Editorial: “ Dreams of a technicolour force“
Fermilab’s Tevatron collider has spotted evidence of new particles that might point to a previously unidentified force of nature
THE Higgs boson, a particle that physicists would give their eye teeth to find, has been pushed out of the limelight.
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The so-called God particle, thought to give other particles mass, is the last undiscovered particle in the standard model, the leading theory for how particles and forces interact. But it was not the Higgs that stirred up the physics world last week. It was a glimpse of a particle that, should it prove to be real, would fall beyond the standard model and could even hint at a new force of nature known as “technicolour”.
“I am extremely excited,” says Pierluigi Catastini of Harvard University, who is part of the team that spotted the candidate particle. “This is the reason why we do this job – to look for the unknown and be ready for the unexpected.”
Brian Cox of the University of Manchester in the UK : “If this stands up to scrutiny and more data … then it is RIP Standard Model :-)”.
The observation is providing a dramatic climax for Fermilab’s Tevatron collider in Batavia, Illinois. The collider is nearly 30 years old and is due to be shut down by the end of September, passing the baton to the newer Large Hadron Collider near Geneva, Switzerland.
Behind all the excitement is a strange signal seen at Fermilab’s CDF experiment, which smashes together protons and antiprotons 2 million times every second. The data, collected over a span of eight years, looks at collisions that produce a W boson, the carrier of the weak nuclear force, and a pair of jets of subatomic particles called quarks.
The proton-antiproton crashes can produce jet pairs that fly out at various energies or, equivalently, as E=mc2 states, at various masses. The standard model predicts that, as the mass of the jet pairs rises, the number of events – where each event is a W boson and a jet pair – produced by the collisions will drop steadily after an initial peak. But the CDF data showed something very different: a bump in the number of events created when the mass of the jet pair is about 145 gigaelectronvolts (see graph).
That suggests the additional events were produced by an unknown particle weighing about 145 GeV, or about 155 times the mass of the proton. “We observe an excess of events concentrated in one region, and it seems to be a bump – the typical signature of a particle,” says Catastini ().
“We observe an excess of events in one region – the typical signature of a particle”
Intriguing as it sounds, there is a 1 in 1000 chance that the bump is simply a statistical fluke. Those odds make it a so-called three-sigma result, falling short of the gold standard for a discovery: five sigma, or a 1 in a million chance of error. “I’ve seen three-sigma effects come and go,” says Kenneth Lane of Boston University (see “The fickle nature of particle physics”). Still, physicists are anxious to pin down the signal’s identity.
Most agree about what it’s not. “It’s definitely not a Higgs-like object,” says Rob Roser, a CDF spokesman at Fermilab. A standard model Higgs boson at this mass should not yet be detectable in this amount of data, he says. Moreover, a Higgs should most often decay into bottom quarks, but no extra bottom quarks appear in the bump.
Lane believes he has the answer. Just over 20 years ago, he and Fermilab physicist Estia Eichten predicted that experiments would see just such a signal. They were working on a theory known as technicolour, which proposes the existence of a fifth fundamental force in addition to the four already known: gravity, electromagnetism, and the strong and weak nuclear forces.
The technicolour force is very similar to the strong force, which binds quarks together in the nuclei of atoms, only it operates at much higher energies. Technicolour fills empty space with a sea of techniquark-antitechniquark pairs, and as particles travel through the sea, they gain mass. Because it can give particles mass, technicolour renders the Higgs unnecessary.
“Technicolour is a force that fills empty space, and as particles travel through it they gain mass”
The new force comes with a zoo of new particles. Lane and Eichten’s model predicted that a technicolour particle called a technirho would often decay into a W boson and another particle called a technipion.
Now Lane, Eichten and Fermilab physicist Adam Martin suggest that a technipion with a mass of about 160 GeV could be the mysterious particle producing the excess jets (). “If this is real, I think people will give up on the idea of looking for the Higgs and begin exploring this rich world of new particles,” Lane says.
Even if technicolour is correct, though, it still leaves questions unanswered. For example, physicists suspect that the high energies found in the early universe unified the fundamental forces into a single superforce. The standard model gets tantalisingly close to such a superforce, suggesting that the dream of unification could be achieved by adding an extra ingredient.
Technicolour has not yet been shown to provide the right ingredient – it is such a powerful force that physicists struggle to model all its interactions. But a theory called supersymmetry, which posits that every particle has a shadow partner, does. For this and other reasons, supersymmetry has long been the leading contender for a theory that extends physics beyond the standard model.
Figuring out if technicolour or supersymmetry – if either – is the theory that takes physics beyond the standard model means combing through more heaps of data to determine if the new signal is real. Even though the Tevatron will shut down this year, the CDF team that made the find is already “sitting on almost twice the data that went into this analysis”, says Roser. “Over the coming months we will redo the analysis with double the data.”
Meanwhile, researchers at DZero, Fermilab’s other detector, will analyse their own data to provide independent corroboration or refutation of the bump. The LHC will soon collect enough data to look for the bump too. “I haven’t been sleeping very well for the past six months,” says Lane, who found out about the bump some time ago. “If this is what we think it is, it’s a whole new world. It’ll be great! And if it’s not, it’s not.”
The fickle nature of particle physics
Hints of new particles come and go – and sometimes come back again.
Oops-Leon In 1976, 12 years before he won the Nobel prize, announced the discovery of a new particle after seeing a mysterious bump in Fermilab data. However, data from the same experiment a year later showed that the signal had been a statistical fluke. The non-existent particle, which Lederman had named upsilon, is now known as “Oops-Leon”.
Pentaquarks Quarks normally come in groups of two or three, but in 2002, observations at the Spring-8 lab in Japan suggested they also cluster fleetingly in parties of five. Other teams soon reported spotting signs of a pentaquark, but curiously not all at the same mass, and still other searches turned up nothing. Today, most physicists believe the pentaquark signal has vanished. of the University of Maryland in College Park says that some of the original reports had claimed a 1 in a million chance of statistical error, and were later seen to be wrong. He says assumptions made about the expected background signals in experiments were incorrect, leading to the apparent discovery of pentaquarks.
Tau lepton In 1975, announced that he had found intriguing evidence for the existence of a new particle similar to an electron. Analysing data from the Stanford Linear Accelerator Center in California, his team said they had “discovered 64 events for which we have no conventional explanation”. It wasn’t until 1977, when data from another accelerator confirmed the result, that the existence of the particle, called the tau, was accepted. Perl won the 1995 Nobel prize in physics for the discovery.
Sterile neutrinos Hints of these ghostly particles, believed to interact only with gravity, appeared in the 1990s in data from the Liquid Scintillator Neutrino Detector (LSND) at Los Alamos National Laboratory in New Mexico. In 2007, Fermilab’s MiniBooNE experiment failed to corroborate the result, leading physicists to believe that the original signal had been a fluke. But just last year, MiniBooNE ran a new experiment that more closely resembled that of the LSND, and found evidence supporting the existence of sterile neutrinos – though whether this latest effect is real remains up in the air. Amanda Gefter and Maggie McKee