arXiv subjects

D. Červenkov

Publications and source records attributed to D. Červenkov.

At least 127 records · Page 7Linked to original sources

arXiv2021

Test of lepton flavor universality and search for lepton flavor violation in $B \to K \ell\ell$ decays

We present measurements of the branching fractions for the decays $B\to K μ^{+}μ^{-}$ and $B\to K e^{+}e^{-}$, and their ratio ($R_{K}$), using a data sample of 711 $fb^{-1}$ that contains $772 \times 10^{6}$ $B\bar{B}$ events. The data were collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The ratio $R_{K}$ is measured in five bins of dilepton invariant-mass-squared ($q^{2}$): $q^{2} \in (0.1, 4.0), (4.0, 8.12), (1.0, 6.0)$, $(10.2, 12.8)$ and ($>14.18) GeV^{2}/c^{4}$, along with the whole $q^2$ region. The $R_{K}$ value for $q^{2} \in (1.0, 6.0) GeV^{2}/c^{4}$ is $1.03^{+0.28}_{-0.24} \pm 0.01$. The first and second uncertainties listed are statistical and systematic, respectively. All results for $R_{K}$ are consistent with Standard Model predictions. We also measure $C\!P$-averaged isospin asymmetries in the same $q^{2}$ bins. The results are consistent with a null asymmetry, with the largest difference of 2.6 standard deviations occurring for the $q^{2}\in(1.0,6.0) GeV^{2}/c^{4}$ bin in the mode with muon final states. The measured differential branching fractions, ${d\cal B}/{dq^{2}}$, are consistent with theoretical predictions for charged $B$ decays, while the corresponding values are below the expectations for neutral $B$ decays. We have also searched for lepton-flavor-violating $B \rightarrow Kμ^{\pm}e^{\mp}$ decays and set $90\%$ confidence-level upper limits on the branching fraction in the range of $10^{-8}$ for $B^{+} \rightarrow K^{+}μ^{\pm}e^{\mp}$, and $B^{0} \rightarrow K^{0}μ^{\pm}e^{\mp}$ modes.

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arXiv2021

Measurements of the branching fractions of $Λ_c^+ \to p η$ and $Λ_c^+ \to p π^0$ decays at Belle

We report measurements of the branching fractions of singly Cabibbo-suppressed decays $Λ_c^+ \to p η$ and $Λ_c^+ \to p π^0$ using the full Belle data sample corresponding to an integrated luminosity of 980.6 $\rm fb^{-1}$. The data were collected by the Belle detector at the KEKB $e^{+}$$e^{-}$ asymmetric-energy collider. A clear $Λ_c^+$ signal is seen in the invariant mass distribution of $p η$. The signal yield of the $Λ_c^+ \to p η$ process is $7734 \pm 263$; from this, we measure the ratio of branching fractions ${\cal B}(Λ_c^+ \to p η)/{\cal B}(Λ_c^+ \to p K^- π^+) = (2.258 \pm 0. 077(\rm stat. ) \pm 0.136(\rm syst. ))\times 10^{-2}$, from which we infer the branching fraction ${\cal B}(Λ_c^+ \to p η) = (1.42 \pm 0.05(\rm stat.) \pm 0.11(\rm syst.)) \times 10^{-3}$. In addition, no significant signal for $Λ_c^+ \to p π^0$ is found so an upper limit on the branching fraction of ${\cal B}(Λ_c^+ \to p π^0)<8.0 \times 10^{-5}$ at 90\% credibility level is set, more than three times better than the best current upper limit.

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arXiv2021

Measurement of branching fractions and $CP$ asymmetries for $D_s^{+} \rightarrow K^+ (η, π^0) $ and $D_s^{+} \rightarrow π^+ (η, π^0)$ decays at Belle

We report measurements of the branching fractions and $CP$ asymmetries for $D_s^{+} \rightarrow K^{+} η$, $D_s^{+} \rightarrow K^{+} π^0 $, and $D_s^{+} \rightarrow π^{+} η$ decays, and the branching fraction for $D_s^{+} \rightarrow π^{+} π^0$. Our results are based on a data sample corresponding to an integrated luminosity of 921 fb$^{-1}$ collected by the Belle detector at the KEKB $e^+e^-$ asymmetric-energy collider. Our measurements of $CP$ asymmetries in these decays are the most precise to-date; no evidence for $CP$ violation is found.

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arXiv2021

$τ$ lepton mass measurement at Belle II

The reconstruction of tau-pair production, $e^{+}e^{-} \to τ^{+}τ^{-}$, from the subsequent 3-prong ($τ^{+} \rightarrow π^{+} π^{-} π^{+} \barν_τ$) and 1-prong ($τ^{-} \to \ell^{-} \barν_{\ell} ν_τ$, $τ^{-} \to h^{-} ν_τ$ or $τ^{-} \to π^{-} π^0 ν_τ$) decays, is presented using 8.8 fb$^{-1}$ of $e^{+}e^{-}$ collision data of Belle II at the center-of-mass energy $\sqrt{s} = m_{Υ(4S)}$. The pseudomass technique developed by the ARGUS experiment is used to measure the $τ$-lepton mass $m_τ$ in the 3-prong $τ^{+} \to π^{+} π^{-} π^{+} \barν_τ $ decay, resulting in $m_τ = 1777.28 \pm 0.75~{\rm (stat.)} \pm 0.33 ~{\rm (sys.)}~{\rm{MeV}/\rm{c}^2}$.

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arXiv2021

Evidence for $X(3872)\rightarrow J/ψπ^+π^-$ produced in single-tag two-photon interactions

We report the first evidence for $X(3872)$ production in two-photon interactions by tagging either the electron or the position in the final state, exploring the highly virtual photon region. The search is performed in $e^+e^- \rightarrow e^+e^-J/ψπ^+π^-$, using 825 fb$^{-1}$ of data collected by the Belle detector operated at the KEKB $e^+e^-$ collider. We observe three $X(3872)$ candidates with an expected background of $0.11\pm 0.10$ events, with a significance of 3.2$σ$. We obtain an estimated value for $\tildeΓ_{γγ}{\cal B}(X(3872)\rightarrow J/ψπ^+π^-$) assuming the $Q^2$ dependence predicted by a $c\bar{c}$ meson model, where $-Q^2$ is the invariant mass-squared of the virtual photon. No $X(3915)\rightarrow J/ψπ^+π^-$ candidates are found.

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arXiv2021

Measurement of Branching Fraction and Search for $CP$ Violation in $B\to ϕϕK$

We report the measurement of branching fractions and $CP$-violation asymmetries in $B\to ϕϕK$ decays based on a $711\,{\rm fb}^{-1}$ data sample containing $772\times 10^6$ $B\bar{B}$ events. The data were recorded at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. For $B^+ \to ϕϕK^+$, the branching fraction and $CP$-violation asymmetry measured below the $η_{c}$ threshold ($m_{ϕϕ}<2.85\,{\rm GeV}/c^2$) are $[3.43^{\,+\,0.48}_{\,-\,0.46}({\rm stat})\pm 0.22({\rm syst})] \times10^{-6}$ and $-0.02\pm0.11({\rm stat})\pm0.01({\rm syst})$, respectively. Similarly, the branching fraction obtained for $B^0 \toϕϕK^0$ below the $η_{c}$ threshold is $[3.02^{\,+\,0.75}_{\,-\,0.66} ({\rm stat})\pm \,0.20({\rm syst})]\times10^{-6}$. We also measure the $CP$-violation asymmetry for $B^+ \toϕϕK^+$ within the $η_{c}$ region ($m_{ϕϕ}\in [2.94,3.02]\,{\rm GeV}/c^2$) to be $+0.12\pm0.12({\rm stat})\pm0.01({\rm syst})$.

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arXiv2021

Search for lepton-number- and baryon-number-violating tau decays at Belle

We search for lepton-number- and baryon-number-violating decays $τ^{-}\to\overline{p}e^{+}e^{-}$, $pe^{-}e^{-}$, $\overline{p}e^{+}μ^{-}$, $\overline{p}e^{-}μ^{+}$, $\overline{p}μ^{+}μ^{-}$, and $pμ^{-}μ^{-}$ using 921 fb$^{-1}$ of data, equivalent to $(841\pm12)\times 10^6$ $τ^{+}τ^{-}$ events, recorded with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. In the absence of a signal, $90\%$ confidence-level upper limits are set on the branching fractions of these decays in the range $(1.8$-$4.0)\times 10^{-8}$. We set the world's first limits on the first four channels and improve the existing limits by an order of magnitude for the last two channels.

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arXiv2020

Search for a doubly-charged $DDK$ bound state in $Υ(1S,2S)$ inclusive decays and via direct production in $e^+e^-$ collisions at $\sqrt{s}$ = 10.520, 10.580, and 10.867 GeV

We report the results of a first search for a doubly-charged $DDK$ bound state, denoted the $R^{++}$, in $Υ(1S)$ and $Υ(2S)$ inclusive decays and via direct production in $e^+e^-$ collisions at $\sqrt{s}$ = 10.520, 10.580, and 10.867 GeV. The search uses data accumulated with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. No significant signals are observed in the $D^{+}D_{s}^{*+}$ invariant-mass spectra of all studied modes. The 90\% credibility level upper limits on their product branching fractions in $Υ(1S)$ and $Υ(2S)$ inclusive decays (${\cal B}(Υ(1S,2S) \to R^{++} + anything) \times {\cal B}(R^{++} \to D^{+} D_{s}^{*+})$), and the product values of Born cross section and branching fraction in $e^+e^-$ collisions ($σ(e^+e^- \to R^{++} + anything) \times {\cal B}(R^{++} \to D^{+} D_{s}^{*+})$) at $\sqrt{s}$ = 10.520, 10.580, and 10.867 GeV under different assumptions of $R^{++}$ masses varying from 4.13 to 4.17 GeV/$c^2$, and widths varying from 0 to 5 MeV are obtained.

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arXiv2020

Measurement of time-dependent CP violation parameters in $B^0 \to K_0^SK_0^SK_0^S$ decays at Belle

We measure the time-dependent $C\!P$ violation parameters in $B^{0} \to K^{0}_{S}K^{0}_{S}K^{0}_{S}$ decays using $772 \times 10^6$ $B {\mkern 1.5mu\overline{\mkern-1.5mu B \mkern-0.1mu}\mkern 0.1mu}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. The obtained mixing-induced and direct $C\!P$ asymmetries are $-0.71 \pm 0.23~{\rm(stat)} \pm 0.05~{\rm(syst)}$ and $0.12 \pm 0.16~{\rm(stat)} \pm 0.05~{\rm(syst)}$, respectively. These values are consistent with the Standard Model predictions. The significance of $C\!P$ violation differs from zero by 2.5 standard deviations.

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arXiv2020

Measurement of the charm-mixing parameter $y_{CP}$ in $D^{0}\to K^{0}_{S}ω$ decays at Belle

We report the first measurement of the charm-mixing parameter $y_{CP}$ in $D^{0}$ decays to the $CP$-odd final state $K^{0}_{S}ω$. The study uses the full Belle $e^{+}e^-$ annihilation data sample of $976\rm~ fb^{-1}$ taken at or near the $Υ(4S)$ centre-of-mass energy. We find $y_{CP} = (0.96 \pm 0.91 \pm 0.61 {}^{+0.17}_{-0.00})\%$, where the first uncertainty is statistical, the second is systematic due to event selection and background, and the last is due to possible presence of $CP$-even decays in the data sample.

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arXiv2020

Search for Axion-Like Particles produced in $e^+e^-$ collisions at Belle II

We present a search for the direct production of a light pseudoscalar $a$ decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process ${e^+e^-\toγa, a \toγγ}$ in the mass range ${0.2} \,< m_a < {9.7}\,{\text{GeV/$c$}^2}$ using data corresponding to an integrated luminosity of $(445\pm 3)\,\text{pb}^{-1}$. Light pseudoscalars interacting predominantly with standard model gauge bosons (so-called axion-like particles or ALPs) are frequently postulated in extensions of the standard model. We find no evidence for ALPs and set 95% confidence level upper limits on the coupling strength $g_{aγγ}$ of ALPs to photons at the level of $10^{-3}\,{\text{GeV}^{-1}}$. The limits are the most restrictive to date for $0.2\,<\,m_a\,<\,1\,{\text{GeV/$c$}^2}$.

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arXiv2020

Search for transitions from $Υ(4S)$ and $Υ(5S)$ to $η_b(1S)$ and $η_b(2S)$ with emission of an $ω$ meson

Using data collected in the Belle experiment at the KEKB asymmetric-energy $e^+e^-$ collider we search for transitions $Υ(4S) \rightarrow η_b(1S)ω$, $Υ(5S) \rightarrow η_b(1S)ω$ and $Υ(5S) \rightarrow η_b(2S)ω$. No significant signals are observed and we set 90\% confidence level upper limits on the corresponding visible cross sections: $0.2 ~\textrm{pb}, 0.4 ~\textrm{pb}$ and $1.9 ~\textrm{pb}$, respectively.

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arXiv2020

Measurements of branching fractions and CP-violating charge asymmetries in charmless $B$ decays reconstructed in 2019--2020 Belle~II data

We report on first measurements of branching fractions~($\mathcal{B}$) and CP-violating charge asymmetries~($\mathcal{A}$) in charmless $B$ decays at Belle~II. We use a sample of electron-positron collisions collected in 2019 and 2020 at the $Υ(4S)$ resonance and corresponding to $34.6$\,fb$^{-1}$ of integrated luminosity. We use simulation to determine optimized event selections. The $ΔE$ distributions of the resulting samples, restricted in $M_{\rm bc}$, are fit to determine signal yields. Signal yields are corrected for efficiencies determined from simulation and control data samples to obtain branching fractions and CP-violating asymmetries for flavour-specific channels. These are the first measurements in charmless decays reported by Belle~II. Results are compatible with known determinations and show detector performance comparable with the best Belle results offering a reliable basis to assess projections for future reach.

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arXiv2020

Search for the Decay $B_s^0 \rightarrow η^\prime η$

In the Standard Model (SM) charmless hadronic decays $B_s^0 \rightarrow η^\prime η$ proceed via tree-level $b\to u$ and penguin $b\to s$ transitions. Penguin transitions are sensitive to Beyond-the-Standard-Model (BSM) physics scenarios and could affect the branching fractions and {\it CP} asymmetries in such decays. Once branching fractions for two-body decays $B_s \to ηη, ηη^{\prime}, η^{\prime}η^{\prime} $ are measured, and the theoretical uncertainties are reduced, it would be possible to extract {\it CP} violating parameters from the data using the formalism based on SU(3)/U(3) symmetry. To achieve this goal, at least four of these six branching fractions need to be measured. Only the branching fraction for $B_s^0 \to η^{\prime}η^{\prime}$ has been measured so far.

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arXiv2020

Measurement of Hadronic Mass Moments $\langle M_X^n \rangle $ in $B \rightarrow X_c \ell ν$ Decays at Belle II

We present measurements of the first six hadronic mass moments in semileptonic $B \rightarrow X_c \ell ν$ decays. The hadronic mass moments, together with other observables of inclusive $B$ decays, can be used to determine the CKM matrix element $|{V_{cb}}|$ and mass of the $b$-quark $m_b$ in the context of Heavy Quark Expansions of QCD. The Belle~II data recorded at the $Υ(4S)$ resonance in 2019 and 2020 (March-July), corresponding to an integrated luminosity of $34.6\;\mathrm{fb}^{-1}$, is used for this measurement. The decay $Υ(4S) \rightarrow B \overline{B}$ is reconstructed by applying the hadronic tagging algorithm provided by the Full Event Interpretation to fully reconstruct one $B$ meson. The second $B$ meson is reconstructed inclusively by selecting a high-momentum lepton. The $X_c$ system is identified by the remaining reconstructed tracks and clusters in the electromagnetic calorimeter. We report preliminary results for the hadronic mass moments $\langle M_X^n \rangle $ with $n=1,\dots,6$, measured as a function of a lower cut on the lepton momentum in the signal $B$ rest frame.

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arXiv2020

Exclusive $B^0 \to π^- \ell^+ ν_\ell$ Decays with Hadronic Full Event Interpretation Tagging in 34.6 fb$^{-1}$ of Belle II Data

We present the results of the re-discovery of the decay $B^0 \to π^- \ell^+ ν_\ell$ in 34.6 fb$^{-1}$ of Belle II data using hadronic $B$-tagging via the Full Event Interpretation algorithm. We observe 21 signal events on a background of 155 in a fit to the distribution of the square of the missing mass, $M_{\mathrm{miss}}^2$, with a significance of 5.69$σ$, and determine a total branching fraction of (1.58 $\pm$ 0.43$_{\mathrm{stat}}$ $\pm$ 0.07$_{\mathrm{sys}}$) $\times 10^{-4}$.

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arXiv2020

Studies of the semileptonic $\bar B^0\to D^{*+}\ell^-\barν_\ell$ and $B^-\to D^{0}\ell^-\barν_\ell$ decay processes with 34.6 fb$^{-1}$ of Belle II data

We report measurements of the $\bar{B}^0 \to D^{*+} \ell^{-} \barν_l$ and $B^- \to D^{0} \ell^{-} \barν_l$ processes using 34.6 fb$^{-1}$ of collision events recorded by the Belle II experiment at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. For the $B^-\to D^{0}\ell^-\barν_\ell$ channel, we present first studies that isolate this decay from other semileptonic processes and backgrounds. We report a measurement of the $\bar{B}^0 \to D^{*+} \ell^{-} \barν_l$ branching fraction and obtain ${\cal B}(\bar{B}^0 \to D^{*+} \ell^{-} \barν_l) = \left(4.60 \pm 0.05_{\mathrm{stat}}\pm0.17_{\mathrm{syst}} \pm 0.45_{π_s}\right) \%$, in agreement with the world average. Here, the uncertainties are statistical, systematic, and related to slow pion reconstruction, respectively. The systematic uncertainties are limited by the statistics of auxiliary measurements and will improve in the future. We also report differential branching fractions in five bins of the hadronic recoil parameter $w$ for $\bar{B}^0 \to D^{*+} \ell^{-} \barν_l$, unfolded to account for resolution and efficiency effects.

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arXiv2020

A calibration of the Belle II hadronic tag-side reconstruction algorithm with $B \rightarrow X\ell ν$ decays

Tag-side reconstruction is an important method for reconstructing $B$ meson decays with missing energy. The Belle II tag-side reconstruction algorithm, Full Event Interpretation, relies on a hierarchical reconstruction of $B$ meson decays with multivariate classification employed at each stage of reconstruction. Given the large numbers of classifiers employed and decay chains reconstructed, the performance of the algorithm on data and simulation differs significantly. Here, calibration factors are derived for hadronic tag-side $B$ decays by measuring a signal side decay, $B \rightarrow X\ell ν$, in $34.6$ fb$^{-1}$ of Belle II data. For a very loose selection on the tag-side $B$ multivariate classifier, the calibration factors are $0.65 \pm 0.02$ and $0.83 \pm 0.03$ for tag-side $B^{+}$ and $B^{0}$ mesons, respectively.

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