Letter of Intent for KEK Super B Factory Part I: Physics

A natural place to investigate a wide range of FCNC processes is in B meson decays, as
the bottom quark belongs to the third generation and hence its decay involves all existing
generations of quarks. In addition to B 0 − ¯ B 0 mixing, which is an analog of the traditional
K 0 − ¯ K 0 mixing, there are many FCNC decay processes induced by so-called penguin diagrams,
such as the radiative decay b → sγ, the semileptonic decay b → sℓ + ℓ − , and the hadronic decays
b → dq¯q and b → sq¯q. All of these processes are suppressed in the Standard Model by the GIM
mechanism, and therefore the effect of new physics may be relatively enhanced. The Higher
Luminosity B Factory is a machine designed to explore such interesting B decay processes.
In the summer of 2001 the presence of CP violation in the B meson system was established
by the Belle collaboration [3–6] (and simultaneously by the BaBar collaboration [7–10]) through
the measurement of the time dependent asymmetry in the decay process B 0 ( ¯ B 0 ) → J/ψK 0 S .
This measurement was the main target of the present asymmetric e + e − B Factory, and it was
achieved as originally planned. The experimental data indicated that the Kobayashi-Maskawa
mechanism, which is now a part of the Standard Model of elementary particles, is indeed the
dominant source of the observed CP violation in Nature.
The Belle experiment also proved its ability to measure a number of decay modes of the B
meson and to extract Cabibbo-Kobayashi-Maskawa (CKM) matrix elements and other interest-
ing observables. For instance, the precision of the measurement of the angle φ1 of the unitarity
time-dependent asymmetry reached the 10% level [11]; a CP
triangle through the B0 → J/ψK0 S
asymmetry was observed in B0 → π + π− decay, from which one can extract the angle φ2 [12];
the angle φ3 could also be measured through the processes B → DK and Dπ [14–16]; the
semi-leptonic FCNC processes B → Kℓ + ℓ− [17], B → K∗ℓ + ℓ− [18], and even the corresponding
inclusive decay B → Xsℓ + ℓ− [19] were observed. Furthermore, the recently observed disagreement
between the values of the angle φ1 measured in the penguin process B → φK0 S and the
precisely measured value in B → J/ψK0 S suggests the existence of a new CP phase in the pen-
guin process b → sq¯q [20]. By collecting many such observations we may probe new physics, and
once its existence is established these measurements will determine the properties of the new
physics. This is only possible by upgrading KEKB’s luminosity by a substantial amount. As we
discuss in the following sections, a factor of 50 improvement will greatly enhance the possibility
to discover new physics.
In the program of quark flavor physics, one way to explore physics beyond the Standard
Model is to substantially improve the measurement of the CKM matrix elements. Their determination
can be done in many different ways, and any inconsistency with the Standard Model
predictions would imply new physics. In this report we discuss the precision we expect to achieve
at the Higher Luminosity B Factory for various determinations of the CKM matrix elements.
These consist of measurements of the three angles and three sides of the Unitarity Triangle.
Another way to search for the effect of new physics is to look at loop-induced rare processes
for which the Standard Model contribution is extremely suppressed. Such processes may provide
an immediate signature of new physics, which also contributes through loops but in a different
manner. We describe several such future measurements including the mixing induced b → sγ
asymmetry, b → sℓ + ℓ − forward-backward asymmetry, and flavor changing tau decay τ → µγ.
The B physics program is also being pursued at hadron machines including the ongoing
Tevatron experiments [21] and the B physics programs at the Large Hadron Collider (LHC) [22],
which is scheduled to start operation in 2007. Because of the very large B ¯ B production cross
section in the hadron environment, some of the quantities we are planning to measure at the
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