Search for: All records

We report the first observation of the radiative decay $D_{s0}^{*}(2317{)}^{+}\to D_s^{*+}\gamma$with a statistical significance exceeding 10 standard deviations. The signal is observed in the continuum $e^{+}{e}^{-}\to c\overline{c}$process, using combined data samples of $980.4{\mathrm{fb}}^{-1}$from Belle and $427.9{\mathrm{fb}}^{-1}$from Belle II, collected at the KEKB and SuperKEKB asymmetric-energy $e^{+}{e}^{-}$colliders, respectively. The branching fraction ratio $\mathcal{B}\left[D_{s0}^{*}\right(2317{)}^{+}\to D_s^{*+}\gamma ]/\mathcal{B}[D_{s0}^{*}(2317{)}^{+}\to D_s^{+}{\pi }^0]$is measured to be $[7.13\pm 0.70(\mathrm{stat})\pm 0.26(\mathrm{syst}\left)\right]\%$. This result provides crucial discrimination between theoretical models of the $D_{s0}^{*}(2317{)}^{+}$structure. 

We report a measurement of the branching fraction for the leptonic decay $B^{+}\to {\mu }^{+}{\nu }_{\mu }$. This work presents the first $B^{+}\to {\mu }^{+}{\nu }_{\mu }$result using Belle II data, an updated Belle measurement that supersedes the previous result, and their combination, which yields the most precise search to date. The analysis is based on $1076{\mathrm{fb}}^{-1}$of $e^{+}{e}^{-}$collision data collected at a center-of-mass energy of 10.58 GeV with the Belle and Belle II detectors at the KEKB and SuperKEKB colliders, respectively. We measure $\mathcal{B}(B^{+}\to {\mu }^{+}{\nu }_{\mu })=(4.4\pm 1.9\pm 1.0)\times {10}^{-7}$, where the first uncertainty is statistical and the second systematic. The observed significance relative to the background-only hypothesis is 2.4 standard deviations. We set a 90% confidence level upper limit of $\mathcal{B}(B^{+}\to {\mu }^{+}{\nu }_{\mu })<6.7\times {10}^{-7}$using a frequentist approach and a 90% credibility level upper limit of $\mathcal{B}(B^{+}\to {\mu }^{+}{\nu }_{\mu })<7.2\times {10}^{-7}$using a Bayesian approach. These are the most stringent limits to date. The result is interpreted as an exclusion region in the parameter space of type II and type III two-Higgs-doublet models. We search for stable sterile neutrinos with masses $m_N\in [0,1.5]\mathrm{GeV}$. No signal is observed, and the resulting exclusion on the squared mixing parameter $|U_{\mu N}{|}^2$provides improvement over previous limits. We report a measurement of the partial branching fraction of semileptonic $B\to X_u\ell {\nu }_{\ell }$decays with $p_{\mu }^B>2.2\mathrm{GeV}$, obtaining $\mathrm{\Delta }\mathcal{B}(B\to X_u\ell {\nu }_{\ell })=(2.72\pm 0.05\pm 0.29)\times {10}^{-4}$. We present a model-dependent study of weak annihilation decays using the muon momentum spectrum. We observe a signal of 2.4 standard deviations above the background-only hypothesis in regions where the distribution resembles that of $B\to X_u\ell {\nu }_{\ell }$decays. 

A<sc>bstract</sc> Using data samples collected by the Belle and Belle II experiments at the Υ(4S) resonance with integrated luminosities of 571 fb⁻¹and 365 fb⁻¹, respectively, we measure the pseudoscalarB-meson mass difference to bem(B⁰) −m(B⁺) = (0.495 ± 0.024 ± 0.005) MeV/c². The results are based on a simultaneous fit to the variable$${\overline{M} }_{bc}$$, which is related to theBmomentum, forB⁰andB⁺candidates; and to the energy dependence of$$\mathcal{R}=\sigma ({e}^{+}{e}^{-}\to {B}^{0}{\overline{B} }^{0}) / \sigma ({e}^{+}{e}^{-}\to {B}^{+}{B}^{-})$$, which is measured using changes in the average center-of-mass energy over the data taking periods. The phase-space hypothesis$$\mathcal{R}={({p}_{{B}^{0}} /{p}_{{B}^{+}})}^{3}$$, upon which previous measurements rely, is strongly disfavored by our fit; the measured mass-difference value for the phase-space hypothesis also differs significantly from our measurement. We constrain$$\mathcal{R}$$in a broader energy range than covered by the direct measurement and extract the energy dependence of$$\mathcal{R}$$in the range from the$$B\overline{B }$$threshold up to 10.59 GeV. We interpret the results using a phenomenological model and constrain the parameters of the$$B\overline{B }$$potential in the isovector channel. 

We measure the time- and phase-space-integrated $CP$asymmetry ${\mathcal{A}}_{CP}$in $D^0\to {\pi }^{+}{\pi }^{-}{\pi }^0$decays reconstructed in $e^{+}{e}^{-}\to c\overline{c}$events collected by the Belle II experiment from 2019 to 2022. This sample corresponds to an integrated luminosity of $428{\mathrm{fb}}^{-1}$. We require $D^0$mesons to be produced in $D^{*+}\to D^0{\pi }^{+}$decays to determine their flavor at production. Control samples of $D^0\to K^{-}{\pi }^{+}$decays are used to correct for reconstruction-induced asymmetries. The result, ${\mathcal{A}}_{CP}\left(D^0\to {\pi }^{+}{\pi }^{-}{\pi }^0\right)=(0.29\pm 0.27\pm 0.13)\%$, where the first uncertainty is statistical and the second systematic, is the most precise result to date and is consistent with $CP$conservation. 

Using data samples of $988.4{\mathrm{fb}}^{-1}$and $427.9{\mathrm{fb}}^{-1}$collected with the Belle and Belle II detectors, we present a study of the singly Cabibbo-suppressed decays ${\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }\eta$, $\mathrm{\Lambda }{\eta }^{\prime }$, and $\mathrm{\Lambda }{\pi }^0$. We observe the decay  ${\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }\eta$and find evidence for the decay ${\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\eta }^{\prime }$, with corresponding branching ratios determined to be $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }\eta )/\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})=(4.16\pm 0.91\pm 0.23)\%$and $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\eta }^{\prime })/\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})=(2.48\pm 0.82\pm 0.12)\%$, respectively. We find no significant signal in the ${\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\pi }^0$decay mode and set an upper limit at the 90% credibility level of $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\pi }^0)/\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})<3.5\%$. Multiplying these ratios by the world-average branching fraction of the normalization channel, $\mathcal{B}({\mathrm{\Xi }}_c^0\to {\mathrm{\Xi }}^{-}{\pi }^{+})=(1.43\pm 0.27)\%$, we obtain the absolute branching fractions of $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }\eta )=(5.95\pm 1.30\pm 0.32\pm 1.13)\times {10}^{-4}$, $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\eta }^{\prime })=(3.55\pm 1.17\pm 0.17\pm 0.68)\times {10}^{-4}$, and an upper limit at the 90% credibility level on the absolute branching fraction of $\mathcal{B}({\mathrm{\Xi }}_c^0\to \mathrm{\Lambda }{\pi }^0)<5.2\times {10}^{-4}$. The quoted first and second uncertainties are statistical and systematic, respectively, while the third uncertainties arise from the branching fraction of the normalization mode. These results are consistent with most theoretical predictions and further the understanding of the underlying decay mechanisms. 

We present a determination of the Cabibbo-Kobayashi-Maskawa matrix element $\left|V_{cb}\right|$from the decay $B\to D\ell {\nu }_{\ell }$using a $365{\mathrm{fb}}^{-1}$ $e^{+}{e}^{-}\to \mathrm{\Upsilon }\left(4S\right)\to B\overline{B}$data sample recorded by the Belle II experiment at the SuperKEKB collider. The semileptonic decay of one $B$meson is reconstructed in the modes $B^0\to D^{-}(\to K^{+}{\pi }^{-}{\pi }^{-}){\ell }^{+}{\nu }_{\ell }$and $B^{+}\to {\overline{D}}^0(\to K^{+}{\pi }^{-}){\ell }^{+}{\nu }_{\ell }$, where $\ell$denotes either an electron or a muon. Charge conjugation is implied. The second $B$meson in the $\mathrm{\Upsilon }\left(4S\right)$event is not reconstructed explicitly. Using an inclusive reconstruction of the unobserved neutrino momentum, we determine the recoil variable $w=v_B·v_D$, where $v_B$and $v_D$are the four-velocities of the $B$and $D$mesons. We measure the total decay branching fractions to be $\mathcal{B}(B^0\to D^{-}{\ell }^{+}{\nu }_{\ell })=(2.06\pm 0.05(\mathrm{stat})\pm 0.10(\mathrm{sys}\left)\right)\%$and $\mathcal{B}(B^{+}\to {\overline{D}}^0{\ell }^{+}{\nu }_{\ell })=(2.31\pm 0.04(\mathrm{stat})\pm 0.09(\mathrm{sys}\left)\right)\%$. We probe lepton flavor universality by measuring $\mathcal{B}(B\to De{\nu }_e)/\mathcal{B}(B\to D\mu {\nu }_{\mu })=1.020\pm 0.020\left(\mathrm{stat}\right)\pm 0.022\left(\mathrm{sys}\right)$. Fitting the partial decay branching fraction as a function of $w$and using the average of lattice QCD calculations of the $B\to D$form factor, we obtain $\left|V_{cb}\right|=(39.2\pm 0.4(\mathrm{stat})\pm 0.6(\mathrm{sys})\pm 0.5(\mathrm{th}\left)\right)\times {10}^{-3}$. 

A<sc>bstract</sc> We present the result of a search for the charged-lepton-flavor violating decaysτ⁻→ e^∓ℓ^±ℓ⁻, whereℓis a muon or an electron, using a data sample with an integrated luminosity of 428 fb⁻¹recorded by the Belle II experiment at the SuperKEKBe⁺e⁻collider. The selection ofe⁺e⁻→ τ⁺τ⁻events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background. Upper limits on the branching fractions between 1.3 and 2.5 × 10⁻⁸are set at the 90% confidence level. These results are the most stringent bounds to date for four of the modes. 

A<sc>bstract</sc> An angular analysis ofB⁰→ K^*0e⁺e⁻decays is presented using proton-proton collision data collected by the LHCb experiment at centre-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of 9 fb⁻¹. The analysis is performed in the region of the dilepton invariant mass squared of 1.1–6.0 GeV²/c⁴. In addition, a test of lepton flavour universality is performed by comparing the obtained angular observables with those measured inB⁰→ K^*0μ⁺μ⁻decays. In general, the angular observables are found to be consistent with the Standard Model expectations as well as with global analyses of otherb → sℓ⁺ℓ⁻processes, whereℓis either a muon or an electron. No sign of lepton-flavour-violating effects is observed. 

A<sc>bstract</sc> A search for the decay$$ {B}_c^{+} $$ $B_c^{+}$→ χ_c1(3872)π⁺is reported using proton-proton collision data collected with the LHCb detector between 2011 and 2018 at centre-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb⁻¹. No significant signal is observed. Using the decay$$ {B}_c^{+} $$ $B_c^{+}$→ψ(2S)π⁺as a normalisation channel, an upper limit for the ratio of branching fractions$$ {\mathcal{R}}_{\psi (2S)}^{\chi_{c1}(3872)}=\frac{{\mathcal{B}}_{B_c^{+}\to {\chi}_{c1}(3872){\pi}^{+}}}{{\mathcal{B}}_{B_c^{+}\to \psi (2S){\pi}^{+}}}\times \frac{{\mathcal{B}}_{\chi_{c1}(3872)\to J/\psi {\pi}^{+}{\pi}^{-}}}{{\mathcal{B}}_{\psi (2S)\to J/\psi {\pi}^{+}{\pi}^{-}}}<0.05(0.06), $$ $R_{\psi \left(2S\right)}^{{\chi }_{c1}\left(3872\right)}=\frac{B_{B_c^{+}\to {\chi }_{c1}\left(3872\right){\pi }^{+}}}{B_{B_c^{+}\to \psi \left(2S\right){\pi }^{+}}}\times \frac{B_{{\chi }_{c1}\left(3872\right)\to J/\psi {\pi }^{+}{\pi }^{-}}}{B_{\psi \left(2S\right)\to J/\psi {\pi }^{+}{\pi }^{-}}}<0.05\left(0.06\right),$ is set at the 90 (95)% confidence level.