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So. So thank you very much.

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I am delighted to speak at this meeting and when I was invited to speak, the email said, uh,

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we would like you to give a talk about cosmic rays and dark matter, two topics that Tom Perkins was very excited and interested in.

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And we'd like to tell you, we'd like you to tell us about subsequent developments and what's happening in the field today.

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So that's what I'll organise my talk around.

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So I know Dom Perkins, but I taught undergraduate for many years from his wonderful textbook on particle astrophysics.

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And so we've heard a little bit about how he's great.

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All his textbook on high energy physics was so the kind of last chapter of that book he expanded into this wonderful textbook,

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which I think he was bringing up to date,

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and revising really enormous revisions between the first of it edition and in 2002 and the second edition in 2009.

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So I think in the preface that he wrote to the final edition in 2009, you can really hear his great enthusiasm for this subject area.

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So he he writes about the first part, that in the first part of this book, discussing the Standard model of particle physics,

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you could say that extremely well understood subject was agreement between theory and experiment better than one part per million in the case of QED,

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and whatever the form might be, of an ultimate theory of everything,

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if there is one, the standard model of particle physics will surely be part of it,

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even if it only accounts for a paltry 4% of the energy density of the universe at large.

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The second part, he writes about the great questions and mysteries in cosmology, chief among them the nature of dark matter.

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So that's what I'll talk about today.

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And in the third part, he writes, this is concerned with the study of the particles and radiation which bombard us from outer space.

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We encounter here some of the most energetic and bizarre processes in the universe,

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with new experimental discoveries being made on an almost daily basis.

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So you can really hear his enthusiasm for the field as he was writing in 2009.

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So I'm going to talk about how the tools of particle astrophysics tell us both about the larger scales and exotic objects,

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and also about smallest particle physics distance scales in the universe.

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So I'll talk a little bit about cosmic rays, and I'll talk about how all of these tools are brought to bear on the search for dark matter.

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So we've found all of these things except for the last one in this picture.

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And so I'll talk about the search. So to kind of set the context.

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The evidence for dark matter comes from a wide range of distant scales in the universe,

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from measurements of stars in spiral galaxies through the motions of galaxies within galaxy clusters,

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through looking at observations of the cosmic microwave background at the scale of the whole sky.

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The evidence for dark matter comes from observations that are made 400,000 years after the Big Bang,

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all the way up through measurements that are made today. 13.7 billion years.

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And this picture, putting together all of the evidence across these astrophysical scales, combines into what is now the concordance model,

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the Lambda CDM standard model of cosmology, where our picture of the energy density of the universe looks like this.

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Something like 70% of it is made up of dark energy, roughly 25% is dark matter,

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and just under 5% is made of the particles we know and love that are described by the Standard Model of particle physics.

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So there's clearly a big opportunity here for discovery.

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So what could the dark matter be?

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Well, if you look at, you know, the the enormous range of possibilities and ask what could be consistent with the astrophysical evidence,

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you can narrow down this huge mass range in the top axis here into a paltry 40 orders of magnitude and mass shown in the bottom axis here.

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And there are many, many theoretical possibilities for what dark matter could be.

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And I think, you know, it's fair to say at this point that there's probably a flat prior.

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Dark matter is equally likely across the entire 14 orders of magnitude of this bottom axis.

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And so what these coloured blocks show are different categories of theoretical models.

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And I'll talk a little bit about what we know about the search in most of these.

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And I think since the time when Don Perkins was writing the preface to his wonderful book in 2009.

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There's kind of a new sociology at this point.

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We know dark matter definitely exists, and perhaps it's sufficient for a theory simply to explain dark matter on its own.

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It might not also need to do another job. It might not need to explain the solution to nationalist problems.

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Maybe it's enough just for it to have a good dark matter candidate. So I'm going to talk about what we do in sort of three areas.

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The first is indirect detection, where we're looking for a dark matter particle to find another dark matter particle,

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annihilate and produce things we can see. I'll talk about direct detection, which is my own field,

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where we look for dark matter to interact with an atom in a very sensitive terrestrial detector.

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And I'll make one comment at the end about complementarity with collider production, which you'll hear more about from Daniela later.

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So indirect detection strategies are concerned with using the study of particles and radiation,

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which bombard us from outer space to look for dark matter, rather than to look for the characteristics of some objects producing them.

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And there are three main channels for this, and I'll focus on the first one, which is self-annihilation.

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So we can imagine weakly interacting massive particle or WIMP dark matter candidates

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annihilating through some process that produces perhaps gamma rays or neutrinos,

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or proton and antiproton pairs, or matter and anti-matter pairs.

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This is not the only possibility. There are also experiments at.

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I'll show one result from that look at decay, producing perhaps sterile neutrinos or conversion of axion producing photons.

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But I'll focus on this first category. So the self annihilation signal that an experiment might hope to observe using cosmic

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rays that signal as a function of energy depends on the annihilation cross-section.

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That's the thing we're trying to measure. It depends on the integral along the line of sight for the instrument,

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and it depends on the energy spectrum produced by those annihilating dark matter particles.

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And so what this figure shows is that annihilation spectrum as a function of energy.

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And what you see along this kind of top row here is a range of different particle candidates.

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And so you see there's a huge range of energies that are spans here.

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And what the vertical axis shows is the signal energy that a particular detector might observe.

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And we can interpret that as a frequency. And if we interpret that as a frequency,

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this list of names on the side are various experiments that are studying different parts of this frequency band.

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The coloured blocks in this region show different signal channels.

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So for example, I'll show plots later that look at B bar annihilation, which is down here in dark blue.

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So there are various channels that different experiments look at. And depending on what band and energy and experiment is looking at,

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that determines the sensitivity to which category of dark matter model it's exploring.

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So this is sort of the current status of self-annihilation searches.

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We can plot this in terms of this annihilation cross-section on the vertical axis versus the mass of the dark matter candidate on the horizontal axis.

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And you know, what's allowed at this point is this white range in the middle.

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So this grey region is excluded because there would be too much dark matter to

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be consistent with the astrophysical data that I introduced at the beginning.

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This purple region is excluded by unitarity, conservation of probability and of scattering.

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And this green region is where we have current experimental constraints where.

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And I'll talk about where these come from. The experiments can exclude dark matter with annihilation cross-sections within this green range,

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because they would produce too much dark matter to be consistent with what we observe.

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So those constraints come from observations of dwarf small galaxies.

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So the Fermi experiment looks at that and can set constraints depending on the final state,

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excluding annihilation cross-sections above and to the right of these coloured lines.

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And this dashed line in this figure is the same as the dashed line in the bottom.

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So we want to reach down to that line in order to probe the range that's compatible.

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And so what you see is this excludes masses below about 100 GeV in the strongest constraints.

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There are also constraints coming from TeV gamma rays.

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And so this shows TeV gamma ray constraints and prospects.

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And what you see is that TV gamma rays can kind of reach down to this thermal relic density line up to just above the 100 GB,

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right up to just above the the, uh, TV scale.

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The hope, the great hope for probing most of this mass range up to the highest masses comes from the triangle telescope array,

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where what you see is that predictions for future sensitivity of this instrument in different final

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states will cover all the way down to this thermal relic line up to something like tens of TeV.

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Now these are all visible final states, but of course there's an important contribution from invisible final states,

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uh, in, you know, neutrino telescope experiments. Uh, so this shows the sensitivity projection for super K and hyperkalemia kind of experiments,

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where the signal is that dark matter annihilates producing neutrino antineutrino pairs, which then go on to interact in super K or hyper K.

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And what you see is that these kinds of neutrino constraints can test masses up to about the ten GeV scale.

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So this is kind of the prospect for testing dark matter across this range.

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And if I sort of sum up and integrate over all the different experiments and just ask, you know, what does this look like for wimps?

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Well, for the WIMP mass Range, I think we have a good chance in the next decade of exploring most of this region up to the unitarity limit.

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You might ask, could there be a signal here? Well, what these, uh,

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sort of brown bar and blue bars show are where you might expect to see a signal in different kinds of supersymmetric dark matter models.

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So there's absolutely a a fair chance at discovery. And if we look at invisible final states.

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So neutrino experiment contributions and there are prospects that are very interesting far

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beyond this just WIMP theory mass range going to much lighter and also much heavier masses.

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Neutrino experiments will have a lot to say.

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So for example, the Dune experiment, uh, has interesting sensitivity down here in the kind of GeV scale mass range.

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And you can see the constraints I showed up there in the sort of tens of TeV, and there's a wide range,

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and I should say here that current experiments are excluding the regions that are in the kind of shaded bars.

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And the dashed lines show where sort of the next generation of experiments are aiming to reach.

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So at this point, I'm going to move on and talk a little bit about what are the prospects for seeing

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dark matter directly interact by scattering off an atom in your sensitive detector?

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And here the scattering kinematics are really determined by the relative velocity of an atom in

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your sensitive detector relative to a dark matter particle in the galactic dark matter halo.

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And the scattering kinematics have a beta that's less than ten to the minus three.

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And so this basically looks like classical non-relativistic to body elastic scattering.

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And so in this two body elastic scattering process we expect to see the recoil travel forward relative to the incident particle direction.

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So because this relative velocity is small the kinetic energy of the dark matter particle is small.

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The momentum transfer of the q squared in the interaction is small.

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And so the wavelength of the propagator is large,

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which means that the incident dark matter particle can scatter coherently off of all of the nucleons in the target nucleus.

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And so we get a coherent sum of amplitudes.

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This process can happen in a way that's independent of spin, and can also happen in a way that depends on spin.

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And I'd like to highlight here, I'm kind of picking up right where the last speaker left off.

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So the first calculation of this coherent initial elastic scattering process was done by Dan Friedmann in, I think, 1973 and published in 1974.

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And he starts out by saying if there's a weak neutral current, then that's a dot.

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This coherent elastic scattering process is possible. And he goes on to calculate it in this paper.

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And this is the foundation of this entire strategy of direct detection.

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So in the first sentence of this paper,

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he begins with citing the recent experimental evidence from CERN and Fermilab suggesting the process of neutral current interactions.

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So in fact, it was this, you know,

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foundational work that went on to see this entire field of direct detection looking for coherent elastic scattering interactions.

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So what we expect to see for the scattering rate as a function of momentum transfer depends on the cross-section at zero q squared shown here segment.

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That's the thing we're trying to measure.

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It depends on the energy density of dark matter and the velocity distribution of that dark matter that our detector is moving through.

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And it depends on, of course, the mass of the dark matter and the target.

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And it depends on a form factor describing the suppression of coherence as the momentum transfer increases.

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What an experiment sees in this kind of search is, of course, not the dark matter particle directly,

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but it sees the particle that it hit and what that particle does, and the detector.

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It may deposit energy by heat or via ionisation energy loss, producing ionisation and scintillation signals.

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And the rate at which this happens is incredibly small.

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So if I pick a pair of parameters that the candidate, the dark matter particle mass is 100 times the mass of the proton,

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and I pick a cross-section out of the air of ten to the -45cm², which is just above current constraints.

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What we see is that the rate of interactions we expect is of order one per ton of detector mass per year of doing the experiments,

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and this event distribution is falling exponentially as we look at increasing recoil energy.

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And so experiments to look for this signal need to have large target mass.

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They need to have very low energy threshold sensitivity to UKTV of energy.

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And they need to have incredible control over things like neutron backgrounds,

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because you're looking for one event per year per ton, and backgrounds can be much more numerous than that.

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And so that's just kind of the current status of direct detection WIMP searches.

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I'm going to show a bunch of plots that look like this,

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where we have the dark matter nucleon interaction cross-section on the vertical axis and the mass of the dark matter candidate on the horizontal axis.

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And so what you see is there's sort of, uh, two regions.

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There are regions where we can look at cross-sections that are far below the weak scale.

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We're operating experiments or testing cross-sections down to about ten to the -48cm².

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And there are experiments being planned to go another order of magnitude beyond that.

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And then there are experiments at kind of later dark matter masses below the GeV scale, which have much less stringent limits.

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And so we can ask, how are we making progress in this? And what are the one of the limitations here?

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So at some point we run into the point where neutrino coherence, elastic scattering becomes an irreducible background.

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And these searches limiting the sensitivity. And that's what's shown in this grey fog down here.

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So the objective more or less is to reach down to this point. So how do we get down to these kind of lower end of masses.

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Well, this is a really active area which Oxford has had a big role in, uh, in developing new technologies.

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So the tricky thing about this coherent elastic scattering process is that the energy of

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the recoil is at most about a millionth of the rest mass of the dark matter particle.

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And so if you want to see a GeV, uh, mass dark matter candidate, you need to have a sensitivity of Kev to recoil energies.

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And so that's, you know, where current experiments are using liquid Noble's and indeed bubble chambers.

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And for reaching lower dark matter mass sensitivity,

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one needs to also invent detector technologies capable of reaching lower recoil energy sensitivity.

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And so there are great ideas about this in kind of some of the most ambitious ideas.

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Deploy quantum sensors using macroscopic quantum states like superfluid helium three,

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in order to reach sensitivities to much lower recoil energies, which give sensitivity to much lower dark matter masses.

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So we're aiming to reach the milli EV recoil energy sensitivity scale to reach movie scale.

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Dark matter mass sensitivity. The other direction we can go in this parameter space is to lower and lower cross section in.

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The strategy for getting here is to build bigger and bigger detectors.

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So the leading constraint in this mass range now comes from the LC experiment which Oxford has a big role in.

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So this is a beautiful result from 2022. And I want to say this is an exquisite instrument.

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I think it's the 10th iteration of the dual phase liquid xenon time projection chamber.

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I think it's worth noting that the very first dual phase GPC was flown in a high altitude balloon to look at exotic gamma ray sources in the sky,

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the LCS egret experiment.

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So in a sense, this kind of, you know, cosmic ray driven technology is what's driving the frontier in direct detection today.

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And these experiments today are fairly large instruments. They're no longer, you know, small scale.

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Uh, so this shows some of the infrastructure associated with the Xenon program at the ground source a laboratory.

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And you can see that the detector more or less occupies the space of a three story building.

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So these are getting to be rather large instruments. The alternative strategy for reaching very large detectors is using liquid argon.

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And this is what I work on. And I'll show some results from this program later.

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So we've had several iterations getting bigger and bigger.

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And at the moment we're building this instrument which is called Darkside 20 K Underground at Gran Sasso.

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This object is eight metres on a side. You can sort of see for scale.

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Here are some nice pictures of people in front of our cryostat under construction.

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So you get a sense of how large it is. And we are aiming in Five Sigma discovery rates to go more than an order of magnitude beyond the reach

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of currently running experiments in the region above the centre of mass energy reach of the LHC.

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And the ultimate aim of this program is to build a kiloton scale detector.

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So at this point I've talked about WIMP sensitivity, which is just this one little blob in the middle of these 40 orders of magnitude.

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So what's happening in the rest of the space? Well, let's kind of start at the lowest mass end and think about actions.

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So actions were positive to solve the strong CP problem.

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So it's a dark matter candidate that also solves other issues in particle physics.

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And at this point this is a huge industry. There's an enormous range of experimental techniques that are trying to detect axion photon coupling ideas.

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And axion converts to a photon and you detect that photon detector.

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There are experiments looking with LHC magnets at axion coming from the sun,

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their experiments looking at axons coming from the gravitational halo of our dark matter halo of our galaxy,

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their experiments trying to produce axioms and then detect them.

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Uh, and then there's kind of a big effort in detecting axion induced RF.

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And so what this figure shows are the constraints on the axion photon coupling as a function of the mass of the action.

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And this yellow band shows the region where the QCD and the dark matter axion could be the same thing.

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So the interesting point is that experiments are kind of just starting to bite into this region where we can look at the axiom and,

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you know, current constraints are shown in the solid colours.

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And, you know, future projected sensitivities are shown in these kind of lighter, uh, pink colours.

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And so you can see that experiments are working hard to reach down to that QCD axion line.

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And looking for axion photon coupling coupling. It's not the only thing you can do.

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You can also look for axion electron coupling.

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And this is an area where there's a big deployment and a big effort on bringing the tools of quantum technology to bear on particle physics problems.

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So, you know, Oxford here leads the ion effort. And this shows the sensitivity to axion, a left electron coupling as a function of the mass.

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And you can see that this is a strategy that can reach perhaps as low even as ten to the -18 GV.

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So moving up in this mass scale. I'll talk next about sterile neutrino candidates.

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So sterile neutrino dark matter could make up something like a third of the dark matter based on astrophysical constraints.

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And it's possible that sterile neutrino dark matter could scatter with electrons because of sterile neutrino electron neutrino mixing.

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There are strong constraints on the electron sterile neutrino mixing angle U4 squared from beta decay.

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So in a beta decay, if the new E mixes with the sterile neutrino, you would expect to see deformation in the shape of the end point.

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And so based on looking for that deformation and various beta decay experiments that

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direct constraints shown in the top here on U4 squared that are greater than about 1%.

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There are also strong constraints from indirect detection experiments, which look at the X-ray spectra coming from,

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for example, dwarfs for little galaxies and look for deformation in that spectrum.

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And these constraints are very strong. So these constraints are down at the level of maybe ten to the minus ten on U4 squared.

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However, it's worth noting that there's also an anomaly. There's an excess that's reported at three and a half kV by several targets.

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And that's a interesting and hot topic in the field, what that might be.

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And so we actually just published the first search for sterile neutrino electron scattering

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in a direct detection experiment where we're sensitive in sort of the 10 to 20 uh Kev range,

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where the sensitivity from a 50 kilogram dark matter instrument actually exceeds that of the beta decay searches.

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And so we're now building a 50 ton experiment.

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So it's going to will be quite interesting to look for this channel in the next generation of direct detection searches,

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which are a factor of a thousand larger and target mass. Okay, so that's sterile neutrinos moving up in mass scale.

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Another possibility, of course, is that perhaps dark matter.

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And, you know, might have self interactions. Perhaps there is some good reason why the ratio of dark matter to baryonic matter is about 3 to 1.

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And there are models that seek to explain that. Those are typically kind of termed asymmetric dark matter models.

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And those kinds of candidates predict dark matter in the Kev to GeV or perhaps just above GeV scale.

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And so experiments can look for absorption processes of Kev scale,

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dark matter or scattering processes of movie scale dark matter or GeV scale dark matter searches.

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The most sensitive results come from looking for not only the effect of the dark matter scattering, but also nuclear dissertation.

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And so all of those kinds of searches are pushing down into this interesting parameter space.

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And again, this is a very hot topic, but we're reaching couplings that are at the scale of perhaps ten to the -12 to 10 to the -14,

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uh, comparable to some of the accident results that I showed earlier.

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And in the kind of slightly higher mass range, we're sort of reaching down to cross-sections that are at the weak interaction scale.

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So another interesting and creative idea that I think Firkins might have been quite interested in is, well,

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if we have all this dark matter around and we have all these cosmic rays raining down on us,

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what would the underground muon telescope see if dark matter and cosmic rays interact?

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And there are fun ideas about this. So one possibility, of course, is that cosmic rays scattering on dark matter would lose energy.

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And this would modify the cosmic rays spectrum that you measured.

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So this shows the electron data from AMS.

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And if you include dark matter electron scattering that changes the prediction for the spectral shape you get this blue dashed line.

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And so you can actually use this as a kind of reverse direct detection.

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So if dark matter were there scattering with a cross section above x, how would it change the cosmic ray spectrum.

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And you can ask how consistent is this with what we observe. And those kinds of experiments can set constraints on dark matter at the Kev scale.

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You could also say, well, if dark matter is scattering off these cosmic rays, it must be gaining energy.

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So you have dark matter up scattering.

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And the effect of that is that it could boost a small fraction of the dark matter to an energy above the detection threshold in a neutrino experiment.

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And so you can ask what kinds of constraints you get if you consider those effects.

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And in this range, actually super K has some of the strongest constraints on movie scale dark matter.

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And then kind of moving up above the mass region, there could be very, very heavy dark matter all the way up to the Planck scale.

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And that kind of candidate can be produced non thermally in grand unified theories like those that

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motivated Sudan to could be produced in primordial black hole radiation or extended thermal production.

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And the signature here is that those kinds of interactions may scatter many times as they travel through your large detector.

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And so this shows the current constraints on that cross-section versus mass.

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And you can kind of see we're reaching all the way up to the Planck scale here.

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So at this point, I'll wrap up and just make one remark about complementarity with colliders.

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So in collider experiments, the signature, of course,

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is trying to produce dark matter candidates in collisions of ultrahigh energy proton proton collisions.

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And you don't see the dark matter particles that are produced flying out,

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but you see some missing energy or some single jet showing momentum imbalance.

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And if we try to convert limits on branching ratios and translate those into limits on cross-section versus mass,

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the kinds of plots I've been showing, the direct detection constraints are shown in these solid shaded regions.

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These are for two different flavours of production models and the science reach of kind of future direct detection experiments.

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The ultimate direct detection experiments we can imagine are shown in these dashed curves.

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And the science reach of future accelerators. High luminosity LHC are shown in the solid coloured curves.

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And what I wanted to illustrate with this is the comment that with these kinds of experiments,

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we can look above the centre of mass energy, even of the most ambitious future accelerator we can imagine.

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And so I'll close with this thought about complementarity,

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which I've stolen from Chris Llewellyn Smith's excellent essay at the end of the Festschrift that was given in 1993 when Perkins retired.

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And I want to say thank you very much to Mandy Cooper Sarkar for lending me this book where I read this very,

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very interesting essay, um, where in an essay on particle physics in the future,

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uh, the, the comment was made here that it's possible that the long term future of experimental particle

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physics may be with non accelerator experiments with which Don Perkins began his career.

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Uh, and I'll insert my own, uh, aside here because they can reach, you know,

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ultimately higher energies than anything we can imagine building here on Earth.

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And this was a trend presciently anticipated by Don towards the end of his career, who moved in that direction.

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And so I'll close with final thought. Uh, you know, as a researcher looking for dark matter, uh,

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May Perkins is optimism for particles and radiations which bombard us from outer space with new discoveries being made on an almost daily basis,

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continue to be justified in the search for dark matter. Thank you.