STFC-RAL-CR03 R61: Bye.
STFC-RAL-CR03 R61: Okay, yeah, we are pleased to see so many people joining the seminar. Welcome, everyone. We would like this continuation of exploring the cross-disciplinary topics.
STFC-RAL-CR03 R61: And technology at, 3 extra years from Excel, for fundamental searches. So we'd like to welcome today, give a warm welcome to our colleague from CLF, Jack and Dave.
STFC-RAL-CR03 R61: Thank you very much. I want to start by kind of introducing myself and saying I'm a staff scientist here at the lab. I'm also a plasma physicist. And so, for me, a lot of the physics that I'm talking about today is actually stuff I'm quite new to.
STFC-RAL-CR03 R61: So I guess bear that in mind in terms of the introduction, please bear with me in those places. This is a project that I started when I was a postdoc at the University of Oxford in a group of John McGregori.
STFC-RAL-CR03 R61: That I continue to collaborate with, along with Bob Bingham here at the lab.
STFC-RAL-CR03 R61: You might also see from the PPD side a few familiar names at the bottom of the authors list.
STFC-RAL-CR03 R61: In particular, Ian Shipsy, Solomonist, who sort of was involved in the conception of this work, and Sue Visitar, who I think might be a name that a few people are aware of as well.
STFC-RAL-CR03 R61: In terms of an overview of what I'm going to discuss, I'll begin by introducing axioms and axion-like particles, and giving an overview of
STFC-RAL-CR03 R61: How people look for them.
STFC-RAL-CR03 R61: I'll talk through the experiment design for this particular search.
STFC-RAL-CR03 R61: And I'll run through some experimental data from some initial work from the European pre-electron Laser.
STFC-RAL-CR03 R61: I'll touch briefly on a follow-up experiment that we performed, actually, just last year in October, at Sakla in Japan, but the analysis is still ongoing for that, so it's going to be quite brief.
STFC-RAL-CR03 R61: And then at the very end, I will switch gears and use the first four bullet points to motivate some ideas about fundamental physics with high-powered lasers.
STFC-RAL-CR03 R61: So…
STFC-RAL-CR03 R61: In terms of an introduction, axions are a hypothetical particle not included in the standard model, which solves the throng CP, or charge parity problem.
STFC-RAL-CR03 R61: And are a cold dark matter candidate.
STFC-RAL-CR03 R61: So these are things that were initially postulated to explain the absence of charge parity violation in strong interactions.
STFC-RAL-CR03 R61: We know this is a conserved quantity, because otherwise, there would be a neutron dipole moment, which constrain this to be small experimentally.
STFC-RAL-CR03 R61: It arises from a spontaneous breaking of PQ symmetry.
STFC-RAL-CR03 R61: And on the right-hand side, you can kind of see a bounds plot, which is like a map of this field.
STFC-RAL-CR03 R61: Sorry for PPD colleagues, this might be a bit low level, but I can see a few faces from different departments in the audience, so I'll kind of explain what this actually is. The x-axis follows the axiom mass, this kind of encodes the energy scale associated with an interaction involving axiom.
STFC-RAL-CR03 R61: The y-axis is the axion-photon coupling, so this tells you something about the probability of producing an axion in an interaction of two photons.
STFC-RAL-CR03 R61: And drawn on this plot are areas of exclusion.
STFC-RAL-CR03 R61: So these are regions where people have looked by various different means, and they haven't found any accidents.
STFC-RAL-CR03 R61: And the way that you make progress in this field is you go and look somewhere, and if you don't find anything, you draw a new area on this diagram.
STFC-RAL-CR03 R61: And then if enough people look at a place where they don't find anything, then I guess eventually people will just stop looking. Another thing to notice on this diagram is this vertical or diagonal band going through it, labeled the QCD axiom band, and that's because in QCD,
STFC-RAL-CR03 R61: Axioms follow this defined scheme. But string theory predicts axiom particles or else, and these kind of appear anywhere on this parameter space.
STFC-RAL-CR03 R61: In terms of a bit of history, in this field, people have typically seen light axioms with masses of around a microEV as a natural dark matter candidate.
STFC-RAL-CR03 R61: And laboratory therapies have traditionally targeted this light axiom window.
STFC-RAL-CR03 R61: A motivation for the study that I'm talking about today
STFC-RAL-CR03 R61: is, some theoretical interest in heavier accents. Which comes about
STFC-RAL-CR03 R61: through a recent suggestion that if B2 symmetry is broken after inflation, so you tweak the cosmological timeframes.
STFC-RAL-CR03 R61: Then axioms would also be produced in the decay of so-called domain walls.
STFC-RAL-CR03 R61: And taking this initial contribution into account.
STFC-RAL-CR03 R61: shifts the favored mass scale for axioms to be dark matter above 10 to the minus 2EB. So I can't go into much more detail about that, but this paper here by Subir is a good place to start, I think, if you want to kind of see more about that sort of theoretical motivation for this stuff.
STFC-RAL-CR03 R61: In terms of the lay of the land when it comes to search strategies, there are a variety of complementary methods to look for axioms within this field, and these range from the astrophysical.
STFC-RAL-CR03 R61: So, lab detections and lab generation.
STFC-RAL-CR03 R61: As for eclipseical bounds, as shown on the plot on the right-hand side in blue.
STFC-RAL-CR03 R61: These involve things like, involve introducing Axie and loss terms to seller models and seeing when they start to break down.
STFC-RAL-CR03 R61: Lab detection models are shown in red and purple.
STFC-RAL-CR03 R61: And these are, techniques where you look for axioms which are produced from, say, the sun, or some…
STFC-RAL-CR03 R61: Astrophysical source, but you try and look for a signature of them in the lab.
STFC-RAL-CR03 R61: At the very bottom of the scale, you have lab generation experiments. These are techniques where you seek to both generate and possibly detect axios, really.
STFC-RAL-CR03 R61: And I think the way that I certainly see this field is these things exist complementary to one another, in the sense that astrophysical bounds are by far and away the most sensitive, but lab bounds have the lowest model dependence, so it's worthwhile using all these different techniques to look for axioms if you want to make some progress.
STFC-RAL-CR03 R61: I'm going to run through some examples of different axion search strategies, and the first couple of these will be, what I call lab detection experiments. So these are where we look for axions produced elsewhere with some terrestrial apparatus.
STFC-RAL-CR03 R61: And in the first case, we're talking about CAST, or the CERN Axiom Telescope. This is a so-called helioscope, in the sense it looks at axioms produced in the Sun.
STFC-RAL-CR03 R61: And the way that it does this is through something called the Primacup Effect.
STFC-RAL-CR03 R61: And you can understand that through this interaction that I'm doing for the software you see here.
STFC-RAL-CR03 R61: And what this tells you is that if you have parallel electromagnetic fields, then there is some probability to generate an axiom.
STFC-RAL-CR03 R61: Or by the inverse process, if you have a strong magnetic field and an axiom has some probability you'll generate a photon.
STFC-RAL-CR03 R61: And that's exactly how this thing you see on the right-hand side here works. This is effectively an electromagnet, that's a blue thing.
STFC-RAL-CR03 R61: And it's arranged on some rig that's got, like, a telescope, so that it can trace the path of the sun in the sky to something like an hour a day.
STFC-RAL-CR03 R61: And it's during that time when axiom propagates down the bore of the magnet.
STFC-RAL-CR03 R61: There's some probability it generates an axiom.
STFC-RAL-CR03 R61: And you detect this, with an optical detector at the bottom of the magnetic.
STFC-RAL-CR03 R61: So this is great in the sense that you have very competitive bounds, you see in red at the bottom there.
STFC-RAL-CR03 R61: But the geometry of the setup suppresses sensitivity to axioms above an EV. So for this heavy axiom window that I motivated earlier.
STFC-RAL-CR03 R61: This is currently not a useful strategy.
STFC-RAL-CR03 R61: There have also been people who've argued that model dependence associated with the high temperature and finite plasma frequency in solar plasma might suppress axiom production, and so the bounds there will carry a degree of model dependence.
STFC-RAL-CR03 R61: Another example of one of these lab detection experiments is ADMX.
STFC-RAL-CR03 R61: This is a telescope that looks for axioms from the local galactic halo, and the idea is you have a microwave cavity surrounded by superconducting electromagnets, and you tune the Q-factor of that cavity to scale through different axion masses.
STFC-RAL-CR03 R61: You can see the bounds plot for this particular experiment at the bottom here, and it's got very impressive sensitivity down to QCD relevant couplings for this light axial.
STFC-RAL-CR03 R61: So, those are a couple of examples of detection experiments. I'll discuss now a couple of generation experiments.
STFC-RAL-CR03 R61: The first, and possibly the most famous of these, is a so-called light shining through a wall scheme.
STFC-RAL-CR03 R61: And the motivation for this, right, the express motivation, was that generating any vaccine in the lab removes model dependence from the generation mechanism.
STFC-RAL-CR03 R61: You can see a…
STFC-RAL-CR03 R61: diagram of the scheme here. Sorry, I'm not being disappointed before this end up screwing up. It effectively consists of an optical laser.
STFC-RAL-CR03 R61: which passes down the core of an electron effect. And again, through the Primarkov effect, there's some probability that you generate an axiom through the electric field of that photon and the magnetic field of the magnet.
STFC-RAL-CR03 R61: These axioms propagate along the same path as the laser, and somewhere in the setup, there's a little paper. The laser's blocked, but the axioms don't care, they go straight through.
STFC-RAL-CR03 R61: And there's some probability that in a second segment of the electromagnet, through the inverse process, photons are regenerated, and you detect these.
STFC-RAL-CR03 R61: So, you reduce model dependence, but you suppress your sensitivity, right? Because there's a generation and a regeneration process, or
STFC-RAL-CR03 R61: Reconversion process.
STFC-RAL-CR03 R61: You suppress the sensitivity by a factor that goes with something like the company tester, which is quite a small number.
STFC-RAL-CR03 R61: For this optical scheme, they're not sensitive to, the high masses that, again, I motivated earlier, because you're using visible photons.
STFC-RAL-CR03 R61: Another example of a lab generation experiment is a so-called vacuum pyrofering scheme, and the idea here is that you look for induced electricity in a laser that propagates through a magnetic field using interaction with the vacuum.
STFC-RAL-CR03 R61: This is sensitive to some QED effect, so the original motivation, but axion-boton coupling can also introduce, some elasticity here. So, I guess for CLF colleagues, this is like doing parallel rotation on the vacuum.
STFC-RAL-CR03 R61: you're sensitive to the generation of axons, but not the regeneration of photons, so you don't have that, suppression factor I mentioned previously.
STFC-RAL-CR03 R61: But there's a hard limit on the overall sensitivity, because at some point, the 3D effects come in, and those will certainly signal.
STFC-RAL-CR03 R61: So, that was maybe a bit whirlwind, but that was kind of my broad overview of the field, as I view this, at least. The next thing I want to discuss is exactly how the scheme that I'm talking about works.
STFC-RAL-CR03 R61: And the motivation of this scheme is effectively to take that optical light showing for a wall experiment and extend it up to X-ray energies.
STFC-RAL-CR03 R61: So again, we rely on the primacop effects, but in this case, we have an x-ray laser.
STFC-RAL-CR03 R61: And I think for the purpose of this talk, you can just view the X-ray laser as a coherent source of plane waves that are in the X-ray energy range.
STFC-RAL-CR03 R61: And we have those impinge on this double bristle setup that you can see here.
STFC-RAL-CR03 R61: So you've got those plane waves impinging on the crystal, they have a defined polarization, which is directed up, down in the diagram, and then you have your crystal.
STFC-RAL-CR03 R61: And we diffract our crystal planes that are perpendicular.
STFC-RAL-CR03 R61: to the surface of the crystal.
STFC-RAL-CR03 R61: To create this part, the photons that you see here.
STFC-RAL-CR03 R61: The electric field comes from the interatomic field between the lattice planes in the crystal. So this field is very strong, it's on the order of 10 to 11 volts per meter. The equivalent magnetic field will be a kilotesla. So for the half millimeter of germanium crystal that we use for the UX valve.
STFC-RAL-CR03 R61: We get a path integrated field, which is equivalent to 25 pesimetres, which is about as good as you can access with the best selection.
STFC-RAL-CR03 R61: The way the scheme works, I should say, is effectively, you generate actions in the first crystal, these co-propagate with the photons to the shield. Again, the shield blocks the X-rays, the axons don't have, they propagate straight through. And in the second crystal, they recomvert to photons, X-ray photons in this case, which you detect out through.
STFC-RAL-CR03 R61: This work builds on previous experiments performed on a synchrotron by Emmaggio.
STFC-RAL-CR03 R61: But the studies that I'm looking at were the first to use X-ray lasers to do this kind of process. You can see at the bottom here a picture of the setup.
STFC-RAL-CR03 R61: There's your generation crystal and your reconversion crystal, and the shield will sit somewhere between them. The overall spatial scale of it is very much tabletop. It's 4cm, 5cm or so. But that's a bit misleading, because of course, in order to generate the X-rays, you need a big old linear accelerometer. So, for those of you that aren't aware.
STFC-RAL-CR03 R61: an expo.
STFC-RAL-CR03 R61: It's an X-ray light source which is based on an electron limit.
STFC-RAL-CR03 R61: You need about 10 GeV electrons, and these are directed through what are called angulators, which are essentially periodic dipole magnets that cause the electron beam to oscillate, emitting radiation.
STFC-RAL-CR03 R61: And through a process which is called SARSI, or self-amplified Spontaneous Emission.
STFC-RAL-CR03 R61: The emitted radiation interacts with the electrons, causing microbunching.
STFC-RAL-CR03 R61: The microbunching amplifies the light, so you end up with this coherent emission process, and it's characterized by very short pulse durations on the order of 10 femtoseconds or so.
STFC-RAL-CR03 R61: You can add to this kind of scheme something called self-seeding.
STFC-RAL-CR03 R61: And the idea here is that you put an X-ray monochromator in the beam, as shown just here.
STFC-RAL-CR03 R61: The monochromator,
STFC-RAL-CR03 R61: only lets X-rays of a certain energy through, so you impose a preferred length scale on the micro brunches, and you end up with, broadly speaking, monochromatic light, as you see here.
STFC-RAL-CR03 R61: People talk about the quality of these instruments in terms of a unit, which I
STFC-RAL-CR03 R61: uses the name Brilliant, which is a bit of a strange one, but effectively this just says lots of photons per second per unit area, per unit angle, per 1% bandwidth. So, lots of photons at one color.
STFC-RAL-CR03 R61: At one time, in one space.
STFC-RAL-CR03 R61: These are, high-impact science machines, which are used for things from drug discovery to fundamental physics.
STFC-RAL-CR03 R61: And the way that you get time on them is you bid for access in really quite competitive pools.
STFC-RAL-CR03 R61: So, we used a configuration at the European Free Electron Laser, in particular the HED or HIVEP instrument, and we got 3 days, so that's 72 straight hours, 24 hours a day, of beam time awarded over the Easter bank holiday in 2023.
STFC-RAL-CR03 R61: We operate the instrument in a self-seeded mode. The photon, or the X-ray parameters, are shown here. I think probably the key ones to note for this discussion are that it was about 10 kiloh photons.
STFC-RAL-CR03 R61: And the pulse duration is really quite short, on the order of 10 femtoseconds or so.
STFC-RAL-CR03 R61: The instrumentation which you have available for you at HED
STFC-RAL-CR03 R61: Is useful in the sense that it allows you to
STFC-RAL-CR03 R61: place constraints on input, output… input and output X-ray flux and absolute sounds. So you can see a kind of a cartoon of the overall instrumentation that was installed here. You've got
STFC-RAL-CR03 R61: The incoming X-ray beam from the left-hand side.
STFC-RAL-CR03 R61: And this is directed through an X-ray gas monitor, which is a passive measure of energy per pulse impinging on our crystal setup that you see here.
STFC-RAL-CR03 R61: The crystal setup for EU Expo was immersed in a 2-meter scale vacuum chamber, that's this thing you see here.
STFC-RAL-CR03 R61: And any regenerated photons that we might create through axioms were made to propagate through these baffles into a 6-meter scale flight tube. So this was just an evacuated tube to reduce background.
STFC-RAL-CR03 R61: onto a pixel… hybrid pixel detector called a Young Crowd4,
STFC-RAL-CR03 R61: Which was the primary detoxer for this scheme of work.
STFC-RAL-CR03 R61: This slide is a bit dense, I'm sorry about that, but it's very… quite difficult to condense this to less information than I've got here. And what you see here are a couple of equations that encode the probability of axiom conversion and the mass range for a search for axioms.
STFC-RAL-CR03 R61: If we look at the probability equation, first of all, you can see that the probability, of course, depends on the coupling constant.
STFC-RAL-CR03 R61: The effective electric field within the crystals.
STFC-RAL-CR03 R61: And then the length scale associated with photons propagating through the crystal, and then the cosine of the bracket.
STFC-RAL-CR03 R61: So, looking at these equations, this equation here, I think the key thing to pull out of it is the majority of the terms of this equation you have no control over. These are just aspects of the solid-state physics that it takes with a fraction through the crystal.
STFC-RAL-CR03 R61: The one thing that, as an experimentalist, you can change is the crystal thickness. And so that's our one dial we have to drive up the sensitivity of surface.
STFC-RAL-CR03 R61: The second set of equations encode the mass range for a search. So an interesting aspect of this double-crystal monochromator-type axion detection instrument is the fact that you're sensitive to two different sorts of mass range, depending on how you set it up.
STFC-RAL-CR03 R61: In the first instance, if you set both crystals to their Bragg angle, you're sensitive to a broad range of axiom masses, which is set by M gamma, which is the plasma frequency of the valence electrons within the crystal, so again, just some sort of, say, physics effect.
STFC-RAL-CR03 R61: And also, K gamma, which is the photon revision.
STFC-RAL-CR03 R61: So, this will give you a broadband sensitivity with some cutoff that just depends on the plasma frequency of your valence electrons, after which you're not going to get generation.
STFC-RAL-CR03 R61: If you detune both crystals by an equal amount from the Bragg angle way, you end up, in the regime which is encoded by this equation here, where you're sensitive to a narrow band of axiom masses, which is… increases in proportion to the square root of the detuning angle.
STFC-RAL-CR03 R61: And in the work that I'm discussing here, we'll exploit both of these regimes, both the broadband on-brag search and the off-brag narrowband search regimes.
STFC-RAL-CR03 R61: This, again, I think might be targeted more towards colleagues that are used to X-ray light source work, but a question that you often get asked when you present this kind of stuff is, why do we opt for this isoteric Maui geometry diffraction regime?
STFC-RAL-CR03 R61: And in this Lowe regime, we scatter off lattice planes that are normal to the crystal surfaces shown here, and due to an effect called the Bormann effect, we get anomalous extinction within the crystals, so the photons can propagate quite a long way before they're absorbed, compared to the conventional mass attenuation.
STFC-RAL-CR03 R61: If you compare that to the Bragg diffraction case.
STFC-RAL-CR03 R61: Where you scatter off planes parallel to the crystal surface, then you end up
STFC-RAL-CR03 R61: losing all your photons, they get diffracted, essentially, within a very short length scale. So just by swift swapping to this Laureue geometry, you end up in a regime where you increase your sensitivity quite dramatically.
STFC-RAL-CR03 R61: I should say, actually, that this figure here, this was the original proposal for an x-ray-based
STFC-RAL-CR03 R61: Axiomed light shining through a wall experiment.
STFC-RAL-CR03 R61: From a fella called…
STFC-RAL-CR03 R61: Buckmeller, and Bookmeller, as a kind of a weird throwaway paragraph in the final part of this paper, does actually say, well, we could do this, we could do the Lowe case. So, I think Bookmello is the person that should really be attributed as the person who came up with this scheme that we're using today. But…
STFC-RAL-CR03 R61: We tend to attribute Yamashi, who was the guy who did this on a single front posture.
STFC-RAL-CR03 R61: Another bit of diffraction physics, I promise this is getting somewhere, so just bear with me, is for the…
STFC-RAL-CR03 R61: expel work.
STFC-RAL-CR03 R61: We have to consider the fact that the pulse duration is short compared to the timescale for
STFC-RAL-CR03 R61: X-ray diffraction through the crystal.
STFC-RAL-CR03 R61: And that's relevant because it basically emerges that you have to consider two measures of
STFC-RAL-CR03 R61: the… what's called the, angular spread, I guess, of X-rays transmitted through the crystal. So the way to view this is through this diagram that you see on the left-hand side here. You have light that impinges on the crystal, and you scan through the Breck angle.
STFC-RAL-CR03 R61: And you measure the output, like, you end up with something like this here, which is like a transmission curve.
STFC-RAL-CR03 R61: And conventionally in the field, people have called this Both the rocking curve, and the dog.
STFC-RAL-CR03 R61: In this work, we define the rocking curve as the actual angular spread of X-rays which can make it through the crystal.
STFC-RAL-CR03 R61: Whereas the Darwin width would be the predicted angular spread, if you use a theory that neglects any sense of time dependence within the crystal.
STFC-RAL-CR03 R61: Now, you could use a dynamical, simulation to back out.
STFC-RAL-CR03 R61: exact values for this rocking curve width, but that gets really into the weeds. And so the approach that we take in this work is we assume a transform limited pulse, and in that case here, the products of the expel pulse duration and the rocking curve width
STFC-RAL-CR03 R61: is equal to a factor of order unity of this expression here, where lambda is just the X… the photon, sorry, the wavelength of the X-ray then binging on the Bristol.
STFC-RAL-CR03 R61: And the reason I've gone through that, is effectively because the original theory, which I introduced a couple of slides ago by Yamagi et al, et al. before that, assumes, that these two quantities, the rocking curve and the Darwin curve.
STFC-RAL-CR03 R61: are the site.
STFC-RAL-CR03 R61: And so, if we take into account time dependence.
STFC-RAL-CR03 R61: we have to modify the generation… the axon generation equation, as you see here.
STFC-RAL-CR03 R61: And what that ends up doing is introducing this extra factor shown in purple, which amplifies the probability of conversion by something like a factor of 100. So, that brilliance number that I talked about before.
STFC-RAL-CR03 R61: What that does is it actually amplifies the probability of action generation compared to the synchron case, which is quite nice.
STFC-RAL-CR03 R61: So, that's kind of the idea in principle. What did it look like in practice? I'm now going to go through some data from our initial runs of the European electron laser.
STFC-RAL-CR03 R61: And the first set of plots I'm going to show you are actually a relatively negative story that we found in the initial hours of that beam time. And this was that maintaining the alignment of that double crystal setup that I showed you was really quite challenging, due to the heat load which was imposed on them by the exfil.
STFC-RAL-CR03 R61: And that point is made… oops, sorry.
STFC-RAL-CR03 R61: is made, I think quite clear by this plot here, which shows the power transmitted through the double crystal setup as a function of time, with that shield removed.
STFC-RAL-CR03 R61: And what you see is that in a time scale in the order of minutes, if you have the full flux of the fail, you lose alignment, and the setup ceases to be sensitive to axials.
STFC-RAL-CR03 R61: We did find we were able to maintain stable alignment
STFC-RAL-CR03 R61: If we reduce the flux on the crystals by something like a factor on the order of 10 to the 3, but the complications associated with finding that stable operating machine reduced the available search time below that 3 days that we had, which already, frankly, wasn't very long to do this kind of work.
STFC-RAL-CR03 R61: And so we limited our search to acquiring data at the Bragg angle, and four discrete, detuning angles.
STFC-RAL-CR03 R61: I've got a throwaway comment on this slide saying we can mitigate this in various ways, and I propose active calling here. That wasn't what we did, but we used a different technique at Sackler, so we'll come back to that towards the end of the talk.
STFC-RAL-CR03 R61: So because of this heating, we had to use a multi-step process for data collection, where we effectively characterized the transmission through the double crystal setup, with the shield out, both crystals teaching for the Bragg angle.
STFC-RAL-CR03 R61: Then detuned the search angle, put the radiation shield in, collected some data for somewhere 10 minutes, and then did the whole thing in reverse and got some more characterization data, and used this
STFC-RAL-CR03 R61: to, run by run, characterize the efficiency of transmission through the setup. So this is just in the systematics, but it's really explaining, in those non-ideal conditions, how we're able to continuously monitor the efficiency of this process in order to get a set of bounds out.
STFC-RAL-CR03 R61: Now, kind of, I guess, the key thing to note is that we didn't observe any events consistent with Axiom production through the entire beam time, and that point is made with these two plots you see here.
STFC-RAL-CR03 R61: The one on the left-hand side is a histogram, which shows,
STFC-RAL-CR03 R61: All events that you can associate with photons above one hectare electron volt for…
STFC-RAL-CR03 R61: All the ones where we're looking for axions, and this is compared in black to a dark count, which is just 24 hours of…
STFC-RAL-CR03 R61: Thermal noise of the sector, essentially.
STFC-RAL-CR03 R61: And what you see is that these two things match. There's very little signal, except maybe in this pedestal here, which you can associate with the bell at all.
STFC-RAL-CR03 R61: On the right-hand side, you see what's called a hitmap. So this just shows, as the colored points, any photons which impinge on the region of the detector where we would expect axioms to fall.
STFC-RAL-CR03 R61: And overlaid on that is what I've called the X-ray spot. So this is effectively just a spot which you see when you tune both crystals to the Bragg angle.
STFC-RAL-CR03 R61: And you remove the… the shield.
STFC-RAL-CR03 R61: So, an event consistent with X, with,
STFC-RAL-CR03 R61: that's in production should have the same energy as the seed photons, and it should also lie on that X-ray spot. We didn't see any events that are consistent with both of those two criteria during the entire beat.
STFC-RAL-CR03 R61: So, based on that null result, we were able to impose, 90% confidence interval bounds on the coupling strength, and the way that we did that was effectively, as described in this paper here.
STFC-RAL-CR03 R61: Assuming Poisson statistics, taking a 90% confidence interval with a mean of zero, which works out as 2.3 detected events per search.
STFC-RAL-CR03 R61: And putting that all together, you get the set of bounds that you see here. So just to remind you, this setup is sensitive to
STFC-RAL-CR03 R61: Axiom masses in huge, discrete ranges.
STFC-RAL-CR03 R61: First of all, we have a broadband range, which is, accessed when we tune both crystals at the Bragg angle.
STFC-RAL-CR03 R61: And then we have these narrowband ranges that you see here, which are what we get when we detune both crystals the same amount in the break angle.
STFC-RAL-CR03 R61: And the Excel result here is compared to a number of different lab detection searches. For this heavy mass range in particular.
STFC-RAL-CR03 R61: You can see that there are two accelerated searches, NA64 and BAR, which were a little more sensitive.
STFC-RAL-CR03 R61: That's, that's…
STFC-RAL-CR03 R61: Which were a little more sensitive than this initial experiment that we carried out at the European Expo.
STFC-RAL-CR03 R61: And I just want to say a few words about those particular experiments, in the sense that I really think they're quite complementary to the work that we did.
STFC-RAL-CR03 R61: So, BA Bar.
STFC-RAL-CR03 R61: It's a scheme which involves, recasting some bounds on a dark photon experiment to look for axioms.
STFC-RAL-CR03 R61: And this is a scheme which is sensitive to spontaneous acts in the case, so you're looking for processes encoded in the Feynman diagrams, like the ones you see here.
STFC-RAL-CR03 R61: The NA64 experiment, again, it's a beam dump experiment in this case, so you…
STFC-RAL-CR03 R61: take 100 GeV electrons, and in the NA64 beam dump, you produce axioms for the primoguff effect.
STFC-RAL-CR03 R61: And you search, again, for the signature of spontaneous acts indicate, and also missing energy in the calorie data. And so I guess the point I'm making here is because we directly reconvert to photons in the light shining through a wall scheme, it's really quite a complementary scheme to these two that you see here.
STFC-RAL-CR03 R61: In terms of this work, the final comment I want to make is we have… I was going to…
STFC-RAL-CR03 R61: two, three months ago, got back from Sappler in Japan, where we performed a follow-up experiment building directly on the EUX file configuration.
STFC-RAL-CR03 R61: In this setup, we operated passively
STFC-RAL-CR03 R61: in air, so we didn't put the crystals in a vacuum. And this increased our background, but it was a deliberate trade-off in the sense that because of the passive cooling, we could increase our, photon flux by a factor of, let's say, 10 to the 3, not 10 to the 4.
STFC-RAL-CR03 R61: Compared to the EU Expo search.
STFC-RAL-CR03 R61: In addition to that.
STFC-RAL-CR03 R61: We also used thicker germanium samples, so we went to 1.5 millimeters as opposed to the half millimeter of germanium that we had in the previous search.
STFC-RAL-CR03 R61: And taking those two steps together yields a set of bounds that you see here. So again, in pink, we have the Sappler bounds, this is on Bragg, this is off Bragg.
STFC-RAL-CR03 R61: There's a zoomed-in view of the off-brag searches here.
STFC-RAL-CR03 R61: And you see that we're really getting quite, quite close to the QCD axiom band in this kind of 3KEV range.
STFC-RAL-CR03 R61: And also that in the ONBRAG case, we're also… we're able to achieve higher sensitivity than any existing direct detection search in the,
STFC-RAL-CR03 R61: Let's say 1 to 10 EV range.
STFC-RAL-CR03 R61: So you could improve this in future, you could improve the signal to noise, and that would give you a factor of few in terms of sensitivity, and you could obviously implement a continuous scan of angles within some range, if you were maybe targeting some gap in the cosmological bounds, for example.
STFC-RAL-CR03 R61: But it's not going to be significantly better. That's where I think this setup as it currently stands is going to… going to stand.
STFC-RAL-CR03 R61: So that is everything I want to say about expels. The next little bit of the talk is kind of a change of gear.
STFC-RAL-CR03 R61: But I do think the two sections sit pretty well together, in the sense that what I just showed you was a scheme to look for axions, so to exploit a particular coupling.
STFC-RAL-CR03 R61: Which, it is quite challenging to access with a conventional research accelerator.
STFC-RAL-CR03 R61: But what we saw from the expel work is that this was actually relatively low-hanging fruit for an X-ray light source. It was a coupling that was not that difficult to access, and I say that in the sense that it's not that we didn't work hard, it's that we're a relatively small team, and we had 3 days of time to set this up and collect all our data.
STFC-RAL-CR03 R61: And I think when we move into the next few years with high-powered lasers, perhaps we as a community should be thinking more about ways in which we can exploit couplings that are challenging on accelerators, but might represent lower-hanging fruit for different drivers. That's the kind of… the parallel between the two schemes that I see here.
STFC-RAL-CR03 R61: And as a bit of background, I kind of want to motivate this in terms of my own story. The reason that I came to the Brotherhood, one of the reasons, was because we are currently engaged in building a new high-powered laser, that's what all the building work behind you is.
STFC-RAL-CR03 R61: And this is called Wellcome 2020. It's an optical laser system, so micron light, that's equivalent to about an EV, both on energy.
STFC-RAL-CR03 R61: It will deliver 1 petawatt per pulse in a… in a 25 length.
STFC-RAL-CR03 R61: Has a large aperture, focused down to large aperture initially, but this is focused down to 5 microns, enabling peak intensities on the order of 10 to the 23 watts per centimeter squared.
STFC-RAL-CR03 R61: The design explores splitting the beam into equal arms, enabling photon-photon interactions, for example. And the main 20 petawatt beam is also complemented by a 1 petawatt class laser, which is optimized for laser wakefield acceleration.
STFC-RAL-CR03 R61: which would give you about 1 GeV electrons, and a beam which is temporally synchronized to the optical pulse, and also has a comparable size and duration. I think that's probably a motivation for using a weight field here as opposed to a conventional accelerator.
STFC-RAL-CR03 R61: So, to kind of give it some context in terms of this field of high-power laser physics, there are a growing number of petawatt-class laser systems worldwide.
STFC-RAL-CR03 R61: One that's, I guess, close to some people's hearts here is TAP, which was the laser system that we just decommissioned here at the Rutherford, which I think was the first of these pedal class lasers, or one of the first.
STFC-RAL-CR03 R61: And…
STFC-RAL-CR03 R61: At the minute, we're kind of pushing the bounds from that sort of petawatt class system to multi-pedawatt class systems, and here you can see a plot of planned, 10-plus petawatt class lasers.
STFC-RAL-CR03 R61: Clf, we're building up in 2020.
STFC-RAL-CR03 R61: There's a project called NSF Opal, which is going on in New York State in the US, and there are the various Eli lasers, throughout Europe, which are at a higher stage of development compared to both Vulcan and Opal.
STFC-RAL-CR03 R61: And here, I've just picked some experiments
STFC-RAL-CR03 R61: which one might consider exploring with these multi-padawatt lasers. Now, this isn't supposed to be some kind of broad map of the field. I'm not trying to pose some phenomenological model to think, what should we try and target? I'm just saying, as an experimentalist, what grabbed my interest?
STFC-RAL-CR03 R61: And so you can see here a scheme for simulated all optical photo scattering involving three laser beams, and this is something that,
STFC-RAL-CR03 R61: Opal, the latest system in the States, is really targeting as part of their science case.
STFC-RAL-CR03 R61: Quantum radiation reaction is this one here, which is interesting in the sense that work is being done. So on the Gemini laser system here at the CLF, colleagues from Imperial College used a scheme where they drove fast electrons against a colliding laser to impose bounds on various models of quantum radiation reaction.
STFC-RAL-CR03 R61: Axiom photon and axion electron coupling, I'm going to talk about that very briefly for a couple of slides before I wrap up.
STFC-RAL-CR03 R61: And I'm sure Bob would want me to call out these, kind of.
STFC-RAL-CR03 R61: longer-term directions in terms of things like exotic atoms, post-Uutanium gravity, gravitational wave detection. I think the list probably goes on.
STFC-RAL-CR03 R61: So, as an example of what you might do, this is a scheme which has been proposed to look for axioms with optical lasers. It involves two orthogonally polarized lasers to generate axioms, then a wall, then a third laser to regenerate photons.
STFC-RAL-CR03 R61: And projected bounds that are relevant for a multi-pack watt laser in Prague.
STFC-RAL-CR03 R61: Shown here, but you could adapt this to other laser facilities, including the one that we're building.
STFC-RAL-CR03 R61: And also, here's a scheme for…
STFC-RAL-CR03 R61: Axion electron coupling, which basically involves directly accelerating electrons in the
STFC-RAL-CR03 R61: Electric fields of two counterpropagating lasers.
STFC-RAL-CR03 R61: And generating axioms by analogy with radiation from accelerated charge.
STFC-RAL-CR03 R61: the projected sensitivities you should look at are really… actually, the blue ones here, that's kind of next generation lasers, the purple is next, next generation. You can see, again, even for this next generation of lasers, you're accessing bounds that I think are competitive with accelerator-based, or reactor-based in this case.
STFC-RAL-CR03 R61: And through different mechanisms, again, so there's that multiple… multitude of approaches type justification.
STFC-RAL-CR03 R61: That is everything I wanted to say, so thanks very much for your attention, and I'd really like any questions you've got.
STFC-RAL-CR03 R61: Yes. Any questions?
STFC-RAL-CR03 R61: Yeah, so clearly your crystals are giving you,
STFC-RAL-CR03 R61: bending power, magnetic fields integrated in a short distance, but they are suffering from heat. If somebody lent you a couple of LHC dipoles, how could you…
STFC-RAL-CR03 R61: So, the kind of conventional light shining through a wall experiments, you could implement a similar scheme with an X-ray laser. The problem would be
STFC-RAL-CR03 R61: Locating that length of electromagnet.
STFC-RAL-CR03 R61: with an expo, and I think…
STFC-RAL-CR03 R61: that's kind of the limitation of this kind of driver, right? They're multi-purpose machines, so you end up with end stations that are designed for, you know, a whole bunch of different jobs, and you get 3 days on it, you get maybe a week on it, maybe 6 weeks if you're lucky. So…
STFC-RAL-CR03 R61: That does really touch on the limitation of this kind of scheme, is that because you're using multi… multi-purpose machines, that kind of permanent installation is something that's challenging. You'd have to build a whole science case around adding extra installations and so on, I guess.
STFC-RAL-CR03 R61: Thanks for the question.
STFC-RAL-CR03 R61: on that, Is it worth…
STFC-RAL-CR03 R61: putting that case forward for a UK expo, and saying, when you build this, or if you build it, we need these stations. Yeah, and some of this stuff could be quite passive, right? So the,
STFC-RAL-CR03 R61: the UK Excel proposal was talking about multiplexed end stations, for example, so you could go to multiple different places at the same time, and you could imagine having this running
STFC-RAL-CR03 R61: For a long time in that kind of scheme.
STFC-RAL-CR03 R61: Thank you.
STFC-RAL-CR03 R61: Probably not a question for you, I suspect, but maybe for somebody else in the audience. Williams published a paper on the 10th of January, arguing that topological features the DeVar Zero naturally without needing an axion at all.
STFC-RAL-CR03 R61: As I've seen her comment on this, I've not published.
STFC-RAL-CR03 R61: I've seen no comment from anybody else. Nobody knows whether it's reasonable or constant.
STFC-RAL-CR03 R61: As far as I'm concerned, whoosh. I'm sorry.
STFC-RAL-CR03 R61: You said there's no need for it, is it? That's what Williams argued, he said. He said, it comes out zero if you take into account topology at QCD, but I didn't… I couldn't follow them. Interesting.
STFC-RAL-CR03 R61: Have you got… you could send us the paper.
STFC-RAL-CR03 R61: Any film, buddy?
STFC-RAL-CR03 R61: I have a really, really naive question. On the heating of the crystals,
STFC-RAL-CR03 R61: And you said you reduced the flux. Is that… how are you reducing the flux? Oh, you just take a bit of silicon in the beam, and that attenuates the number of X-rays, so it's mass attenuation in the silicon. Yeah, makes sense.
STFC-RAL-CR03 R61: Can you actively cool the crystals? Yes.
STFC-RAL-CR03 R61: So, we actually… In parallel with the SACNA proposal, which was designed to be a relatively
STFC-RAL-CR03 R61: Easy, easy way of doing it, so just the passive cooling and air is a
STFC-RAL-CR03 R61: straightforward modification. We submitted a proposal to EUXL to do the same thing with actively called Bristols.
STFC-RAL-CR03 R61: We didn't get anywhere with that particular proposal. I think what it would require is some dedicated synchron time, just looking at heating and trying to understand how we can limit that, and we could use that to
STFC-RAL-CR03 R61: motivate a bid for next felletime, but…
STFC-RAL-CR03 R61: It is a con… ask anyone who works on this kind of machine, the constant kind of…
STFC-RAL-CR03 R61: driver is really getting time. Those three days you really work for, and then you've got to make him count.
STFC-RAL-CR03 R61: You probably said, but what's the physical dimension of these crystals? They're… so the EUX, though, were half a millimetre thick by centimetre by centimeter in area. Okay. You use a small fraction of that. In terms of the area, you use microns.
STFC-RAL-CR03 R61: What sort of temperature did they get to? It doesn't take very much, so, kind of, I had this in the plot.
STFC-RAL-CR03 R61: Crime.
STFC-RAL-CR03 R61: Only back, but…
STFC-RAL-CR03 R61: Here we go. So this is… this is actually the Darwin cut. In reality, it's even thinner, but this is the… the angular acceptance of one of these things in a steady-state fashion.
STFC-RAL-CR03 R61: And you can see we're really talking about a very small fraction of a degree here, so you need to expand your lattice planes by something which is a small fraction of the interatomic space before you start to lose this. And so we were looking at significantly less than a Kelvin degree of heating, it's just that was too much already.
STFC-RAL-CR03 R61: So, I know we chatted about this a little bit before, but the European Expo, because they use that rapid train, there's a big break and stuff, whereas other expos have that continuous train.
STFC-RAL-CR03 R61: would the heating problem go away? Would that all be better with other scales? Because you have that break, you don't have that rapid? I think the equilibrium timescale would be on the order of microseconds to microseconds, probably, so my guess is that's not something you could access with the
STFC-RAL-CR03 R61: The bunch structure on different fellas is not necessarily relevant to the timescale of the cooling process, but active cooling is… there's lots of work to be done in terms of active cooling if we want to push it in that way.
STFC-RAL-CR03 R61: So, in… you've showed various limit plots and so on, in sort of…
STFC-RAL-CR03 R61: future mega-ambitions, have you got any plots for where one could get to?
STFC-RAL-CR03 R61: I think in that heavy region, realistically, you can probably get halfway down the QCD axiom band. That's… without some step change in the way that we do this, that's the limit of the technique. But that doesn't mean… I mean, you saw those couple of papers that I showed you at the end.
STFC-RAL-CR03 R61: that's John Leaker's group at Oxford, really thinking of different ways to do it, so there are people, including John Leeker, working on
STFC-RAL-CR03 R61: ways to change these setups, so you end up with those step changes. But again, this is… I'm looking at Axioms to some degree as an observer from this perspective. A lot of these schemes, they have this thing where you have a really nice idea.
STFC-RAL-CR03 R61: you can do it for 3 days, you can do it for 3 years. It doesn't hugely, hugely change the picture, it's really just the underlying physics of whatever you're using to make your X's.
STFC-RAL-CR03 R61: Sorry, it would… my technique is it does have to be a new scheme if you want to go any further than that.
STFC-RAL-CR03 R61: So, are you planning to request, more beam time at, XL? I think…
STFC-RAL-CR03 R61: We are currently working on the paper, so this… the paper on the Sackler stuff.
STFC-RAL-CR03 R61: There's a draft which has just gone out to our co-authors, and we want to see what the reaction for that is.
STFC-RAL-CR03 R61: The next thing would be to kind of target
STFC-RAL-CR03 R61: a bit of physics that we want to do differently the next time around, and that will be used to motivate a proposal. So, maybe 6 months from now, that's what we'll be trying to do, but I don't know if it will be going for sensitivity or going for
STFC-RAL-CR03 R61: you know, the Umagi paper that was… that people did this on a synchrotron, they scanned continuously in the off… off-brand case. They got a broadband range of masses. So you might want to do something like that instead, where you go for a broadband in masses, so, you know, not just looking at these little spikes in parameter space you're looking at.
STFC-RAL-CR03 R61: A fuller parameter space up to a given sensitivity.
STFC-RAL-CR03 R61: And, yeah, one question. On these kind of experiments, when you said that the model dependency is removed, I was wondering whether
STFC-RAL-CR03 R61: you have an estimate of how many, signal events eventually you would expect, or how is systematics, estimated in those exclusion bands? That's my ignorance.
STFC-RAL-CR03 R61: Well, so, for the XFEL UXPL experiment, I think systematics were…
STFC-RAL-CR03 R61: relatively trivial. We had to worry about asystem at it, but not in the sense that people usually mean in terms of the changing efficiency of the setup, because we were constantly fiddling with it.
STFC-RAL-CR03 R61: But there was no… we didn't see any background, right? There was not a single photon in the whole beam time that you could associate with Axion production.
STFC-RAL-CR03 R61: And that came down to precise time gating, so we gated the detector around the X-ray pulse, and it came down to spatial gating in the sense that we have this spatially resolved detector, and we know where on that detector the face one should end up, and it came round down to energy resolution in the sense that
STFC-RAL-CR03 R61: because the detector is single photon counting, we can go down to… this was a 10KV photon, this was a 3KV photon, so check away the 3KV photons. And all those things together reduce your systematics very long.
STFC-RAL-CR03 R61: For the Sackler experiment, because we were operating in air, the background was higher.
STFC-RAL-CR03 R61: And so in Sackler, what we ended up doing was we saw, like, this uniform distribution of scatter.
STFC-RAL-CR03 R61: which made it through the baffles and the flight tube and so on. And we ended up doing an approach where we took a mixed model where we fit a Gaussian to the, axion signal.
STFC-RAL-CR03 R61: and fit a uniform to the… to the… to the scatter. And we took a 90% confidence interval on the amplitude of that Gaussian. Now, we claimed it was a null result in the sense that if we used a
STFC-RAL-CR03 R61: if we compared the two models through, like, a Bayes factor, so, like, a Bayesian analysis to work out which model was better out of just the uniform distribution for the scatter versus a uniform distribution with some Gaussian contribution for the axioms.
STFC-RAL-CR03 R61: The… the result of that was that the uniform distribution was a profoundly better fit for the data than the mixed model, so we still claimed it was a null result in that case.
STFC-RAL-CR03 R61: Excellent. But that's… I mean, we still need to go through that, right? We've just written out the paper, we're going to co-authors now, and I'm sure they will have some…
STFC-RAL-CR03 R61: Some comeback on that particular analysis.
STFC-RAL-CR03 R61: And on the petawatt laser facilities, what do you think would be the first
STFC-RAL-CR03 R61: topic from the fundamental physics research list that you mentioned that is… would be
STFC-RAL-CR03 R61: more… most attractive, or more easily accessible? I think…
STFC-RAL-CR03 R61: this is a controversial statement, and there might be people in the audience who disagree, but the work that Stuart Mangles and so on are doing
STFC-RAL-CR03 R61: at the minute on Gemini demonstrates a pathway to achieve results that are relevant to the community. So this sort of stuff.
STFC-RAL-CR03 R61: It's useful in the sense that
STFC-RAL-CR03 R61: You're getting results that are relevant, but also you're learning about things like the systematics and how they're different.
STFC-RAL-CR03 R61: And so, for me, going forwards, one of the things that I think… I'm thinking about is, can you submit proposals that really, even if you don't have particularly competitive bounds, if you're saying, well, we're trying to understand the systematics associated with these optical laser experiments in order to facilitate the design of future facilities, or understand how we might improve it.
STFC-RAL-CR03 R61: then that's the motivation, right? Yeah. Yeah. I think getting more papers out of it, and having, like, a killer experiment will attract also more proposals from our community, from article theses. Yes, that's a good point. I think…
STFC-RAL-CR03 R61: Certainly in the community, for all… for optical lasers, this all-optical photon-photon scattering type work is seen as one of the
STFC-RAL-CR03 R61: things that would be of the highest impact, but it also, and I'm now speaking as a laser designer, rather than a laser user, happens to be one of the most challenging to pull off in terms of the design of your laser.
STFC-RAL-CR03 R61: Can I ask a silly question? When you said you measured no events, which were an axiom, suppose you've seen what?
STFC-RAL-CR03 R61: I'm thinking here of, what was it called, the Kensington event, where they found a charge of a third for a charge, and nobody knew what to do about it. Nobody saw one. If he saw one, I mean.
STFC-RAL-CR03 R61: And this is… for me, coming from a field where null results are not the norm, that's something which I feel a bit uncomfortable about. In reality, what you do is you claim that it was a null result.
STFC-RAL-CR03 R61: And you set your part-time distribution with a mean of 1, rather than a mean of 0, and take your 90% confidence interval.
STFC-RAL-CR03 R61: But I… I share your…
STFC-RAL-CR03 R61: You didn't say it, but I have a… I am uncomfortable about that kind of thought process.
STFC-RAL-CR03 R61: It's a good argument for more beam time, isn't it? Yeah, that's what I was expecting to say. If you got a bunch, you know, not just a single event, but a bunch which were, you know, if you started to see a signal that you thought might be meaningful, so you were saying, well, this is one sigma, can we push it to three sigma? They're very often.
STFC-RAL-CR03 R61: that's how you motivate more time with a lot of these things, right? Like, me and Bob and others are grappling with this from the other perspective, in the sense we want to access an end station as an accelerator, where there's currently a dark matter experiment that's saying, well, we've got three sigma, we might be able to push it to 4.
STFC-RAL-CR03 R61: I'm not sure I fully understood, like…
STFC-RAL-CR03 R61: how you can, like, rule this stuff out, like, definitively. If you do an experiment for just, like, just 3 days.
STFC-RAL-CR03 R61: is the probability of this reaction high enough that you can be like, right, there's definitely nothing there? Like, if you then ran your experiment for, like, 5 weeks, continuously… Well, I mean, practically what that… we're not saying there's nothing now, we're saying the probability of an interaction is less than something if accident exists, right? So…
STFC-RAL-CR03 R61: What that would mean is, if we ran it for 5 weeks, then we would…
STFC-RAL-CR03 R61: shift that down a bit. Oh, I see, I see. So you're… okay, so you're working bounds on the property, exactly.
STFC-RAL-CR03 R61: This is off the topic, but just to mention, there was a workshop a few weeks ago on, you know, other options if the FCC is unaffordable. And there was a really exciting session there with,
STFC-RAL-CR03 R61: Tim Barclay and others. I mean, there's been the latenacial acceleration, the sort of long-term thing, and the real problem with that has been that it looks very promising for accelerating electrons, but positrons much harder. And it all came together with a session where Tim Barclay was kind of running a show and so on, and, where, but Emil Nani and this girl who's recently come to serve
STFC-RAL-CR03 R61: from, Berkeley, I forgot her name.
STFC-RAL-CR03 R61: And, the idea is that you have two electron accelerators, and then you do it as a gamma-gamma collider. A 10TV gamma-gamma collider, no worrying about positrons, and it just looks beautiful. I was really impressed with it.
STFC-RAL-CR03 R61: I'm sure you know.
STFC-RAL-CR03 R61: So, I don't see more questions in the room, but we are going to have a lunch with Jack, so please join if you have more.
STFC-RAL-CR03 R61: questions to discuss. So, thank you very much again for the…
STFC-RAL-CR03 R61: Okay, hope people are fully closed the seminar. No, thank you.