Short Cuts

David Kaiser

Particle physics is at once the most elegant and brutish of sciences. Elegant because of its sweeping symmetries and exquisite mathematical structures. Brutish because the principal means of acquiring information about the subatomic realm is revving up tiny bits of matter to extraordinary energies and then smashing them together. Imagine trying to discern the hidden inner workings of a pocketwatch – carefully gauged springs and gears, all arranged just so – by hurling it at a wall and watching the detritus as it flies apart. In the case of particle physics, there’s an added twist: some of the detritus was never contained within the original matter, so it’s as if in addition to springs and gears, the bits from the smashed watch also included pulleys, ropes, the odd pound coin and a yo-yo or two. When smashing subatomic particles, the new objects that come flying out are coagulations of raw energy: some of the energy carried by the two colliding particles becomes transmuted, in accordance with Einstein’s famous equation, E = mc2, into little chunks of matter. These interactions occur billions of times per second in hulking machines like the Large Hadron Collider near Geneva and the Tevatron, soon to be decommissioned, at the Fermi National Accelerator Laboratory outside Chicago.

Most of the physicists currently using these machines are looking for one particle in particular: the Higgs boson. (‘Boson’ is a generic label for particles that carry integer-number units of ‘spin’, or intrinsic angular momentum, such as a photon of light. Most of the particles that make up ordinary matter, such as protons and electrons, carry half-integer units of spin and are known as ‘fermions’.) The Higgs has been dubbed ‘the God particle’, though I have never understood why this particular bit of matter is presumed to be holier than all the others. I prefer the more descriptive nickname, the ‘billion-dollar boson’, since for more than a quarter-century the prospect of finding the Higgs has been a major argument in favour of building huge particle accelerators.

To date, the Higgs boson remains entirely hypothetical. No decisive experimental evidence has ever accrued that would establish its existence. It is the only particle of the so-called ‘Standard Model’ of particle physics yet to be found. The Standard Model consists of 17 particles: familiar electrons and their cousins; the quarks that reside deep within protons and neutrons; the force-carrying bosons that are responsible for the nuclear and electromagnetic forces that keep matter bound together and occasionally make it fall apart; and the elusive Higgs. Those particles interact according to specific laws of force, the mathematical expression of which makes extensive use of certain types of symmetry. The theoretical contraption is by no means beautiful – to many theoretical physicists the Standard Model has more than a whiff of Rube Goldberg about it, and most believe it will be subsumed under some more elaborate, less arbitrary structure – but it has worked unbelievably well when confronted with experimental data. In fact, the Standard Model must surely be the most quantitatively precise theory in the history of science: several of its theoretical predictions match experimental data to more than a dozen decimal places.

The Higgs boson is believed to play an important role: it bestows mass on what would otherwise be massless particles. The Higgs is what gives the others their heft. The symmetries that govern the laws of force cannot be satisfied by particles that have their own intrinsic masses. So we assume that the particles have no mass at all, but that they are all swimming in a sea of Higgs bosons. As if caught in molasses, the particles’ interactions with the Higgs slow their natural rate of moving from point A to point B – the particles lumber along as if encumbered by a large mass.

The idea behind the Higgs mechanism has been around for nearly 50 years. It is a remarkably clever answer to the mathematical challenge of accommodating masses to the reigning symmetries. It is a central pillar of the Standard Model. And we still have no evidence that it is correct.

What would it take to supply that evidence? The basic idea is to smash protons together in a huge particle accelerator at such speeds that Higgs bosons (along with a great deal of other stuff) coagulate from the residual energy. The energies with which the beams of protons collide in the LHC should be high enough to create Higgs particles. Even so, finding them will be tricky. Current limits suggest that a single Higgs particle should be a bit smaller than an atom of gold. Incredible as it may sound, powerful microscopes can actually image individual atoms of gold. But unlike gold atoms, Higgs particles should be remarkably evanescent, with a lifetime of roughly a trillion-trillionth of a second: they simply won’t sit still long enough to be photographed. Neither would they leave much of a track. The furthest they could travel before decaying into other particles would be about a trillionth of a centimetre.

The only hope of finding evidence of Higgs particles is by sifting through their decay products, or, more likely, the decay products of their decay products, amid all the other flotsam that comes streaming away from the collision region. This is a bit like trying to infer the existence of a particular long-dead grandmother – and measure her height and weight – by sifting the data in the national census. One must look for statistical deviations from the expected patterns of ordinary particles that get trapped in the detectors. Are there more particles of a particular type, with particular energies, than would be expected in the absence of a Higgs boson that had decayed into them?

We can expect no shriek of ‘Eureka!’ from this type of search, no ‘golden event’ that could be captured in a single photograph. Rather, by necessity, claims of discovery will depend on complicated statistical arguments. Physicists must collect terabytes of data from all the trillions of scatterings and interactions and stack them into histograms (bar graphs showing the numbers of events of a given type at a given energy). Then they must carefully subtract away the ‘background’, or expected patterns of particle decays from known processes other than the creation and decay of a Higgs boson, and check whether any excess signal remains. (One of my favourite books in graduate school was The Higgs Hunter’s Guide, filled with the arcana of calculating the expected signals and backgrounds for Higgs decay at various energy scales. The title made me feel like Indiana Jones.) Higgs hunting is a game of looking for bumps in the night: tiny but otherwise unexplainable deviations in those histograms. Last month, two different teams working at the Large Hadron Collider announced that they had detected possible bumps in their data that could correspond to the decay of a Higgs boson of mass in the expected range; a group from the US Tevatron also reported some possibilities at about the same spot. But none of those bumps has as yet cleared a major hurdle: they all remain less than ‘three sigma’ events with more than a one in a thousand chance of being statistical hiccups. By convention, no discovery will be accredited until the signal-to-noise ratio exceeds ‘five sigma’, which is to say, about a one in two million chance that the excess of decay products could have resulted from a random statistical fluke in the absence of a genuine Higgs.

There will be widespread elation among the world’s physicists if – when? – decisive evidence of the Higgs boson accumulates. But as a glass-half-filled kind of guy, I would also be excited if no Higgs materialises. Nothing beats the thrill of the hunt.