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Institute
Two-particle correlations on transverse rapidity in Au+Au collisions at √sNN = 200 GeV at STAR
(2022)
Two-particle correlation measurements projected onto two-dimensional, transverse rapidity coordinates (yT1,yT2), allow access to dynamical properties of the QCD medium produced in relativistic heavy-ion collisions that angular correlation measurements are not sensitive to. We report non-identified charged-particle correlations for Au + Au minimum-bias collisions at sNN−−−√ = 200 GeV taken by the STAR experiment at the Relativistic Heavy-Ion Collider (RHIC). Correlations are presented as 2D functions of transverse rapidity for like-sign, unlike-sign and all charged-particle pairs, as well as for particle pairs whose relative azimuthal angles lie on the near-side, the away-side, or at all relative azimuth. The correlations are constructed using charged particles with transverse momentum pT≥0.15 GeV/c, pseudorapidity from −1 to 1, and azimuthal angles from −π to π. The significant correlation structures that are observed evolve smoothly with collision centrality. The major correlation features include a saddle shape plus a broad peak with maximum near yT≈3, corresponding to pT≈ 1.5 GeV/c. The broad peak is observed in both like- and unlike-sign charge combinations and in near- and away-side relative azimuthal angles. The all-charge, all-azimuth correlation measurements are compared with the theoretical predictions of {\sc hijing} and {\sc epos}. The results indicate that the correlations for peripheral to mid-central collisions can be approximately described as a superposition of nucleon + nucleon collisions with minimal effects from the QCD medium. Strong medium effects are indicated in mid- to most-central collisions.
We report a measurement of cumulants and correlation functions of event-by-event proton multiplicity distributions from fixed-target Au+Au collisions at sNN−−−√ = 3 GeV measured by the STAR experiment. Protons are identified within the rapidity (y) and transverse momentum (pT) region −0.9<y<0 and 0.4<pT<2.0 GeV/c in the center-of-mass frame. A systematic analysis of the proton cumulants and correlation functions up to sixth-order as well as the corresponding ratios as a function of the collision centrality, pT, and y are presented. The effect of pileup and initial volume fluctuations on these observables and the respective corrections are discussed in detail. The results are compared to calculations from the hadronic transport UrQMD model as well as a hydrodynamic model. In the most central 5\% collisions, the value of proton cumulant ratio C4/C2 is negative, drastically different from the values observed in Au+Au collisions at higher energies. Compared to model calculations including Lattice QCD, a hadronic transport model, and a hydrodynamic model, the strong suppression in the ratio of C4/C2 at 3 GeV Au+Au collisions indicates an energy regime dominated by hadronic interactions.
A linearly polarized photon can be quantized from the Lorentz-boosted electromagnetic field of a nucleus traveling at ultra-relativistic speed. When two relativistic heavy nuclei pass one another at a distance of a few nuclear radii, the photon from one nucleus may interact through a virtual quark-antiquark pair with gluons from the other nucleus forming a short-lived vector meson (e.g. ρ0). In this experiment, the polarization was utilized in diffractive photoproduction to observe a unique spin interference pattern in the angular distribution of ρ0→π+π− decays. The observed interference is a result of an overlap of two wave functions at a distance an order of magnitude larger than the ρ0 travel distance within its lifetime. The strong-interaction nuclear radii were extracted from these diffractive interactions, and found to be 6.53±0.06 fm (197Au) and 7.29±0.08 fm (238U), larger than the nuclear charge radii. The observable is demonstrated to be sensitive to the nuclear geometry and quantum interference of non-identical particles.
Utilizing the data set corresponding to an integrated luminosity of 3.19 fb−1 collected by the BESIII detector at a center-of-mass energy of 4.178 GeV, we perform an amplitude analysis of the D+s→π+π−π+ decay. The sample contains 13,797 candidates with a signal purity of ∼80%. The amplitude and phase of the contributing ππ S wave are measured based on a quasi-model-independent approach, along with the amplitudes and phases of the P and D waves parametrized by Breit-Wigner models. The fit fractions of different intermediate decay channels are also reported.
The strong force, as one of the four fundamental forces at work in the universe, governs interactions of quarks and gluons, and binds together the atomic nucleus. Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange, a full understanding of the strong interaction remains a major challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distance scales on the order of the size of the proton. Here we show that in relativistic heavy-ion collisions, where quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely ϕ and K∗0, emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for ϕ is unexpectedly large, while that for K∗0 is consistent with zero. The observed spin-alignment pattern and magnitude for the ϕ cannot be explained by conventional mechanisms, while a model with strong force fields accommodates the current data. This is the first time that the strong force field is experimentally supported as a key mechanism that leads to global spin alignment. We extract a quantity proportional to the intensity of the field of the strong force. Within the framework of the Standard Model, where the strong force is typically described in the quark and gluon language of Quantum Chromodynamics, the field being considered here is an effective proxy description. This is a qualitatively new class of measurement, which opens a new avenue for studying the behaviour of strong force fields via their imprint on spin alignment.
The strong force, as one of the four fundamental forces at work in the universe, governs interactions of quarks and gluons, and binds together the atomic nucleus. Notwithstanding decades of progress since Yukawa first developed a description of the force between nucleons in terms of meson exchange, a full understanding of the strong interaction remains a major challenge in modern science. One remaining difficulty arises from the non-perturbative nature of the strong force, which leads to the phenomenon of quark confinement at distance scales on the order of the size of the proton. Here we show that in relativistic heavy-ion collisions, where quarks and gluons are set free over an extended volume, two species of produced vector (spin-1) mesons, namely ϕ and K∗0, emerge with a surprising pattern of global spin alignment. In particular, the global spin alignment for ϕ is unexpectedly large, while that for K∗0 is consistent with zero. The observed spin-alignment pattern and magnitude for the ϕ cannot be explained by conventional mechanisms, while a model with strong force fields accommodates the current data. This is the first time that the strong force field is experimentally supported as a key mechanism that leads to global spin alignment. We extract a quantity proportional to the intensity of the field of the strong force. Within the framework of the Standard Model, where the strong force is typically described in the quark and gluon language of Quantum Chromodynamics, the field being considered here is an effective proxy description. This is a qualitatively new class of measurement, which opens a new avenue for studying the behaviour of strong force fields via their imprint on spin alignment.
The Cabibbo-allowed weak radiative decay Λ+c→Σ+γ has been searched for in a sample of Λ+cΛ¯−c pairs produced in e+e− annihilations, corresponding to an integrated luminosity of 4.5fb−1 collected with the BESIII detector at center-of-mass energies between 4.60 and 4.70 GeV. No excess of signal above background is observed, and we set an upper limit on the branching fraction of this decay to be B(Λ+c→Σ+γ)<4.4×10−4 at a confidence level of 90\%, which is in agreement with Standard Model expectations.
The Cabibbo-allowed weak radiative decay Λ+c→Σ+γ has been searched for in a sample of Λ+cΛ¯−c pairs produced in e+e− annihilations, corresponding to an integrated luminosity of 4.5fb−1 collected with the BESIII detector at center-of-mass energies between 4.60 and 4.70 GeV. No excess of signal above background is observed, and we set an upper limit on the branching fraction of this decay to be B(Λ+c→Σ+γ)<4.4×10−4 at a confidence level of 90\%, which is in agreement with Standard Model expectations.
Cross sections for the process e+e−→K0SK0SJ/ψ at center-of-mass energies from 4.128 to 4.950 GeV are measured using data samples with a total integrated luminosity of 21.2 fb−1 collected by the BESIII detector operating at the BEPCII storage ring. The Y(4230) state is observed in the energy dependence of the e+e−→K0SK0SJ/ψ cross section for the first time with a statistical significance of 26.0σ. In addition, an enhancement around 4.710 GeV, called the Y(4710), is seen with a statistical significance of 4.2σ. There is no clear structure around 4.484 GeV. Using a fit with a coherent sum of three Breit-Wigner functions, we determine the mass and width of the Y(4230) state to be 4226.9±6.6±21.9 MeV/c2 and 71.7±16.2±31.4 MeV, respectively, and the mass and width of the Y(4710) state to be 4704.0±52.3±69.5 MeV/c2 and 183.2±114.0±90.8 MeV, respectively, where the first uncertainties are statistical and the second are systematic. In addition, the average Born cross section ratio of e+e−→K0SK0SJ/ψ to e+e−→K+K−J/ψ is measured to be 0.388+0.035−0.028±0.016, or 0.426+0.038−0.031±0.018 if three-body phase space is considered.
We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at 3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity (y) and transverse momentum (pT) within −0.5<y<0 and 0.4<pT<2.0 GeV/c. In the most central 0--5\% collisions, a proton cumulant ratio is measured to be C4/C2=−0.85±0.09 (stat.)±0.82 (syst.), which is less than unity, the Poisson baseline. The hadronic transport UrQMD model reproduces our C4/C2 in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in C4/C2 is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3\,GeV.
We report cumulants of the proton multiplicity distribution from dedicated fixed-target Au+Au collisions at 3.0 GeV, measured by the STAR experiment in the kinematic acceptance of rapidity (y) and transverse momentum (pT) within −0.5<y<0 and 0.4<pT<2.0 GeV/c. In the most central 0--5\% collisions, a proton cumulant ratio is measured to be C4/C2=−0.85±0.09 (stat.)±0.82 (syst.), which is less than unity, the Poisson baseline. The hadronic transport UrQMD model reproduces our C4/C2 in the measured acceptance. Compared to higher energy results and the transport model calculations, the suppression in C4/C2 is consistent with fluctuations driven by baryon number conservation and indicates an energy regime dominated by hadronic interactions. These data imply that the QCD critical region, if created in heavy-ion collisions, could only exist at energies higher than 3\,GeV.
In high-energy heavy-ion collisions, partonic collectivity is evidenced by the constituent quark number scaling of elliptic flow anisotropy for identified hadrons. A breaking of this scaling and dominance of baryonic interactions is found for identified hadron collective flow measurements in sNN−−−√ = 3 GeV Au+Au collisions. In this paper, we report measurements of the first- and second-order azimuthal anisotropic parameters, v1 and v2, of light nuclei (d, t, 3He, 4He) produced in sNN−−−√ = 3 GeV Au+Au collisions at the STAR experiment. An atomic mass number scaling is found in the measured v1 slopes of light nuclei at mid-rapidity. For the measured v2 magnitude, a strong rapidity dependence is observed. Unlike v2 at higher collision energies, the v2 values at mid-rapidity for all light nuclei are negative and no scaling is observed with the atomic mass number. Calculations by the Jet AA Microscopic Transport Model (JAM), with baryonic mean-field plus nucleon coalescence, are in good agreement with our observations, implying baryonic interactions dominate the collective dynamics in 3 GeV Au+Au collisions at RHIC.
The Born cross section of the process e+e−→ηJ/ψ at a center-of-mass energy s√=3.773 GeV is measured to be (8.89±0.88±0.42) pb, using a data sample collected with the BESIII detector operating at the BEPCII storage ring. The decay ψ(3770)→ηJ/ψ is observed for the first time with a statistical significance of 7.4σ. From a fit to the dressed cross-section line-shape of e+e−→ηJ/ψ from s√=3.773 to 4.600 GeV we obtain the branching fraction of the decay ψ(3770)→ηJ/ψ to be (11.6±6.1±1.0)×10−4 when the ψ(3770) decay amplitude is added coherently to the other contributions, and (7.9±1.0±0.7)×10−4 when it is added incoherently. Here the first uncertainties are statistical and the second are systematic.
Using 7.33 fb−1 of e+e− collision data collected by the BESIII detector at center-of-mass energies between 4.128 and 4.226~GeV, we observe for the first time the decay D±s→ωπ±η with a statistical significance of 7.6σ. The measured branching fraction of this decay is (0.54±0.12±0.04)%, where the first uncertainty is statistical and the second is systematic.
We present cross sections for the reaction e+e−→K0SK0L at center-of-mass energies ranging from 3.51 GeV to 4.95 GeV using data samples collected in the BESIII experiment, corresponding to a total integrated luminosity of 26.5 fb−1. The ratio of neutral-to-charged kaon form factors at large momentum transfers (12 GeV2<Q2<25 GeV2) is determined to be 0.21±0.01, which indicates a small but significant effect of flavor-SU(3) breaking in the kaon wave function, and consequently excludes the possibility that flavor-SU(3) breaking is the primary reason for the strong experimental violation of the pQCD prediction |F(π±)|/|F(K±)|=f2π/f2K, where F(π±) and F(K±) are the form factors, and fπ and fK are the decay constants of charged pions and kaons, respectively. We also observe a significant signal for the charmless decay ψ(3770)→K0SK0L for the first time. Within a 1σ contour of the likelihood value, the the branching fraction for ψ(3770)→K0SK0L is determined to be B=(2.63+1.40−1.59)×10−5, and the relative phase between the continuum and ψ(3770) amplitudes is ϕ=(−0.39+0.05−0.10)π. The branching fraction is in good agreement with the S- and D-wave charmonia mixing scheme proposed in the interpretation of the "ρπ puzzle" between J/ψ and ψ(3686) decays.
A light scalar X0 or vector X1 particles have been introduced as a possible explanation for the (g−2)μ anomaly and dark matter phenomena.
Using (8.998±0.039)×109 $\jpsi$ events collected by the BESIII detector, we search for a light muon philic scalar X0 or vector X1 in the processes J/ψ→μ+μ−X0,1 with X0,1 invisible decays. No obvious signal is found, and the upper limits on the coupling g′0,1 between the muon and the X0,1 particles are set to be between 1.1×10−3 and 1.0×10−2 for the X0,1 mass in the range of 1<M(X0,1)<1000~MeV/c2 at 90% confidence level.
Using a data sample corresponding to an integrated luminosity of 11.3 fb−1 collected at center-of-mass energies from 4.23 to 4.70 GeV with the BESIII detector, we observe the process e+e− → π0π0ψ2(3823) for the first time with a statistical significance of 6.0 standard deviations. The ratio of average cross sections for e+e− → π0π0ψ2(3823) and π+π−ψ2(3823) is determined to be R = σ[e+e− → π0π0ψ2(3823)] σ[e+e−→π+π−ψ2(3823)] = 0.57 ± 0.14 ± 0.05, which is consistent with expectations from isospin symmetry. Here and below, the first uncertainties are statistical and the second are systematic. The mass of the ψ2(3823) is measured to be M[ψ2(3823)] = 3824.5±2.4±1.0 MeV/c2. Due to the limited data sample, an upper limit of 18.8 MeV at 90% confidence level is set on the intrinsic width of ψ2(3823).
Using an 𝑒+𝑒− collision data sample of (27.08±0.14)×108 𝜓(3686) events collected by the BESIII detector, we report the first observation of 𝜒𝑐𝐽→Ω−¯Ω+ (𝐽=0, 1, 2) decays with significances of 5.6𝜎, 6.4𝜎, and 18𝜎, respectively, where the 𝜒𝑐𝐽 mesons are produced in the radiative 𝜓(3686) decays. The branching fractions are determined to be ℬ(𝜒𝑐0→Ω−¯Ω+) = (3.51±0.54±0.29)×10−5, ℬ(𝜒𝑐1→Ω−¯Ω+)=(1.49±0.23±0.10)×10−5, and ℬ(𝜒𝑐2→Ω−¯Ω+)=(4.52±0.24±0.18)×10−5, where the first and second uncertainties are statistical and systematic, respectively.
Using e+e− annihilation data corresponding to an integrated luminosity of 2.93 fb−1 taken at the center-of-mass energy s√=3.773~GeV with the BESIII detector, a joint amplitude analysis is performed on the decays D0→π+π−π+π− and D0→π+π−π0π0(non-η). The fit fractions of individual components are obtained, and large interferences among the dominant components of D0→a1(1260)π, D0→π(1300)π, D0→ρ(770)ρ(770) and D0→2(ππ)S are found in both channels. With the obtained amplitude model, the CP-even fractions of D0→π+π−π+π− and D0→π+π−π0π0(non-η) are determined to be (75.2±1.1stat.±1.5syst.)% and (68.9±1.5stat.±2.4syst.)%, respectively. The branching fractions of D0→π+π−π+π− and D0→π+π−π0π0(non-η) are measured to be (0.688±0.010stat.±0.010syst.)% and (0.951±0.025stat.±0.021syst.)%, respectively. The amplitude analysis provides an important model for binning strategy in the measurements of the strong phase parameters of D0→4π when used to determine the CKM angle γ(ϕ3) via the B−→DK− decay.
Using data samples with an integrated luminosity of 22.42 fb−1 collected by the BESIII detector operating at the BEPCII storage ring, we measure the cross sections of the $e^{+}e^{-}\rightarrow\etaJ/\psi$ process at center-of-mass energies from 3.808 to 4.951 GeV. Three structures are observed in the line shape of the measured cross sections. A maximum-likelihood fit with ψ(4040), two additional resonances, and a non-resonant component is performed. The mass and width of the first additional state are (4219.7±2.5±4.5)MeV/c2 and (80.7±4.4±1.4)MeV, respectively, consistent with the ψ(4230). For the second state, the mass and width are (4386±13±17)MeV/c2 and (177±32±13)MeV, respectively, consistent with the ψ(4360). The first uncertainties are statistical and the second ones are systematic. The statistical significance of ψ(4040) is 8.0σ and those for ψ(4230) and ψ(4360) are more than 10.0σ.