Magnetic field phenomenology: from the Universe to laboratory experiments

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


Magnetic fields are an ubiquitous feature of astrophysical and laboratory plasmas, as revealed by diffuse radio-synchrotron emission and Faraday rotation observations. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. How such fields are created and amplified remains a mystery. It is believed that turbulent dynamo action can efficiently amplify magnetic fields and this is supported by novel laboratory experiments that we have performed in the past few years. Thus the magnetic fields observed in astronomical bodies today could plausibly have arisen from tiny initial seeds. Still, the origin of such seeds remains unclear - and a variety of plasma processes have been proposed, Moreover, such amplification process cannot occur in cosmic voids, where the plasma density is too small for dynamo to become operative.
These considerations are part of a more fundamental question of how the energy injected at large scales into a compressible plasma is partitioned, in a turbulent cascade, between small-scale motions, magnetic and compressive fluctuations, and dissipated via cosmic rays. Indeed, the presence of energetic particles in the Universe is a well established fact. The exact mechanism that leads to such high energy particles remains controversial. Although many different processes may result in cosmic ray acceleration, the current understanding is that turbulence and magnetic fields play an essential role in energizing both the electrons and ions present in the interstellar medium.
We plan to tackle this problem using a multi-strategy approach that focuses on:
1. Developing novel theoretical models for magnetic field generation in the Universe that applies plasma-physics processes (such as returns currents, baroclinic effects and/or turbulent dynamo) and go beyond them (for example, using non-standard model physics). We will investigate processes that occurs in the early Universe as well those that occurs in supernova remnants.
2. Embed these models into simulation codes - particularly particle-in cell (OSIRIS and/or EPOCH) and predict measurable quantities that can be tested against observations and laboratory experiment. Examples are the spectrum of the magnetic field, or secondary processes such as photon (x-ray) production.
3. Understand the interplay between magnetic field generation, amplification and particle acceleration - for example second order Fermi acceleration - and develop phenomenological models that can be validated with experiments on high-power laser facilities.
This project fits "Plasmas and lasers"


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publication icon
King B (2019) Axion-like-particle decay in strong electromagnetic backgrounds in Journal of High Energy Physics

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1947611 Studentship EP/N509711/1 01/10/2017 31/03/2021 Konstantin Beyer
Description The prospect to investigate stochastic particle acceleration at high power laser facilities has been investigated. This would be an interesting, controlled laboratory test of a process possibly responsible for accelerating particles hitting the earth as cosmic rays. It was concluded that the technology for performing such an experiment is already operative and an attempt at performing such a measurement has been done by other researchers in the group. The stochastic acceleration mechanism intimately relies on the existence of turbulent magnetic fields.

Similar, high power lasers can be used to search for specific, light dark matter particles, axions or axion like particles (ALPs). Those are specific, well motivated candidate particles and extensively searched for at multiple experiments. We have performed theoretical studies to design a purely laboratory search for such particles which does not rely on a specific model of ALPs (they can have different properties like mass, interactions with different particles and so on). This is an important endeavour as little is known about dark matter and thus comparison between different experiments making individual assumptions is challenging.
Typically the search method depends on the assumed mass of the new particle. We have found an experimental design utilizing lasers which places stronger bounds than purely laboratory based searches in the specific mass range of operation.

In collaboration with Dr Ben King and Dr Barry Dillon we have investigated the behaviour of ALPs in strong magnetic fields, specifically the decay of them into electron positron pairs. This addresses the challenge of measuring the presence of ALPs. Since by virtue of being a dark matter candidate and having escaped detection to date, their cross-sections are very small. It is thus necessary to convert ALPs into Standard Model particles which can easily be detected. In this paper we have chosen to consider another common coupling, the ALP-electron/positron interaction. We have also addressed the effect which is made by the edge of a magnetic field. This is a technical point which accounts for the complications of dealing with large gradients.

In collaboration with Prof Subir Sarkar we are looking at cosmological implications of ALPs. It turns out that these have fascinating cosmological history, which might tell us more about their properties by putting constraints due to their presence at well understood and thoroughly tested cosmological eras. Depending on the specific properties of the ALPs there exist additional cosmological complications and we are investigating a possible solution.
Exploitation Route The experimental designs can be taken forward in a straightforward manner by performing the proposed experiments. The theoretical studies we have conducted can then be utilized to analyse the findings. Generally the further path will depend on the outcome of the experiment, however negative results like the absence of an ALP signal will still allow us to narrow down the search space.
The general Phenomenological investigations of ALP cosmology will be helpful in identifying ALP parameters of interest (e.g. where a single type of ALP would explain the entire dark matter abundance). Additionally we will have additional consistency checks which might rule out parameters.
Sectors Other

Description Axion OSIRIS 
Organisation Technical University of Lisbon
Country Portugal 
Sector Academic/University 
PI Contribution I came to give insight into Axions.
Collaborator Contribution I was given access to OSIRIS and we jointly included an Axion module into the code.
Impact No outcomes yet.
Start Year 2018
Description Strong Field Axion 
Organisation University of Plymouth
Country United Kingdom 
Sector Academic/University 
PI Contribution We have jointly worked on different aspects of strong fields and their application to Axion searches/physics.
Collaborator Contribution I have been given training and insight into strong field physics. We have then jointly worked on different aspects of strong fields and their application to Axion searches/physics.
Impact 10.1007/JHEP12(2019)162
Start Year 2018