不稳定和衰变的尾部：流体动力学观点
Tails of Instability and Decay: a Hydrodynamic Perspective
摘要：
在量子场论（QFT）中，不稳定粒子与非物理快度空间中两体散射矩阵的复值极点相关联。布雷特-维格纳公式将这个极点与粒子的质量和寿命联系起来，在散射事件中观察到。在本文中，我们发现了新的，不稳定的激发动力学签名，并表明它们有很强的影响QFT的非平衡性质。针对1+ 1维可积模型，利用广义流体力学理论，通过分析热物质向低温环境中的释放，研究了不稳定粒子的形成和衰变。我们观察到尾部的形成和发射的非线性波的衰减，在没有不稳定的激发的情况下形成鲜明对比。我们还发现了一种新的现象，即不稳定粒子的稳定种群的波可以持续很长时间而不衰减。我们希望这些不稳定粒子存在的特征具有很大程度的普遍性。我们的研究表明，多体系统的非平衡动力学不仅受到谱的强烈影响，而且还受到有限寿命激励的强烈影响。
英文摘要：
In the context of quantum field theory (QFT), unstable particles are associated with complex-valued poles of two-body scattering matrices in the unphysical sheet of rapidity space. The Breit-Wigner formula relates this pole to the mass and life-time of the particle, observed in scattering events. In this paper, we uncover new, dynamical signatures of unstable excitations and show that they have a strong effect on the non-equilibrium properties of QFT. Focusing on a 1+1D integrable model, and using the theory of Generalized Hydrodynamics, we study the formation and decay of unstable particles by analysing the release of hot matter into a low-temperature environment. We observe the formation of tails and the decay of the emitted nonlinear waves, in sharp contrast to the situation without unstable excitations. We also uncover a new phenomenon by which a wave of a stable population of unstable particles may persist without decay for long times. We expect these signatures of the presence of unstable particles to have a large degree of universality. Our study shows that the out-of-equilibrium dynamics of many-body systems can be strongly affected not only by the spectrum, but also by excitations with finite life-times.
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