Three decades of theorising and calculating. Entire careers spent constructing ideas. Nine billion dollars invested in an underground ring that spans two nations. Ten thousand dedicated scientists and engineers looking for the particle physics equivalent of a needle in a haystack. It’s all been leading to this moment. Small wonder that amid bated breath, you can hear a lot of nervous laughter.
“It’s got to be there, damn it!” Nobel prizewinning physicist chuckles in his office at the Massachusetts Institute of Technology in Cambridge. He’s talking about supersymmetry, endearingly known as SUSY, a theory that most physicists believe will lead them beyond the of particle physics, the tried-and-true model of how particles and forces interact, and one big step closer to understanding how reality works.
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Physicists are doggedly searching for it in the debris of particle collisions from ATLAS and CMS, two experiments at CERN’s Large Hadron Collider near Geneva, Switzerland. A year into their runs, neither have glimpsed so much as a hint of SUSY particles at masses up to 700 gigaelectronvolts – well within the range theorists expect it to lurk (, , , ).
Rumours are spreading of SUSY’s demise, and alternative theories are already waiting in the wings (see box below). But for many physicists like Wilczek, SUSY is just too beautiful to be wrong. “It would be really cruel of nature to get us this far, and have the next step in sight, and then it’s all just a joke on us.”
Supersymmetry suggests that the two basic types of particles that make up our world – fermions, the matter particles such as electrons and quarks, and bosons, the force-carrying particles such as photons and gluons – are merely two aspects of a single particle.
Perplexing problems
It’s an elegant idea and if correct, could solve some of the most perplexing problems in physics. It endows the elusive Higgs particle, which is believed to be responsible for giving every other known particle its mass, with just the right mass of its own to keep the whole edifice of particle physics from crumbling around us. Without SUSY, the Higgs mass is heavily influenced by the quantum behaviour of the vacuum.
As it interacts with the vacuum’s virtual particles, its mass skyrockets, growing so large that the standard model breaks down. SUSY saves the day – for every virtual interaction that drives up the Higgs mass, there is a svirtual interaction that drives it back down.
Just as importantly, SUSY unifies the three fundamental forces of the standard model, suggesting that electromagnetism and the strong and weak nuclear forces merge into a single superforce at high energies (see diagram).
Dark matter
What’s more, it provides an ideal candidate for the mysterious dark matter that seems to be holding galaxies together, accounting for approximately 80 per cent of all the matter in the universe. It even appears to be an essential ingredient in string theory, physicists’ leading contender for a theory of everything that will finally unite gravity with the other three forces.
No competing theory is able to solve all four problems in one fell swoop. That’s what makes SUSY so compelling and explains why many physicists are on tenterhooks.
Not everyone, though. “I never really believed in SUSY anyway,” says physicist of University College London, who works on the LHC’s ATLAS experiment. Butterworth admits, though, that the LHC’s search has only just begun. “It would have been something of a surprise if it had shown up by now,” he says, explaining that the LHC will gather 20 times as much data by the end of the year, and another factor of 10 by the end of 2012. “There’s plenty of room for SUSY to show up.”
of Boston University in Massachusetts agrees. “The suspicions of the death of supersymmetry are premature,” he says. “But that’s the only nice thing I’ll say about it.”
Fifth force
Lane prefers an alternative theory. With physicist Estia Eichten of Batavia, Illinois, Lane showed that particles could come by their masses without a Higgs boson if there is a fifth force in addition to the four we know about: technicolour. It is similar to the strong force, which binds quarks together, but operates at much higher energies. “There’s already a precedent for it in nature,” Lane says, adding that it could also provide a new candidate for dark matter.
The LHC will be able to put the theory to the test. Just as quarks pair up to form mesons, techniquarks pair up to form technimesons with masses ranging from 250 GeV to 700 GeV – well within the LHC’s reach. If technimesons exist, the LHC should find them within the next few years.
Lane has already made a bet that this underdog theory will prevail. At a 1994 conference, Lane was out to dinner with Nobel laureates and . “We drank a lot of wine and David and I made a bet about whether SUSY would be found at the LHC after they had a certain amount of data,” Lane says. “The loser has to take everyone to dinner at a three-star restaurant.”
Ultimate theory
For his part, Butterworth is betting on something totally unforeseen. “I think maybe there’s a whole new set of forces,” he says. “I just think nature is more likely to surprise us than to fit in with our guesses.”
But Wilczek is putting his money behind SUSY. “I’ll happily give even money, and probably better odds than that if pressed, that we’ll see some form of SUSY within 10 years.”
That could help shed light on another mystery of supersymmetry – why it’s not perfectly symmetric. If it were, “sparticles” would weigh as much as their normal cousins – and would have been seen by now. Instead, physicists believe supersymmetry is broken, with sparticles weighing more than their standard-model partners.
“There’s no consensus on how SUSY is broken,” says Wilczek. Many models implicate gravity in the process, so if the LHC does find signs of SUSY, it could usher in a way to merge gravity with the other fundamental forces, providing an ultimate theory of everything.
If supersymmetry isn’t real
SUSY solves four puzzles at once, but other theories can attack them too
Makes the Higgs work
Technicolour This is a fifth force, similar to the strong force that binds quarks but at higher energies, endowing particles with mass without the need for a Higgs
Holographic technicolour If the technicolour force has the same strength across a range of energy scales, it is mathematically equivalent to a warped space-time geometry in five dimensions, simplifying the technicolour equations
Little Higgs This new kind of symmetry gives the standard model’s Higgs the right mass. Fermions are partnered with new, heavier fermions and bosons with new, heavier bosons
Unifies electromagnetic, weak and strong forces
Randall-Sundrum model Our universe exists on a 4D membrane surrounded by five warped dimensions. Inside the warped 5D world, the forces all have the same strength; they seem different to us because of our limited perspective
Multiverse If there are an infinite number of universes, we might just happen to live in one with forces at the strengths we observe – possibly because it’s one of the few universes that could support life. If so, the forces might not be unified at all
Candidates for dark matter
Technicolour Dark matter would be made of a neutron-like particle made of “technifermions”
Axions Hypothetical particles 500 million times lighter than an electron
Modified Newtonian Dynamics (MOND) In this theory, there is no dark matter at all. Instead, Newton’s law of gravity is tweaked to explain the observed motions of galaxies
Key ingredient in string theory
Loop quantum gravity It’s not clear that SUSY is required for string theory, but lack of SUSY would bolster alternatives like LQG, which suggests that space is built of discrete units of geometry
