Published online 3 November 2008 | Nature | doi:10.1038/news.2008.1203

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Spectral particles spook physicsts

Ghostly visitors appear inside Tevatron accelerator.

CDFThe Collision Detector at Fermilab (CDF) has seen some mysterious extra muons that hint at very strange physics.CDF/Fermilab

Unexplained 'ghost particles' are mysteriously appearing inside a US-based high-energy physics experiment.

Bloggers and theorists are already lining up explanations that involve unseen particles, hypothetical strings, or modifications of conventional physics. The finding is so controversial that about one-third of the 600-person experiment that detected it are refusing to put their names on the 69-page paper purporting its discovery, unveiled on the arXiv preprint server on 29 October1.

Whatever the spectres may be, their effect has been seen in roughly 100,000 events — enough to make it more than just a statistical fluke. Particle physics experiments sometimes see odd signals in their data, but it is rare that an effect persists long enough to be published by the entire collaboration.

The ghostly visitors are appearing at the Collider Detector at Fermilab (CDF), one of two building-sized experiments at the Tevatron, a proton-antiproton collider located at the Fermi National Accelerator Laboratory in Batavia, Illinois. "I've been in CDF since 1992, but I've never seen something this strange," says Tommaso Dorigo, a physicist at the University of Padua in Italy who collaborated on the experiment. "We need to try and understand what it is."

Debris slide

CDF is designed to hunt for exotic, massive particles rarely seen in nature. Like most high-energy physics experiments, it does not detect these particles directly, but instead watches for evidence of their decay in the showers of lighter particles that make up the debris from its collisions.

By precisely measuring the trajectories and types of light particles spewing from the centre of the machine, CDF physicists can infer which of the heavier particles their collider has produced.

The whole process works because of a precise set of equations known as the standard model of particle physics — a mathematical description of the quantum world. The standard model gives scientists the power to figure out what's happening at the centre of CDF with startling accuracy.

But CDF physicists are flummoxed by a surplus of muons seen in their detector. Muons are heavy cousins of electrons and one of the most common by-products of particle collisions. An interpretation of CDF's data has seen a "much larger than expected" number of decays that produce excess muons.

"It just doesn't add up," says Jacobo Konigsberg, a physicist at the University of Florida at Gainesville and a CDF spokesperson. Konigsberg says that the collaboration struggled for months to explain away the effect, but in the end felt it was better to publish their data for others to see and debate. "It wouldn't have been responsible to sit on this for much longer," he says.

Brane power

Theorists are already coming up with ideas about what might be producing the excess muons. One possibility is that they stem from the decay of a heavier, yet-to-be-discovered particle — perhaps related to dark matter, an unseen material that is believed to make up some 85% of matter in the Universe.

Another idea from string theory evokes seven-dimensional 'branes' — theoretical surfaces that are inhabited by exotic particles manifested as strings. These higher-dimensional branes might be home to force-carrying particles that interact weakly with our three-dimensional world and create a faint, but traceable, signal in the data.

But Adam Falkowski, a theorist at CERN, Europe's particle accelerator laboratory near Geneva, Switzerland, says that the explanations need some work, and cautions against attempting to force the data to fit into particular theories.

Others remain sceptical that the CDF experiment has in fact seen something new. Dorigo notes that the showers of particles created by the Tevatron collisions are often messy and difficult to understand. Although the CDF collaboration has gone to great lengths to remove systematic errors from the experiment, the signal may yet be explained by an unknown decay chain, or it may even be other particles masquerading as muons. "I want to see more before I believe it," he says.

Konigsberg agrees. "It could end up being something very mundane," he cautions. He hopes other experiments, including the D0 detector at Fermilab and others at the Large Hadron Collider in CERN, will be able to reproduce the apparitions. 

  • References

    1. CDF Collaboration. Published at http://arxiv.org/abs/0810.5357 (2008).
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