在158 A GeV铟-铟碰撞中产生质量大于1 GeV/c ^2的热μ介子对的证据
Evidence for the production of thermal muon pairs with masses above 1 GeV/c^2 in 158A GeV Indium-Indium Collisions
摘要：
在重离子碰撞中产生的不变质量区1<M<2.5 GeV/c^2的μ子对的产额明显超过了两个预期贡献之和，即来自D介子对衰变的Drell-Yan双μ子和μ子对。这些来源正确地解释了质子-核碰撞中产生的双μ子。在本文中，我们表明，在158 A GeV的In-In碰撞中也产生了多余的双μ子。我们还观察到，通过标记的双μ子顶点，这是多余的不是由于增强D介子生产，但由提示μ子对，从特定的高能核-核碰撞的热双μ子源的预期。这种过剩的产量显着增加从外围到中心碰撞，无论是相对于Drell-Yan产量和参与碰撞的核子数。此外，过剩双μ子的横向质量分布很好地描述了一个指数函数，与反斜率值约190 MeV。这些值与质量无关，并且明显低于质量低于1 GeV/c^2时的值，由于径向流的作用，这些值会上升到250 MeV。这表明，当径向流尚未形成时，能量超过1 GeV/c^2的热双μ子的发射源主要是部分子源。
英文摘要：
The yield of muon pairs in the invariant mass region 1<M<2.5 GeV/c^2 produced in heavy-ion collisions significantly exceeds the sum of the two expected contributions, Drell-Yan dimuons and muon pairs from the decays of D meson pairs. These sources properly account for the dimuons produced in proton-nucleus collisions. In this paper, we show that dimuons are also produced in excess in 158 A GeV In-In collisions. We furthermore observe, by tagging the dimuon vertices, that this excess is not due to enhanced D meson production, but made of prompt muon pairs, as expected from a source of thermal dimuons specific to high-energy nucleus-nucleus collisions. The yield of this excess increases significantly from peripheral to central collisions, both with respect to the Drell-Yan yield and to the number of nucleons participating in the collisions. Furthermore, the transverse mass distributions of the excess dimuons are well described by an exponential function, with inverse slope values around 190 MeV. The values are independent of mass and significantly lower than those found at masses below 1 GeV/c^2, rising there up to 250 MeV due to radial flow. This suggests the emission source of thermal dimuons above 1 GeV/c^2 to be of largely partonic origin, when radial flow has not yet built up.
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