Higgs Pt. 7: What a Higgs boson looks like

-September 21, 2012

We have everything we need to look at screenshots from the detectors and make sense of the evidence for the Higgs boson.

In the prevailing theory, called the Standard Model of Particle Physics, the Higgs field interacts with particles to give them mass. There’s a bit of confusion here that comes from E=mc2 which was covered in the last term of your freshman physics course. The equation says that anything with energy has mass; this is called the “relativistic mass.” In practice, physicists use E2 = p2c2 + m2c4 rather than E=mc2 so that the momentum, p, is explicit and m is the less ambiguous rest mass, rather than the variable relativistic mass.

The Standard Model covers everything that has been experimentally verified about the basic building blocks of nature, but it would be perfectly content and consistent if all the building blocks were massless. Instead of having the theory provide those masses, they have to be measured and put in by hand – one reason why it’s called a “model” rather than a “theory.” We don’t tolerate such inadequacies of theories.

Mass is the inertia of an object. The heavier it is, the harder it is to turn or accelerate. The Higgs mechanism describes a field of particles, not unlike electromagnetic fields between the plates of a capacitor, that saturates the universe and interacts with particles in a way that imposes inertia on them.

Just as you don’t expect a photon to pop out of the field between the plates of a capacitor unless you kick it with something (like enough voltage to cause discharge), the Higgs field doesn’t produce Higgs bosons unless we inject some energy. When the Large Hadron Collider smashes protons into each other it provides that kick.

The detector features – high signal-to-noise in detecting muons and photons – combine with the possible decay products of the Higgs boson to provide a few “gold-plated” decay channels. In particular:

* The four muon channel: H → Z0Z0*µ+µ-µ+µ-, is a nice clean channel if one pair of opposite sign muons is consistent with the decay of a Z boson (Z bosons, discovered in 1983, are like photons only they’re quite heavy).
* The two photon channel H →  γ γ, has two very high energy isolated photons.
* The two muon, two electron channel: H → Z0Z0*µ+µ-e+e-, is like the four muon channel, but one of the Zs decays to electrons whose signal to noise ratio is less than that of the muons.

Remember from Part 6 that the ATLAS experiment dedicates a great deal of hardware to muon detection. Here is a screenshot of a four muon event:

Caption: ATLAS screen shot of a four muon Higgs candidate (photo courtesy of CERN).

The red lines indicate the direction of the outgoing muons. Notice all the orange tracks in the central detectors, if this is a Higgs event, then those come from the remnants of the colliding protons and/or a second Higgs boson that didn’t decay to an easily observed final state like those listed above.

The backgrounds are ominous and that’s why these measurements rely on statistical certainty. It’s nearly impossible to look at a single event and state conclusively that it is or is not from a Higgs.

Here’s another look at this event.

Caption: ATLAS screen shot of a four muon Higgs candidate (photo courtesy of CERN).

This view is from a perspective where you can see how the red tracks light up the muon chambers, green on the end cap, and blue in the barrel.

This is an ATLAS screenshot of a µ+µ-e+e- event:

Caption: ATLAS screen shot of a two muon, two electron Higgs candidate (photo courtesy of CERN).

The muons are easy to detect because they penetrate all the way to the outer edge of the detector, the two blue tracks. The electrons are absorbed in the electromagnetic calorimeter, the bright green deposits, and are associated with high momentum tracks, the other two blue lines.

Here’s a CMS screen shot of a µ+µ-e+e- event:

Caption: CMS screen shot of a two muon, two electron Higgs candidate (photo courtesy of CERN).

The muons are marked in red. One exits the far end cap of the detector and the other is on the right side of the screen shot. The electrons are the large electromagnetic deposits, marked in green on the sides of the detector.

Remember from Part 5 that CMS has a high resolution lead-tungstate crystal electromagnetic calorimeter. Using a crystal calorimeter was a big design risk because radiation can damage the crystals. Radiation doses continuously accumulate in the detector as it operates, but the real risk is if the proton beams are mismanaged. If the accelerator loses control and dumps a proton beam in the heart of the CMS detector, the resulting radiation damage will form clouds in the crystals, reducing their resolution. The reason CMS took that risk was to get high quality images like this:

Caption: CMS screen shot of a two photon Higgs candidate (photo courtesy of CERN).

The two photon signature couldn’t be much clearer, even with all the junk in the central tracker. Notice how the green lines indicate the vast energy deposited in the crystal calorimeter. The dotted lines extend back to the beam pipe and don’t coincide with charged tracks, indicating that they are very high energy photons.

You can see more ATLAS screen shots here: www.atlas.ch/photos/events.html  and more CMS screen shots here: www.cern.ch/go/9wnC

Next time, we’ll conclude this series by answering the question: Is the particle observed at CERN really the Higgs boson?

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