.0 sigma confidence. After 48 years, the Higgs boson has at long last been discovered! Phil Plait has a nice summary. A more detailed blow-by-blow account can be found in Sean Carroll’s live blog of the event. The most concise summary ever can be seen here.

H-Week is here! H-Day arrives on July 4 with the Atlas and CMS teams delivering a press conference at CERN in Geneva, Switzerland at 2:00 AM CDT. (A webcast will be available here; and, just in case the webcast craters under the load as it did for the December event, Sean Carroll will be live-blogging from the event on his Cosmic Variance blog.)  And the rumor mill has been working in overdrive, with speculation running rampant that the discovery of the Higgs boson at a 5-sigma confidence level will be formally announced. If that happens, you know what that means: Nobel prizes (among other things).

Of course, the smart money waits for the actual announcement before popping the Champagne corks. It wasn’t until very recently that the data was “unblinded” for full analysis, and the physicists at CERN have been busily pouring over the numbers. Despite the rumors, even they don’t yet know what the outcome will actually be yet. Whatever is announced, though, progress will follow. More on that in a bit.

“What? That still isn’t settled?”

Not quite. Back in December, results of the analysis for the LHC’s 2011 data run were announced. The ATLAS team reported seeing a 3.6 sigma signal consistent with the Higgs boson at around 126 GeV. The CMS team found a 2.4 sigma signal at around 124 GeV. Those results were promising, but not strong enough to definitively claim a discovery. That requires a 5 sigma signal.  Much more data is required to either drive the signal up to 5 sigma, or have the existing signals whither away as statistical anomalies.

Here we are, halfway through the 2012 run, but already the ATLAS and CMS teams have collected more data than they had during the 2011 run. How is this possible? First of all, the LHC is now operating at higher energies than it did last year. Secondly, the proton bunches in the LHC beam are being produced more frequently.

“What’s a sigma?” I hear you cry. In statistics-speak, a sigma is one standard deviation and is an overall measure of how close the data points in a given collection of data are to the mean (average). In the case of the hunt for the Higgs, the Higgs boson cannot be observed directly, since it is rather massive and consequently decays rapidly. However, it can be observed indirectly by its decay products. The complication here is that the decay products for the various decay channels predicted for the Higgs boson match up with the decay products of other known processes. The trick is to calculate (via a combination of simulations and experimental data) the expected rates at which those other processes will create those decay products to establish a statistical baseline. Subtract that from the actual data recorded by the ATLAS and CMS detectors, and whatever is left is the suspected Higgs signal. (To complicate matters, the decay channels that are the least Standard Model background signal are the rarest decays.) The number of sigmas away from the baseline indicates the likelihood (and I am being very sloppy with terminology here) of the signal being real and not just a statistical fluke. The standard in particle physics for proclaiming the discovery of a real effect is 5 sigmas.

By way of analogy, for a coin flip the expected outcome for multiple flips is that 50% of flips will result in heads and 50% in tails. That is the expected baseline. But if I flip a coin 10 times, there is a statistical chance that I might get, say, 7 heads and 3 tails. That isn’t enough of a sampling to say that the coin is not a fair one. But the more flips I make, the breakdown of outcomes should gradually converge to the expected 50/50 baseline. Otherwise, if the breakdown continues to hover around 70/30, we have a signal indicating an anomaly. At that point, we can legitimately suspect that the coin is an unfair one.  The increased amount of data being collected in the current run can be though of as being like increasing the number of coin flips in our data sample.

The Higgs boson: What is it?

In the general press the Higgs boson is frequently referred to as being the particle responsible for giving other particles their mass, but that isn’t quite right. It is also frequently referred to in popular media as “the God particle,” but that isn’t how physicists refer to it. That particular moniker was dreamt up by some publishing marketing guru as the title for Leon Lederman’s book on the topic. It should be called the Godot particle. We’ve been waiting for it for about 50 years.

Part of the confusion here is that there are three distinct but interrelated concepts involved bearing the name of Peter Higgs: the Higgs boson, the Higgs field, and the Higgs mechanism. A complete description is well beyond the scope of this posting (and warrants a full article all to itself), but, in summary, it is through interactions with the Higgs field via the Higgs mechanism that certain elementary particles (particularly W and Z bosons) are thought to acquire mass in the Standard Model of particle physics. The more strongly a given particle interacts with the Higgs field, the more mass it has.

The Higgs mechanism plays a critical role in electroweak theory. In that theory, the electromagnetic and weak forces were indistinguishable in the high-energy conditions of the early universe. Photons and W and Z bosons were identical, symmetric with respect to one another. Then, as the universe cooled, the Higgs mechanism imbued the W and Z bozons with mass, causing them to be distinct from photons. This process of spontaneous symmetry breaking, made possible by the Higgs mechanism, forms a critical part of the Standard Model.

As for the Higgs boson, it doesn’t really have much to do with the Higgs mechanism itself (other than in terms of virtual Higgs bosons interacting with the other particles). Remember that, in the Standard Model at least, and in quantum field theory in general, all elementary particles are merely localized excitations of underlying fields. The Higgs boson is an excitation of the Higgs field, which Peter Higgs predicted could be used as a means of verifying the existence of the Higgs field.

“And the winner is…”

Well, that is all well and good, but none of it really has anything to do with what this posting is REALLY about: credit for the discovery! (My High School English teacher would have been so dismayed with me for taking so long to get to the point, but those preliminaries were pretty important.)

I’ve discussed in a previous posting the sometimes tricky nature of establishing credit (or “priority”) for discoveries in science. Who discovered the Hubble expansion formula: Hubble or Lemaître? What about Special Relativity: Einstein, Lorentz, or Poincaré? Or General Relativity: Einstein or Hilbert? (See this article on the Relativity priority dispute.) The Calculus: Newton or Leibniz?

Giving credit where credit is due is a tricky proposition, but an important one. At stake is one’s professional reputation, a place in the history books, funding, and, of course, awards. Already, there are discussions of the possibility of a Nobel Prize in Physics regarding the Higgs boson should a solid discovery be announced (either on the 4th or at the end of this year). Actually, there is the potential for two: one for its theoretical prediction and one for its actual discovery.

One issue with awarding a Nobel Prize is that a single award can be split among at most three individuals. Regarding the discovery, this is a little problematic.  There are literally THOUSANDS of individuals working on the project.  In publications coming out of high energy particle physics experiments such as the LHC, the author listings generally fill up the first several pages, with hundreds or even thousands of authors being listed for an individual paper. However, for such “big science” situations, the precedent is to award the prize to the Principle Investigators for the experiment. Problem solved, nice and tidy.

For the theoretical work, it isn’t so simple. Peter Higgs did not come up with all of this all on his own. There were also contributions made by Phillip Anderson (who first actually proposed the mechanism, but without working out the details), Francois Englert, Robert Brout, Gerald Guralnik, C.R. Hagen, and Tom Kibble.

All of these gentlemen arguably stand on an equal footing with respect to the theoretical development of the Higgs mechanism. But the Nobel Prize can only be awarded to up to three people. Who? Brout has passed away. Will the Nobel Committee sit on its hands and see if the field gets winnowed more?

For more on the history of the theory of the Higgs mechanism: Englert-Brout-Higgs-Guralnik-Hagen-Kibble mechanism (history) – Scholarpedia

What comes next?

Regardless of who gets the credit, Wednesday will be a big day. If results are still shy of 5 sigma, we’ll have to wait until the end of the year to see what happens. If the signal found earlier has faded away, it would actually be a more interesting discovery.  Physicists love it when experimental outcomes do not match expectations (aside from cases where the unexpected outcome is due to error, as in the recent OPERA superluminal neutrino kerfuffle). Getting the expected result is boring. Unexpected results are an opportunity for new physics!

If a 5 sigma discovery is reported, the work is only just beginning. The teams at CERN will need to continue making measurements to measure other properties of the Higgs aside from its mass. They will need to refine their measurements of the branching ratios of its decay modes in order to answer an even bigger question: Is this the Standard Model Higgs, or one of many proposed variants?

Come Wednesday, we may very well have a clearer picture of the next steps.

For more information:

Important papers related to the history of the Higgs mechanism

Note that Higg’s paper was simply the first to present a relativistic version of a model proposed earlier by Anderson (applying concepts from the BCS theory of superconductivity) and others:

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About Glen Mark Martin

MCSE-Messaging. Exchange Administrator at the University of Texas at Austin. Unrepentant armchair physicist.
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One Response to Higgs: What’s In A Name?

  1. Pingback: Nobel Prize Rumors and the Higgs Boson | Whiskey…Tango…Foxtrot?

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