STAR Experiment Research Articles

Polarized antimatter in the proton from a global QCD analysis

We present the first simultaneous global QCD analysis of spin-dependent parton distribution functions alongside their spin-averaged counterparts and pion, kaon, and unidentified hadron fragmentation functions. This analysis includes all data relevant for constraining the polarized light quark sea asymmetry $\mathrm{\ensuremath{\Delta}}\overline{u}\ensuremath{-}\mathrm{\ensuremath{\Delta}}\overline{d}$, in particular the latest polarized $W$-lepton production data from the STAR Collaboration at RHIC and semi-inclusive deep inelastic scattering data from COMPASS, allowing the most robust extraction available with minimal theoretical assumptions. We also extract a self-consistent set of antiquark polarization ratios $\mathrm{\ensuremath{\Delta}}\overline{u}/\overline{u}$ and $\mathrm{\ensuremath{\Delta}}\overline{d}/\overline{d}$ and determine the signs of the truncated contributions to the proton spin from the light antiquarks.

Evidence for Nonlinear Gluon Effects in QCD and Their Mass Number Dependence at STAR.

The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-π^{0}s produced at forward pseudorapidities (2.6<η<4.0) in p+p, p+Al, and p+Au collisions at a center-of-mass energy of 200GeV. We observe a clear suppression of the correlated yields of back-to-back π^{0} pairs in p+Al and p+Au collisions compared to the p+p data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. The larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale Q_{s}^{2} on the mass number A. A linear scaling of the suppression with A^{1/3} is observed with a slope of -0.09±0.01.

Light Nuclei Production in Au+Au Collisions at $\sqrt{s_{\mathrm{NN}}}$ = 3 GeV from the STAR experiment

Light nuclei production is expected to be sensitive to baryon density fluctuations and can be used to probe the signatures of QCD critical point and/or a first-order phase transition in heavy-ion collisions. In this proceedings, we present the spectra and yields of protons (\bm{p}𝐩) and light nuclei (\bm{d}𝐝, \bm{t}𝐭, \bm{^{3}\mathrm{He}}3He, \bm{^{4}\mathrm{He}}4He) in Au+Au collisions at \bm{\sqrt{s_{\mathrm{NN}}}}𝐬NN = 3 GeV by the STAR experiment. Finally, it is found that the kinetic freeze-out dynamics (temperature \bm{T_{kin}}𝐓𝐤𝐢𝐧\bm{vs.}𝐯𝐬. average radial flow velocity \langle\beta_{T}\rangle⟨βT⟩) at \bm{\sqrt{s_{\mathrm{NN}}}}𝐬NN = 3 GeV extracted with the blast-wave model deviate from the trends at high energies (\bm{\sqrt{s_{\mathrm{NN}}}}𝐬NN = 7.7 - 200 GeV), indicating a different medium equation of state.

Measurements of $\phi$ meson and $\Xi^-$ hyperon production in Au+Au collisions at $\sqrt{s_{NN}}$ = 3 GeV from STAR experiment

In this proceedings, we present our first measurements of \phiϕ meson and \Xi^-Ξ− hyperon production in Au+Au collisions at \sqrt{s_{NN}}sNN = 3 GeV. Various models including thermal and transport model calculations are compared to data, these results imply that the matter produced in the 3 GeV Au+Au collisions is considerably different from that at higher energies.

Measurement of Intermittency for Charged Particles in Au + Au Collisions at $\sqrt{s_\mathrm{NN}}$ = 7.7-200 GeV from STAR

Local density fluctuations near the QCD critical point can be probed by intermittency analysis of scaled factorial moments in relativistic heavy-ion collisions. We report the first measurement of intermittency for charged particles in Au + Au collisions at \sqrt{s_\mathrm{NN}}sNN = 7.7-200 GeV from the STAR experiment at RHIC. We observe scaling behaviors in central Au + Au collisions, with the extracted scaling exponent decreasing from mid-central to the most central Au + Au collisions. Furthermore, the scaling exponent exhibits a non-monotonic energy dependence with a minimum around \sqrt{s_\mathrm{NN}}sNN = 20-30 GeV in central Au + Au collisions.

First indication on self-similarity of strangeness production in Au+Au collisions at RHIC: Search for signature of phase transition in nuclear matter

New results of analysis of K^0_S-KS0−meson spectra measured over a wide range of energy \sqrt {s_{NN}}=7.7-200sNN=7.7−200 GeV and centrality in Au+AuAu+Au collisions by the STAR Collaboration at RHIC using the zz-scaling approach are presented. Indication on self-similarity of fractal structure of nuclei and fragmentation processes with K^0_SKS0 probe is demonstrated. The energy loss as a function of the collision energy, centrality and transverse momentum of the inclusive strange meson is estimated.

Probing the high-density nuclear symmetry energy with the Ξ−/Ξ0 ratio in heavy-ion collisions at sNN≈3 GeV

Recent beam energy scan experiments at the BNL Relativistic Heavy Ion Collider by the STAR Collaboration [Phys. Lett. B 827, 137003 (2022) and Phys. Rev. Lett. 128, 202303 (2022)] found that hadronic interactions dominate the collective flow and the proton cumulant ratios are driven by baryon number conservation in a region of high baryon density in $\sqrt{{s}_{NN}}=3$ GeV $\mathrm{Au}+\mathrm{Au}$ reactions, indicating that the dense medium formed in such collisions is likely hadronic matter. Within an updated a relativistic transport model with momentum dependent isoscalar and isovector single-nucleon mean-field potentials corresponding to different symmetry energies at suprasaturation densities, the $n/p, {\ensuremath{\pi}}^{\ensuremath{-}}/{\ensuremath{\pi}}^{+}, {K}_{s}^{0}/{K}^{+}, {\mathrm{\ensuremath{\Sigma}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Sigma}}}^{+}$, and ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Xi}}}^{0}$ ratios are studied for central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=3$ GeV, where the maximum central density reaches about $(3.6--4.0){\ensuremath{\rho}}_{0}$. The doubly strange ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Xi}}}^{0}$ ratio is found to have the strongest sensitivity to the variation of high-density nuclear symmetry energy. Thus, the ${\mathrm{\ensuremath{\Xi}}}^{\ensuremath{-}}/{\mathrm{\ensuremath{\Xi}}}^{0}$ ratio in relativistic heavy-ion reactions at $\sqrt{{s}_{NN}}\ensuremath{\sim}3$ GeV may help probe sensitively the poorly known symmetry energy of dense neutron-rich matter critically important for understanding various properties of neutron stars.

Dynamical development of proton cumulants and correlation functions in Au+Au collisions at sNN=7.7 GeV from a multiphase transport model

Higher-order cumulants of the distributions of conserved charges, such as net-baryon number, are sensitive to the quantum chromodynamics(QCD) phase transition and the QCD critical point. We calculate the cumulants and correlation functions of proton, antiproton, and net-proton multiplicity distributions in Au+Au collisions at $\sqrt{s_{NN}} = 7.7$ GeV using a multiphase transport model(AMPT). The AMPT model can basically describe the trends of cumulants, cumulant ratios, (normalized) correlation functions of the proton and net-proton measured by the STAR experiment. The multiproton (baryon) correlations in the AMPT model are consistent with the expectation from baryon number conservation. We demonstrate that multiproton (baryon) correlations suffer the dynamical evolution of heavy-ion collisions. Our results provide a baseline for searching for the possible critical behaviors at the critical end point in relativistic heavy-ion collisions.

Impact of nuclear structure on the background in the chiral magnetic effect in Ru4496+Ru4496 and Zr4096+Zr4096 collisions at sNN=7.7–200 GeV from a multiphase transport model

Impacts of nuclear structure on multiplicity $({N}_{\mathrm{ch}})$ and anisotropic flows $({v}_{2}$ and ${v}_{3})$ in the isobaric collisions of $_{44}^{96}\mathrm{Ru}+_{44}^{96}\mathrm{Ru}$ and $_{40}^{96}\mathrm{Zr}+_{40}^{96}\mathrm{Zr}$ at $\sqrt{{s}_{NN}}$ = 7.7, 27, 62.4, and 200 GeV are investigated by using the string melting version of a multiphase transport (AMPT) model. In comparison with the experimental data released recently by the STAR Collaboration, it is found that the impact of quadrupole deformation ${\ensuremath{\beta}}_{2}$ on the ${v}_{2}$ difference is mainly manifested in the most central collisions, while the octupole deformation ${\ensuremath{\beta}}_{3}$ is in the near-central collisions, and the neutron skin effect dominates in the mid-central collisions. Viewing from the energy dependence, these effects are magnified at lower energies.

Measurements of $W$ and $Z/γ^*$ cross sections and their ratios in $pp$ collisions at STAR

While the unpolarized parton distribution functions of the valence quarks (dd and uu) are well determined from DIS experiments, the sea quark counterparts, \bar{d}d‾ and \bar{u}u‾, are much less constrained, in particular, near the valence region. Measurements of W^+/W^-W+/W− production ratio in pppp collider experiments, such as the STAR experiment at RHIC, can probe the \bar{d}/\bar{u}d‾/u‾ ratio at a large Q^2Q2 set by the WW mass. These proceedings will discuss recent results of WW and ZZ cross-section measurements using the STAR pppp collision data at a center-of-mass energy of \sqrt{s} = 500/510\;\mathrm{GeV}s=500/510GeV collected in 2011, 2012, 2013 and 2017.

Measurement of charged-particle production in single diffractive proton-proton collisions with the STAR detector

We present results on the inclusive and identified (pion, kaon, proton and their antiparticles) charged-particle production in single diffractive (SD) dissociation process in proton-proton collisions at \sqrt{s}=200s=200 GeV with the STAR detector at RHIC. The forward-scattered proton is measured in the Roman Pot (RP) system, while the charged particle tracks are reconstructed in the STAR Time Projection Chamber. The proton-antiproton production asymmetry is measured to study the baryon number transfer over a large rapidity interval in SD process. In addition, K/\piK/π ratio is measured, showing a larger strangeness production at p_T&gt;0.5pT&gt;0.5 GeV/\mathit{c}𝑐 compared to measurements in inclusive proton-proton collisions.

Recent results on Central Exclusive Production with the STAR detector

We present results on the Central Exclusive Production of charged particle pairs h^{+}h^{-}h+h− (h = \pi, K, ph=π,K,p), pp\to p^\prime+h^{+}h^{-}+p^\primepp→p′+h+h−+p′, obtained with the STAR experiment at RHIC in proton-proton collisions at a center-of-mass energy of \sqrt{s} = 200s=200~GeV. All final-state particles were reconstructed, including forward-scattered protons detected in the Roman Pot system. As a result, the Double Pomeron Exchange (DPE) events were selected and the non-exclusive backgrounds were efficiently rejected.Differential fiducial cross sections were measured as functions of observables related to the central hadronic final state and to the forward-scattered protons. The measured cross sections were compared to phenomenological model predictions based on the DPE.We also present preliminary results on the measurement of the same physics process at higher collision energy \sqrt{s} = 510s=510~GeV. The data demonstrate features similar to those observed at \sqrt{s} = 200s=200~GeV.

Using coherent dipion photoproduction to image gold nuclei

Vector meson photoproduction offers the opportunity to image target nuclei. The two-dimensional Fourier transform d\sigma_{coherent}/dtdσcoherent/dt of coherent vector meson photoproduction gives the two-dimensional distribution of interaction sites in the target. Since vector meson photoproduction occurs, at lowest order, via two-gluon exchange, this is sensitive to gluon shadowing. We present an analysis of \pi^+\pi^-π+π− photoproduction using data from the STAR detector and a study of d\sigma_{coherent}/dtdσcoherent/dt, with an emphasis on probing the nuclear shape and its systematic uncertainties.

Differential measurements of jet sub-structure observables and their correlation in p+p collisions at √s = 200 GeV in STAR

This analysis extends recent measurements of the jet sub-structure observables based on the SoftDrop algorithm in pp+pp collisions at \sqrt{s}s = 200 GeV in the STAR experiment. We present fully unfolded multi-differential measurements of jet sub-structure observables at the first split and their correlations for jets of different transverse momenta and radii. We compare our measurements to various Monte Carlo models.

Di-$\pi^0$ correlations in p+p, p+Al and p+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV at STAR

The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-\pi^0π0 produced at forward pseudorapidity (2.6&lt;\eta&lt;4.02.6&lt;η&lt;4.0) in p+p+pp, p+p+\mathrm{Al}Al and p+p+\mathrm{Au}Au collisions at a center-of-mass energy per nucleon-nucleon pair of 200 GeV. A clear suppression of the correlated away-side yields is observed in p+p+\mathrm{Au}Au and for the first time in p+p+\mathrm{Al}Al collisions, compared with the pp++pp data. The enhanced suppression found in p+p+\mathrm{Au}Au with respect to p+p+\mathrm{Al}Al collisions exhibits the saturation scale (Q_s^2Qs2) dependence on the mass number AA. The observed suppression of back-to-back pairs as a function of event activity and transverse momentum points to non-linear gluon dynamics arising at high parton densities.

Reweighting the quark Sivers function with STAR jet data

The Bayesian reweighting procedure is applied for the first time to a TMD distribution, the quark Sivers function extracted from SIDIS data. By exploiting the recent published single spin asymmetry data for the inclusive jet production in p^\uparrow pp↑p collisions from the STAR collaboration at RHIC, we show how such a procedure allows to incorporate the information contained in the new data set, without the need of re-fitting, and to explore a much wider xx region compared to SIDIS measurements. The reweighting method is also extended to the case of asymmetric errors, and the results show a significant improvement on the knowledge of the quark Sivers function.

Sea Asymmetry from Polarized W Boson Production

We present the results of a global QCD analysis of helicity parton distribution functions (PDFs) that includes the latest polarized W-lepton production data from the STAR collaboration at the Relativistic Heavy-Ion Collider (RHIC). This data allows the first extraction of a nonzero helicity light-quark sea asymmetry within a simultaneous global QCD analysis of unpolarized and helicity PDFs.

NNLO constraints on proton PDFs from the SeaQuest and STAR experiments and other developments in the CTEQ-TEA global analysis

We review progress in the global QCD analysis by the CTEQ-TEA group since the publication of CT18 parton distribution functions (PDFs) in the proton. Specifically, we discuss comparisons of CT18 NNLO predictions with the LHC 13 TeV measurements as well as with the FNAL SeaQuest and BNL STAR data on lepton pair production. The specialized CT18X PDFs approximating saturation effects are compared with the CT18sx PDFs obtained using NLL/NLO small-$x$ resummation. Short summaries are presented for the special CT18 parton distributions with fitted charm and with lattice QCD inputs. A recent comparative analysis of the impact of deuteron nuclear effects on the parton distributions by the CTEQ-JLab and CTEQ-TEA groups is summarized.

Local and global polarization of Λ hyperons across RHIC-BES energies: The roles of spin hall effect, initial condition, and baryon diffusion

We perform a systematic study on the local and global spin polarization of $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ hyperons in relativistic heavy-ion collisions at beam energy scan energies via the ($3+1$)-dimensional CLVisc hydrodynamics model with a multiphase transport (AMPT) and simulating many accelerated strongly interacting hadron (SMASH) initial conditions. Following the quantum kinetic theory, we decompose the polarization vector as the parts induced by thermal vorticity, shear tensor and the spin Hall effect (SHE). We find that the polarization induced by the SHE and the total polarization strongly depends on the initial conditions. At $7.7\phantom{\rule{0.28em}{0ex}}\mathrm{GeV}$, the SHE gives a sizable contribution and even flips the sign of the local polarization along the beam direction for the AMPT initial condition, which is not observed for the SMASH initial condition. Meanwhile, the local polarization along the out-of-plane direction induced by the SHE with the AMPT initial condition does not always increase with decreasing collision energies. Next, we find that the polarization along the beam direction is sensitive to the baryon diffusion coefficient, but the local polarization along the out-of-plane direction is not. Our results for the global polarization of $\mathrm{\ensuremath{\Lambda}}$ and $\overline{\mathrm{\ensuremath{\Lambda}}}$ agree well with the data of the STAR Collaboration. Interestingly, the global polarization of $\overline{\mathrm{\ensuremath{\Lambda}}}$ is not always larger than that of $\mathrm{\ensuremath{\Lambda}}$ due to various competing effects. Our findings are helpful for understanding the polarization phenomenon and the detailed structure of quark-gluon plasma in relativistic heavy-ion collisions.

Self-similarity of [formula omitted]-meson production in Au + Au collisions from BES-I at STAR and anomaly of “specific heat” and entropy

We present results of analysis of KS0-meson spectra in Au+Au collisions obtained by the STAR Collaboration in the first phase of the Beam Energy Scan program at RHIC using the z-scaling approach. The analyzed data on inclusive cross sections were measured over a wide range of collision energy sNN=7.7−200 GeV and transverse momentum of produced particles for different centralities in the rapidity range |y|<0.5. A scaling behavior of the spectra in z-presentation is observed. The concept of the z-scaling is based on fundamental principles of self-similarity, locality, and fractality of hadron interactions at a constituent level. Structure of the colliding nuclei and fragmentation process in final state are described by fractal dimensions δA and ϵAA, respectively. Produced medium is characterized by a “specific heat” cAA. Onset of constant δA, smooth increase of ϵAA and anomalous behavior of cAA with collision energy in the scaling regime are observed. The self-similarity of fractal structure of nuclei and fragmentation processes with KS0 probe is demonstrated. The energy loss as a function of the collision energy, centrality and transverse momentum of the inclusive particle is estimated. An anomaly in the entropy behavior of system configurations in the region sNN∼30−40 GeV at low pT in the central collisions is found. A possible signature of phase transition indicated by the peculiarities of specific heat and entropy as ingredients of the z-scaling of KS0-meson production is discussed.