Hunting Higgs

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By Alan Scrivner

The mediators of the weak force; so named because although they are stronger than gravity, are only effective at very short distances (10-18 m). The W and Z bosons, are both roughly 100 times more massive than the proton, while the photon, which mediates the electromagnetic force, is massless. Beta decay is just one example of the weak force.  During beta decay a neutron disappears and is replaced by a proton, an electron, and a neutrino (or anti-electron).  Also, during this process, a down quark disappears and an up quark is produced which eventually turns into the electron and neutrino.

Based on symmetry principles, which are central to particle physics, one would expect the W, Z, and photon (collectively called gauge bosons) to all be massless. This however is not true. The W and Z bosons are among the heavyweights of the elementary particles. With masses of 80.4 GeV/c2 and 91.2 GeV/c2 [1], respectively, the W and Z bosons are almost 100 times as massive as the proton. The masses of these bosons are significant because they act as the force carriers of the weak force and their high masses thus limit the range of this nuclear force. By way of contrast, the electromagnetic force has an infinite range because its force carrier, the photon, has zero rest mass.

In order to break the expected symmetry, physicists had to invent a particular set of new particles and interactions, which ultimately provide an explanation of the origin of mass of the elementary particles. The simplest way to do this, and the one that is incorporated in the standard model of particle physics, predicts the existence of a particle called the Higgs boson. The interactions of the Higgs boson with the quarks, leptons, and gauge bosons of the standard model are uniquely predicted by the theory, but the mass of the Higgs boson is a free parameter that can only be determined by experiment.

The most central missions of the Large Hadron Collider (LHC) is to ascertain the origin of mass by discovering the Higgs boson (or, if no Higgs is found, perhaps by discovering some alternative phenomena that can be attributed to the symmetry-breaking mechanism). The data collected up to this time has shown that the Higgs boson of the standard model, if it exists, must lay in a narrow range of masses around 130 GeV/c2.

What came out of the news conference on July 4 was that an entirely new particle has been found. It was found where Higgs was expected to be found, at about 126 GeV/c2, but more data will be required before they can absolutely confirm that it is the long-awaited Higgs boson.

The problem with claiming victory lies with the fact that when scientists search for new physics, they compare what they observe to what theories predict. If an experiment sees something that doesn't match the theory, it could be evidence of something new, or it could be merely a result of random fluctuations in the data. Scientists use the statistical measure sigma to express the probability of a statistical fluke as large as the observed mismatch between theory and experiment. Particle physicists typically do not declare an official discovery until the signal surpasses 5 sigma, meaning that it has just a 0.00006% probability that the result is due to pure chance. At 4.9 sigma, today’s announcement today puts us on the very edge of discovery and all we need is more data and thus the statistical nudge to throw us across the threshold.


[1] Since E=mc2, m=E/c2, physicists typically refer to masses in terms of energy expressed in electron volts (~1.602×10−19 joule) divided by the speed of light squared.


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