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Y. Guan

Publications and source records attributed to Y. Guan.

At least 37 records · Page 2Linked to original sources
arXiv2024

Measurements of the branching fraction, polarization, and $CP$ asymmetry for the decay $B^0\rightarrow ωω$

We present a measurement of $B^{0} \rightarrow ωω$, a charmless decay into two vector mesons, using 772 $\times 10^6$ $B\overline{B}$ pairs collected with the Belle detector at the KEKB $e^+e^-$ collider. The decay is observed with a significance of 7.9 standard deviations. We measure a branching fraction $\mathcal{B} = (1.53 \pm 0.29 \pm 0.17) \times 10^{-6}$, a fraction of longitudinal polarization $f_L = 0.87 \pm 0.13 \pm 0.13$, and a time-integrated $CP$ asymmetry $A_{CP}$ = $-0.44 \pm 0.43 \pm 0.11$, where the first uncertainties listed are statistical and the second are systematic. This is the first observation of $B^{0} \rightarrow ωω$, and the first measurements of $f_L$ and $A_{CP}$ for this decay.

hep-ex↗
arXiv2023

Study of Silicon Photomultiplier External Cross-Talk

Optical cross-talk is a critical characteristic of Silicon Photomultipliers (SiPMs) and represents a significant source of the excess noise factor, exerting a substantial influence on detector performance. During the avalanche process of SiPMs, photons generated can give rise to both internal cross-talk within the same SiPM and external cross-talk when photons escape from one SiPM and trigger avalanches in others. In scenarios where SiPMs are arranged in a compact configuration and positioned facing each other, the external cross-talk could even dominate the cross-talk phenomenon. This paper investigates two distinct methods for measuring external cross-talk: the counting method, which involves operating SiPMs face-to-face and measuring their coincident signals, and the reflection method, which employs a highly reflective film attached to the surface of the SiPMs. External cross-talk measurements have been conducted on several types of SiPMs, including Vacuum Ultra-Violet (VUV) sensitive SiPMs that Fondazione Bruno Kessler (FBK) and Hamamatsu Photonics Inc (HPK) produced for nEXO as well as visible-sensitive SiPMs provided by FBK, HPK and SensL Technologies Ltd (SenSL) for JUNO-TAO. The results reveal a significant presence of external cross-talk in all tested SiPMs, with HPK's SiPMs exhibiting a dominant external cross-talk component due to the implementation of optical trenches that effectively suppress internal cross-talk. Furthermore, we found that the number of fired pixels resulting from internal cross-talk can be described by combining Geometric and Borel models for all tested SiPMs, while the external cross-talk can be predicted using a pure Borel model. These distinct probability distributions lead to different excess noise factors, thereby impacting the detector performance in varying ways.

physics.ins-det↗
arXiv2023

Search for $CP$ violation in $D^{+}_{(s)}\rightarrow K^{+}K^{0}_{S}h^{+}h^{-}$ $(h=K,π)$ decays and observation of the Cabibbo-suppressed decay $D^{+}_{s}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+}$

We search for $CP$ violation by measuring a $T$-odd asymmetry in the Cabibbo-suppressed $D^{+}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-} $ decay, and in the Cabibbo-favored $D^{+}_{s}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-}$ and $D^{+}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+}$ decays. We use 980 ${\rm fb}^{-1}$ of data collected by the Belle detector running at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. The $C\!P$-violating $T$-odd parameter ${a}^{T\text{-}\rm{odd}}_{CP}$ is measured to be ${a}^{T\text{-}\rm{odd}}_{CP}(D^{+}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-})=(0.34\pm0.87\pm0.32)\%,$ ${a}^{T\text{-}\rm{odd}}_{CP}(D^{+}_{s}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-})=(-0.46\pm0.63\pm0.38)\%,$ and ${a}^{T\text{-}\rm{odd}}_{CP}(D^{+}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+})=(-3.34\pm2.66\pm0.35)\%,$ where the first uncertainty is statistical and the second is systematic. We also report the first observation of the Cabibbo-suppressed decay $D^{+}_{s}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+}$. The branching fraction is measured relative to that of the analogous Cabibbo-favored decay : $B(D^{+}_{s}\rightarrow K^{+}K^{-}K^{0}_{S}π^{+}) / B(D^{+}_{s}\rightarrow K^{+}K^{0}_{S}π^{+}π^{-}) = (1.36\pm 0.15\pm 0.04)\%$.

hep-ex↗
arXiv2023

Search for $B{}^0_s \rightarrow \ell^{\mp} τ^{\pm}$ with the Semi-leptonic Tagging Method at Belle

We present a search for the lepton-flavor-violating decays $B{}^0_s \rightarrow \ell^{\mp}τ^{\pm}$, where $\ell = e, μ$, using the full data sample of $121~\mathrm{fb}^{-1}$ collected at the $Υ(5S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We use $B{}^0_s \overline{B}{}^0_s$ events in which one $B{}^0_s$ meson is reconstructed in a semileptonic decay mode and the other in the signal mode. We find no evidence for $B{}^0_s \rightarrow \ell^{\mp}τ^{\pm}$ decays and set upper limits on their branching fractions at $90\%$ confidence level as $\mathcal{B}(B{}^0_s \rightarrow e^{\mp}τ^{\pm}) < 14 \times 10^{-4}$ and $\mathcal{B}(B{}^0_s \rightarrow μ^{\mp}τ^{\pm}) < 7.3 \times 10^{-4}$. Our result represents the first upper limit on the $B{}^0_s \rightarrow e^{\mp}τ^{\pm}$ decay rate.

hep-ex↗
arXiv2023

Evidence for $B^0 \to p\barΣ^0π^-$ at Belle

We search for the $B^0\to p\barΣ^0π^-$ decay with $\barΣ^0 \to \barΛγ$, where the $γ$ is not measured, using a data sample corresponding to an integrated luminosity of 711 $\rm{fb^{-1}}$ which contains 772 $\times$ $10^{6}$ $B\bar{B}$ pairs, collected around the $Υ$(4S) resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We measure for the first time the $B^0\to p\barΣ^0π^-$ branching fraction to be $\mathcal{B}(B^0 \to p \barΣ^0 π^-) = (1.17^{+0.43}_{-0.40}(\text{stat})\pm 0.07(\text{syst})) \times 10^{-6}$ with a significance of $3.0σ$. We simultaneously measure the branching fraction for the related channel $B^{0}\to p\barΛπ^{-}$ with much improved precision.

hep-ex↗
arXiv2023

First Simultaneous Determination of Inclusive and Exclusive $\left|V_{ub}\right|$

The first simultaneous determination of the absolute value of the Cabibbo-Kobayashi-Maskawa matrix element $V_{ub}$ using inclusive and exclusive decays is performed with the full Belle data set at the $Υ(4S)$ resonance, corresponding to an integrated luminosity of 711 fb${}^{-1}$. We analyze collision events in which one $B$ meson is fully reconstructed in hadronic modes. This allows for the reconstruction of the hadronic $X_u$ system of the semileptonic $b \to u \ell \bar ν_\ell$ decay. We separate exclusive $B \to π\, \ell\, \bar ν_{\ell}$ decays from other inclusive $B \to X_u \, \ell\, \bar ν_{\ell}$ and backgrounds with a two-dimensional fit, that utilizes the number of charged pions in the $X_u$ system and the four-momentum transfer $q^2$ between the $B$ and $X_u$ system. Combining our measurement with information from lattice QCD and QCD calculations of the inclusive partial rate as well as external experimental information on the shape of the $B \to π\, \ell\, \bar ν_{\ell}$ form factor, we determine $\left|V_{ub}^{\mathrm{excl.}} \right| = (3.78 \pm 0.23 \pm 0.16 \pm 0.14)\times 10^{-3}$ and $\left|V_{ub}^{\mathrm{incl.}} \right| = (3.88 \pm 0.20 \pm 0.31 \pm 0.09)\times 10^{-3}$, respectively, with the uncertainties being the statistical error, systematic errors, and theory errors. The ratio of $\left|V_{ub}^{\mathrm{excl.}} \right| / \left|V_{ub}^{\mathrm{incl.}} \right| = 0.97 \pm 0.12$ is compatible with unity.

hep-ex↗
arXiv2023

Search for the double-charmonium state with $η_c J/ψ$ at Belle

We measure the cross section of $e^+e^-\rightarrowη_c J/ψ$ at the $Υ(nS) (n=1$ -- $5)$ on-resonance and 10.52 GeV off-resonance energy points using the full data sample collected by the Belle detector with an integrated luminosity of $955~\rm fb^{-1}$. We also search for double charmonium production in $e^+e^-\rightarrowη_c J/ψ$ via initial state radiation near the $η_c J/ψ$ threshold. No evident signal of the double charmonium state is found, but evidence for the $e^+e^-\rightarrowη_c J/ψ$ process is found with a statistical significance greater than $3.3σ$ near the $η_c J/ψ$ threshold. The average cross section near the threshold is measured and upper limits of cross sections are set for other regions.

hep-ex↗
arXiv2023

Observation of Kekulé vortices induced in graphene by hydrogen adatoms

Fractional charges are one of the wonders of the fractional quantum Hall effect, a liquid of strongly correlated electrons in a large magnetic field. Fractional excitations are also anticipated in two-dimensional crystals of non-interacting electrons under time-reversal symmetry, as bound states of a rotating bond order known as Kekulé vortex. However, the physical mechanisms inducing such topological defects remain elusive, preventing experimental realisations. Here, we report the observation of Kekulé vortices in the local density of states of graphene under time-reversal symmetry. The vortices result from intervalley scattering on chemisorbed hydrogen adatoms and have a purely electronic origin. Their 2π winding is reminiscent of the Berry phase π of the massless Dirac electrons. Remarkably, we observe that point scatterers with different symmetries such as divacancies can also induce a Kekulé bond order without vortex. Therefore, our local-probe study further confirms point defects as versatile building blocks for the control of graphene's electronic structure by kekulé order.

cond-mat.mes-hall↗
arXiv2023

Study of the muon decay-in-flight in the $τ^-\toμ^-\barν_μν_τ$ decay to measure the Michel parameter $ξ^\prime$

We present the first measurement of the Michel parameter $ξ^\prime$ in the $τ^-\toμ^-\barν_μν_τ$ decay using the full data sample of $988\,\text{fb}^{-1}$ collected by the Belle detector operating at the KEKB asymmetric energy $e^+ e^-$ collider. The method is based on the reconstruction of the $μ^- \to e^- \barν_eν_μ$ decay-in-flight in the Belle central drift chamber and relies on the correlation between muon spin and its daughter electron momentum. We study the main sources of the background that can imitate the signal decay, such as kaon and pion decays-in-flight and charged particle scattering on the detector material. Highly efficient methods of their suppression are developed and applied to select 165 signal-candidate events. We obtain $ξ^\prime=0.22\pm0.94\pm0.42$ where the first uncertainty is statistical, and the second one is systematic. The result is in agreement with the Standard Model prediction of $ξ^\prime=1$.

hep-ex↗
arXiv2023

First measurement of the Michel parameter $ξ^\prime$ in the $τ^-\toμ^-\barν_μν_τ$ decay at Belle

We report the first measurement of the Michel parameter $ξ^\prime$ in the $τ^-\toμ^-\barν_μν_τ$ decay with a new method proposed just recently. The measurement is based on the reconstruction of the $τ^-\toμ^-\barν_μν_τ$ events with subsequent muon decay-in-flight in the Belle central drift chamber. The analyzed data sample of $988\,\text{fb}^{-1}$ collected by the Belle detector corresponds to approximately $912\times10^6$ $τ^+ τ^-$ pairs. We measure $ξ^\prime=0.22\pm0.94(\text{stat})\pm0.42(\text{syst})$, which is in agreement with the standard model prediction of $ξ^\prime=1$. Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.

hep-ex↗
arXiv2023

Search for $C\!P$ violation using $T$-odd correlations in $D_{(s)}^{+}\to K^{+} K^{-}π^{+}π^{0}$, $D_{(s)}^{+}\to K^{+} π^{-}π^{+}π^{0}$, and $D^{+}\to K^{-}π^{+}π^{+}π^{0}$ decays

We search for $C\!P$ violation using $T$-odd correlations in five $D_{(s)}^{+}$ and $D_{(s)}^{-}$ four-body decays. Our analysis is based on 980 $\rm fb^{-1}$ of data collected by the Belle detector at the KEKB energy-asymmetric $e^+e^-$ collider. Our results for the $T$-odd $C\!P$-violating parameter $a^{T\text{-odd}}_{C\!P}$ are: $a^{T\text{-odd}}_{C\!P}({D^{+}\to K^{-}K^{+}π^{+}π^{0}}) = (+2.6\pm 6.6\pm 1.3 )\times10^{-3}$, $a^{T\text{-odd}}_{C\!P}({D^{+}\to K^{+}π^{-}π^{+}π^{0}}) = (-1.3\pm 4.2\pm 0.1 )\times10^{-2}$, $a^{T\text{-odd}}_{C\!P}({D^{+}\to K^{-}π^{+}π^{+}π^{0}}) = (+0.2\pm 1.5\pm 0.8 )\times10^{-3}$, $a^{T\text{-odd}}_{C\!P}({D_s^{+}\to K^{+}π^{-}π^{+}π^{0}}) = (-1.1\pm 2.2\pm 0.1 )\times10^{-2}$, and $a^{T\text{-odd}}_{C\!P}({D_s^{+}\to K^{-}K^{+}π^{+}π^{0}}) = (+2.2\pm 3.3\pm 4.3 )\times10^{-3}$, where the uncertainties are statistical and systematic, respectively. These results are the first such measurements and are all consistent with zero. They include the first measurement for a $D^+_s$ singly Cabibbo-suppressed decay, and the first measurement for a $D$ meson doubly Cabibbo-suppressed decay. We also measure $a^{T\text{-odd}}_{C\!P}$ in different subregions of phase space, where the decays are dominated by different intermediate resonance states such as $D^+\toϕρ^+$, $\bar{K}^{*0}K^{*+}$, and $\bar{K}^{*0}ρ^+$; and $D_s^+\to K^{*+}ρ^{0}$, $K^{*0}ρ^{+}$, $ϕρ^+$, and $\bar{K}^{*0}K^{*+}$. No evidence for $C\!P$ violation is found.

hep-ex↗
arXiv2023

First measurement of the $B^{+}\toπ^{+}π^{0}π^{0}$ branching fraction and \textit{CP} asymmetry

We study $B^{+}\to π^{+}π^{0}π^{0}$ using 711 $\rm{fb}^{-1}$ of data collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We measure an inclusive branching fraction of $(19.0\pm 1.5\pm 1.4)\times 10^{-6}$ and an inclusive \textit{CP} asymmetry of $(9.2 \pm 6.8 \pm 0.7)\%$, where the first uncertainties are statistical and the second are systematic; and a $B^{+}\to ρ(770)^{+}π^{0}$ branching fraction of $(11.2\pm 1.1\pm 0.9 ^{+0.8}_{-1.6})\times 10^{-6}$, where the third uncertainty is due to possible interference with $B^{+}\to ρ(1450)^{+}π^{0}$. We present the first observation of a structure around 1 GeV/$c^{2}$ in the $π^{0}π^{0}$ mass spectrum, with a significance of 6.4$σ$, and measure a branching fraction to be $(6.9\pm 0.9\pm 0.6)\times 10^{-6}$. We also report a measurement of local \textit{CP} asymmetry in this structure.

hep-ex↗
arXiv2023

Search for lepton-flavor-violating $τ$ decays into a lepton and a vector meson using the full Belle data sample

Charged-lepton-flavor-violation is predicted in several new physics scenarios. We update the analysis of $τ$ lepton decays into a light charged lepton ($\ell$ = $e^{\pm}$ or $μ^{\pm}$) and a vector meson ($V^0$ = $ρ^0$, $ϕ$, $ω$, $K^{\ast0}$, or $\overline{K}{}^{\ast0}$) using 980 fb$^{-1}$ of data collected with the Belle detector at the KEKB collider. No significant excess of such signal events is observed, and thus 90% credibility level upper limits are set on the $τ\rightarrow \ell V^0$ branching fractions in the range of (1.7--$4.3) \times 10^{-8}$. These limits are improved by 30% on average from the previous results.

hep-ex↗
arXiv2023

Search for $C\!P$ violation and measurement of branching fractions and decay asymmetry parameters for $Λ_c^+\toΛh^+$ and $Λ_c^+\toΣ^{0} h^+$ ($h\!=\!K,\,π$)

We report a study of $Λ_c^+\toΛh^+$ and $Λ_c^+\toΣ^{0} h^+$ ($h\!=\!K,\,π$) decays based on a data sample of 980~${\rm fb}^{-1}$ collected with the Belle detector at the KEKB energy-asymmetric $e^+e^-$ collider. The first results of direct $C\!P$ asymmetry in two-body singly Cabibbo-suppressed (SCS) decays of charmed baryons are measured, $A_{C\!P}^{\rm{dir}}(Λ_c^+\toΛK^+)\!=\!+0.021\pm0.026\pm0.001$ and $A_{C\!P}^{\rm{dir}}(Λ_c^+\toΣ^0K^+)\!=\!+0.025\pm0.054\pm0.004$. We also make the most precise measurement of the decay asymmetry parameters ($α$) for the four modes of interest and search for $C\!P$ violation via the $α$-induced $C\!P$ asymmetry ($A_{C\!P}^α$). We measure $A_{C\!P}^α(Λ_c^+\toΛK^+)\!=\!{-0.023\pm0.086\pm0.071}$ and $A_{C\!P}^α(Λ_c^+\toΣ^0K^+)\!=\!{+0.08\pm 0.35\pm 0.14}$, which are the first $A_{C\!P}^α$ results for SCS decays of charmed baryons. We search for $Λ$-hyperon $C\!P$ violation in $Λ_c^+\to(Λ,\,Σ^0)π^+$ and find $A_{C\!P}^α(Λ\to pπ^{-})\!=\!{+0.013\pm0.007\pm0.011}$. This is the first time that hyperon $C\!P$ violation has been measured via Cabibbo-favored charm decays. No evidence of baryon $C\!P$ violation is found. We also obtain the most precise branching fractions for two SCS $Λ_c^+$ decays, $\mathcal{B}(Λ_c^+\toΛK^+)\!=\!(6.57\pm0.17\pm0.11\pm0.35)\times10^{-4}$ and $\mathcal{B}(Λ_c^+\toΣ^0K^+)\!=\!(3.58\pm0.19\pm0.06\pm0.19)\times10^{-4}$. The first uncertainties are statistical and the second systematic, while the third uncertainties come from the uncertainties on the world average branching fractions of $Λ_c^+\to(Λ,\,Σ^0)π^+$.

hep-ex↗
arXiv2023

Search for the decay $B^0_s \rightarrow π^0 π^0$ at Belle

We report the results of the first search for the decay $B_s^0\rightarrowπ^0π^0$ using $121.4\ \rm fb^{-1}$ of data collected at the $Υ(5\rm S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We observe no signal and set a 90\% confidence level upper limit of $7.7\times 10^{-6}$ on the $B_s^0\rightarrowπ^0π^0$ decay branching fraction.

hep-ex↗
arXiv2023

Search for rare decays $B^{+} \to D_{s}^{(\ast)+}η$, $D_{s}^{(\ast)+}\bar{K}^{0}$, $D^{+}η$, and $D^{+}K^{0}$

We present a study of rare decay modes $B^{+} \to D_{s}^{+}h^{0}$, $B^{+} \to D_{s}^{\ast+}h^{0}$, and $B^{+} \to D^{+}h^{0}$, where $h^{0}$ denotes the neutral mesons $η$ or $K^{0}$, using a data sample of $(772 \pm 10 ) \times 10^{6}$ $B\bar{B}$ events produced at the $Υ(4S)$ resonance. The data were collected by the Belle detector operating at the asymmetric-energy KEKB collider. We observe no evidence for these decays, so we provide upper limits at the 90$\%$ confidence level on the branching fractions of $B^{+} \to D_{s}^{+}h^{0}$, $D_{s}^{\ast+}h^{0}$, and $D^{+}h^{0}$ decay modes. Along with rare decay modes, we report improved measurements of the color-suppressed decay branching fractions $\mathcal{B}(\bar{B}^{0} \to D^{0}η)$ = (26.6 $\pm$ 1.2 $\pm$ 2.1) $\times$ $10^{-5}$ and $\mathcal{B}(\bar{B}^{0} \to D^{0}\bar{K}^{0})$ = (5.6 $\pm$ 0.5 $\pm$ 0.2) $\times$ $10^{-5}$. The first and second uncertainties are statistical and systematic, respectively.

hep-ex↗
arXiv2022

Neutrinoless Double Beta Decay

This White Paper, prepared for the Fundamental Symmetries, Neutrons, and Neutrinos Town Meeting related to the 2023 Nuclear Physics Long Range Plan, makes the case for double beta decay as a critical component of the future nuclear physics program. The major experimental collaborations and many theorists have endorsed this white paper.

nucl-ex↗
arXiv2022

Measurement of branching fractions of $Λ_c^+\to{}pK_S^0K_S^0$ and $Λ_c^+\to{}pK_S^0η$ at Belle

We present a study of a singly Cabibbo-suppressed decay $Λ_c^+\to{}pK_S^0K_S^0$ and a Cabibbo-favored decay $Λ_c^+\to{}pK_S^0η$ based on 980 $\rm fb^{-1}$ of data collected by the Belle detector, operating at the KEKB energy-asymmetric $e^+e^-$ collider. We measure their branching fractions relative to $Λ_c^+\to{}pK_S^0$: $\mathcal{B}(Λ_c^+\to{}pK_S^0K_S^0)/\mathcal{B}(Λ_c^+\to{}pK_S^0)={(1.48 \pm 0.08 \pm 0.04)\times 10^{-2}}$ and $\mathcal{B}(Λ_c^+\to{}pK_S^0η)/\mathcal{B}(Λ_c^+\to{}pK_S^0)={(2.73\pm 0.06\pm 0.13)\times 10^{-1}}$. Combining with the world average $\mathcal{B}(Λ_c^+\to{}pK_S^0)$, we have the absolute branching fractions: $\mathcal{B}(Λ_c^+\to{}pK_S^0K_S^0) = {(2.35\pm 0.12\pm 0.07 \pm 0.12 )\times 10^{-4}}$ and $\mathcal{B}(Λ_c^+\to{}pK_S^0η) = {(4.35\pm 0.10\pm 0.20 \pm 0.22 )\times 10^{-3}}$. The first and second uncertainties are statistical and systematic, respectively, while the third ones arise from the uncertainty on $\mathcal{B}(Λ_c^+\to{}pK_S^0)$. The mode $Λ_c^+\to{}pK_S^0K_S^0$ is observed for the first time and has a statistical significance of $>\!10σ$. The branching fraction of $Λ_c^+\to{}pK_S^0η$ has been measured with a threefold improvement in precision over previous results and is found to be consistent with the world average.

hep-ex↗