We present results for the measurement of $\phi$ meson production via its charged kaon decay channel $\phi \to K^+K^-$ in Au+Au collisions at $\sqrt{s_{_{NN}}}=62.4$, 130, 200 GeV, and in p+p and d+Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV from the STAR experiment at RHIC. The mid-rapidity ($|y|<0.5$) $\phi$ meson spectra in central Au+Au collisions are found to be well described by a single exponential distribution. On the other hand, the spectra from p+p, d+Au and peripheral Au+Au collisions show power-law tails at intermediate and high transverse momenta ($p_{T}$) and are described better by Levy distributions. The constant $\phi/K^-$ yield ratio vs. beam species, collision centrality and colliding energy is in contradiction with expectations from models having kaon coalescence as the dominant mechanism for $\phi$ production at RHIC. The $\Omega/\phi$ yield ratio as a function of $p_{T}$ is consistent with a model based on the recombination of thermal $s$ quarks up to $p_{T}\sim 4$ GeV/c, but disagrees at higher transverse momenta. The measured nuclear modification factor, $R_{dAu}$, for the $\phi$ meson increases above unity at intermediate $p_{T}$, similar to that for pions and protons, while $R_{AA}$ is suppressed due to jet quenching in central Au+Au collisions. Number of constituent quark scaling of both $R_{cp}$ and $v_{2}$ for the $\phi$ meson with respect to other hadrons in Au+Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV at intermediate $p_{T}$ is observed. These observations support quark coalescence as being the dominant mechanism of hadronization in the intermediate $p_{T}$ region at RHIC.
Background-subtracted invariant mass distributions at 0.4 < $p_{T}$ < 0.8 GeV/c in d + Au 200 GeV collisions (0–100%) with (solid points) and without (open points) the $\delta$-dipangle cut. The dashed curves show a Breit-Wigner (see the text for details) + linear background function fit to the case with the $\delta$-dip-angle cut.
Background-subtracted invariant mass distributions at 0.4 < $p_{T}$ < 0.8 GeV/c in d + Au 200 GeV collisions (0–100%) with (solid points) and without (open points) the $\delta$-dipangle cut. The dashed curves show a Breit-Wigner (see the text for details) + linear background function fit to the case with the $\delta$-dip-angle cut.
Upper panels: same-event (full points) and mixed-event (solid line) $K^{+}K^{-}$ invariant mass distributions at 0.6 < $p_{T}$ < 1.4 GeV/c in p + p 200 GeV collisions (a), 0.8 < $p_{T}$ < 1.2 GeV/c in Au + Au 62.4 GeV collisions (60–80%) (c), and 0.8 < $p_{T}$ < 1.2 GeV/c in Au + Au 200 GeV collisions (0–10%) (e). Lower panels: the corresponding $\phi$ meson mass peaks after subtracting the background. Dashed curves show a Breit-Wigner + linear background function fit in (b), (d). In (f), both linear and quadratic backgrounds are shown as dashed and dot-dashed lines, respectively.
Identified charged particle spectra of $\pi^{\pm}$, $K^{\pm}$, $p$ and $\pbar$ at mid-rapidity ($|y|<0.1$) measured by the $\dedx$ method in the STAR-TPC are reported for $pp$ and d+Au collisions at $\snn = 200$ GeV and for Au+Au collisions at 62.4 GeV, 130 GeV, and 200 GeV. ... [Shortened for arXiv list. Full abstract in manuscript.]
Uncorrected charged particle multiplicity distribution measured in the TPC in $|\eta| < 0.5$ for Au+Au collisions at 62.4 GeV and 200 GeV. The shaded regions indicate the centrality bins used in the analysis. The 200 GeV data are scaled by a factor 5 for clarity.
Uncorrected charged particle multiplicity distribution measured in the TPC in $|\eta| < 0.5$ for Au+Au collisions at 62.4 GeV and 200 GeV. The shaded regions indicate the centrality bins used in the analysis. The 200 GeV data are scaled by a factor 5 for clarity.
Uncorrected charged particle multiplicity distribution measured in the E-FTPC (Au-direction) within $−3.8 < |\eta| < −2.8$ in d+Au collisions at 200 GeV. The shaded regions indicate the centrality bins used in the analysis.
We present measurements of net charge fluctuations in $Au + Au$ collisions at $\sqrt{s_{NN}} = $ 19.6, 62.4, 130, and 200 GeV, $Cu + Cu$ collisions at $\sqrt{s_{NN}} = $ 62.4, 200 GeV, and $p + p$ collisions at $\sqrt{s} = $ 200 GeV using the dynamical net charge fluctuations measure $\nu_{+-{\rm,dyn}}$. We observe that the dynamical fluctuations are non-zero at all energies and exhibit a modest dependence on beam energy. A weak system size dependence is also observed. We examine the collision centrality dependence of the net charge fluctuations and find that dynamical net charge fluctuations violate $1/N_{ch}$ scaling, but display approximate $1/N_{part}$ scaling. We also study the azimuthal and rapidity dependence of the net charge correlation strength and observe strong dependence on the azimuthal angular range and pseudorapidity widths integrated to measure the correlation.
(Color online) Dynamical net charge fluctuations, $\nu_{+−,dyn}$, of particles produced within pseudorapidity $|\eta|$ < 0.5, as function of the number of participating nucleons.
(Color online) Corrected values of dynamical net charge fluctuations ($\nu^{corr}_{+−,dyn}$) as a function of $\sqrt{s_{NN}}$. See text for details.
(Color online) Dynamical net charge fluctuations, $\nu_{+−,dyn}$, of particles produced with pseudorapidity $|\eta|$ < 0.5 scaled by (a) the multiplicity, $dN_{ch}/d\eta$. The dashed line corresponds to charge conservation effect and the solid line to the prediction for a resonance gas, (b) the number of participants, and (c) the number of binary collisions.
We measure directed flow ($v_1$) for charged particles in Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}} =$ 200 GeV and 62.4 GeV, as a function of pseudorapidity ($\eta$), transverse momentum ($p_t$) and collision centrality, based on data from the STAR experiment. We find that the directed flow depends on the incident energy but, contrary to all existing models, not on the size of the colliding system at a given centrality. We extend the validity of the limiting fragmentation concept to different collision systems, and investigate possible explanations for the observed sign change in $v_1(p_t)$.
Charged particle $v_1(\eta)$ for 0-5 % centrality in Au+Au collisions at 200 GeV.
$<P_x>/<P_t>$ of charged particles as a function of pseudorapidity, for centrality 0-5% in Au+Au collisions at 200 GeV.
Charged particle $v_1(\eta)$ for 5-40 % centrality in Au+Au collisions at 200 GeV.
Three-particle azimuthal correlation measurements with a high transverse momentum trigger particle are reported for pp, d+Au, and Au+Au collisions at 200 GeV by the STAR experiment. The acoplanarities in pp and d+Au indicate initial state kT broadening. Larger acoplanarity is observed in Au+Au collisions. The central Au+Au data show an additional effect signaling conical emission of correlated charged hadrons.
FIG. 1: (a) Raw two-particle correlation signal $Y_2$ (red), background $aB_{inc}F_2$ (solid histogram), and background systematic uncertainty from a (dashed histograms). (b) Background-subtracted two-particle correlation $\hat{Y}_2$ (red), and systematic uncertainties due to a (dashed histograms) and flow (blue histograms). (c) Raw three-particle correlation $Y_3$. (d) $ba^2Y_{inc}^2$ . (e) Sum of trig-corr-bkgd and trigger flow. Data are from 12% central Au+Au collisions. Statistical errors in (a,b) are smaller than the point size. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.
FIG. 1: (a) Raw two-particle correlation signal $Y_2$ (red), background $aB_{inc}F_2$ (solid histogram), and background systematic uncertainty from a (dashed histograms). (b) Background-subtracted two-particle correlation $\hat{Y}_2$ (red), and systematic uncertainties due to a (dashed histograms) and flow (blue histograms). (c) Raw three-particle correlation $Y_3$. (d) $ba^2Y_{inc}^2$ . (e) Sum of trig-corr-bkgd and trigger flow. Data are from 12% central Au+Au collisions. Statistical errors in (a,b) are smaller than the point size. NOTE: For points with invisible error bars, the point size was considered as an absolute upper limit for the uncertainty.
FIG. 1: (a) Raw two-particle correlation signal $Y_2$ (red), background $aB_{inc}F_2$ (solid histogram), and background systematic uncertainty from a (dashed histograms). (b) Background-subtracted two-particle correlation $\hat{Y}_2$ (red), and systematic uncertainties due to a (dashed histograms) and flow (blue histograms). (c) Raw three-particle correlation $Y_3$. (d) $ba^2Y_{inc}^2$ . (e) Sum of trig-corr-bkgd and trigger flow. Data are from 12% central Au+Au collisions. Statistical errors in (a,b) are smaller than the point size.
We report precision measurements of the Feynman-x dependence, and first measurements of the transverse momentum dependence, of transverse single spin asymmetries for the production of \pi^0 mesons from polarized proton collisions at \sqrt{s}=200 GeV. The x_F dependence of the results are in fair agreement with perturbative QCD model calculations that identify orbital motion of quarks and gluons within the proton as the origin of the spin effects. Results for the p_T dependence at fixed x_F are not consistent with pQCD-based calculations.
Analyzing powers A_N(x_F) in x_F bins at < eta > =3.7 and x_F > 0.
Analyzing powers A_N(x_F) in x_F bins at < eta > =3.3 and x_F > 0.
Analyzing powers A_N(-x_F) in x_F bins at < eta > =3.7 and x_F < 0.
We present STAR results on the elliptic flow v_2 of charged hadrons, strange and multi-strange particles from sqrt(s_NN) = 200 GeV Au+Au collisions at RHIC. The detailed study of the centrality dependence of v_2 over a broad transverse momentum range is presented. Comparison of different analysis methods are made in order to estimate systematic uncertainties. In order to discuss the non-flow effect, we have performed the first analysis of v_2 with the Lee-Yang Zero method for K_s^0 and Lambda. In the relatively low p_T region, p_T <= 2 GeV/c, a scaling with m_T - m is observed for identified hadrons in each centrality bin studied. However, we do not observe v_2(p_T) scaled by the participant eccentricity to be independent of centrality. At higher p_T, 2 GeV/c <= p_T <= 6 GeV/c, v_2 scales with quark number for all hadrons studied. For the multi-strange hadron Omega, which does not suffer appreciable hadronic interactions, the values of v_2 are consistent with both m_T -m scaling at low p_T and number-of-quark scaling at intermediate p_T. As a function of collision centrality, an increase of p_T-integrated v_2 scaled by the participant eccentricity has been observed, indicating a stronger collective flow in more central Au+Au collisions.
$v_{2}(\eta)$ for charged hadrons, $0.15 < p_{T} < 2.0$ GeV/c, centrality $10-40\%$, from $Au+Au$ collisions at $\sqrt{s_{NN}} = 200$ GeV.
$v_{2}$ as a function of $p_{T}$ for charged hadrons with $|\eta| < 1.0$ in 10–40$%$ $Au+Au$ collisions, at $\sqrt{s_{NN}} = 200$ GeV, from the Event Plane method (open circles).
$v_{2}$ as a function of $p_{T}$ for charged hadrons with $|\eta| < 1.0$ in 10–40$%$ $Au+Au$ collisions, at $\sqrt{s_{NN}} = 200$ GeV, from the 4-particle cumulant method (solid squares).
We present the first measurements of the $\rho(770)^0$, $K^*$(892), $\Delta$(1232)$^{++}$, $\Sigma$(1385), and $\Lambda$(1520) resonances in $d$+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV, reconstructed via their hadronic decay channels using the STAR detector at RHIC. The masses and widths of these resonances are studied as a function of transverse momentum ($p_T$). We observe that the resonance spectra follow a generalized scaling law with the transverse mass ($m_T$). The $
rho0 reconstruction efficiency times detector acceptance as a function of the invariant mass for minimum bias d+Au. The error shown is due to the available statistics in the simulation.
K*0 reconstruction efficiency times detector acceptance as a function of pT for minimum bias d+Au and three different centralities.
K*+- reconstruction efficiency times detector acceptance as a function of pT for minimum bias d+Au and three different centralities.
We present the first spin alignment measurements for the $K^{*0}(892)$ and $\phi(1020)$ vector mesons produced at mid-rapidity with transverse momenta up to 5 GeV/c at $\sqrt{s_{NN}}$ = 200 GeV at RHIC. The diagonal spin density matrix elements with respect to the reaction plane in Au+Au collisions are $\rho_{00}$ = 0.32 $\pm$ 0.04 (stat) $\pm$ 0.09 (syst) for the $K^{*0}$ ($0.8
The spin-density matrix elements $\rho_{00}$ with respect to the reaction plane in midcentral $Au+Au$ collisions at $\sqrt{s_{NN}}=200$ GeV versus $p_{T}$ of the vector meson. The sizes of the statistical uncertainties are indicated by error bars, and the systematic uncertainties by caps. The $K^{∗0}$ data points have been shifted slightly in $p_{T}$ for clarity. The dashed horizontal line indicates the unpolarized expectation $\rho_{00}=1/3$. The bands and continuous horizontal lines show predictions discussed in the text.
The dependence of $\rho_{00}$ with respect to the reaction plane on the number of participants at midrapidity in $Au+Au$ collisions at $\sqrt{s_{NN}}=200$ GeV. The sizes of the statistical uncertainties are indicated by error bars and the systematic uncertainties by caps. The $\phi$ data for $p_{T} > 2$ GeV/$c$ and the $K^{∗0}$ data points have been shifted slightly in $\langle N_{\scriptsize{\mbox{part}}}\rangle$ for clarity. The dashed horizontal line indicates the unpolarized expectation $\rho_{00}=1/3$.
The spin-density matrix elements $\rho_{00}$ with respect to the production plane in midcentral $Au+Au$ and $p+p$ collisions at $\sqrt{s_{NN}}=200$ GeV versus $p_{T}$ of the vector meson. The sizes of the statistical uncertainties are indicated by error bars and the systematic uncertainties by caps. The $K^{*0}$ and the $\phi$ $p+p$ data points have been shifted slightly in $p_{T}$ for clarity. The dashed horizontal line indicates the unpolarized expectation $\rho_{00}=1/3$.
Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR collaboration presents a measurement of rho^0 and direct pi^+pi^- photoproduction in ultra-peripheral relativistic heavy ion collisions at sqrt(s_{NN})=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross-section of sigma(AuAu) -> Au^*Au^*rho^0 = 530 pm 19 (stat.) pm 57 (syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The rho^0 transverse momentum spectrum (p_{T}^2) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus/ we find sigma_{inc}/sigma_{coh} = 0.29 pm 0.03 (stat.) pm 0.08 (syst.). The ratio of direct pi^+pi^- to rho^0 production is comparable to that observed in gamma p collisions at HERA, and appears to be independent of photon energy. Finally, the measured rho^0 spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.
ZDC spectra obtained with the minimum bias sample after the $\rho^{0}$ selection cuts are applied, and fit with three Gaussians. The east ZDC is shown on the left and the west ZDC is shown on the right. The ratio of numbers of candidates in the West ZDC of 1n:2n:3n is 1: 0.48 $\pm$ 0.03: 0.42 $\pm$ 0.03, while in the East ZDC, we find 1n:2n:3n is 1: 0.46 $\pm$ 0.03: 0.42 $\pm$ 0.03.
ZDC spectra obtained with the minimum bias sample after the $\rho^{0}$ selection cuts are applied, and fit with three Gaussians. The east ZDC is shown on the left and the west ZDC is shown on the right. The ratio of numbers of candidates in the West ZDC of 1n:2n:3n is 1: 0.48 $\pm$ 0.03: 0.42 $\pm$ 0.03, while in the East ZDC, we find 1n:2n:3n is 1: 0.46 $\pm$ 0.03: 0.42 $\pm$ 0.03.
The invariant mass distribution for the coherently produced $\rho^{0}$ candidates from the minimum bias sample with the cut on the $\rho^{0}$ transverse momentum $p_{T}$ < 150 MeV/c. The hatched area is the contribution from the combinatorial background. The solid line corresponds to Eq. 3 which encompasses the Breit-Wigner (dashed), the mass independent contribution from the direct $\pi^{+}\pi^{-}$ production (dash-dotted), and the interference term(dotted).
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