On a weather map, the surface temperature is described by assigning a number to each point on the map. On a surface wind map, an arrow is assigned to each point that describes the wind speed and direction at that point. Both are examples of a field. Mathematical descriptions of how field values change in space-time are ubiquitous in physics. In this post, I focus on the famous “Higgs field” and chart the efforts that led to its remarkable discovery in 2012.

In the Beginning

The “Higgs field” pervades the universe, and we should be thankful that it does because life on Earth would be unthinkable without it, largely because a universe with zero Higgs field is one without mass!

So, how did the Higgs field come about? It’s story goes back to deep time, in fact, to the “Big Bang”, at the time of the Universe’s creation as envisaged by physicists. Moments after the Big Bang, the Higgs field was zero (and hence, there was no mass to speak of). But as the universe cooled and the temperature fell below a critical value, the Higgs field grew spontaneously so that any particle interacting with it acquired a mass. The more a particle interacts with the Higgs field, the heavier it is. From that implausible beginning, virtually everything that exists in the universe – right down to atoms – have mass. Everything except subatomic particles like photons which are massless because they don’t interact with the Higgs field.

The Higgs field is named after the Scottish physicist, Peter W. Higgs, who first predicted the existence of the eponymous field in a paper published in 1964. Like all fundamental fields, the Higgs field has an associated particle – the Higgs boson or simply the Higgs particle. The Higgs particle is the visible manifestation of the Higgs field, rather like waves on the surface of the sea. Given its all important role in endowing particles with mass, the Higgs particle has been christened, the “God particle.”

Finding the “God Particle”

I now chart the immense scientific effort that led to the empirical discovery of the Higgs particle, the event for which Peter Higgs was awarded the 2013 Nobel Prize in Physics [1]. To discover the Higgs particle, one must do two things. First, one must produce enough of them, and second, one must document evidence of their (fleeting) existence. Both steps are challenging for reasons I will explain in a minute.

To produce heavy elementary particles, you must concentrate a lot of energy into a very small volume (Einstein’s famous equation requires it). That is why scientists have been building bigger and more powerful high-energy particle accelerators such as the Large Electron Positron collider (LEP) located at the CERN laboratory near Geneva where beams of rapidly moving protons and other particles are made to collide with target materials or with one another to release tons of fundamental particles, among which is the hoped for Higgs particle. Needless to say, that is a tall order. In the years prior to 2012, Higgs particle searches were mounted with a succession of ever-higher energy concentrations, but each effort came up empty. In hindsight, we now know why: these machines simply didn’t crank up enough energy to get the job done, until the completion of an even more powerful accelerator: the suitably-named Large Hadron Collider (LHC) in 2008.

A Smashing Time

The home of the LHC is a circular underground tunnel measuring about 27 km (17 miles) around, beneath a rural area straddling France and Switzerland. When the LHC is operating, two narrow beams of protons traverse the tunnel in opposite directions within a pipe that threads it. Moving at nearly the speed of light, the protons make 11,000 orbits per second. At four points, the beams cross. Only a small fraction of protons collides, but this still amounts to nearly a billion collisions per second, an indication of the kind of firepower necessary to find the Higgs boson.

The next task is to detect the Higgs particles that “fall out”. Enormous, instrumented detectors surround the crossing points. One of them, the ATLAS detector, is more than twice as large as the Parthenon. The detectors track the energies, charges, and masses of the particles that emerge from the collisions, as well as their directions of motion. They feed all this information, at the rate of 25 million gigabytes per second, to a worldwide grid of supercomputers numbering in the thousands.

Let’s pause for a moment and ask: why is all that information necessary? Here’s the answer, explained in bullet points:

[1] The events are complicated. Typically, ten or more particles stream out from each collision.

[2] Few of the events – less than one in a billion – ever contained Higgs particles.

[3] Those events that do contain them don’t last long. The lifetime of a Higgs particle is about seconds, or a tenth of a trillionth of a billionth of a second.

[4] The rare events that briefly contained Higgs particles also contain a lot of other stuff.

In short, to have a good shot at detecting the Higgs particle, scientists have to intensely monitor a host of things that get spewed out from the high-energy collisions. Otherwise, they will end up getting a lot of false positives. This is how secretive nature can be, as though she is willing to reveal only rarely, and for a very short time.

The immense effort that went into the construction and operation of the LHC was finally rewarded on July 4, 2012, when the tell-tale signatures of the Higgs particle – an excess of high-energy photon pairs- emerged. Such pairs were predicted to arise from Higgs particle decays, and the excess swamped any other plausible explanations. It was reliable evidence that the elusive God particle revealed itself, to the relief and excitement of physicists. This epochal discovery reaffirmed the glorious achievements that human minds are capable of when the truth is pursued with tenacity. Truth in this case is nothing short of fundamental, because finding evidence of the Higgs particle is finding the very source of all mass.

Notes

[1] although Peter W. Higgs was the first to explicitly predict the particle that would eventually acquire his name in 1964, other physicists can also lay claim to the idea of a mass-generating boson. In August the same year, Robert Brout and François Englert independently detail how the mass-generation mechanism could work. Another group – Dick Hagen, Gerald Guralnik and Tom Kibble – also produce similar ideas independently, publishing shortly after Higgs in November. The Nobel Committee ultimately decided to award the 2013 Nobel Prize in Physics to Peter Higgs and François Englert.