Update on 31 May: The CDF team has reportedly analysed more data and continues to find the bump

Hints of new physics at the Tevatron (Image: Fermilab) The vertical axis shows the number of events – where each event is the production of a W boson and a pair of particle jets – in proton-antiproton collisions at the Tevatron. The horizontal axis shows the mass of the pair of jets. Physicists expected the number of events to fall off as the mass of the jet pairs rose (red), but instead they observed a bump. The extra events may have been created by mysterious new particles (Image: CDF collaboration)


The physics world is buzzing with news of an unexpected sighting at Fermilab’s Tevatron collider in Illinois – a glimpse of an unidentified particle that, should it prove to be real, will radically alter physicists’ prevailing ideas about how nature works and how particles get their mass.

The candidate particle may not belong to the standard model of particle physics, physicists’ best theory for how particles and forces interact. Instead, some say it might be the first hint of a new force of nature, called technicolour, which would resolve some problems with the standard model but would leave others unanswered.

The observation was made by 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.

Physicists predicted that the number of these events – producing a W boson and a pair of jets – would fall off as the mass of the jet pair increased. But the CDF data showed something strange (see graph): a bump in the number of events when the mass of the jet pair was about 145 GeV.

Just a fluke?

That suggests that the additional jet pairs were produced by a new particle weighing about 145 GeV. “We expected to see a smooth shape that decreases for increasing values of the mass,” says CDF team member Pierluigi Catastini of Harvard University in Cambridge, Massachusetts. “Instead we observe an excess of events concentrated in one region, and it seems to be a bump – 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 in Massachusetts. Still, physicists are 99.9 per cent sure it is not a fluke, so they are understandably anxious to pin down the particle’s identity.

Most agree that the mysterious particle is not the long-sought Higgs boson, believed by many to endow particles with mass. “It’s definitely not a Higgs-like object,” says Rob Roser, a CDF spokesperson at Fermilab. If it were, the bump in the data would be 300 times smaller. What’s more, a Higgs particle should most often decay into bottom quarks, which do not seem to make an appearance in the Fermilab data.

Fifth force

“There’s no version of a Higgs in any model that I know of where the production rate would be this large,” says Lane. “It has to be something else.” And Lane is confident that he knows exactly what it is.

Just over 20 years ago, Lane, along with Fermilab physicist Estia Eichten, predicted that experiments would see just such a signal. Lane and Eichten 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. Technicolour is very similar to the strong force, which binds quarks together in the nuclei of atoms, only it operates at much higher energies. It is also able to give particles their mass – rendering the Higgs boson unnecessary.

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.

In a new paper, 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 two 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.

Future tests

But if technicolour is correct, it would not be able to resolve all the questions left unanswered by the standard model. For example, physicists believe that at the high energies found in the early universe, the fundamental forces of nature were unified into a single superforce. Supersymmetry, physicists’ leading contender for a theory beyond the standard model, paves a way for the forces to unite at high energies, but technicolour does not.

Figuring out which theory – if either – is right means combing through more heaps of data to determine if the new signal is real. Budget constraints mean the Tevatron will shut down this year, but fortunately the CDF team, which 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, DZero, Fermilab’s other detector, will analyse its own data to provide independent corroboration or refutation of the bump. And at CERN’s Large Hadron Collider near Geneva, Switzerland, physicists will soon collect enough data to perform their own search. In their paper, Lane and his colleagues suggest ways to look for other techniparticles.

“I haven’t been sleeping very well for the past six months,” says Lane, who found out about the bump long before the team went public with the result. “If this is what we think it is, it’s a whole new world beyond quarks and leptons. It’ll be great! And if it’s not, it’s not.”

Journal reference: arxiv.org/abs/1104.0699