A
bstract
We present measurements of the branching fractions for the decays
B
→
Kμ
+
μ
−
and
B
→
Ke
+
e
−
, and their ratio (
R
K
), using a data sample of 711 fb
−
1
that contains 772 × 10
6
B
B
¯
...events. The data were collected at the ϒ(4
S
) resonance with the Belle detector at the KEKB asymmetric-energy
e
+
e
−
collider. The ratio
R
K
is measured in five bins of dilepton invariant-mass-squared (
q
2
):
q
2
∈ (0
.
1
,
4
.
0)
,
(4
.
00
,
8
.
12)
,
(1
.
0
,
6
.
0), (10
.
2
,
12
.
8) and (
>
14
.
18) GeV
2
/c
4
, along with the whole
q
2
region. The
R
K
value for
q
2
∈ (1
.
0
,
6
.
0) GeV
2
/c
4
is
1.03
−
0.24
+
0.28
± 0
.
01. The first and second uncertainties listed are statistical and systematic, respectively. All results for
R
K
are consistent with Standard Model predictions. We also measure
CP
-averaged isospin asymmetries in the same
q
2
bins. The results are consistent with a null asymmetry, with the largest difference of 2.6 standard deviations occurring for the
q
2
∈ (1
.
0
,
6
.
0) GeV
2
/c
4
bin in the mode with muon final states. The measured differential branching fractions,
d
ℬ
/dq
2
, are consistent with theoretical predictions for charged
B
decays, while the corresponding values are below the expectations for neutral
B
decays. We have also searched for lepton-flavor-violating
B
→
Kμ
±
e
∓
decays and set 90% confidence-level upper limits on the branching fraction in the range of 10
−
8
for
B
+
→
K
+
μ
±
e
∓
, and
B
0
→
K
0
μ
±
e
∓
modes.
Abstract We present a measurement of the Cabibbo-Kobayashi-Maskawa unitarity triangle angle ϕ 3 (also known as γ) using a model-independent Dalitz plot analysis of B + → D ( K S 0 $$ {K}_S^0 $$ h + h ...− )h +, where D is either a D 0 or D ¯ $$ \overline{D} $$ 0 meson and h is either a π or K. This is the first measurement that simultaneously uses Belle and Belle II data, combining samples corresponding to integrated luminosities of 711 fb −1 and 128 fb −1, respectively. All data were accumulated from energy-asymmetric e + e − collisions at a centre-of-mass energy corresponding to the mass of the Υ(4S) resonance. We measure ϕ 3 = (78.4 ± 11.4 ± 0.5 ± 1.0)°, where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is from the uncertainties on external measurements of the D-decay strong-phase parameters.
A
bstract
Charged-lepton-flavor-violation is predicted in several new physics scenarios. We update the analysis of
τ
lepton decays into a light charged lepton (
ℓ
=
e
±
or
μ
±
) and a vector meson (
...V
0
=
ρ
0
,
ϕ
,
ω
,
K
*0
, or
K
¯
*0
) using 980 fb
−
1
of data collected with the Belle detector at the KEKB collider. No significant excess of such signal events is observed, and thus 90% credibility level upper limits are set on the
τ
→
ℓV
0
branching fractions in the range of (1.7–4
.
3) × 10
−
8
. These limits are improved by 30% on average from the previous results.
A
bstract
We report a search for the charged-lepton flavor violation in Υ(2
S
) →
ℓ
∓
τ
±
(
ℓ
=
e, μ
) decays using a 25 fb
−
1
Υ(2
S
) sample collected by the Belle detector at the KEKB
e
+
e
−
...asymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions (
B
) at 90% confidence level. We obtain the most stringent upper limits:
B
(Υ(2
S
)
→ μ
∓
τ
±
)
<
0
.
23
×
10
−
6
and
B
(Υ(2
S
)
→ e
∓
τ
±
)
<
1
.
12
×
10
−
6
.
We report the first measurement of the Michel parameter ξ′ in the τ−→μ−¯νμντ decay with a new method proposed just recently. The measurement is based on the reconstruction of the τ−→μ−¯νμντ events ...with subsequent muon decay in flight in the Belle central drift chamber. The analyzed data sample of 988 fb−1 collected by the Belle detector corresponds to approximately 912×106 τ+τ− pairs. We measure ξ′=0.22±0.94(stat)±0.42(syst), which is in agreement with the standard model prediction of ξ′=1. Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.
We present a search for the lepton-flavour-violating decays $B^+ \to K^+ \tau^\pm \ell^\mp$, with $\ell = (e, \mu)$, using the full data sample of $772 \times 10^6$ $B\overline{B}$ pairs recorded by ...the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We use events in which one $B$ meson is fully reconstructed in a hadronic decay mode. We find no evidence for $B^\pm \to K^\pm \tau \ell$ decays and set upper limits on their branching fractions at the 90% confidence level in the $(1$-$3) \times 10^{-5}$ range. The obtained limits are the world's best results.
A bstract We report the first measurement of the inclusive e + e − → $$ b\overline{b} $$ b b ¯ → $$ {D}_s^{\pm } $$ D s ± X and e + e − → $$ b\overline{b} $$ b b ¯ → D 0 / $$ {\overline{D}}^0 $$ D ¯ ...0 X cross sections in the energy range from 10 . 63 to 11 . 02 GeV. Based on these results, we determine σ ( e + e − → $$ {B}_s^0{\overline{B}}_s^0 $$ B s 0 B ¯ s 0 X ) and σ ( e + e − → $$ B\overline{B} $$ B B ¯ X ) in the same energy range. We measure the fraction of $$ {B}_s^0 $$ B s 0 events at Υ(10860) to be f s = ( $$ {22.0}_{-2.1}^{+2.0} $$ 22.0 − 2.1 + 2.0 )%. We determine also the ratio of the $$ {B}_s^0 $$ B s 0 inclusive branching fractions $$ \mathcal{B} $$ B ( $$ {B}_s^0 $$ B s 0 → D 0 / $$ {\overline{D}}^0 $$ D ¯ 0 X ) / $$ \mathcal{B} $$ B ( $$ {B}_s^0 $$ B s 0 → $$ {D}_s^{\pm } $$ D s ± X ) = 0 . 416 ± 0 . 018 ± 0 . 092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energy e + e − collider.
The $L_{\mu}-L_{\tau}$ extension of the standard model predicts the existence of a lepton-flavor-universality-violating $Z^{\prime}$ boson that couples only to the heavier lepton families. We search ...for such a $Z^\prime$ through its invisible decay in the process $e^+ e^- \to \mu^+ \mu^- Z^{\prime}$. We use a sample of electron-positron collisions at a center-of-mass energy of 10.58GeV collected by the Belle II experiment in 2019-2020, corresponding to an integrated luminosity of 79.7fb$^{-1}$. We find no excess over the expected standard-model background. We set 90$\%$-confidence-level upper limits on the cross section for this process as well as on the coupling of the model, which ranges from $3 \times 10^{-3}$ at low $Z^{\prime}$ masses to 1 at $Z^{\prime}$ masses of 8$GeV/c^{2}$.
We study the processes $e^+e^-\to\omega\chi_{bJ}(1P)$ ($J$ = 0, 1, or 2) using samples at center-of-mass energies $\sqrt{s}$ = 10.701, 10.745, and 10.805 GeV, corresponding to 1.6, 9.8, and 4.7 ...fb$^{-1}$ of integrated luminosity, respectively. These data were collected with the Belle II detector during special operations of the SuperKEKB collider above the $\Upsilon(4S)$ resonance. We report the first observation of $\omega\chi_{bJ}(1P)$ signals at $\sqrt{s}$ = 10.745 GeV. By combining Belle II data with Belle results at $\sqrt{s}$ = 10.867 GeV, we find energy dependencies of the Born cross sections for $e^+e^-\to \omega\chi_{b1,b2}(1P)$ to be consistent with the shape of the $\Upsilon(10753)$ state. These data indicate that the internal structures of the $\Upsilon(10753)$ and $\Upsilon(10860)$ states may differ. Including data at $\sqrt{s}$ = 10.653 GeV, we also search for the bottomonium equivalent of the $X(3872)$ state decaying into $\omega\Upsilon(1S)$. No significant signal is observed for masses between 10.45 and 10.65 GeV/$c^2$.