Gotcha!

“WE HAVE a discovery.” Rolf Heuer, the director-general of CERN, was in no doubt. He left none of the wiggle-room with which physicists often hedge their announcements when he summed up the results of his organisation's search for the Higgs boson. These were presented in detail on July 4th by Joe Incandela and Fabiola Gianotti, the leaders of the two experiments that have been looking for the elusive particle. CMS, run by Dr Incandela, and ATLAS, run by Dr Gianotti, are fitted to the Large Hadron Collider (LHC), the principal piece of equipment at Europe's main particle-physics laboratory, near Geneva, which CERN runs. Both have found conclusive evidence for a particle of the right type and mass to be the Higgs. If it is not actually the Higgs, that will be the biggest upset in physics for a century.

It has taken five decades, billions of dollars and millions of man-hours. But, at long last, Peter Higgs, a British physicist (pictured above), and four other, less well-known individuals—François Englert, Gerald Guralnik, Tom Kibble and Carl Hagen—can crack open a bottle of champagne. They are the ones who, in 1964, plucked what has come to be known (unfairly in some eyes) as the Higgs boson from formulae they were working on to fix a niggle in quantum theory. Another co-originator, Robert Brout, died last year.

The discovery puts the finishing flourish on the Standard Model, the best explanation to date for how the universe works—except in the domain of gravity, which is governed by the general theory of relativity. The model comprises 17 particles. Of these, 12 are fermions such as quarks (which coalesce into neutrons and protons in atomic nuclei) and electrons (which whizz around those nuclei). They make up matter. A further four particles, known as gauge bosons, transmit forces and so allow fermions to interact: photons convey electromagnetism, which holds electrons in orbit around atoms; gluons link quarks into protons and neutrons via the strong nuclear force; W and Z bosons carry the weak nuclear force, which is responsible for certain types of radioactive decay. And then there is the Higgs.

The Higgs, though a boson (meaning it has a particular sort of value of a quantum-mechanical property known as spin), is not a gauge boson. Physicists need it not to transmit a force but to give mass to other particles. Two of the 16 others, the photon and the gluon, are massless. But without the Higgs, or something like it, there is no explanation of where the mass of the other particles comes from.

For fermions this is no big deal. The Standard Model's rules would let mass be ascribed to them without further explanation. But the same trick does not work with bosons. In the absence of a Higgs, the rules of the Standard Model demand that bosons be massless. The W and Z are not. They are very heavy indeed, weighing almost as much as 100 protons. This makes the Higgs the keystone of the Standard Model. Slot it in and the structure stands. Take it out and it topples. Little wonder that physicists were getting impatient.

The 48-year itch

There are several reasons why it has taken nearly half a century to nab the Higgs. For a start, theory suggests the particle's own mass (which it gets by interacting with itself) should be huge. Since, as Einstein showed, energy and mass are the same thing, a heavy particle takes more energy to produce. That meant bigger, more powerful and more expensive machines, like the LHC, which smashes together protons travelling in opposite directions in a circular tunnel 27km (17 miles) in circumference.

To complicate matters, the Standard Model is non-committal about what a Higgs should weigh, so physicists had to look across a broad range of possible masses. That meant having to sift through thousands of trillions of collisions.

Nor were they looking for the Higgs per se. Higgs bosons are so unstable that they can never be observed directly. Rather, ATLAS and CMS, which are located on opposite sides of the LHC's loop, are designed to detect patterns of observable particles that theory suggests the Higgs should break down into. Unfortunately, such patterns are not specific to the Higgs; other subatomic processes produce similar traces.

The experiments could not, therefore, simply identify a Higgs signal. Instead, they looked for an excess of possible signals, amounting to a fraction of a percent over what would have been expected were the Higgs not real. They have both found it at a mass of around 125 giga-electron-volts, in the arcane units used to measure how heavy subatomic particles are. At one chance in 3m of being a random fluctuation, the findings leave no room for doubt. A new particle has been observed.

The next step is to ascertain that it really is the sort of Higgs the inventors envisaged. Although the Standard Model says little about how heavy Higgses ought to be, it is quite specific about how mass affects the ways they decay. Measuring the proportions of the different observed decay modes, and comparing them with these predictions, should show whether the newly discovered particle is the garden variety of Higgs dreamed up back in 1964, or something more exotic.

Here, the data are more equivocal. Some observed decays—in particular one where the Higgs turns into two photons—crop up more often than the Standard Model says they should, though this could still prove to be a statistical fluke.

To many physicists, an exotic Higgs would be good news. For all its explanatory prowess, the Standard Model cannot be the last word in physics. A humdrum Higgs would shore up that venerable theory but offer few clues as to what might replace it.

The constant gardener

One problem is that, as it stands, the model requires its 20 or so constants to be exactly what they are to an uncomfortable 32 decimal places. Insert different values and the upshot is nonsensical predictions, like phenomena occurring with a likelihood of more than 100%.

Nature could, of course, turn out to be this fastidious. But physicists have learned to take the need for such fine-tuning, as the precision fiddling is known in the argot, as a sign that something important is missing from their picture of the world.

One way to look beyond the Standard Model is to question the Higgs's status as an elementary particle. According to an idea called technicolour, if it were instead made up of all-new kinds of quark held together by a new interaction, akin to but distinct from the strong force, the need for fine-tuning disappears.

Alternatively, the Higgs can maintain its elementary status, but gain siblings. This is a consequence of an idea called supersymmetry, or susy for short. Just as all the known particles of matter have antimatter versions in the Standard Model, in the world of susy every known boson, including the Higgs, has one or more fermion partners, and every known fermion has one or more associated bosons.

Crucially, both theories have a purchase on much that the Standard Model leaves unanswered—why the universe is full of matter and not antimatter, say, or why different fundamental forces have wildly different strengths. Answers to these questions flow naturally out of supersymmetric and technicolour maths. So do candidates for the particles that make up dark matter, a shadowy substance whose presence can be deduced from its gravitational pull (see article) but which does not interact much via the three Standard-Model forces. The lightest of these dark-matter particles might pop up in the LHC.

The problem with these and other proposals has been a conspicuous lack of evidence regarding which, if any of them is a good description of reality. A decade ago physicists were confidently predicting that supersymmetric particles or “techniquarks” would be discovered in short order at the Tevatron (a less powerful American accelerator which was shut down last year) even before the LHC was up and running. This hope failed to materialise. Time and again experiments conformed neatly with the Standard Model, giving no inkling of what might lie beyond it, to the chagrin of many in the field. That may at last change as they start unpicking the Higgs.

The discovery of the boson, then, is rightly hailed as the crowning achievement of one of history's most successful scientific theories. But it is also almost certainly the beginning of that theory's undoing, and its replacement by something better. In science, with its constant search for the truth, this is something to celebrate.