arXiv subjects

V. Chobanova

Publications and source records attributed to V. Chobanova.

At least 37 records · Page 2Linked to original sources

arXiv2017

Precise determination of the CKM matrix element $\left| V_{cb}\right|$ with $\bar B^0 \to D^{*\,+} \, \ell^- \, \bar ν_\ell$ decays with hadronic tagging at Belle

The precise determination of the CKM matrix element $\left| V_{cb}\right|$ is important for carrying out tests of the flavour sector of the Standard Model. In this article we present a preliminary analysis of the $\bar B^0 \to D^{*\,+} \, \ell^- \, \bar ν_\ell$ decay mode and its charge conjugate, selected in events that contain a fully reconstructed $B$-meson, using 772 million $e^+ \, e^- \to Υ(4S) \to B \bar B$ events recorded by the Belle detector at KEKB. Unfolded differential decay rates of four kinematic variables fully describing the $\bar B^0 \to D^{*\,+} \, \ell^- \, \bar ν_\ell$ decay in the $B$-meson rest frame are presented. We measure the total branching fraction $\mathcal{B}( \bar B^0 \to D^{*\,+} \, \ell^- \, \bar ν_\ell ) = \left(4.95 \pm 0.11 \pm 0.22 \right) \times 10^{-2}$, where the errors are statistical and systematic respectively. The value of $\left|V_{cb} \right|$ is determined to be $\left( 37.4 \pm 1.3 \right) \times 10^{-3}$. Both results are in good agreement with current world averages.

hep-ex↗

arXiv2017

Search for $D^{0}$ decays to invisible final states at Belle

We report the result from the first search for $D^0$ decays to invisible final states. The analysis is performed on a data sample of 924 $\rm{fb}^{-1}$ collected at and near the $Υ(4S)$ and $Υ(5S)$ resonances with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The absolute branching fraction is determined using an inclusive $D^0$ sample, obtained by fully reconstructing the rest of the particle system including the other charmed particle. No significant signal yield is observed and an upper limit of $9.4\times 10^{-5}$ is set on the branching fraction of $D^0$ to invisible final states at 90\% confidence level.

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arXiv2016

Likelihood Analysis of the Minimal AMSB Model

We perform a likelihood analysis of the minimal Anomaly-Mediated Supersymmetry Breaking (mAMSB) model using constraints from cosmology and accelerator experiments. We find that a wino-like or a Higgsino-like neutralino LSP, $m_{\tilde χ^0_{1}}$, may provide the cold dark matter (DM) with similar likelihood. The upper limit on the DM density from Planck and other experiments enforces $m_{\tilde χ^0_{1}} \lesssim 3~TeV$ after the inclusion of Sommerfeld enhancement in its annihilations. If most of the cold DM density is provided by the $\tilde χ_0^1$, the measured value of the Higgs mass favours a limited range of $\tan β\sim 5$ (or for $μ> 0$, $\tan β\sim 45$) but the scalar mass $m_0$ is poorly constrained. In the wino-LSP case, $m_{3/2}$ is constrained to about $900~TeV$ and ${m_{\tilde χ^0_{1}}}$ to $2.9\pm0.1~TeV$, whereas in the Higgsino-LSP case $m_{3/2}$ has just a lower limit $\gtrsim 650TeV$ ($\gtrsim 480TeV$) and $m_{\tilde χ^0_{1}}$ is constrained to $1.12 ~(1.13) \pm0.02~TeV$ in the $μ>0$ ($μ<0$) scenario. In neither case can the anomalous magnetic moment of the muon, ${(g-2)_μ}$, be improved significantly relative to its Standard Model (SM) value, nor do flavour measurements constrain the model significantly, and there are poor prospects for discovering supersymmetric particles at the LHC, {though there} are some prospects for direct DM detection. On the other hand, if the ${m_{\tilde χ^0_{1}}}$ contributes only a fraction of the cold DM density, {future LHC $E_T$-based searches for gluinos, squarks and heavier chargino and neutralino states as well as disappearing track searches in the wino-like LSP region will be relevant}, and interference effects enable ${\rm BR}(B_{s, d} \to μ^+μ^-)$ to agree with the data better than in the SM in the case of wino-like DM with $μ> 0$.

hep-ph↗

arXiv2016

Measurement of the branching ratio of $\bar{B}^0 \rightarrow D^{+} τ^- \barν_τ$ relative to $\bar{B}^0 \rightarrow D^{+} \ell^- \barν_{\ell}$ decays with a semileptonic tagging method

We report a measurement of the ratio ${\cal R}(D^*) = {\cal B}(\bar{B}^0 \rightarrow D^{*+} τ^- \barν_τ)/{\cal B}(\bar{B}^0 \rightarrow D^{*+} \ell^- \barν_{\ell})$, where $\ell$ denotes an electron or a muon. The results are based on a data sample containing $772\times10^6$ $B\bar{B}$ pairs recorded at the $Υ(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. We select a sample of $B^0 \bar{B}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{*\mp} \ell^{\pm}$. We measure ${\cal R}(D^*)= 0.302 \pm 0.030({\rm stat)} \pm 0.011({\rm syst)}$, which is within $1.6 σ$ of the Standard Model theoretical expectation, where the standard deviation $σ$ includes systematic uncertainties. We use this measurement to constrain several scenarios of new physics in a model-independent approach.

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arXiv2016

Search for a dark vector gauge boson decaying to $π^+ π^-$ using $η\rightarrow π^+π^- γ$ decays

We report a search for a dark vector gauge boson $U^\prime$ that couples to quarks in the decay chain $D^{*+} \to D^0 π^+, D^0 \to K^0_S η, η\to U^\prime γ$, $U^\prime \to π^+ π^-$. No signal is found and we set a mass-dependent limit on the baryonic fine structure constant of $10^{-3} - 10^{-2}$ in the $U^\prime$ mass range of 290 to 520 MeV/$c^2$. This analysis is based on a data sample of 976 fb$^{-1}$ collected by the Belle experiment at the KEKB asymmetric-energy $e^+e^-$ collider.

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arXiv2016

Observation of Transverse $Λ/\barΛ$ Hyperon Polarization in $e^+e^-$ Annihilation at Belle

We report the first observation of the polarization of $Λ/\barΛ$ hyperons transverse to its production plane in $e^+e^-$ annihilation. We observe a significant polarization that rises with the fractional energy carried by the hyperon as well as its transverse momentum. To define the production plane, we use the direction of the hyperon momentum together with either the thrust axis in the event or the momentum vector of a hadron in the opposite hemisphere. Furthermore, we investigate the contributions to the hyperon polarization from the feed-down from $Σ^0/\barΣ^0$ and $Λ_c^{\pm}$ decays. This measurement uses a dataset of 800.4fb$^{-1}$ collected by the Belle experiment at or near a center-of-mass energy of 10.58 GeV.

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arXiv2016

Study of $\mathbf{B^{0}\rightarrowρ^{+}ρ^{-}}$ decays and implications for the CKM angle $\mathbf{ϕ_2}$

We present a measurement of the branching fraction and the longitudinal polarization fraction of $B^{0}\rightarrowρ^+ρ^-$ decays, as well as the time-dependent $CP$ violating parameters in decays into longitudinally polarized $ρ^{+}ρ^{-}$ pairs with Belle's final data set of $772\times 10^6$ $B\bar{B}$ pairs, at the $Υ(4S)$ resonance, collected at the asymmetric-energy $e^+ e^-$ collider KEKB. We obtain ${\cal B}(B^0 \rightarrow ρ^+ ρ^-)=(28.3\pm 1.5\;(\rm stat) \pm 1.5\;(\rm syst))\times 10^{-6}$, $f_L = 0.988\pm 0.012\;(\rm stat ) \pm0.023\;(\rm syst)$, ${\cal A}_{CP} = 0.00\pm0.10\;(\rm stat ) \pm0.06\;(\rm syst)$, ${\cal S}_{CP} = -0.13\pm0.15\;(\rm stat ) \pm0.05\;(\rm syst).$ We perform an isospin analysis to constrain the CKM angle $ϕ_2$ and obtain two solutions with \begin{eqnarray} ϕ_{2} = (93.7\pm10.6)^{\circ},\nonumber \end{eqnarray} being most compatible with other Standard-Model based fits to the data.

hep-ex↗

arXiv2016

Study of Two-Body $e^+e^- \to B_s^{()}\bar{B}_s^{()}$ Production in the Energy Range from 10.77 to 11.02 GeV

We report results on the studies of the $e^+e^-\to B_s^{(*)}\bar{B}_s^{(*)}$ processes. The results are based on a $121.4$ fb$^{-1}$ data sample collected with the Belle detector at the center-of-mass energy near the $Υ(10860)$ peak and $16.4$ fb$^{-1}$ of data collected at 19 energy points in the range from 10.77 to 11.02 GeV. We observe a clear $e^+e^-\toΥ(10860)\to B_s^{(*)}\bar{B}_s^{(*)}$ signal, with no statistically significant signal of $e^+e^-\to Υ(11020)\to B_s^{(*)}\bar{B}_s^{(*)}$. The relative production ratio of $B_s^*\bar{B}_s^*$, $B_s\bar{B}_s^{*}$, and $B_s\bar{B}_s$ final states at $\sqrt{s}=10.866$ GeV is measured to be $7:$ $0.856\pm0.106(stat.)\pm0.053(syst.):$ $0.645\pm0.094(stat.)^{+0.030}_{-0.033}(syst.)$. An angular analysis of the $B_s^*\bar{B}_s^*$ final state produced at the $Υ(10860)$ peak is also performed.

hep-ex↗

arXiv2016

Measurement of Michel Parameters ($\barη$, $ξκ$) in the radiative leptonic decay of tau at Belle

We present the first measurement of the Michel parameters $\barη$ and $ξκ$ in the radiative leptonic decay of the $τ$ lepton using 703 f$\mathrm{b}^{-1}$ of data collected with the Belle detector at the KEKB $e^+e^-$ collider. The Michel parameters are measured by an unbinned maximum likelihood fit to the kinematic information of $e^+e^-\rightarrowτ^+τ^-\rightarrow (π^+π^0 \barν)(l^-ν\barνγ)$ $(l=e$ or $μ)$. The preliminary values of the measured Michel parameters are $\barη = -2.0 \pm 1.5 \pm 0.8$ and $ξκ= 0.6 \pm 0.4 \pm 0.2$, where the first error is statistical and the second is systematic.

hep-ex↗

arXiv2016

Measurement of $CP$ asymmetry in the $D^{0} \to K^0_S K^0_S$ decay at Belle

We report a measurement of the time-integrated $CP$ asymmetry in the neutral charm meson decay $D^0 \to K^0_S K^0_S$ using 921~fb$^{-1}$ data collected at the $Υ(4S)$ and $Υ(5S)$ resonances with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The observed asymmetry is $$ A_{CP}(D^0 \to K^0_S K^0_S) = (-0.02 \pm 1.53 \pm 0.17) \%, $$ where the first uncertainty is statistical and the second systematic. This latter uncertainty is dominated by the error of the normalisation channel. The result is consistent with Standard Model expectations and improves the uncertainty with respect to previous measurement of this quantity by more than a factor of three.

hep-ex↗

arXiv2016

Measurements of the $Υ(10860)$ and $Υ(11020)$ resonances via $σ(e^+e^-\rightarrowΥ(n{\rm S})π^+π^-)$

We report new measurements of the total cross sections for $e^+e^-\to Υ(n{\rm S})π^+π^-$ ($n$ = 1, 2, 3) and $e^+e^-\to b\bar b$ from a high-luminosity fine scan of the region $\sqrt{s} = 10.63$-$11.05$ GeV with the Belle detector. We observe that the $Υ(n{\rm S})π^+π^-$ spectra have little or no non-resonant component and extract from them the masses and widths of $Υ(10860)$ and $Υ(11020)$ and their relative phase. We find $M_{10860}=(10891.1\pm3.2^{+0.6}_{-1.7})$ MeV/$c^2$ and $Γ_{10860}=(53.7^{+7.1}_{-5.6}\,^{+1.3}_{-5.4})$ MeV and report first measurements $M_{11020}=(10987.5^{+6.4}_{-2.5}\,^{+9.0}_{-2.1})$ MeV/$c^2$, $Γ_{11020}=(61^{+9}_{-19}\,^{+2}_{-20})$ MeV, and $ϕ_{\rm 11020}-ϕ_{\rm 10860} = (-1.0\pm0.4\,^{+1.4}_{-0.1})$ rad.

hep-ex↗

arXiv2016

Evidence for the decay $B^{0}\to ηη$

We report a search for $B^{0}\to ηη$ with a data sample corresponding to an integrated luminosity of $698 \,{\rm fb}^{-1}$ containing $753 \times 10^{6}$ $B\bar{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The branching fraction is measured to be $\mathcal{B}(B^{0} \to ηη) = (7.6^{+2.7 +1.4}_{-2.3 -1.6}) \times 10^{-7}$ at the level of 3.3 standard deviations above zero, which provides the first evidence for the decay $B^{0} \to ηη$.

hep-ex↗

arXiv2016

Measurement of the $τ$ lepton polarization in the decay ${\bar B} \rightarrow D^* τ^- {\bar ν_τ}$

We report the first measurement of the $τ$ lepton polarization in the decay ${\bar B} \rightarrow D^* τ^- {\barν_τ}$ as well as a new measurement of the ratio of the branching fractions $R(D^{*}) = \mathcal{B}({\bar B} \rightarrow D^* τ^- {\barν_τ}) / \mathcal{B}({\bar B} \rightarrow D^* \ell^- {\barν_{\ell}})$, where $\ell^-$ denotes an electron or a muon, with the decays $τ^- \rightarrow π^- ν_τ$ and $τ^- \rightarrow ρ^- ν_τ$. We use the full data sample of $772 \times 10^6$ $B{\bar B}$ pairs accumulated with the Belle detector at the KEKB electron-positron collider. Our preliminary results, $R(D^*) = 0.276 \pm 0.034{\rm (stat.)} ^{+0.029} _{-0.026}{\rm (syst.)}$ and $P_τ = -0.44 \pm 0.47 {\rm (stat.)} ^{+0.20} _{-0.17} {\rm (syst.)}$, are consistent with the theoretical predictions of the Standard Model within $0.6$ standard deviation.

hep-ex↗

arXiv2016

Measurement of the CKM angle $φ_1$ in $B^0\to\bar{D}{}^{(*)0}h^0$, $\bar{D}{}^0\to K_S^0π^+π^-$ decays with time-dependent binned Dalitz plot analysis

We report a measurement of the CP violation parameter $φ_1$ obtained in a time-dependent analysis of $B^0\to\bar{D}{}^{(*)0}h^0$ decays followed by $\bar{D}{}^0\to K_S^0π^+π^-$ decay. A model-independent measurement is performed using the binned Dalitz plot technique. The measured value is $φ_1 = 11.7^{\circ}\pm7.8^{\circ}({\rm stat.})\pm 2.1^{\circ}({\rm syst.})$. Treating $\sin{2φ_1}$ and $\cos{2φ_1}$ as independent parameters, we obtain $\sin{2φ_1} = 0.43\pm 0.27({\rm stat.})\pm 0.08({\rm syst.})$ and $\cos{2φ_1} = 1.06\pm 0.33({\rm stat.})^{+0.21}_{-0.15}({\rm syst.})$. The results are obtained with a full data sample of $772 \times 10^6 B\bar{B}$ pairs collected near the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider.

hep-ex↗

arXiv2016

Measurement of $e^+e^- \to π^+π^-ψ(2S)$ via Initial State Radiation at Belle

We report measurement of the cross section of $e^+e^-\to π^+π^-ψ(2S)$ between 4.0 and $5.5 {\rm GeV}$, based on an analysis of initial state radiation events in a $980 \rm fb^{-1}$ data sample recorded with the Belle detector. The properties of the $Y(4360)$ and $Y(4660)$ states are determined. Fitting the mass spectrum of $π^+π^-ψ(2S)$ with two coherent Breit-Wigner functions, we find two solutions with identical mass and width but different couplings to electron-positron pairs: $M_{Y(4360)} = (4347\pm 6\pm 3) {\rm MeV}/c^2$, $Γ_{Y(4360)} = (103\pm 9\pm 5) {\rm MeV}$, $M_{Y(4660)} = (4652\pm10\pm 8) {\rm MeV}/c^2$, $Γ_{Y(4660)} = (68\pm 11\pm 1) \rm MeV$; and ${\cal{B}}[Y(4360)\to π^+π^-ψ(2S)]\cdot Γ_{Y(4360)}^{e^+e^-} = (10.9\pm 0.6\pm 0.7) \rm eV$ and ${\cal{B}}[Y(4660)\to π^+π^-ψ(2S)]\cdot Γ_{Y(4660)}^{e^+e^-} = (8.1\pm 1.1\pm 0.5) \rm eV$ for one solution; or ${\cal{B}}[Y(4360)\to π^+π^-ψ(2S)]\cdot Γ_{Y(4360)}^{e^+e^-} = (9.2\pm 0.6\pm 0.6) \rm eV$ and ${\cal{B}}[Y(4660)\to π^+π^-ψ(2S)]\cdot Γ_{Y(4660)}^{e^+e^-} = (2.0\pm 0.3\pm 0.2) \rm eV$ for the other. Here, the first errors are statistical and the second systematic. Evidence for a charged charmoniumlike structure at $4.05 {\rm GeV}/c^2$ is observed in the $π^{\pm}ψ(2S)$ intermediate state in the $Y(4360)$ decays.

hep-ex↗

arXiv2016

Energy scan of the $e^+e^- \to h_b(nP)π^+π^-$ $(n=1,2)$ cross sections and evidence for $Υ(11020)$ decays into charged bottomonium-like states

Using data collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider, we measure the energy dependence of the $e^+e^- \to h_b(nP)π^+π^-$ $(n=1,2)$ cross sections from thresholds up to $11.02\,$GeV. We find clear $Υ(10860)$ and $Υ(11020)$ peaks with little or no continuum contribution. We study the resonant substructure of the $Υ(11020) \to h_b(nP)π^+π^-$ transitions and find evidence that they proceed entirely via the intermediate isovector states $Z_b(10610)$ and $Z_b(10650)$. The relative fraction of these states is loosely constrained by the current data: the hypothesis that only $Z_b(10610)$ is produced is excluded at the level of 3.3 standard deviations, while the hypothesis that only $Z_b(10650)$ is produced is not excluded at a significant level.

hep-ex↗

arXiv2016

Study of χ_{bJ}(1P) Properties in the Radiative Υ(2S) Decays

We report a study of radiative decays of χ_{bJ}(1P)(J=0,1,2) mesons into 74 hadronic final states comprising charged and neutral pions, kaons, protons; out of these, 41 modes are observed with at least 5 standard deviation significance. Our measurements not only improve the previous measurements by the CLEO Collaboration but also lead to first observations in many new modes. The large sample allows us to probe the total decay width of the χ_{b0}(1P). In the absence of a statistically significant result, a 90% confidence-level upper limit is set on the width at Γ_{total}< 2.4 MeV. Our results are based on 24.7 fb^{-1} of e+e- collision data recorded by the Belle detector at the Υ(2S) resonance, corresponding to (157.8\pm3.6)\times10^6 Υ(2S) decays.

hep-ex↗

arXiv2016

Studies of charmed strange baryons in the $ΛD$ final state at Belle

We report the discovery of $Ξ_{c}(3055)^{0}$, observed by its decay into the final state $ΛD^{0}$, and present the first observation and evidence of the decays of $Ξ_c(3055)^{+}$ and $Ξ_c(3080)^{+}$ into $ΛD^{+}$. We also perform a combined analysis of the $ΛD^{+}$ with the $Σ_{c}^{++}K^{-}$ and $Σ_{c}^{\ast ++}K^{-}$ decay modes to measure the ratios of branching fractions, masses and widths with improved accuracy. We measure the ratios of branching fractions ${\cal B}(Ξ_{c}(3055)^{+} \to ΛD^{+})/{\cal B}(Ξ_{c}(3055)^{+} \to Σ_{c}^{++}K^{-})=5.09\pm1.01\pm0.76$, ${\cal B}(Ξ_{c}(3080)^{+} \to ΛD^{+})/{\cal B}(Ξ_{c}(3080)^{+} \to Σ_{c}^{++}K^{-})=1.29\pm0.30\pm0.15$, and ${\cal B}(Ξ_{c}(3080)^{+} \to Σ_{c}^{\ast ++}K^{-})/{\cal B}(Ξ_{c}(3080)^{+} \to Σ_{c}^{++}K^{-})=1.07\pm0.27\pm0.01$, where the uncertainties are statistical and systematic. The analysis is performed using a 980 fb$^{-1}$ data sample collected with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider.

hep-ex↗