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Register a new userLambda and Antilambda polarization in Au-Au collisions at RHIC
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Celso de Camargo Barros Jr.
Publication date
2010
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English
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Recent experiments at RHIC have shown that in 200 GeV Au-Au collisions, the Lambda and Antilambda hyperons are produced with very small polarizations [1], almost consistent with zero. These results can be understood in terms of a model that we recently proposed [2]. In this work, we show how this model may be applied in such collisions, and also will discuss the relation of our results with other models, in order to explain the experimental data.
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We present results from Experiment E917 for antilambda and antiproton production in Au+Au collisions at 11.7 A GeV. We have measured invariant spectra and yields for both species in central and peripheral collisions. We find that the antilambda/antiproton ratio near mid-rapidity increases from 0.26+0.19-0.15 in peripheral collisions to 3.6+4.7-1.8 in central collisions, a value that is substantially larger than current theoretical estimates.
In the PHOBOS experiment, charged particles are measured in almost the full solid angle. This enables the study of fluctuations and correlations in the particle production over a very wide kinematic range. In this paper, we show results of a direct search for fluctuations identified by an unusual shape of the pseudorapidity distribution. In addition, we use analysis of correlations of the multiplicity in similar pseudorapidity bins, placed symmetrically in the forward and backward hemispheres, to test the hypothesis of production of particles in clusters.
With a Yang-Mills field, stratified shear flow initial state and a high resolution (3+1)D Particle-in-Cell Relativistic (PICR) hydrodynamic model, we calculate the $Lambda$ polarization for peripheral Au+Au collisions at RHIC energy of $sqrt{S_{NN}}=200$ GeV. The obtained longitudinal polarization in our model agrees with the experimental signature and the quadrupole structure on transverse momentum plane. It is found that the relativistic correction (2nd term), arising from expansion and from the time component of the thermal vorticity, plays a crucial role in our results. This term is changing the signature and exceeds the first term, arising from the classical vorticity. Finally, the global polarization in our model shows no significant dependence on rapidity, which agrees with the experimental data. It is also found that the second term flattens the sharp peak arising from the classical vorticity (1st term).
Collisions between prolate uranium nuclei are used to study how particle production and azimuthal anisotropies depend on initial geometry in heavy-ion collisions. We report the two- and four-particle cumulants, $v_2{2}$ and $v_2{4}$, for charged hadrons from U+U collisions at $sqrt{s_{rm NN}}$ = 193 GeV and Au+Au collisions at $sqrt{s_{rm NN}}$ = 200 GeV. Nearly fully overlapping collisions are selected based on the amount of energy deposited by spectators in the STAR Zero Degree Calorimeters (ZDCs). Within this sample, the observed dependence of $v_2{2}$ on multiplicity demonstrates that ZDC information combined with multiplicity can preferentially select different overlap configurations in U+U collisions. An initial-state model with gluon saturation describes the slope of $v_2{2}$ as a function of multiplicity in central collisions better than one based on Glauber with a two-component multiplicity model.
PHOBOS is one of four experiments studying the Au-Au interactions at RHIC. The data collected during the first few weeks after the RHIC start-up, using the initial configuration of the PHOBOS detector, were sufficient to obtain the first physics results for the most central collisions of Au nuclei at the center of mass energy of 56 and 130 AGeV. The pseudorapidity density of charged particles near midrapidity is shown and compared with data at lower energies and from $pp$ and $pbar{p}$ collisions. The progress of the analysis of the data is also presented.PHOBOS is one of four experiments studying the Au-Au interactions at RHIC. The data collected during the first few weeks after the RHIC start-up, using the initial configuration of the PHOBOS detector, were sufficient to obtain the first physics results for the most central collisions of Au nuclei at the center of mass energy of 56 and 130 AGeV. The pseudorapidity density of charged particles near midrapidity is shown and compared with data at lower energies and from $pp$ and $pbar{p}$ collisions. The progress of the analysis of the data is also presented.
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