Two B mesons violate CP

N is the number of events determined by a fit for each decay mode. The first uncertainty is statistical, and the second is systematic.

In 1957 Lev Landau postulated that the laws of physics are symmetric under charge parity. This means that a mirror reflection of a particle with a particular charge and handedness would yield a particle of opposite charge and handedness. For example, a left-handed electron is transformed under charge parity to a right-handed antielectron.

In 1964 Cronin, Fitch and collaborators observed that charge parity, or CP, invariance was broken by a small amount in the decay of a neutral kaon. This means the decay, in producing particles of opposite charge, does not always produce particles of opposite handedness.

It was only in 2001 that CP violation was observed in B meson experiments done at the B factories such as SLAC’s BaBar experiment and KEK’s Belle experiment. In today’s result, CDF physicists looked for CP violation in the decays of neutral hadrons containing a b quark, in particular, a neutral B meson (made of a d quark and a b antiquark), a strange B meson (made of an s quark and a b antiquark) and a Λ baryon (made of an up, down and b quark).

For example, we look at the decay rate for a neutral B meson to decay into a positive kaon and a negative pion. We also look for the corresponding antimatter decay. The asymmetry is given by the difference between the two rates divided by their sum.

The experiment is difficult because the occurrence of this particular decay chain is rare: The neutral B meson decays into a positive kaon and negative pion 2 times out of every 100,000 decays. Another difficulty is particle identification — the separation of pions from kaons. Scientists identify the particles by measuring the energy deposited in the detector’s drift chambers. The experiment must also determine how efficiently the detector measured the particles’ decays as compared with their corresponding antiparticle states.

The results are summarized in the above table. No asymmetry is seen for the two different decay modes of the Λb, so there is no evidence of CP violation in the decay of this particle. The asymmetry of the B0 → K+π decay is measured with a significance of greater than five sigma. The measurement is in agreement with those at the B factories and the LHCb experiment at the LHC, thus corroborating evidence of CP violation in this decay. We also confirm the recent result by LHCb of the first measurement of the asymmetry for the strange B meson system. This large asymmetry is consistent with the Standard Model.

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edited by Andy Beretvas

These CDF physicists contributed to this data analysis. From left: Michael Morello (INFN, Pisa, Scuola Normale Superiore), Giovanni Punzi (INFN, Pisa) and Fabrizio Ruffini (INFN, Siena).