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Read more: 鈥�Crunch time for physics: What鈥檚 next?鈥�

IT WAS February 1964. The Beatles were poised to take the US by storm, and potent stuff was coursing through the brain of theoretical physicist . What if the protons and neutrons that make up matter themselves consist of smaller stuff 鈥� 鈥渜uarks鈥�? The name came about simply because Gell-Mann liked the sound of the word, which he pronounced like quarts 鈥� of alcohol. The spelling was supplied by relating to seagulls, sex and a publican.

At the time, physics was badly in need of radical ideas. Dozens of exotic new particles were turning up with seemingly no rhyme or reason in cosmic rays. Gell-Mann鈥檚 invention allowed protons, neutrons and all these new upstarts to be portrayed as combinations of two or three of these more fundamental entities.

The idea was too far out for most physicists. The new particles broke established rules by having fractional electrical charges of +2/3 or -1/3, and could also never be seen alone. Why should reality conform to such a whim?

Why shouldn鈥檛 it? 鈥淓verybody says this and that is impossible, but perhaps there is no good reason 鈥� perhaps it鈥檚 just baloney,鈥� says Gell-Mann, now at the Santa Fe Institute in New Mexico. And so it proved. Quarks became a foundation of one of the best-tested theoretical models in all of science: the standard model of particle physics. In four decades, the standard model has demonstrated an uncanny ability to turn theorists鈥� desires into dutifully confirmed reality. The discovery of the Higgs boson at CERN鈥檚 Large Hadron Collider, announced last year, is just the latest, most spectacular example.

Yet whispers of defeat surround this apparently resounding victory. With the Higgs, a manifestly incomplete model is complete, and experiments have failed to give much of a clue what sleeker model might remedy its deficiencies. Once again, particle physics is in urgent need of something radical.

The name 鈥渟tandard model鈥� was a considered understatement, says theorist of the University of Texas at Austin, who coined the term in 1974. 鈥淚 didn鈥檛 want it to become a dogma, but more a basis for conversation and experiment that might lead to the discovery that it is wrong.鈥� Its basics can be written on a postcard: six quarks arranged in pairs to make three 鈥済enerations鈥� identical in all but mass; six leptons, such as electrons and neutrinos, arranged similarly; and a handful of bosons that transmit nature鈥檚 fundamental forces between them (鈥渟ee diagram鈥�).

The essential thing about all these entities is that they are quantum particles. Quantum theory grew from radical discoveries at the beginning of the 20th century, which showed that the wavelengths of radiation emitted and absorbed by atoms could be explained only by assuming that energy is bundled in discrete amounts, or 鈥渜uanta鈥�. That unleashed an absurd duality at the smallest scales whereby a particle is also a wave and vice versa. These nebulous wave-particles do not move according to the tidy rules of classical, Newtonian mechanics, but dance to probabilities bounded by bizarre rules in an abstract mathematical space.

Quantum mechanics was largely in place by the mid 1920s, and it has yet to fail an experimental test. But when in the late 1920s Paul Dirac and others started to hook up quantum mechanics with Einstein鈥檚 special relativity 鈥� a vital step in describing particles that shunt around at near-light speeds 鈥� things began to take on a life of their own. Dirac鈥檚 relativistic equation for the electron had more than one solution, and seemed to predict that a particle existed just like the electron, but with opposite electric charge. The positron duly turned up in cosmic rays five years later. Antimatter had been invented from a theorist鈥檚 pen.

鈥�Antimatter came from a theorist鈥檚 pen: the positron duly turned up later in cosmic rays鈥�

Quantum field theory, the basis of the standard model, represents the culmination of this logic. The idea of a field transmitting forces goes back to Michael Faraday in the 19th century, but the mathematical structure of quantum fields gives them an odd property: they can create particles from empty space and destroy them again almost at will. Thus, according to the theory of quantum electrodynamics, two electrons repel each other thanks to a photon 鈥� the quantum particle of the electromagnetic field 鈥� that appears from nowhere and passes from one electron to the other. An infinite series of such 鈥渧irtual鈥� particle fluctuations shift properties of classical or 鈥渂are鈥� electrons by tiny amounts 鈥� shifts confirmed with stunning accuracy by many experiments since the 1940s.

It took a little longer for quantum theory to tame the other forces. The weak nuclear force, which transmutes one particle into another in radioactive decay, was plagued by unruly infinities that made calculations of all but the simplest effects impossible. The way forward, taken during the 1960s by Weinberg and others, was to mash it up with electromagnetism into a unified electroweak force that manifests itself at very high energies, such as those in the early universe.

Just as Dirac鈥檚 equation predicted antimatter, this theory presaged particles that had never been seen: the massive W and Z bosons to transmit today鈥檚 separated, short-range weak force; and the Higgs boson. The Higgs was needed to ensure that during the breakdown of the unified electroweak force the W and Z particles acquired mass, confining the weak force to atomic distances, while the photons of electromagnetism did not, allowing them to zip across the universe.

At the same time, the quantum field theory of the strong nuclear force, which holds atomic nuclei together, was evolving 鈥渇rom farce to triumph鈥�, in the words of the theory鈥檚 co-inventor of the University of California, Santa Barbara. Quantum chromodynamics, another term coined by Gell-Mann, finally made quarks respectable by describing their interactions by the exchange of eight gluons that carry a 鈥渃olour鈥� charge, and showed how, uniquely, this force gets stronger the further you pull two quarks apart. 鈥淚t could both explain why protons looked as if they were made of quarks and why these quarks could never be pulled out of protons,鈥� says Gross.

And that, largely, was it. By 1973, the Beatles had split up and, following a period of mind-boggling theoretical invention, the standard model was in place. There was the unified electroweak theory, to which all particles succumb; and quantum chromodynamics, which affects only quarks and gluons. The model wasn鈥檛 just clever, it was beautiful. Its equations had a powerful symmetry that dictated the character of nature鈥檚 forces, and told physicists what sort of new particles to look for and where.

And, sure enough, the bumps in particle-collider data soon began to appear 鈥� together with goosebumps on the skin of the theorists. Evidence for three quarks had already been established in experiments the late 1960s, but by the end of the 1970s physicists in the US had inferred the existence of a fourth and fifth and finally, in 1995, the sixth, 鈥渢op鈥�, quark. By 2000, the tau neutrino, the last of the leptons to be discovered, had also been bagged. On the other side of the pond, the gluon was snared at the DESY laboratory outside Hamburg, Germany, in 1979; the W and Z bosons in 1983 at CERN. And finally, last year, the Higgs 鈥� the last outstanding particle predicted by the standard model.

For Weinberg, the standard model鈥檚 triumphant march has been something quite special. 鈥淭o fool around at your desk with mathematical ideas and then find that, after spending a few billion dollars, experimentalists have confirmed them鈥� there really isn鈥檛 anything comparable to it,鈥� he says. So why aren鈥檛 he and others like him rejoicing?

Puzzling features

For many reasons. Some are aesthetic. Why, for example, do particles come in three generations, with the heaviest quark weighing 75,000 times more than the lightest? The standard model鈥檚 equations might be elegant, but to give them their predictive power, they must be fed more than 20 鈥渇ree鈥� parameters, such as particle masses, by hand. A truly fundamental theory would use the power of quantum theory, or perhaps some deeper idea that nobody has yet thought of, to prune that thicket.

Then there is the fact that the standard model does not technically unify the strong force. Electroweak theory and quantum chromodynamics are bolted together, rather than mixed up at the level of quantum fields as the weak and electromagnetic forces are 鈥� the first hurdle of many on the difficult route towards an ultimate theory of everything (see 鈥�Trouble with physics: Desperately seeking everything鈥�). That鈥檚 before we even talk about gravity, which is described by the distinctly non-quantum general theory of relativity. And while we are not talking about gravity, why is this force so phenomenally weak compared with the others? This 鈥渉ierarchy problem鈥� is one of the standard model鈥檚 most puzzling features.

There are also hints from experiments that all is not well. The supposedly massless neutrinos do in fact have a small mass. That blots the standard model鈥檚 mathematical consistency and is, perhaps, a first pointer to new physics beyond (see 鈥�Trouble with physics: Time to snare neutrinos?鈥�, left). More mysterious still are dark matter and dark energy, those stuffs that astronomers suggest make up almost 96 per cent of everything out there (see 鈥�Trouble with physics: Cosmology鈥檚 dark crisis鈥�). The standard model remains silent on their identity.

Faced with such gaping holes, theorists have turned to the solution that has served them so well up till now: plug the gaps with new particles and symmetries. But reality seems to have stopped playing ball. No particle collider has yet delivered more than a hint of anything unexpected and exotic 鈥� not even the LHC, although the machine has yet to reach its full beam energies. 鈥淭here is a real possibility that the LHC will simply go on confirming the standard model,鈥� says Weinberg.

What then? In short, we don鈥檛 know. Without further guidance from the LHC or elsewhere, we could find ourselves in the position of the Greek philosopher Democritus when he reasoned that matter could not be sliced up indefinitely 鈥� an idea only vindicated by experiment over 2000 years later. It is worth remembering that the first 鈥渁toms鈥� to meet his description were not the last word. For all the standard model鈥檚 successes, we are far from knowing whether Gell-Mann鈥檚 quarks will be either.

This article appeared in print under the headline 鈥淔lawed genius鈥�