arXiv 0812.30982008-12-16nucl-ex

高能核碰撞中的热双轻子

Thermal dileptons in high-energy nuclear collisions

Sanja Damjanovic

备注：: 7 pages, 6 figures, Proceedings of the "International Workshop on Nuclear Physics", Erice 2008, to appear in Prog. Part. Nucl. Phys
DOI：: 10.1016/j.ppnp.2008.12.031

摘要：

长期以来，在SPS发现了超过已知来源的双轻子的明显迹象。然而，直到最近NA 60才真正澄清了这些观测结果，在158 A GeV的In-In碰撞中以前所未有的精度测量了双μ子。在M<1 GeV区域的过剩质量谱与π + π-> ρ-> μ + μ-湮没的主要贡献一致。相关的ρ谱函数显示出强烈的加宽，但基本上没有质量的变化。在区域M>1 GeV，过剩被发现是提示，而不是由于增强的魅力生产。与横向动量谱相关的反斜率参数Teff随着质量上升到ρ，然后突然下降。虽然最初的上升，加上强子冻结出的层次，指向强子衰变源的径向流，上述下降信号过渡到低流量源，推测部分起源。质谱显示了类普朗克辐射特征的向低质量的急剧上升。在Collins Soper坐标系下，过剩态的极化是各向同性的。所有的观测结果都与全球对过量热辐射的解释一致。最后，我们简短地讨论了直接光子的可能联系。

英文摘要：

Clear signs of excess dileptons above the known sources were found at the SPS since long. However, a real clarification of these observations was only recently achieved by NA60, measuring dimuons with unprecedented precision in 158A GeV In-In collisions. The excess mass spectrum in the region M<1 GeV is consistent with a dominant contribution from pi+pi- -> rho -> mu+mu- annihilation. The associated rho spectral function shows a strong broadening, but essentially no shift in mass. In the region M>1 GeV, the excess is found to be prompt, not due to enhanced charm production. The inverse slope parameter Teff associated with the transverse momentum spectra rises with mass up to the rho, followed by a sudden decline above. While the initial rise, coupled to a hierarchy in hadron freeze-out, points to radial flow of a hadronic decay source, the decline above signals a transition to a low-flow source, presumably of partonic origin. The mass spectra show the steep rise towards low masses characteristic for Planck-like radiation. The polarization of the excess referred to the Collins Soper frame is found to be isotropic. All observations are consistent with a global interpretation of the excess as thermal radiation. We conclude with a short discussion of a possible link to direct photons.

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