arXiv subjects

L. Li

Publications and source records attributed to L. Li.

At least 163 records · Page 9Linked to original sources

arXiv2017

Search for the $0^{--}$ Glueball in $Υ(1S)$ and $Υ(2S)$ decays

We report the first search for the $J^{PC}=0^{--}$ glueball in $Υ(1S)$ and $Υ(2S)$ decays with data samples of $(102\pm2)$ million and $(158\pm4)$ million events, respectively, collected with the Belle detector. No significant signals are observed in any of the proposed production modes, and the 90\% credibility level upper limits on their branching fractions in $Υ(1S)$ and $Υ(2S)$ decays are obtained. The inclusive branching fractions of the $Υ(1S)$ and $Υ(2S)$ decays into final states with a $χ_{c1}$ are measured to be $\BR(Υ(1S)\to χ_{c1}+ anything) = (1.90\pm 0.43(stat.)\pm 0.14(syst.))\times 10^{-4}$ with an improved precision over prior measurements and $\BR(Υ(2S)\to χ_{c1}+ anything) = (2.24\pm 0.44(stat.)\pm 0.20(syst.))\times 10^{-4}$ for the first time.

hep-ex↗

arXiv2016

POLAR: Final Calibration and In-Flight Performance of a Dedicated GRB Polarimeter

Gamma-ray polarimetry is a new powerful tool to study the processes responsible for the emission from astrophysical sources and the environments in which this emission takes place. Few successful polarimetric measurements have however been performed thus far in the gamma-ray energy band due to the difficulties involved. POLAR is a dedicated polarimeter designed to perform high precision measurements of the polarization of the emission from gamma-ray burst in the 50-500 keV energy range. This new polarimeter is expected to detect approximately 50 gamma-ray bursts per year while performing high precision polarization measurements on approximately 10 bursts per year. The instrument was launched into lower earth orbit as part of the second Chinese space lab, the Tiangong-2, on September 15th 2016 and has been taking data successfully since being switched on one week after. The instrument uses a segmented scintillator array consisting of 1600 plastic scintillator bars, read out by 25 flat-panel multi-anode photomultipliers, to measure the Compton scattering angles of incoming photons. The small segmentation and relatively large uniform effective area allow the instrument to measure the polarization of a large number of transient events, such as gamma-ray bursts, with an unprecedented precision during its two year life-time. The final flight model underwent detailed calibration prior to launch as well as intensive space qualification tests, a summary of which will be presented in this paper. The instrument design will be discussed first followed by an overview of the on-ground tests, finally the in-orbit behavior as measured during the first weeks of the mission will be presented.

astro-ph.IM↗

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.

hep-ex↗

arXiv2016

Improving quantum-transition temperatures in BaFe2As2-based crystals by removing local-lattice strain & electronic-structure disorder

Quantum materials such as antiferromagnets or superconductors are complex in that chemical, electronic, and spin phenomena at atomic scales can manifest in their collective properties. Although there are some clues for designing such materials, they remain mainly unpredictable. In this work, we find that enhancement of transition temperatures in BaFe2As2-based crystals are caused by removing local-lattice strain and electronic-structure disorder by thermal annealing. While annealing improves Neel-ordering temperature in BaFe2As2 crystal (TN=132 K to 136 K) by removing in-plane electronic defects and overall reduction of a-lattice parameter, it increases superconducting-ordering temperature in optimally cobalt-doped BaFe2As2 crystal (Tc=23 to 25 K) by precipitating-out the cobalt dopants and giving larger overall a-lattice parameter. Although annealing promotes local chemical and electronic uniformity resulting in higher TN in the parent, it results in nanoscale phase separation in the superconductor resulting in lower disparity and strong superconducting band gaps in the dominant crystalline regions, which lead to both higher overall Tc and critical-current-density, Jc.

cond-mat.supr-con↗

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.

hep-ex↗

arXiv2016

Attosecond Control of Relativistic Electron Bunches using Two-Colour Fields

Energy coupling during relativistically intense laser-matter interactions is encoded in the attosecond motion of strongly driven electrons at the pre-formed plasma-vacuum boundary. Studying and controlling this motion can reveal details about the microscopic processes that govern a vast array of light-matter interaction physics and applications. These include research areas right at the forefront of extreme laser-plasma science such as laser-driven ion acceleration1, bright attosecond pulse generation2,3 and efficient energy coupling for the generation and study of warm dense matter4. Here we demonstrate attosecond control over the trajectories of relativistic electron bunches formed during such interactions by studying the emission of extreme ultraviolet (XUV) harmonic radiation. We describe how the precise addition of a second laser beam operating at the second harmonic of the driving laser pulse can significantly transform the interaction by modifying the accelerating potential provided by the fundamental frequency to drive strong coherent emission. Numerical particle-in-cell code simulations and experimental observations demonstrate that this modification is extremely sensitive to the relative phase of the two beams and can lead to significant enhancements in the resulting harmonic yield. This work also reveals that the ability to control these extreme interactions with attosecond precision is an essential requirement for generation of ultra-bright, high temporal contrast attosecond radiation for atomic and molecular pump-probe experiments5,6.

physics.plasm-ph↗

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 Excited $Ξ_c$ States Decaying into $Ξ_c^0$ and $Ξ_c^+$ Baryons

Using a data sample of 980 ${\rm fb}^{-1}$ of $e^+e^-$ annihilation data taken with the Belle detector operating at the KEKB asymmetric-energy $e^+e^-$ collider, we report the results of a study of excited $Ξ_c$ states that decay, via the emission of photons and/or charged pions, into $Ξ_c^0$ or $Ξ_c^+$ ground state charmed-strange baryons. We present new measurements of the masses of all members of the $Ξ_c^{\prime}$, $Ξ_c(2645)$, $Ξ_c(2790)$, $Ξ_c(2815)$, and $Ξ_c(2980)$ isodoublets, measurements of the intrinsic widths of those that decay strongly, and evidence of previously unknown transitions.

hep-ex↗

arXiv2016

Detection of HIV-1 antigen based on magnetic tunnel junction sensor and magnetic nanoparticles

In recent years, it is evidenced that the individuals newly infected HIV are transmitting the virus prior to knowing their HIV status. Identifying individuals that are early in infection with HIV antibody negative (window period) remains problematic. In the newly infected individuals, HIV antigen p24 is usually present in their serum or plasma 7-10 days before the HIV antibody. After antibody production initiates, the p24 antigen is bound into immune complexes. That means the detectable p24 antigens in serum/plasma are short-lived, and their amount is in the pg/ml range. Thus, a rapid quantitative bio-detection system with high-sensitivity is required to achieve early disease diagnosis. Magnetoresistive (MR) biosensor with ultra-high sensitivity possesses great potential in this area. In this study, a p24 detection assay using MgO-based magnetic tunnel junction (MTJ) sensor and 20-nm magnetic nanoparticles is reported.

cond-mat.mes-hall↗

arXiv2016

In-situ Instrumental Setup for Influence Study of Hard-axis Bias Magnetic Field on MR transfer curves of sing MTJ sensor and MTJs array sensor

Establishment of home-made measurement setups for the characterization of MR sensor is proposed and described here. The MR loops of MR sensors can be obtained with the instrument using two-point probe measurement and four-point probe measurement. Two pairs of Helmholtz coils can supply a hard-axis magnetic field and a soft-axis magnetic field for the sensor. The single MTJ sensor and MTJs array sensor in Wheatstone bridge were characterized and compared here. The influence of hard-bias magnetic field on MR transfer curves of sing MTJ sensor and MTJs array sensor are investigated. The corresponding optimal hard-axis magnetic fields were obtained through Helmholtz coils to eliminate the hysteresis for linear response of single MTJ sensor and MTJs array sensor.

cond-mat.mes-hall↗

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

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

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↗

arXiv2016

Proper motions of 15 pulsars: a comparison between Bayesian and frequentist algorithms

We present proper motions for 15 pulsars which are observed regularly by the Nanshan 25-m radio telescope. Two methods, the frequentist method (Coles et al.2011) and the Bayesian (Lentati et al. 2014) method, are used and the results are compared. We demonstrate that the two methods can be applied to young pulsar data sets that exhibit large amounts of timing noise with steep spectral exponents and give consistent results. The measured positions also agree with very-long-baseline interferometric positions. Proper motions for four pulsars are obtained for the first time, and improved values are obtained for five pulsars.

astro-ph.IM↗

arXiv2016

Search for a massive invisible particle $X^0$ in $B^{+}\to e^{+}X^{0}$ and $B^{+}\to μ^{+}X^{0}$ decays

We present a search for a non-Standard-Model invisible particle $X^0$ in the mass range $0.1\textrm{-}1.8 \,{\rm GeV}/{c^2}$ in $B^{+}\to e^{+} X^{0}$ and $B^{+}\to μ^{+} X^{0}$ decays. The results are obtained from a $711~{\rm fb}^{-1}$ data sample that corresponds to $772 \times 10^{6} B\bar{B}$ pairs, collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. One $B$ meson is fully reconstructed in a hadronic mode to determine the momentum of the lepton of the signal decay in the rest frame of the recoiling partner $B$ meson. We find no evidence of a signal and set upper limits on the order of $10^{-6}$.

hep-ex↗

arXiv2016

Imprints of Electron-positron Winds on the Multi-wavelength Afterglows of Gamma-ray Bursts

Optical re-brightenings in the afterglows of some gamma-ray bursts (GRBs) are unexpected within the framework of the simple external shock model. While it has been suggested that the central engines of some GRBs are newly born magnetars, we aim to relate the behaviors of magnetars to the optical re-brightenings. A newly born magnetar will lose its rotational energy in the form of Poynting-flux, which may be converted into a wind of electron-positron pairs through some magnetic dissipation processes. As proposed by Dai (2004), this wind will catch up with the GRB outflow and a long-lasting reverse shock would form. By applying this scenario to GRB afterglows, we find that the reverse shock propagating back into the electron-positron wind can lead to an observable optical re-brightening and a simultaneous X-ray plateau (or X-ray shallow decay). In our study, we select four GRBs, i.e., GRB 080413B, GRB 090426, GRB 091029, and GRB 100814A, of which the optical afterglows are well observed and show clear re-brightenings. We find that they can be well interpreted. In our scenario, the spin-down timescale of the magnetar should be slightly smaller than the peak time of the re-brightening, which can provide a clue to the characteristics of the magnetar.

astro-ph.HE↗

arXiv2016

Search for $XYZ$ states in $Υ(1S)$ inclusive decays

The branching fractions of the $Υ(1S)$ inclusive decays into final states with a $J/ψ$ or a $ψ(2S)$ are measured with improved precision to be $\BR(Υ(1S)\to J/ψ+ {\rm anything})=(5.25\pm 0.13(\mathrm{stat.})\pm 0.25(\mathrm{syst.}))\times 10^{-4}$ and $\BR(Υ(1S)\to ψ(2S) + {\rm anything})=(1.23\pm 0.17(\mathrm{stat.})\pm 0.11(\mathrm{syst.}))\times 10^{-4}$. The first search for $Υ(1S)$ decays into $XYZ$ states that decay into a $J/ψ$ or a $ψ(2S)$ plus one or two charged tracks yields no significant signals for $XYZ$ states in any of the examined decay modes, and upper limits on their production rates in $Υ(1S)$ inclusive decays are determined.

hep-ex↗

arXiv2016

Observation of the decay $B_s^0\rightarrow K^0\overline{K}^0$

We measure the decay $B_s^0\rightarrow K^0\overline{K}^0$ using data collected at the $Υ(5S)$ resonance with the Belle detector at the KEKB $e^+e^-$ collider. The data sample used corresponds to an integrated luminosity of 121.4 ${\rm fb^{-1}}$. We measure a branching fraction $\mathcal{B}(B_s^0\rightarrow K^0\overline{K}^0) = [19.6\,^{+5.8}_{-5.1}({\rm stat.})\,\pm1.0({\rm sys.})\,\pm 2.0(N^{}_{B_s^0\overline{B}_s^0})]\times 10^{-6}$ with a significance of 5.1 standard deviations. This measurement constitutes the first observation of this decay.

hep-ex↗