Article

# Pion interferometry in Au+Au and Cu+Cu collisions at √sNN=62.4 and 200 GeV

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(Impact Factor: 3.73). 08/2009; 80(2). DOI: 10.1103/PhysRevC.80.024905

ABSTRACT

We present a systematic analysis of two-pion interferometry in Au+Au collisions at √sNN=62.4 GeV and Cu+Cu collisions at √sNN=62.4 and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider (RHIC). The multiplicity and transverse momentum dependences of the extracted correlation lengths (radii) are studied. The scaling with charged particle multiplicity of the apparent system volume at final interaction is studied for the RHIC energy domain. The multiplicity scaling of the measured correlation radii is found to be independent of colliding system and collision energy.

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##### Article: Global conservation laws and femtoscopy at RHIC
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ABSTRACT: It is increasingly important to understand, in details the space and momentum observables in elementary particle collisions (e.g. p + p collisions), as they should serve as a reference to the same observables in heavy-ion collisions. Such a comparison is crucial to claim a discovery of new phenomena in the big system. However, in low-multiplicity systems, global conservation laws generate significant N-body correlations in addition to other physics effects. We discuss a formalism to analytically calculate these effects on single-particle distributions and multi-particle correlation functions. Transverse mass distributions in relativistic heavy ion collisions provide valuable information about the dynamics of the system. The comparison of the spectra from big systems with analogous distribution from p + p collisions led to a claims of discovery of strong collective flow dominating the low momentum part of the spectra in heavy ion collisions. However, we question such a comparison by pointing out the risk of ignoring conservation laws when comparing high- (e.g. Au + Au) and low-multiplicity (e.g. p + p) collisions. Then, we argue that a correct treatment of the effects due to energy and momentum conservation may account for most of the difference between spectra in small and big system. As a result, we show that after this effect is considered, p + p collisions have similar amount of radial flow as Au + Au collisions at RHIC. The effect of phase-space constraints due to energy and momentum conservation project onto two-particle space in a non-trivial way, affecting the shape of the two-particle correlation functions, and therefore, complicating the femtoscopic analysis. We also present results from p + p collisions at s =200 GeV, d + Au collisions at sNN =200 GeV and Au + Au collisions at sNN =19.6 GeV from the STAR Experiment at RHIC. The sizes of homogeneity regions are extracted through femtoscopic analysis of the pion correlations. In small system, we see a significant effect of phase-space constraints due to the energy and momentum conservations and we use our formalism to treat these non-femtoscopic correlations. For the first time, we compare RHIC femtoscopic results from Au + Au collisions at sNN =19.6 GeV with previously published results from SPS experiments at very similar energy of the collision. We put STAR results from small systems in the context of world data from femtoscopic studies in elementary particle collisions and observe trends seen in the data. We also directly compare STAR results from heavy-ion and p + p collisions, under identical analysis, detector acceptance and performance. We identify that the multiplicity and the transverse mass dependence of femtoscopic radii in small systems is surprisingly similar to what is seen in heavy ion collisions. Based on these similarities between spectra and femtoscopic results from small and big systems, we speculate that there is as strong radial flow in p + p collisions as observed in Au + Au collisions.
• ##### Article: Femtoscopy of pp collisions at root s=0.9 and 7 TeV at the LHC with two-pion Bose-Einstein correlations
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ABSTRACT: We report on the high statistics two-pion correlation functions from pp collisions at root s = 0.9 TeV and root s = 7 TeV, measured by the ALICE experiment at the Large Hadron Collider. The correlation functions as well as the extracted source radii scale with event multiplicity and pair momentum. When analyzed in the same multiplicity and pair transverse momentum range, the correlation is similar at the two collision energies. A three-dimensional femtoscopic analysis shows an increase of the emission zone with increasing event multiplicity as well as decreasing homogeneity lengths with increasing transverse momentum. The latter trend gets more pronounced as multiplicity increases. This suggests the development of space-momentum correlations, at least for collisions producing a high multiplicity of particles. We consider these trends in the context of previous femtoscopic studies in high-energy hadron and heavy-ion collisions and discuss possible underlying physics mechanisms. Detailed analysis of the correlation reveals an exponential shape in the outward and longitudinal directions, while the sideward remains a Gaussian. This is interpreted as a result of a significant contribution of strongly decaying resonances to the emission region shape. Significant nonfemtoscopic correlations are observed, and are argued to be the consequence of "mini-jet"-like structures extending to low p(t). They are well reproduced by the Monte-Carlo generators and seen also in pi(+)pi(-) correlations.
Physical Review D 01/2011; 84(11):22. DOI:10.1103/PhysRevD.84.112004 · 4.64 Impact Factor
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##### Article: In-medium reduction of the η′ mass √sNN = 200 GeV Au+Au collisions
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ABSTRACT: A reduction of the mass of the \eta'(958) meson may indicate the restoration of the UA(1) symmetry in a hot and dense hadronic matter, corresponding to the return of the 9th, "prodigal" Goldstone boson. We report on an analysis of a combined PHENIX and STAR data set on the intercept parameter of the two-pion Bose-Einstein correlation functions, as measuremed in \sqrt{s_NN} = 200 GeV Au+Au collisions at RHIC. To describe this combined PHENIX and STAR dataset, an in-medium \eta' mass reduction of at least 200 MeV is needed, at the 99.9 % confidence level in a broad model class of resonance multiplicities. Energy, system size and centrality dependence of the observed effect is also discussed.
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