While engineers at the Large Hadron Collider (LHC) race to fix its teething problems and start looking for new particles, its ageing predecessor is refusing go silently into the night.
Last week, physicists announced that the Tevatron particle accelerator at Fermilab in Batavia, Illinois, has produced particles that they are unable to explain. Could it be a sign of new physics?
The Collider Detector at Fermilab (CDF) monitors the particles that spew from collisions between protons and anti-protons, which are accelerated and smashed head-on by the Tevatron. The collision occurs inside the 1.5-centimetre-wide "beam pipe" that confines the protons and anti-protons, and the particles created are tracked by surrounding layers of electronics.
In this instance, the CDF was looking at bottom quarks and bottom anti-quarks that decay into, among other things, at least two charged particles called muons.
The team was in for a big surprise. First, they saw far more muons coming from the collisions than expected. But crucially, some of these muons seemed to have been created outside of the beam pipe: they had left no trace in the innermost layer of the detector.
The CDF team says it is unable to explain such muons using the standard model of particle physics, or from what they know of their detector.
However, "we haven't ruled out a mundane explanation for this, and I want to make that very clear", says CDF spokesperson Jacobo Konigsberg, who adds that it is important that other experiments verify the effect.
While the CDF team is circumspect, theoreticians are more willing to speculate. If the signal is not spurious, this means that some unknown particle with a lifetime of about 20 picoseconds was produced in the collision, travelled about 1 centimetre, through the side of the beam pipe, and then decayed into muons.
"A centimetre is a long way for most kinds of particles to make it before decaying," says Dan Hooper of Fermilab. "It's too early to say much about this. That being said, if it turns out that a new 'long-lived' particle exists, it would be a very big deal."
Neal Weiner of New York University agrees. "If this is right, it is just incredibly exciting," he says. "It would be an indication of physics perhaps even more interesting than we have been guessing beforehand."
So what could it be? As it happens, Weiner and Nima Arkani-Hamed of the Institute for Advanced Study in Princeton, New Jersey, and colleagues have developed a theory of dark matter – the enigmatic stuff thought to make up a large proportion of the universe – to explain recent observations of radiation and anti-particles from the Milky Way.
Their model posits dark matter particles that interact among themselves by exchanging "force-carrying" particles with a mass of about 1 gigaelectronvolts.
The CDF muons appear to have come from the decay of a particle with a mass of about 1 GeV. So could they be a signature of dark matter? "We are trying to figure that out," says Weiner. "But I would be excited by the CDF data regardless."