Low energy pion-hyperon interaction

Junior, Celso de Camargo Barros; Hama, Yogiro

doi:10.1103/physrevc.63.065203

Cited by 25 publications

(56 citation statements)

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“…Neglecting hyperon production in the hadronic phase, the dominant hadronic processes involving hyperons will be hyperon-π scattering. The scattering amplitude in general involves both scalar and vector channels [16]. One can show that global hyperon polarization from the vector channel is along while polarizations from the scalar channel and all the interferences are against the global quark polarization.…”

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confidence: 99%

Globally Polarized Quark-Gluon Plasma in NoncentralA+ACollisions

2005

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Produced partons have large local relative orbital angular momentum along the direction opposite to the reaction plane in the early stage of non-central heavy-ion collisions. Parton scattering is shown to polarize quarks along the same direction due to spin-orbital coupling. Such global quark polarization will lead to many observable consequences, such as left-right asymmetry of hadron spectra, global transverse polarization of thermal photons, dileptons and hadrons. Hadrons from the decay of polarized resonances will have azimuthal asymmetry similar to the elliptic flow. Global hyperon polarization is studied within different hadronization scenarios and can be easily tested.PACS numbers: 13.88.+e, 12.38.Mh, 25.75.Nq Strong transverse polarization of hyperons has been observed in unpolarized p + p and p + A collisions since the 1970's [1]. Given the beam and hyperon momenta p and p H , hyperons produced in the beam fragmentation region are found transversely polarized in the direction perpendicular to the hyperon production plane,Polarizations of Λ, Ξ andΞ are negative while Σ andΣ's are positive. In the meantime,Λ and Ω are not transversely polarized. Although the origin for such striking transverse hyperon polarization is still in debate, one can relate it to the single-spin left-right asymmetries observed in hadron-hadron collisions with transversely polarized beam [2], which in turn can be attributed to the orbital angular momenta (o.a.m.) of the valence quarks in a polarized nucleon [3,4,5], or fragmentation functions of transverse polarized quarks [6] as well as twist-3 parton correlations in nucleons [7]. It has also been suggested [8,9] that hyperon polarization could disappear due to the formation of QGP.In this Letter, we show that parton interaction in noncentral heavy-ion collisions leads to a global quark polarization along the opposite direction of the reaction plane,as determined by the nuclear impact parameter b. This global polarization is essentially a local manifestation of the global angular momentum of the colliding system through interaction of spin-orbital coupling in QCD. It will have far reaching consequences in non-central heavyion collisions, such as left-right asymmetry of hadron spectra in the reaction plane, global transverse polarization of direct photons, dileptons and hadrons with spin. Within different hadronization scenarios, we will discuss hyperon polarization as a result of the global quark polarization. Possible contribution from final state hadronic interaction will also be discussed. Let us consider two colliding nuclei with the beam projectile moving in the direction of the z axis, as illustrated in Fig. 1. We define the impact parameter b (alongx) as the transverse distance of the projectile from the target nucleus and the reaction plane as given by n b (alongŷ) in Eq. (1). Partons produced in the overlapped region of the collision will carry a global angular momentum along the direction opposite to the reaction plane (−ŷ). A thermalized QGP requires final state par...

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Globally Polarized Quark-Gluon Plasma in NoncentralA+ACollisions

2005

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“…in 1995 and his Ph.D.Thesis in 1991, on hyperon polarization [31], under the supervision of Prof. Hama. His work was motivated by a proposal by Prof. Hama and T. Kodama [32] that part of the (anti)hyperon polarization comes from its interaction with the medium that surrounds it.…”

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confidence: 99%

Working in hydrodynamics with Yogiro Hama

Grassi¹

2007

Braz. J. Phys.

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“…1, 2 and 3. The expressions resulting from the calculations of the diagrams (and a review of the formalism) may be found in [13], where the sigma term was introduced simply as a parametrization. In recent studies [12], [14], it was shown that the σ term may be understood in terms of loops of pions, and then, in the calculation of the diagrams 1c, 2c and 3c, the expressions from [15], [16] have been used.…”

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“…Thus, this seems to be the most reliable treatment to be followed in studying the low energy πY interactions [13].…”

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Antihyperon polarization in inclusive processes at high energies

Barros¹,

Hama²

2004

Braz. J. Phys.

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We propose a model that we believe is the main source of the antihyperon polarization in high-energy protonnucleus inclusive reactions. The polarization is originated by the final-state interactions between the antihyperons and other produced particles in these collisions (predominantly pions). The model is based on two elements: the low-energy pion-hyperon interaction (described by chiral effective Lagrangians) and the statistical fluctuations plus expansion of the background matter.In 1976, Bunce and collaborators [1], studying the Λ inclusive productionwith 200 GeV protons, where X represents the not observed particles, verified that the Λ polarization was significantly different from zero (and negative, reaching values close to 20%). These results were totally unexpected [2], because there were indications that polarization effects should vanish at high energies and hadron polarization was believed to be a low energy phenomenon. Since then, many similar experiments have been performed [3]- [6], that confirmed these results and have shown also that both hyperons and antihyperons produced are polarized in this kind of reaction. However, the variety of behavior of the polarizations has caused many problems to the theoretical understanding of the experimental data ([7]-[9] for example), specially in the antihyperon case. Since these models are based in direct mechanisms for hyperon production (the initial proton becomes a hyperon, by some mechanism, such as quark recombination [7], for example) the final antihyperon polarization practically vanishes.The first model that was able to produce polarized antihyperons, was the one proposed by Y. Hama and T. Kodama [10] and it was based in the hydrodynamical model. The hyperon is produced inside a medium composed of hot hadronic matter (produced in the collision process) and interacts with it. This interaction was represented by an optic potential. With this model, the antihyperon data were quite well reproduced. Nevertheless, the model left some questions in open, such as the need of different potentials for different hyperons and their interpretations.Aiming to study these questions, a new model was developed, [11], based in [10]. The hyperon was considered to be produced unpolarized (as an average effect) in the interior of a hot hadronic matter, now considered as a fluid composed of particles (predominantly pions). The hyperon (or antihyperon) interacts with the other hadrons (instead of considering a potential as it was done in [10]), and becomes polarized due to these final state interactions. The remainder of this work will be devoted to the presentation of the main features of this model, and the results of the Λ and Ξ + polarizations (the calculations to the Σ − polarization are in progress). To build a model with the cited characteristics, two elements must be considered: the microscopic interactions, that happen inside the hadronic matter, and the calculation of averages, considering the fluid in expansion. A brief description of these elements will be ma...

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Low energy pion-hyperon interaction

Cited by 25 publications

References 30 publications

Globally Polarized Quark-Gluon Plasma in NoncentralA+ACollisions

Globally Polarized Quark-Gluon Plasma in NoncentralA+ACollisions

Working in hydrodynamics with Yogiro Hama

Antihyperon polarization in inclusive processes at high energies

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