Evidence for D⁰-D⁰bar Mixing Found at the Belle Experiment

The transformation of particles into their antiparticles, a phenomenon referred to as mixing, has been observed in several systems of neutral mesons: neutral kaons, B_d and most recently B_s mesons. The only system where this phenomenon is possible, but has not yet been observed, is the neutral D meson system.

In the Standard Model of particle physics D meson mixing is highly suppressed. As a result it is sensitive to the contribution of new, as of now unobserved processes and particles. Many searches for D mixing have been carried out at electron-positron colliders, proton-antiproton colliders and fixed target experiments since the discovery of D mesons in 1976.

Evidence has now been found for D⁰ mixing by a team of Slovenian researchers in the Belle experiment (M. Starič, B. Golob, U. Bitenc, P. Križan, A. Zupanc), all members of the Belle working group for charm physics. The Belle experiment, a collaborative effort of scientists from universities and laboratories in America, Asia, Australia, and Europe, operates at the KEK High Energy Physics Laboratory in Tsukuba, Japan, which is home to the world's highest luminosity particle accelerator.

On March 13, 2007, M. Starič presented evidence for D⁰-D⁰bar mixing at the Rencontres de Moriond, an international high energy physics conference in LaThuile, Italy. Mixing modifies the decay times of D mesons by a small but observable amount. From a careful comparison of decay times in three decay modes of D mesons, a flavour-specific D⁰->K^- pi⁺ decay, and two decays to CP eigenstates K⁺ K^- and pi⁺ pi^-, Belle measured the relative lifetime difference to be (1.31 +- 0.32 +- 0.25)%.

A non-zero value of a lifetime difference between different decay modes represents clear evidence for the transformation of the D⁰ particle into its anti-partner. Belle's measurement can now be compared with Standard Model theoretical predictions in order to search for new physics.

A small difference in the lifetimes of two sets of D meson modes is the first evidence for D⁰-D⁰bar mixing.

Figure (a) shows the time distributions for D⁰->K^- pi⁺ and the sum of D⁰->K^- K⁺ and D⁰->pi⁺ pi^-, (b) shows the ratio of the two time distributions. The deviation from zero slope in (b) is visual evidence for D⁰-D⁰bar mixing

Further reading

M. Starič et al. http://belle.kek.jp/belle/talks/moriondQCD07/Staric.pdf

Belle research group: http://belle.kek.jp

Experimental particle physics at FMF in J. Stefan Institute: http://www-f9.ijs.si