B-factories pick up penguins

Antimatter mirrors matter — but not quite. The violation of charge–parity symmetry ('CP violation') in interactions mediated by the weak force means that particles and their antiparticles don't behave as exactly equal opposites. For example, a neutral B meson (made of a pair of quarks) constantly evolves between its particle and antiparticle states — but the rate of change of matter into antimatter is not the same as the rate for antimatter into matter. New analyses from the BaBar and Belle collaborations, published in Physical Review Letters, delve further into this asymmetry, investigating in particular a process described, rather entertainingly, by a 'penguin' diagram.

B mesons can decay in several ways, one possibility being the penguin process pictured — so called for its resemblance to the antarctic bird. (As the story goes, theorist John Ellis coined the name after losing a barroom bet, the forfeit being to work the word 'penguin' into his next paper, on B decay.) Here, an antibottom quark (b̄) from the B meson decays to produce an antistrange quark (s̄) through a loop that incudes a W boson and antitop quarks (t̄). Gluon radiation (g) from that loop can produce a quark–antiquark pair (s and s̄). Then, including the 'spectator' down quark (d) from the original B meson, the final-state quarks pair up into a neutral K meson and a particle known as η′, as indicated.

What makes the penguin process so interesting is the loop in the diagram: the particles contained within it are virtual and can have very high masses; it's possible that undiscovered massive particles could enter the loop, and hence be revealed through their influence on the process. But this penguin process is also relatively rare, and it takes a careful analysis of a large data sample to isolate its signature.

The BaBar and Belle collaborations — sited at the PEP-II collider at SLAC, California, and the KEKB collider at KEK, Japan, respectively — have been taking data for several years on B-meson production in electron–positron collisions. The energy of the collisions is chosen to maximize the rate of production of BB̄ pairs, hence the experiments have been called 'B-factories'. Following up on earlier analyses, both BaBar and Belle have now found a clear signal for the penguin process B⁰ → η′K⁰ in their huge datasets, comprising nearly 400 million Bs̄ pairs for BaBar (B. Aubert et al. Phys. Rev. Lett. 98, 031801; 2007), and more than 500 million BB̄ pairs for Belle (K.-F. Chen et al. Phys. Rev. Lett. 98, 031802; 2007).

The CP violation in the process is clear: using this decay signature, BaBar and Belle record matter–antimatter asymmetries between B and B̄ that have a significance of 5.5 and 5.6 standard deviations, respectively. More data is needed to probe whether there are new particles appearing within the virtual loop of the process, but as yet the possibility cannot be ruled out — leaving everything to play for in particle searches at the Large Hadron Collider, which turns on at CERN, Geneva, later this year.