We present a search for the lepton flavor violating decays B^{+}→K^{+}τ^{±}ℓ^{∓}, with ℓ=(e,μ), using the full data sample of 772×10^{6} BBover ¯ 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^{±}→K^{±}τℓ decays and set upper limits on their branching fractions at the 90% confidence level in the (1-3)×10^{-5} range. The obtained limits are the world's best results.
We report on a search for a heavy Majorana neutrino in the decays τ^{-}→π^{-}ν_{h}, ν_{h}→π^{±}ℓ^{∓}, ℓ=e, μ. The results are obtained using the full data sample of 988 fb^{-1} collected with the ...Belle detector at the KEKB asymmetric energy e^{+}e^{-} collider, which contains 912×10^{6} ττ pairs. We observe no significant signal and set 95% CL upper limits on the couplings of the heavy right-handed neutrinos to the conventional standard model left-handed neutrinos in the mass range 0.2-1.6 GeV/c^{2}. This is the first study of a mixed couplings of heavy neutrinos to τ leptons and light-flavor leptons.
We present the study of Bover ¯^{0}→Σ_{c}(2455)^{0,++}π^{±}pover ¯ decays based on 772×10^{6} BBover ¯ events collected with the Belle detector at the KEKB asymmetric-energy e^{+}e^{-} collider. The ...Σ_{c}(2455)^{0,++} candidates are reconstructed via their decay to Λ_{c}^{+}π^{∓} and Λ_{c}^{+} decays to pK^{-}π^{+}, pK_{S}^{0}, and Λπ^{+} final states. The corresponding branching fractions are measured to be B(Bover ¯^{0}→Σ_{c}(2455)^{0}π^{+}pover ¯)=(1.09±0.06±0.07)×10^{-4} and B(Bover ¯^{0}→Σ_{c}(2455)^{++}π^{-}pover ¯)=(1.84±0.11±0.12)×10^{-4}, which are consistent with the world average values with improved precision. A new structure is found in the M_{Σ_{c}(2455)^{0,++}π^{±}} spectrum with a significance of 4.2σ including systematic uncertainty. The structure is possibly an excited Λ_{c}^{+} and is tentatively named Λ_{c}(2910)^{+}. Its mass and width are measured to be (2913.8±5.6±3.8) MeV/c^{2} and (51.8±20.0±18.8) MeV, respectively. The products of branching fractions for the Λ_{c}(2910)^{+} are measured to be B(Bover ¯^{0}→Λ_{c}(2910)^{+}pover ¯)×B(Λ_{c}(2910)^{+}→Σ_{c}(2455)^{0}π^{+})=(9.5±3.6±1.6)×10^{-6} and B(Bover ¯^{0}→Λ_{c}(2910)^{+}pover ¯)×B(Λ_{c}(2910)^{+}→Σ_{c}(2455)^{++}π^{-})=(1.24±0.35±0.10)×10^{-5}. Here, the first and second uncertainties are statistical and systematic, respectively.
We present the first comprehensive tests of light-lepton universality 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 $\Upsilon\left(4S\right)\to{}B \overline{B}$ decays in data corresponding to $189~\mathrm{fb}^{-1}$ 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 × 103 $D^+_s$ → 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 on a search for a resonance
X
decaying to a pair of muons in
e
+
e
−
→
μ
+
μ
−
X
events in the
0.212
–
9.000
GeV
/
c
2
mass range, using
178
fb
−
1
of data collected by the Belle II ...experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models of
X
beyond the standard model: a
Z
′
vector boson in the
L
μ
−
L
τ
model and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for the
L
μ
−
L
τ
model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below
6
GeV
/
c
2
, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for the
L
μ
−
L
τ
model and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search.
<supplementary-material>
<copyright-statement>Published by the American Physical Society</copyright-statement>
2024
</supplementary-material>
We present the results of a search for the b → d ℓ + ℓ − flavor-changing neutral-current rare decays B + , 0 → ( η , ω , π + , 0 , ρ + , 0 ) e + e − and B + , 0 → ( η , ω , π 0 , ρ + ) μ + μ − using ...a 711 fb − 1 data sample 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. We find no evidence for signal and set upper limits on branching fractions at the 90% confidence level in the range ( 3.8 – 47 ) × 10 − 8 depending on the decay channel. The obtained limits are the world’s best results. This is the first search for the channels B + , 0 → ( ω , ρ + , 0 ) e + e − and B + , 0 → ( ω , ρ + ) μ + μ − . Published by the American Physical Society 2024
