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Higgs Pt. 8: Is the particle observed at CERN really the Higgs boson?

-September 25, 2012

In this, the final installment on the discovery of the Higgs Boson, we’re going to take a quantitative look at the evidence. In Part 7, we saw the qualitative evidence – screen shots from particle detectors of proton interactions that showed some of the signatures of the Higgs boson:

* The four muon channel: H → Z0Z0 →µ+µ-µ+µ-
* The two photon channel H → γγ
* The two muon, two electron channel: H → Z0Z0 →µ+µ-e+e-
* The four electron channel: H → Z0Z0 → e+e-e+e-


Here’s a graph from the ATLAS experiment:



ATLAS measurement of the reconstructed masses of events that have four electrons, four muons, or a pair of each. The measured data have error bars and the histograms are expectations for different hypotheses of the Higgs mass based on simulation (photo courtesy of CERN).

It’s a tricky graph to look at. The red and purple histograms indicate the expected background. The light blue, orange, and gray histograms are simulations for different hypotheses of the Higgs mass. No obvious signal leaps out. Though there’s an accumulation at 125 GeV that’s consistent with the light blue 125 GeV hypothesis, it’s not nearly significant enough to claim discovery.

Here’s the CMS graph of the same quantity:


CMS measurement of the reconstructed masses of events that have four electrons, four muons, or a pair of each. The blue and green histograms are expected backgrounds and the red histogram is the prediction for a Higgs of mass 126 GeV (photo courtesy of CERN).

The evidence in the CMS graph is only slightly more compelling. The blue histogram is the expected background; the big peak on the left is due to production of the “neutral intermediate Z vector boson” which was a big deal when it was discovered in 1983. The possible Higgs consists of the four data points with error bars that aligns with the red hypothetical Higgs signal.

It’s easy to imagine the four data points around 125 GeV fluctuating down into the light blue background. It’s at most 6 events above a background of 16. Tantalizing, but is it evidence?

The best single-channel evidence for the Higgs comes from the CMS analysis of the two photon channel. Remember from Part 5 that CMS has a lead-tungstate crystal electromagnetic calorimeter and very high magnetic filed in the inner detector. The combination gives CMS tremendous resolution and background discrimination in the two photon channel and produces the clearest evidence for "Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC":


CMS measurement of the reconstructed masses of pairs of high energy photons, the bump at 125 GeV is evidence for the Higgs boson (photo courtesy of CERN).

To make this graph, the masses of photon pairs are calculated from all of the data with at least one pair of photons. The little bump above 125 GeV is evidence for a new particle. There are other data points that diverge from the smooth background, but they don’t cluster together into a signal. This graph is also a great way to see that the vast majority of pairs of photons come from other processes, only the tiny fraction in the red curve above the dotted background are from the “new boson at a mass of 125 GeV.”

Now that we see the actual data, you can get a sense of how it feels for the physicists as they watch the data accumulate. The fact that they stare at thousands of graphs like the three shown here is the reason that they use stringent statistical criteria before using words like “observation” or “discovery.”

Before they announced the “observation of a new boson at 125 GeV” all the data were combined and submitted to rigorous statistical analysis. The announcement came after the probability that what was observed could result from random fluctuations was less than one part in 1.75 million – which corresponds to an outlier of more than 5 standard deviations from the mean of a Gaussian distribution.

Here are links to the published results:

Let’s be straight about what we know and what we don’t know. The ATLAS and CMS experiments at CERN have seen evidence for a new particle at a mass of about 125 GeV, but is it the Higgs?

It’s consistent with the Higgs because it decays in the predicted channels. In order to determine if it has all the other properties expected of the Higgs boson, they’ll need enough events to determine the precise breakdown of how often it decays to the different final states and its angular distribution.

The word “boson” refers to a particle’s angular momentum. Electrons are “fermions” and have spin-half, photons are “bosons” and have spin-one. A particle’s angular momentum dictates the angular distribution of its decay products. The “minimal Standard Model Higgs” is a spin-zero object, but other hypotheses, like the highly favored supersymmetric model which would give the first hint of evidence for Super Strings, predicts other Higgs Bosons with other spins.

Look at that last graph, the two-photon masses from CMS. What if that little fluctuation near 136 GeV started to shape up into evidence for something else? Something new? That’s when it will get interesting. In the mean time, they’ll be busy sorting out properties of the Higgs and tending to a laundry list of analysis topics that will never make the mainstream news.

But what does it mean to you? Why should you be pouring so many Euros into these experiments? It’s a good question that’s difficult to answer. In high tech, our careers are built on classical electrodynamics with generous contributions from quantum physics in its application to diodes, transistors, and so forth.

When Faraday, Maxwell, Ampere, Volt, Ohm, and all those guys formulated electrodynamics, they didn’t predict television. When Planck, Einstein, Rutherford, Schroedinger, Heisenberg and all those guys developed quantum mechanics and atomic physics, they didn’t predict the iPad, but that’s where it came from.

Investments in basic research like the LHC experiments takes a long time to pay dividends, but when the results pay off, they pay off big.

Is it a shame that the Superconducting Super Collider was canceled in 1994? If it had gone along as planned, we’d have discovered the Higgs several years ago in Texas. It would have been nice to do it in our own backyard, but personally, I don’t think a few years here and there matter very much and since nothing that would have been done at the SSC would have been kept secret – the growth of science is due to the freedom to share results, after all – it doesn’t seem that it would matter very much. Besides, it’s hard to imagine that the SSC cafeteria could have been as good or as cheap as the CERN cafeteria.

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