Desperately seeking CP violation

One of the main objectives of B physics has been to complete the mapping of the CKM matrix. The B-mesons are analogous to K-mesons: the strange quark is simply replaced by a b-quark. Just as neutral K-mesons proved to be a powerful tool, the spotlight is now on the neutral B-mesons.

Studies with neutral B-mesons are particularly useful because results are sensitive to CKM elements related to top quarks as well (even though the top quark is not produced physically). This is especially the case for the mixing of neutral B particles and antiparticles, where tremendous progress has been made during the last decade at high-energy colliders like LEP at CERN and the Tevatron at Fermilab.

Despite these advances, a nagging question is that of CP violation: a tiny effect only observed in neutral K-mesons. After 35 years of delicate experiments with K-mesons, progress has been limited. The good news is that not only do B-mesons offer a new arena to study CP violation, but, according to the CKM model, there should be many manifestations of the violation and the effects should in some cases be large. The problem is to produce enough B-mesons, and even LEP falls short.

The need for more B particles has resulted in an armada of new facilities around the globe. These include HERA­B at DESY (wire target in a proton beam), performance upgrades of the venerable CESR electron-positron ring at Cornell, new dedicated electron­positron "B-factories" at KEK (Japan) and SLAC (Stanford), the Tevatron proton­antiproton collider upgrade with the new main injector and, ultimately, the LHC at CERN.

Two paths to decay

The centrepiece of these studies is to look for an asymmetry between the decays of the two neutral B-mesons (which are a particle and an antiparticle of each other) into a J/psi particle and a short-lived K-meson. This "golden" mode is ideal in several respects: the resulting state has well defined CP properties; the relationship between the decay asymmetry and CKM parameters is not plagued by theoretical uncertainties, as is the case for some other decays; and, experimentally, the decay is easy to identify when the J/psi subsequently decays into a pair of leptons. It also has a relatively large rate, but still fewer than one neutral B in a thousand will decay in this way.

The CP violation manifests itself via the unequal decay rates of neutral B particles and antiparticles. This effect should be starkly exposed because of subtle interference effects from neutral B mixing. A neutral B can decay directly, or it may oscillate into its antiparticle before decaying (see figure). These two different paths are expected to have a slightly different phase, and the interference between them produces an asymmetry.

Whereas the classic CP violation in neutral K-mesons amounts to approximately one violation in 500 decays, the expected asymmetry in the new case means that, at the times of maximal interference in the neutral B oscillation cycle, the difference in decay rates is one out of eight.

If observed, such CP violation will not be an obscure subtlety. Although a massive attack on this problem is about to commence with the ongoing start-up of the B-factories, the opening salvos have already been fired. Last year the OPAL experiment at LEP was the first to publish results of an investigation of CP violation with a sample of about a dozen golden mode decays. Taking advantage of the clean LEP environment and powerful "tagging" techniques to distinguish neutral B particles from their antiparticles, the experiment obtained an asymmetry of 3.2, but with an error of ±2. Mathematically, such an asymmetry must lie between ­1.0 and 1.0. Furthermore, the errors are large compared with this allowed range. It was none the less intriguing to obtain such a large number, as it is somewhat unlikely for the measurement to fluctuate to so high a value if CP is conserved. Overall, this was an impressive achievement given so few events.

Hot on OPAL's heels was CDF at the Fermilab Tevatron. The advantage of a hadron collider is the much larger B production rate. From existing data samples, CDF was able to muster several hundred golden mode decays. Last summer, CDF announced a measurement of CP asymmetry of 1.8 ±1.1 using a single method to separate neutral B particle and antiparticle events. Again, this is an unusually large positive asymmetry, but with half of the uncertainty of the OPAL result. Is a pattern emerging?

In February, CDF released results from an expanded analysis using all available data and tagging methods to squeeze the uncertainty. The preliminary asymmetry result was 0.79 ±0.43 ­ large and physically possible. This result is strikingly close to the expected asymmetry of about 3/4 from experimental constraints on the CKM matrix. The limit excluding no asymmetry is only a little better than before, but the uncertainty is halved. This is still too large to claim that CP violation has finally been observed outside the K-meson system, but it is tantalizingly close. The uncertainty shrinks: the evidence mounts.

Prospects look bright for the new generation of B experiments to look for large CP violating effects. In a matter of months we can hope for new signs of CP violation, especially from the new B factories at KEK and SLAC; and next year the Tevatron experiments will return to the hunt where CDF left off.

In the years to come, an extensive array of measurements around the globe will study various manifestations of CP violation in B-mesons and stringently test the CKM model. This will hopefully enable us to move beyond the phenomenological CKM description and unveil some of the mystery surrounding the emergence of a matter-dominated universe that is so convenient for our existence.