Higgs Pt. 9: What makes King Carl XVI Gustaf think it’s the Higgs Boson?

-November 04, 2013

Two of the guys who installed the Higgs mechanism into the Standard Model of Particle Physics, Peter Higgs and Francois Englert, will be awarded the Nobel Prize in physics on 10-December-2013 by King Carl XVI Gustaf of Sweden.

A year ago, I posed the question: Is the particle observed at CERN really the Higgs boson? The geezers in Stockholm on the Nobel Prize committee decided that it was; this article describes what led them to conclude that the particle observed last summer by the ATLAS and CMS LHC experiments at 125 GeV is indeed the Higgs boson predicted by the Standard Model of Particle Physics.

Bust of Alfred Nobel, the man who conceived prizes.

This article should make sense even if you missed my complete coverage of the Higgs discovery last year, but if you’re interested, see the set of links below:

Thousands of physicists, collaborating on two experiments were searching for the Higgs, so why think it might be something else? Surely several thousand Ph.D.s wouldn’t be so arrogant as to assume that what they see is what they sought, right?

With the data in hand, hordes of graduate students and post-docs guided by bands of professors and research scientists all over the world set to work comparing data to theory, 99% of them hoping against hope that they’d find evidence for a hole in the abiding theory. It appears as though, as usual, the 1% won.

The bare-bones Higgs is unique among all the other fundamental particles in the standard model. The quarks—up, down, strange, charm, top, and bottom—as well as the leptons—electron, muon, tau, and the three neutrinos that accompany each of them—are all "spin-half" Fermions. Their defining characteristic is that no two Fermions can be in exactly the same state. Dig into your memory and recall how the periodic table is built by filling atomic shells with electrons. No two electrons in exactly the same state!

The other fundamental particles are the force carriers—photon for the electromagnetic force, eight different gluons for the strong nuclear force, and three heavy particles for the weak nuclear force (W+, Z, W-)—all of which are “spin-one” Bosons. Spin-one means that they can be polarized. Individual photons, for example, are either right-circular polarized or left-circular polarized.

The Standard Model Higgs is a “spin-zero” Boson. Spin-zero means that it’s not polarizable, it just has the one boring orientation. No spin, no angular momentum at all.

A particle’s spin is its intrinsic angular momentum. To change the angular momentum of a system, like a bicycle or a spinning top, you have to apply a torque. It’s the same deal with linear momentum, to change it you have to apply a force—Newton’s laws of motion.

When a particle decays, the total angular momentum of the decay particles has to be the same as the angular momentum that the parent particle had. In other words, the angular momenta from the spins of the decay products combined with the angular momentum of their relative motion has to add up to the spin of the original particle.

Another constraint that affects the direction of decay particles is kind of weird. It’s called parity and has to do with a really cool property of the weak nuclear force that seems to explain why this universe has so much more matter than antimatter. Unfortunately, I can’t think of a way to explain it that (a) fits in the < 900 words I’m allowed here, (b) would actually make sense, and (c) wouldn’t put you to sleep. Suffice it to say that there are two possible states of parity, positive and, you guess it, negative. The standard model Higgs is positive. Parity adds an additional constraint to the directions of the decay products.

Yes, it gets complicated, but it boils down to this: the angular momentum and parity of the decaying particle constrains the directions that different decay products can travel into the detector.

All that said, the minimal standard model Higgs has the most boring decay distribution possible. It’s flat: equal probability for decay products to go in any relative direction, none of the lumpiness you’d expect of a spin-one or spin-two decay. Flat. Like a top resting on your desk, no spin, no fun.

To determine if the observed decays of the X(125) exhibit a flat distribution, the data are compared to simulations of different Higgs decays, different types of Higgs particles, like the supersymmetric Higgs that have different spin and parity, as well as background processes already observed and predicted by the Standard Model. Statistical analyses leads to likelihoods and, voila, the X(125) is deemed consistent with a spin-zero, positive parity object, just like the Higgs boson.

A simpler constraint is the frequency with which the Higgs decays into specific particles. Thorough confirmation of the identity of the X(125) requires comparison of the fractions of decays through all possible channels. As with any fresh discovery, neither experiment has the huge data sample necessary to perform a thorough analysis of these “branching fractions,” but analyses of the data they have is perfectly consistent with those predicted by the Standard Model.

The conclusion of the Nobel Committee, and most people at CERN, is that the X(125) is, indeed, the minimal Standard Model Higgs boson.

King Carl XVI Gustaf will hand out the prizes.

But could this be an altogether different spin-zero particle that coincidentally decays into the prescribed channels at the prescribed rates? It could, but that woman you used to kiss every night before bedtime who always looked like your mother was probably your mother every time.

That said, the LHC will start up again next summer and gather far more data at even higher energies. Hopefully the experiments will see something, anything, other than what the Standard Model predicts. It’s possible that they’ll resolve more than one particle right around 125 GeV or maybe they’ll see something weird in the branching fractions or, better yet, maybe with enough data, they’ll see a whole bunch of particles at different masses.

Also See
Part 1: Herding cats on the Franco-Swiss Border—an overview of the experiments and the technical challenges they face.
Part 2: What they detect—the experimental hardware consists of a particle collider, the LHC, and two huge detectors, this article describes the particles that live long enough to be directly observed.
Part 3: What they actually measure—just three properties, electric charge, energy and momentum, are measured in the experiments, this article explains what and introduces how.
Part 4: Indentifying the stuff in the detectors—how the detectors work using minimal jargon.
Part 5: ATLAS and CMS–the biggest T&M devices on earth—a look at the hardware that distinguishes the two experiments and how it all functions.
Part 6: ATLAS and CMS – catching muons and neutrinos—concludes Part 5, describing how two of the more important Higgs decay particles are measured.
Part 7: What a Higgs boson looks like—my personal favorite, with lots of screenshots from the experiments, it gives you a feel for how the experimentalists work, though this one depends a lot on the previous 6.
Part 8: Is the particle observed at CERN really the Higgs boson?—something was observed in both experiments at a mass around 125GeV, but was it really the Higgs?

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