Welcome, everyone. It's a great pleasure to see so many here.
My name is Felix Khalifa and I'm going to single out one of the many aspects that Sebas just told us about, which is the hunt for dark matter.
And that is why I chose this background picture to remind you of the pie charts that CBS showed earlier,
which represents our current understanding of the energy density, the energy content of the universe.
And really this understanding is that everything we are made of all atoms or gas,
planets and stars, that just constitutes 5% of the universe and a much larger fraction,
something around 27% is in an entirely different form of matter matter that doesn't emit light,
that doesn't absorb light, and that we therefore call dark matter.
And both of these metal components drift in a sea of dark energy with negative pressure, which causes the accelerated expansion of the universe.
So I think the point that should be made clear is that everything we know about the universe and we've learned from observing radiation,
radiation emitted from astrophysical or cosmological objects, anything from light to X-rays to cosmic rays.
So if it no turns out that the largest part of the universe is dark so that all of these detection techniques fail,
but that that really is a worrying perspective.
So how can we possibly understand anything about the universe if it turns out to be invisible to the to the instruments that we have?
And the point of my talk is to convince you that the situation is not quite that bad.
So the first part of my talk will be to to present evidence for why we actually think that dark matter is that.
And then the second to the second part will be to try and describe various experiments and various models and
theories that we have to make sense of this evidence to actually interpret dark matter and pin down its properties.
So the evidence for dark matter really comes from the gravitational effects.
It comes from gravitational effects that we see that, but that we cannot explain just from the visible objects that we have.
And one of the simplest examples for that is when we measure rotation.
So we measure the velocities of stars that orbit the centre of a galaxy.
And as you know, when you have something orbiting on a circle, the faster the object goes.
The larger force is required to keep that object on track.
So if we measure the velocity at a certain distance, we therefore know the gravitational force.
We know the amount of mass required within that radius to keep the object on track.
And we can therefore make these measurements at different distances from the centre of the galaxy and compare the observed,
the inferred mass distribution to what we observe, what we observe, invisible matter.
And it turns out that we have a spectacular mismatch between the prediction and the observation.
That's what's shown on the right hand side here. This is one of these rotation curves.
We have velocity as a function of distance. The dashed line.
Here is the prediction for what this velocity should look like just from the matter in the disk.
So that's essentially the stars.
And and there's another contribution here from gas, but that is nearly enough to bring the velocities up to where we actually see it.
So what we need to what is missing is a mass component in a galaxy which is three dimensional,
which lives in a spherical halo rather than just the disk,
and which becomes dominant at large distances, and therefore supports this constant rotation curve.
And this is really the first evidence that there is something missing in in what we see.
Now, if we want to understand if this kind of argument is a good argument to make, if it is actually a good idea to to infer missing mass,
in that way, we can go back in history and see if this argument has worked before.
And in fact, it has.
So a very famous example is the discovery of the planet Neptune that was predicted essentially on a mathematical basis by John Adams,
because the orbit of Uranus didn't match with the prediction of Newtonian gravity.
It was a slight deviation. And this deviation, he figured out, pointed towards a missing mass in the Milky Way.
And indeed the planet was discovered precisely at the position where he predicted that it should be.
However, there are also counter examples in history,
and most famously a similar discrepancy was observed in Mercury, the anomalous precession of Mercury.
And when the VVA postulated that therefore there should be a new planet which he named Vulcan even closer to the sun.
Now you all know this pleasant planet doesn't exist.
And in fact, just a few years later,
it became clear that the reason for the anomalous position was that we can't use Newtonian gravity and strong gravitational fields.
Instead, we need a more complicated theory of gravity, which is Einstein's theory of general relativity.
And that perfectly explains the anomalous precession. So we should be worried that exactly the same thing is happening again here, that in fact,
the galactic rotation curves that we observe are explained by a deviation from Newtonian gravity.
Not in strong fields, but in very weak fields.
And such a theory exists. It's called modified Newtonian dynamics, and it perfectly explains the galactic rotation curves.
So if we want to be sure about dark matter, we better get more evidence and we better get evidence at different scales.
And one of the other things that we can do is actually exploit the predictions of general relativity.
And one of the central predictions is that matter bends space time.
It bends spacetime in such a way that photons no longer travel in straight lines.
Photons travel on curves. And therefore, if we have a mass distribution between us and an object that we see on the sky,
this object will be distorted just as it would be by by an optical lens, which is why this effect is called gravitational lensing.
And this allows us to infer mass distributions that we cannot directly see, provided we have some object behind it that we can see.
And and one spectacular application of this approach is shown here.
This is the so-called bullet cluster, which is a collision of two galaxy clusters.
And what you see in colour here in red and yellow, this is the X-ray emitting gas.
So in the cluster collision, the gas heated up and we can now see it in X-rays.
But the green circles here, this is what we get from gravitational lensing.
This is where we see the mass of the system to be. And you see very clearly that there's a mismatch.
And this gas, I should say, this is believed to be the dominant contribution to the visible mass.
So there must be invisible mass, which leads to this offset.
Here again, we have at least a piece in the puzzle pointing towards dark matter.
And it turns out that the more pieces we gather, the more consistent the picture actually becomes.
Another very famous piece of evidence we get from large scale structure formation.
So this is our attempt at understanding how structures actually formed in the universe,
why there are so many structures starting from stars to galaxies to galaxy clusters, and how that came into being.
And if we start so slow to do that, we have to start from a primordial density perturbations.
So we have to figure out how the first things clumped together in the early universe.
And it turns out that if we start just with the particles, we know, for example, protons and electrons,
they can't do that because they scatter too frequently and they get heated up whenever they interact with photons,
and therefore any structures that they would form would immediately get erased again.
So. So what we need to form structures sufficiently early is some kind of matter which doesn't have these interactions,
which doesn't scatter so much, which doesn't interact with photons and which can fall,
which can clump together and which can create potential wealth, which then serve for all the visible matter to fall into and form stars and galaxies.
And really what we need is, is a different form of mass, which is called which means non relativistic and very weakly interacting.
So summarising everything that I've said, the conclusion really is that we only understand about 20% of of the matter in the universe.
Probably less than that. And and the only thing that we can say with certainty about dark matter is
that it must be fundamentally different from the matter that we are made of,
which has exactly these problems that it interacts with photons, heats up, scatters and so on.
And since the interactions of dark matter must be so weak, our belief is that it should be a completely new, a yet undiscovered particle.
This is of course, this is an enormous hypothesis. Just from astrophysical observations, we are essentially now postulating a new particle.
But actually there are very good motivations to do that.
And and the motivations are that that series which have been introduced,
which have been proposed for completely different reasons, predict these kinds of particles.
So so this goes back to what's to be mentioned about extensions for the standard model, for example, to address the hierarchy problem.
And when you do this, you typically obtain predictions for new stable particles.
The most discussed example is the so-called weakly interacting, massive particle abbreviated to wimp,
which comes out as a generic prediction, for example, from supersymmetry or from from theories with extra dimensions.
And the amazing thing is that these theories don't only predict the particle to have the right properties,
but they even predict how these particles came into being in the early universe.
And the idea is that that the early universe, when it was still hot and dense,
actually was hot enough to keep these dark matter particles in thermal equilibrium with all other particles.
And only when the universe expanded and cooled down and the dark matter decoupled from the thermal path.
And this prediction allows us to calculate the amount of dark matter that we expect to see in the present universe.
And that prediction agrees with what we see. This goes by the name of the wimp miracle.
So. So these theories allow us to understand why there should be dark matter and also why there should be so much of it.
I should mention just in passing, that this is by far not the only proposal that we have.
There are interesting alternative theories for what dark matter could be.
One idea is axioms which would not be produced from the firm Abbas, but but in fact, non-family.
There could be a new kind of neutrinos, which would not be called dark matter, but warm dark matter.
Or there could be dark matter with new strong interactions.
So dark matter, which actually is relatively similar to two protons.
And in that case, we would try to or we would imagine that dark matter carries the same matter anti matter asymmetry,
which we just talked about a few minutes ago. So. So if dark matter is she has this property with baryons but it does not interact with light,
then that could very well account for the dark matter we have in the universe.
So we are left with these ideas. We are left with the evidence for gravitational interactions,
but really a lack of information about the details about the particle nature of dark matter.
So therefore, the next part of my talk will be how to make this darkness visible.
How to how to set up and perform experiments that can tell us the details,
that can give us information such as the mass, the interactions, the spin of the dark matter particle.
And these experiments are based essentially on the idea that I mentioned,
which is that at some level, dark matter particles should interact with standard model particles.
If we want to understand how they came into being in the early universe and we represent
them as this kind of diagram where the details here are deliberately left vague.
So the only thing that matters is that we have two dark matter particles on the
one hand side of the diagram and two standard model fermions on the other side.
And there are three different ways to think about this diagram,
and they directly correspond to the three different techniques that we have to search for dark matter.
If I read the diagram from left to right, that is too dark matter particles colliding and going into two standard model fermions.
That is dark matter and the relation.
If I read the diagram from top to bottom, that's a dark matter particle colliding with the standard model particle and both of them coming out again.
So that represents dark matter scattering. And finally, if I reverse the directions, if I start from two standard model particles,
and if I bring them to collision, they might actually be able to lead to dark matter production.
So going through these techniques one by one. Let's start with direct detection of dark matter.
So direct detection wants to exploit the scattering of dark matter particles and standard model particles.
And the idea is that if the Milky Way indeed has a dark matter halo,
then dark matter particles will be constantly passing through through the earth, through everything.
For example, the rough numbers are that ten to the seven of these dark matter particles pass through every one of you in every single second.
Of course, we don't feel any of that because they have a tiny, tiny, tiny chance of interacting.
But this interaction rate should be non-zero.
So occasionally there will be scattering between dark matter particles and nuclei.
And if such a scattering process actually happens in a controlled environment, in a dedicated detector with very low backgrounds,
then we can convert the recoil energy transferred to the nucleus into visible signals.
So that's sketched here on the right hand side.
And, and the recoil energy, for example, can go into scintillation light, can go into ionised electrons or simply into heat into phonons.
And that will then be visible. So then we can see essentially the interaction of a dark matter particle.
But. The point I want to make here is that this is really an enormous a very,
very brave experimental enterprise, because when you actually do the calculations,
when you write down the event rate that you expect to see,
it turns out that the event rates that you expect to something like one event per kilogram detect a material per year.
So if you if you set up a ten kilogram detector, it will see ten dark matter particles in the year.
And to make things even worse, it turns out that these dark matter particles are not very fast because their velocity must be
smaller than the galactic escape velocity if they are to be bound in the in the Milky Way halo.
And that means that the typical energy transfer in a collision will be maybe ten.
K.V. And now compare that to a radioactive decay.
Radioactive decay can easily give you tens or hundreds of of that amount of energy and radioactive decay happen all the time,
ten to the four times per second in our bodies. So we are trying to see something which happens a few times a year and actually is
not very impressive compared to things that happens thousands of times per second.
So so this this really is a challenge.
And if we want to make any progress with this challenge, the first thing we have to do is we have to go deep underground.
So these experiments are somewhere in old mines or in tunnels under mountain ranges just to be shielded from cosmic rays.
And then you need to to select materials which have very low radioactivity just to get
all the backgrounds down to a level where you could hope to see dark matter scattering.
And this indeed is possible. And it has been. It has been attempted by quite a few number of experiments.
And they are really getting to these levels of sensitivity that we need.
And in fact, some of these experiments, for example, the Cress two experiment,
which is very closely linked to Oxford, already claims to see something which which they don't think is background.
Nevertheless, there are other experiments, for example, Xenon 100 here,
which says that the signal is in agreement with the number of background events they would expect from radioactivity.
So the experimental situation here is not quite resolved yet,
and I will get back later to to the kinds of questions that that these experiments lead us to.
For now, moving on to the different techniques that we have.
The next one is indirect detection. And indirect means we do not see dark matter directly, but we see the products of its end relation.
So so what we do is we set up satellites or balloons as in the old days or in fact very large ground based telescopes.
And we look for things that might be produced when dark matter particles, any light, for example, gamma rays, antiprotons, positrons or neutrinos.
And the name of the game is to to know where to look.
So try to understand where large dark matter densities are, and also to know where not to look.
So to understand where you would suffer large astrophysical backgrounds.
Just to give you one example, these are gamma ray searches.
Searches for very highly energetic photons.
And this is done, for example, by the Fermi Space Telescope.
And it turns out that a particularly exciting place to look with this telescope is the Galactic Centre,
because the Galactic Centre has the highest dark matter density around us.
And the problem is that we really need to understand the Milky Way in a lot of detail.
So we really have to figure out its dynamics and how it came into being.
To understand the astrophysical backgrounds and to understand if, for example,
a bump like this one here is due to dark matter or it's just a background that we haven't understood yet.
So maybe the cleanest thing that we can do is actually to try to make our own dark matter in the laboratory.
And this is exactly what the LHC aims to do.
So the LHC collides protons at very high energies in order to reproduce the conditions very early after the Big Bang.
And we can therefore hope that we produce dark matter in exactly the same way that it was produced in the early universe.
But as you already know, dark matter is so weakly interacting that if we make it in a collider,
it would just escape and we are not going to see any trace of it.
So the trick is we don't only want to produce dark matter particles, but we also want to produce some visible particles.
And if those two are produced at the same time, it will look as if momentum conservation is violated in the process.
A very simple example for that are so-called monitored searches.
This is the atlas detector at the LHC. And this is one so-called mono jet event where you have a single jet of particles to this many corks,
gluons and things like that, going out in one way.
So this this plane is perpendicular to the line of the beam.
So you have something coming out in the perpendicular direction and you have nothing coming out on the other end.
So so this observation immediately tells us that there must be an invisible particle, must be missing momentum going that way.
Unfortunately, we already know a particle which gives exactly the signature.
That's neutrinos, because neutrinos also don't leave a trace in detectors.
And therefore, we expect to see some of these events from neutrinos already.
So again, the point where theorists get involved is to calculate very precise predictions of what we would expect
to see from neutrinos so that experimentalists can then compare what they see and search for in excess.
The the summary for for all of what I've just said really is that that we have a lot of complementary,
a lot of different techniques, a lot of different angles from which we can attack the problem.
But all of these have in common the problem that we fight with backgrounds.
We are looking for something which is almost completely invisible, which has very weak interactions.
And and therefore, we need to understand anything that could possibly mimic such a signal,
be it radioactivity, be it astrophysical sources or just simply neutrinos.
And in fact, one of the most interesting aspects of this hunt is to come up with new signatures,
come up with signatures that would suffer less if these backgrounds that would
give a more unambiguous signal than than the simplest searches that you could do.
I would like to give you one particular example, which I have been working on a lot,
which is the idea of an annual modulation in the direct detection signal.
So the idea is the following. We have the sun going at a velocity of about 220 kilometres per second around the Galactic Centre,
and on top of that you have the rotation of the earth around the sun.
And it turns out that the inclination angle is such that in June the velocity of earth and sun are partially aligned,
whereas in December the velocities of the two are partially anti aligned.
So what that means is that in summer the earth will go through the Milky Way at a higher velocity than in winter.
And that means that in summer we are going to encounter more dark matter particles and
we are going to encounter dark matter particles at higher velocities than in winter.
So therefore, we would expect to see more signal in summer than in winter.
And there are indeed experiments which look for these kinds of modulations experiments which attempt to be have
extremely stable running conditions over several years so that they can pick up this modulation of the event rate.
And there are two experiments which have been doing that called dama and Cogent.
And both of these experiments see, at least to some degree, the significance is still debateable.
But there seems to be some evidence in both of these for a modulation of the signal interface that agrees with the prediction.
Now, the problem with this observation is, of course, that we need to be able to interpret it.
In particular, we need to make a robust prediction for what the modulation fraction should be.
A modulation fraction is the ratio of the modulation amplitude to the total signal strength.
And it turns out, first of all, this is to be expected that the modulation fraction depends on the velocity of the dark matter particles.
So that's depicted here where you have velocity here and modulation fraction on the Y axis.
But what is worse is that the prediction also depends on how exactly we model the Milky Way,
how exactly we try to describe the mass distribution of the matter in the Milky Way.
And this is, of course, worrying, because this limits the amount of information that we can get from the experimental data.
Therefore, one of the things that we have been involved in very actively over the past few years is to develop methods to avoid these uncertainties,
to be able to interpret direct detection experiments independently of the assumed properties of the dark matter halo.
And and the method just very roughly is to map different experiments into a common parameter space
where you do not need to make these assumptions in order to interpret the experimental data.
And the conclusions that we reach is, in fact, that these two experiments, dama and cogent, are in agreement with each other,
but are actually in tension with other experiments, for example, seen on hundreds, which I mentioned earlier.
We can, in fact, go even further.
We can not only compare different direct detection experiments with each other without making too strong assumptions,
but we can even compare completely different search strategies.
And this again exploits a technique that Subbiah introduced earlier, which is the idea of effective field theories.
So let me give you just one example. If we want to study dipole dipole interactions between dark matter and standard model fermions,
that means we want to consider an effective operator which looks like this.
So this covers the dark matter dipole moment to essentially a a nucleus dipole moment.
And because this is a dimension six operator,
this has to come with a suppression mass scale of one over M stars with where M star is this scale where we expect new physics to become relevant.
And this approach allows us to immediately calculate the different experimental signatures, calculate the the scattering cross-section,
the energy lation cross-section, and so on, just in terms of the dark matter mass and the scale of new physics.
So with without assuming any details of what exactly creates this interaction, what the mediator looks like and so on,
and these are for this particular operator,
we obtained these predictions and we can then use that to interpret completely independent sets of experiments.
These are some of the results that we have obtained.
So on the left hand side here, this is now comparing these two experiments cogent in Dama and Xenon 100 with balance that we obtained from the LHC.
So so a completely different search. This is this is in fact, from the monitored search that I mentioned earlier,
the way to present these results conventionally is to show the dark matter mass on the x axis
and on the right hand side and the left hand side to show the scattering cross-section.
So this means, of course, that everything towards larger cross-sections, so everything above these lines is experimentally excluded.
A different way to to show the same thing is to plot everything in terms of this new physics scale.
So now it's the other way around. Now everything below the line, everything towards smaller scales is excluded.
And t on the right hand side, we, we not only show the bone from the LHC and from Xenon 100,
but now here this also two lines from from searches done with the Fermi satellite.
So you can see how all these different pieces come together.
And the most the most interesting the most exciting thing is that you can actually see that they are complementary.
So. So depending on what exactly your your motive for dark matter is, different searches will give you the strongest sensitivity.
So so the point I want to make here is that really what we need is a lot of different ways to attack this problem.
We really ideally want to be able to combine all of these different bits of information that we get from different
searches in order to then cover essentially the entire range of possibilities as completely as possible.
Now, of course, ideally what we hope to find is, in the end, a consistent picture for all of the different experiments.
But. What happens more often is that actually we see tension.
We see that the different experiments don't give a consistent picture.
And what that means is that we actually have to go back and review all the assumptions that we have made along the way.
And and this is this is really a very important task for us theorists to make sure that actually the theory assumptions that
we make are not overly strong or overly constraining or too optimistic and questions that we have been working on this.
For example, how would experimental signatures change if dark matter is not a wimp?
And what at times what types of experiments would be needed in that case to cover alternative scenarios?
Just put one plot here for one very particular example,
where the assumption that we relax is the assumption about how dark matter couples to protons and neutrons.
So so typically and the the usual assumption is that because dark matter doesn't have electromagnetic interactions,
it should be blind to the charge of the proton and therefore it should see protons and neutrons in exactly the same way.
And this is what everyone normally uses to interpret searches.
But of course we can very easily relax those assumptions and see how the implications of different
experiments would change if we go to two different ratios of coupling of neutron and proton.
So this brings me to my conclusions. I hope to have convinced you that we do have a very good case for dark matter,
that we have very strong and very convincing evidence for the statement that I made in
the beginning that we have so much more dark matter in the universe than visible matter.
And in fact I hope I have made clear that would be need is really something
completely new something completely different from what we what we already have.
And this points towards a new elementary particle, which remains to be discovered.
We have good models that predict the properties that this particle should have, but this is still a hypothesis.
And we can test this hypothesis by by searching, for example, in direct detection experiments,
indirect detection experiments, or to actually produce dark matter in colliders.
And the name of the game really is to reduce experimental backgrounds,
in particular to identify clean signals such as the annual modulation of the direct detection rate.
There have already been some claims in recent years.
There have also been counter claims. So the experimental situation at the moment is not clear.
But the good news is that there are a lot more experiments coming.
So the next few years will see an enormous increase of the number of experiments and the sensitivity of these experiments.
So we have very good hopes that the next few years will be exciting.
Thank you very much.