arXiv 0801.14252008-01-09hep-ph

FAIR能核碰撞中的直接光子

Direct Photons in Nuclear Collisions at FAIR Energies

S. M. Kiselev

备注：: 9 pages, 5 figures, Talk at the session of Russian Academy of Sciences, ITEP, Moscow, 26 - 30 November 2007
DOI：: 10.1134/S1063778809030168

摘要：

利用现有数据的外推估计已在FAIR能量的提示光子生产。在$y=y_{c.m.}$上，在25 AGeV处，每个中心Au+Au事件的$p_{t}>$ 1.5 GeV/c的瞬发光子的快度密度估计为$\sim 10^{-4}$。利用每秒的计划光束强度$10^{9}$和1%的相互作用概率，对于10%的大多数中心事件，可以预期每秒的即时光子速率$\sim 10^{2}$光子。本文分析了强子弦动力学（HSD）输运方法和介子散射$πρ\toπγ, ππ\toργ$产生的离子碰撞强子方案中的直接光子。在碰撞的密集阶段期间衰减的短寿命共振（例如$ω\to π^{0} γ$）的光子应被视为直接光子。它们在$p_{t}=0.5 - 1$ GeV/c的直接光子谱中有显著的贡献。在FAIR能量25 AGeV的Au+Au中心碰撞中，HSD发生器预测，作为较低的估计，$γ_{direct}/γ_{π^{0}} \simeq$ 0.5%在区域$p_{t}=0.5 - 1$ GeV/c。在$p_{t}=1.5 - 2$ GeV/c $γ_{prompt}/γ_{π^{0}} \simeq$ 2%时。热直接光子已与Bjorken流体动力学（BHD）模型进行了评估。BHD光谱与HSD预测有很大不同。直接光子谱对模型的初始温度参数$T_{0}$非常敏感。$T_{0}$值增加10 MeV导致$\sim$光子产额增加2倍。

英文摘要：

Using the extrapolation of existing data estimations of prompt photon production at FAIR energies have been made. At $y=y_{c.m.}$ the rapidity density of prompt photons with $p_{t}>$ 1.5 GeV/c per central Au+Au event at 25 AGeV is estimated as $\sim 10^{-4}$ . With the planed beam intensity $10^{9}$ per second and 1% interaction probability, for 10% of most central events one can expect the prompt photon rate $\sim 10^{2}$ photons per second. Direct photons from the hadron scenario of ion collisions generated by the Hadron-String-Dynamics (HSD) transport approach with implemented meson scatterings $πρ\toπγ, ππ\toργ$ have been analyzed. Photons from short-living resonances (e.g. $ω\to π^{0} γ$) decaying during the dense phase of the collision should be considered as direct photons. They contribute significantly in the direct photon spectrum at $p_{t}=0.5 - 1$ GeV/c. At the FAIR energy 25 AGeV in Au+Au central collisions the HSD generator predicts, as a lower estimate, $γ_{direct}/γ_{π^{0}} \simeq$ 0.5% in the region $p_{t}=0.5 - 1$ GeV/c. At $p_{t}=1.5 - 2$ GeV/c $γ_{prompt}/γ_{π^{0}} \simeq$ 2%. Thermal direct photons have been evaluated with the Bjorken Hydro-Dynamics (BHD) model. The BHD spectra differ strongly from the HSD predictions. The direct photon spectrum is very sensitive to the initial temperature parameter $T_{0}$ of the model. The 10 MeV increase in the $T_{0}$ value leads to $\sim$ 2 times higher photon yield.

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摘要：

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