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X. Chen

Publications and source records attributed to X. Chen.

At least 379 records · Page 21Linked to original sources
arXiv2015

Measurement of the $\mathrm e^+\mathrm e^-\rightarrow\mathrmπ^+\mathrmπ^-$ Cross Section between 600 and 900 MeV Using Initial State Radiation

We extract the $e^+e^-\rightarrow π^+π^-$ cross section in the energy range between 600 and 900 MeV, exploiting the method of initial state radiation. A data set with an integrated luminosity of 2.93 fb$^{-1}$ taken at a center-of-mass energy of 3.773 GeV with the BESIII detector at the BEPCII collider is used. The cross section is measured with a systematic uncertainty of 0.9%. We extract the pion form factor $|F_π|^2$ as well as the contribution of the measured cross section to the leading order hadronic vacuum polarization contribution to $(g-2)_μ$. We find this value to be $a_μ^{ππ,\rm LO}(600-900\;\rm MeV) = (368.2 \pm 2.5_{\rm stat} \pm 3.3_{\rm sys})\cdot 10^{-10}$.

hep-ex↗
arXiv2015

Confirmation of a charged charmoniumlike state $Z_c(3885)^{\mp}$ in $e^+e^-\toπ^{\pm}(D\bar{D}^*)^\mp$ with double $D$ tag

We present a study of the process $e^+e^-\toπ^{\pm}(D\bar{D}^*)^{\mp}$ using data samples of 1092~pb$^{-1}$ at $\sqrt{s}=4.23$~GeV and 826~pb$^{-1}$ at $\sqrt{s}=4.26$~GeV collected with the BESIII detector at the BEPCII storage ring. With full reconstruction of the $D$ meson pair and the bachelor $π^{\pm}$ in the final state, we confirm the existence of the charged structure $Z_c(3885)^{\mp}$ in the $(D\bar{D}^*)^{\mp}$ system in the two isospin processes $e^+e^-\toπ^+D^0D^{*-}$ and $e^+e^-\toπ^+D^-D^{*0}$. By performing a simultaneous fit, the statistical significance of $Zc(3885)^{\mp}$ signal is determined to be greater than 10$σ$, and its pole mass and width are measured to be $M_{\rm{pole}}$=(3881.7$\pm$1.6(stat.)$\pm$1.6(syst.))~MeV/$c^2$ and $Γ_{\rm{pole}}$=(26.6$\pm$2.0(stat.)$\pm$2.1(syst.))~MeV, respectively. The Born cross section times the $(D\bar{D}^*)^{\mp}$ branching fraction ($σ(e^+e^-\toπ^{\pm}Z_{c}(3885)^{\mp}) \times Br(Z_{c}(3885)^{\mp}\to(D\bar{D}^*)^{\mp})$) is measured to be $(141.6\pm7.9(\text{stat.})\pm12.3(\text{syst.}))~\text{pb}$ at $\sqrt{s}=4.23$~GeV and $(108.4\pm6.9(\text{stat.})\pm8.8(\text{syst.}))~\text{pb}$ at $\sqrt{s}=4.26$~GeV. The polar angular distribution of the $π^{\pm}$-$Z_c(3885)^{\mp}$ system is consistent with the expectation of a quantum number assignment of $J^P=1^+$ for $Z_c(3885)^{\mp}$.

hep-ex↗
arXiv2015

Azimuthal anisotropy in U+U and Au+Au collisions at RHIC

Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle cumulants, $v_2\{2\}$ and $v_2\{4\}$, for charged hadrons from U+U collisions at $\sqrt{s_{\rm NN}}$ = 193 GeV and Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. Nearly fully overlapping collisions are selected based on the amount of energy deposited by spectators in the STAR Zero Degree Calorimeters (ZDCs). Within this sample, the observed dependence of $v_2\{2\}$ on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in U+U collisions. An initial-state model with gluon saturation describes the slope of $v_2\{2\}$ as a function of multiplicity in central collisions better than one based on Glauber with a two-component multiplicity model.

nucl-ex↗
arXiv2015

Measurement of the center-of-mass energies at BESIII via the di-muon process

From 2011 to 2014, the BESIII experiment collected about 5 fb$^{-1}$ data at center-of-mass energies around 4 GeV for the studies of the charmonium-like and higher excited charmonium states. By analyzing the di-muon process $e^{+}e^{-}\rightarrowγ_{\rm ISR/FSR}μ^{+}μ^{-}$, the center-of-mass energies of the data samples are measured with a precision of 0.8 MeV. The center-of-mass energy is found to be stable for most of time during the data taking.

hep-ex↗
arXiv2015

Di-Hadron Correlations with Identified Leading Hadrons in 200 GeV Au+Au and d+Au Collisions at STAR

The STAR collaboration presents for the first time two-dimensional di-hadron correlations with identified leading hadrons in 200 GeV central Au+Au and minimum-bias d+Au collisions to explore hadronization mechanisms in the quark gluon plasma. The enhancement of the jet-like yield for leading pions in Au+Au data with respect to the d+Au reference and the absence of such an enhancement for leading non-pions (protons and kaons) are discussed within the context of a quark recombination scenario. The correlated yield at large angles, specifically in the \emph{ridge region}, is found to be significantly higher for leading non-pions than pions. The consistencies of the constituent quark scaling, azimuthal harmonic model and a mini-jet modification model description of the data are tested, providing further constraints on hadronization.

nucl-ex↗
arXiv2015

Measurement of the branching fractions of $D_{s}^{+}\rightarrow η'X$ and $D_{s}^{+}\rightarrow η'ρ^{+}$ in $e^+e^-\to D^+_{s}D^-_{s}$

We study $D_{s}^{+}$ decays to final states involving the $η'$ with a 482$\,$pb$^{-1}$ data sample collected at $\sqrt{s}$ = 4.009$\,$GeV with the \mbox{BESIII} detector at the BEPCII collider. We measure the branching fractions $\mathcal{B}(D^+_{s}\rightarrow η'X)$ = (8.8$\pm$1.8$\pm$0.5)$\%$ and $\mathcal{B}(D_{s}^{+}\rightarrow η'ρ^{+})$ = ($5.8\pm1.4\pm0.4$)$\%$ where the first uncertainty is statistical and the second is systematic. In addition, we estimate an upper limit on the non-resonant branching ratio $\mathcal{B}(D_{s}^{+}\rightarrow η'π^+π^0)<5.1\%$ at the 90$\%$ confidence level. Our results are consistent with CLEO's recent measurements and help to resolve the disagreement between the theoretical prediction and CLEO's previous measurement of $\mathcal{B}(D_{s}^{+}\rightarrow η'ρ^{+})$.

hep-ex↗
arXiv2015

Measurement of the absolute branching fraction for $Λ^+_{c}\to Λe^+ν_e$

We report the first absolute measurement of the branching fraction of $Λ^+_{c}\rightarrow Λe^+ν_e$. This measurement is based on 567 pb$^{-1}$ of $e^+e^-$ annihilation data produced at $\sqrt{s}=4.599$ GeV, which is just above the $Λ^+_c\barΛ^-_c$ threshold. The data were collected with the BESIII detector at the BEPCII storage rings. The branching fraction is determined to be $\mathcal B({Λ^+_c\rightarrow Λe^+ν_e})=(3.63\pm0.38({\rm stat})\pm0.20({\rm syst}))\%$, representing a more than twofold improvement in precision upon previously published results. As the branching fraction for $Λ^+_{c}\rightarrow Λe^+ν_e$ is the benchmark for those of other $Λ^+_c$ semileptonic channels, our result provides a unique test of different theoretical models, which is the most stringent to date.

hep-ex↗
arXiv2015

Observation of a neutral charmoniumlike state $Z_c(4025)^0$ in $e^{+} e^{-} \to (D^{*} \bar{D}^{*})^{0} π^0$

We report a study of the process $e^{+} e^{-} \to (D^{*} \bar{D}^{*})^{0} π^0$ using $e^+e^-$ collision data samples with integrated luminosities of $1092 \rm{pb}^{-1}$ at $\sqrt{s}=4.23 \rm{GeV}$ and $826 \rm{pb}^{-1}$ at $\sqrt{s}=4.26 \rm{GeV}$ collected with the BESIII detector at the BEPCII storage ring. We observe a new neutral structure near the $(D^{*} \bar{D}^{*})^{0}$ mass threshold in the $π^0$ recoil mass spectrum, which we denote as $Z_{c}(4025)^{0}$. Assuming a Breit-Wigner line shape, its pole mass and pole width are determined to be $(4025.5^{+2.0}_{-4.7}\pm3.1) \rm{MeV}/c^2$ and $(23.0\pm 6.0\pm 1.0) \rm{MeV}$, respectively. The Born cross sections of $e^{+}e^{-}\to Z_{c}(4025)^{0} π^0\to (D^{*} \bar{D}^{*})^{0}π^0$ are measured to be $(61.6\pm8.2\pm9.0) \rm{pb}$ at $\sqrt{s}=4.23 \rm{GeV}$ and $(43.4\pm8.0\pm5.4) \rm{pb}$ at $\sqrt{s}=4.26 \rm{GeV}$. The first uncertainties are statistical and the second are systematic.

hep-ex↗
arXiv2015

Search for new phenomena in events with a photon and missing transverse momentum in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

Results of a search for new phenomena in events with an energetic photon and large missing transverse momentum with the ATLAS experiment at the LHC are reported. Data were collected in proton--proton collisions at a center-of-mass energy of 8 TeV and correspond to an integrated luminosity of 20.3 fb$^{-1}$. The observed data are well described by the expected Standard Model backgrounds. The expected (observed) upper limit on the fiducial cross section for the production of such events is 6.1 (5.3) fb at 95% confidence level. Exclusion limits are presented on models of new phenomena with large extra spatial dimensions, supersymmetric quarks, and direct pair production of dark-matter candidates.

hep-ex↗
arXiv2015

Science Highlights from VERITAS

The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is a ground-based array located at the Fred Lawrence Whipple Observatory in southern Arizona and is one of the world's most sensitive gamma-ray instruments at energies of 85 GeV to $>$30 TeV. VERITAS has a wide scientific reach that includes the study of extragalactic and Galactic objects as well as the search for astrophysical signatures of dark matter and the measurement of cosmic rays. In this paper, we will summarize the current status of the VERITAS observatory and present some of the scientific highlights from the last two years, focusing in particular on those results shown at the 2015 ICRC in The Hague, Netherlands.

astro-ph.HE↗
arXiv2015

JUNO Conceptual Design Report

The Jiangmen Underground Neutrino Observatory (JUNO) is proposed to determine the neutrino mass hierarchy using an underground liquid scintillator detector. It is located 53 km away from both Yangjiang and Taishan Nuclear Power Plants in Guangdong, China. The experimental hall, spanning more than 50 meters, is under a granite mountain of over 700 m overburden. Within six years of running, the detection of reactor antineutrinos can resolve the neutrino mass hierarchy at a confidence level of 3-4$σ$, and determine neutrino oscillation parameters $\sin^2θ_{12}$, $Δm^2_{21}$, and $|Δm^2_{ee}|$ to an accuracy of better than 1%. The JUNO detector can be also used to study terrestrial and extra-terrestrial neutrinos and new physics beyond the Standard Model. The central detector contains 20,000 tons liquid scintillator with an acrylic sphere of 35 m in diameter. $\sim$17,000 508-mm diameter PMTs with high quantum efficiency provide $\sim$75% optical coverage. The current choice of the liquid scintillator is: linear alkyl benzene (LAB) as the solvent, plus PPO as the scintillation fluor and a wavelength-shifter (Bis-MSB). The number of detected photoelectrons per MeV is larger than 1,100 and the energy resolution is expected to be 3% at 1 MeV. The calibration system is designed to deploy multiple sources to cover the entire energy range of reactor antineutrinos, and to achieve a full-volume position coverage inside the detector. The veto system is used for muon detection, muon induced background study and reduction. It consists of a Water Cherenkov detector and a Top Tracker system. The readout system, the detector control system and the offline system insure efficient and stable data acquisition and processing.

physics.ins-det↗
arXiv2015

Energy dependence of acceptance-corrected dielectron excess mass spectrum at mid-rapidity in Au+Au collisions at $\sqrt{s_{NN}} = 19.6$ and 200 GeV

The acceptance-corrected dielectron excess mass spectra, where the known hadronic sources have been subtracted from the inclusive dielectron mass spectra, are reported for the first time at mid-rapidity $|y_{ee}|<1$ in minimum-bias Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6 and 200 GeV. The excess mass spectra are consistently described by a model calculation with a broadened $ρ$ spectral function for $M_{ee}<1.1$ GeV/$c^{2}$. The integrated dielectron excess yield at $\sqrt{s_{NN}}$ = 19.6 GeV for $0.4<M_{ee}<0.75$ GeV/$c^2$, normalized to the charged particle multiplicity at mid-rapidity, has a value similar to that in In+In collisions at $\sqrt{s_{NN}}$ = 17.3 GeV. For $\sqrt{s_{NN}}$ = 200 GeV, the normalized excess yield in central collisions is higher than that at $\sqrt{s_{NN}}$ = 17.3 GeV and increases from peripheral to central collisions. These measurements indicate that the lifetime of the hot, dense medium created in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV is longer than those in peripheral collisions and at lower energies.

hep-ex↗
arXiv2015

Planck intermediate results. XXV. The Andromeda Galaxy as seen by Planck

The Andromeda Galaxy (M31) is one of a few galaxies that has sufficient angular size on the sky to be resolved by the Planck satellite. Planck has detected M31 in all of its frequency bands, and has mapped out the dust emission with the High Frequency Instrument, clearly resolving multiple spiral arms and sub-features. We examine the morphology of this long-wavelength dust emission as seen by Planck, including a study of its outermost spiral arms, and investigate the dust heating mechanism across M31. We find that dust dominating the longer wavelength emission ($\gtrsim 0.3\,$mm) is heated by the diffuse stellar population (as traced by 3.6$\,μ$m emission), with the dust dominating the shorter wavelength emission heated by a mix of the old stellar population and star-forming regions (as traced by 24$\,μ$m emission). We also fit spectral energy distributions (SEDs) for individual 5' pixels and quantify the dust properties across the galaxy, taking into account these different heating mechanisms, finding that there is a linear decrease in temperature with galactocentric distance for dust heated by the old stellar population, as would be expected, with temperatures ranging from around 22$\,$K in the nucleus to 14$\,$K outside of the 10$\,$kpc ring. Finally, we measure the integrated spectrum of the whole galaxy, which we find to be well-fitted with a global dust temperature of ($18.2\pm1.0$)$\,$K with a spectral index of $1.62\pm0.11$ (assuming a single modified blackbody), and a significant amount of free-free emission at intermediate frequencies of 20-60$\,$GHz, which corresponds to a star formation rate of around $0.12$M$_\odot\,$yr$^{-1}$. We find a $2.3\,σ$ detection of the presence of spinning dust emission, with a 30$\,$GHz amplitude of $0.7\pm0.3\,$Jy, which is in line with expectations from our Galaxy.

astro-ph.GA↗
arXiv2015

Large Fermi Surface of Heavy Electrons at the Border of Mott Insulating State in NiS2

One of the early triumphs of quantum physics is the explanation why some materials are metallic whereas others are insulating. While a treatment based on single electron states correctly predicts the character of most materials this approach can fail spectacularly, when the electrostatic repulsion between electrons causes strong correlations. Not only can these favor new and subtle forms of order in metals, such as magnetism or superconductivity, they can even cause the electrons in a half-filled energy band to lock into position altogether, producing a correlated, or Mott insulator. Arguably the most extreme manifestation of electronic correlations in dense electronic matter, the transition into the Mott insulating state raises a number of fundamental questions. Foremost among these is the fate of the electronic Fermi surface and the associated charge carrier mass, as the Mott transition is approached at low temperature, which have been particularly controversial in the high temperature superconducting cuprates. We report the first direct observation of the Fermi surface on the metallic side of a Mott insulating transition by high pressure quantum oscillatory measurements in NiS2. Our results point at a large Fermi surface consistent with Luttinger's theorem and a strongly enhanced carrier effective mass, suggesting that electron localization occurs via a diverging effective mass and concomitant slowing down of charge carriers as predicted theoretically.

cond-mat.str-el↗
arXiv2015

Observation and Spin-Parity Determination of the $X(1835)$ in $J/ψ\rightarrowγK^0_S K^0_Sη$

We report an observation of the process $J/ψ\rightarrowγX(1835)\rightarrowγK^0_S K^0_Sη$ at low $K^0_S K^0_S$ mass with a statistical significance larger than 12.9$σ$ using a data sample of $1.31 \times 10^{9}$ $J/ψ$ events collected with the BESIII detector. In this region of phase space the $K^0_S K^0_S$ system is dominantly produced through the $f_0(980)$. By performing a partial wave analysis, we determine the spin-parity of the $X(1835)$ to be $J^{PC}=0^{-+}$. The mass and width of the observed $X(1835)$ are $1844\pm9(\text{stat})^{+16}_{-25}(\text{syst})$~\MeV~and $192^{+20}_{-17}(\text{stat})^{+62}_{-43}(\text{syst})$~MeV, respectively, which are consistent with the results obtained by BESIII in the channel $J/ψ\rightarrowγπ^+π^-η^{\prime}$.

hep-ex↗
arXiv2015

Study of Dynamics of $D^0 \to K^- e^+ ν_{e}$ and $D^0\toπ^- e^+ ν_{e}$ Decays

In an analysis of a 2.92~fb$^{-1}$ data sample taken at 3.773~GeV with the BESIII detector operated at the BEPCII collider, we measure the absolute decay branching fractions to be $\mathcal B(D^0 \to K^-e^+ν_e)=(3.505\pm 0.014 \pm 0.033)\%$ and $\mathcal B(D^0 \to π^-e^+ν_e)=(0.295\pm 0.004\pm 0.003)\%$. From a study of the differential decay rates we obtain the products of hadronic form factor and the magnitude of the CKM matrix element $f_{+}^K(0)|V_{cs}|=0.7172\pm0.0025\pm 0.0035$ and $f_{+}^π(0)|V_{cd}|=0.1435\pm0.0018\pm 0.0009$. Combining these products with the values of $|V_{cs(d)}|$ from the SM constraint fit, we extract the hadronic form factors $f^K_+(0) = 0.7368\pm0.0026\pm 0.0036$ and $f^π_+(0) = 0.6372\pm0.0080\pm 0.0044$, and their ratio $f_+^π(0)/f_+^{K}(0)=0.8649\pm 0.0112\pm 0.0073$. These form factors and their ratio are used to test unquenched Lattice QCD calculations of the form factors and a light cone sum rule (LCSR) calculation of their ratio. The measured value of $f_+^{K(π)}(0) |V_{cs(d)}|$ and the lattice QCD value for $f^{K(π)}_+(0)$ are used to extract values of the CKM matrix elements of $|V_{cs}|=0.9601 \pm 0.0033 \pm 0.0047 \pm 0.0239$ and $|V_{cd}|=0.2155 \pm 0.0027 \pm 0.0014 \pm 0.0094$, where the third errors are due to the uncertainties in lattice QCD calculations of the form factors. Using the LCSR value for $f_+^π(0)/f_+^K(0)$, we determine the ratio $|V_{cd}|/|V_{cs}|=0.238\pm 0.004\pm 0.002\pm 0.011$, where the third error is from the uncertainty in the LCSR normalization. In addition, we measure form factor parameters for three different theoretical models that describe the weak hadronic charged currents for these two semileptonic decays. All of these measurements are the most precise to date.

hep-ex↗
arXiv2015

Energy Dependence of $Kπ$, $pπ$, and $Kp$ Fluctuations in Au+Au Collisions from $\rm \sqrt{s_{NN}}$ = 7.7 to 200 GeV

A search for the quantum chromodynamics (QCD) critical point was performed by the STAR experiment at the Relativistic Heavy Ion Collider, using dynamical fluctuations of unlike particle pairs. Heavy-ion collisions were studied over a large range of collision energies with homogeneous acceptance and excellent particle identification, covering a significant range in the QCD phase diagram where a critical point may be located. Dynamical $Kπ$, $pπ$, and $Kp$ fluctuations as measured by the STAR experiment in central 0-5\% Au+Au collisions from center-of-mass collision energies $\rm \sqrt{s_{NN}}$ = 7.7 to 200 GeV are presented. The observable $\rm ν_{dyn}$ was used to quantify the magnitude of the dynamical fluctuations in event-by-event measurements of the $Kπ$, $pπ$, and $Kp$ pairs. The energy dependences of these fluctuations from central 0-5\% Au+Au collisions all demonstrate a smooth evolution with collision energy.

nucl-ex↗