Looking for the unexpected

Some of the Feynman diagrams that contribute to the decay of a D0 (a meson composed of a charm quark and up anti-quark) to a pair of muons. The loops (square or triangle) in the diagrams are an indication that the processes they represent are suppressed. This decay has no processes without loops in the Standard Model. But new physics can have processes without loops and these can dominate over the Standard Model contribution, so this decay becomes an interesting probe for new physics.

Scientists can see new particles either directly by producing them in collisions, or indirectly by observing their effect on another process. Indirect processes are sensitive to particles with masses much greater than those that can be produced directly in detectors.

CDF has been looking for the decay of a neutral charm meson (D0) to a pair of oppositely charged muons. This process conserves energy, momentum, and quantum numbers such as lepton number and baryon number. Conservation means that if you start with a certain set of quantum numbers, the sum of the particles produced in a collision must have the same net quantum numbers. However, the Standard Model rarely permits this process and therefore scientists predict this decay process to occur in fewer than one in 10 trillion decays of D0 mesons.

When a Standard Model process is so strongly suppressed, CDF would be lucky to observe a single decay. On the other hand, the observation of even a handful of events would be a smoking gun. It would indirectly reveal the presence of new and yet unknown physics processes, which might lurk far beyond energies available for direct exploration at even the most powerful accelerators.

CDF physicists searched through data selected from 360 inverse picobarns of integrated luminosity, which is estimated to contain about 50 million D0s. If the Standard Model is correct, than the probability of a real D0 decaying into two muons in this sample is low and there are possible sources of background that might mimic the signal, confounding the measurement. After a careful study, physicists expect 8.6 background events.

Finally, after counting the events in their actual data sample, CDF physicists observed, just four events, which are consistent with no signal. The measurement set a limit that fewer than 2.1 out of 10 million D0 meson decays would decay into two muons.

This limit is close to the best limit ever obtained using dedicated b-factory experiments at other laboratories. CDF has much larger samples yet to be analyzed in this particular way. Through more results similar to this one, CDF physicists hope to update this analysis and either find this rare decay and herald new physics, or exclude it with unprecedented sensitivity.

More information.

— edited by Andy Beretvas

These individuals contributed to this analysis. From left to right: Edmund Berry, University of Chicago; Ivan Furic, University of Florida; Robert Harr, Wayne State; Fermilab Deputy Director Young-Kee Kim, University of Chicago.