Monday, 22 January 2018

Hello, World

Well, here goes. I've wanted to start a blog for a long time and late January of a new year is as good a time as any to finally do it. What do I intend to write about?

  • Why theoretical high energy physics is exciting. I've been increasingly upset by the negative effect that a few presumably well-meaning bloggers have had on the HEP community, and want to help show that we're not all suffering from groupthink and wasting taxpayers' money going up blind alleys.
  • Why doing physics in France is great!
  • Life as a British scientist living in France as Brexit unfolds. 
Hopefully I'll find time to write things of interest to both my fellow physicists and anyone else. 

So, to start with, what's the title of the blog about? The self-energy of a field in quantum field theory denotes the quantum corrections to the propagation of a particle. There is a nice technical description on wikipedia but it's a complex quantity (which may be a matrix ...) describing each particle's self-interactions as it moves about. Remember the cartoons about the Higgs boson bumping into other particles to give them mass? Well, instead now imagine a distressed parent walking through a ball-pit, where the more the pit is filled and the larger the person the higher their mass, because it's harder to walk. This is like a particle moving through the Higgs field, which we call the "tree level" mass. Now imagine that children (and other parents) keep jumping in and out of the pit. They bump into our hero and then wade off elsewhere, hopefully giggling. These interactions are like the self-energy, and they make it harder for everyone to walk, but usually not as much as the plastic balls. Of course, we can imagine situations where the self-energy is more important than the tree-level effect too, and indeed some models have the tree-level mass as zero. The relative importance depends also on the strength of the interactions, so some types of particles contribute more than others; for instance, the top quark gives a large contribution to the Higgs self-energy.

So that's a basic idea. But the self-energy contains both a real and an imaginary part, and the imaginary part tells us about the decays of the particle. So I don't want to be negative and blog about decay, but rather the real part, which tells us about the particle's mass. And it so happens that I do a lot of work calculating the Higgs mass ... 


  1. Well done, I look forward to reading your blog regularly, thank you. Hopefully the Seine will recede soon, and the RER C will be back to normal.