Phys. Rev. Lett. 131, 121802 (2023) 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.
Journal of High Energy Physics 09(2023)146 We measure $C\!P$ asymmetries and branching-fraction ratios for $B^\pm \to
DK^\pm$ and $D\pi^\pm$ decays with $D\to K^0_{\rm S} K^\pm\pi^\mp$, where $D$
is ...a superposition of $D^0$ and $\bar{D}^0$. We use the full data set of the
Belle experiment, containing $772\times 10^6~B\bar{B}$ pairs, and data from the
Belle~II experiment, containing $387\times 10^6~B\bar{B}$ pairs, both collected
in electron-positron collisions at the $\Upsilon(4S)$ resonance. Our results
provide model-independent information on the unitarity triangle angle $\phi_3$.
Additional spin-0 particles appear in many extensions of the standard model.
We search for long-lived spin-0 particles $S$ in $B$-meson decays mediated by a
$b\to s$ quark transition in $e^+e^-$ ...collisions at the $\Upsilon(4S)$
resonance at the Belle II experiment. Based on a sample corresponding to an
integrated luminosity of $189 \mathrm{\,fb}^{-1}$, we observe no evidence for
signal. We set model-independent upper limits on the product of branching
fractions $\mathrm{Br}(B^0\to K^*(892)^0(\to K^+\pi^-)S)\times \mathrm{Br}(S\to
x^+x^-)$ and $\mathrm{Br}(B^+\to K^+S)\times \mathrm{Br}(S\to x^+x^-)$, where
$x^+x^-$ indicates $e^+e^-, \mu^+\mu^-, \pi^+\pi^-$, or $K^+K^-$, as functions
of $S$ mass and lifetime at the level of $10^{-7}$.
We present a measurement of the $\tau$-lepton mass using a sample of about
175 million $e^+e^- \to \tau^+\tau^-$ events collected with the Belle II
detector at the SuperKEKB $e^+e^-$ collider at a ...center-of-mass energy of
$10.579\,\mathrm{Ge\kern -0.1em V}$. This sample corresponds to an integrated
luminosity of $190\,\mathrm{fb^{-1}}$. We use the kinematic edge of the $\tau $
pseudomass distribution in the decay ${\tau^-\to\pi^-\pi^+\pi^-\nu_\tau}$ and
measure the $\tau$ mass to be $1777.09 \pm 0.08 \pm 0.11 \,\mathrm{Me\kern
-0.1em V\!/c^2}$, where the first uncertainty is statistical and the second
systematic. This result is the most precise to date.
PRL 131, 111803 (2023) We report a measurement of the $C\!P$-violating parameters $A$ and $S$ in
$B^{0}\to K_{S}^{0} \pi^{0}$ decays at Belle II using a sample of $387\times
10^{6}$ $B\bar{B}$ events ...recorded in $e^{+}e^{-}$ collisions at a
center-of-mass energy corresponding to the $\Upsilon(4S)$ resonance. These
parameters are determined by fitting the proper decay-time distribution of a
sample of 415 signal events. We obtain $A = 0.04^{+0.15}_{-0.14}\pm 0.05$ and
$S = 0.75^{+0.20}_{-0.23}\pm 0.04$, where the first uncertainties are
statistical and the second are systematic.
We report a search for lepton-flavor-violating decays $\tau^- \to \ell^-
\phi$ ($\ell^- =e^-,\mu^-$) at the Belle II experiment, using a sample of
electron-positron data produced at the SuperKEKB ...collider in 2019-2021 and
corresponding to an integrated luminosity of 190 fb$^{-1}$. We use a new
untagged selection for $e^+e^- \to \tau^+\tau^-$ events, where the signal
$\tau$ is searched for as a neutrinoless final state of a single charged lepton
and a $\phi$ meson and the other $\tau$ is not reconstructed in any specific
decay mode, in contrast to previous measurements by the BaBar and Belle
experiments. We find no evidence for $\tau^- \to \ell^- \phi$ decays and obtain
upper limits on the branching fractions at 90% confidence level of 23 $\times
10^{-8}$ and 9.7$\times 10^{-8}$ for $\tau^- \rightarrow e^-\phi$ and $\tau^-
\rightarrow \mu^-\phi$, respectively
Belle II is a new-generation B-factory experiment, dedicated to exploring new physics beyond the standard model of elementary particles in the flavor sector. Belle~II started data-taking in April ...2018, using a synchronous data acquisition (DAQ) system based on pipelined trigger flow control. The Belle II DAQ system is designed to handle a 30-kHz trigger rate with approximately 1% of dead time, under the assumption of a raw event size of 1 MB. The DAQ system is reliable, and the overall data-taking efficiency reached 84.2% during the run period of January 2020 to June 2020. The current readout system cannot be operated in the term of 10 years from the viewpoint of DAQ maintainability; meanwhile, the readout system is obstructing high-speed data transmission. A solution involving a PCI-express-based readout module with high data throughput of up to 100 Gb/s was adopted to upgrade the Belle II DAQ system. We particularly focused on the design of firmware and software based on this new generation of readout board, called PCIe40, with an Altera Arria 10 field-programmable gate array chip. Forty-eight GBT (GigaBit Transceiver) serial links, PCI-express hard IP-based DMA architecture, interface of timing and trigger distribution system, and slow control system were designed to integrate with the current Belle II DAQ system. This paper describes the performances accomplished during the data readout and slow control tests conducted using a test bench and a demonstration performed using on-site front-end electronics, specifically involving Belle II TOP and KLM sub-detectors.
We present a measurement of the branching fractions of four \(B^{0,-}\to D^{(*)+,0} K^- K^{0}_S\) decay modes. The measurement is based on data from SuperKEKB electron-positron collisions at the ...\(\Upsilon(4S)\) resonance collected with the Belle II detector and corresponding to an integrated luminosity of \({362~\text{fb}^{-1}}\). The event yields are extracted from fits to the distributions of the difference between expected and observed \(B\) meson energy to separate signal and background, and are efficiency-corrected as a function of the invariant mass of the \(K^-K_S^0\) system. We find the branching fractions to be: \ \text{B}(B^-\to D^0K^-K_S^0)=(1.89\pm 0.16\pm 0.10)\times 10^{-4}, \ \ \text{B}(\overline B{}^0\to D^+K^-K_S^0)=(0.85\pm 0.11\pm 0.05)\times 10^{-4},\ \ \text{B}(B^-\to D^{*0}K^-K_S^0)=(1.57\pm 0.27\pm 0.12)\times 10^{-4}, \ \ \text{B}(\overline B{}^0\to D^{*+}K^-K_S^0)=(0.96\pm 0.18\pm 0.06)\times 10^{-4},\ where the first uncertainty is statistical and the second systematic. These results include the first observation of \(\overline B{}^0\to D^+K^-K_S^0\), \(B^-\to D^{*0}K^-K_S^0\), and \(\overline B{}^0\to D^{*+}K^-K_S^0\) decays and a significant improvement in the precision of \(\text{B}(B^-\to D^0K^-K_S^0)\) compared to previous measurements.
We propose a new algorithm for the identification of the production flavor of neutral \(D\) mesons in the Belle II experiment. The algorithm exploits the correlation between the flavor of a ...reconstructed neutral \(D\) meson (signal \(D\) meson) and the electric charges of particles reconstructed in the rest of the \(e^+e^-\to c \bar{c}\) event. These include those originating from the decay of the other charm hadron produced in the event, as well as those possibly produced in association with the signal \(D\) meson. We develop the algorithm using simulation and calibrate it in data using decay modes that identify the flavor of the decaying neutral \(D\) meson. We use a data sample of \(e^+e^-\) collisions, corresponding to \(362\,\mathrm{fb}^{-1}\) of integrated luminosity, collected by Belle II at center-of-mass energies near the \(\Upsilon(4S)\) mass. The effective tagging efficiency in data is \((47.91 \pm 0.07 \mathrm{(stat)} \pm 0.51 \mathrm{(syst)})\%\), independent of the neutral-\(D\)-meson decay mode. This charm flavor tagger will approximately double the effective sample size of many \(CP\)-violation and charm-mixing measurements that so far have exclusively relied on neutral \(D\) mesons originating from \(D^{*\pm}\) decays. While developed for Belle II, the basic principles underlying the charm flavor tagger can be used in other experiments, including those at hadron colliders.
We report measurements of the branching fraction and $\it CP$ asymmetry in
$B^{0} \to \pi^{0} \pi^{0}$ decays reconstructed at Belle II in an
electron-positron collision sample containing $198 \times ...10^{6}$
$B\overline{B}$ pairs. We measure a branching fraction $\mathcal{B}(\Bpipi) =
(1.38 \pm 0.27 \pm 0.22) \times 10^{-6}$ and a $\it CP$ asymmetry $\Acp(\Bpipi)
= 0.14 \pm 0.46 \pm 0.07$, where the first uncertainty is statistical and the
second is systematic.
