THE Universe that we know is only a half-truth. Every known particle has a doppelgänger, whose shadowy presence affects everything from the minutiae of matter to the motions of the heavens.
So says the theory called supersymmetry. This idea may plug the holes in the current theory of fundamental physics, help unify the four forces of nature, and explain the origins of the “dark matter” needed to hold the galaxies together. With so much going for it, supersymmetry might seem like a sure-fire winner were it not for one stark fact – we have no evidence that that these extra particles exist.
Until now. At Brookhaven National Laboratory, a particle called a muon has been misbehaving; and at CERN in Geneva, there have been traces of a long-sought particle known as the Higgs boson. The two apparently unconnected results mesh in a way that points towards supersymmetry, says Gordon Kane, a theoretical physicist at the University of Michigan in Ann Arbor. “They’re completely consistent down to the numerical details.”
Advertisement
If the results hold up, particle physics will enter a new golden age, says Lee Roberts, a physicist at Boston University. “The discovery of supersymmetry would open up new horizons for particle physics, which seems to have been banging its head against the wall for years.”
That wall is the standard model of particle physics, a theory that encompasses everything from chemical reactions to radioactive decay to the shining of the Sun. These phenomena, along with almost everything else around us, are caused by three fundamental forces – electromagnetism, the strong nuclear force and the weak nuclear force – acting on matter.
According to the standard model, all matter is made of particles called fermions, which include quarks, electrons and neutrinos. The idea that matter is built up of little particles doesn’t sound strange; less obvious is the idea that forces are also made of particles. Fermions exert forces on each other by exchanging force particles called bosons, which differ from fermions in how much they spin.
For example, quarks attract each other by exchanging gluons, the bosons that carry the strong nuclear force. This powerful attraction binds trios of quarks together to form neutrons and protons, the basic constituents of every atomic nucleus. A negatively charged electron and the positively charged nucleus of an atom bind by exchanging photons – the bosons that carry the electromagnetic force. The weak nuclear force, which causes a form of radioactivity, is carried by massive particles called W and Z bosons. The final piece of the standard model is the still undiscovered Higgs boson. Thanks to Heisenberg’s uncertainty principle, Higgs bosons could flash in and out of existence even in empty space, and these virtual particles would drag on the W and Z and make them weighty.
The standard model can explain nearly every particle interaction ever seen. Yet it has considerable shortcomings. For example, it completely ignores the fourth force of nature, gravity. It cannot account for dark matter. And it even tends to trip itself up: unless theorists tune the parameters of the model very carefully, the mass of the Higgs becomes outrageously large, at which point the whole theory falls apart.
Supersymmetry can help to solve these problems. The idea is to make a theory that is more symmetrical. For every standard model fermion, supersymmetry supposes that there is a heavier, but otherwise identical boson, and for every standard model boson it supposes a heavier, but otherwise identical fermion.
The extra particles are known as superpartners. The electron partners the selectron, the top quark partners the stop squark, the photon partners the photino, the W boson partners the Wino, and so on.
For years such superpartners were considered mere mathematical abstractions, until researchers discovered how useful they could be. First, supersymmetry keeps the mass of the Higgs down (see “Big Higgs”). What’s more, it could help to wrap the four forces into one unified force.
Between any two particles, the four forces act with very different strengths. The strong overpowers electromagnetic, the electromagnetic overpowers the weak, and all three overpower gravity. But physicists think that the four forces are really different manifestations of the same single “superforce”, which ruled the ultra-hot Universe just after the big bang, and dictated how much and what kind of matter emerged. When two particles colide with very high energies, the forces should act with the same strength.
Feel the force
Physicists can calculate how the strong, weak and electromagnetic forces tend to change strength as the energy of particle collisions increases. These changes are produced by virtual particles that enshroud the colliding real particles. If the researchers only include the standard model particles in their calculations, all three forces never have the same strength at a single energy. But adding supersymmetric particles makes the forces equalise at energies about a million billion times higher than those achieved in particle accelerators so far. This suggests that any theory that rolls these forces together must ultimately include supersymmetry, says Frank Wilczek, a theorist at the Massachusetts Institute of Technology. One example that does this is superstring theory, which proposes that everything is made of tiny vibrating strings.
Supersymmetry could also explain dark matter, the mysterious stuff that is thought to account for 90 per cent of the mass of the Universe and provide the gravity needed to hold galaxies together. The lightest superpartner should be a heavy, stable particle that rarely interacts with ordinary matter. This makes it an ideal candidate for dark matter. Moreover, supersymmetry predicts the right amount of dark matter coming out of the big bang, says John Ellis, a theoretical physicist at CERN.
Despite all this promise, supersymmetry is untested and vague. The theory doesn’t say exactly how heavy the superpartners should be. All that physicists know for sure is that they haven’t seen one yet, even in the highest energy particle collisions in the lab.
But there are subtler ways to stalk supersymmetry. In February, researchers at Brookhaven National Laboratory in New York announced that a particle called the muon was slightly more magnetic than expected. The tiny discrepancy – four parts in a billion – could be accounted for by new particles, they say.
A muon traveling through a magnetic field emits and reabsorbs virtual particles, and each virtual particle interacts with the magnetic field and effectively alters the magnetism of the original muon. “Any particle that is there can contribute,” says Roberts. The standard model allows theorists to count up the contributions from the known fermions and bosons and calculate the muon’s magnetism to less than one part in a billion.
At Brookhaven, the team studied the motion and decay of roughly a billion muons in a precisely measured magnetic field, which enabled them to measure the magnetism of the muon with an uncertainty of only 1.5 parts per billion. The result disagrees with the standard model calculation, indicating that the familiar particles aren’t the only ones flitting in and out of existence around the muon. There is a roughly 1 in 100 chance that the experiment would have produced such a large discrepancy by a statistical fluke.
Given the number of experiments carried out in particle accelerators every year, those are not yet compelling odds. And some physicists are sceptical for other reasons too. The reported discrepancy may not be real, says Francisco Ynduráin, a theoretical physicist at the Independent University of Madrid, because the calculation of the muon’s magnetism may not be reliable. Different calculations yield half a dozen different values, he says. “They have chosen the one that’s farthest away from their number, and that’s not fair.” But Michel Davier, a physicist at Paris-Sud University, says that the value he and colleague Andreas Höcker calculated is the most reliable, so the Brookhaven experimenters were right to use it alone. “The ones that Ynduráin is proposing are clearly obsolete in my view,” says Davier.
The Brookhaven result, if true, shows that at least one new particle exists, but it doesn’t prove that the new particle is a superpartner. However, that interpretation gains strength from results reported at CERN a few months earlier.
In September, physicists working on the Large Electron Positron Collider, or LEP, reported a handful of particle collisions that may have produced the long-sought Higgs boson. The purported Higgs had a mass of 115 billion electronvolts (115 GeV), roughly 123 times the mass of the proton. The Brookhaven results suggest superpartners that are lighter than 400 GeV. From supersymmetry theory, such a light superpartner would mean the Higgs must have a mass below 130 GeV, so the two results tie in nicely.
Relatively light superpartners would suggest that theorists might be on the right track to unifying the four forces. If the superpartners were too heavy, they wouldn’t make the strengths of the forces converge.
The Brookhaven results should also encourage experimenters hoping to see dark matter particles. A light superpartner will be relatively easy to spot in the underground detectors at the Soudan salt mine in Minnesota, the Boulby mine in Yorkshire, and the Gran Sasso Laboratory in Italy.
The evidence for supersymmetry could grow much stronger within a year. Brookhaven experimenters have already collected more data, which should allow them to measure the magnetism of the muon even more precisely. Meanwhile, after four years of refurbishment, Fermilab’s powerful Tevatron collider started producing data in March. If the LEP result is right, a Higgs boson should fall right in the sweet spots of Fermilab’s two big particle detectors, says Sharon Hagopian of Florida State University in Tallahassee. “If it really is at 115 or 120 GeV,” she says, “we have an excellent chance of finding it.” A firm detection of such a light Higgs would back up the supersymmetry interpretation.
Of course, a direct detection of a supersymmetric particle would be better. Researchers disagree on whether the lightest superpartner is likely to surface first at Fermilab or CERN’s more powerful Large Hadron Collider, which will power up in 2006. It could simply be too massive for the Tevatron to make, says Simona Rolli of Tufts University in Medford, Massachusetts. And experimenters can’t be sure what they’re looking for. “With the search for the top quark, we knew the mass range and we knew it had to be there,” she says. “In this case, we don’t really have a hint from the theory.” But Kane believes that the evidence from Brookhaven and LEP makes the discovery of a superpartner at Fermilab likely.
It is possible that supersymmetry simply doesn’t exist and that other phenomena, such as new forces or new dimensions that open up only for very high energy particles, will solve the standard model’s problems. But that is relatively unlikely, says Howard Haber, a theorist at the University of California, Santa Cruz. These other phenomena seem to be inconsistent with experiments performed at LEP and elsewhere, he says.
Even if supersymmetry doesn’t exist, physicists generally agree that as experimenters push to energies beyond 100 GeV the standard model will begin to crack. At some point a mechanism will be found that limits the Higgs mass. Or if the Higgs itself doesn’t appear, some other phenomenon must emerge to account for the masses of the W and Z bosons.
All of this means that a slew of new particles wait just over the high-energy horizon. The deeper truths of the Universe may soon emerge from the shadows of the standard model.
Big Higgs
By accounting for the mass of W and Z bosons, the Higgs solves one problem, but immediately creates another. That’s because the standard model tends to make the Higgs supermassive. A real Higgs cruising through space can emit and reabsorb virtual particles, such as a virtual Z. Virtual particles contribute to the Higgs mass – and they should drive it up to 10 billion billion gigaelectronvolts. Unfortunately, the entire theory goes haywire if the Higgs mass climbs past 1000 GeV, so physicists have to carefully adjust many parameters in the standard model theory to keep the Higgs mass under control.
Or they can invoke supersymmetry, which solves the problem far more elegantly by taking advantage of a crucial difference between each particle and its superpartner. The Higgs can, for example, emit and reabsorb either a Z boson or its fermion superpartner, a Zino. And a fundamental principle of quantum mechanics requires the contributions of the otherwise identical boson and fermion to cancel. The two contributions can be thought of as waves, and the boson and fermion waves have to line up peak to trough, so that they counteract one another. Thus, supersymmetry practically guarantees that the Higgs’s mass stays believably small, says Frank Wilczek, a theorist at MIT: “The bosons make it bigger, the fermions make it smaller, and in supersymmetry the two cancel.”
