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.
A
bstract
We report a new measurement of the
e
+
e
−
→
ϒ(
nS
)
π
+
π
−
(
n
= 1
,
2
,
3) cross sections at energies from 10
.
52 to 11
.
02 GeV using data collected with the Belle detector at the KEKB ...asymmetric-energy
e
+
e
−
collider. We observe a new structure in the energy dependence of the cross sections; if described by a Breit-Wigner function its mass and width are found to be
M
=
10752.7
±
5.9
−
1.1
+
0.7
MeV
/
c
2
and
Γ
=
35.5
−
11.3
−
3.3
+
17.6
+
3.9
MeV, where the first error is statistical and the second is systematic. The global significance of the new structure including systematic uncertainty is 5.2 standard deviations. We also find evidence for the
e
+
e
−
→
ϒ (1
S
)
π
+
π
−
process at the energy 10
.
52 GeV, which is below the
B
B
¯
threshold.
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 the measurement of the two-photon decay width of
χ
c
2
(1
P
) in two-photon processes at the Belle experiment. We analyze the process
γγ
→
χ
c
2
(1
P
) →
J/ψγ
,
J/ψ
→
ℓ
+
ℓ
−
(
ℓ
...=
e
or
μ
) using a data sample of 971 fb
−
1
collected with the Belle detector at the KEKB
e
+
e
−
collider. In this analysis, the product of the two-photon decay width of
χ
c
2
(1
P
) and the branching fraction is determined to be
Γ
γγ
χ
c
2
1
P
B
χ
c
2
1
P
→
J
/
ψγ
B
J
/
ψ
→
ℓ
+
ℓ
−
=
14.8
±
0.3
stat
.
±
0.7
syst
.
eV, which corresponds to Γ
γγ
(
χ
c
2
(1
P
)) = 653
±
13(stat.)
±
31(syst.)
±
17(B.R.) eV, where the third uncertainty is from
B
(
χ
c
2
(1
P
)
→ J/ψγ
) and
B
(
J/ψ → ℓ
+
ℓ
−
).
A
bstract
We present the analysis of two-particle angular correlations using coordinate systems defined with the conventional beam axis and the event thrust axis. We propose the latter as a good ...representation for the correlation structure interpretation in the
e
+
e
−
collision system. The
e
+
e
−
collisions to hadronic final states at center-of-mass energies of
s
= 10
.
52 GeV and 10.58 GeV are recorded by the Belle detector at KEKB. In this paper, results on the first dataset are supplementary to the previous Belle publication
1
. At the same time, the latter is the first two-particle correlation measurement at collision energy on the
Υ
(4
S
) resonance and is sensitive to its decay products. Measurements are reported as a function of the charged-particle multiplicity. Finally, a qualitative understanding of the correlation structure is discussed using a combination of Monte Carlo simulations and experimental data.
We measure the branching fractions and CP asymmetries for the singly Cabibbo-suppressed decays $D^0 → π^+π^-η, D^0 → K^+K^-η$, and $D^0 → Φη$, using 980 fb-1 of data from the Belle experiment at the ...KEKB e+e- collider. We obtain $$\mathcal{B}(D^0 → π^+π^-η = \mathrm{1.22 ± 0.02 (stat) ± 0.02(syst) ± 0.03}(\mathcal{B}_{\mathrm{ref}}) \mathrm{x 10^{-3}}$$ $$\mathcal{B}(D^0 → K^+K^-η = \mathrm{1.80^{+0.07}_{-0.06}(stat) ± 0.04 (syst) ± 0.05}(\mathcal{B}_{\mathrm{ref}}) \mathrm{x 10^{-4}}$$ $$\mathcal{B}(D^0 → Φη = \mathrm{1.84 ± 0.09 (stat) ± 0.0 (syst) ± 0.05}(\mathcal{B}_{\mathrm{ref}}) \mathrm{x 10^{-4}}$$ where the third uncertainty ($\mathcal{B}_{\mathrm{ref}}$) is from the uncertainty in the branching fraction of the reference mode $D^0 → K^-π^+η$. The color-suppressed decay $D^0 → Φη$ is observed for the first time, with very high significance. The results for the CP asymmetries are $$A_{CP} (D^0 → π^+π^-η) = \mathrm{0.9 ± 1.2 (stat) ± 0.5 (syst)}\%$$ $$A_{CP} (D^0 → K^+K^-η) = \mathrm{-1.4 ± 3.3 (stat) ± 1.1 (syst)}\%$$ $$A_{CP} (D^0 → Φη) = \mathrm{-1.9 ± 4.4 (stat) ± 0.6 (syst)}\%$$ The results for $D^0 → π^+π^-η$ are a significant improvement over previous results. The branching fraction and ACP results for $D^0 → K^+K^-η$, and the ACP result for $D^0 → Φη$, are the first such measurements. No evidence for CP violation is found in any of these decays.
Beam background study for the Belle II Silicon Vertex Detector Tanigawa, H.; Adamczyk, K.; Aihara, H. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
12/2020, Letnik:
982
Journal Article
Recenzirano
The Belle II experiment aims to accumulate 50ab−1 of e+e− collision data at the SuperKEKB asymmetric energy collider (Tsukuba, Japan). The first physics data using all Belle II detectors were taken ...in spring 2019.
In the vast physics program of the Belle II experiment, the vertex detector plays a crucial role for the determination of the B-meson decay vertices. It consists of two inner layers of pixelated silicon detectors and four outer layers of double-sided silicon strip detectors (SVD).
To achieve a design luminosity of 8×1035cm−2s−1, 40 times higher than the recorded luminosity of its predecessor, the SuperKEKB collider squeezes the beams to a vertical size of 50 nm (“nano-beam scheme”) and doubles the beam currents.
Therefore, the detectors are required to tolerate intense beam induced background due to the very high luminosity. During the 2019 spring run we measured the occupancy rate in the SVD to estimate the level of the beam induced background. With the low initial luminosity, the observed beam induced background mostly originated from Touschek processes and beam-gas scattering within individual beams. Since these different background contributions depend differently on accelerator conditions, such as the beam current, beam size and pressure, they can be disentangled. We estimate the background rate of each contribution and compare them with simulated ones. The results enable us to predict the background levels at increased beam currents and luminosity in the coming years. They also hint at background mitigation measures for running at higher luminosity. In this proceeding we present the results of our study of the beam induced background in the SVD and the prospects for future operation.
A
bstract
The first dedicated search for the
η
c
2
(1
D
) is carried out using the decays
B
+
→ η
c
2
(1
D
)
K
+
,
B
0
→
η
c
2
(1
D
)
K
S
0
,
B
0
→ η
c
2
(1
D
)
π
−
K
+
, and
B
+
→ η
c
2
(1
D
)
π
+
K
...S
0
with
η
c
2
(1
D
)
→ h
c
γ
. No significant signal is found. For the
η
c
2
(1
D
) mass range between 3795 and 3845 MeV
/c
2
, the branching-fraction upper limits are determined to be ℬ(
B
+
→ η
c
2
(1
D
)
K
+
)
×
ℬ(
η
c
2
(1
D
)
→ h
c
γ
)
<
3
.
7
×
10
−
5
, ℬ(
B
0
→ η
c
2
(1
D
)
K
0
)
×
ℬ(
η
c
2
(1
D
)
→ h
c
γ
)
<
3
.
5
×
10
−
5
, ℬ(
B
0
→ η
c
2
(1
D
)
π
−
K
+
)
×
ℬ(
η
c
2
(1
D
)
→ h
c
γ
)
<
1
.
0
×
10
−
4
, and ℬ(
B
+
→ η
c
2
(1
D
)
π
+
K
S
0
)
×
ℬ(
η
c
2
(1
D
)
→ h
c
γ
)
<
1
.
1
×
10
−
4
at 90% C.L. The analysis is based on the 711 fb
−
1
data sample collected on the ϒ(4
S
) resonance by the Belle detector, which operated at the KEKB asymmetric-energy
e
+
e
−
collider.
Performance of the Belle II Silicon Vertex Detector Tanigawa, H.; Adamczyk, K.; Aihara, H. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
08/2020, Letnik:
972
Journal Article
Recenzirano
The Belle II experiment at the SuperKEKB collider of KEK (Japan) started recording physics data in spring 2019 with all its subdetectors installed and with the goal of accumulating 50ab−1 of e+e− ...collision events at the unprecedented instantaneous luminosity of 8×1035cm−2s−1, about 40 times larger than its predecessor. The Belle II vertex detector plays a crucial role in the broad Belle II physics program, especially for time-dependent CP measurements. It consists of two layers of DEPFET-based pixels and four layers of double-sided silicon strip detectors (SVD).
The experience gained from the first period of SVD operation can be summarized as smooth and reliable running of the detector, with high stability of noise levels and calibration parameters obtained from local calibration runs. No major problem has been experienced. The detector even survived a few serious radiation accidents in which the beam was lost due to failure in the machine focusing quadrupoles without any notable damage. The SVD performance were carefully studied with these first physics data. The SVD showed excellent hit and tracking efficiency. Moreover, cluster energy and signal to noise ratio as well as the hit time and spatial resolutions measured on data showed a fair agreement with the expected performance.
•Belle II silicon vertex detector operated during the first year of the experiment.•All sensors worked with stable and excellent hit efficiencies above 99 %.•Signal-to-noise ratios between 15 and 30, cluster time resolution better than 3 ns.•First effects of irradiation visible in leakage currents.
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