Abstract
We report the first observation of the flavor-changing neutral current decay
B→
K
∗
ℓ
+
ℓ
−
and an improved measurement of the decay
B→K
ℓ
+
ℓ
−
, where
ℓ
represents an electron or a muon, with a data sample of 140 fb
−1
accumulated at the
Υ(4S)
resonance with the Belle detector at KEKB. The results for the branching fractions are
B(B→
K
∗
ℓ
+
ℓ
−
)=(
11.5
+2.6
−2.4
±0.8±0.2)×
10
−7
and
B(B→K
ℓ
+
ℓ
−
)=(
4.8
+1.0
−0.9
±0.3±0.1)×
10
−7
, where the first error is statistical, the second is systematic and the third is from model dependence.