arXiv subjects

R. Kumar

Publications and source records attributed to R. Kumar.

At least 109 records · Page 6Linked to original sources

arXiv2021

Measurement of Differential Branching Fractions of Inclusive ${B \to X_u \, \ell^+\, ν_{\ell}}$ Decays

The first measurements of differential branching fractions of inclusive semileptonic ${B \to X_u \, \ell^+\, ν_{\ell}}$ decays are performed using the full Belle data set of 711 fb$^{-1}$ of integrated luminosity at the $Υ(4S)$ resonance and for $\ell = e, μ$. Differential branching fractions are reported as a function of the lepton momentum, the four-momentum-transfer squared, light-cone momenta, the hadronic mass, and the hadronic mass squared. They are obtained by subtracting the backgrounds from semileptonic ${B \to X_c \, \ell^+\, ν_{\ell}}$ decays and other processes, and corrected for resolution and acceptance effects. The measured distributions are compared to predictions from inclusive and hybrid ${B \to X_u \, \ell^+\, ν_{\ell}}$ calculations.

hep-ex↗

arXiv2021

Search for $B^0$ meson decays into $Λ$ and missing energy with a hadronic tagging method at Belle

We present a search for the decays of $B^0$ mesons into a final state containing a $Λ$ baryon and missing energy. These results are obtained from a $711\text{fb}^{-1}$ data sample that contains $772 \times 10^6$ $B{\kern 0.18em}\overline{\kern -0.18em B}$ pairs and was collected near the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We use events in which one $B$ meson is fully reconstructed in a hadronic decay mode and require the remainder of the event to consist of only a single $Λ$. No evidence for these decays is found and we set $90\%$ confidence level upper limits on the branching fractions in the range $2.1$-$3.8\times 10^{-5}$. This measurement provides the world's most restrictive limits, with implications for baryogenesis and dark matter production.

hep-ex↗

arXiv2021

Classical Spin Liquid State in the $S=\frac{5}{2}$ Heisenberg Kagomé Antiferromagnet Li$_9$Fe$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$

We investigate the low temperature magnetic properties of a $S=\frac{5}{2}$ Heisenberg kagomé antiferromagnet, the layered monodiphosphate Li$_9$Fe$_3$(P$_2$O$_7$)$_3$(PO$_4$)$_2$, using magnetization measurements and $^{31}$P nuclear magnetic resonance. An antiferromagnetic-type order sets in at $T_{\rm N}=1.3$ K and a characteristic magnetization plateau is observed at 1/3 of the saturation magnetization below $T^* \sim 5$ K. A moderate $^{31}$P NMR line broadening reveals the development of anisotropic short-range correlations within the plateau phase concomitantly with a gapless spin-lattice relaxation time $T_1 \sim k_B T / \hbar S$, which both point to the presence of a semiclassical nematic spin liquid state predicted for the Heisenberg kagomé antiferromagnetic model or to the persistence of the zero-energy modes of the kagome lattice under large magnetic fields.

cond-mat.str-el↗

arXiv2021

Measurement of the branching fraction of $Λ_c^+ \to p ω$ decay at Belle

Using 980.6 $\rm fb^{-1}$ of data collected with the Belle detector operating at the KEKB asymmetric-energy $e^+e^-$ collider, we present a measurement of the branching fraction of the singly Cabibbo-suppressed decay $Λ_c^+ \to p ω$. A clear $Λ_c^+$ signal is observed for $Λ_c^+ \to p ω$ with a statistical significance of 9.1 standard deviations, and we measure the ratio of branching fractions ${\cal B}(Λ_c^+ \to p ω)/{\cal B}(Λ_c^+ \to p K^- π^+) = (1.32 \pm 0.12 (\rm stat) \pm 0.10 (\rm syst))\times 10^{-2}$, from which we infer the branching fraction ${\cal B}(Λ_c^+ \to p ω) = (8.27 \pm 0.75 (\rm stat) \pm 0.62 (\rm syst) \pm 0.42 (\rm ref))\times 10^{-4}$. The first quoted uncertainty is statistical, the second systematic, and the third from the reference mode $Λ_c^+ \to p K^- π^+$.

hep-ex↗

arXiv2021

Search for lepton-flavor-violating tau-lepton decays to $\ellγ$ at Belle

Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays $τ^{\pm}\rightarrow\ell^{\pm}γ$ ($\ell=e,μ$) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via $τ^{\pm}\rightarrow\ell^{\pm}γ$ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb$^{-1}$, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, $\mathcal{B}(τ^{\pm}\rightarrow μ^{\pm}γ)$ $\leq$ $4.2\times10^{-8}$ and $\mathcal{B}(τ^{\pm}\rightarrow e^{\pm}γ)$ $\leq$ $5.6\times10^{-8}$, are set at 90\% confidence level.

hep-ex↗

arXiv2021

Measurements of the branching fractions of the semileptonic decays $Ξ_{c}^{0} \to Ξ^{-} \ell^{+} ν_{\ell}$ and the asymmetry parameter of $Ξ_{c}^{0} \to Ξ^{-} π^{+}$

Using data samples of 89.5 and 711 fb$^{-1}$ recorded at energies of $\sqrt{s}=10.52$ and $10.58$ GeV, respectively, with the Belle detector at the KEKB $e^+e^-$ collider, we report measurements of branching fractions of semileptonic decays $Ξ_{c}^{0} \to Ξ^{-} \ell^{+} ν_{\ell}$ ($\ell=e$ or $μ$) and the $CP$-asymmetry parameter of $Ξ_{c}^{0} \to Ξ^{-} π^{+}$ decay. The branching fractions are measured to be ${\cal B}(Ξ_{c}^{0} \to Ξ^{-} e^{+} ν_{e})=(1.31 \pm 0.04 \pm 0.07 \pm 0.38)\%$ and ${\cal B}(Ξ_{c}^{0} \to Ξ^{-} μ^{+} ν_μ)=(1.27 \pm 0.06 \pm 0.10 \pm 0.37)\%$, and the decay parameter $α_{Ξπ}$ is measured to be $0.63 \pm 0.03 \pm 0.01$ with much improved precision compared to the current world average. The corresponding ratio ${\cal B}(Ξ_{c}^{0} \to Ξ^{-} e^{+} ν_{e})/{\cal B}(Ξ_{c}^{0} \to Ξ^{-} μ^{+} ν_μ)$ is $1.03 \pm 0.05\pm 0.07$, which is consistent with the expectation of lepton flavor universality. The first measured asymmetry parameter ${\cal A}_{CP} = (α_{Ξ^{-}π^{+}} + α_{\barΞ^{+}π^{-}})/(α_{Ξ^{-}π^{+}} - α_{\barΞ^{+}π^{-}}) = 0.024 \pm 0.052 \pm 0.014$ is found to be consistent with zero. The first and the second uncertainties above are statistical and systematic, respectively, while the third ones arise due to the uncertainty of the $Ξ_{c}^{0} \to Ξ^{-} π^+$ branching fraction.

hep-ex↗

arXiv2021

Study of $e^+e^-\to Υ(\rm1S,2S)η$ and $e^+e^-\to Υ(\rm 1S)η^{\prime}$ at $\sqrt{s}=10.866$ GeV with the Belle detector

We report the first observation of the processes $e^+e^-\toΥ(\rm 1S,2S)η$ at $\sqrt{s}=10.866$~GeV with a $10.2σ$ and $16.5σ$ significance respectively. The measured Born cross sections are $σ(e^+e^- \to Υ(2S)η)=2.07 \pm 0.21 \pm 0.19$~pb, and $σ(e^+e^- \to Υ(\rm 1S)η)=0.42 \pm 0.08 \pm 0.04$~pb. We also set the upper limit on the cross section of the process $e^+e^- \to Υ(\rm 1S)η^{\prime}$ to be $σ(e^+e^- \to Υ(\rm 1S)η^{\prime})<0.035$~pb at $90\%$ CL. The results are obtained with the data sample collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider in the energy range from $10.63$~GeV to $11.02$~GeV.

hep-ex↗

arXiv2021

Search for $B_{s}^{0} \rightarrow η^{\prime} X_{s\bar{s}}$ at Belle using a semi-inclusive method

We report the first search for the penguin-dominated process $B_{s}^{0} \rightarrow η^{\prime} X_{s\bar{s}}$ using a semi-inclusive method. A 121.4 $\mathrm{fb}^{-1}$ integrated luminosity $Υ(5S)$ data set collected by the Belle experiment, at the KEKB asymmetric-energy $e^+e^-$ collider, is used. We observe no statistically significant signal and including all uncertainties, we set a 90\% confidence level upper limit on the partial branching fraction at 1.4 $\times$ 10$^{-3}$ for $M(X_{s\bar{s}})$ $\leq$ 2.4 GeV/$c^{2}$.

hep-ex↗

arXiv2021

Measurement of the Branching Fraction of the Decay $\boldsymbol{B^{+}\toπ^{+}π^{-}\ell^{+}ν_\ell}$ in Fully Reconstructed Events at Belle

We present an analysis of the exclusive $B^{+}\toπ^{+}π^{-}\ell^{+}ν_{\ell}$ decay, where $\ell$ represents an electron or a muon, with the assumption of charge-conjugation symmetry and lepton universality. The analysis uses the full $Υ(4S)$ data sample collected by the Belle detector, corresponding to 711 fb$^{-1}$ of integrated luminosity. We select the events by fully reconstructing one $B$ meson in hadronic decay modes, subsequently determining the properties of the other $B$ meson. We extract the signal yields using a binned maximum-likelihood fit to the missing-mass squared distribution in bins of the invariant mass of the two pions or the momentum transfer squared. We measure a total branching fraction of ${{\cal B}(B^{+}\to π^{+}π^{-}\ell^{+}ν_{\ell})= [22.7 ^{+1.9}_{-1.6} (\mathrm{stat}) \pm 3.5(\mathrm{syst}) ]\times 10^{-5}}$, where the uncertainties are statistical and systematic, respectively. This result is the first reported measurement of this decay.

hep-ex↗

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.

hep-ex↗

arXiv2021

Overview of KAGRA: Calibration, detector characterization, physical environmental monitors, and the geophysics interferometer

KAGRA is a newly built gravitational wave observatory, a laser interferometer with a 3 km arm length, located in Kamioka, Gifu, Japan. In this series of articles, we present an overview of the baseline KAGRA, for which we finished installing the designed configuration in 2019. This article describes the method of calibration (CAL) used for reconstructing gravitational wave signals from the detector outputs, as well as the characterization of the detector (DET). We also review the physical environmental monitors (PEM) system and the geophysics interferometer (GIF). Both are used for characterizing and evaluating the data quality of the gravitational wave channel. They play important roles in utilizing the detector output for gravitational wave searches. These characterization investigations will be even more important in the near future, once gravitational wave detection has been achieved, and in using KAGRA in the gravitational wave astronomy era.

gr-qc↗

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 $+0.12\pm0.12({\rm stat})\pm0.01({\rm syst})$.

hep-ex↗

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.

hep-ex↗

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.

hep-ex↗

arXiv2020

Supersymmetric Quantum Spherical Model: A Model for Hodge Theory

We discuss various symmetry properties of the N = 2 supersymmetric quantum spin model in one (0 + 1)-dimension of spacetime and provide their relevance in the realm of the mathematics of differential geometry. We show one-to-one mapping between the continuous symmetry transformations (and corresponding generators) and de Rham cohomological operators of differential geometry. One of the novel observations is the existence of discrete symmetry transformations which play a crucial role in providing the physical realization of the Hodge duality ($\star$) operation. Thus, the present model provides a toy model for the Hodge theory.

hep-th↗

arXiv2020

Study of Electromagnetic Decays of Orbitally Excited $Ξ_c$ Baryons

Using 980 $fb^-1$ of data collected with the Belle detector operating at the KEKB asymmetric-energy e^+e^- collider, we report a study of the electromagnetic decays of excited {charmed baryons} $Ξ_c(2790)$ and $Ξ_c(2815)$. A clear signal (8.6 standard deviations) is observed for $Ξ_c(2815)^0 \to Ξ_c^0γ$, and we measure: $B[Ξ_c(2815)^0 \to Ξ_c^0γ]/B[Ξ_c(2815)^0 \to Ξ_c(2645)^+π^- \to Ξ_c^0π^+π^-] = 0.41 \pm 0.05 \pm 0.03$. We also present evidence (3.8 standard deviations) for the similar decay of the $Ξ_c(2790)^0$ and measure: $B[Ξ_c(2790)^{0}\toΞ_c^{0}γ]/B[Ξ_c(2790)^0\toΞ_c^{\prime +}π^{-}\toΞ_c^{+}γπ^-] = 0.13 \pm 0.03 \pm 0.02$. The first quoted uncertainties are statistical and the second systematic. We find no hint of the analogous decays of the $Ξ_c(2815)^+$ and $Ξ_c(2790)^+$ baryons and set upper limits at the 90% confidence level of: $B[Ξ_c(2815)^{+}\toΞ_c^{+}γ]/B[Ξ_c(2815)^+\toΞ_c(2645)^0π^+\toΞ_c^+π^-π^+] < 0.09,$ and $B[Ξ_c(2790)^{+}\toΞ_c^{+}γ]/B[Ξ_c(2790)^+\toΞ_c^{\prime 0}π^{+}\toΞ_c^{0}γπ^+] < 0.06.$ Approximate values of the partial widths of the decays are extracted, which can be used to discriminate between models of the underlying quark structure of these excited states.

hep-ex↗

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.

hep-ex↗

arXiv2020

Test of lepton-flavor universality in ${B\to K^\ast\ell^+\ell^-}$ decays at Belle

We present a measurement of $R_{K^{\ast}}$, the branching fraction ratio ${{\cal B}(B\to K^\ast μ^+ μ^-)}$/ ${{\cal B}(B\to K^\ast e^+ e^-)}$, for both charged and neutral $B$ mesons. The ratio for the charged case, $R_{K{^{\ast +}}}$, is the first measurement ever performed. In addition, we report absolute branching fractions for the individual modes in bins of the squared dilepton invariant mass, $q^2$. The analysis is based on a data sample of $711~\mathrm{fb}^{-1}$, containing $772\times 10^{6}$ $B\bar B$ events, recorded at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. The obtained results are consistent with Standard Model expectations.

hep-ex↗