Higgs at Last

David Kaiser

I wasn't the only person in the United States who counted an extra reason to enjoy the parades and festivities this week. The announcement at CERN that two independent experimental groups had each collected convincing evidence that the long-sought Higgs particle had been found prompted the physicist and blogger Matthew Strassler to declare 4 July ‘IndependHiggs Day’. I couldn't imagine a better reason for fireworks.

The Higgs particle is among the plainest of elementary particles: no electric charge (unlike the garden-variety electron); no intrinsic angular momentum (unlike nearly all known elementary particles); zero values for such abstruse quantum properties as strangeness, colour charge and lepton number.

Plain though it may be, it has stood for decades at the centre of the Standard Model of particle physics, the linchpin that has made the rest of the system hold together, at least on paper. According to the Standard Model, the entire universe is awash in Higgs particles. When other elementary particles try to move from one spot to another, they get mired in the Higgs goo, which slows their motion. We interpret their slowed pace as being due to their mass: heavy objects lumber around more slowly than lightweight ones. Frank Wilczek has called the Higgs particle the ‘quantum of ubiquitous resistance’. John Ellis has made an analogy with people moving across a snow-covered meadow.

It's one thing to posit a universal medium which all other matter trudges through; quite another to find direct evidence that it exists – in essence, to break off tiny pieces of that medium (individual Higgs particles) and measure their properties. And that, it now seems, is precisely what the two experimental groups at CERN have managed to do.

Last December, representatives of both experiments showed evidence that they were on the trail of the Higgs particle. But they had to stop shy of claiming an actual discovery. They had both collected and analysed impressive amounts of data from trillions of collisions among sub-atomic particles at CERN's Large Hadron Collider (LHC), but not enough to rule out statistical flukes. That changed, in a big way, between December and June.

The physicists needed to collect information on trillions more scatterings in order to clarify that the tiny bumps in their data were due to a new particle and not merely unlikely (but not impossible) events from ordinary, non-Higgs particles masquerading as the real deal. Toss a coin ten times and it may well come up heads six times. In fact, you can expect to get six heads out of ten about 20 per cent of the time – a not uncommon statistical fluke. Such a limited data set – only ten coin tosses – is not large enough to determine with confidence whether you are using a fair coin or a biased coin that favours heads over tails. But if you tossed the coin 10,000 times and got more than 5000 heads, that would point more convincingly towards some real effect – an inherent bias towards heads – rather than an ordinary coin on a lucky streak.

It’s the same with sifting for sub-atomic particles. Higgs particles are inherently unstable (according to our reigning theory): they should decay into other types of matter in fractions of a second. So the experimenters had to collect information on the flotsam and jetsam that comes flying out when revved-up particles collide inside the LHC, and painstakingly reconstruct whether the debris they collected is consistent with known processes: decays of other particles independent of the Higgs. If they found evidence of more decays into certain types of particle than they’d expect to see from other, known processes, that could be evidence that Higgs particles had been created and then decayed within the experiment. Or it could be that ordinary, non-Higgs particles had gone on a lucky streak, popping up and decaying into the measured debris more often than expected, like a coin coming up heads more than half the time.

Years ago, particle physicists adopted the convention that to claim the discovery of a new particle requires statistical significance of at least five standard deviations (or five sigma). That means that the odds that the observed events could have been due to mundane particles on a lucky streak rather than from a genuinely new particle are about three million to one. Neither of the experimental teams at CERN had sufficient data to reach the five-sigma mark last December. Both teams announced on Wednesday that they had crossed the five-sigma mark with data collected through June.

Press accounts have often measured the effort to find the Higgs in terms of lucre: billions of dollars spent on the ill-fated Superconducting Supercollider (SSC), which was under construction in Texas until scrapped by the US Congress late in 1993; billions more expended at CERN on the LHC. Those are important measures – not least during times of financial hardship – but they’re not the only ones.

A different kind of accounting may help to explain the physicists' reverie about this week's announcement. The main ideas about the Higgs particle date back fifty years. One of the standard databases of scientific publications includes more than 16,000 articles on the Higgs particle over that period. More than 90 per cent of them have been published since 1990, and nearly 1000 appeared last year. Those 16,000 papers have almost 11,000 authors: physicists around the world who have been focusing on the Higgs particle, its theoretical roles and possible experimental detection, for decades. Five hundred of them have each published at least 55 articles on the topic, dedicating a large portion of their entire careers to the Higgs particle. (Four of my papers show up on that list, less than 0.03 per cent of the global effort. John Ellis leads the pack with 150 articles: when he compares the Higgs field to a snow-covered meadow, you can be sure he knows what he’s talking about.)

Hence the anticipation, and now the celebration, of the latest news from the LHC. Much remains to be understood about the fundamental nature of matter and its interactions, including what roles (if any) the Higgs particle might have played during the earliest moments after the big bang. The particle's discovery provides even more compelling evidence that the Standard Model should be a reliable guide to these further investigations. Hats off to the dedicated teams at CERN, and to the tens of thousands of physicists around the world who have contributed to the quest over half a century. Cue the marching bands, light the fireworks: we have found the Higgs at last.