Cosmological phase transitions: field theoretical and numerical improvements

宇宙学中最大的谜团之一是宇宙中观测到的重子不对称：为什么观测到的重子比反重子多得多？原则上，从基本粒子物理学的角度，通过早期宇宙中发生的电弱相变，应该可以解释这一点。然而，标准模型在这方面失败了，尽管它包含了所有必需的结构成分。因此，它需要扩展。在这个项目中，我们将首先开发现场理论和数值方法，以改进这种相变的计算。它可以大致分为三个步骤：(i)成核阶段，在这个阶段，具有能量有利相的气泡成核；（ii）这些气泡的膨胀；（iii）气泡的碰撞和合并。我们将在2pi形式中计算所谓的反弹解，它用于描述非平衡现象。通过这种方式，我们获得了气泡壁特定于模型的起始轮廓，而不依赖于通常的arctan-ansatz。此外，还可以检查是否确实接近热平衡，就像文献中通常假设的那样。这与标准模型的扩展特别相关。在第二步中，我们将把这些技术应用于其中一些扩展。我们将研究具有扩展希格斯扇区的特定模型、中性自然性模型以及考虑到最新可用的lhc数据的复合希格斯模型。

One of the biggest riddles in cosmology is the one of the observed baryon-asymmetry in the Universe: why does one observe much more baryons than anti-baryons. In principle it should be possible to explain this from the point of view of elementary particle physics via the electroweak phase transition which took place in the early Universe. However, the Standard Model fails in this respect despite the fact that it contains all required structural ingredients. Thus, it needs to be extended.In this project we will first develop fieldtheoretical and numerical methods, which allow for an improved calculation of this phase transition. It can roughly be divided into three steps: (i) the nucleation phase, in which bubbles with the energetically favourable phase nucleate, (ii) the expansion of these bubbles and (iii) collisions and coalescence of the bubbles. We will calculate the so-called bounce-solution within the 2PI-formalism, which is used to describe out-of-equilibrium phenomena. In this way we obtain a model-specific starting profile for the bubble wall and we do not rely on the usual arctan-ansatz. Moreover, one can check if one is indeed close to thermal equilibrium as it is usually assumed in the literature. This is in particular relevant for extensions of the Standard Model. In the second step we will apply these techniques to some of these extensions. We will study in particular models with an extended Higgs sector, neutral naturalness models as well as composite Higgs models taking into account the latest available LHC-data.