Global polarization of Ξ and Ω hyperons has been measured for the first time in Au + Au collisions at √sNN = 200 GeV. The measurements of the Ξ− and ... hyperon polarization have been performed by ...two independent methods, via analysis of the angular distribution of the daughter particles in the parity violating weak decay Ξ → Λ + π, as well as by measuring the polarization of the daughter Λ hyperon, polarized via polarization transfer from its parent. The polarization, obtained by combining the results from the two methods and averaged over Ξ− and ... is measured to be ⟨PΞ⟩ = 0.47 ± 0.10(stat) ± 0.23(syst)% for the collision centrality 20%–80%. The ⟨PΞ⟩ is found to be slightly larger than the inclusive Λ polarization and in reasonable agreement with a multiphase transport model. The ⟨PΞ⟩ is found to follow the centrality dependence of the vorticity predicted in the model, increasing toward more peripheral collisions. The global polarization of Ω, ⟨PΩ⟩ = 1.11 ± 0.87(stat) ± 1.97(syst)% was obtained by measuring the polarization of daughter Λ in the decay Ω → Λ + K, assuming the polarization transfer factor CΩΛ = 1.
We present results from a harmonic decomposition of two-particle azimuthal correlations measured with the STAR detector in Au+Au collisions for energies ranging from sqrts_{NN}=7.7 to 200 GeV. The ...third harmonic v_{3}^{2}{2}=⟨cos3(ϕ_{1}-ϕ_{2})⟩, where ϕ_{1}-ϕ_{2} is the angular difference in azimuth, is studied as a function of the pseudorapidity difference between particle pairs Δη=η_{1}-η_{2}. Nonzero v_{3}^{2}{2} is directly related to the previously observed large-Δη narrow-Δϕ ridge correlations and has been shown in models to be sensitive to the existence of a low viscosity quark gluon plasma phase. For sufficiently central collisions, v_{3}^{2}{2} persist down to an energy of 7.7 GeV, suggesting that quark gluon plasma may be created even in these low energy collisions. In peripheral collisions at these low energies, however, v_{3}^{2}{2} is consistent with zero. When scaled by the pseudorapidity density of charged-particle multiplicity per participating nucleon pair, v_{3}^{2}{2} for central collisions shows a minimum near sqrts_{NN}=20 GeV.
A
bstract
We report on the measurement of the Central Exclusive Production of charged particle pairs
h
+
h
−
(
h
=
π, K, p
) with the STAR detector at RHIC in proton-proton collisions at
s
= 200 GeV. ...The charged particle pairs produced in the reaction
pp → p
′ +
h
+
h
−
+
p
′ are reconstructed from the tracks in the central detector and identified using the specific energy loss and the time of flight method, while the forward-scattered protons are measured in the Roman Pot system. Exclusivity of the event is guaranteed by requiring the transverse momentum balance of all four final-state particles. Differential cross sections are measured as functions of observables related to the central hadronic final state and to the forward-scattered protons. They are measured in a fiducial region corresponding to the acceptance of the STAR detector and determined by the central particles’ transverse momenta and pseudorapidities as well as by the forward-scattered protons’ momenta. This fiducial region roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range 0
.
04 GeV
2
< −t
1
, −t
2
<
0
.
2 GeV
2
, invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range
|η| <
0
.
7. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of
π
+
π
−
and
K
+
K
−
pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to
π
+
π
−
production. For
π
+
π
−
production, the fiducial cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the
f
0
(980),
f
2
(1270) and
f
0
(1500), with a possible small contribution from the
f
0
(1370). Fits to the extrapolated differential cross section as a function of
t
1
and
t
2
enable extraction of the exponential slope parameters in several bins of the invariant mass of
π
+
π
−
pairs. These parameters are sensitive to the size of the interaction region.
The chiral magnetic wave (CMW) has been theorized to propagate in the deconfined nuclear medium formed in high-energy heavy-ion collisions, and to cause a difference in elliptic flow (v2) between ...negatively and positively charged hadrons. Experimental data consistent with the CMW have been reported by the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC), based on the charge asymmetry dependence of the pion v2 from Au+Au collisions at $\sqrt{s_{NN}}$ = 27 to 200 GeV. In this comprehensive study, we present the STAR measurements of elliptic flow and triangular flow of charged pions, along with the v2 of charged kaons and protons, as a function of charge asymmetry in Au+Au collisions at $\sqrt{s_{NN}}$ = 27, 39, 62.4 and 200 GeV. The slope parameters extracted from the linear dependence of the v2 difference on charge asymmetry for different particle species are reported and compared in different centrality intervals. In addition, the slopes of v2 for charged pions in small systems, i.e., p+Au and d+Au at $\sqrt{s_{NN}}$ = 200 GeV, are also presented and compared with those in large systems, i.e., Au+Au at $\sqrt{s_{NN}}$ = 200 GeV and U+U at 193 GeV. Our results provide new insights for the possible existence of the CMW, and further constrain the background contributions in heavy-ion collisions at RHIC energies.
We report on measurements of sequential ϒ suppression in Au+Au collisions at sqrts_{NN}=200 GeV with the STAR detector at the Relativistic Heavy Ion Collider (RHIC) through both the dielectron and ...dimuon decay channels. In the 0%-60% centrality class, the nuclear modification factors (R_{AA}), which quantify the level of yield suppression in heavy-ion collisions compared to p+p collisions, for ϒ(1S) and ϒ(2S) are 0.40±0.03(stat)±0.03(sys)±0.09(norm) and 0.26±0.08(stat)±0.02(sys)±0.06(norm), respectively, while the upper limit of the ϒ(3S) R_{AA} is 0.17 at a 95% confidence level. This provides experimental evidence that the ϒ(3S) is significantly more suppressed than the ϒ(1S) at RHIC. The level of suppression for ϒ(1S) is comparable to that observed at the much higher collision energy at the Large Hadron Collider. These results point to the creation of a medium at RHIC whose temperature is sufficiently high to strongly suppress excited ϒ states.
Test of charge conjugation invariance Nefkens, B M K; Prakhov, S; Gårdestig, A ...
Physical review letters,
02/2005, Letnik:
94, Številka:
4
Journal Article
Recenzirano
We report on the first determination of upper limits on the branching ratio (BR) of eta decay to pi0pi0gamma and to pi0pi0pi0gamma. Both decay modes are strictly forbidden by charge conjugation (C) ...invariance. Using the Crystal Ball multiphoton detector, we obtained BR(eta-->pi0pi0gamma)<5 x 10(-4) at the 90% confidence level, in support of C invariance of isoscalar electromagnetic interactions of the light quarks. We have also measured BR(eta-->pi0pi0pi0gamma)<6 x 10(-5) at the 90% confidence level, in support of C invariance of isovector electromagnetic interactions.
We present STAR measurements of strange hadron ( K S0, Λ , Λ ¯, Ξ -, Ξ ¯+, Ω -, Ω ¯+ , and Φ ) production at midrapidity ( |y|<0.5 ) in Au+Au collisions at sNN = 7.7–39 GeV from the Beam Energy Scan ...Program at the Relativistic Heavy Ion Collider (RHIC). Transverse-momentum spectra, averaged transverse mass, and the overall integrated yields of these strange hadrons are presented versus the centrality and collision energy. Antibaryon-to-baryon ratios ( Λ ¯/ Λ , Ξ ¯+/ Ξ -, Ω ¯+/ Ω - ) are presented as well and used to test a thermal statistical model and to extract the temperature normalized strangeness and baryon chemical potentials at hadronic freeze-out ( μB/Tch and μS/Tch ) in central collisions. Strange baryon-to-pion ratios are compared to various model predictions in central collisions for all energies. The nuclear modification factors ( RCP ) and antibaryon-to-meson ratios as a function of transverse momentum are presented for all collision energies. The K S0 RCP shows no suppression for pT up to 3.5 GeV/c at energies of 7.7 and 11.5 GeV. The Λ ¯/ K S0 ratio also shows baryon-to-meson enhancement at intermediate pT ( ≈2.5 GeV/c ) in central collisions at energies above 19.6 GeV. Lastly, both observations suggest that there is likely a change of the underlying strange quark dynamics at collision energies below 19.6 GeV.
The polarization of Λ and Λover ¯ hyperons along the beam direction has been measured relative to the second and third harmonic event planes in isobar Ru+Ru and Zr+Zr collisions at sqrts_{NN}=200 ...GeV. This is the first experimental evidence of the hyperon polarization by the triangular flow originating from the initial density fluctuations. The amplitudes of the sine modulation for the second and third harmonic results are comparable in magnitude, increase from central to peripheral collisions, and show a mild p_{T} dependence. The azimuthal angle dependence of the polarization follows the vorticity pattern expected due to elliptic and triangular anisotropic flow, and qualitatively disagrees with most hydrodynamic model calculations based on thermal vorticity and shear induced contributions. The model results based on one of existing implementations of the shear contribution lead to a correct azimuthal angle dependence, but predict centrality and p_{T} dependence that still disagree with experimental measurements. Thus, our results provide stringent constraints on the thermal vorticity and shear-induced contributions to hyperon polarization. Comparison to previous measurements at RHIC and the LHC for the second-order harmonic results shows little dependence on the collision system size and collision energy.
PDF
