The Kobayashi-Maskawa Model is Validated
by the Observation of CP Violation in Neutral B Meson Decay
by the Belle* Experiment  at the KEKB B-factory

Although at first glance the left side of this picture may look like a it is a mirror image of the right side, it isn't, quite.  This image conjures up what might come to mind when you hear the term "B Factory", which is used to describe the accelerator system that creates the millions of B mesons that must be generated in order to study CP violation in their decays.  The fanciful 3x3 matrix of "V's" represents the CKM matrix, which quantifies some of the weak interactions between quarks.  According to the Standard Model, the nine elements of the CKM matrix are interrelated closely in very specific ways.  Prior to the present experiment, CP violation was predicted to exist in B decays based on the values of the CKM matrix established through other experimental data.
 

The matrix in its current form was first presented by M. Kobayashi and T. Maskawa in 1973 as part of a model where CP violation occurs naturally in the theory and could be used to explain the single experimental observation of CP violation to that date (in K meson decays, 1964).  The centerpiece of the model was the existence of six quarks, a daring proposition at a time when only three, u,d, and s, were known to exist.  The subsequent discoveries of the c, b, and t quarks, and the natural existence of CP violation in the model, led to the incorporation of the K-M mechanism into the Standard Model, even though it had not been conclusively tested experimentally.     Why is CP Violation Important? If we look out in space as far as we can see, all evidence says that the universe is made up of matter - protons, neutrons, electrons. In the laboratory it is possible to create antimatter (e.g., antiprotons, antineutrons, positrons) which appears to be identical to matter, except that in close contact with matter both can annihilate. Since we now believe that the universe was born with equal amounts of both, why should this change over the universe's lifetime? We now know that fundamental properties of matter (and antimatter) determine how the universe evolves and that at least some elementary particles must exhibit a property known as CP violation if the universe does indeed consist dominantly of matter. Simply put, matter and antimatter do not behave identically.  How has it been observed in B decay? If we assume that the Standard Model is correct, then previous measurements have constrained the CKM matrix enough to make a rough estimate of where and how to expect CP violation in B decays.  The current experiment is designed to be able to make the measurements well enough to really determine whether the Standard Model is indeed correct; then, if it turns out that the model isn't correct, we will know it.    These first studies involve rate differences that depend on when a neutral B (B0) or anti-B0 decays.  The decay is so fast, though, that no timing device exists that could measure it; the average B0 decays in one-trillionth of a second.  Instead, the B0 is made to move rapidly, and the distance between its birth and its decay is measured: time=distance/velocity. B and anti-B pairs are produced by colliding electrons with positrons (the antimatter partners of electrons) at specific energies such that they annihilate completely, produce a meson known as the Upsilon(4S) (which contains a b and a b-bar quark), which decays almost immediately to a B and an anti-B meson pair.  If the electrons and positrons are at different energies, the Upsilon(4S) is moving, as are the B and anti-B. An interesting characteristic of using the Upsilon(4S) as a source of B mesons is that the B pairs are produced in a coherent quantum mechanical state (like Schrodinger's cat or single atoms in a trap).  The B mesons begin their life in a superposition of quantum mechanical states.  When one of the pair decays, its partner is forced into a definite state.  That this is indeed the case is amply demonstrated in the current data, where both decays are timed.   The expected CP violation or asymmetry occurs in the difference in decay time between the decay of a B0 (or anti-B0) to an identifiable B0 (or anti-B0) mode and its partner B in a decay type that could come from either B0 or anti-B0.  The asymmetry is the difference divided by the sum of rates for B0 and anti-B0. and is expected to oscillate as a function of the decay time difference.

The Belle result supports Kobayashi and Maskawa's theory, which predicts large CP violation effects in the neutral B meson system.  However, the details are still controversial.  At what exact point in the space of quark parameters does CP violation occur?  Will other, different measurements be consistent with this result?  Are there other, previously unknown, forces that also contain CP violation?  All of these questions await the accumulation of more data from the B factories. click on figures to enlarge Raw data - rates in modes expected to show opposite asymmetries (red, blue). Asymmetries including expected oscillatory behavior, for (a) all data combined, (b) data expected to have one sign of asymmetry, (c) data expected to have opposite sign of asymmetry, (d) data expected to show no asymmetry. An event used in the Belle CP violation measurement.