arXiv subjects

B. G. Cheon

Publications and source records attributed to B. G. Cheon.

At least 271 records · Page 15Linked to original sources

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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arXiv2020

Rediscovery of $B \to ϕK^{()}$ decays and measurement of the longitudinal polarization fraction $f_L$ in $B \to ϕK^{}$ decays using the Summer 2020 Belle II dataset

We utilize a sample of 34.6 fb$^{-1}$, collected by the Belle II experiment at the SuperKEKB asymmetric energy $e^+e^-$ collider, to search for the $B^+ \to ϕK^+$, $B^+ \to ϕK^{*+}$, $B^0 \to ϕK^0_S$, and $B^0 \to ϕK^{*0}$ decays. Charmless hadronic $B$ decays represent an important part of the Belle II physics program, and are an ideal benchmark to test the detector capabilities in terms of tracking efficiency, charged particle identification, vertexing, and advanced analysis techniques. Each channel is observed with a significance that exceeds 5 standard deviations, and we obtain measurements of their branching ratios that are in good agreement with the world averages. For the $B \to ϕK^*$ modes, we also perform a measurement of the longitudinal polarization fraction $f_L$.

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arXiv2020

Measurement of two-particle correlations in hadronic $e^{+}e^{-}$ collisions at Belle

The enhancement of charged-particle pairs with large pseudorapidity difference and small azimuthal angle difference, often referred to as the ``ridge signal'', is a phenomenon widely observed in high multiplicity proton-proton, proton-ion and deutron-ion collisions, which is not yet fully understood. In heavy-ion collisions, the hydrodynamic expansion of the Quark-Gluon Plasma is one of the possible explanations of the origin of the ridge signal. Measurements in the $e^+e^-$ collision system, without the complexities introduced by hadron structure in the initial state, can serve as a complementary probe to examine the formation of a ridge signal. The first measurement of two-particle angular correlation functions in high multiplicity $e^+e^-$ collisions at $\sqrt{s}=10.52$ GeV is reported. The hadronic $e^+e^-$ annihilation data collected by the Belle detector at KEKB are used in this study. Two-particle angular correlation functions are measured over the full azimuth and large pseudorapidity intervals which are defined by either the electron beam axis or the event thrust as a function of charged particle multiplicity. The measurement in the event thrust analysis, with mostly outgoing quark pairs determining the reference axis, is sensitive to the region of additional soft gluon emissions. No significant ridge signal is observed with either coordinates analyses. Near side jet correlations appear to be absent in the thrust axis analysis. The measurements are compared to predictions from various event generators and expected to provide new constraints to the phenomenological models in the low energy regime.

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arXiv2020

First flavor tagging calibration using 2019 Belle II data

We report on the first calibration of the standard Belle II $B$-flavor tagger using the full data set collected at the $Υ(4{\rm S})$ resonance in 2019 with the Belle II detector at the SuperKEKB collider, corresponding to 8.7 fb$^{-1}$ of integrated luminosity. The calibration is performed by reconstructing various hadronic charmed $B$-meson decays with flavor-specific final states. We use simulation to optimize our event selection criteria and to train the flavor tagging algorithm. We determine the tagging efficiency and the fraction of wrongly identified tag-side $B$~candidates from a measurement of the time-integrated $B^0-\overline{B}^0$ mixing probability. The total effective efficiency is measured to be $\varepsilon_{\rm eff} = \big(33.8 \pm 3.6(\text{stat}) \pm 1.6(\text{sys})\big)\%$, which is in good agreement with the predictions from simulation and comparable with the best one obtained by the Belle experiment. The results show a good understanding of the detector performance and offer a basis for future calibrations.

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arXiv2020

First Determination of the Spin and Parity of a Charmed-Strange Baryon, $Ξ_{c}(2970)^+$

We report results from a study of the spin and parity of $Ξ_{c}(2970)^+$ using a $980~\mathrm{fb^{-1}}$ data sample collected by the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The decay angle distributions in the chain $Ξ_{c}(2970)^+ \to Ξ_c(2645)^{0}π^{+} \to Ξ_c^{+}π^{-}π^{+}$ are analyzed to determine the spin of this charmed-strange baryon. The angular distributions strongly favor the $Ξ_{c}(2970)^+$ spin $J =1/2$ over $3/2$ or $5/2$, under an assumption that the lowest partial wave dominates in the decay. We also measure the ratio of $Ξ_{c}(2970)^+$ decay branching fractions $R={\mathcal{B}[ Ξ_{c}(2970)^+ \to Ξ_c(2645)^{0}π^{+} ]} / { \mathcal{B}[ Ξ_{c}(2970)^+ \to Ξ_c^{\prime0}π^{+} ]} =1.67 \pm 0.29\mathrm{(stat.)}^{ +0.15}_{ -0.09}\mathrm{(syst.)} \pm 0.25\mathrm{(IS)}$, where the last uncertainty is due to possible isospin-symmetry-breaking effects. This $R$ value favors the spin-parity $J^P=1/2^+$ with the spin of the light-quark degrees of freedom $s_{l}=0$. This is the first determination of the spin and parity of a charmed-strange baryon.

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arXiv2020

Search for Large-scale Anisotropy on Arrival Directions of Ultra-high-energy Cosmic Rays Observed with the Telescope Array Experiment

Motivated by the detection of a significant dipole structure in the arrival directions of ultrahigh-energy cosmic rays above 8 EeV reported by the Pierre Auger Observatory (Auger), we search for a large-scale anisotropy using data collected with the surface detector array of the Telescope Array Experiment (TA). With 11 years of TA data, a dipole structure in a projection of the right ascension is fitted with an amplitude of 3.3+- 1.9% and a phase of 131 +- 33 degrees. The corresponding 99% confidence-level upper limit on the amplitude is 7.3%. At the current level of statistics, the fitted result is compatible with both an isotropic distribution and the dipole structure reported by Auger.

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arXiv2020

Evidence for a Supergalactic Structure of Magnetic Deflection Multiplets of Ultra-High Energy Cosmic Rays

Evidence for a large-scale supergalactic cosmic ray multiplet (arrival directions correlated with energy) structure is reported for ultra-high energy cosmic ray (UHECR) energies above 10$^{19}$ eV using seven years of data from the Telescope Array (TA) surface detector and updated to 10 years. Previous energy-position correlation studies have made assumptions regarding magnetic field shapes and strength, and UHECR composition. Here the assumption tested is that, since the supergalactic plane is a fit to the average matter density of the local Large Scale Structure (LSS), UHECR sources and intervening extragalactic magnetic fields are correlated with this plane. This supergalactic deflection hypothesis is tested by the entire field-of-view (FOV) behavior of the strength of intermediate-scale energy-angle correlations. These multiplets are measured in spherical cap section bins (wedges) of the FOV to account for coherent and random magnetic fields. The structure found is consistent with supergalactic deflection, the previously published energy spectrum anisotropy results of TA (the hotspot and coldspot), and toy-model simulations of a supergalactic magnetic sheet. The seven year data post-trial significance of this supergalactic structure of multiplets appearing by chance, on an isotropic sky, is found by Monte Carlo simulation to be 4.2$σ$. The ten years of data post-trial significance is 4.1$σ$. Furthermore, the starburst galaxy M82 is shown to be a possible source of the TA Hotspot, and an estimate of the supergalactic magnetic field using UHECR measurements is presented.

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arXiv2020

Dalitz analysis of $D^{0}\to K^{-}π^{+}η$ decays at Belle

We present the results of the first Dalitz plot analysis of the decay $D^{0}\to K^{-}π^{+}η$. The analysis is performed on a data set corresponding to an integrated luminosity of 953 $\rm{fb}^{-1}$ collected by the Belle detector at the asymmetric-energy $e^{+}e^{-}$ KEKB collider. The Dalitz plot is well described by a combination of the six resonant decay channels $\bar{K}^{*}(892)^0η$, $K^{-}a_0(980)^+$, $K^{-}a_2(1320)^+$, $\bar{K}^{*}(1410)^0η$, $K^{*}(1680)^-π^{+}$ and $K_2^{*}(1980)^-π^{+}$, together with $Kπ$ and $Kη$ S-wave components. The decays $K^{*}(1680)^{-}\to K^{-}η$ and $K_{2}^{*}(1980)^{-}\to K^{-}η$ are observed for the first time. We measure ratio of the branching fractions, $\frac{\mathcal{B}(D^{0}\to K^{-}π^{+}η)}{\mathcal{B}(D^{0}\to K^{-}π^{+})}=0.500\pm0.002{\rm(stat)}\pm0.020{\rm(syst)}\pm0.003{\rm (\mathcal{B}_{PDG})}$. Using the Dalitz fit result, the ratio $\frac{\mathcal{B}(K^{*}(1680)\to Kη)}{\mathcal{B}(K^{*}(1680)\to Kπ)}$ is measured to be $0.11\pm0.02{\rm(stat)}^{+0.06}_{-0.04}{\rm(syst)}\pm0.04{\rm(\mathcal{B}_{\text{PDG}})}$; this is much lower than the theoretical expectations ($\approx1$) made under the assumption that $K^{*}(1680)$ is a pure $1^{3}D_1$ state. The product branching fraction $\mathcal{B}(D^0\to [K_2^{*}(1980)^-\to K^{-}η]π^{+})=(2.2^{+1.7}_{-1.9})\times10^{-4}$ is determined. In addition, the $πη^{\prime}$ contribution to the $a_0(980)^{\pm}$ resonance shape is confirmed with 10.1$σ$ statistical significance using the three-channel Flatté model. We also measure $\mathcal{B}(D^0\to\bar{K}^{*}(892)^0η)=(1.41^{+0.13}_{-0.12})\%$. This is consistent with, and more precise than, the current world average $(1.02\pm0.30)\%$, deviates with a significance of more than $3σ$ from the theoretical predictions of (0.51-0.92)%.

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arXiv2020

Measurement of the branching fraction ${\cal B}(\bar B^0\to D^{*+}\ell^-\barν_\ell)$ with early Belle II data

We measure the branching fractions for the decays $\bar{B}^0 \to D^{*+} e^{-} \barν_e$ and $\bar{B}^0 \to D^{*+} μ^{-} \barν_μ$ using $8.70 \pm 0.09~{\rm fb}^{-1}$ of data collected by the Belle II experiment at the SuperKEKB asymmetric-energy $e^+ e^-$ collider. Candidate signal decays are reconstructed with the subsequent decays $D^{*+}\to D^0 π^+$ and $D^0\to K^-π^+$. We obtain the results ${\cal B}(\bar{B}^0 \to D^{*+} e^{-} \barν_e)$ = $(4.55\pm0.14(\mathrm{stat})\pm0.35 (\mathrm{syst})) \%$ and ${\cal B}(\bar{B}^0 \to D^{*+} μ^{-} \barν_μ)$ = $(4.84\pm 0.13(\mathrm{stat})\pm0.37(\mathrm{syst})) \%$, in agreement with the world averages. The measurements serve to validate the full chain of detector operation and calibration, data collection and processing, and production of physics results in the case of semileptonic $B$-meson decays.

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arXiv2020

Measurement of the Proton-Air Cross Section with Telescope Array's Black Rock Mesa and Long Ridge Fluorescence Detectors, and Surface Array in Hybrid Mode

Ultra high energy cosmic rays provide the highest known energy source in the universe to measure proton cross sections. Though conditions for collecting such data are less controlled than an accelerator environment, current generation cosmic ray observatories have large enough exposures to collect significant statistics for a reliable measurement for energies above what can be attained in the lab. Cosmic ray measurements of cross section use atmospheric calorimetry to measure depth of air shower maximum ($X_{\mathrm{max}}$), which is related to the primary particle's energy and mass. The tail of the $X_{\mathrm{max}}$ distribution is assumed to be dominated by showers generated by protons, allowing measurement of the inelastic proton-air cross section. In this work the proton-air inelastic cross section measurement, $σ^{\mathrm{inel}}_{\mathrm{p-air}}$, using data observed by Telescope Array's Black Rock Mesa and Long Ridge fluorescence detectors and surface detector array in hybrid mode is presented. $σ^{\mathrm{inel}}_{\mathrm{p-air}}$ is observed to be $520.1 \pm 35.8$[Stat.] $^{+25.0}_{-40}$[Sys.]~mb at $\sqrt{s} = 73$ TeV. The total proton-proton cross section is subsequently inferred from Glauber formalism and is found to be $σ^{\mathrm{tot}}_{\mathrm{pp}} = 139.4 ^{+23.4}_{-21.3}$ [Stat.]$ ^{+15.0}_{-24.0}$[Sys.]~mb.

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arXiv2020

Charmless $B$ decay reconstruction in 2019 Belle II data

We report on the reconstruction of various charmless $B$ decays from electron-positron collisions at the energy corresponding to the $Υ(4S)$ resonance collected with the Belle II detector at the SuperKEKB collider. We use simulation to devise optimized event selections and apply them to the full data set collected in 2019, corresponding to 8.7\,fb$^{-1}$ of integrated luminosity. We fit the difference between half of the collision energy and the $B$ candidate energy (in the $Υ(4S)$ frame) for events restricted to a signal-rich range in beam-energy-constrained mass to search for charmless signals. Signal yields of approximately 80, 15, 20, 30, 90, and 160 decays are reconstructed for the channels $B^0 \to K^+π^-$, $B^0 \to π^+π^-$, $B^+ \to K^0_S(\to π^+π^-)π^+$, $B^+ \to K^+π^0(\to γγ)$, $B^+ \to K^+K^-K^+$, and $B^+ \to K^+π^-π^+$, respectively. Yields and background contaminations are compatible with those expected from simulation and comparable with those obtained by the Belle experiment. The results show a good understanding of the detector performance and offer a reliable basis to assess projections for future reach.

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arXiv2020

Measurement of the $B^0$ lifetime using fully reconstructed hadronic decays in the 2019 Belle II dataset

This document presents the measurement of $B^0$ meson lifetimes using the 2019 Belle II dataset that corresponds to an integrated luminosity of $8.7 \pm 0.2$ fb$^{-1}$. Each candidate is fully reconstructed with hadronic decay final states on the signal side, while the rest-of-event technique allows to infer the decay vertex position on the other (tag) side. $B^0$ lifetime is extracted from an unbinned maximum likelihood fit to the distribution of the difference between the signal side $B^0$ candidate and the tag side decay times. The measured lifetime is $τ_{B^0} = 1.48 \pm 0.28 \pm 0.06 $ ps, where the first uncertainty is statistical and the second is systematic.

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arXiv2020

Search for Ultra-High-Energy Neutrinos with the Telescope Array Surface Detector

We present an upper limit on the flux of ultra-high-energy down-going neutrinos for $E > 10^{18}\ \mbox{eV}$ derived with the nine years of data collected by the Telescope Array surface detector (05-11-2008 -- 05-10-2017). The method is based on the multivariate analysis technique, so-called Boosted Decision Trees (BDT). Proton-neutrino classifier is built upon 16 observables related to both the properties of the shower front and the lateral distribution function.

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arXiv2020

First search for the $η_{c2}(1D)$ in $B$ decays at Belle

The first dedicated search for the $η_{c2}(1D)$ is carried out using the decays $B^+ \rightarrow η_{c2}(1D) K^+$, $B^0 \rightarrow η_{c2}(1D) K^0_S$, $B^0 \rightarrow η_{c2}(1D) π^- K^+$, and $B^+ \rightarrow η_{c2}(1D) π^+ K^0_S$ with $η_{c2}(1D) \to h_c γ$. No significant signal is found. For the $η_{c2}(1D)$ mass range between $3795$ and $3845\ \mathrm{MeV}/c^2$, the branching-fraction upper limits are determined to be $\mathcal{B}(B^+ \rightarrow η_{c2}(1D) K^+) \times \mathcal{B}(η_{c2}(1D) \to h_c γ) < 3.7 \times 10^{-5}$, $\mathcal{B}(B^0 \rightarrow η_{c2}(1D) K^0_S) \times \mathcal{B}(η_{c2}(1D) \to h_c γ) < 3.5 \times 10^{-5}$, $\mathcal{B}(B^0 \rightarrow η_{c2}(1D) π^- K^+) \times \mathcal{B}(η_{c2}(1D) \to h_c γ) < 1.0 \times 10^{-4}$, and $\mathcal{B}(B^+ \rightarrow η_{c2}(1D) π^+ K^0_S) \times \mathcal{B}(η_{c2}(1D) \to h_c γ) < 1.1 \times 10^{-4}$ at 90% C. L. The analysis is based on the 711 $\mathrm{fb}^{-1}$ data sample collected on the $Υ(4S)$ resonance by the Belle detector, which operated at the KEKB asymmetric-energy $e^+ e^-$ collider.

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arXiv2020

Evidence for a vector charmonium-like state in $e^+e^- \to D^+_sD^*_{s2}(2573)^-+c.c.$

We report the measurement of $e^+e^- \to D^+_sD^*_{s2}(2573)^-+c.c.$ via initial-state radiation using a data sample of an integrated luminosity of 921.9 fb$^{-1}$ collected with the Belle detector at the $Υ(4S)$ and nearby. We find evidence for an enhancement with a 3.4$σ$ significance in the invariant mass of $D^+_sD^*_{s2}(2573)^- +c.c.$ The measured mass and width are $(4619.8^{+8.9}_{-8.0}({\rm stat.})\pm2.3({\rm syst.}))~{\rm MeV}/c^{2}$ and $(47.0^{+31.3}_{-14.8}({\rm stat.})\pm4.6({\rm syst.}))~{\rm MeV}$, respectively. The mass, width, and quantum numbers of this enhancement are consistent with the charmonium-like state at 4626 MeV/$c^2$ recently reported by Belle in $e^+e^-\to D^+_sD_{s1}(2536)^-+c.c.$ The product of the $e^+e^-\to D^+_sD^*_{s2}(2573)^-+c.c.$ cross section and the branching fraction of $D^*_{s2}(2573)^-\to{\bar D}^0K^-$ is measured from $D^+_sD^*_{s2}(2573)^-$ threshold to 5.6 GeV.

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arXiv2020

Search for point sources of ultra-high energy photons with the Telescope Array surface detector

The surface detector (SD) of the Telescope Array (TA) experiment allows one to indirectly detect photons with energies of order $10^{18}$ eV and higher and to separate photons from the cosmic-ray background. In this paper we present the results of a blind search for point sources of ultra-high energy (UHE) photons in the Northern sky using the TA SD data. The photon-induced extensive air showers (EAS) are separated from the hadron-induced EAS background by means of a multivariate classifier based upon 16 parameters that characterize the air shower events. No significant evidence for the photon point sources is found. The upper limits are set on the flux of photons from each particular direction in the sky within the TA field of view, according to the experiment's angular resolution for photons. Average 95% C.L. upper limits for the point-source flux of photons with energies greater than $10^{18}$, $10^{18.5}$, $10^{19}$, $10^{19.5}$ and $10^{20}$ eV are $0.094$, $0.029$, $0.010$, $0.0073$ and $0.0058$ km$^{-2}$yr$^{-1}$ respectively. For the energies higher than $10^{18.5}$ eV, the photon point-source limits are set for the first time. Numerical results for each given direction in each energy range are provided as a supplement to this paper.

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