Axion domain wall network evolution in the early universe

早期宇宙中轴子畴壁网络的演化

• 批准号: 2602033
• 金额: --
• 依托单位: University of Oxford
• 依托单位国家: 英国
• 项目类别: Studentship
• 财政年份: 2021
• 资助国家: 英国
• 起止时间: 2021 至 无数据
• 项目状态: 未结题
• 来源: https://gtr.ukri.org/projects?ref=studentship-2602033
• 关键词: Axion; domain; wall; network; evolution

An attractive candidate dark matter (DM) particle is the axion. This is the pseudo Nambu-Goldstone boson from the spontaneous symmetry breaking of a chiral U(1)_PQ introduced by Peccei & Quinn to solve the 'strong CP' problem, viz. that the strong interactions in the Standard Model (SM) do not violate CP symmetry as is evidenced by the vanishing electric dipole moment of the neutron. While the original 'Weinberg-Wilczek' axion corresponding to PQ symmetry breaking at the electroweak scale was never found, it was soon realised that the relevant scale can be much higher, thus yielding an 'invisible axion' with very suppressed (derivative) couplings to SM fields. Nevertheless collective oscillations of the relic axion field has the same equation of state as non-relativistic particles so relic axions can account for the DM of the universe for a PQ scale around 10^{10-11} GeV. The cosmological evolution of such an axion is interesting because of a sequence of symmetry breaking which produces potentially stable topological defects. Below the Peccei-Quinn scale f_PQ the vacuum manifold is not simply connected. This implies the existence of closed paths in physical space which get mapped onto non-trivial paths in field space winding around the origin. Such field configurations correspond to cosmic strings. When the temperature drops to A_QCD ~ 300 MeV, non-perturbative QCD effects generate a mass for the axion which breaks the symmetry to a Z(N) with N the number of quarks charged under U(1)_PQ. The vacuum manifold of Z(N) is disconnected which implies the existence of paths in physical space which map onto paths interpolating between two vacuum states in field space. Such paths necessarily leave the vacuum manifold and the resulting structure is a domain wall.Topologically each string must be connected by N domain walls once the axion gets a mass. Due to the surface energy of domain walls, a network of strings and domain walls with N= 1 would be unstable and collapse. However networks with N >1 are stable in principle and can lead to a cosmological catastrophe if they come to dominate the energy density of the universe after they form [. The standard argument for evading the 'domain wall problem' when N >1 is to invoke non-renormalisable Plank-scale suppressed operators reflecting quantum gravity effects which explicitly break the initial U(1)PQ and lift the degeneracy of the vacuum states, resulting in an effective pressure term which causes the domain of true vacuum to grow. Such explicit breaking of U(1)PQ is however experimentally constrained as it reintroduces the CP violation which is required by experimental upper limits on the neutron electromagnetic dipole moment (EDM) to be negligibly small. By requiring axion domain walls to disappear in time to avoid cosmological catastrophe one can thus infer a lower bound on the neutron EDM. Therefore the `tilt' solution to the domain wall problem is potentially testable by future neutron EDM experiments. We propose an alternative mechanism to render domain walls unstable by instead introducing a statistical `bias' in the population of the vacuum states. Such bias is naturally generated by the evolution of the axion field during inflation, under the assumption that inflation lasts long enough for the universe to cool down below A_QCD during inflation so the axion acquires a mass. This possibility has recently been exploited to open up previously excluded axion parameter space. It has been demonstrated that such bias leads for Z(2) models to exponential decay of the co-moving domain wall energy density. The impact of this on the most likely mass for axions to make up dark matter will be studied, as this is particularly relevant to ongoing searches for axions including in the UK 'Quantum Sensors for the Hidden Sector' programme.

一个有吸引力的候选暗物质（DM）粒子是轴子。这是Peccei和Quinn为了解决“强CP”问题而引入的来自手征U（1）_PQ的自发对称破缺的伪Nambu-Goldstone玻色子，即标准模型（SM）中的强相互作用不违反CP对称性，这一点可以通过中子的电偶极矩消失来证明。虽然在电弱尺度下对应PQ对称性破缺的原始“温伯格-威尔切克”轴子从未被发现，但人们很快意识到相关的尺度可以高得多，从而产生一个“不可见的轴子”，与SM场的耦合非常受抑制（衍生）。然而，遗留轴子场的集体振荡与非相对论粒子具有相同的状态方程，因此遗留轴子可以解释10^{10-11} GeV附近PQ尺度的宇宙DM。这种轴子的宇宙学演化是有趣的，因为一系列的对称性破缺会产生潜在的稳定拓扑缺陷。在Peccei-Quinn标度f_PQ以下，真空歧管不是单连通的。这意味着物理空间中存在闭合路径，这些路径映射到场空间中绕原点的非平凡路径上。这样的场结构对应于宇宙弦。当温度降到A-QCD ~ 300 MeV时，非微扰QCD效应产生了一个质量，破坏了对称性Z（N），其中N是U（1）-PQ下带电夸克的个数。Z（N）的真空流形是不连通的，这意味着存在物理空间中的路径，这些路径映射到场空间中两个真空态之间的路径。这样的路径必然离开真空流形，所产生的结构是一个畴壁。拓扑上，一旦轴子得到质量，每个弦必须由N个畴壁连接。由于畴壁的表面能，具有N= 1的弦和畴壁的网络将是不稳定的和崩溃的。然而，N >1的网络在原则上是稳定的，如果它们在形成后控制了宇宙的能量密度，就可能导致宇宙学灾难。当N >1时，避免“畴壁问题”的标准论点是调用不可重正化的普朗克尺度抑制算子，反映量子引力效应，显着打破初始U（1）PQ并提升真空态的简并性，导致有效压力项，导致真真空的畴增长。然而，U（1）PQ的这种显式破缺在实验上受到限制，因为它重新引入了CP破坏，而CP破坏是中子电磁偶极矩（EDM）的实验上限所要求的，可以忽略不计。通过要求轴子畴壁及时消失以避免宇宙学灾难，人们可以由此推断出中子EDM的下限。因此，畴壁问题的“倾斜”解决方案可能通过未来的中子EDM实验进行测试。我们提出了一种替代机制，使畴壁不稳定，而不是引入统计的“偏见”的人口的真空状态。这种偏差是由膨胀过程中轴子场的演化自然产生的，假设膨胀持续足够长的时间，使宇宙在膨胀过程中冷却到A_QCD以下，因此轴子获得质量。这种可能性最近被利用，以开辟以前排除的轴子参数空间。已经证明，这种偏差导致Z（2）模型的同向移动畴壁能量密度的指数衰减。这对构成暗物质的轴子最可能的质量的影响将被研究，因为这与正在进行的轴子搜索特别相关，包括在英国的“隐藏部门的量子传感器”计划中。