1 00:00:00,150 --> 00:00:02,640 Welcome, everyone. It's a great pleasure to see so many here. 2 00:00:02,880 --> 00:00:11,400 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. 3 00:00:11,880 --> 00:00:17,970 And that is why I chose this background picture to remind you of the pie charts that CBS showed earlier, 4 00:00:18,330 --> 00:00:24,030 which represents our current understanding of the energy density, the energy content of the universe. 5 00:00:24,870 --> 00:00:29,250 And really this understanding is that everything we are made of all atoms or gas, 6 00:00:29,340 --> 00:00:36,299 planets and stars, that just constitutes 5% of the universe and a much larger fraction, 7 00:00:36,300 --> 00:00:42,900 something around 27% is in an entirely different form of matter matter that doesn't emit light, 8 00:00:42,900 --> 00:00:46,080 that doesn't absorb light, and that we therefore call dark matter. 9 00:00:46,470 --> 00:00:54,450 And both of these metal components drift in a sea of dark energy with negative pressure, which causes the accelerated expansion of the universe. 10 00:00:56,090 --> 00:01:03,829 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, 11 00:01:03,830 --> 00:01:10,700 radiation emitted from astrophysical or cosmological objects, anything from light to X-rays to cosmic rays. 12 00:01:11,820 --> 00:01:18,710 So if it no turns out that the largest part of the universe is dark so that all of these detection techniques fail, 13 00:01:19,290 --> 00:01:21,810 but that that really is a worrying perspective. 14 00:01:21,820 --> 00:01:31,380 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? 15 00:01:32,070 --> 00:01:36,300 And the point of my talk is to convince you that the situation is not quite that bad. 16 00:01:36,310 --> 00:01:43,230 So the first part of my talk will be to to present evidence for why we actually think that dark matter is that. 17 00:01:43,860 --> 00:01:51,269 And then the second to the second part will be to try and describe various experiments and various models and 18 00:01:51,270 --> 00:01:57,990 theories that we have to make sense of this evidence to actually interpret dark matter and pin down its properties. 19 00:02:00,020 --> 00:02:05,569 So the evidence for dark matter really comes from the gravitational effects. 20 00:02:05,570 --> 00:02:13,190 It comes from gravitational effects that we see that, but that we cannot explain just from the visible objects that we have. 21 00:02:13,820 --> 00:02:18,680 And one of the simplest examples for that is when we measure rotation. 22 00:02:18,920 --> 00:02:23,030 So we measure the velocities of stars that orbit the centre of a galaxy. 23 00:02:23,570 --> 00:02:28,040 And as you know, when you have something orbiting on a circle, the faster the object goes. 24 00:02:28,040 --> 00:02:31,340 The larger force is required to keep that object on track. 25 00:02:31,790 --> 00:02:36,680 So if we measure the velocity at a certain distance, we therefore know the gravitational force. 26 00:02:36,890 --> 00:02:41,510 We know the amount of mass required within that radius to keep the object on track. 27 00:02:42,260 --> 00:02:48,740 And we can therefore make these measurements at different distances from the centre of the galaxy and compare the observed, 28 00:02:48,830 --> 00:02:53,870 the inferred mass distribution to what we observe, what we observe, invisible matter. 29 00:02:54,500 --> 00:02:59,690 And it turns out that we have a spectacular mismatch between the prediction and the observation. 30 00:03:00,170 --> 00:03:03,620 That's what's shown on the right hand side here. This is one of these rotation curves. 31 00:03:03,620 --> 00:03:07,699 We have velocity as a function of distance. The dashed line. 32 00:03:07,700 --> 00:03:12,650 Here is the prediction for what this velocity should look like just from the matter in the disk. 33 00:03:13,250 --> 00:03:15,040 So that's essentially the stars. 34 00:03:15,050 --> 00:03:22,610 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. 35 00:03:23,270 --> 00:03:28,219 So what we need to what is missing is a mass component in a galaxy which is three dimensional, 36 00:03:28,220 --> 00:03:31,370 which lives in a spherical halo rather than just the disk, 37 00:03:31,700 --> 00:03:37,670 and which becomes dominant at large distances, and therefore supports this constant rotation curve. 38 00:03:38,270 --> 00:03:43,790 And this is really the first evidence that there is something missing in in what we see. 39 00:03:46,360 --> 00:03:55,509 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, 40 00:03:55,510 --> 00:03:59,560 in that way, we can go back in history and see if this argument has worked before. 41 00:04:00,340 --> 00:04:01,480 And in fact, it has. 42 00:04:01,570 --> 00:04:09,100 So a very famous example is the discovery of the planet Neptune that was predicted essentially on a mathematical basis by John Adams, 43 00:04:09,460 --> 00:04:14,060 because the orbit of Uranus didn't match with the prediction of Newtonian gravity. 44 00:04:14,080 --> 00:04:21,070 It was a slight deviation. And this deviation, he figured out, pointed towards a missing mass in the Milky Way. 45 00:04:21,700 --> 00:04:27,220 And indeed the planet was discovered precisely at the position where he predicted that it should be. 46 00:04:28,470 --> 00:04:31,440 However, there are also counter examples in history, 47 00:04:31,440 --> 00:04:39,180 and most famously a similar discrepancy was observed in Mercury, the anomalous precession of Mercury. 48 00:04:39,630 --> 00:04:46,380 And when the VVA postulated that therefore there should be a new planet which he named Vulcan even closer to the sun. 49 00:04:47,130 --> 00:04:50,160 Now you all know this pleasant planet doesn't exist. 50 00:04:50,610 --> 00:04:52,319 And in fact, just a few years later, 51 00:04:52,320 --> 00:05:00,330 it became clear that the reason for the anomalous position was that we can't use Newtonian gravity and strong gravitational fields. 52 00:05:00,360 --> 00:05:06,179 Instead, we need a more complicated theory of gravity, which is Einstein's theory of general relativity. 53 00:05:06,180 --> 00:05:15,009 And that perfectly explains the anomalous precession. So we should be worried that exactly the same thing is happening again here, that in fact, 54 00:05:15,010 --> 00:05:20,860 the galactic rotation curves that we observe are explained by a deviation from Newtonian gravity. 55 00:05:21,220 --> 00:05:24,220 Not in strong fields, but in very weak fields. 56 00:05:24,520 --> 00:05:33,580 And such a theory exists. It's called modified Newtonian dynamics, and it perfectly explains the galactic rotation curves. 57 00:05:34,390 --> 00:05:39,940 So if we want to be sure about dark matter, we better get more evidence and we better get evidence at different scales. 58 00:05:41,020 --> 00:05:46,179 And one of the other things that we can do is actually exploit the predictions of general relativity. 59 00:05:46,180 --> 00:05:50,379 And one of the central predictions is that matter bends space time. 60 00:05:50,380 --> 00:05:54,940 It bends spacetime in such a way that photons no longer travel in straight lines. 61 00:05:55,330 --> 00:06:02,410 Photons travel on curves. And therefore, if we have a mass distribution between us and an object that we see on the sky, 62 00:06:02,770 --> 00:06:10,210 this object will be distorted just as it would be by by an optical lens, which is why this effect is called gravitational lensing. 63 00:06:10,870 --> 00:06:20,700 And this allows us to infer mass distributions that we cannot directly see, provided we have some object behind it that we can see. 64 00:06:21,580 --> 00:06:25,790 And and one spectacular application of this approach is shown here. 65 00:06:25,810 --> 00:06:29,980 This is the so-called bullet cluster, which is a collision of two galaxy clusters. 66 00:06:30,670 --> 00:06:35,500 And what you see in colour here in red and yellow, this is the X-ray emitting gas. 67 00:06:35,530 --> 00:06:40,060 So in the cluster collision, the gas heated up and we can now see it in X-rays. 68 00:06:40,870 --> 00:06:44,590 But the green circles here, this is what we get from gravitational lensing. 69 00:06:44,710 --> 00:06:49,990 This is where we see the mass of the system to be. And you see very clearly that there's a mismatch. 70 00:06:50,230 --> 00:06:55,090 And this gas, I should say, this is believed to be the dominant contribution to the visible mass. 71 00:06:55,480 --> 00:06:58,700 So there must be invisible mass, which leads to this offset. 72 00:06:58,720 --> 00:07:04,600 Here again, we have at least a piece in the puzzle pointing towards dark matter. 73 00:07:06,130 --> 00:07:10,870 And it turns out that the more pieces we gather, the more consistent the picture actually becomes. 74 00:07:11,380 --> 00:07:17,040 Another very famous piece of evidence we get from large scale structure formation. 75 00:07:17,050 --> 00:07:21,820 So this is our attempt at understanding how structures actually formed in the universe, 76 00:07:21,820 --> 00:07:29,050 why there are so many structures starting from stars to galaxies to galaxy clusters, and how that came into being. 77 00:07:29,260 --> 00:07:34,989 And if we start so slow to do that, we have to start from a primordial density perturbations. 78 00:07:34,990 --> 00:07:39,220 So we have to figure out how the first things clumped together in the early universe. 79 00:07:39,790 --> 00:07:44,530 And it turns out that if we start just with the particles, we know, for example, protons and electrons, 80 00:07:44,800 --> 00:07:51,040 they can't do that because they scatter too frequently and they get heated up whenever they interact with photons, 81 00:07:51,040 --> 00:07:55,660 and therefore any structures that they would form would immediately get erased again. 82 00:07:56,880 --> 00:08:03,600 So. So what we need to form structures sufficiently early is some kind of matter which doesn't have these interactions, 83 00:08:03,600 --> 00:08:08,850 which doesn't scatter so much, which doesn't interact with photons and which can fall, 84 00:08:08,970 --> 00:08:17,970 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. 85 00:08:18,990 --> 00:08:28,110 And really what we need is, is a different form of mass, which is called which means non relativistic and very weakly interacting. 86 00:08:30,060 --> 00:08:39,300 So summarising everything that I've said, the conclusion really is that we only understand about 20% of of the matter in the universe. 87 00:08:39,450 --> 00:08:45,329 Probably less than that. And and the only thing that we can say with certainty about dark matter is 88 00:08:45,330 --> 00:08:48,719 that it must be fundamentally different from the matter that we are made of, 89 00:08:48,720 --> 00:08:53,820 which has exactly these problems that it interacts with photons, heats up, scatters and so on. 90 00:08:55,240 --> 00:09:04,750 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. 91 00:09:06,200 --> 00:09:14,690 This is of course, this is an enormous hypothesis. Just from astrophysical observations, we are essentially now postulating a new particle. 92 00:09:15,470 --> 00:09:18,620 But actually there are very good motivations to do that. 93 00:09:18,830 --> 00:09:23,959 And and the motivations are that that series which have been introduced, 94 00:09:23,960 --> 00:09:28,430 which have been proposed for completely different reasons, predict these kinds of particles. 95 00:09:28,460 --> 00:09:36,620 So so this goes back to what's to be mentioned about extensions for the standard model, for example, to address the hierarchy problem. 96 00:09:37,160 --> 00:09:42,050 And when you do this, you typically obtain predictions for new stable particles. 97 00:09:42,770 --> 00:09:47,720 The most discussed example is the so-called weakly interacting, massive particle abbreviated to wimp, 98 00:09:48,560 --> 00:09:55,910 which comes out as a generic prediction, for example, from supersymmetry or from from theories with extra dimensions. 99 00:09:57,080 --> 00:10:02,510 And the amazing thing is that these theories don't only predict the particle to have the right properties, 100 00:10:02,870 --> 00:10:07,280 but they even predict how these particles came into being in the early universe. 101 00:10:07,970 --> 00:10:14,330 And the idea is that that the early universe, when it was still hot and dense, 102 00:10:14,720 --> 00:10:21,020 actually was hot enough to keep these dark matter particles in thermal equilibrium with all other particles. 103 00:10:21,560 --> 00:10:28,250 And only when the universe expanded and cooled down and the dark matter decoupled from the thermal path. 104 00:10:29,000 --> 00:10:35,690 And this prediction allows us to calculate the amount of dark matter that we expect to see in the present universe. 105 00:10:36,500 --> 00:10:41,600 And that prediction agrees with what we see. This goes by the name of the wimp miracle. 106 00:10:41,790 --> 00:10:48,140 So. So these theories allow us to understand why there should be dark matter and also why there should be so much of it. 107 00:10:50,150 --> 00:10:54,200 I should mention just in passing, that this is by far not the only proposal that we have. 108 00:10:54,830 --> 00:10:59,000 There are interesting alternative theories for what dark matter could be. 109 00:10:59,750 --> 00:11:05,630 One idea is axioms which would not be produced from the firm Abbas, but but in fact, non-family. 110 00:11:06,530 --> 00:11:12,230 There could be a new kind of neutrinos, which would not be called dark matter, but warm dark matter. 111 00:11:12,770 --> 00:11:16,459 Or there could be dark matter with new strong interactions. 112 00:11:16,460 --> 00:11:21,230 So dark matter, which actually is relatively similar to two protons. 113 00:11:21,980 --> 00:11:29,690 And in that case, we would try to or we would imagine that dark matter carries the same matter anti matter asymmetry, 114 00:11:29,690 --> 00:11:39,560 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, 115 00:11:39,950 --> 00:11:44,000 then that could very well account for the dark matter we have in the universe. 116 00:11:45,590 --> 00:11:51,320 So we are left with these ideas. We are left with the evidence for gravitational interactions, 117 00:11:51,680 --> 00:11:57,590 but really a lack of information about the details about the particle nature of dark matter. 118 00:11:58,160 --> 00:12:02,180 So therefore, the next part of my talk will be how to make this darkness visible. 119 00:12:02,210 --> 00:12:08,270 How to how to set up and perform experiments that can tell us the details, 120 00:12:08,270 --> 00:12:13,160 that can give us information such as the mass, the interactions, the spin of the dark matter particle. 121 00:12:15,280 --> 00:12:20,380 And these experiments are based essentially on the idea that I mentioned, 122 00:12:20,680 --> 00:12:25,959 which is that at some level, dark matter particles should interact with standard model particles. 123 00:12:25,960 --> 00:12:31,120 If we want to understand how they came into being in the early universe and we represent 124 00:12:31,120 --> 00:12:35,350 them as this kind of diagram where the details here are deliberately left vague. 125 00:12:35,830 --> 00:12:39,459 So the only thing that matters is that we have two dark matter particles on the 126 00:12:39,460 --> 00:12:43,780 one hand side of the diagram and two standard model fermions on the other side. 127 00:12:44,590 --> 00:12:47,229 And there are three different ways to think about this diagram, 128 00:12:47,230 --> 00:12:51,790 and they directly correspond to the three different techniques that we have to search for dark matter. 129 00:12:52,700 --> 00:12:59,870 If I read the diagram from left to right, that is too dark matter particles colliding and going into two standard model fermions. 130 00:13:00,110 --> 00:13:01,820 That is dark matter and the relation. 131 00:13:02,800 --> 00:13:10,320 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. 132 00:13:10,330 --> 00:13:17,709 So that represents dark matter scattering. And finally, if I reverse the directions, if I start from two standard model particles, 133 00:13:17,710 --> 00:13:23,260 and if I bring them to collision, they might actually be able to lead to dark matter production. 134 00:13:24,800 --> 00:13:30,670 So going through these techniques one by one. Let's start with direct detection of dark matter. 135 00:13:30,770 --> 00:13:37,130 So direct detection wants to exploit the scattering of dark matter particles and standard model particles. 136 00:13:37,140 --> 00:13:40,940 And the idea is that if the Milky Way indeed has a dark matter halo, 137 00:13:41,270 --> 00:13:46,580 then dark matter particles will be constantly passing through through the earth, through everything. 138 00:13:47,660 --> 00:13:54,950 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. 139 00:13:55,790 --> 00:14:00,500 Of course, we don't feel any of that because they have a tiny, tiny, tiny chance of interacting. 140 00:14:00,980 --> 00:14:04,190 But this interaction rate should be non-zero. 141 00:14:04,200 --> 00:14:08,690 So occasionally there will be scattering between dark matter particles and nuclei. 142 00:14:10,490 --> 00:14:17,180 And if such a scattering process actually happens in a controlled environment, in a dedicated detector with very low backgrounds, 143 00:14:17,540 --> 00:14:22,429 then we can convert the recoil energy transferred to the nucleus into visible signals. 144 00:14:22,430 --> 00:14:24,950 So that's sketched here on the right hand side. 145 00:14:24,950 --> 00:14:33,050 And, and the recoil energy, for example, can go into scintillation light, can go into ionised electrons or simply into heat into phonons. 146 00:14:33,530 --> 00:14:38,840 And that will then be visible. So then we can see essentially the interaction of a dark matter particle. 147 00:14:40,720 --> 00:14:47,280 But. The point I want to make here is that this is really an enormous a very, 148 00:14:47,280 --> 00:14:51,120 very brave experimental enterprise, because when you actually do the calculations, 149 00:14:51,120 --> 00:14:53,670 when you write down the event rate that you expect to see, 150 00:14:54,390 --> 00:15:00,150 it turns out that the event rates that you expect to something like one event per kilogram detect a material per year. 151 00:15:00,180 --> 00:15:05,160 So if you if you set up a ten kilogram detector, it will see ten dark matter particles in the year. 152 00:15:06,120 --> 00:15:12,239 And to make things even worse, it turns out that these dark matter particles are not very fast because their velocity must be 153 00:15:12,240 --> 00:15:17,250 smaller than the galactic escape velocity if they are to be bound in the in the Milky Way halo. 154 00:15:18,210 --> 00:15:22,440 And that means that the typical energy transfer in a collision will be maybe ten. 155 00:15:22,440 --> 00:15:26,040 K.V. And now compare that to a radioactive decay. 156 00:15:26,110 --> 00:15:34,110 Radioactive decay can easily give you tens or hundreds of of that amount of energy and radioactive decay happen all the time, 157 00:15:34,110 --> 00:15:40,979 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 158 00:15:40,980 --> 00:15:44,970 not very impressive compared to things that happens thousands of times per second. 159 00:15:45,930 --> 00:15:48,180 So so this this really is a challenge. 160 00:15:48,780 --> 00:15:54,839 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. 161 00:15:54,840 --> 00:16:02,720 So these experiments are somewhere in old mines or in tunnels under mountain ranges just to be shielded from cosmic rays. 162 00:16:02,730 --> 00:16:09,000 And then you need to to select materials which have very low radioactivity just to get 163 00:16:09,000 --> 00:16:13,380 all the backgrounds down to a level where you could hope to see dark matter scattering. 164 00:16:14,190 --> 00:16:20,430 And this indeed is possible. And it has been. It has been attempted by quite a few number of experiments. 165 00:16:20,880 --> 00:16:24,700 And they are really getting to these levels of sensitivity that we need. 166 00:16:25,410 --> 00:16:29,729 And in fact, some of these experiments, for example, the Cress two experiment, 167 00:16:29,730 --> 00:16:37,170 which is very closely linked to Oxford, already claims to see something which which they don't think is background. 168 00:16:38,430 --> 00:16:42,480 Nevertheless, there are other experiments, for example, Xenon 100 here, 169 00:16:42,810 --> 00:16:49,890 which says that the signal is in agreement with the number of background events they would expect from radioactivity. 170 00:16:50,580 --> 00:16:54,120 So the experimental situation here is not quite resolved yet, 171 00:16:54,120 --> 00:17:00,660 and I will get back later to to the kinds of questions that that these experiments lead us to. 172 00:17:02,070 --> 00:17:05,220 For now, moving on to the different techniques that we have. 173 00:17:05,640 --> 00:17:13,590 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. 174 00:17:14,220 --> 00:17:22,230 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. 175 00:17:22,620 --> 00:17:30,740 And we look for things that might be produced when dark matter particles, any light, for example, gamma rays, antiprotons, positrons or neutrinos. 176 00:17:32,010 --> 00:17:35,910 And the name of the game is to to know where to look. 177 00:17:36,330 --> 00:17:42,110 So try to understand where large dark matter densities are, and also to know where not to look. 178 00:17:42,120 --> 00:17:47,730 So to understand where you would suffer large astrophysical backgrounds. 179 00:17:49,200 --> 00:17:53,040 Just to give you one example, these are gamma ray searches. 180 00:17:53,280 --> 00:17:56,340 Searches for very highly energetic photons. 181 00:17:56,760 --> 00:17:59,820 And this is done, for example, by the Fermi Space Telescope. 182 00:18:00,390 --> 00:18:04,980 And it turns out that a particularly exciting place to look with this telescope is the Galactic Centre, 183 00:18:05,100 --> 00:18:09,270 because the Galactic Centre has the highest dark matter density around us. 184 00:18:10,140 --> 00:18:16,930 And the problem is that we really need to understand the Milky Way in a lot of detail. 185 00:18:16,930 --> 00:18:20,580 So we really have to figure out its dynamics and how it came into being. 186 00:18:20,970 --> 00:18:24,540 To understand the astrophysical backgrounds and to understand if, for example, 187 00:18:24,540 --> 00:18:30,420 a bump like this one here is due to dark matter or it's just a background that we haven't understood yet. 188 00:18:32,330 --> 00:18:37,940 So maybe the cleanest thing that we can do is actually to try to make our own dark matter in the laboratory. 189 00:18:38,390 --> 00:18:40,880 And this is exactly what the LHC aims to do. 190 00:18:40,910 --> 00:18:47,900 So the LHC collides protons at very high energies in order to reproduce the conditions very early after the Big Bang. 191 00:18:48,760 --> 00:18:54,640 And we can therefore hope that we produce dark matter in exactly the same way that it was produced in the early universe. 192 00:18:55,450 --> 00:19:00,310 But as you already know, dark matter is so weakly interacting that if we make it in a collider, 193 00:19:00,520 --> 00:19:03,700 it would just escape and we are not going to see any trace of it. 194 00:19:04,390 --> 00:19:11,410 So the trick is we don't only want to produce dark matter particles, but we also want to produce some visible particles. 195 00:19:11,830 --> 00:19:18,280 And if those two are produced at the same time, it will look as if momentum conservation is violated in the process. 196 00:19:19,350 --> 00:19:22,560 A very simple example for that are so-called monitored searches. 197 00:19:22,830 --> 00:19:31,559 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, 198 00:19:31,560 --> 00:19:34,590 gluons and things like that, going out in one way. 199 00:19:34,620 --> 00:19:37,650 So this this plane is perpendicular to the line of the beam. 200 00:19:37,710 --> 00:19:43,560 So you have something coming out in the perpendicular direction and you have nothing coming out on the other end. 201 00:19:44,190 --> 00:19:52,290 So so this observation immediately tells us that there must be an invisible particle, must be missing momentum going that way. 202 00:19:53,750 --> 00:19:57,530 Unfortunately, we already know a particle which gives exactly the signature. 203 00:19:57,530 --> 00:20:01,640 That's neutrinos, because neutrinos also don't leave a trace in detectors. 204 00:20:02,090 --> 00:20:05,930 And therefore, we expect to see some of these events from neutrinos already. 205 00:20:06,410 --> 00:20:12,830 So again, the point where theorists get involved is to calculate very precise predictions of what we would expect 206 00:20:12,830 --> 00:20:18,920 to see from neutrinos so that experimentalists can then compare what they see and search for in excess. 207 00:20:21,920 --> 00:20:29,719 The the summary for for all of what I've just said really is that that we have a lot of complementary, 208 00:20:29,720 --> 00:20:33,800 a lot of different techniques, a lot of different angles from which we can attack the problem. 209 00:20:34,960 --> 00:20:38,640 But all of these have in common the problem that we fight with backgrounds. 210 00:20:38,650 --> 00:20:43,000 We are looking for something which is almost completely invisible, which has very weak interactions. 211 00:20:43,650 --> 00:20:48,280 And and therefore, we need to understand anything that could possibly mimic such a signal, 212 00:20:48,280 --> 00:20:53,260 be it radioactivity, be it astrophysical sources or just simply neutrinos. 213 00:20:54,820 --> 00:21:00,399 And in fact, one of the most interesting aspects of this hunt is to come up with new signatures, 214 00:21:00,400 --> 00:21:03,490 come up with signatures that would suffer less if these backgrounds that would 215 00:21:03,490 --> 00:21:09,820 give a more unambiguous signal than than the simplest searches that you could do. 216 00:21:10,900 --> 00:21:14,560 I would like to give you one particular example, which I have been working on a lot, 217 00:21:14,890 --> 00:21:18,790 which is the idea of an annual modulation in the direct detection signal. 218 00:21:19,480 --> 00:21:28,000 So the idea is the following. We have the sun going at a velocity of about 220 kilometres per second around the Galactic Centre, 219 00:21:28,540 --> 00:21:32,710 and on top of that you have the rotation of the earth around the sun. 220 00:21:33,160 --> 00:21:40,030 And it turns out that the inclination angle is such that in June the velocity of earth and sun are partially aligned, 221 00:21:40,480 --> 00:21:44,590 whereas in December the velocities of the two are partially anti aligned. 222 00:21:45,010 --> 00:21:50,440 So what that means is that in summer the earth will go through the Milky Way at a higher velocity than in winter. 223 00:21:51,190 --> 00:21:55,540 And that means that in summer we are going to encounter more dark matter particles and 224 00:21:55,540 --> 00:22:00,010 we are going to encounter dark matter particles at higher velocities than in winter. 225 00:22:00,490 --> 00:22:05,050 So therefore, we would expect to see more signal in summer than in winter. 226 00:22:06,430 --> 00:22:13,239 And there are indeed experiments which look for these kinds of modulations experiments which attempt to be have 227 00:22:13,240 --> 00:22:20,170 extremely stable running conditions over several years so that they can pick up this modulation of the event rate. 228 00:22:20,590 --> 00:22:24,190 And there are two experiments which have been doing that called dama and Cogent. 229 00:22:24,610 --> 00:22:30,730 And both of these experiments see, at least to some degree, the significance is still debateable. 230 00:22:31,060 --> 00:22:37,780 But there seems to be some evidence in both of these for a modulation of the signal interface that agrees with the prediction. 231 00:22:40,070 --> 00:22:45,970 Now, the problem with this observation is, of course, that we need to be able to interpret it. 232 00:22:45,980 --> 00:22:50,809 In particular, we need to make a robust prediction for what the modulation fraction should be. 233 00:22:50,810 --> 00:22:56,150 A modulation fraction is the ratio of the modulation amplitude to the total signal strength. 234 00:22:56,840 --> 00:23:04,969 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. 235 00:23:04,970 --> 00:23:10,700 So that's depicted here where you have velocity here and modulation fraction on the Y axis. 236 00:23:11,650 --> 00:23:16,830 But what is worse is that the prediction also depends on how exactly we model the Milky Way, 237 00:23:16,840 --> 00:23:25,540 how exactly we try to describe the mass distribution of the matter in the Milky Way. 238 00:23:25,990 --> 00:23:32,290 And this is, of course, worrying, because this limits the amount of information that we can get from the experimental data. 239 00:23:33,340 --> 00:23:41,290 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, 240 00:23:41,290 --> 00:23:48,040 to be able to interpret direct detection experiments independently of the assumed properties of the dark matter halo. 241 00:23:48,160 --> 00:23:55,030 And and the method just very roughly is to map different experiments into a common parameter space 242 00:23:55,660 --> 00:24:00,730 where you do not need to make these assumptions in order to interpret the experimental data. 243 00:24:01,610 --> 00:24:07,610 And the conclusions that we reach is, in fact, that these two experiments, dama and cogent, are in agreement with each other, 244 00:24:08,060 --> 00:24:13,760 but are actually in tension with other experiments, for example, seen on hundreds, which I mentioned earlier. 245 00:24:15,930 --> 00:24:17,770 We can, in fact, go even further. 246 00:24:17,790 --> 00:24:25,680 We can not only compare different direct detection experiments with each other without making too strong assumptions, 247 00:24:26,010 --> 00:24:29,540 but we can even compare completely different search strategies. 248 00:24:29,550 --> 00:24:37,170 And this again exploits a technique that Subbiah introduced earlier, which is the idea of effective field theories. 249 00:24:37,410 --> 00:24:44,340 So let me give you just one example. If we want to study dipole dipole interactions between dark matter and standard model fermions, 250 00:24:44,850 --> 00:24:48,670 that means we want to consider an effective operator which looks like this. 251 00:24:48,690 --> 00:24:54,960 So this covers the dark matter dipole moment to essentially a a nucleus dipole moment. 252 00:24:55,380 --> 00:24:57,690 And because this is a dimension six operator, 253 00:24:57,960 --> 00:25:07,740 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. 254 00:25:08,430 --> 00:25:15,120 And this approach allows us to immediately calculate the different experimental signatures, calculate the the scattering cross-section, 255 00:25:15,120 --> 00:25:21,120 the energy lation cross-section, and so on, just in terms of the dark matter mass and the scale of new physics. 256 00:25:21,120 --> 00:25:28,770 So with without assuming any details of what exactly creates this interaction, what the mediator looks like and so on, 257 00:25:29,190 --> 00:25:31,559 and these are for this particular operator, 258 00:25:31,560 --> 00:25:38,250 we obtained these predictions and we can then use that to interpret completely independent sets of experiments. 259 00:25:39,750 --> 00:25:42,060 These are some of the results that we have obtained. 260 00:25:42,360 --> 00:25:51,810 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. 261 00:25:52,590 --> 00:25:59,220 So so a completely different search. This is this is in fact, from the monitored search that I mentioned earlier, 262 00:26:00,820 --> 00:26:05,850 the way to present these results conventionally is to show the dark matter mass on the x axis 263 00:26:06,210 --> 00:26:10,830 and on the right hand side and the left hand side to show the scattering cross-section. 264 00:26:11,970 --> 00:26:19,530 So this means, of course, that everything towards larger cross-sections, so everything above these lines is experimentally excluded. 265 00:26:20,770 --> 00:26:27,060 A different way to to show the same thing is to plot everything in terms of this new physics scale. 266 00:26:27,090 --> 00:26:32,820 So now it's the other way around. Now everything below the line, everything towards smaller scales is excluded. 267 00:26:33,390 --> 00:26:38,760 And t on the right hand side, we, we not only show the bone from the LHC and from Xenon 100, 268 00:26:39,090 --> 00:26:43,960 but now here this also two lines from from searches done with the Fermi satellite. 269 00:26:43,980 --> 00:26:46,590 So you can see how all these different pieces come together. 270 00:26:47,340 --> 00:26:53,210 And the most the most interesting the most exciting thing is that you can actually see that they are complementary. 271 00:26:53,220 --> 00:27:03,180 So. So depending on what exactly your your motive for dark matter is, different searches will give you the strongest sensitivity. 272 00:27:03,220 --> 00:27:10,650 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. 273 00:27:10,650 --> 00:27:15,840 We really ideally want to be able to combine all of these different bits of information that we get from different 274 00:27:15,840 --> 00:27:23,280 searches in order to then cover essentially the entire range of possibilities as completely as possible. 275 00:27:25,210 --> 00:27:31,720 Now, of course, ideally what we hope to find is, in the end, a consistent picture for all of the different experiments. 276 00:27:33,070 --> 00:27:38,300 But. What happens more often is that actually we see tension. 277 00:27:38,320 --> 00:27:42,940 We see that the different experiments don't give a consistent picture. 278 00:27:43,390 --> 00:27:49,590 And what that means is that we actually have to go back and review all the assumptions that we have made along the way. 279 00:27:49,650 --> 00:27:58,719 And and this is this is really a very important task for us theorists to make sure that actually the theory assumptions that 280 00:27:58,720 --> 00:28:05,620 we make are not overly strong or overly constraining or too optimistic and questions that we have been working on this. 281 00:28:05,620 --> 00:28:10,000 For example, how would experimental signatures change if dark matter is not a wimp? 282 00:28:10,870 --> 00:28:17,170 And what at times what types of experiments would be needed in that case to cover alternative scenarios? 283 00:28:17,770 --> 00:28:21,490 Just put one plot here for one very particular example, 284 00:28:21,700 --> 00:28:28,030 where the assumption that we relax is the assumption about how dark matter couples to protons and neutrons. 285 00:28:28,600 --> 00:28:36,910 So so typically and the the usual assumption is that because dark matter doesn't have electromagnetic interactions, 286 00:28:37,090 --> 00:28:43,690 it should be blind to the charge of the proton and therefore it should see protons and neutrons in exactly the same way. 287 00:28:44,530 --> 00:28:50,259 And this is what everyone normally uses to interpret searches. 288 00:28:50,260 --> 00:28:56,530 But of course we can very easily relax those assumptions and see how the implications of different 289 00:28:56,530 --> 00:29:02,770 experiments would change if we go to two different ratios of coupling of neutron and proton. 290 00:29:04,700 --> 00:29:12,259 So this brings me to my conclusions. I hope to have convinced you that we do have a very good case for dark matter, 291 00:29:12,260 --> 00:29:17,870 that we have very strong and very convincing evidence for the statement that I made in 292 00:29:17,870 --> 00:29:22,070 the beginning that we have so much more dark matter in the universe than visible matter. 293 00:29:22,880 --> 00:29:28,100 And in fact I hope I have made clear that would be need is really something 294 00:29:28,100 --> 00:29:32,150 completely new something completely different from what we what we already have. 295 00:29:32,810 --> 00:29:37,760 And this points towards a new elementary particle, which remains to be discovered. 296 00:29:38,210 --> 00:29:44,510 We have good models that predict the properties that this particle should have, but this is still a hypothesis. 297 00:29:44,510 --> 00:29:50,989 And we can test this hypothesis by by searching, for example, in direct detection experiments, 298 00:29:50,990 --> 00:29:55,730 indirect detection experiments, or to actually produce dark matter in colliders. 299 00:29:56,420 --> 00:30:00,500 And the name of the game really is to reduce experimental backgrounds, 300 00:30:01,400 --> 00:30:07,760 in particular to identify clean signals such as the annual modulation of the direct detection rate. 301 00:30:08,870 --> 00:30:12,409 There have already been some claims in recent years. 302 00:30:12,410 --> 00:30:17,360 There have also been counter claims. So the experimental situation at the moment is not clear. 303 00:30:17,690 --> 00:30:20,690 But the good news is that there are a lot more experiments coming. 304 00:30:20,690 --> 00:30:28,070 So the next few years will see an enormous increase of the number of experiments and the sensitivity of these experiments. 305 00:30:28,820 --> 00:30:32,810 So we have very good hopes that the next few years will be exciting. 306 00:30:33,080 --> 00:30:33,950 Thank you very much.