March 13, 2007
Hot on arXiv: BaBar claims 3.9 σ evidence for D0 — D0 mixing, by measuring the time modulation of the “wrong sign” decay D0 → K+π–. All the significance comes from y’ = (9.7 ± 4.4 ± 3.1) × 10-3. (The x’2 value, (-0.22 ± 0.30 ± 0.21) × 10-3, is not significantly nonzero, though it is highly correlated with y’.) They see no evidence of CP violation.
For charm quark enthusiasts, this is exciting, even though nobody’s claiming “discovery” yet. Nobody knew whether D mixing would happen at a rate large enough to be observable (estimates differed by several orders of magnitude), and this suggests that the answer may be yes. That being said, a recent Belle result, using the same experimental idea, about the same amount of data, and similar errors, doesn’t see a particularly significant signal.
This is an incredibly difficult measurement, both theoretically and experimentally. From the theory side, it is not really a probe of “short distance” physics, the sort of diagrams involving the top quark which dominate the rate for B mixing. (This is a consequence of the famous “GIM mechanism.”) Instead, “long distance” effects are believed to be more important: for example, both the D0 and the D0 can decay to K+K–, so a D0 can turn into a D0 by going through an intermediate K+K– state. Unfortunately, that’s all hadronic physics (while for B mixing it’s electroweak physics that counts), so predictions and interpretation are hard.
On the experimental side, the D0 — D0 mixing rate (the frequency of the oscillation between the two states) is small, relative to the lifetime of the D0. The mesons usually don’t have time to oscillate before they decay, so the effects you are trying to observe are tiny. (In contrast, the recent Bs mixing measurements had the opposite problem; their oscillations are so fast that a typical meson changes back and forth many times before decaying, and the challenge is to measure how many switches it makes.) In this particular analysis, the idea was to measure the interference between the processes D0 → K+π– and D0 → D0 → K+π– as a function of time since the production of the D0.
This result is tantalizing, but inconclusive; we really need to see mixing in other channels with different systematic uncertainties to be sure. Still, it’s very impressive work.
Update: Alexey Petrov reports from Moriond that Belle also reports 3.2 σ evidence for mixing in a different process (Belle talk here). The analysis compares the lifetimes of the “flavor eigenstate” decay D0 → K–π+ and the “CP eigenstate” decays D0 → π+π–, K+K–. They see y = (1.31 ± 0.32 ± 0.25)%. This y differs from the BaBar y’ by a rotation by an unknown angle δ which is believed to be near zero (CLEO-c is measuring this now!) — if we assume δ ≈ 0, then y’ ≈ y, and the two results are consistent.