Speaker
Dr
Claudio Gatti
(LNF - INFN, Frascati)
Description
Semileptonic kaon decays offer possibly the cleanest way to
obtain an
accurate value of the Cabibbo angle, or better, $V_{us}$. At
present,
the largest uncertainty in calculating $V_{us}$ from the decay
rate, is due to
the difficulties in computing the matrix element of the $K
\rightarrow \pi$ transition.
The matrix element of $K_L \rightarrow \pi \mu \nu$ decay is
expressed in terms of
kaon and
pion four-momenta, $P$ and $p$ respectively, and using form
factors $f_+(t)$ and
$f_0(t)$,
where $t=(P-p)^2$.
It is customary to expand the scalar form factor $f_0(t)$ in
powers of $t$ as
$f_0(t)=f_+(0)\left[1+\lambda_0^{\prime} t/m^2+..\right]$, where
$m$ is the mass of the
carged pion, and only the linear term is retained. The form
factor at zero momentum
transfer,
$f_+(0)$, is evaluated from theory, while the form factor slope,
$\lambda_0^{\prime}$, has to be
determined experimentally from $K_L \rightarrow \pi \mu \nu$
decay spectra.
The best sensitivity to $\lambda_0^{\prime}$ is achieved in KLOE
by using the neutrino
energy spectrum. Such a measurement is possible because of the
tagging technique,
consisting of identifying $K_L$ decays through the selection of
$K_S \rightarrow
\pi^+\pi^-$
decay near the $e^+ e^-$ interaction point. This strategy allows
to measure $K_L$
momentum
with good precision. We present the results of this analysis,
based on 330 pb$^{-1}$
of data acquired during years 2001 and 2002.
Primary author
Collaboration KLOE
(INFN/LNF)
