arXiv subjects

D. Epifanov

Publications and source records attributed to D. Epifanov.

At least 55 records · Page 3Linked to original sources

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

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

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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arXiv2024

Search for a heavy neutral lepton that mixes predominantly with the tau neutrino

We report a search for a heavy neutral lepton (HNL) that mixes predominantly with $ν_τ$. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy $e^+ e^-$ collider. The data sample was collected at and just below the center-of-mass energies of the $Υ(4S)$ and $Υ(5S)$ resonances and has an integrated luminosity of $915~\textrm{fb}^{-1}$, corresponding to $(836\pm 12)\times 10^6$ $e^+e^\toτ^+τ^-$ events. We search for production of the HNL (denoted $N$) in the decay $τ^-\to π^- N$ followed by its decay via $N \to μ^+μ^- ν_τ$. The search focuses on the parameter-space region in which the HNL is long lived, so that the $μ^+μ^-$ originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the $τ$ neutrino as a function of the HNL mass.

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arXiv2024

Evidence for $B^{+}\to K^{+}ν\barν$ decays

We search for the rare decay $B^{+}\rightarrow K^{+}ν\barν$ in a $362\ \rm{fb}^{-1}$ sample of electron-positron collisions at the $Υ(4S)$ resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying $B$ meson in $Υ(4S) \to B\kern 0.18em\overline{\kern -0.18em B}{}$ events to suppress background from other decays of the signal $B$ candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying $B$ meson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for the $B^{+}\rightarrow K^{+}ν\barν$ branching fraction of $\left[2.7\pm 0.5(\mathrm{stat})\pm 0.5(\mathrm{syst})\right] \times 10^{-5}$ and $\left[1.1^{+0.9}_{-0.8}(\mathrm{stat}){}^{+0.8}_{-0.5}(\mathrm{syst})\right] \times 10^{-5}$, respectively. Combining the results, we determine the branching fraction of the decay $B^{+}\rightarrow K^{+}ν\barν$ to be $\left[2.3 \pm 0.5(\mathrm{stat})^{+0.5}_{-0.4}(\mathrm{syst})\right]\times 10^{-5}$, providing the first evidence for this decay at $3.5$ standard deviations. The combined result is $2.7$ standard deviations above the standard model expectation.

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arXiv2024

First Measurement of $R(X_{τ/\ell})$ as an Inclusive Test of the $b \to c τν$ Anomaly

We measure the tau-to-light-lepton ratio of inclusive $B$-meson branching fractions $R(X_{τ/\ell}) \equiv \mathcal{B}(B\to X τν)/\mathcal{B}(B \to X \ell ν)$, where $\ell$ 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 $189~\mathrm{fb}^{-1}$ of electron-positron collision data collected with the Belle II detector. We find $R(X_{τ/\ell}) = 0.228 \pm 0.016~(\mathrm{stat}) \pm 0.036~(\mathrm{syst})$, in agreement with standard-model expectations. This is the first direct measurement of $R(X_{τ/\ell})$.

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arXiv2024

Search for Two-Body $B$ Meson Decays to $Λ^{0}$ and $Ω^{(*)0}_{c}$

We report the results of the first search for Standard Model and baryon-number-violating two-body decays of the neutral $B$ mesons to $Λ^{0}$ and $Ω^{(*)0}_c$ using 711~${\rm fb^{-1}}$ of data collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We observe no evidence of signal from any such decays and set 95\% confidence-level upper limits on the products of $B^0$ and $\bar{B}^0$ branching fractions for these two-body decays with $\mathcal{B}(Ω_{c}^{0} \to π^+ Ω^-)$ in the range between 9.5~$\times 10^{-8}$ and 31.2~$\times 10^{-8}$.

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arXiv2024

Search for baryon and lepton number violating decays $\boldsymbol{D \rightarrow p\ell}$

We search for the baryon and lepton number violating charm decays, $D \rightarrow p\ell$, where $D$ is either a $D^0$ or a $\overline{D}^0$ and $\ell$ is a muon or an electron, using a data sample of $921\,\mathrm{fb}^{-1}$ collected by the Belle detector at the KEKB asymmetric energy $e^{+}e^{-}$ collider. In the absence of significant signals, we set upper limits on the branching fractions in the range $(5 - 8) \times 10^{-7}$ at a 90\% confidence level, depending on the decay mode.

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arXiv2024

Search for charged-lepton flavor violation in $Υ(2S) \to \ell^\mpτ^\pm$ ($\ell=e,μ$) decays at Belle

We report a search for the charged-lepton flavor violation in $Υ(2S) \to \ell^\mpτ^\pm$ ($\ell=e,μ$) decays using a $25~\fbi$ $Υ(2S)$ sample collected by the Belle detector at the KEKB $e^{+}$$e^-$ asymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions ($\mathcal{B}$) at 90\% confidence level. We obtain the most stringent upper limits: $\mathcal{B}(\Ytomutau) < 0.23 \times 10^{-6}$ and $\mathcal{B}(\Ytoetau) < 1.12 \times 10^{-6}$.

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arXiv2024

Search for a dark leptophilic scalar produced in association with $τ^+τ^-$ pair in $e^+e^-$ annihilation at center-of-mass energies near 10.58 GeV

A dark leptophilic scalar $(ϕ_L)$ is a hypothetical particle that couples only to leptons rather than quarks. We report on a search for $ϕ_L$ in the $e^+e^- \to τ^+ τ^- ϕ_L, ~ϕ_L \to \ell^+ \ell^- ~(\ell = e, μ)$ process using 626~\invfb of data collected by the Belle experiment near the \Y4S resonance. We validate the backgrounds with multiple control regions in data, using a novel multiclass multivariate event classifier. In absence of a signal, we quote upper limits at the 90\% confidence level on the coupling between $ϕ_L$ and leptons. Our bounds, obtained in a blinded approach, are 19\% more constraining than the previous limits, averaged over the mass range $0.04 \leq m_{ϕ_L} \leq 6.5~\gev$. We exclude the parameter space below 4~\gev favored by measurement of the anomalous magnetic moment of the muon.

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arXiv2024

Measurements of the branching fraction, polarization, and $CP$ asymmetry for the decay $B^0\rightarrow ωω$

We present a measurement of $B^{0} \rightarrow ωω$, a charmless decay into two vector mesons, using 772 $\times 10^6$ $B\overline{B}$ pairs collected with the Belle detector at the KEKB $e^+e^-$ collider. The decay is observed with a significance of 7.9 standard deviations. We measure a branching fraction $\mathcal{B} = (1.53 \pm 0.29 \pm 0.17) \times 10^{-6}$, a fraction of longitudinal polarization $f_L = 0.87 \pm 0.13 \pm 0.13$, and a time-integrated $CP$ asymmetry $A_{CP}$ = $-0.44 \pm 0.43 \pm 0.11$, where the first uncertainties listed are statistical and the second are systematic. This is the first observation of $B^{0} \rightarrow ωω$, and the first measurements of $f_L$ and $A_{CP}$ for this decay.

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arXiv2024

A test of lepton flavor universality with a measurement of $R(D^{*})$ using hadronic $B$ tagging at the Belle II experiment

The ratio of branching fractions $R(D^{*}) = \mathcal{B}(\overline{B} \rightarrow D^{*} τ^{-} \overlineν_τ)$/$\mathcal{B} (\overline{B} \rightarrow D^{*} \ell^{-} \overlineν_{\ell})$, where $\ell$ is an electron or muon, is measured using a Belle~II data sample with an integrated luminosity of $189~\mathrm{fb}^{-1}$ at the SuperKEKB asymmetric-energy $e^{+} e^{-}$ collider. Data is collected at the $Υ(\mathrm{4S})$ resonance, and one $B$ meson in the $Υ(\mathrm{4S})\rightarrow B\overline{B}$ decay is fully reconstructed in hadronic decay modes. The accompanying signal $B$ meson is reconstructed as $\overline{B}\rightarrow D^{*} τ^{-}\overlineν_τ$ using leptonic $τ$ decays. The normalization decay, $\overline{B}\rightarrow D^{*} \ell^{-} \overlineν_{\ell}$, where $\ell$ is an electron or muon, produces the same observable final state particles. The ratio of branching fractions is extracted in a simultaneous fit to two signal-discriminating variables in both channels and yields $R(D^{*}) = 0.262~_{-0.039}^{+0.041}(\mathrm{stat})~_{-0.032}^{+0.035}(\mathrm{syst})$. This result is consistent with the current world average and with standard model predictions.

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