arXiv subjects

X. Chen

Publications and source records attributed to X. Chen.

At least 307 records · Page 17Linked to original sources

arXiv2017

Analysis of $D^+\to\bar K^0e^+ν_e$ and $D^+\toπ^0e^+ν_e$ Semileptonic Decays

Using 2.93~fb$^{-1}$ of data taken at 3.773 GeV with the BESIII detector operated at the BEPCII collider, we study the semileptonic decays $D^+ \to \bar K^0e^+ν_e$ and $D^+ \to π^0 e^+ν_e$. We measure the absolute decay branching fractions $\mathcal B(D^+ \to \bar K^0e^+ν_e)=(8.60\pm0.06\pm 0.15)\times10^{-2}$ and $\mathcal B(D^+ \to π^0e^+ν_e)=(3.63\pm0.08\pm0.05)\times10^{-3}$, where the first uncertainties are statistical and the second systematic. We also measure the differential decay rates and study the form factors of these two decays. With the values of $|V_{cs}|$ and $|V_{cd}|$ from Particle Data Group fits assuming CKM unitarity, we obtain the values of the form factors at $q^2=0$, $f^K_+(0) = 0.725\pm0.004\pm 0.012$ and $f^π_+(0) = 0.622\pm0.012\pm 0.003$. Taking input from recent lattice QCD calculations of these form factors, we determine values of the CKM matrix elements $|V_{cs}|=0.944 \pm 0.005 \pm 0.015 \pm 0.024$ and $|V_{cd}|=0.210 \pm 0.004 \pm 0.001 \pm 0.009$, where the third uncertainties are theoretical.

hep-ex↗

arXiv2017

Observation of $Λ^+_{c}\to nK^0_Sπ^+$

We report the first direct measurement of decays of the $Λ^+_c$ baryon involving the neutron. The analysis is performed using 567 pb$^{-1}$ of $e^+e^-$ collision data collected at $\sqrt{s}=4.599$ GeV with the BESIII detector at the BEPCII collider. We observe the decay $Λ^+_{c}\rightarrow n K^0_Sπ^+$ and measure the absolute branching fraction to be $\mathcal{B}({Λ^+_{c}\rightarrow n K^0_Sπ^+})=(1.82\pm0.23({\rm stat})\pm0.11({\rm syst}))\%$. A comparison to $\mathcal{B}({Λ^+_{c}\rightarrow p(\bar{K}π)^0})$ provides an important test of isospin symmetry and final state interactions.

hep-ex↗

arXiv2017

Elliptic flow of electrons from heavy-flavor hadron decays in Au+Au collisions at $\sqrt{s_{\rm NN}} = $ 200, 62.4, and 39 GeV

We present measurements of elliptic flow ($v_2$) of electrons from the decays of heavy-flavor hadrons ($e_{HF}$) by the STAR experiment. For Au+Au collisions at $\sqrt{s_{\rm NN}} = $ 200 GeV we report $v_2$, for transverse momentum ($p_T$) between 0.2 and 7 GeV/c using three methods: the event plane method ($v_{2}${EP}), two-particle correlations ($v_2${2}), and four-particle correlations ($v_2${4}). For Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 62.4 and 39 GeV we report $v_2${2} for $p_T< 2$ GeV/c. $v_2${2} and $v_2${4} are non-zero at low and intermediate $p_T$ at 200 GeV, and $v_2${2} is consistent with zero at low $p_T$ at other energies. The $v_2${2} at the two lower beam energies is systematically lower than at $\sqrt{s_{\rm NN}} = $ 200 GeV for $p_T < 1$ GeV/c. This difference may suggest that charm quarks interact less strongly with the surrounding nuclear matter at those two lower energies compared to $\sqrt{s_{\rm NN}} = 200$ GeV.

hep-ex↗

arXiv2017

MHD modeling of a DIII-D low-torque QH-mode discharge and comparison to observations

Extended-MHD modeling of DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] quiescent H-mode (QH-mode) discharges with nonlinear NIMROD [C. R. Sovinec et al., J. Comput. Phys. 195, 355 (2004)] simulations saturates into a turbulent state but does not saturate when the steady-state flow inferred from measurements is not included. This is consistent with the experimental observations of the quiescent regime on DIII-D. The simulation with flow develops into a saturated turbulent state where the n=1 and 2 toroidal modes become dominant through an inverse cascade. Each mode in the range of n=1-5 is dominant at a different time. Consistent with experimental observations during QH-mode, the simulated state leads to large particle transport relative to the thermal transport. Analysis shows that the amplitude and phase of the density and temperature perturbations differ resulting in greater fluctuation-induced convective particle transport relative to the convective thermal transport. Comparison to magnetic-coil measurements shows that rotation frequencies differ between the simulation and experiment, which indicates that more sophisticated extended-MHD two-fluid modeling is required.

physics.plasm-ph↗

arXiv2017

Evidence of Two Resonant Structures in $e^+ e^- \to π^+ π^- h_c$

The cross sections of $e^{+}e^{-}\toπ^{+}π^{-}h_c$ at center-of-mass energies from 3.896 to 4.600 GeV are measured using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The cross sections are found to be of the same order of magnitude as those of $e^{+}e^{-}\toπ^{+}π^{-}J/ψ$ and $e^{+}e^{-}\toπ^{+}π^{-}ψ(2S)$, but the line shape is inconsistent with the $Y$ states observed in the latter two modes. Two structures are observed in the $e^{+}e^{-}\toπ^{+}π^{-}h_c$ cross sections around 4.22 and 4.39 GeV/$c^{2}$, which we call $Y(4220)$ and $Y(4390)$, respectively. A fit with a coherent sum of two Breit-Wigner functions results in a mass of $(4218.4^{+5.5}_{-4.5}\pm0.9)$ MeV/$c^{2}$ and a width of $(66.0^{+12.3}_{-8.3}\pm0.4)$ MeV for the $Y(4220)$, and a mass of $(4391.6^{+6.3}_{-6.8}\pm1.0)$ MeV/$c^{2}$ and a width of $(139.5^{+16.2}_{-20.6}\pm0.6)$ MeV for the $Y(4390)$, where the first uncertainties are statistical and the second ones systematic. The statistical significance of $Y(4220)$ and $Y(4390)$ is 10$σ$ over one structure assumption.

hep-ex↗

arXiv2017

Search for the Radiative Leptonic Decay $D^{+}\to γe^{+} ν_{e}$

Using an electron-positron collision data sample of 2.93 fb$^{-1}$ collected at a center-of-mass energy of $\sqrt{s}=3.773$ GeV with the BESIII detector, we present the first search for the radiative leptonic decay $D^{+} \rightarrow γe^{+}ν_{e}$. The analysis is performed with a double tag method. We do not observe a significant $D^{+} \rightarrow γe^{+}ν_{e}$ signal, and obtain an upper limit on the branching fraction of $D^{+} \rightarrow γe^{+}ν_{e}$ decay with the energy of radiative photon larger than 10 MeV of $3.0\times10^{-5}$ at the 90% confidence level.

hep-ex↗

arXiv2017

Amplitude analysis of the $χ_{c1} \to ηπ^+π^-$ decays

Using $448.0 \times 10^6$~$ψ(3686)$ events collected with the BESIII detector, an amplitude analysis is performed for $ψ(3686)\toγχ_{c1}$, $χ_{c1}\toηπ^+π^-$ decays. The most dominant two-body structure observed is $a_0(980)^{\pm}π^{\mp}$; $a_0(980)^{\pm}\toηπ^{\pm}$. The $a_0(980)$ line shape is modeled using a dispersion relation, and a significant non-zero $a_0(980)$ coupling to the $η^{\prime}π$ channel is measured. We observe $χ_{c1}\to a_2(1700)π$ production for the first time, with a significance larger than 17$σ$. The production of mesons with exotic quantum numbers, $J^{PC}=1^{-+}$, is investigated, and upper limits for the branching fractions $χ_{c1}\to π_1(1400)^{\pm}π^{\mp}$, $χ_{c1}\to π_1(1600)^{\pm}π^{\mp}$, and $χ_{c1}\toπ_1(2015)^{\pm}π^{\mp}$, with subsequent $π_1(X)^{\pm} \to ηπ^{\pm}$ decay, are determined.

hep-ex↗

arXiv2017

Measurement of the cross section and longitudinal double-spin asymmetry for di-jet production in polarized $pp$ collisions at $\sqrt{s}$ = 200 GeV

We report the first measurement of the longitudinal double-spin asymmetry $A_{LL}$ for mid-rapidity di-jet production in polarized $pp$ collisions at a center-of-mass energy of $\sqrt{s} = 200$ GeV. The di-jet cross section was measured and is shown to be consistent with next-to-leading order (NLO) perturbative QCD predictions. $A_{LL}$ results are presented for two distinct topologies, defined by the jet pseudorapidities, and are compared to predictions from several recent NLO global analyses. The measured asymmetries, the first such correlation measurements, support those analyses that find positive gluon polarization at the level of roughly 0.2 over the region of Bjorken-$x > 0.05$.

hep-ex↗

arXiv2017

Coherent diffractive photoproduction of $ρ^{0}$ mesons on gold nuclei at RHIC

The STAR Collaboration reports on the photoproduction of $π^+π^-$ pairs in gold-gold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly-real photon emitted by one ion scatters from the other ion. We fit the $π^+π^-$ invariant mass spectrum with a combination of $ρ$ and $ω$ resonances and a direct $π^+π^-$ continuum. This is the first observation of the $ω$ in ultra-peripheral collisions, and the first measurement of $ρ-ω$ interference at energies where photoproduction is dominated by Pomeron exchange. The $ω$ amplitude is consistent with the measured $γp\rightarrow ωp$ cross section, a classical Glauber calculation and the $ω\rightarrowπ^+π^-$ branching ratio. The $ω$ phase angle is similar to that observed at much lower energies, showing that the $ρ-ω$ phase difference does not depend significantly on photon energy. The $ρ^0$ differential cross section $dσ/dt$ exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with 2 minima visible. The positions of the diffractive minima agree better with the predictions of a quantum Glauber calculation that does not include nuclear shadowing than with a calculation that does include shadowing.

nucl-ex↗

arXiv2017

Luminosity measurements for the R scan experiment at BESIII

By analyzing the large-angle Bhabha scattering events $e^{+}e^{-}$ $\to$ ($γ$)$e^{+}e^{-}$ and diphoton events $e^{+}e^{-}$ $\to$ $γγ$ for the data sets collected at center-of-mass (c.m.) energies between 2.2324 and 4.5900 GeV (131 energy points in total) with the upgraded Beijing Spectrometer (BESIII) at the Beijing Electron-Positron Collider (BEPCII), the integrated luminosities have been measured at the different c.m. energies, individually. The results are the important inputs for R value and $J/ψ$ resonance parameter measurements.

hep-ex↗

arXiv2017

Switchable Particle Statistics with an Embedding Quantum Simulator

We propose the implementation of a switch of particle statistics with an embedding quantum simulator. By encoding both Bose-Einstein and Fermi-Dirac statistics into an enlarged Hilbert space, the statistics of quantum particles may be changed in situ during the time evolution, from bosons to fermions and from fermions to bosons, as many times as desired. We illustrate our proposal with few-qubit examples, although the protocol is straightforwardly extendable to larger numbers of particles. This proposal can be implemented on different quantum platforms such as trapped ions, quantum photonics, and superconducting circuits, among others. The possibility to implement permutation symmetrization and antisymmetrization of quantum particles enhances the toolbox of quantum simulations, for unphysical operations as well as for symmetry transformations.

quant-ph↗

arXiv2017

Measurements of jet quenching with semi-inclusive hadron+jet distributions in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

The STAR Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum hadron trigger, in central and peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Charged jets are reconstructed with the anti-kT algorithm for jet radii R between 0.2 and 0.5 and with low infrared cutoff of track constituents ($p_T>0.2$ GeV/c). A novel mixed-event technique is used to correct the large uncorrelated background present in heavy ion collisions. Corrected recoil jet distributions are reported at mid-rapidity, for charged-jet transverse momentum $p_T^\mathrm{jet,ch}<30$ GeV/c. Comparison is made to similar measurements for Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV, to calculations for p+p collisions at $\sqrt{s}$ = 200 GeV based on the PYTHIA Monte Carlo generator and on a Next-to-Leading Order perturbative QCD approach, and to theoretical calculations incorporating jet quenching. The recoil jet yield is suppressed in central relative to peripheral collisions, with the magnitude of the suppression corresponding to medium-induced charged energy transport out of the jet cone of $2.8\pm0.2\mathrm{(stat)}\pm1.5\mathrm{(sys)}$ GeV/c, for $10<p_T^\mathrm{jet,ch}<20$ GeV/c and R = 0.5. No medium-induced change in jet shape is observed for $R<0.5$. The azimuthal distribution of low-$p_T^\mathrm{jet,ch}$ recoil jets may be enhanced at large azimuthal angles to the trigger axis, due to scattering off quasi-particles in the hot QCD medium. Measurement of this distribution gives a 90% statistical confidence upper limit to the yield enhancement at large deflection angles in central Au+Au collisions of $50\pm30\mathrm{(sys)\%}$ of the large-angle yield in p+pcollisions predicted by PYTHIA.

nucl-ex↗

arXiv2017

Precise measurement of the $e^+e^-\to π^+π^-J/ψ$ cross section at center-of-mass energies from 3.77 to 4.60 GeV

The cross section for the process $e^+e^-\to π^+π^-J/ψ$ is measured precisely at center-of-mass energies from 3.77 to 4.60~GeV using 9~fb$^{-1}$ of data collected with the BESIII detector operating at the BEPCII storage ring. Two resonant structures are observed in a fit to the cross section. The first resonance has a mass of $(4222.0\pm 3.1\pm 1.4)$~MeV/$c^2$ and a width of $(44.1\pm 4.3\pm 2.0)$~MeV, while the second one has a mass of $(4320.0\pm 10.4 \pm 7.0)$~MeV/$c^2$ and a width of $(101.4^{+25.3}_{-19.7}\pm 10.2)$~MeV, where the first errors are statistical and second ones are systematic. The first resonance agrees with the $Y(4260)$ resonance reported by previous experiments. The precision of its resonant parameters is improved significantly. The second resonance is observed in $e^+e^-\to π^+π^-J/ψ$ for the first time. The statistical significance of this resonance is estimated to be larger than $7.6σ$. The mass and width of the second resonance agree with the $Y(4360)$ resonance reported by the $BABAR$ and Belle experiments within errors. Finally, the $Y(4008)$ resonance previously observed by the Belle experiment is not confirmed in the description of the BESIII data.

hep-ex↗

arXiv2017

Cross section measurements of $e^{+}e^{-}\rightarrow p \bar{p} π^{0}$ at center-of-mass energies between 4.008 and 4.600 GeV

Based on $e^+e^-$ annihilation data samples collected with the BESIII detector at the BEPCII collider at 13 center of mass energies from 4.008 to 4.600 GeV, measurements of the Born cross section of $e^{+}e^{-}\rightarrow p \bar{p} π^{0}$ are performed. No significant resonant structure is observed in the measured energy-dependent cross section. The upper limit on the Born cross section of $e^{+}e^{-}\rightarrow Y(4260) \rightarrow p \bar{p} π^{0}$ at the 90\% C.L. is determined to be 0.01 pb.

hep-ex↗

arXiv2017

Subdwarf B stars from the common envelope ejection channel

From the canonical binary scenario, the majority of sdBs are produced from low-mass stars with degenerate cores where helium is ignited in a way of flashes. Due to numerical difficulties, the models of produced sdBs are generally constructed from more massive stars with non-degenerate cores, leaving several uncertainties on the exact characteristics of sdB stars. Employing MESA, we systematically studied the characteristics of sdBs produced from the common envelope (CE) ejection channel, and found that the sdB stars produced from the CE ejection channel appear to form two distinct groups on the effective temperature-gravity diagram. One group (the flash-mixing model) almost has no H-rich envelope and crows at the hottest temperature end of the extremely horizontal branch (EHB), while the other group has significant H-rich envelope and spreads over the whole canonical EHB region. The key factor for the dichotomy of the sdB properties is the development of convection during the first helium flash, which is determined by the interior structure of the star after the CE ejection. For a given initial stellar mass and a given core mass at the onset of the CE, if the CE ejection stops early, the star has a relatively massive H-rich envelope, resulting in a canonical sdB generally. The fact of only a few short-orbital-period sdB binaries being in the flash-mixing sdB region and the lack of He-rich sdBs in short-orbital-period binaries indicate that the flash mixing is not very often in the products of the CE ejection. A falling back process after the CE ejection, similar to that happened in nova, is an appropriate way of increasing the envelope mass, then prevents the flash mixing.

astro-ph.SR↗

arXiv2017

Amplitude Analysis of the Decays $η^\prime \rightarrow π^+π^-π^0$ and $η^\prime \rightarrow π^0π^0π^0$

Based on a sample of $1.31 \times 10^9$ $J/ψ$ events collected with the BESIII detector, an amplitude analysis of the isospin-violating decays $η^\prime \rightarrow π^+π^-π^0$ and $η^\prime \rightarrow π^0π^0π^0$ is performed. A significant $P$-wave contribution from $η^\prime \rightarrow ρ^{\pm} π^{\mp}$ is observed for the first time in $η^\prime \rightarrow π^+π^-π^0$. The branching fraction is determined to be ${\mathcal B}(η^\prime \rightarrow ρ^{\pm}π^{\mp})=(7.44\pm0.60\pm1.26\pm1.84)\times 10^{-4}$, where the first uncertainty is statistical, the second systematic, and the third model dependent. In addition to the nonresonant $S$-wave component, there is a significant $σ$ meson component. The branching fractions of the combined $S$-wave components are determined to be ${\mathcal B}(η^\prime \rightarrow π^+π^-π^0)_S=(37.63\pm0.77\pm2.22\pm4.48)\times 10^{-4}$ and ${\mathcal B}(η^\prime \rightarrow π^0π^0π^0)=(35.22\pm0.82\pm2.54)\times 10^{-4}$, respectively. The latter one is consistent with previous BESIII measurements.

hep-ex↗

arXiv2017

A luminous and isolated gamma-ray flare from the blazar B2 1215+30

B2 1215+30 is a BL Lac-type blazar that was first detected at TeV energies by the MAGIC atmospheric Cherenkov telescopes, and subsequently confirmed by the VERITAS observatory with data collected between 2009 and 2012. In 2014 February 08, VERITAS detected a large-amplitude flare from B2 1215+30 during routine monitoring observations of the blazar 1ES 1218+304, located in the same field of view. The TeV flux reached 2.4 times the Crab Nebula flux with a variability timescale of < 3.6 h. Multiwavelength observations with Fermi-LAT, Swift, and the Tuorla observatory revealed a correlated high GeV flux state and no significant optical counterpart to the flare, with a spectral energy distribution where the gamma-ray luminosity exceeds the synchrotron luminosity. When interpreted in the framework of a one-zone leptonic model, the observed emission implies a high degree of beaming, with Doppler factor > 10, and an electron population with spectral index < 2.3.

astro-ph.HE↗

arXiv2016

A possible giant planet orbiting the cataclysmic variable LX Ser

LX Ser is a deeply eclipsing cataclysmic variable with an orbital period of $0.^d 1584325$. Sixty two new eclipse times were determined by our observations and the AAVSO International Data base. Combining all available eclipse times, we analyzed the O-C behavior of LX Ser. We found that the O-C diagram of LX Ser shows a sinusoidal oscillation with a period of 22.8 yr and an amplitude of 0.00035 days. Two mechanisms (i.e., the Applegate mechanism and the light travel time effect) are applied to explain the cyclic modulation. We found that the Applegate mechanism is difficult to explain the cyclic oscillation in the orbital period. Therefore, the cyclic period change is most likely to be caused by the light travel time effect due to the presence of a third body. The mass of the tertiary component was determined to be $M_3\sim7.5 M_{Jup}$. We supposed that the tertiary companion is plausible a giant planet. The stability of the giant planet was checked, and we found that the multiple system is stable.

astro-ph.EP↗