arXiv subjects

M. Bessner

Publications and source records attributed to M. Bessner.

At least 91 records · Page 5Linked to original sources

arXiv2024

Search for a long-lived spin-0 mediator in $b\to s$ transitions at the Belle II experiment

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 $Υ(4S)$ 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 $\mathrm{Br}(B^0\to K^*(892)^0(\to K^+π^-)S)\times \mathrm{Br}(S\to x^+x^-)$ and $\mathrm{Br}(B^+\to K^+S)\times \mathrm{Br}(S\to x^+x^-)$, where $x^+x^-$ indicates $e^+e^-, μ^+μ^-, π^+π^-$, or $K^+K^-$, as functions of $S$ mass and lifetime at the level of $10^{-7}$.

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arXiv2024

Measurement of the integrated luminosity of data samples collected during 2019-2022 by the Belle II experiment

A series of data samples was collected with the Belle~II detector at the SuperKEKB collider from March 2019 to June 2022. We determine the integrated luminosities of these data samples using three distinct methodologies involving Bhabha ($e^+e^- \to e^+e^-(nγ)$), digamma ($e^+e^- \to γγ(nγ)$), and dimuon ($e^+e^- \to μ^+ μ^- (nγ)$) events. The total integrated luminosity obtained with Bhabha, digamma, and dimuon events is ({426.88} $\pm$ 0.03 $\pm$ {2.61})~fb$^{-1}$, ({429.28} $\pm$ 0.03 $\pm$ {2.62})~fb$^{-1}$, and ({423.99} $\pm$ 0.04 $\pm$ {3.83})~fb$^{-1}$, where the first uncertainties are statistical and the second are systematic. The resulting total integrated luminosity obtained from the combination of the three methods is ({427.87 $\pm$ 2.01})~fb$^{-1}$.

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arXiv2024

Angular analysis of $B \to K^* e^+ e^-$ in the low-$q^2$ region with new electron identification at Belle

We perform an angular analysis of the $B\to K^* e^+ e^-$ decay for the dielectron mass squared, $q^2$, range of $0.0008$ to $1.1200 ~\text{GeV}^2 /c^4$ using the full Belle data set in the $K^{*0} \to K^+ π^-$ and $K^{*+} \to K_S^0 π^+$ channels, incorporating new methods of electron identification to improve the statistical power of the data set. This analysis is sensitive to contributions from right-handed currents from physics beyond the Standard Model by constraining the Wilson coefficients $\mathcal{C}_7^{(\prime)}$. We perform a fit to the $B\to K^* e^+ e^-$ differential decay rate and measure the imaginary component of the transversality amplitude to be $A_T^{\rm Im} = -1.27 \pm 0.52 \pm 0.12$, and the $K^*$ transverse asymmetry to be $A_T^{(2)} = 0.52 \pm 0.53 \pm 0.11$, with $F_L$ and $A_T^{\rm Re}$ fixed to the Standard Model values. The resulting constraints on the value of $\mathcal{C}_7^{\prime}$ are consistent with the Standard Model within a $2σ$ confidence interval.

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arXiv2024

Search for Rare $b \to d\ell^+\ell^-$ Transitions at Belle

We present the results of a search for the $b \to d\ell^+\ell^-$ flavor-changing neutral-current rare decays $B^{+, 0} \to (η, ω, π^{+,0}, ρ^{+, 0}) e^+e^-$ and $B^{+, 0} \to (η, ω, π^{0}, ρ^{+}) μ^+μ^-$ using a $711$ fb$^{-1}$ data sample that contains $772 \times 10^{6}$ $B\overline{B}$ events. The data were collected at the $Υ(4S)$ 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 $(3.8 - 47) \times 10^{-8}$ depending on the decay channel. The obtained limits are the world's best results. This is the first search for the channels $B^{+, 0} \to (ω, ρ^{+,0}) e^+e^-$ and $B^{+, 0} \to (ω, ρ^{+})μ^+μ^-$.

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arXiv2024

Measurement of the branching fractions of $\bar{B}\to D^{()} K^- K^{()0}_{(S)}$ and $\bar{B}\to D^{(*)}D_s^{-}$ decays at Belle II

We present measurements of the branching fractions of eight $\overline B{}^0\to D^{(*)+} K^- K^{(*)0}_{(S)}$, $B^{-}\to D^{(*)0} K^- K^{(*)0}_{(S)}$ decay channels. The results are based on data from SuperKEKB electron-positron collisions at the $Υ(4S)$ resonance collected with the Belle II detector, 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, and are efficiency-corrected as a function of $m(K^-K^{(*)0}_{(S)})$ and $m(D^{(*)}K^{(*)0}_{(S)})$ in order to avoid dependence on the decay model. 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 the other channels compared to previous measurements. The helicity-angle distributions and the invariant mass distributions of the $K^- K^{(*)0}_{(S)}$ systems are compatible with quasi-two-body decays via a resonant transition with spin-parity $J^P=1^-$ for the $K^-K_S^0$ systems and $J^P= 1^+$ for the $K^-K^{*0}$ systems. We also present measurements of the branching fractions of four $\overline B{}^0\to D^{(*)+} D_s^-$, $B^{-}\to D^{(*)0} D_s^- $ decay channels with a precision compatible to the current world averages.

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arXiv2024

Test of light-lepton universality in $τ$ decays with the Belle II experiment

We present a measurement of the ratio $R_μ= \mathcal{B}(τ^-\to μ^-\barν_μν_τ) / \mathcal{B}(τ^-\to e^-\barν_eν_τ)$ of branching fractions $\mathcal{B}$ of the $τ$ 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 $362\!\pm\!2\,\text{fb}^{-1}$ at a centre-of-mass energy of $10.58\,\text{GeV}$. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result, $R_μ= 0.9675 \pm 0.0007 \pm 0.0036$, 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 $τ$ decays of $0.9974 \pm 0.0019$, in agreement with the standard model expectation of unity.

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arXiv2024

Search for the decay $B^{0}\toγγ$ using Belle and Belle II data

We report the result of a search for the rare decay $B^{0} \to γγ$ using a combined dataset of $753\times10^{6}$ $B\bar{B}$ pairs collected by the Belle experiment and $387\times10^{6}$ $B\bar{B}$ pairs collected by the Belle II experiment from decays of the $\rm Υ(4S)$ resonance produced in $e^{+}e^{-}$ collisions. A simultaneous fit to the Belle and Belle II data sets yields $11.0^{+6.5}_{-5.5}$ signal events, corresponding to a 2.5$σ$ significance. We determine the branching fraction $\mathcal{B}(B^{0} \to γγ) = (3.7^{+2.2}_{-1.8}(\rm stat)\pm0.5(\rm syst))\times10^{-8}$ and set a 90% credibility level upper limit of $\mathcal{B}(B^{0} \to γγ) < 6.4\times10^{-8}$.

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arXiv2024

Determination of the CKM angle $ϕ_{3}$ from a combination of Belle and Belle II results

We report a determination of the CKM angle $ϕ_{3}$, also known as $γ$, from a combination of measurements using samples of up to 711~fb$^{-1}$ from the Belle experiment and up to 362~fb$^{-1}$ from the Belle II experiment. We combine results from analyses of $B^+\to DK^+, B^+\to Dπ^+$, and $B^+ \to D^{*}K^+$ decays, where $D$ is an admixture of $D^0$ and $\overline{D}{}^{0}$ mesons, in a likelihood fit to obtain $ϕ_{3} = (78.6^{+7.2}_{-7.3})^{\circ}$. We also briefly discuss the interpretation of this result.

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arXiv2024

Search for charmed baryons in the $Λ_c^+η$ system and measurement of the branching fractions of $Λ_c(2880)^+$ and $Λ_c(2940)^+$ decaying to $Λ_c^+η$ and $pD^0$ relative to $Σ_c(2455)π$

We search for excited charmed baryons in the $Λ_c^+η$ system using a data sample corresponding to an integrated luminosity of 980 $\rm fb^{-1}$. The data were collected by the Belle detector at the KEKB $e^{+}$$e^{-}$ asymmetric-energy collider. No significant signals are found in the $Λ_c^+η$ mass spectrum, including the known $Λ_c(2880)^+$ and $Λ_c(2940)^+$. Clear $Λ_c(2880)^+$ and $Λ_c(2940)^+$ signals are observed in the $pD^0$ mass spectrum. We set upper limits at 90\% credibility level on ratios of branching fractions of $Λ_c(2880)^+$ and $Λ_c(2940)^+$ decaying to $Λ_c^+η$ relative to $Σ_c(2455)π$ of $<0.13$ for the $Λ_c(2880)^+$ and $<1.11$ for the $Λ_c(2940)^+$. We measure ratios of branching fractions of $Λ_c(2880)^+$ and $Λ_c(2940)^+$ decaying to $pD^0$ relative to $Σ_c(2455)π$ of $0.75 \pm 0.03(\text{stat.}) \pm 0.07(\text{syst.})$ for the $Λ_c(2880)^+$ and $3.59 \pm 0.21(\text{stat.}) \pm 0.56(\text{syst.})$ for the $Λ_c(2940)^+$.

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arXiv2024

Determination of $|V_{ub}|$ from simultaneous measurements of untagged $B^0\toπ^- \ell^+ ν_{\ell}$ and $B^+\toρ^0 \ell^+ν_{\ell}$ decays

We present a measurement of $|V_{ub}|$ from a simultaneous study of the charmless semileptonic decays $B^0\toπ^- \ell^+ ν_{\ell}$ and $B^+\toρ^0 \ell^+ν_{\ell}$, where $\ell = e, μ$. This measurement uses a data sample of 387 million $B\overline{B}$ meson pairs recorded by the Belle~II detector at the SuperKEKB electron-positron collider between 2019 and 2022. The two decays are reconstructed without identifying the partner $B$ mesons. We simultaneously measure the differential branching fractions of $B^0\toπ^- \ell^+ ν_{\ell}$ and $B^+\toρ^0 \ell^+ν_{\ell}$ decays as functions of $q^2$ (momentum transfer squared). From these, we obtain total branching fractions $B(B^0\toπ^- \ell^+ ν_{\ell}) = (1.516 \pm 0.042 (\mathrm{stat}) \pm 0.059 (\mathrm{syst})) \times 10^{-4}$ and $B(B^+\toρ^0 \ell^+ν_{\ell}) = (1.625 \pm 0.079 (\mathrm{stat}) \pm 0.180 (\mathrm{syst})) \times 10^{-4}$. By fitting the measured $B^0\toπ^- \ell^+ ν_{\ell}$ partial branching fractions as functions of $q^2$, together with constraints on the non-perturbative hadronic contribution from lattice QCD calculations, we obtain $|V_{ub}|$ = $(3.93 \pm 0.09 \pm 0.13 \pm 0.19) \times 10^{-3}$. Here, the first uncertainty is statistical, the second is systematic, and the third is theoretical.

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arXiv2024

New graph-neural-network flavor tagger for Belle II and measurement of $\sin2ϕ_1$ in $B^0 \to J/ψK^0_\text{S}$ decays

We present GFlaT, a new algorithm that uses a graph-neural-network to determine the flavor of neutral $B$ mesons produced in $Υ(4S)$ decays. It improves previous algorithms by using the information from all charged final-state particles and the relations between them. We evaluate its performance using $B$ decays to flavor-specific hadronic final states reconstructed in a 362 $\text{fb}^{-1}$ sample of electron-positron collisions collected at the $Υ(4S)$ resonance with the Belle II detector at the SuperKEKB collider. We achieve an effective tagging efficiency of $(37.40 \pm 0.43 \pm 0.36) \%$, where the first uncertainty is statistical and the second systematic, which is $18\%$ better than the previous Belle II algorithm. Demonstrating the algorithm, we use $B^{0}\to J/ψK^0_\text{S}$ decays to measure the mixing-induced and direct $CP$ violation parameters, $S = (0.724 \pm 0.035 \pm 0.009)$ and $C = (-0.035 \pm 0.026 \pm 0.029)$.

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arXiv2024

Measurement of branching fractions, CP asymmetry, and isospin asymmetry for $\boldsymbol{B\rightarrowργ}$ decays using Belle and Belle II data

We present measurements of $B^{+}\rightarrowρ^{+}γ$ and $B^{0}\rightarrowρ^{0}γ$ decays using a combined data sample of $772 \times 10^6$ $B\overline{B}$ pairs collected by the Belle experiment and $387\times 10^6$ $B\overline{B}$ pairs collected by the Belle II experiment in $e^{+}e^{-}$ collisions at the $Υ(4S)$ resonance. After an optimized selection, a simultaneous fit to the Belle and Belle II data sets yields $114\pm 12$ $B^{+}\rightarrowρ^{+}γ$ and $99\pm 12$ $B^{0}\rightarrowρ^{0}γ$ decays. The measured branching fractions are $(13.1^{+2.0 +1.3}_{-1.9 -1.2})\times 10^{-7}$ and $(7.5\pm 1.3^{+1.0}_{-0.8})\times 10^{-7}$ for $B^{+}\rightarrowρ^{+}γ$ and $B^{0}\rightarrowρ^{0}γ$ decays, respectively, where the first uncertainty is statistical and the second is systematic. We also measure the isospin asymmetry $A_{\rm I}(B\rightarrowργ)=(10.9^{+11.2 +7.8}_{-11.7 -7.3})\%$ and the direct CP asymmetry $A_{CP}(B^{+}\rightarrowρ^{+}γ)=(-8.2\pm 15.2^{+1.6}_{-1.2})\%$.

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arXiv2024

Search for the decay $B_s^0\to J/ψπ^0$ at Belle experiment

We have analyzed 121.4 fb$^{-1}$ of data collected at the $Υ(5S)$ resonance by the Belle experiment using the KEKB asymmetric-energy $e^+e^-$ collider to search for the decay $B_s^0\to J/ψπ^0$. We observe no signal and report an upper limit on the branching fraction $\mathcal{B}(B_s^0\to J/ψπ^0)$ of $1.21\times 10^{-5}$ at 90\% confidence level. This result is the most stringent, improving the previous bound by two orders of magnitude.

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arXiv2024

Search for the baryon number and lepton number violating decays $τ^-\to Λπ^-$ and $τ^-\to \barΛπ^-$ at Belle II

We present a search for the baryon number $B$ and lepton number $L$ violating decays $τ^- \rightarrow Λπ^-$ and $τ^- \rightarrow \barΛ π^-$ produced from the $e^+e^-\to τ^+τ^-$ process, using a 364 fb$^{-1}$ data sample collected by the Belle~II experiment at the SuperKEKB collider. No evidence of signal is found in either decay mode, which have $|Δ(B-L)|$ equal to $2$ and $0$, respectively. Upper limits at 90\% credibility level on the branching fractions of $τ^- \rightarrow Λπ^-$ and $τ^- \rightarrow \barΛπ^-$ are determined to be $4.7 \times 10^{-8}$ and $4.3 \times 10^{-8}$, respectively.

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arXiv2024

The Belle II Detector Upgrades Framework Conceptual Design Report

We describe the planned near-term and potential longer-term upgrades of the Belle II detector at the SuperKEKB electron-positron collider operating at the KEK laboratory in Tsukuba, Japan. These upgrades will allow increasingly sensitive searches for possible new physics beyond the Standard Model in flavor, tau, electroweak and dark sector physics that are both complementary to and competitive with the LHC and other experiments.

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arXiv2024

Evidence of $h_{b}(\text{2P}) \to Υ(\text{1S})η$ decay and search for $h_{b}(\text{1P,2P}) \to Υ(\text{1S})π^0$ with the Belle detector

We report the first evidence for the $h_{b}(\text{2P}) \to Υ(\text{1S})η$ transition with a significance of $3.5$ standard deviations. The decay branching fraction is measured to be $\mathcal{B}[h_{b}(\text{2P}) \to Υ(\text{1S})η]=(7.1 ~^{+3.7} _{-3.2}\pm 0.8)\times10^{-3}$, which is noticeably smaller than expected. We also set upper limits on $π^0$ transitions of $\mathcal{B}[h_{b}(\text{2P}) \to Υ(\text{1S})π^0] < 1.8\times10^{-3}$, and $\mathcal{B}[h_{b}(\text{1P})\to Υ(\text{1S})π^0] < 1.8\times10^{-3}$, at the $90\%$ confidence level. These results are obtained with a $131.4$~fb$^{-1}$ data sample collected near the $Υ(\text{5S})$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider.

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arXiv2024

Measurement of the branching fraction of the decay $B^- \to D^0 ρ(770)^-$ at Belle II

We measure the branching fraction of the decay $B^- \to D^0 ρ(770)^-$ using data collected with the Belle II detector. The data contain 387 million $B\overline{B}$ pairs produced in $e^+e^-$ collisions at the $Υ(4S)$ resonance. We reconstruct $8360\pm 180$ decays from an analysis of the distributions of the $B^-$ energy and the $ρ(770)^-$ helicity angle. We determine the branching fraction to be $(0.939 \pm 0.021\mathrm{(stat)} \pm 0.050\mathrm{(syst)})\%$, in agreement with previous results. Our measurement improves the relative precision of the world average by more than a factor of two.

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arXiv2024

Search for a $μ^+μ^-$ resonance in four-muon final states at Belle II

We report on a search for a resonance $X$ decaying to a pair of muons in $e^{+}e^{-}\rightarrow μ^+ μ^- X$ events in the 0.212-9.000 GeV/$c^{2}$ mass range, using 178 fb$^{-1}$ of data collected by the BelleII 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 $Z^{\prime}$ 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.

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