Okay, so okay, very good. I have to click on continue. So I take over at that point. It's

a pleasure to welcome Felix Kahlhüfer for today's colloquium. Let me maybe shortly

introduce him. He has studied physics in Cambridge and Heidelberg and then has done his PhD on

the phenomenology of dark matter in Oxford in the group of Subhiyazad Khar. Felix, you

then went to DASY as a theory fellow and subsequently acquired your Emmy Noota Young

Researches group, which is located in Aachen, where you are now also a junior professor,

if I understand that correctly. Your field of research is anything that has to do with

dark matter particles and cosmology in the interaction between theory and experiment.

And that is also the field about which you will tell us today how to analyze hints for

dark matter. And I think I don't go any further into this topic because you will introduce

it anyway. So please go ahead.

Great. Hello, everyone. Before I start, I just posted a link to my slides in the chat

window in case you want to go through the slides at your own speed. And then, of course,

thanks very much for the introduction. I'm very happy to give this talk about a topic

that I've been thinking about and working on for quite a few years now. Of course, it's

a pity that I can only give this talk via Zoom. But I very much hope that we nevertheless

manage to have a bit of discussion. So also feel free to interrupt if at any point something

is unclear and otherwise we can talk more at the end.

So the outline of what I want to talk about today is fairly broad. I want to start with

a general introduction on why I think dark matter is an interesting research topic. So

this starts with the evidence that we have for the existence of dark matter. So why this

is a problem that requires solving. And then I will switch to the particle physics perspective.

I'm a particle physicist myself. So I like to think about the signatures of elementary

particles and how we can search for them in experiments. The specific focus that I've

picked for today is searches for axions, a specific class of dark matter or hypothetical

dark matter particles. And I will link this to a recent experimental signal, both because

this signal is interesting and as a kind of illustration of the more general point of

how we can approach this kind of journey towards the eventual discovery of a dark matter particle.

I will then switch gears a bit and talk more broadly about how we can analyze such datasets.

There will be a couple of slides on statistics, in particular on Bayesian statistics, which

is a framework that is maybe not so common in the toolbox of particle physicists, but

in my opinion offers some interesting new perspectives. And then I will talk a bit more

about the kind of technical aspects and in particular the work of the Gambit community

of which I am a member. And then coming back to kind of the beginning, I will talk specifically

about the xenon-one tonic sets. I will give an example for a few models that can potentially

explain the success and how we should think about this in the comparison of different

kinds of datasets in a global way. So let's get started with the first part, which is

about dark matter very broadly. And when we talk about dark matter, what we essentially

mean is missing matter. So matter that we cannot account for through observations of

visible matter. So if we want to look for this kind of matter, the essential strategy

is very simple. Basically what we do is we pick an astrophysical system and we map out

the distribution of visible matter in that system. Once we have a kind of survey of the

matter that we expect, we can calculate the gravitational potential corresponding to this

matter and then we can calculate the motion of objects in this potential. And this allows

us to make a prediction for how the system should evolve and we can compare that to observations.

So this strategy is quite old. It was first implemented by Fritz Zwicky in the 1930s,

who applied it using the virial theory. So basically comparing potential and kinetic

energy in a galaxy cluster called the Coma cluster. And what he concluded, let me just

quote from his original paper, is that the kind of the observations don't fit. So to

have the required velocity dispersion, you need 400 times more matter than what you get