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
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
.
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.
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 search for lepton-number- and baryon-number-violating decays τ− → ¯pe+e−, pe−e−, ¯pe+μ−, ¯pe−μ+, ¯pμ+μ−, and pμ−μ− using 921 fb−1 of data, equivalent to (841 ± 12) × 106 τ+τ− events, recorded with ...the Belle detector at the KEKB asymmetric-energy e+e− collider. In the absence of a signal, 90% confidence-level upper limits are set on the branching fractions of these decays in the range (1.8 − 4.0) × 10−8. We set the world's first limits on the first four channels and improve the existing limits by an order of magnitude for the last two channels.
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/$c^2$.
PDF
