About BaBar

A plot of the number of neutral D meson decays vs. time. Evidence for charm mixing is seen as a deviation in the decay rates of charm mesons from the ordinary exponential behavior expected for direct particle decay. Plot (a) shows the number of neutral D mesons remaining vs. time (in white) along with various background processes (shaded). Plot (b) shows the difference between the data (points) and the value expected if there is no mixing (dotted, flat line) The mixing effect is shown as a solid line and is seen to fit the data points much better than the no-mixing hypothesis.

At the 2007 Moriond Electroweak conference in Italy on March 13, the BABAR Collaboration presented the first evidence for charm meson mixing, a process in which an electrically-neutral charm meson is produced as a particle but decays as its anti-particle partner. The existence of this process has been the subject of debate by physicists for over thirty years. Many current and former high-energy physics experiments have searched for it, including CLEO, Belle, E791, FOCUS, and BABAR. The two-year long analysis was performed by the international team of 600 physicists and engineers who make up the BABAR Collaboration and required the efforts of many personnel at SLAC, where the charm mesons were produced in matter-anti-matter electron collisions at the PEP-II storage rings.

The evidence for charm-meson mixing, also known as D⁰-D⁰ mixing since it converts a D⁰meson into a D⁰ meson (or vice versa), confirms that mixing occurs in all sectors of the quark model. Meson mixing was first observed in the 1950s between neutral K mesons, in the 1980s between neutral B mesons, and in 2006 between neutral B_S mesons. The search for for D⁰-D⁰ mixing is particularly challenging since it is expected to occur infrequently and because the time the D meson lives is about the same as the time required for mixing to begin to develop. This means that many of the particles will decay before mixing can contribute significantly. Of approximately 1.2 million D⁰ and D⁰ mesons studied, only some 600 show evidence for mixing.

This result leaves open an intriguing question about the origin of the effect. Is it due to the known physics of the "Standard Model", or is it due to new processes previously unseen? Further work will be required to sort this out in the coming months and years, including efforts from other high-energy experiments such as CLEO and Belle. This illustrates the collaborative nature of particle physics; while it is exciting to "be the first" to produce a new result, the real game is in understanding nature at a deeper level, and that is more than enough of a challenge for all of us put together.

BaBar Finds First Evidence for Charm-Meson Mixing