CORDIS - EU research results

The Electroweak Phase Transition Beyond the Standard Model

Final Report Summary - EWPTBSM (The Electroweak Phase Transition Beyond the Standard Model)

The Electroweak Phase Transition Beyond the Standard Model

During this two-year Marie Curie intra-European fellowship, I have been exploring the physics of the universe around the time the Higgs boson got its mass. In the Standard Model of particle physics, this is a fairly gentle process, termed a crossover. However, we also know that the Standard Model is incomplete – it does not explain, for example, the matter-antimatter asymmetry we observe. It is reasonable to consider extensions of the Standard Model and their observational consequences.

One particularly exciting observational consequence is the production of gravitational waves. These "ripples in spacetime" were finally directly detected in autumn 2015, with the results announced in early 2016 – although rumours surfaced almost immediately. This has substantially raised interest in possible cosmological sources of gravitational waves. One such source would be the violent collisions of bubbles and associated plasma shockwaves in extensions of the Standard Model, a so-called thermal first order phase transition.

Two key areas of research for this project were studying certain extensions of the Standard Model to determine their properties, and better understanding the observed gravitational waves from such a scenario.

Main results

In the first few months of the two-year fellowship, I applied for a PRACE (Partnership for Advanced Computing in Europe) Tier-0 computing grant as Principal Investigator. This application was successful and the additional computing resources – around 17 million core-hours – were very useful.

The PRACE computer time was principally used to study the expected gravitational wave signal resulting from the collisions of bubbles created at a thermal phase transition. The resources allocated allowed me to perform some of the largest simulations of the early universe ever attempted, in order to separate out as many different physical length scales in the system as possible. This was successful, and with my collaborators I was able to identify central features in the data which can be extrapolated to give generic predictions of the signal. These extrapolations will be further refined in future publications.

While this was ongoing, I also wanted to consider what gravitational waves would be seen in an alternative scenario, where the bubbles were not associated with substantial shells of plasma. This has been previously studied with the "envelope approximation" (which had earlier also been applied to thermal phase transitions, even though it neglects the thickness of the plasma shocks). I successfully compared the envelope approximation with a first-principles numerical simulation of colliding bubbles.

All my work in this area has been closely coordinated with the eLISA Cosmology Working Group, of which I am a member. This is a group of scientists working to refine cosmological predictions of gravitational waves from the future LISA space-based gravitational wave detector. Indeed, in 2015 during my fellowship I organised a workshop for this group, in Stavanger.

During the fellowship I also worked on simulating the underlying physics models, to make precision measurements of their properties. Focussing on one such model in particular, the so-called "real Higgs singlet model", I implemented a multicanonical algorithm for simulating the model – as well as independently reproducing for the first time the seminal work of the mid-1990s that showed that the Standard Model phase transition is a crossover, and therefore creates no bubbles.

The first, analytic stage – a so-called "dimensional reduction" of the model to make it easier to simulate – is now complete, and the simulations are also nearly finished.

Conclusions and their potential impact

The refined predictions of expected gravitational wave signals from first-order thermal phase transitions are the most important outcome of the project. The results were incorporated into a report of the eLISA Cosmology Working Group, essentially setting a benchmark prediction of the gravitational wave spectrum from nearly any phenomenological model of physics beyond the Standard Model. This has the potential for very considerable impact, for example on the LISA science case.

From the results of simulating the real Higgs singlet model, we can conclude that the range of parameter space permitting a thermal first-order phase transition is rather smaller than expected from perturbative calculations.

Impact beyond the scientific community

During the fellowship, I gave colloquia at the University of Stavanger to a broad audience; and helped to prepare and install an exhibition about particle physics and cosmology at the local science museum ("Vitenfabrikken"). Furthermore, I was jointly responsible for administration and technical development of our Facebook page, helping to substantially increase our reach to the local community and communicate our work.

Several press releases were written by communications staff about my research and experiences as a Marie Curie Fellow in Stavanger, some resulting in media attention. These also received attention within the university community, and helped to raise the profile of the Marie Curie Actions as a way of attracting talented staff to Stavanger from abroad.

I also contributed to a multimedia presentation developed by Kip Thorne, a physicist; Hans Zimmer, a film-score composer; and Paul Franklin, a special-effects expert. The three had worked together previously on, for example, 'Interstellar'. The aim was to create a presentation combining visualisations of physical processes, speech, and music; I was asked to contribute a short movie of one of my simulations (see attached image).

Caption for attached image: Still from an HD movie created for "Warped Side of the Universe", a multimedia presentation by Kip Thorne, Paul Franklin and Hans Zimmer. A 60 second movie was produced, showing the a slice through one of the simulations of a first-order thermal phase transition carried out for this project.

Conclusions for policymakers

The work carried out during this fellowship has demonstrated that the observability of gravitational waves from first-order phase transitions is perhaps greater than previously thought. This should help to bolster the science case for the ESA New Gravitational wave Observatory (NGO), for which LISA is the likely mission.