We report on measurements of dielectron ($e^+e^-$) production in Au$+$Au collisions at a center-of-mass energy of 200 GeV per nucleon-nucleon pair using the STAR detector at RHIC. Systematic measurements of the dielectron yield as a function of transverse momentum ($p_{\rm T}$) and collision centrality show an enhancement compared to a cocktail simulation of hadronic sources in the low invariant-mass region ($M_{ee}<$ 1 GeV/$c^2$). This enhancement cannot be reproduced by the $\rho$-meson vacuum spectral function. In minimum-bias collisions, in the invariant-mass range of 0.30 $-$ 0.76 GeV/$c^2$, integrated over the full $p_{\rm T}$ acceptance, the enhancement factor is 1.76 $\pm$ 0.06 (stat.) $\pm$ 0.26 (sys.) $\pm$ 0.29 (cocktail). The enhancement factor exhibits weak centrality and $p_{\rm T}$ dependence in STAR's accessible kinematic regions, while the excess yield in this invariant-mass region as a function of the number of participating nucleons follows a power-law shape with a power of 1.44 $\pm$ 0.10. Models that assume an in-medium broadening of the $\rho$ meson spectral function consistently describe the observed excess in these measurements. Additionally, we report on measurements of $\omega$ and $\phi$-meson production through their $e^+e^-$ decay channel. These measurements show good agreement with Tsallis Blast-Wave model predictions as well as, in the case of the $\phi$-meson, results through its $K^+K^-$ decay channel. In the intermediate invariant-mass region (1.1$<M_{ee}<$ 3 GeV/$c^2$), we investigate the spectral shapes from different collision centralities. Physics implications for possible in-medium modification of charmed hadron production and other physics sources are discussed.
Estimated electron purity vs. momentum in 200 GeV Au + Au collisions.
Acceptance correction factor for unlike-sign and like-sign pair difference from 200 GeV Au+Au minimum-bias collisions.
Ratio of the same-event like-sign to the mixed event unlike-sign distributions.
We present measurements of $\pi^-$ and $\pi^+$ elliptic flow, $v_2$, at midrapidity in Au+Au collisions at $\sqrt{s_{_{\rm NN}}} =$ 200, 62.4, 39, 27, 19.6, 11.5 and 7.7 GeV, as a function of event-by-event charge asymmetry, $A_{ch}$, based on data from the STAR experiment at RHIC. We find that $\pi^-$ ($\pi^+$) elliptic flow linearly increases (decreases) with charge asymmetry for most centrality bins at $\sqrt{s_{_{\rm NN}}} = \text{27 GeV}$ and higher. At $\sqrt{s_{_{\rm NN}}} = \text{200 GeV}$, the slope of the difference of $v_2$ between $\pi^-$ and $\pi^+$ as a function of $A_{ch}$ exhibits a centrality dependence, which is qualitatively similar to calculations that incorporate a chiral magnetic wave effect. Similar centrality dependence is also observed at lower energies.
The distribution of observed charge asymmetry from STAR data.
Pion $v_2${2} as a function of observed charge asymmetry.
$v_2$ difference between $\pi^-$ and $\pi^+$ as a function of charge asymmetry with the tracking efficiency correction, for 30-40% central Au+Au collisions at 200 GeV. The errors are statistical only.
The relative production yields and transverse mass spectra for Λ, Λ , Ξ − and Ξ + hyperons in proton-tungsten interactions are presented and compared with the WA85 results from central sulphur-tungsten interactions. A study of the negative particle yield has also been undertaken and the ratio of Λ hyperons to negative particles has been calculated.
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The transverse momentum distribution at 90° of pions, protons and antiprotons have been measured at the CERN intersecting storage rings for C.M. energies between 23.2 and 52.7 GeV. In this energy range, the pion and proton distributions are almost energy independent. The antiproton production rises by a factor of two between 23.2 and 52.7 GeV.
The invariant cross section was fitted by CONST*EXP(-SLOPE*PT).
The invariant cross section was fitted by CONST*EXP(-SLOPE(C=1)*PT+SLOPE(C=2)*PT**2).
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Strange and multistrange baryon and antibaryon production is a useful probe into the dynamics of the hot hadronic matter created in central heavy ion interactions. Relative production yields and transverse mass spectra are presented for Λ, Λ , Ξ − and Ξ + hyperons produced in central sulphur-tungsten interactions at 200 GeV/ c per nucleon.
Distributions are fitted with (1/MT**1.5)*DN/DMT=CONST*EXP(-MT/SLOPE).
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Preliminary results from WA97 measurements on Λ, Ξ and Ω production in lead-lead and proton-lead collisions are presented, along with a comparison of WA97 proton-lead data with previous WA85 proton-tungsten results. The ratio Ω gX seems to be enhanced in lead initiated reactions compared to proton initiated reactions.
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PRELIMINARI DATA.
The WA94 experiment uses the production of strange particles and antiparticles to investigate the properties of hot hadronic matter created in heavy-ion interactions. Λ, Λ , Ξ − and Ξ + particle yields and transverse mass spectra are presented for pS interactions. These results are compared with those from SS interactions. Our results are also compared with those from pW and SW interactions of the WA85 experiment.
The fit with formula (1/MT**1.5)*D(SIG)/D(MT) = CONST*EXP(MT/SLOPE).
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We have measured ρ0, ω (combined) and ϕ electroproduction over a range of virtual-photon four-momentum Q2 from 0.4 to 2.2 GeV2 and for photon energies ν from 2.7 to 8.6 GeV. We find that the slope of the t (momentum transfer) dependence of the ρ0 and ω forward peak decreases with increasing Q2 to less than half of the photoproduction slope.
The cross section for virtual photon are derived from E- P cross section bydividing on the virtual-photon flux factor.
Results of fitting the differential distributions in x F and p T 2 of D mesons produced in 400 GeV/ c p-p interactions to the form d 2 σ d x F d p T 2 ∝(1−x F ) n exp [−(p T 2 /〈p T 2 〉)] are discussed. The D + distribution is found to be relatively hard [ n =3.1±0.8〈 P t 2 〉=1.32±0.27 (GeV/ c ) 2 ] and the D̄ 0 distribution relatively soft [ n =8.1±1.9,〈 p T 2 〉=0.62±0.14 (GeV/ c ) 2 ] compared to the average for all D's [ n =4.9±0.5,〈 p T 2 〉=0.99±0.10 (GeV/ c ) 2 ]. It is suggested that these distributions could reflect contribution of leading di-quarks in pp collisions. Comparison is made with evidence for leading quarks in charm production in 360 GeV/ cπ − p interactions.
The invariant (C=INV) and non-invariant (C=NON-INV) distributions are fitted to (1-XL)**POWER. Pt distribution is fitted to EXP(-PT**2/SLOPE).
Multi-strange baryon and anti-baryon production is expected to be a useful probe in the search for Quark-Gluon Plasma formation. We present the transverse mass distributions of negative particles, K o s, Λs, Λ s, and Ξ − s produced in sulphurtungsten interactions at 200 GeV/c per nucleon and give the corrected ratios Λ Λ, Ξ − Λ and Ξ − /Λ . We note that our ratio Ξ − / Λ appears large in comparison to that from p p interactions.
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