The latest results from the KEK B Factory
- Observation of a new interaction in elementary particle physics -
June 30, 2005, Belle Collaboration
Quarks are the most fundamental constituents of material, and it is
widely known that six types exist in nature. The purpose of a B
Factory is to produce an enormous number of the second heaviest, called
the b quark, and clarify the fundamental laws of the universe by
detailed studies of its behavior. The international experimental group
called Belle has been carrying out these studies at KEKB, the
electron-positron colliding beam accelerator at KEK in Tsukuba, Japan,
and has made many important contributions to our understanding of
nature at its most fundamental level. These include the discovery
of the violation of particle-antiparticle symmetry in b quark decays,
the experimental confirmation of the Kobayashi-Maskawa theory that
predicted this symmetry violation, and the discovery of many new
particle states.
In the last year, the performance of the KEKB accelerator has continued
to improve, reaching the world's highest luminosity of 1.5x10^34
/cm2/sec, and delivering 390 million pairs of B and anti-B mesons to
the Belle experiment. (These mesons contain a b quark or its
antiparticle.) The Belle group has investigated the decays of these B
mesons in detail, and has observed a new type of interaction wherein
the b quark turns into one of the lightest quarks, the d quark. This
phenomenon was confirmed by finding 35 events where the B meson decays
into either a omega or an rho meson with an accompanying photon, and 30
events where the B meson disintegrates into two K mesons.
This type of interaction is predicted in the standard model of
elementary particles. Its rarity, according to the parameters of the
successful Kobayashi-Maskawa formulation, is attributed to the
constraint that the b quark transform into a d quark via a two-step
transition---a so-called ``penguin process.'' The low probability
explains why no evidence for the b -> d + photon process was
observed in previous experiments.
Observation of this very rare phenomenon allows us to study if this
occurs only through the penguin process as predicted by the standard
theory, or through some hitherto unobserved way. This opens an exciting
new window into our understanding of elementary particle physics: for
example, new physics models that incorporate so-called supersymmetry
also predict b to d quark transitions, and our announced observation
will provide valuable feedback to these models. On the other hand,
if we assume that the standard model holds, then our observation
provides a novel measurement of Vtd, one of the parameters in the
Kobayashi-Maskawa theory, and leads to a more complete understanding of
the standard model.
The new results will be announced at the Symposium on Lepton Photon
Interactions at High Energies to be held in Uppsala, Sweden between
June 30th and July 5th.
Fig.1 The new results show B meson decays into a lighter meson such as
rho or omega with an accompanying photon. This is the first
observation of a b-> d transition.
Fig.2 B mesons disintegrate into two K
mesons with a probability on the order of 10^-6. This is also
evidence for the existence of b -> d transitions.
Fig.3 In the standard model of elementary particle theory,
a b quark turns into a W particle and a top quark for a very short
time, which causes a transition from b quark to d quark. This process
is sometimes called a "penguin process".