arXiv subjects

V. Bansal

Publications and source records attributed to V. Bansal.

At least 91 records · Page 5Linked to original sources

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.

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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.

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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.

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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.

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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.

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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 $B^{0} \to ηη$.

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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.

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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.

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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.

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arXiv2016

Angular analysis of $B^0 \to K^\ast(892)^0 \ell^+ \ell^-$

We present a measurement of angular observables, $P_4'$, $P_5'$, $P_6'$, $P_8'$, in the decay $B^0 \to K^\ast(892)^0 \ell^+ \ell^-$, where $\ell^+\ell^-$ is either $e^+e^-$ or $μ^+μ^-$. The analysis is performed on a data sample corresponding to an integrated luminosity of $711~\mathrm{fb}^{-1}$ containing $772\times 10^{6}$ $B\bar B$ pairs, collected at the $Υ(4S)$ resonance with the Belle detector at the asymmetric-energy $e^+e^-$ collider KEKB. Four angular observables, $P_{4,5,6,8}'$ are extracted in five bins of the invariant mass squared of the lepton system, $q^2$. We compare our results for $P_{4,5,6,8}'$ with Standard Model predictions including the $q^2$ region in which the LHCb collaboration reported the so-called $P_5'$ anomaly.

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arXiv2016

Inclusive and exclusive measurements of $B$ decays to $χ_{c1}$ and $χ_{c2}$ at Belle

We report inclusive and exclusive measurements for $χ_{c1}$ and $χ_{c2}$ production in $B$ decays. We measure $\mathcal{B}(B \to χ_{c1} X)$= $(3.03 \pm 0.05(\mbox{stat}) \pm 0.24(\mbox{syst})) \times 10^{-3}$ and $\mathcal{B}(B \to χ_{c2} X)$= $(0.70 \pm 0.06(\mbox{stat}) \pm 0.10(\mbox{syst})) \times 10^{-3}$. For the first time, $χ_{c2}$ production in exclusive $B$ decays in the modes $B^0 \to χ_{c2}π^- K^+$ and $B^+ \to χ_{c2} π^+ π^- K^+$ has been observed, along with first evidence for the $B^+ \to χ_{c2} π^+ K_S^0$ decay mode. For $χ_{c1}$ production, we report the first observation in the $B^+ \to χ_{c1} π^+ π^- K^+$, $B^0 \to χ_{c1} π^+ π^- K_S^0$ and $B^0 \to χ_{c1} π^0 π^- K^+$ decay modes. Using these decay modes, we observe a difference in the production mechanism of $χ_{c2}$ in comparison to $χ_{c1}$ in $B$ decays. In addition, we report searches for $X(3872)$ and $χ_{c1}(2P)$ in the $B^+ \to (χ_{c1} π^+ π^-) K^+$ decay mode. The reported results use $772 \times 10^{6}$ $B\overline{B}$ events collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider.

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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 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 $σ$ is the standard deviation including systematic uncertainties.

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arXiv2016

Measurement of the decay $B\to D\ellν_\ell$ in fully reconstructed events and determination of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cb}|$

We present a determination of the magnitude of the Cabibbo-Kobayashi-Maskawa matrix element $|V_{cb}|$ using the decay $B\to D\ellν_\ell$ ($\ell=e,μ$) based on 711 fb$^{-1}$ of $e^+e^-\to Υ(4S)$ data recorded by the Belle detector and containing $772 \times 10^6$ $B\bar{B}$ pairs. One $B$ meson in the event is fully reconstructed in a hadronic decay mode while the other, on the signal side, is partially reconstructed from a charged lepton and either a $D^+$ or $D^0$ meson in a total of 23 hadronic decay modes. The isospin-averaged branching fraction of the decay $B\to D\ellν_\ell$ is found to be $\mathcal{B}(B^0 \to D^- \ell^+ ν_{\ell})=(2.31\pm 0.03(\mathrm{stat})\pm 0.11(\mathrm{syst}))\%$. Analyzing the differential decay rate as a function of the hadronic recoil with the parameterization of Caprini, Lelouch and Neubert and using the form-factor prediction $\mathcal{G}(1)=1.0541\pm 0.0083$ calculated by FNAL/MILC, we obtain $η_\mathrm{EW}|V_{cb}|=(40.12\pm 1.34)\times 10^{-3}$, where $η_\mathrm{EW}$ is the electroweak correction factor. Alternatively, assuming the model-independent form-factor parameterization of Boyd, Grinstein and Lebed and using lattice QCD data from the FNAL/MILC and HPQCD collaborations, we find $η_\mathrm{EW}|V_{cb}|=(41.10 \pm 1.14)\times 10^{-3}$.

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arXiv2015

Measurement of the branching ratio of $\bar{B} \to D^{(\ast)} τ^- \barν_τ$ relative to $\bar{B} \to D^{(\ast)} \ell^- \barν_\ell$ decays with hadronic tagging at Belle

We report a measurement of the branching fraction ratios R(D(*)) of Bbar -> D(*) tau- nubar_tau relative to Bbar -> D()* l- nubar_l (where l = e or mu) using the full Belle data sample of 772 x 10^6 BBbar pairs collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. The measured values are R(D)= 0.375 +- 0.064(stat.) +- 0.026(syst.) and R(D*) = 0.293 +- 0.038(stat.) +- 0.015(syst.). The analysis uses hadronic reconstruction of the tag-side B meson and purely leptonic tau decays. The results are consistent with earlier measurements and do not show a significant deviation from the standard model prediction.

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arXiv2015

Observation of $X(3872)$ in $B \to X(3872) Kπ$ decays

We report the first observation of $B^0 \to X(3872) (K^{+}π^{-})$ and evidence for $B^+ \to X(3872) (K^{0}π^{+})$. We measure the product of branching fractions for the former to be ${\cal B}(B^0 \to X(3872) (K^+ π^-)) \times {\cal B}(X(3872) \to J/ψπ^+ π^-) = (7.9 \pm 1.3(\mbox{stat.})\pm 0.4(\mbox{syst.})) \times 10^{-6}$ and find that $B^{0}\to X(3872) K^{*}(892)^{0}$ does not dominate the $B^{0}\to X(3872)K^{+}π^{-}$ decay mode. We also measure ${\cal B}(B^+ \to X(3872) (K^0 π^+)) \times {\cal B}(X(3872) \to J/ψπ^+ π^-) = (10.6 \pm 3.0(\mbox{stat.}) \pm 0.9(\mbox{syst.})) \times 10^{-6}$. This study is based on the full data sample of 711~fb$^{-1}$ ($772\times 10^6 B\bar B$ pairs) collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB collider.

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arXiv2015

Measurement of the branching fraction of B^+ -> tau^+ nu_tau decays with the semileptonic tagging method

We report a measurement of the branching fraction of B^+ -> tau^+ nu_tau decays using a data sample of 772 x 10^6 BBbar pairs, collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We reconstruct the accompanying B meson in a semileptonic decay and detect the recoiling B candidate in the decay channel B^+ -> tau^+ nu_tau. We obtain a branching fraction of BR(B^+ -> tau^+ nu_tau) = [1.25 +- 0.28 (stat.) +- 0.27 (syst.)] x 10^-4. This result is in good agreement with previous measurements and the expectation from the calculation based on the Standard Model.

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