The charming socialites

The figure shows the decay of the D*⁺ into a D⁰ and a soft (low-momentum) pion. The D⁰ then decays into a negative kaon and a positive pion. This is called a right-sign decay. Another decay mode called a wrong-sign, which is very rare, can occur. In this mode, the D⁰ decays into a positive kaon and a negative pion.

Since the discovery of the charm quark in 1974, physicists have postulated a rare process in which a charm particle spontaneously changes into its antiparticle. Evidence for this unique “social” behavior, called charm mixing, was uncovered more than three decades later by experiments at the B-factory accelerators at SLAC in California, KEK in Japan and the CDF experiment at Fermilab. Conclusive observation of the process has emerged this year from the LHCb experiment at CERN, with 9.1 sigma significance, and from this CDF experiment, with 6.1 sigma significance.

In these recent experiments, neutral D mesons (D⁰), which are bound states of a charm quark and an anti-up quark, are seen to change into anti-D mesons. Using the Standard Model of particle physics, it is difficult to make a precise prediction of the mixing rate. The observed rate could be due solely to Standard Model physics but may also indicate the presence of new particles that mediate the mixing process.

The results of the recent CDF measurement using the full data set for Run II are shown in the lower figure. A ratio R_m of a rare D⁰ decay mode to a more common one is plotted versus decay time. If the mixing process did not occur, the ratio would remain unchanged with time. The data clearly show the contrary behavior: The ratio increases with time. The plot also shows the difficulty of the measurement. The rare decays take place only about five times for every thousand decays. Furthermore, some of the decays originate from beauty particle decays (non-prompt) and not from direct (prompt) production in the proton-antiproton collisions of the Tevatron. The decays from the prompt D⁰ mesons follow the red curve in the figure, whereas the blue curve has contributions from prompt and non-prompt decays.

Now that charm mixing is clearly established, more precise measurements and improvements in theoretical interpretation are needed. The next generation of measurements will include a search for differences in the mixing rates of D⁰ versus anti-D⁰ mesons. Such a difference could elucidate the puzzle of why particle matter in the universe overwhelmingly dominates over antimatter.

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—edited by Andy Beretvas

The ratio of the rare D⁰ decay mode (K⁺π^–) to a more common one (K^–π⁺) versus decay time, measured in units of the D⁰ lifetime. If the mixing process did not occur, the ratio would remain unchanged with time. The measured ratio includes decays from prompt and non-prompt D⁰ mesons (blue curve). The ratio using only prompt decays is shown by the red curve.

Top row from left: Paul Karchin and Mark Mattson (both at Wayne State University). Bottom row from left: Satyajit Behari (Fermilab) and Paolo Maestro (University of Siena and INFN- Pisa)