So. So thank you very much.
I am delighted to speak at this meeting and when I was invited to speak, the email said, uh,
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.
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.
So that's what I'll organise my talk around.
So I know Dom Perkins, but I taught undergraduate for many years from his wonderful textbook on particle astrophysics.
And so we've heard a little bit about how he's great.
All his textbook on high energy physics was so the kind of last chapter of that book he expanded into this wonderful textbook,
which I think he was bringing up to date,
and revising really enormous revisions between the first of it edition and in 2002 and the second edition in 2009.
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.
So he he writes about the first part, that in the first part of this book, discussing the Standard model of particle physics,
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,
and whatever the form might be, of an ultimate theory of everything,
if there is one, the standard model of particle physics will surely be part of it,
even if it only accounts for a paltry 4% of the energy density of the universe at large.
The second part, he writes about the great questions and mysteries in cosmology, chief among them the nature of dark matter.
So that's what I'll talk about today.
And in the third part, he writes, this is concerned with the study of the particles and radiation which bombard us from outer space.
We encounter here some of the most energetic and bizarre processes in the universe,
with new experimental discoveries being made on an almost daily basis.
So you can really hear his enthusiasm for the field as he was writing in 2009.
So I'm going to talk about how the tools of particle astrophysics tell us both about the larger scales and exotic objects,
and also about smallest particle physics distance scales in the universe.
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.
So we've found all of these things except for the last one in this picture.
And so I'll talk about the search. So to kind of set the context.
The evidence for dark matter comes from a wide range of distant scales in the universe,
from measurements of stars in spiral galaxies through the motions of galaxies within galaxy clusters,
through looking at observations of the cosmic microwave background at the scale of the whole sky.
The evidence for dark matter comes from observations that are made 400,000 years after the Big Bang,
all the way up through measurements that are made today. 13.7 billion years.
And this picture, putting together all of the evidence across these astrophysical scales, combines into what is now the concordance model,
the Lambda CDM standard model of cosmology, where our picture of the energy density of the universe looks like this.
Something like 70% of it is made up of dark energy, roughly 25% is dark matter,
and just under 5% is made of the particles we know and love that are described by the Standard Model of particle physics.
So there's clearly a big opportunity here for discovery.
So what could the dark matter be?
Well, if you look at, you know, the the enormous range of possibilities and ask what could be consistent with the astrophysical evidence,
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.
And there are many, many theoretical possibilities for what dark matter could be.
And I think, you know, it's fair to say at this point that there's probably a flat prior.
Dark matter is equally likely across the entire 14 orders of magnitude of this bottom axis.
And so what these coloured blocks show are different categories of theoretical models.
And I'll talk a little bit about what we know about the search in most of these.
And I think since the time when Don Perkins was writing the preface to his wonderful book in 2009.
There's kind of a new sociology at this point.
We know dark matter definitely exists, and perhaps it's sufficient for a theory simply to explain dark matter on its own.
It might not also need to do another job. It might not need to explain the solution to nationalist problems.
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.
The first is indirect detection, where we're looking for a dark matter particle to find another dark matter particle,
annihilate and produce things we can see. I'll talk about direct detection, which is my own field,
where we look for dark matter to interact with an atom in a very sensitive terrestrial detector.
And I'll make one comment at the end about complementarity with collider production, which you'll hear more about from Daniela later.
So indirect detection strategies are concerned with using the study of particles and radiation,
which bombard us from outer space to look for dark matter, rather than to look for the characteristics of some objects producing them.
And there are three main channels for this, and I'll focus on the first one, which is self-annihilation.
So we can imagine weakly interacting massive particle or WIMP dark matter candidates
annihilating through some process that produces perhaps gamma rays or neutrinos,
or proton and antiproton pairs, or matter and anti-matter pairs.
This is not the only possibility. There are also experiments at.
I'll show one result from that look at decay, producing perhaps sterile neutrinos or conversion of axion producing photons.
But I'll focus on this first category. So the self annihilation signal that an experiment might hope to observe using cosmic
rays that signal as a function of energy depends on the annihilation cross-section.
That's the thing we're trying to measure. It depends on the integral along the line of sight for the instrument,
and it depends on the energy spectrum produced by those annihilating dark matter particles.
And so what this figure shows is that annihilation spectrum as a function of energy.
And what you see along this kind of top row here is a range of different particle candidates.
And so you see there's a huge range of energies that are spans here.
And what the vertical axis shows is the signal energy that a particular detector might observe.
And we can interpret that as a frequency. And if we interpret that as a frequency,
this list of names on the side are various experiments that are studying different parts of this frequency band.
The coloured blocks in this region show different signal channels.
So for example, I'll show plots later that look at B bar annihilation, which is down here in dark blue.
So there are various channels that different experiments look at. And depending on what band and energy and experiment is looking at,
that determines the sensitivity to which category of dark matter model it's exploring.
So this is sort of the current status of self-annihilation searches.
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.
And you know, what's allowed at this point is this white range in the middle.
So this grey region is excluded because there would be too much dark matter to
be consistent with the astrophysical data that I introduced at the beginning.
This purple region is excluded by unitarity, conservation of probability and of scattering.
And this green region is where we have current experimental constraints where.
And I'll talk about where these come from. The experiments can exclude dark matter with annihilation cross-sections within this green range,
because they would produce too much dark matter to be consistent with what we observe.
So those constraints come from observations of dwarf small galaxies.
So the Fermi experiment looks at that and can set constraints depending on the final state,
excluding annihilation cross-sections above and to the right of these coloured lines.
And this dashed line in this figure is the same as the dashed line in the bottom.
So we want to reach down to that line in order to probe the range that's compatible.
And so what you see is this excludes masses below about 100 GeV in the strongest constraints.
There are also constraints coming from TeV gamma rays.
And so this shows TeV gamma ray constraints and prospects.
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,
right up to just above the the, uh, TV scale.
The hope, the great hope for probing most of this mass range up to the highest masses comes from the triangle telescope array,
where what you see is that predictions for future sensitivity of this instrument in different final
states will cover all the way down to this thermal relic line up to something like tens of TeV.
Now these are all visible final states, but of course there's an important contribution from invisible final states,
uh, in, you know, neutrino telescope experiments. Uh, so this shows the sensitivity projection for super K and hyperkalemia kind of experiments,
where the signal is that dark matter annihilates producing neutrino antineutrino pairs, which then go on to interact in super K or hyper K.
And what you see is that these kinds of neutrino constraints can test masses up to about the ten GeV scale.
So this is kind of the prospect for testing dark matter across this range.
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?
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.
You might ask, could there be a signal here? Well, what these, uh,
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.
So there's absolutely a a fair chance at discovery. And if we look at invisible final states.
So neutrino experiment contributions and there are prospects that are very interesting far
beyond this just WIMP theory mass range going to much lighter and also much heavier masses.
Neutrino experiments will have a lot to say.
So for example, the Dune experiment, uh, has interesting sensitivity down here in the kind of GeV scale mass range.
And you can see the constraints I showed up there in the sort of tens of TeV, and there's a wide range,
and I should say here that current experiments are excluding the regions that are in the kind of shaded bars.
And the dashed lines show where sort of the next generation of experiments are aiming to reach.
So at this point, I'm going to move on and talk a little bit about what are the prospects for seeing
dark matter directly interact by scattering off an atom in your sensitive detector?
And here the scattering kinematics are really determined by the relative velocity of an atom in
your sensitive detector relative to a dark matter particle in the galactic dark matter halo.
And the scattering kinematics have a beta that's less than ten to the minus three.
And so this basically looks like classical non-relativistic to body elastic scattering.
And so in this two body elastic scattering process we expect to see the recoil travel forward relative to the incident particle direction.
So because this relative velocity is small the kinetic energy of the dark matter particle is small.
The momentum transfer of the q squared in the interaction is small.
And so the wavelength of the propagator is large,
which means that the incident dark matter particle can scatter coherently off of all of the nucleons in the target nucleus.
And so we get a coherent sum of amplitudes.
This process can happen in a way that's independent of spin, and can also happen in a way that depends on spin.
And I'd like to highlight here, I'm kind of picking up right where the last speaker left off.
So the first calculation of this coherent initial elastic scattering process was done by Dan Friedmann in, I think, 1973 and published in 1974.
And he starts out by saying if there's a weak neutral current, then that's a dot.
This coherent elastic scattering process is possible. And he goes on to calculate it in this paper.
And this is the foundation of this entire strategy of direct detection.
So in the first sentence of this paper,
he begins with citing the recent experimental evidence from CERN and Fermilab suggesting the process of neutral current interactions.
So in fact, it was this, you know,
foundational work that went on to see this entire field of direct detection looking for coherent elastic scattering interactions.
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.
That's the thing we're trying to measure.
It depends on the energy density of dark matter and the velocity distribution of that dark matter that our detector is moving through.
And it depends on, of course, the mass of the dark matter and the target.
And it depends on a form factor describing the suppression of coherence as the momentum transfer increases.
What an experiment sees in this kind of search is, of course, not the dark matter particle directly,
but it sees the particle that it hit and what that particle does, and the detector.
It may deposit energy by heat or via ionisation energy loss, producing ionisation and scintillation signals.
And the rate at which this happens is incredibly small.
So if I pick a pair of parameters that the candidate, the dark matter particle mass is 100 times the mass of the proton,
and I pick a cross-section out of the air of ten to the -45cm², which is just above current constraints.
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,
and this event distribution is falling exponentially as we look at increasing recoil energy.
And so experiments to look for this signal need to have large target mass.
They need to have very low energy threshold sensitivity to UKTV of energy.
And they need to have incredible control over things like neutron backgrounds,
because you're looking for one event per year per ton, and backgrounds can be much more numerous than that.
And so that's just kind of the current status of direct detection WIMP searches.
I'm going to show a bunch of plots that look like this,
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.
And so what you see is there's sort of, uh, two regions.
There are regions where we can look at cross-sections that are far below the weak scale.
We're operating experiments or testing cross-sections down to about ten to the -48cm².
And there are experiments being planned to go another order of magnitude beyond that.
And then there are experiments at kind of later dark matter masses below the GeV scale, which have much less stringent limits.
And so we can ask, how are we making progress in this? And what are the one of the limitations here?
So at some point we run into the point where neutrino coherence, elastic scattering becomes an irreducible background.
And these searches limiting the sensitivity. And that's what's shown in this grey fog down here.
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.
Well, this is a really active area which Oxford has had a big role in, uh, in developing new technologies.
So the tricky thing about this coherent elastic scattering process is that the energy of
the recoil is at most about a millionth of the rest mass of the dark matter particle.
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.
And so that's, you know, where current experiments are using liquid Noble's and indeed bubble chambers.
And for reaching lower dark matter mass sensitivity,
one needs to also invent detector technologies capable of reaching lower recoil energy sensitivity.
And so there are great ideas about this in kind of some of the most ambitious ideas.
Deploy quantum sensors using macroscopic quantum states like superfluid helium three,
in order to reach sensitivities to much lower recoil energies, which give sensitivity to much lower dark matter masses.
So we're aiming to reach the milli EV recoil energy sensitivity scale to reach movie scale.
Dark matter mass sensitivity. The other direction we can go in this parameter space is to lower and lower cross section in.
The strategy for getting here is to build bigger and bigger detectors.
So the leading constraint in this mass range now comes from the LC experiment which Oxford has a big role in.
So this is a beautiful result from 2022. And I want to say this is an exquisite instrument.
I think it's the 10th iteration of the dual phase liquid xenon time projection chamber.
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,
the LCS egret experiment.
So in a sense, this kind of, you know, cosmic ray driven technology is what's driving the frontier in direct detection today.
And these experiments today are fairly large instruments. They're no longer, you know, small scale.
Uh, so this shows some of the infrastructure associated with the Xenon program at the ground source a laboratory.
And you can see that the detector more or less occupies the space of a three story building.
So these are getting to be rather large instruments. The alternative strategy for reaching very large detectors is using liquid argon.
And this is what I work on. And I'll show some results from this program later.
So we've had several iterations getting bigger and bigger.
And at the moment we're building this instrument which is called Darkside 20 K Underground at Gran Sasso.
This object is eight metres on a side. You can sort of see for scale.
Here are some nice pictures of people in front of our cryostat under construction.
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
of currently running experiments in the region above the centre of mass energy reach of the LHC.
And the ultimate aim of this program is to build a kiloton scale detector.
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.
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.
So actions were positive to solve the strong CP problem.
So it's a dark matter candidate that also solves other issues in particle physics.
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.
And axion converts to a photon and you detect that photon detector.
There are experiments looking with LHC magnets at axion coming from the sun,
their experiments looking at axons coming from the gravitational halo of our dark matter halo of our galaxy,
their experiments trying to produce axioms and then detect them.
Uh, and then there's kind of a big effort in detecting axion induced RF.
And so what this figure shows are the constraints on the axion photon coupling as a function of the mass of the action.
And this yellow band shows the region where the QCD and the dark matter axion could be the same thing.
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,
you know, current constraints are shown in the solid colours.
And, you know, future projected sensitivities are shown in these kind of lighter, uh, pink colours.
And so you can see that experiments are working hard to reach down to that QCD axion line.
And looking for axion photon coupling coupling. It's not the only thing you can do.
You can also look for axion electron coupling.
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.
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.
And you can see that this is a strategy that can reach perhaps as low even as ten to the -18 GV.
So moving up in this mass scale. I'll talk next about sterile neutrino candidates.
So sterile neutrino dark matter could make up something like a third of the dark matter based on astrophysical constraints.
And it's possible that sterile neutrino dark matter could scatter with electrons because of sterile neutrino electron neutrino mixing.
There are strong constraints on the electron sterile neutrino mixing angle U4 squared from beta decay.
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.
And so based on looking for that deformation and various beta decay experiments that
direct constraints shown in the top here on U4 squared that are greater than about 1%.
There are also strong constraints from indirect detection experiments, which look at the X-ray spectra coming from,
for example, dwarfs for little galaxies and look for deformation in that spectrum.
And these constraints are very strong. So these constraints are down at the level of maybe ten to the minus ten on U4 squared.
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.
And that's a interesting and hot topic in the field, what that might be.
And so we actually just published the first search for sterile neutrino electron scattering
in a direct detection experiment where we're sensitive in sort of the 10 to 20 uh Kev range,
where the sensitivity from a 50 kilogram dark matter instrument actually exceeds that of the beta decay searches.
And so we're now building a 50 ton experiment.
So it's going to will be quite interesting to look for this channel in the next generation of direct detection searches,
which are a factor of a thousand larger and target mass. Okay, so that's sterile neutrinos moving up in mass scale.
Another possibility, of course, is that perhaps dark matter.
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.
And there are models that seek to explain that. Those are typically kind of termed asymmetric dark matter models.
And those kinds of candidates predict dark matter in the Kev to GeV or perhaps just above GeV scale.
And so experiments can look for absorption processes of Kev scale,
dark matter or scattering processes of movie scale dark matter or GeV scale dark matter searches.
The most sensitive results come from looking for not only the effect of the dark matter scattering, but also nuclear dissertation.
And so all of those kinds of searches are pushing down into this interesting parameter space.
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,
uh, comparable to some of the accident results that I showed earlier.
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.
So another interesting and creative idea that I think Firkins might have been quite interested in is, well,
if we have all this dark matter around and we have all these cosmic rays raining down on us,
what would the underground muon telescope see if dark matter and cosmic rays interact?
And there are fun ideas about this. So one possibility, of course, is that cosmic rays scattering on dark matter would lose energy.
And this would modify the cosmic rays spectrum that you measured.
So this shows the electron data from AMS.
And if you include dark matter electron scattering that changes the prediction for the spectral shape you get this blue dashed line.
And so you can actually use this as a kind of reverse direct detection.
So if dark matter were there scattering with a cross section above x, how would it change the cosmic ray spectrum.
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.
You could also say, well, if dark matter is scattering off these cosmic rays, it must be gaining energy.
So you have dark matter up scattering.
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.
And so you can ask what kinds of constraints you get if you consider those effects.
And in this range, actually super K has some of the strongest constraints on movie scale dark matter.
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.
And that kind of candidate can be produced non thermally in grand unified theories like those that
motivated Sudan to could be produced in primordial black hole radiation or extended thermal production.
And the signature here is that those kinds of interactions may scatter many times as they travel through your large detector.
And so this shows the current constraints on that cross-section versus mass.
And you can kind of see we're reaching all the way up to the Planck scale here.
So at this point, I'll wrap up and just make one remark about complementarity with colliders.
So in collider experiments, the signature, of course,
is trying to produce dark matter candidates in collisions of ultrahigh energy proton proton collisions.
And you don't see the dark matter particles that are produced flying out,
but you see some missing energy or some single jet showing momentum imbalance.
And if we try to convert limits on branching ratios and translate those into limits on cross-section versus mass,
the kinds of plots I've been showing, the direct detection constraints are shown in these solid shaded regions.
These are for two different flavours of production models and the science reach of kind of future direct detection experiments.
The ultimate direct detection experiments we can imagine are shown in these dashed curves.
And the science reach of future accelerators. High luminosity LHC are shown in the solid coloured curves.
And what I wanted to illustrate with this is the comment that with these kinds of experiments,
we can look above the centre of mass energy, even of the most ambitious future accelerator we can imagine.
And so I'll close with this thought about complementarity,
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.
And I want to say thank you very much to Mandy Cooper Sarkar for lending me this book where I read this very,
very interesting essay, um, where in an essay on particle physics in the future,
uh, the, the comment was made here that it's possible that the long term future of experimental particle
physics may be with non accelerator experiments with which Don Perkins began his career.
Uh, and I'll insert my own, uh, aside here because they can reach, you know,
ultimately higher energies than anything we can imagine building here on Earth.
And this was a trend presciently anticipated by Don towards the end of his career, who moved in that direction.
And so I'll close with final thought. Uh, you know, as a researcher looking for dark matter, uh,
May Perkins is optimism for particles and radiations which bombard us from outer space with new discoveries being made on an almost daily basis,
continue to be justified in the search for dark matter. Thank you.