We report charged-particle pair correlation analyses in the space of Delta -phi (azimuth) and Delta -eta (pseudo-rapidity), for central Au + Au collisions at sqrt{s_{NN}} = 200 GeV in the STAR detector. The analysis involves unlike-sign charge pairs and like-sign charge pairs, which are transformed into charge-dependent (CD) signals and charge-independent (CI) signals. We present detailed parameterizations of the data. A model featuring dense gluonic hot spots as first proposed by van Hove predicts that the observables under investigation would have sensitivity to such a substructure should it occur, and the model also motivates selection of transverse momenta in the range 0.8 < p_t < 2.0$ GeV/c. Both CD and CI correlations of high statistical significance are observed and possible interpretations are discussed.
FIG. 1: a) left side: The $\Delta\phi$ - $\Delta\eta$ correlation data for unlike-sign charge particle pairs from the Star central trigger dataset shown in a 2-dimensional (2-D) perspective plot. The particle tracks have 0.8 GeV/c < $p_t$ < 2.0 GeV/c and |$\eta$| < 1.0. The structure that looks like tiles on a roof is due to the readout boundary effects of the 12 sector TPC. b) right side: The similar correlation data for like-sign charge particle pairs is shown.
FIG. 1: a) left side: The $\Delta\phi$ - $\Delta\eta$ correlation data for unlike-sign charge particle pairs from the Star central trigger dataset shown in a 2-dimensional (2-D) perspective plot. The particle tracks have 0.8 GeV/c < $p_t$ < 2.0 GeV/c and |$\eta$| < 1.0. The structure that looks like tiles on a roof is due to the readout boundary effects of the 12 sector TPC. b) right side: The similar correlation data for like-sign charge particle pairs is shown.
FIG. 2: a) left side: The correlation data for the ratio of the histograms of same-event-pairs to mixed-event-pairs for unlike-sign charged pairs, shown in a two-dimensional (2-D) perspective plot $\Delta\phi$ - $\Delta\eta$. The plot was normalized to a mean of 1. b) right side: The similar correlation data for like-sign charge pairs.
The STAR collaboration presents jet substructure measurements related to both the momentum fraction and the opening angle within jets in \pp and \AuAu collisions at \sqrtsn $= 200$ GeV. The substructure observables include SoftDrop groomed momentum fraction (\zg), groomed jet radius (\rg), and subjet momentum fraction (\zsj) and opening angle (\tsj). The latter observable is introduced for the first time. Fully corrected subjet measurements are presented for \pp collisions and are compared to leading order Monte Carlo models. The subjet \tsj~distributions reflect the jets leading opening angle and are utilized as a proxy for the resolution scale of the medium in \AuAu collisions. We compare data from \AuAu collisions to those from \pp which are embedded in minimum-bias \AuAu events in order to include the effects of detector smearing and the heavy-ion collision underlying event. The subjet observables are shown to be more robust to the background than \zg~and \rg. We observe no significant modifications of the subjet observables within the two highest-energy, back-to-back jets, resulting in a distribution of opening angles and the splittings that are vacuum-like. We also report measurements of the differential di-jet momentum imbalance ($A_{\rm{J}}$) for jets of varying \tsj. We find no qualitative differences in energy loss signatures for varying angular scales in the range $0.1 < $\tsj $ < 0.3$, leading to the possible interpretation that energy loss in this population of high momentum di-jet pairs, is due to soft medium-induced gluon radiation from a single color-charge as it traverses the medium.
Global polarizations ($P$) of $\Lambda$ ($\bar{\Lambda}$) hyperons have been observed in non-central heavy-ion collisions. The strong magnetic field primarily created by the spectator protons in such collisions would split the $\Lambda$ and $\bar{\Lambda}$ global polarizations ($\Delta P = P_{\Lambda} - P_{\bar{\Lambda}} < 0$). Additionally, quantum chromodynamics (QCD) predicts topological charge fluctuations in vacuum, resulting in a chirality imbalance or parity violation in a local domain. This would give rise to an imbalance ($\Delta n = \frac{N_{\text{L}} - N_{\text{R}}}{\langle N_{\text{L}} + N_{\text{R}} \rangle} \neq 0$) between left- and right-handed $\Lambda$ ($\bar{\Lambda}$) as well as a charge separation along the magnetic field, referred to as the chiral magnetic effect (CME). This charge separation can be characterized by the parity-even azimuthal correlator ($\Delta\gamma$) and parity-odd azimuthal harmonic observable ($\Delta a_{1}$). Measurements of $\Delta P$, $\Delta\gamma$, and $\Delta a_{1}$ have not led to definitive conclusions concerning the CME or the magnetic field, and $\Delta n$ has not been measured previously. Correlations among these observables may reveal new insights. This paper reports measurements of correlation between $\Delta n$ and $\Delta a_{1}$, which is sensitive to chirality fluctuations, and correlation between $\Delta P$ and $\Delta\gamma$ sensitive to magnetic field in Au+Au collisions at 27 GeV. For both measurements, no correlations have been observed beyond statistical fluctuations.
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Figure 2ab
Figure 2c
Mid-rapidity open charm spectra from direct reconstruction of $D^{0}$($\bar{D^0}$)$\to K^{\mp}\pi^{\pm}$ in d+Au collisions and indirect electron/positron measurements via charm semileptonic decays in p+p and d+Au collisions at \srt = 200 GeV are reported. The $D^{0}$($\bar{D^0}$) spectrum covers a transverse momentum ($p_T$) range of 0.1 $<p_T<$ 3 \GeVc whereas the electron spectra cover a range of 1 $<p_T<$ 4 GeV/$c$. The electron spectra show approximate binary collision scaling between p+p and d+Au collisions. From these two independent analyses, the differential cross section per nucleon-nucleon binary interaction at mid-rapidity for open charm production from d+Au collisions at RHIC is $d\sigma^{NN}_{c\bar{c}}/dy$=0.30$\pm$0.04 (stat.)$\pm$0.09(syst.) mb. The results are compared to theoretical calculations. Implications for charmoniumm results in A+A collisions are discussed.
Inclusive electrons yield versus transverse momentum in D+AU collisions Data points at PT = 2.2, 2.7 and 3.5 GeV/c was obtained using only the TPC (Time Projection Chamber) and cover a pseudo-rapidity range of -1<eta<1, while other points were obtained using both a prototypeTime-of-Flight System and the TPC and cover a pseudo-rapidity range of -1<eta<0.
Inclusive electrons yield versus transverse momentum in P+P collisions.
D0 yield versus transverse momentum in D+AU collisions.
Elliptic flow from nuclear collisions is a hadronic observable sensitive to the early stages of system evolution. We report first results on elliptic flow of charged particles at midrapidity in Au+Au collisions at sqrt(s_NN)=130 GeV using the STAR TPC at RHIC. The elliptic flow signal, v_2, averaged over transverse momentum, reaches values of about 6% for relatively peripheral collisions and decreases for the more central collisions. This can be interpreted as the observation of a higher degree of thermalization than at lower collision energies. Pseudorapidity and transverse momentum dependence of elliptic flow are also presented.
Elliptic flow as a function of centrality defined as nch/nmax. Also given is epsilon, the initial space eccentricity of the overlap region, as well as the cumulative fraction of events starting with the most central. From the results of the study of non-flow contributions by different subevent selections and the maximum magnitudes of the first and higher-order harmonics, we estimate a systematic error for v2 of about 0.007, with somewhat smaller uncertainty for the mid-centralities where the resolution of the event plane is high.
Elliptic flow as a function of transverse momen-tum for minimum bias events
The results from the STAR Collaboration on directed flow (v_1), elliptic flow (v_2), and the fourth harmonic (v_4) in the anisotropic azimuthal distribution of particles from Au+Au collisions at sqrtsNN = 200 GeV are summarized and compared with results from other experiments and theoretical models. Results for identified particles are presented and fit with a Blast Wave model. Different anisotropic flow analysis methods are compared and nonflow effects are extracted from the data. For v_2, scaling with the number of constituent quarks and parton coalescence is discussed. For v_4, scaling with v_2^2 and quark coalescence is discussed.
Directed flow of charged hadrons v1{3} as a function of pseudorapidity for 10–70% centrality.
Directed flow of charged hadrons v1 {EP1,EP2} as a function of pseudorapidity for 20–60% centrality.
Charged hadron v2 for the centrality bins 5 to 10% and in steps of 10% starting at 10, 20, 30, 40, 50, 60, and 70 up to 80% along with min. bias as a function of p_T.
We present a systematic analysis of two-pion interferometry in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV using the STAR detector at RHIC. We extract the HBT radii and study their multiplicity, transverse momentum, and azimuthal angle dependence. The Gaussianess of the correlation function is studied. Estimates of the geometrical and dynamical structure of the freeze-out source are extracted by fits with blast wave parameterizations. The expansion of the source and its relation with the initial energy density distribution is studied.
1D correlation function for different values of SL (antisplitting cut).
1D correlation functions for differeny values of the maximum fraction of merged hits allowed.
Projections of the 3 dimensional correlation function and corresponding fits for negative pions from the 0-5% most central events and k_T = [150,250] MeV/c according to the standard and Bowler-Sinyukov procedures.
We present the transverse momentum (pT) spectra for identified charged pions, protons and anti-protons from p+p and d+Au collisions at \sqrts_NN = 200 GeV. The spectra are measured around midrapidity (|y| < 0.5) over the range of 0.3 < pT < 10 GeV/c with particle identification from the ionization energy loss and its relativistic rise in the Time Projection Chamber and Time-of-Flight in STAR. The charged pion and proton+anti-proton spectra at high pT in p+p and d+Au collisions are in good agreement with a phenomenological model (EPOS) and with the next-to-leading order perturbative quantum chromodynamic (NLO pQCD) calculations with a specific fragmentation scheme and factorization scale. We found that all proton, anti-proton and charged pion spectra in p+p collisions follow xT-scalings for the momentum range where particle production is dominated by hard processes (pT > 2 GeV/c). The nuclear modification factor around midrapidity are found to be greater than unity for charged pions and to be even larger for protons at 2 < pT < 5 GeV/c.
Transverse momentum distribution for $\pi^+$ production in d+Au minbias events in the mid rapidity region, $|y|<0.5$.
Transverse momentum distribution for $\pi^+$ production in p+p NSD events in the mid rapidity region, $|y|<0.5$.
Transverse momentum distribution for $\pi^+$ production in d+Au collisions with centrality 0-20% in the mid rapidity region, $|y|<0.5$.
We present strange particle spectra and yields measured at mid-rapidity in $\sqrt{\text{s}}=200$ GeV proton-proton ($p+p$) collisions at RHIC. We find that the previously observed universal transverse mass ($\mathrm{m_{T}}\equiv\sqrt{\mathrm{p_{T}}^{2}+\mathrm{m}^{2}}$) scaling of hadron production in $p+p$ collisions seems to break down at higher \mt and that there is a difference in the shape of the \mt spectrum between baryons and mesons. We observe mid-rapidity anti-baryon to baryon ratios near unity for $\Lambda$ and $\Xi$ baryons and no dependence of the ratio on transverse momentum, indicating that our data do not yet reach the quark-jet dominated region. We show the dependence of the mean transverse momentum (\mpt) on measured charged particle multiplicity and on particle mass and infer that these trends are consistent with gluon-jet dominated particle production. The data are compared to previous measurements from CERN-SPS, ISR and FNAL experiments and to Leading Order (LO) and Next to Leading order (NLO) string fragmentation model predictions. We infer from these comparisons that the spectral shapes and particle yields from $p+p$ collisions at RHIC energies have large contributions from gluon jets rather than quark jets.
Corrected mid-rapidity (|y| < 0.5) pT spectra for $K^{+}$, $K^{−}$, $K^{0}_{S}$, Λ, Ξ, and Ω. Λ spectra that have been corrected for feed-down are shown as open symbols in the Λ panel. The dashed lines are fits using Equation 11 except for the $\Omega+\overline{\Omega}$ where the fit uses Equation 9. The error bars displayed include systematic errors while the fits were done using statistical errors only for all species except the charged kaons.
Corrected mid-rapidity (|y| < 0.5) pT spectra for $K^{+}$, $K^{−}$, $K^{0}_{S}$, Λ, Ξ, and Ω. Λ spectra that have been corrected for feed-down are shown as open symbols in the Λ panel. The dashed lines are fits using Equation 11 except for the $\Omega+\overline{\Omega}$ where the fit uses Equation 9. The error bars displayed include systematic errors while the fits were done using statistical errors only for all species except the charged kaons.
Corrected mid-rapidity (|y| < 0.5) pT spectra for $K^{+}$, $K^{−}$, $K^{0}_{S}$, Λ, Ξ, and Ω. Λ spectra that have been corrected for feed-down are shown as open symbols in the Λ panel. The dashed lines are fits using Equation 11 except for the $\Omega+\overline{\Omega}$ where the fit uses Equation 9. The error bars displayed include systematic errors while the fits were done using statistical errors only for all species except the charged kaons.
We present a measurement of the first-order azimuthal anisotropy, $v_1(y)$, of deuterons from Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV recorded with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The energy dependence of the $v_1(y)$ slope, $dv_{1}/dy|_{y=0}$, for deuterons, where $y$ is the rapidity, is extracted for semi-central collisions (10-40\% centrality) and compared to that of protons. While the $v_1(y)$ slopes of protons are generally negative for $\sqrt{s_{NN}} >$ 10 GeV, those for deuterons are consistent with zero, a strong enhancement of the $v_1(y)$ slope of deuterons is seen at the lowest collision energy (the largest baryon density) at $\sqrt{s_{NN}} =$ 7.7 GeV. In addition, we report the transverse momentum dependence of $v_1$ for protons and deuterons. The experimental results are compared with transport and coalescence models.
The 1st-order event plane ($\Psi_{1}$) resolution as a function of centrality of Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV. The $\Psi_{1}$ is reconstructed with the BBC detectors and its resolution is estimated by the correlation of sub-$\Psi_{1}$ from east BBC and west BBC. Data presented later (10-40\% centrality) is indicated by the dashed-line box.
Rapidity dependene of $v_1$ for protons(open squares) in 10-40 \%Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV. The lines, dashed-dot line for proton and dashed line for deuteron, at the midrapidity ($|y| < 0.6$) are the fit with linear functions to extract the slopes. The plotted uncertainties are the statistical only
Rapidity dependene of $v_1$ for deuterons(solid circles) in 10-40 \%Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV. The lines, dashed-dot line for proton and dashed line for deuteron, at the midrapidity ($|y| < 0.6$) are the fit with linear functions to extract the slopes. The plotted uncertainties are the statistical only
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