A Clarification: What Exactly Was Discovered?

In the wake of this morning’s big announcement out of CERN, I received a phone call today from a friend asking for some clarification about something. Since I suspect that it is something a great many other folks are probably unclear about, I thought I would share my response here.

Basically, the point of confusion is regarding the phrasing of the official statements out of CERN indicating that they have found a particle that is “Higgs-like.” Why not come right out and say they have found the Higgs boson? Have they or haven’t they?

Yes and no.

The current situation brings to mind a joke that was circulating around December’s announcement of preliminary results:

“We have not yet found the Higgs, and it has a mass of 125 GeV.”

Physicists have to be a little circumspect with their claims.  The researchers at CERN can currently state with a high degree of confidence that they have discovered a new particle, and that particle possesses properties which STRONGLY resemble those of the Higgs particle predicted by the Standard Model of particle physics.

Higgs decay channels:

  • pp → h → bbˉ
  • pp → h → γγ
  • pp → h → τ+τ
  • pp → h → W+W
  • pp → h → ZZ

But the resemblance is not exact. The Standard Model (SM) Higgs can decay in multiple ways, and the Standard Model provides mathematical tools for predicting the probability for the decays to occur along these different channels. (Physicists call these probabilities “branching ratios.”)  The particle observed at CERN is seen to decay with same decay products, but with branching ratios similar to, but not quite the same, as those predicted for the SM Higgs. For example, decays to a pair of photons (di-gamma decays) are being seen slightly more frequently than expected, but decays to tau-antitau lepton pairs are hardly being seen.

To be fair, the tau-antitau decays are tricky to discriminate from the background anyway, so their scarcity (and the di-gamma excess) may merely be a statistical fluke which will fade away as even more data is collected. Analysis of LHC data is very much a numbers game. The more data that is collected, the better. If these discrepancies vanish with the collection of more data, then we will eventually be able to say that we have discovered the SM Higgs.

But there is another possibility. This new particle may not be exactly the SM Higgs. It may be “a” Higgs boson, but slightly different in nature to the one predicted by the Standard Model. And this prospect is actually more exciting to physicists, because that implies new physics beyond the Standard Model. Although the Standard Model has been wildly successful over the last few decades, having been experimentally verified time and time again with excruciating precision, it has holes in it. It doesn’t get along with General Relativity, and thus does not explain gravity. It does not address several recently discovered phenomena such as dark energy, dark matter, and neutrino mass. And it does not address the hierarchy problem, which has to do with why the fundamental interactions of nature (gravity, electromagnetism, the weak nuclear force, and the strong nuclear force) have such different strengths.

Physicists know that they need to move beyond the Standard Model to address these problems, and over the last several decades, many extensions to the Standard Model have been proposed to accomplish this, including string theory, supersymmetry (SUSY), and technicolor theories. As it turns out, most of these alternative theories also incorporate the Higgs interaction, with some variants predicting MULTIPLE types of Higgs boson. String theory has turned out to be agonizingly difficult to test since it is flexible enough to accomodate just about any experimental outcome by twiddling its parameters, and data from the LHC has already eliminated many variants of SUSY and technicolor theory. It remains to be seen if some variant of one of these theories will eventually supplant the Standard Model, or something else entirely, but such an advancement would need to be based upon hard data. Finding a non-SM Higgs boson and measuring its properties would provide that. And finding multiple sorts of Higgs bosons, well, that would be a bonanza, greatly narrowing down the types of extensions to the Standard Model that would work.

In other words, this discovery isn’t the end of the story. It is just the beginning of another chapter.

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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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