Experimental evidence for charm mixing

Great news! Two collaborations, BaBar at SLAC and Belle at KEK announced the evidence for D0-anti-D0 mixing today during the conference in Moriond.  They reported evidence for mixing using two different techniques, which is even better.

First of all, what is mixing? A phenomenon of meson mixing has been observed in several meson systems. As it turns out, electroweak interactions of the Standard Model allow some mesons and anti-mesons mix. This is possible because this process does not violate any conservation laws of Nature. For example, both D0 (made of a charm quark and up antiquark) and anti-D0 (made of a charm antiquark and up quark) are neutral as far as electrical charge is concerned. They are made of quarks of different flavors, but weak interactions do not conserve flavor, so Standard Model allows for diagrams that change D0 into anti-D0. For more information, see my recent review.

Why is it interesting? Well, for starters, this is the last unobserved meson mixing (mixings of neutral kaons, B and B_s mesons have been observed already). Second, this is a system that makes theoretical predictions difficult and thus interesting. I’ve been working on charm mixing for about ten years…

So, what did they see? When meson and antimeson mix, the phenomenon that occurs is very similar to a mechanical phenomenon of double pendula connected by a weak spring: after some time the system will develop oscillations that are described by generalized frequencies, which one can find. Similarly to that, time development of D0 or anti-D0 meson is described by a Schroedinger equation with non-diagonal Hamiltonian. This results in two states composed of both D0 and anti-D0 (the “heavy” and “light” states). So one can ask what the mass and lifetime differences between those  states are. And that’s what they measure! Both those quantities are usually normalized to a meson’s lifetime, so notmalized mass and lifetime differences are called x and y.

Now to the results: 

Belle finds y = (1.31 +- 0.32 +- 0.25) % with a significance of 3.2 sigma including systematics.

BaBar finds y’ = (0.97 +- 0.44 +- 0.31) % and x^’2 = (-0.22 +- 0.31 +- 0.21) x 10^-3 with a significance of 3.9 sigma.

This is consistent with a theoretical prediction (not post-diction) reported here.

And that’s the news! I’ll report about theoretical implications of that later!