We present the first measurement of the ratio of branching fractions of inclusive semileptonic $B$-meson decays, $R(X_{e/\mu}) = \mathcal{B}(B\to X \, e \, \nu) / \mathcal{B}(B\to X \, \mu \, \nu)$, ...a precision test of electron-muon universality, using data corresponding to $189\,\mathrm{fb}^{-1}$ from electron-positron collisions collected with the Belle II detector. In events where the partner $B$ meson is fully reconstructed, we use fits to the lepton momentum spectra above $1.3\,\mathrm{GeV}/c$ to obtain $R(X_{e/\mu}) = 1.007 \pm 0.009~(\mathrm{stat}) \pm 0.019~(\mathrm{syst})$, which is the most precise lepton-universality test of its kind and agrees with the standard-model expectation.
We report the first search for a nonstandard-model resonance decaying into τ pairs in e^{+}e^{-}→μ^{+}μ^{-}τ^{+}τ^{-} 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 axionlike 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 τ pairs, ranging from 0.7 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 axionlike particle model over the entire mass range.
A
bstract
We report results from a study of
B
±
→ DK
±
decays followed by
D
decaying to the
CP
-even final state
K
+
K
−
and CP-odd final state
K
S
0
π
0
, where
D
is an admixture of
D
0
and
D
¯
0
...states. These decays are sensitive to the Cabibbo-Kobayashi-Maskawa unitarity-triangle angle
ϕ
3
. The results are based on a combined analysis of the final data set of 772
×
10
6
B
B
¯
pairs collected by the Belle experiment and a data set of 198
×
10
6
B
B
¯
pairs collected by the Belle II experiment, both in electron-positron collisions at the Υ(4
S
) resonance. We measure the CP asymmetries to be
A
CP
+
= (+12.5
±
5.8
±
1.4)% and
A
CP−
= (
−
16.7
±
5.7
±
0.6)%, and the ratios of branching fractions to be
R
CP
+
= 1.164
±
0.081
±
0.036 and
R
CP−
= 1.151
±
0.074
±
0.019. The first contribution to the uncertainties is statistical, and the second is systematic. The asymmetries
A
CP
+
and
A
CP−
have similar magnitudes and opposite signs; their difference corresponds to 3.5 standard deviations. From these values we calculate 68.3% confidence intervals of (8.5
°
<
ϕ
3
< 16.5
°
) or (84.5
°
<
ϕ
3
< 95.5
°
) or (163.3
°
<
ϕ
3
< 171.5
°
) and 0.321 <
r
B
< 0.465.
A
bstract
We measure
CP
asymmetries and branching-fraction ratios for
B
±
→ DK
±
and
Dπ
±
decays with
D →
K
S
0
K
±
π
∓
, where
D
is a superposition of
D
0
and
D
¯
0
. We use the full data set of the ...Belle experiment, containing 772
×
10
6
B
B
¯
pairs, and data from the Belle II experiment, containing 387 × 10
6
B
B
¯
pairs, both collected in electron-positron collisions at the Υ(4
S
) resonance. Our results provide model-independent information on the unitarity triangle angle
ϕ
3
.
We measure the tau-to-light-lepton ratio of inclusive B-meson branching fractions R(X_{τ/ℓ})≡B(B→Xτν)/B(B→Xℓν), where ℓ indicates an electron or muon, and thereby test the universality of ...charged-current weak interactions. We select events that have one fully reconstructed B meson and a charged lepton candidate from 189 fb^{-1} of electron-positron collision data collected with the Belle II detector. We find R(X_{τ/ℓ})=0.228±0.016(stat)±0.036(syst), in agreement with standard-model expectations. This is the first direct measurement of R(X_{τ/ℓ}).
We present the first comprehensive tests of the universality of the light leptons in the angular distributions of semileptonic B^{0}-meson decays to charged spin-1 charmed mesons. We measure five ...angular-asymmetry observables as functions of the decay recoil that are sensitive to lepton-universality-violating contributions. We use events where one neutral B is fully reconstructed in ϒ(4S)→BBover ¯ decays in data corresponding to 189 fb^{-1} integrated luminosity from electron-positron collisions collected with the Belle II detector. We find no significant deviation from the standard model expectations.
We measure the lifetime of the D_{s}^{+} meson using a data sample of 207 fb^{-1} collected by the Belle II experiment running at the SuperKEKB asymmetric-energy e^{+}e^{-} collider. The lifetime is ...determined by fitting the decay-time distribution of a sample of 116×10^{3} D_{s}^{+}→ϕπ^{+} decays. Our result is τ_{D_{s}^{+}}=(499.5±1.7±0.9) fs, where the first uncertainty is statistical and the second is systematic. This result is significantly more precise than previous measurements.
We report measurements of the $e^+e^- \to B\bar{B}$, $B\bar{B}{}^*$, and
$B^*\bar{B}{}^*$ cross sections at four energies, 10653, 10701, 10746 and 10805
MeV, using data collected by the Belle~II ...experiment. We reconstruct one $B$
meson in a large number of hadronic final states and use its momentum to
identify the production process. In the first $2-5$ MeV above $B^*\bar{B}{}^*$
threshold, the $e^+e^- \to B^*\bar{B}{}^*$ cross section increases rapidly.
This may indicate the presence of a pole close to the threshold.
We present a measurement of the ratio $R_\mu = \mathcal{B}(\tau^-\to
\mu^-\bar\nu_\mu\nu_\tau) / \mathcal{B}(\tau^-\to e^-\bar\nu_e\nu_\tau)$ of
branching fractions $\mathcal{B}$ of the $\tau$ lepton ...decaying to muons or
electrons using data collected with the Belle II detector at the SuperKEKB
$e^+e^-$ collider. The sample has an integrated luminosity of 362 fb$^{-1}$ at
a centre-of-mass energy of 10.58 GeV. Using an optimised event selection, a
binned maximum likelihood fit is performed using the momentum spectra of the
electron and muon candidates. The result, $R_\mu = 0.9675 \pm 0.0007 \pm
0.0036$, where the first uncertainty is statistical and the second is
systematic, is the most precise to date. It provides a stringent test of the
light-lepton universality, translating to a ratio of the couplings of the muon
and electron to the $W$ boson in $\tau$ decays of $0.9974 \pm 0.0019$, in
agreement with the standard model expectation of unity.
