arXiv subjects

Y. Gao

Publications and source records attributed to Y. Gao.

At least 289 records · Page 16Linked to original sources

arXiv2017

Evidence for the singly-Cabibbo-suppressed decay $Λ_{c}^{+} \to pη$ and search for $Λ_{c}^{+} \to pπ^{0}$

We study the singly-Cabibbo-suppressed decays $Λ_c^{+}\to pη$ and $Λ_{c}^{+}\to pπ^{0}$ using $Λ_{c}^{+}Λ_{c}^{+}$ pairs produced by $e^+e^-$ collisions at a center-of-mass energy of $\sqrt{s} = 4.6\; GeV$. The data sample was collected by the BESIII detector at the BEPCII collider and corresponds to an integrated luminosity of 567 $pb^{-1}$. We find the first evidence for the decay $Λ_{c}^{+}\to p η$ with a statistical significance of $4.2σ$ and measure its branching fraction to be $\mathcal{B}(Λ_c^+\to pη)= (1.24\pm0.28({\rm{stat.}})\pm0.10({\rm{syst.}}))\times10^{-3}$. No significant $Λ_c^+ \to pπ^0$ signal is observed. We set an upper limit on its branching fraction $\mathcal{B}(Λ_c^+ \to pπ^{0}) < 2.7 \times 10^{-4}$ at the 90% confidence level.

hep-ex↗

arXiv2017

Observation of the helicity-selection-rule suppressed decay of the $χ_{c2}$ charmonium state

The decays of $χ_{c2}\to K^{+}K^{-}π^{0}$, $K_{S} K^{\pm}π^{\mp}$ and $π^{+}π^{-}π^{0}$ are studied with the $ψ(3686)$ data samples collected with the Beijing Spectrometer (BESIII). For the first time, the branching fractions of $χ_{c2}\to K^{\ast}\overline{K}$, $χ_{c2}\to a_{2}^{\pm}(1320)π^{\mp}/a_{2}^{0}(1320)π^{0}$ and $χ_{c2}\to ρ(770)^{\pm}π^{\mp}$ are measured. Here $K^{\ast}\overline{K}$ denotes both $K^{\ast\pm}K^{\mp}$ and $K^{\ast 0}\overline{K}{}^{0}+c.c.$, and $K^{\ast}$ denotes the resonances $K^{\ast}(892)$, $K^{\ast}_{2}(1430)$ and $K^{\ast}_{3}(1780)$. The observations indicate a strong violation of the helicity selection rule in $χ_{c2}$ decays into vector and pseudoscalar meson pairs. The measured branching fractions of $χ_{c2}\to K^{\ast}(892)\overline{K}$ are more than 10 times larger than the upper limit of $χ_{c2}\to ρ(770)^{\pm}π^{\mp}$, which is so far the first direct observation of a significant $U$-spin symmetry breaking effect in charmonium decays.

hep-ex↗

arXiv2017

Determination of spin and parity of the $Z_c(3900)$

The spin and parity of the $Z_c(3900)^\pm$ state are determined to be $J^P=1^+$ with a statistical significance larger than $7σ$ over other quantum numbers in a partial wave analysis of the process $e^+e^-\to π^+π^-J/ψ$. We use a data sample of 1.92 fb$^{-1}$ accumulated at $\sqrt{s}=4.23$ and 4.26 GeV with the BESIII experiment. When parameterizing the $Z_c(3900)^\pm$ with a Flatte-like formula, we determine its pole mass $M_\textrm{pole}=(3881.2\pm4.2_\textrm{stat}\pm52.7_\textrm{syst})\textrm{MeV}/c^2$ and pole width $Γ_\textrm{pole}=(51.8\pm4.6_\textrm{stat}\pm36.0_\textrm{syst})\textrm{MeV}$. We also measure cross sections for the process $e^+e^-\to Z_c(3900)^+π^-+c.c.\to J/ψπ^+π^-$ and determine an upper limit at the 90\% confidence level for the process $e^+e^-\to Z_c(4020)^+π^-+c.c.\to J/ψπ^+π^-$.

hep-ex↗

arXiv2017

Description of Charged Particle Pseudorapidity Distributions in Pb+Pb Collisions with Tsallis Thermodynamics

The centrality dependence of pseudorapidity distributions for charged particles produced in Au+Au collisions at $\sqrt{s_{NN}}=130$ GeV and 200 GeV at RHIC, and in Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV at LHC are investigated in the fireball model, assuming that the rapidity axis is populated with fireballs following one distribution function. We assume that the particles in the fireball fulfill the Tsallis distribution. The theoretical results are compared with the experimental measurements and a good agreement is found. Using these results, the pseudorapidity distributions of charged particles produced in Pb+Pb central collisions at $\sqrt{s_{NN}}=5.02$ TeV and 10 TeV are predicted.

nucl-th↗

arXiv2017

Study of $J/ψ$ and $ψ(3686)\rightarrowΣ(1385)^{0}\barΣ(1385)^{0}$ and $Ξ^0\barΞ^{0}$

We study the decays of $J/ψ$ and $ψ(3686)$ to the final states $Σ(1385)^{0}\barΣ(1385)^{0}$ and $Ξ^0\barΞ^{0}$ based on a single baryon tag method using data samples of $(1310.6 \pm 7.0) \times 10^{6}$ $J/ψ$ and $(447.9 \pm 2.9) \times 10^{6}$ $ψ(3686)$ events collected with the BESIII detector at the BEPCII collider. The decays to $Σ(1385)^{0}\barΣ(1385)^{0}$ are observed for the first time. The measured branching fractions of $J/ψ$ and $ψ(3686)\rightarrowΞ^0\barΞ^{0}$ are in good agreement with, and much more precise, than the previously published results. The angular parameters for these decays are also measured for the first time. The measured angular decay parameter for $J/ψ\rightarrowΣ(1385)^{0}\barΣ(1385)^{0}$, $α=-0.64 \pm 0.03 \pm 0.10$, is found to be negative, different to the other decay processes in this measurement. In addition, the "12\% rule" and isospin symmetry in the $J/ψ$ and $ψ(3686)\rightarrowΞ\barΞ$ and $Σ(1385)\barΣ(1385)$ systems are tested.

hep-ex↗

arXiv2017

Group divisible (K_4-e)-packings with any minimum leave

A decomposition of $K_{n(g)}\setminus L$, the complete n-partite equipartite graph with a subgraph L (called the leave) removed, into edge disjoint copies of a graph G is called a maximum group divisible packing of $K_{n(g)}$ with G if L contains as few edges as possible. We examine all possible minimum leaves for maximum group divisible $(K_4-e)$-packings. Necessary and sufficient conditions are established for their existences.

math.CO↗

arXiv2017

Evidence for $e^{+}e^{-} \to γη_c(1S)$ at center-of-mass energies between 4.01 and 4.60 GeV

We present first evidence for the process $e^+e^-\to γη_c(1S)$ at six center-of-mass energies between 4.01 and 4.60~GeV using data collected by the BESIII experiment operating at BEPCII. These data sets correspond to a total integrated luminosity of 4.6~fb$^{-1}$. We measure the Born cross section at each energy using a combination of twelve $η_c(1S)$ decay channels. Because the significance of the signal is marginal at each energy ($\le3.0σ$), we also combine all six energies under various assumptions for the energy-dependence of the cross section. If the process is assumed to proceed via the $Y(4260)$, we measure a peak Born cross section $σ_{\mathrm{ peak}}(e^+e^-\toγη_c(1S)) = 2.11 \pm 0.49 (\mathrm{stat.}) \pm 0.36 (\mathrm{syst.})$~pb with a statistical significance of 4.2$σ$.

hep-ex↗

arXiv2017

Observation of $ψ(3686) \rightarrow e^{+}e^{-}χ_{cJ}$ and $χ_{cJ} \rightarrow e^{+}e^{-}J/ψ$

Using $4.479 \times 10^{8}$ $ψ(3686)$ events collected with the BESIII detector, we search for the decays $ψ(3686) \rightarrow e^{+}e^{-}χ_{c0,1,2}$ and $χ_{c0,1,2} \rightarrow e^{+}e^{-}J/ψ$. The decays $ψ(3686) \rightarrow e^{+}e^{-}χ_{c0,1,2}$ and $χ_{c0,1,2} \rightarrow e^{+}e^{-}J/ψ$ are observed for the first time. The measured branching fractions are $\mathcal{B}(ψ(3686) \rightarrow e^{+}e^{-}χ_{c0,1,2}) = (11.7 \pm 2.5 \pm 1.0)\times10^{-4}$, $(8.6 \pm 0.3 \pm 0.6)\times10^{-4}$, $(6.9 \pm 0.5 \pm 0.6)\times10^{-4}$, and $\mathcal{B}(χ_{c0,1,2} \rightarrow e^{+}e^{-}J/ψ) = (1.51 \pm 0.30 \pm 0.13)\times10^{-4}$, $(3.73 \pm 0.09 \pm 0.25)\times10^{-3}$, $(2.48 \pm 0.08 \pm 0.16)\times10^{-3}$. The ratios of the branching fractions $\frac{\mathcal{B}(ψ(3686) \rightarrow e^{+}e^{-}χ_{c0,1,2})}{\mathcal{B}(ψ(3686) \rightarrow γχ_{c0,1,2})}$ and $\frac{\mathcal{B}(χ_{c0,1,2} \rightarrow e^{+}e^{-}J/ψ)}{\mathcal{B}(χ_{c0,1,2} \rightarrow γJ/ψ)}$ are also reported.

hep-ex↗

arXiv2017

Amplitude analysis of $D^{0} \rightarrow K^{-} π^{+} π^{+} π^{-}$

We present an amplitude analysis of the decay $D^{0} \rightarrow K^{-} π^{+} π^{+} π^{-}$ based on a data sample of 2.93 ${\mbox{\,fb}^{-1}}$ acquired by the BESIII detector at the $ψ(3770)$ resonance. With a nearly background free sample of about 16000 events, we investigate the substructure of the decay and determine the relative fractions and the phases among the different intermediate processes. Our amplitude model includes the two-body decays $D^{0} \rightarrow \bar{K}^{*0}ρ^{0}$, $D^{0} \rightarrow K^{-}a_{1}^{+}(1260)$ and $D^{0} \rightarrow K_{1}^{-}(1270)π^{+}$, the three-body decays $D^{0} \rightarrow \bar{K}^{*0}π^{+}π^{-}$ and $D^{0} \rightarrow K^{-}π^{+}ρ^{0}$, as well as the four-body decay $D^{0} \rightarrow K^{-}π^{+}π^{+}π^{-}$. The dominant intermediate process is $D^{0} \rightarrow K^{-}a_{1}^{+}(1260)$, accounting for a fit fraction of $54.6\%$.

hep-ex↗

arXiv2017

Improved measurements of branching fractions for $η_c\toϕϕ$ and $ωϕ$

Using $(223.7\pm1.4)\times10^6$ J/$ψ$ events accumulated with the BESIII detector, we study $η_c$ decays to $ϕϕ$ and $ωϕ$ final states. The branching fraction of $η_c\toϕϕ$ is measured to be $Br(η_c\toϕϕ)=(2.5\pm0.3^{+0.3}_{-0.7}\pm0.6)\times10^{-3}$, where the first uncertainty is statistical, the second is systematic, and the third is from the uncertainty of $Br(J/ψ\toγη_c)$. No significant signal for the double OZI-suppressed decay of $η_c\toωϕ$ is observed, and the upper limit on the branching fraction is determined to be $Br(η_c\toωϕ)<2.5\times 10^{-4}$ at the 90\% confidence level.

hep-ex↗

arXiv2017

Testing the universality of the star formation efficiency in dense molecular gas

Recent studies have suggested that star formation (SF) in dense gas may be governed by essentially the same law in Galactic clouds and external galaxies. This conclusion remains controversial, however, because different tracers have been used to probe the dense gas mass in Galactic and extragalactic studies. We conducted observations of Aquila, Oph., and Orion B in HCN(1-0), HCO$^+$(1-0), and their isotopomers to calibrate the HCN and HCO$^+$ lines used as dense gas tracers in extragalactic studies and to test the possible universality of the star formation efficiency in dense gas, SFE$_{dense}$. H$^{13}$CO$^+$ and H$^{13}$CN were observed to be good tracers of the filaments detected with Herschel. Comparing the luminosities $L_{HCN}$ and $L_{HCO^+}$ with the reference masses $M_{Herschel}^{Av>8}$, the empirical conversion factors $α_{Herschel-HCN}$ and $α_{Herschel-HCO^+}$ were found to be anti-correlated with the local FUV strength. In agreement with Pety et al. (2017), HCN and HCO$^+$ were also found to trace gas down to Av>2. As a result, published extragalactic HCN studies must be tracing all of the moderate density gas. Estimating the contribution of this moderate density gas from the typical column density PDFs in nearby clouds, we obtained the following $G_0$-dependent HCN conversion factor for external galaxies:$α_{Herschel-HCN}^{fit'}=64\times G_0^{-0.34}$. Re-estimating the dense gas masses in external galaxies with $α_{Herschel-HCN}^{fit'}$, we found that SFE$_{dense}$ is remarkably constant over 8 orders of magnitude in dense gas mass. Our results confirm that SFE$_{dense}$ of galaxies is quasi-universal on a wide range of scales from ~1-10pc to >10kpc. Based on the tight link between SF and filamentary structure found in Herschel studies of nearby clouds, we argue that SFE$_{dense}$ is primarily set by the microphysics of SF in filaments.

astro-ph.GA↗

arXiv2017

Measurement of higher-order multipole amplitudes in $ψ(3686)\rightarrowγχ_{c1,2}$ with $χ_{c1,2}\toγJ/ψ$ and search for the transition $η_{c}(2S)\toγJ/ψ$

Using 106~million $ψ(3686)$ events collected with the BESIII detector, we measure multipole amplitudes for the decay $ψ(3686)\rightarrowγχ_{c1,2}\toγγJ/ψ$ beyond the dominant electric-dipole amplitudes. The normalized magnetic-quadrupole amplitude for $ψ(3686)\rightarrowγχ_{c1,2}\rightarrowγγJ/ψ$ and the normalized electric-octupole amplitudes for $ψ(3686)rightarrowγχ_{c2}$,~$χ_{c2}\rightarrowγJ/ψ$ are determined. The M2 amplitudes for $ψ(3686)\rightarrowγχ_{c1}$ and $χ_{c1,2}\rightarrowγJ/ψ$ are found to differ significantly from zero and are consistent with theoretical predictions. We also obtain the ratios of M2 contributions of $ψ(3686)$ and $J/ψ$ decays to $χ_{c1,2}$, $b_{2}^{1}/b_{2}^{2} = 1.35\pm0.72$ and $a_{2}^{1}/a_{2}^{2} = 0.617\pm0.083$, which agree well with theoretical expectations. By considering the multipole contributions of $χ_{c1,2}$, we measure the product branching fractions for the cascade decays $ψ(3686)\rightarrowγχ_{c0,1,2}\toγγJ/ψ$ and search for the process $η_{c}(2S)\toγJ/ψ$ through $ψ(3686)\rightarrowγη_{c}(2S)$. The product branching fraction for $ψ(3686)\rightarrowγχ_{c0}\toγγJ/ψ$ is 3$σ$ larger than published measurements, while those of $ψ(3686)rightarrowγχ_{c1,2}\toγγJ/ψ$ are consistent. No significant signal for the decay $ψ(3686)\toγη_c(2S)\toγγJ/ψ$ is observed, and the upper limit of the product branching fraction at the 90\% confidence level is determined.

hep-ex↗

arXiv2017

Study of $J/ψ$ and $ψ(3686)$ decay to $Λ\barΛ$ and $Σ^0\barΣ^0$ final states

Using $1310.6\times10^6$ $J/ψ$ and $447.9\times10^6$ $ψ(3686)$ events collected with the BESIII detector at the BEPCII $e^{+}e^{-}$ collider, the branching fractions and the angular distributions of $J/ψ$ and $ψ(3686)$ decays to $Λ\barΛ$ and $Σ^0\barΣ^0$ final states are measured. The branching fractions are in agreement with, and much more precise than, the averages of previously published results. The polar angular distributions of $ψ(3686)$ decays are measured for the first time, while those of $J/ψ$ decays are measured with much improved precision. In addition, the ratios of branching fractions $\frac{\mathcal{B}(ψ(3686)\toΛ\barΛ)}{\mathcal{B}(J/ψ\toΛ\barΛ)}$ and $\frac{\mathcal{B}(ψ(3686)\toΣ^0\barΣ^0)}{\mathcal{B}(J/ψ\toΣ^0\barΣ^0)}$ are determined to test the "12\% rule".

hep-ex↗

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

Updated baseline for a staged Compact Linear Collider

The Compact Linear Collider (CLIC) is a multi-TeV high-luminosity linear e+e- collider under development. For an optimal exploitation of its physics potential, CLIC is foreseen to be built and operated in a staged approach with three centre-of-mass energy stages ranging from a few hundred GeV up to 3 TeV. The first stage will focus on precision Standard Model physics, in particular Higgs and top-quark measurements. Subsequent stages will focus on measurements of rare Higgs processes, as well as searches for new physics processes and precision measurements of new states, e.g. states previously discovered at LHC or at CLIC itself. In the 2012 CLIC Conceptual Design Report, a fully optimised 3 TeV collider was presented, while the proposed lower energy stages were not studied to the same level of detail. This report presents an updated baseline staging scenario for CLIC. The scenario is the result of a comprehensive study addressing the performance, cost and power of the CLIC accelerator complex as a function of centre-of-mass energy and it targets optimal physics output based on the current physics landscape. The optimised staging scenario foresees three main centre-of-mass energy stages at 380 GeV, 1.5 TeV and 3 TeV for a full CLIC programme spanning 22 years. For the first stage, an alternative to the CLIC drive beam scheme is presented in which the main linac power is produced using X-band klystrons.

physics.acc-ph↗

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

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↗