The order of the quantum chromodynamics transition predicted by the standard model of particle physics
The order of the quantum chromodynamics transition predicted by the standard model of particle physics
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DOI：
10.1038/nature05120
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发表时间：
2006-10-12
期刊：
影响因子：
64.8
通讯作者：
Szabo, K. K.
中科院分区：
文献类型：
--
作者：
Aoki, Y.;Endrodi, G.;Szabo, K. K.
量子色动力学（QCD）是强相互作用理论，解释了（例如）三个几乎无质量的夸克与一个重得多的质子或中子的结合，从而解释了可见宇宙的大部分质量。粒子物理学的标准模型预测了与早期宇宙演化相关的 QCD 相关转变。在低温下，主要的自由度是强子（例如质子和π介子）的无色束缚态。然而，QCD 是渐近自由的，这意味着在高能量或高温度下相互作用会变得越来越弱(1,2)，导致强子分裂。这种行为是预测的低温强子相和高温夸克-胶子等离子体相之间的宇宙学转变的基础（为简单起见，我们使用“相”一词来描述具有不同主导自由度的区域）。尽管付出了巨大的理论努力，这种有限温度 QCD 跃迁（即一阶、二阶或解析交叉）的性质仍然不明确。在这里，我们使用物理夸克质量的计算要求较高的晶格计算来确定 QCD 跃迁的性质。磁化率外推到三个物理体积的消失晶格间距，其中最小和最大的相差五倍。这确保了真正的转变会导致敏感性的急剧增加。没有观察到这样的行为：我们的有限尺寸尺度分析表明，炎热的早期宇宙中的有限温度 QCD 转变不是真正的相变，而是分析交叉（涉及随着温度变化而发生的快速变化，而不是跳跃）。因此，很难从天文观测中找到这种转变的实验证据。
Quantum chromodynamics (QCD) is the theory of the strong interaction, explaining (for example) the binding of three almost massless quarks into a much heavier proton or neutron-and thus most of the mass of the visible Universe. The standard model of particle physics predicts a QCD-related transition that is relevant for the evolution of the early Universe. At low temperatures, the dominant degrees of freedom are colourless bound states of hadrons (such as protons and pions). However, QCD is asymptotically free, meaning that at high energies or temperatures the interaction gets weaker and weaker(1,2), causing hadrons to break up. This behaviour underlies the predicted cosmological transition between the low-temperature hadronic phase and a high-temperature quark-gluon plasma phase (for simplicity, we use the word 'phase' to characterize regions with different dominant degrees of freedom). Despite enormous theoretical effort, the nature of this finite-temperature QCD transition (that is, first-order, second-order or analytic crossover) remains ambiguous. Here we determine the nature of the QCD transition using computationally demanding lattice calculations for physical quark masses. Susceptibilities are extrapolated to vanishing lattice spacing for three physical volumes, the smallest and largest of which differ by a factor of five. This ensures that a true transition should result in a dramatic increase of the susceptibilities. No such behaviour is observed: our finite-size scaling analysis shows that the finite-temperature QCD transition in the hot early Universe was not a real phase transition, but an analytic crossover (involving a rapid change, as opposed to a jump, as the temperature varied). As such, it will be difficult to find experimental evidence of this transition from astronomical observations.