If physicists weren’t jumping up and down with excitement in April at the announcement that an unknown particle had been glimpsed at Fermilab, they are now.
The news of a possible particle sighting in the debris of proton-antiproton collisions at the Illinois accelerator had been met with a mix of curiosity and scepticism. It was based on an analysis of eight years of data collected by Fermilab’s CDF experiment that looked at collisions that produced a W boson, carrier of the weak nuclear force, along with two jets of quarks.
A suspicious bump in the data showed an unexpected rise in the number of these events clustered around 145 GeV – suggesting that they are being produced by an unidentified particle of the same mass. It was immediately clear that whatever the particle was, it was not predicted by the standard model of physics, the leading theory for how particles and forces interact. To add to the mystery, it was clearly not a Higgs boson, the long-sought particle that gives other particles their mass.
Still, physicists maintained their composure, noting that the results weren’t convincing enough to warrant any impromptu dancing or high-pitched screams. At a robustness of “three sigma”, there was a 1 in 1000 chance that it was just a statistical fluke, making the finding good enough to be considered “evidence” but still far from the five-sigma gold standard for a true discovery.
Now the CDF team has analysed nearly twice the amount of data the first result was based on, and the result has not gone away. In fact, as CDF physicist Giovanni Punzi told a conference this week in Blois, France, the signal has only gotten stronger. It is reportedly at 4.8 sigma, tantalisingly close to five-sigma certainty.
Now that there’s a mere 1 in a million chance that the result is a fluke, it seems there are only two options: either it’s the result of some systematic or detector effect that no one has thought of yet, or it’s real. The CDF team is hard at work trying to figure out which. “We’re still going through all the data, and we’ve got two other teams repeating the analysis in a different way, so we’re not going to publish a five-sigma result until all of our i’s and t’s are dotted and crossed,” says CDF spokesperson Rob Roser.
An independent check will also come from Fermilab’s DZero experiment, which has enough of its own data to corroborate or cast doubt on the particle’s existence. So far, the DZero team is keeping quiet, but it is expected to publish its results in the next few weeks.
The Large Hadron Collider in Switzerland should also be able to test CDF’s result. There’s talk that the LHC hasn’t seen any such bump – but Roser says that doesn’t mean much one way or the other, as the LHC hasn’t collected as much data as Fermilab. “They haven’t really achieved the sensitivity to see this yet,” he explains.
So if the alleged particle is real, what is it? That’s the million dollar question, and theorists are already placing their bets. Some say it’s a particle called the Z-prime, a hypothetical carrier of a new force similar to the electroweak force, though it would have to be an unusual version of a Z-prime to have slipped by unnoticed at CERN’s LEP collider. Others say it might be a sign of supersymmetry – a popular theory that solves some perplexing problems in physics and postulates that every particle has a shadowy partner. The Fermilab bump could therefore be pairs of “squarks” or “selectrons” – supersymmetric partners of quarks and electrons. Still others believe that it is a technipion – a particle that appears in a theory known as technicolour, which posits a new force that is similar to the strong nuclear force but operates at much higher energies.
The CDF team is looking at these and other theoretical models to see which best fits and will be publishing a paper in the next few months with their result. Roser says a leading contender has emerged, but he’s keeping his lips sealed about its identity.
Whatever happens, a five-sigma result is enough to allow physicists to finally let out a little excitement. As we usher in the era of physics beyond the standard model, I am sure that office doors in physics departments around the world are being closed and behind each one, someone is doing a little dance.