The weighty Higgs particle

Posted: January 09, 2012

By David Kaiser

Fifty years ago, particle physicists faced an unexpected challenge. Their best mathematical models could account for some of the natural forces that explain the structure and behavior of matter at a fundamental level, such as electromagnetism and the weak nuclear force responsible for radioactive decay. But the models worked only if the particles inside of atoms had no mass. How could huge conglomerations of such particles - proteins, people, planets - behave as they do if their constituent parts weighed nothing at all?

Some physicists invented a clever workaround. They suggested a type of particle that had never been detected; it was eventually named in honor of the British physicist Peter Higgs. For a half-century, physicists searched for the elusive "Higgs particle." Now, following research conducted at CERN, the sprawling particle-physics laboratory near Geneva, the hunt may soon be over.

At first blush, the idea behind the Higgs particle sounds outlandish. Higgs and his colleagues suggested that every elementary particle really is massless, just as the mathematical models require, and hence all particles would ordinarily zip around at the speed of light. But suppose that everything around us is immersed in a huge, unseen vat of Higgs particles. Whenever most kinds of particles move from point A to point B, they continually bump into Higgs particles, slowing their motion. When we observe them, they appear to lumber along like holiday shoppers in a crowded store. From their slow motion, we infer that they have mass.

While a 50-year search for a hypothetical particle reminiscent of a bizarre fairy tale might seem quixotic, the Higgs particle stands at the center of the Standard Model of particle physics. Every experimental test of the model so far has matched theoretical expectations. In some striking examples, the agreement between prediction and measurement has stretched out to 12 decimal places, making the Standard Model the most accurate scientific theory in human history. The model successfully accounts for three of the four basic forces of nature; only gravity remains beyond its purview.

Higgs particles might have played an even more substantial role at earlier moments in cosmic history. My own research, along with that of physicists around the world, has focused on what effects Higgs particles might have had just fractions of a second after the Big Bang - effects that could explain the shape and fate of the universe.

And yet, for all that, we still have no direct evidence that Higgs particles even exist. According to the Standard Model, Higgs particles scatter off each other, so they, too, should have mass. The latest research indicates that Higgs particles (if they exist) should be among the most massive critters of the subatomic realm, more than 120 times as massive as the familiar proton.

To produce such a particle in the laboratory requires revving up protons to nearly the speed of light and smashing them together, which the Large Hadron Collider at CERN accomplishes trillions of times per second. The energetic collisions produce all manner of debris, which physicists carefully track with huge detectors and sift with sophisticated computer algorithms.

Physicists confront two major hurdles in their hunt for the Higgs. First, they must identify patterns in the debris that could have come from the production and rapid decay of a Higgs particle. The sought-after signal is well understood in principle, given what we know about the Standard Model. So is the background noise from all the other junk that comes flying out when two protons collide with colossal energy. Physicists searching for a few Higgs-like needles in a mind-bogglingly large haystack must comb their data for anomalies in the debris that cannot be accounted for by known processes.

The second difficulty concerns statistics. The rules of quantum theory, on which the Standard Model is built, are at root probabilistic. There will always be statistical flukes in the data, just as any series of coin tosses can produce an unexpected string of seven heads in a row.

To know with confidence that the coin is ordinary, with no hidden features, one must log a sufficiently large number of coin flips and check whether the data include equal numbers of heads and tails over the long run. After thousands of coin tosses, if the data still show a bias toward heads, one may be justified in thinking the coin has some unusual properties.

The same holds true for all of the chaff from the protons' collisions. Before physicists can claim that their anomalies really come from Higgs particles, they must gather enough data to rule out flukes.

At CERN, two independent teams of physicists recently announced that their data were consistent with detection of a Higgs particle, though there remained a 1-in-2,000 chance that the signal came from mundane, non-Higgs processes. So the teams will continue smashing protons together, gathering more data, and sifting for signs of a Higgs.

We might not have the Higgs in hand right now. But the latest news is the strongest indication yet that the 50-year hunt for one of the most fundamental bits of matter might well be coming to a successful conclusion. The next time the CERN teams call a news conference, it could be weighty news indeed.


David Kaiser is a professor of physics and the history of science at Massachusetts Institute of Technology. This was distributed by Project Syndicate.

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