The front-end electronics system for the Belle II Time-Of-Propagation (TOP) counter was fully installed in May 2016. The detector is a novel particle identification device for the barrel region, ...where Cherenkov ring images are reconstructed with precise timing information of each photon. The readout electronics need to have excellent timing performance for single photon detection. To exploit the benefit of high luminosity, the electronics must also be able to cope with a high input trigger rate (30 kHz) and have buffer memory which is deep enough to wait for a trigger decision (>5 μs). The TOP electronics has switching capacitor arrays to sample waveforms with 2.7 GSample/s, which allows a timing resolution of 50 ps for a single photon signal. The programmable logic and the processing system are the effective implementation to meet these requirements, where flexible optimization and step-by-step development of readout logic are possible. The system has been successfully operated in the first accelerator commissioning runs with beam collisions in 2018, where the typical trigger rate was 500Hz. Operation with a 20 kHz trigger rate was also tested.
•Readout electronics for the Belle II Time-Of-Propagation counter was installed.•A single photon is detected with timing resolution better than 100 ps.•The whole system was successfully operated in the physics run.
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.
We report the first search for a non-standard-model resonance decaying into $\tau$ pairs in $e^{+}e^{-}\rightarrow \mu^{+}\mu^{-} \tau^+\tau^-$ events in the 3.6-10 GeV/$c^{2}$ mass range. We use a ...62.8 fb$^{-1}$ sample of $e^+e^-$ collisions collected at a center-of-mass energy of 10.58 GeV by the Belle II experiment at the SuperKEKB collider. The analysis probes three different models predicting a spin-1 particle coupling only to the heavier lepton families, a Higgs-like spin-0 particle that couples preferentially to charged leptons (leptophilic scalar), and an axion-like particle, respectively. We observe no evidence for a signal and set exclusion limits at 90% confidence level on the product of cross section and branching fraction into $\tau$ pairs, ranging from 0.7 fb to 24 fb, and on the couplings of these processes. We obtain world-leading constraints on the couplings for the leptophilic scalar model for masses above 6.5 GeV/$c^2$ and for the axion-like particle model over the entire mass range.
We study $B^{+}\to \pi^{+}\pi^{0}\pi^{0}$ using 711 $\rm{fb}^{-1}$ of data collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We ...measure an inclusive branching fraction of $(19.0\pm 1.5\pm 1.4)\times 10^{-6}$ and an inclusive \textit{CP} asymmetry of $(9.2 \pm 6.8 \pm 0.7)\%$, where the first uncertainties are statistical and the second are systematic; and a $B^{+}\to \rho(770)^{+}\pi^{0}$ branching fraction of $(11.2\pm 1.1\pm 0.9 ^{+0.8}_{-1.6})\times 10^{-6}$, where the third uncertainty is due to possible interference with $B^{+}\to \rho(1450)^{+}\pi^{0}$. We present the first observation of a structure around 1 GeV/$c^{2}$ in the $\pi^{0}\pi^{0}$ mass spectrum, with a significance of 6.4$\sigma$, and measure a branching fraction to be $(6.9\pm 0.9\pm 0.6)\times 10^{-6}$. We also report a measurement of local \textit{CP} asymmetry in this structure.
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.
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.
Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays τ± → ℓ±γ (ℓ = e, μ) are predicted to ...have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via τ± → ℓ±γ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb–1, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, B(τ± → μ±γ) ≤ 4.2 × 10–8 and B(τ± → e±γ) ≤ 5.6 × 10–8, are set at 90% confidence level.
We report the first measurement of the exclusive cross sections e+e- → $B\overline{B}$, e+e- → $B{\overline{B}}^{\ast }$, and e+e- → ${B}^{\ast }{\overline{B}}^{\ast }$ in the energy range from 10.63 ...GeV to 11.02 GeV. The B mesons are fully reconstructed in a large number of hadronic final states and the three channels are identified using a beam-constrained-mass variable. The shapes of the exclusive cross sections show oscillatory behavior with several maxima and minima. The results are obtained using data collected by the Belle experiment at the KEKB asymmetric-energy e+e- collider.
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