Tag Archives: CP Violation

New e+/e- Collider Turns On

SuperKEKB, an upgraded version of the KEK B electron/positron collider at the KEK lab in Tsukuba has started up. There are no collisions yet, but they are starting to run beams around the main ring. The Belle II experiment will use he SuperKEKB facility to study B physics – that is, the physics of B mesons (and other particles involving b quarks). These are particularly interesting because the lifetimes of many of these particles decay weakly with long enough lifetimes to actually measure how far they travelled. B physics allows for high precision tests of various aspects of the Standard Model, including things like CP violation in the quark sector (i.e. matter/antimatter differences), particle spectroscopy (measuring the properties of the various kinds of composite particles), and searches for various kinds of new physics.

New LHCb Results Available

Over the past couple days, the LHCb collaboration has released a number of papers. LHCb is probably the least well known of the four large experiments at the LHC. ATLAS and CMS are general purpose detectors meant to search for a wide variety of physical processes. ALICE focuses on collisions of heavy ions. Unlike these detectors, which try to cover as much of the total 4π solid angle as possible and typically look for events with a large transverse (perpendicular to the beam) momentum, LHCb is a forward spectrometer focusing on events near the beam direction. One of the principal goals of LHCb is to study the properties of B mesons, which are mesons containing a b-quark.

The LHCb papers include one measuring CP violation in a particular decay of the B0s meson, one measuring the lifetime of the B0s using a particular decay channel, and a third measuring the production of two charmonium (charm-anticharm mesons) states in proton proton collisions.

The most interesting paper to non-specialists is the fourth paper, which presents evidence of direct CP violation in the decay of the B+ to a K+ and a proton-antiproton pair. Direct CP violation is basically just looking for differences between particles and their antiparticles. In this case, it was found that in some regions of the kinematic phase space of the final state, decays of the B+ and B mesons seem to occur at different rates. This can occur due to interference between different diagrams leading to the same final state. The result is not significant enough to claim a discovery of direct CP violation in B decays. If the result holds up, it would represent the first measurement of direct CP violation in B decays using a decay channel involving baryons.