| We now know that we need to produce B mesons moving quickly
- at an appreciable fraction of the speed of light - and that we need a
way of identifying the decay of a B0 or
B0-bar to a CP eigenstate. Fortunately,
whatever method is used, a b-quark is nearly always produced with a b-antiquark
(b-bar). So if one can detect the CP eigenstate decay and somehow
identify the partner b or b-bar, one can know whether the CP eigenstate
decay originated with a B0 or B0-bar.
Two basic approaches are being used for producing the
moving B mesons. One is to collide protons with other protons (or
neutrons). With enough energy, many B's are produced, and most are
moving. A difficulty, though, is that only a very small fraction
of the collisions produce B's, at best one in a million. It is a
major challenge to design a detector that is able to collect data at the
appropriate rate. Another approach is to collide electrons with positrons,
which are the antimatter partners of electrons. At the right collision
energy, the electron and positron can annihilate completely and use the
resulting energy to produce a meson known as the Upsilon(4S) (usually printed
with the Greek letter, as  ),
which contains a b and a b-bar quark. The Upsilon(4S) decays almost
immediately to a B and a B-bar meson pair. If the electrons and positrons
are at different energies, then the Upsilon(4S) will be moving and the
B and B-bar will be moving. This is a unique way to produce just
B and B-bar and have them moving. It has a lot of practical appeal
from the point of view of detecting B's; a sizable fraction of collected
events (about 25%) contain B mesons, and the events themselves contain
no extra particles. The big challenge of this approach is in producing
the collisions. To collide beams with different energies is difficult,
and since probability of producing Upsilon(4S) in electron-positron collisions
is not high the beam intensities must be very high. |
