Evidence for the Rare Decay B -> K*ll and Measurement of the B -> Kll Branching Fraction
Abstract
We present evidence for the flavor-changing neutral current decay
B→
K
∗
ℓ
+
ℓ
−
and a measurement of the branching fraction for the related process
B→K
ℓ
+
ℓ
−
, where
ℓ
+
ℓ
−
is either an
e
+
e
−
or
μ
+
μ
−
pair. These decays are highly suppressed in the Standard Model, and they are sensitive to contributions from new particles in the intermediate state. The data sample comprises
123×
10
6
Υ(4S)→B
B
¯
decays collected with the Babar detector at the PEP-II
e
+
e
−
storage ring. Averaging over
K
(∗)
isospin and lepton flavor, we obtain the branching fractions
B(B→K
ℓ
+
ℓ
−
)=(
0.65
+0.14
−0.13
±0.04)×
10
−6
and
B(B→
K
∗
ℓ
+
ℓ
−
)=(
0.88
+0.33
−0.29
±0.10)×
10
−6
, where the uncertainties are statistical and systematic, respectively. The significance of the
B→K
ℓ
+
ℓ
−
signal is over
8σ
, while for
B→
K
∗
ℓ
+
ℓ
−
it is
3.3σ
.