Misplaced muons either mundane or monumental

Physicists are puzzling over a bunch of measly muons. In a series of experiments at the Tevatron, a powerful atom smasher at the Fermi National Accelerator Laboratory in Batavia, Ill., researchers have detected too many of these heavy cousins of electrons in a region where there should be hardly any.

Most physicists believe a mundane explanation exists for the aberrant location of these subatomic particles in this experiment.

But there’s a chance, even if slim, that the muon detections indicate the existence of some new, long-lived elementary particle and perhaps a previously unknown force. Such a finding could revolutionize the understanding of the universe, says physicist Mark Kruse of Duke University in Durham, N.C.

Only two-thirds of the many collaborators on the Tevatron experiment, including Kruse, consented to have their names listed on the online article that announced the muon puzzle October 30 (http://arxiv.org/abs/0810.5357). Many believe the puzzle, upon further analysis, will be solved with ordinary physics — perhaps what produced the muons is some overlooked background process within the Tevatron’s particle detectors.

Kruse himself thinks this will likely be the case, but he and his collaborators nonetheless speculate on the novel types of elementary particles that might be required if the muon riddle endures. They posted their model online November 1 (http://arxiv.org/abs/0810.5730). And a physicist not affiliated with the experiment, Matt Strassler of the Rutgers University campus in Piscataway, N.J., contributes his own musings in a November 11 posting (http://arxiv.org/abs/0811.1560).

“If you saw any significant number of muons appearing outside the beam pipe, pretty much anywhere, and you were sure it wasn’t coming from a [known] source], you’d be excited,” says Strassler.

This particular Tevatron experiment is known as the Collider Detector at Fermilab, or CDF. It crashes together beams of protons and antiprotons moving at nearly the speed of light. Muons are negatively charged and can be produced directly or indirectly by these collisions, but researchers wouldn’t expect the particles to be found where some have now been detected — only a few centimeters from the collision site. The muons’ positively charged anti-particles, anti-muons, were also found in the same abundance and in the same strange place as the muons.

Because muons last for a microsecond — nearly an eternity in an a particle physics experiment — those that are created directly in the collision process would race through the accelerator’s beam pipe and wouldn’t decay or be detected until journeying about half a kilometer outside the pipe. These muons would leave an electronic signal in their wake that allows physicists to trace the particles back to the collision.

Another type of elementary particle produced by the collision, called a pion, can decay into muons, but on average the pions wouldn’t undergo this decay until they were, on average, meters from the collision.

The collisions between protons and anti-protons at CDF can also produce bottom quarks. These short-lived particles would decay into muons only a few millimeters from the collision.

The relatively large population of muons and anti-muons found centimeters from the collision can’t be obviously explained by any of these processes. Moreover, some of the muons appear to be grouped, as if several were produced at the same time, Kruse and his colleagues note in the Nov. 1 paper. That observation makes it less likely that the muons were produced by some random background process in the detectors, Strassler says.

While Strassler adds that he’s agnostic about the CDF results until more data are published, he suggests that some type of hidden particle — a particle that
doesn’t interact with light or that is impervious to the nuclear forces that hold neutrons and protons together — might conceivably explain the muon conundrum.

In a “hidden-valley” model proposed in 2006 by Strassler and by Kathryn Zurek, now at Fermilab, new forces allow a high-energy particle collision to produce hidden particles, which can’t be detected directly. These hidden particles multiply, travel some distance and finally decay back to visible particles, such as the muons. In some hidden valley models, Strassler notes, the visible particles would appear at centimeters or more from the collision point — just as the muons do in the CDF experiment. Perhaps, Strassler and Zurek suggest, the mystery muons decayed from an as yet unknown type of hidden particle.

The hidden particles could be related to the unseen dark matter that astrophysicists invoke to explain how the universe remains intact. Kruse says they might also be linked to a trio of Higgs particles, the hypothetical particles that physicists believe may explain why elementary particles have mass.

For now, physicist remain interested but skeptical, eager to see if the muon riddle appears in another experiment, known as D0, that shares the Tevatron with CDF. An analysis of the D0 data could take several months, Kruse says.