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Y. Guo

Publications and source records attributed to Y. Guo.

At least 163 records · Page 9Linked to original sources
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

The inferred evolution of the cold gas properties of CANDELS galaxies at 0.5 < z < 3.0

We derive the total cold gas, atomic hydrogen, and molecular gas masses of approximately 24 000 galaxies covering four decades in stellar mass at redshifts 0.5 < z < 3.0, taken from the CANDELS survey. Our inferences are based on the inversion of a molecular hydrogen based star formation law, coupled with a prescription to separate atomic and molecular gas. We find that: 1) there is an increasing trend between the inferred cold gas (HI and H2), HI, and H2 mass and the stellar mass of galaxies down to stellar masses of 10^8 Msun already in place at z = 3; 2) the molecular fractions of cold gas increase with increasing stellar mass and look-back time; 3) there is hardly any evolution in the mean HI content of galaxies at fixed stellar mass; 4) the cold gas fraction and relative amount of molecular hydrogen in galaxies decrease at a relatively constant rate with time, independent of stellar mass; 5) there is a large population of low-stellar mass galaxies dominated by atomic gas. These galaxies are very gas rich, but only a minor fraction of their gas is molecular; 6) the ratio between star-formation rate (SFR) and inferred total cold gas mass (HI + H2) of galaxies (i.e., star-formation efficiency; SFE) increases with star-formation at fixed stellar masses. Due to its simplicity, the presented approach is valuable to assess the impact of selection biases on small samples of directly-observed gas masses and to extend scaling relations down to stellar mass ranges and redshifts that are currently difficult to probe with direct measurements of gas content.

astro-ph.GA↗
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↗
arXiv2015

An amplitude analysis of the $π^{0}π^{0}$ system produced in radiative $J/ψ$ decays

An amplitude analysis of the $π^{0}π^{0}$ system produced in radiative $J/ψ$ decays is presented. In particular, a piecewise function that describes the dynamics of the $π^{0}π^{0}$ system is determined as a function of $M_{π^{0}π^{0}}$ from an analysis of the $(1.311\pm0.011)\times10^{9}$ $J/ψ$ decays collected by the BESIII detector. The goal of this analysis is to provide a description of the scalar and tensor components of the $π^0π^0$ system while making minimal assumptions about the properties or number of poles in the amplitude. Such a model-independent description allows one to integrate these results with other related results from complementary reactions in the development of phenomenological models, which can then be used to directly fit experimental data to obtain parameters of interest. The branching fraction of $J/ψ\to γπ^{0}π^{0}$ is determined to be $(1.15\pm0.05)\times10^{-3}$, where the uncertainty is systematic only and the statistical uncertainty is negligible.

hep-ex↗
arXiv2015

Measurement of the form factors in the decay $D^+ \to ωe^+ ν_{e}$ and search for the decay $D^+ \to ϕe^+ ν_{e}$

Using 2.92 fb$^{-1}$ of electron-positron annihilation data collected at a center-of-mass energy of $\sqrt{s}= 3.773$ GeV with the BESIII detector, we present an improved measurement of the branching fraction $\mathcal{B}(D^+ \to ωe^+ ν_{e}) = (1.63\pm0.11\pm0.08)\times 10^{-3}$. The parameters defining the corresponding hadronic form factor ratios at zero momentum transfer are determined for the first time, we measure them to be $r_V = 1.24\pm0.09\pm0.06$ and $r_2 = 1.06\pm0.15 \pm 0.05$. The first and second uncertainties are statistical and systematic, respectively. We also search for the decay $D^+ \to ϕe^+ ν_{e}$. An improved upper limit $\mathcal{B}(D^+ \to ϕe^+ ν_{e}) < 1.3 \times 10^{-5}$ is set at 90\% confidence level.

hep-ex↗
arXiv2015

Observation of $η^{\prime}\toωe^{+} e^{-}$

Based on a sample of \etapr mesons produced in the radiative decay $J/ψ\toγη^{\prime}$ in $1.31\times 10^9$ $J/ψ$ events collected with the BESIII detector, the decay $η^{\prime}\toωe^{+} e^{-}$ is observed for the first time, with a statistical significance of $8σ$. The branching fraction is measured to be $\mathcal{B}(η^{\prime}\toωe^{+} e^{-})=(1.97\pm0.34(\text{stat})\pm0.17(\text{syst}))\times10^{-4}$, which is in agreement with theoretical predictions. The branching fraction of $η^{\prime}\toωγ$ is also measured to be $(2.55\pm0.03(\text{stat})\pm0.16(\text{syst}))\times10^{-2}$, which is the most precise measurement to date, and the relative branching fraction $\frac{\mathcal{B}(η^{\prime}\to ωe^{+}e^{-})}{\mathcal{B}(η^{\prime}\to ωγ)}$ is determined to be $(7.71\pm1.34(\text{stat})\pm0.54(\text{syst}))\times10^{-3}$.

hep-ex↗
arXiv2015

Observation of the isospin-violating decay $J/ψ\to ϕπ^{0}f_{0}(980)$

Using a sample of 1.31 billion $J/ψ$ events collected with the BESIII detector at the BEPCII collider, the decays $J/ψ\to ϕπ^{+}π^{-}π^{0}$ and $J/ψ\to ϕπ^{0}π^{0}π^{0}$ are investigated. The isospin violating decay $J/ψ\to ϕπ^{0} f_{0}(980)$ with $f_{0}(980) \to ππ$, is observed for the first time. The width of the $f_{0}(980)$ obtained from the dipion mass spectrum is found to be much smaller than the world average value. In the $π^{0} f_{0}(980)$ mass spectrum, there is evidence of $f_1(1285)$ production. By studying the decay $J/ψ\to ϕη'$, the branching fractions of $η' \to π^{+}π^{-}π^{0}$ and $η' \to π^{0}π^{0}π^{0}$, as well as their ratio, are also measured.

hep-ex↗
arXiv2015

Search for $Z_c(3900)^\pm\toωπ^\pm$

The decay $Z_c(3900)^\pm\toωπ^\pm$ is searched for using data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies $\sqrt{s}=4.23$ and $4.26$~GeV. No significant signal for the $Z_c(3900)^\pm$ is found, and upper limits at the 90\% confidence level on the Born cross section for the process $e^+e^-\to Z_c(3900)^\pmπ^\mp\toωπ^+π^-$ are determined to be $0.26$ and $0.18$ pb at $\sqrt{s}=4.23$ and 4.26 GeV, respectively.

hep-ex↗
arXiv2015

Searches for isospin-violating transitions $χ_{c0,2} \rightarrow π^{0} η_{c}$

We present the first upper-limit measurement of the branching fractions of the isospin-violating transitions $χ_{c0,2} \rightarrow π^{0} η_{c}$. The measurements are performed using $106\times 10^{6}$ $ψ(3686)$ events accumulated with the BESIII detector at the BEPCII $e^{+}e^{-}$ collider at a center-of-mass energy corresponding to the $ψ(3686)$ mass. We obtained upper limits on the branching fractions at a 90\% confidence level of $B(χ_{c0} \rightarrow π^{0} η_{c}) < 1.6 \times 10^{-3}$ and $B(χ_{c2} \rightarrow π^{0} η_{c}) < 3.2 \times 10^{-3}$.

hep-ex↗
arXiv2015

Can We Determine the Filament Chirality by the Filament Footpoint Location or the Barb-bearing?

We attempt to propose a method for automatically detecting the solar filament chirality and barb bearing. We first introduce the unweighted undirected graph concept and adopt the Dijkstra shortest-path algorithm to recognize the filament spine. Then, we use the polarity inversion line (PIL) shift method for measuring the polarities on both sides of the filament, and employ the connected components labeling method to identify the barbs and calculate the angle between each barb and the spine to determine the bearing of the barbs, i.e., left or right. We test the automatic detection method with H-alpha filtergrams from the Big Bear Solar Observatory (BBSO) H-alpha archive and magnetograms observed with the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Four filaments are automatically detected and illustrated to show the results. The barbs in different parts of a filament may have opposite bearings. The filaments in the southern hemisphere (northern hemisphere) mainly have left-bearing (right-bearing) barbs and positive (negative) magnetic helicity, respectively. The tested results demonstrate that our method is efficient and effective in detecting the bearing of filament barbs. It is demonstrated that the conventionally believed one-to-one correspondence between filament chirality and barb bearing is not valid. The correct detection of the filament axis chirality should be done by combining both imaging morphology and magnetic field observations.

astro-ph.SR↗
arXiv2015

Extreme Ultraviolet Imaging of Three-dimensional Magnetic Reconnection in a Solar Eruption

Magnetic reconnection, a change of magnetic field connectivity, is a fundamental physical process in which magnetic energy is released explosively. It is responsible for various eruptive phenomena in the universe. However, this process is difficult to observe directly. Here, the magnetic topology associated with a solar reconnection event is studied in three dimensions (3D) using the combined perspectives of two spacecraft. The sequence of extreme ultraviolet (EUV) images clearly shows that two groups of oppositely directed and non-coplanar magnetic loops gradually approach each other, forming a separator or quasi-separator and then reconnecting. The plasma near the reconnection site is subsequently heated from $\sim$1 to $\ge$5 MK. Shortly afterwards, warm flare loops ($\sim$3 MK) appear underneath the hot plasma. Other observational signatures of reconnection, including plasma inflows and downflows, are unambiguously revealed and quantitatively measured. These observations provide direct evidence of magnetic reconnection in a 3D configuration and reveal its origin.

astro-ph.SR↗
arXiv2015

Search for $D^0\toγγ$ and improved measurement of the branching fraction for $D^0 \to π^0π^0$

Using $2.92$ fb$^{-1}$ of electron-positron annihilation data collected at $\sqrt{s} = 3.773$~GeV with the BESIII detector, we report the results of a search for the flavor-changing neutral current process $D^0\toγγ$ using a double-tag technique. We find no signal and set an upper limit at $90\%$ confidence level for the branching fraction of $B(D^0\toγγ) < 3.8\times10^{-6}$. We also investigate $D^0$-meson decay into two neutral pions, obtaining a branching fraction of $B(D^0\toπ^0π^0) = (8.24\pm0.21(\text{stat.})\pm0.30(\text{syst.}))\times10^{-4}$, the most precise measurement to date and consistent with the current world average

hep-ex↗
arXiv2015

Microphase transitions of block copolymer/homopolymer under shear flow

Cell dynamics simulation is used to investigate the phase behavior of block copolymer/homopolymer mixture subjected to a steady shear flow. Phase transitions occur from transverse to parallel and then to perpendicular lamellar structure with an increase of shear rate and this is the result of interaction between the shear flow and the concentration fluctuation. Rheological properties, such as normal stress differences and shear viscosity, are all closely related with the direction of the lamellae. Furthermore, we specifically explore the phase behavior and the order parameter under weak and strong shear of two different initial states, and realize the importance of the thermal history. It is necessary to apply the shear field at the appropriate time if we want to get what we want. These results provide an easy method to create ordered, defect-free materials in experiment and engineering technology through imposing shear flow.

cond-mat.soft↗
arXiv2015

Probing Parton Dynamics of QCD Matter with $Ω$ and $ϕ$ Production

We present measurements of $Ω$ and $ϕ$ production at mid-rapidity from Au+Au collisions at nucleon-nucleon center-of-mass energies $\sqrt{s_{NN}}$ = 7.7, 11.5, 19.6, 27 and 39 GeV by the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). Motivated by the coalescence formation mechanism for these strange hadrons, we study the ratios of $N(Ω^{-}+Ω^{+})/(2N(ϕ))$. These ratios as a function of transverse momentum ($p_T$) fall on a consistent trend at high collision energies, but start to show deviations in peripheral collisions at $\sqrt{s_{NN}}$ = 19.6, 27 and 39 GeV, and in central collisions at 11.5 GeV in the intermediate $p_T$ region of 2.4-3.6 GeV/c. We further evaluate empirically the strange quark $p_T$ distributions at hadronization by studying the $Ω/ϕ$ ratios scaled by the number of constituent quarks. The NCQ-scaled $Ω/ϕ$ ratios show a suppression of strange quark production in central collisions at 11.5 GeV compared to $\sqrt{s_{NN}} >= 19.6$ GeV. The shapes of the presumably thermal strange quark distributions in 0-60% most central collisions at 7.7 GeV show significant deviations from those in 0-10% most central collisions at higher energies. These features suggest that there is likely a change of the underlying strange quark dynamics in the transition from quark-matter to hadronic matter at collision energies below 19.6 GeV.

nucl-ex↗
arXiv2015

Measurement of the Matrix Elements for the Decays $η\rightarrow π^{+}π^{-}π^0$ and $η/η^{\prime}\rightarrowπ^0π^0π^0$

Based on a sample of $1.31 \times 10^9$ $J/ψ$ events collected with the BESIII detector at the BEPCII collider, Dalitz plot analyses of selected 79,625 $η\rightarrowπ^{+}π^{-}π^0$ events, 33,908 $η\rightarrowπ^0π^0π^0$ events and 1,888 $η^{\prime}\rightarrowπ^0π^0π^0$ events are performed. The measured matrix elements of $η\rightarrowπ^+π^-π^0$ are in reasonable agreement with previous measurements. The Dalitz plot slope parameters of $η\rightarrowπ^0π^0π^0$ and $η^{\prime}\rightarrowπ^0π^0π^0$ are determined to be $-0.055 \pm 0.014 \pm 0.004$ and $-0.640 \pm 0.046 \pm 0.047$, respectively, where the first uncertainties are statistical and the second systematic. Both values are consistent with previous measurements, while the precision of the latter one is improved by a factor of three. Final state interactions are found to have an important role in those decays.

hep-ex↗