Did They Find the Higgs Particle?

The biggest science news recently has been the almost-discovery of the long-sought Higgs particle or Higgs boson. I call it an almost-discovery because, while the particle seems evident, it does not seem strongly-enough evident. At CERN’s Large Hadron Collider in Switzerland there are detectors around where the beams cross, detectors for detecting the particles produced by collisions. The ones involved in the Higgs hunt have been ATLAS and CMS, each with its own team of physicists who analyze its results. They do so by first looking for distinctive sorts of events, like pairs of energetic photons, which could be produced by Higgs-particle decay. They then plot how many by putative Higgs-particle mass.

On December 13, 2011, the ATLAS and CMS teams announced that they have discovered bumps in their plots, both at masses of about 125 GeV in particle-physics units. However, they are only about 2 to 3 standard deviations or sigmas above average, which is not enough to plausibly claim discovery. Particle physicists prefer a margin of 5 sigma before making such claims. But since the bumps have nearly the same mass, their combined height is about 3 or 4 sigma. Almost, but not quite. It could well be that this result is a false alarm, but that is very improbable.

Why is the Higgs particle so important? Because it is the only particle of the Standard Model that has yet to be found, and because this particle would give masses to most of the other Standard-Model particles. It would have a constant nonzero field value everywhere in the Universe, and this always presence would drag the particles it interacts with, giving them their masses.

The LHC is currently shut down for maintenance, and it will be restarted early in 2012. It should make enough collisions to go past 5 sigma, if the Higgs particle exists. So a year from now, we should know. In 2013, the LHC will be upgraded so that its protons will have an energy of 7 TeV instead of the present 3.5 TeV. Also on tap is increases in the number of particles being accelerated and collided, the “beam luminosity”. That will help not only in determining the properties of this putative Higgs particle, but also in finding other particles or setting improved limits on their masses.

This includes possible additional Higgs particles and supersymmetry partners of known partners, both of which are expected to exist according to some popular theories. So we are on the threshold of discovering some important new physics — or setting strong limits on its existence.

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