Today until the end of the week we're hosting the KUTS 8 workshop here in Paris. It's a small workshop gathering about 20 people to discuss Higgs mass calculations. It's supposed to be entirely focussed on the Minimal Supersymmetric Standard Model (MSSM) and the Next-to-Minimal variant (NMSSM) but in recent times my collaborators and I have been trying to stretch the scope to talk about calculations in general models.

This work is rather technical, involving two- and three-loop calculations, effective field theory techniques, etc, but is important for bridging the gap between top-down theories and the extremely precisely measured value of the Higgs mass, which is known experimentally to within 0.2%. This is much better than the theoretical uncertainty in the above theories! I.e. if I define a theory such as the MSSM and give the masses of the new particles then the state of the art is that we can only calculate the mass of the Higgs to within maybe 1-2%, although that number depends on the parameter choice and is also something that will be (hotly) debated at the meeting ...

I really like small workshops like this one because it's all relevant and no extraneous or less interesting stuff. I get to meet friends and collaborators and discuss the very latest results -- and find out what is about to appear, what they're working on next, and what is important to think about. This is quite an unusual event, since most workshops are a little bigger with more like 30-50 people, and conferences would be upwards of 100; those can be very interesting too but take more time and are more diverse, so generally better for getting the big picture on a whole discipline rather than making progress in one area.

Fortunately, my lab is located in the centre of Paris, so it hasn't been too hard to persuade people to make the trip here. It's only a shame that today is the day that the Seine has risen high enough to disrupt some of the transport ...

Some time I will hopefully write more about the physics behind Higgs mass calculations, but for now here's a link to the webpage where it's also possible to listen in to our videoconference broadcast ...

## Wednesday, 24 January 2018

## 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 ...

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