25 research outputs found
Search for an invisible in a final state with two muons and missing energy at Belle II
The extension of the standard model predicts the existence
of a lepton-flavor-universality-violating boson that couples only
to the heavier lepton families. We search for such a through its
invisible decay in the process . 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. 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 at low masses to 1 at
masses of 8
Measurement of the branching fraction and asymmetry of decays using pairs in Belle II data
We report measurements of the branching fraction and asymmetry in
decays reconstructed at Belle II in an
electron-positron collision sample containing
pairs. We measure a branching fraction \mathcal{B}(\Bpipi) =
(1.38 \pm 0.27 \pm 0.22) \times 10^{-6} and a asymmetry \Acp(\Bpipi)
= 0.14 \pm 0.46 \pm 0.07, where the first uncertainty is statistical and the
second is systematic
Search for a resonance in events with the Belle II experiment
We report the first search for a non-standard-model resonance decaying into
pairs in events in
the 3.6-10 GeV/ mass range. We use a 62.8 fb sample of
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 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/ and for the axion-like particle model over
the entire mass range
Observation of decays using the 2019-2022 Belle II data sample
We present a measurement of the branching fractions of four decay modes. The measurement is based on data from
SuperKEKB electron-positron collisions at the resonance
collected with the Belle II detector and corresponding to an integrated
luminosity of . The event yields are extracted from fits
to the distributions of the difference between expected and observed meson
energy to separate signal and background, and are efficiency-corrected as a
function of the invariant mass of the system. We find the branching
fractions to be: where the first uncertainty is statistical and
the second systematic. These results include the first observation of
, , and decays and a significant improvement in the precision
of compared to previous measurements
Reconstruction of decays identified using hadronic decays of the recoil meson in 2019 -- 2021 Belle II data
We present results on the semileptonic decays and in a sample corresponding to
189.9/fb of Belle II data at the SuperKEKB collider. Signal decays
are identified using full reconstruction of the recoil meson in hadronic
final states. We determine the total branching fractions via fits to the
distributions of the square of the "missing" mass in the event and the dipion
mass in the signal candidate and find and where the dominant
systematic uncertainty comes from modeling the nonresonant contribution
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Observation of e+e−→ωχbJ(1P) and Search for Xb→ωϒ(1S) at s near 10.75 GeV
We study the processes eþ e− → ωχ bJ ð1PÞ (J ¼ 0, 1, or 2) using samples at center-of-mass energies
pffiffiffi
s ¼ 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 ϒð4SÞ resonance. We report the first observation of ωχ bJ ð1PÞ signals at
pffiffiffi
pffiffiffi
s ¼ 10.745 GeV. By combining Belle II data with Belle results at s ¼ 10.867 GeV, we find energy
dependencies of the Born cross sections for eþ e− → ωχ b1;b2 ð1PÞ to be consistent with the shape of the
ϒð10753Þ state. These data indicate that the internal structures of the ϒð10753Þ and ϒð10860Þ states may
pffiffiffi
differ. Including data at s ¼ 10.653 GeV, we also search for the bottomonium equivalent of the Xð3872Þ
state decaying into ωϒð1SÞ. No significant signal is observed for masses between 10.45 and 10.65 GeV=c2
Observation of e+e- → ωχbJ (1P) and Search for Xb → ω ϒ (1S) at s near 10.75 GeV
We study the processes e+e-→ωχbJ(1P) (J=0, 1, or 2) using samples at center-of-mass energies 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 ϒ(4S) resonance. We report the first observation of ωχbJ(1P) signals at s=10.745 GeV. By combining Belle II data with Belle results at s=10.867 GeV, we find energy dependencies of the Born cross sections for e+e-→ωχb1,b2(1P) to be consistent with the shape of the ϒ(10753) state. These data indicate that the internal structures of the ϒ(10753) and ϒ(10860) states may differ. Including data at s=10.653 GeV, we also search for the bottomonium equivalent of the X(3872) state decaying into ωϒ(1S). No significant signal is observed for masses between 10.45 and 10.65 GeV/c2