 Hello everyone and welcome to the sixth webinar of these Latin American webinars and today we have Paulo Paola Arias from Universidad Santiago de Chile and she will be talking about... Hello everyone and welcome to the sixth webinar of these Latin American webinars and today we have Paulo Paola Arias from Universidad Santiago de Chile and she will be talking about... Hello everyone and welcome to the sixth webinar of these Latin American webinars. Hello Nico, I'm sorry, I had to... Well, I'm back. Sorry. Well, welcome. We are here and the coordinators of the webinar and Roberto Lineros in Valencia. You have Nicolas in Sao Paulo, Joel in Peru, Federico is in Medellin, and Paola here in Santiago. Let me tell you a little bit about Paola to introduce her work. She did the PhD studies here in Chile in Universidad de Santiago de Chile and then she was a postdoc in Desi and then postdoc in Universidad Catolica de Chile. Right now she's an associate professor in Usage in Universidad de Santiago de Chile. She returned to her university and well she works on hidden photons and today she will be talking about shining light into the hidden sector. Well, I will let you with Paola, hello Paola, can you hear us, Paola, can you hear us? Hi, can you hear me? Yeah, yeah, we can hear you now. Sorry, sorry. Yeah, as I was saying, can you hear me, yes? Yeah, yeah, we can. Yeah, thank you and sorry for that, this is new for me and thank you very much for the kind invitation to this webinar and as Hermann said, I will tell you about shining light into the hidden sector. This is mainly about two hypothetical particles, axions and hidden photons, so I will tell you a bit about what they are and why we think they are there and how can we look for them in the experiments. So this is the overview of my talk, first I will motivate the existence of weakly interacting slim particles or the abbreviation wisps, then I will tell you how can they be called dark matter candidates, some detection techniques and the parameter space that is currently available for these particles, then I will tell you about new ideas that we are developing to search for these particles and I will finish with an outlook. So going beyond the standard model, probably I don't have to convince you so much that we need to go beyond the standard model and probably there are two ways to do that. I have some pictures here, one of them is to go into power, so to build big accelerators and hope that these very massive particles that we cannot see appear in these colliders. But we also have an alternative to go beyond the standard model and that would be to go into the realm of low energy experiments but very high precision experiments. So I will tell you about how can we search for these dragons that might be not accessible by colliders but they can be accessible by low energy experiments and this low energy of course is a way to speak but low energy in the sense of colliders. So the first motivation I want to do is from a bottom up approach or just waving my hands that why we think that should be more particles than the ones of the standard model. So from a very phenomenological point of view, one can name the so-called strong CP problem that probably you have heard about it. So it's observed that the strong interactions are even by CP transformations but in the Lagrangian we have a term that actually violates CP which is given by this expression that I wrote here that is proportional to the field strength of the gluons and this term that violates CP is conformed by the sum of two different terms, one of them comes from the solution of the U1A anomaly and the other one comes from the quark sector from the mass of the quarks and therefore one can constrain this theta bar basically that is the sum of this angle that comes from solving the U1A anomaly plus this contribution from quarks. One can constrain this parameter by looking into the electric dipole moment of the neutron and realizing that so far we haven't observed an electron dipole moment of the neutron. This means that probably this parameter should be zero, this theta bar. So currently the bound is about less than 10 to the minus 9 but probably as the experiments get more reliable we will find that these numbers should be really, really small. So the question is why is theta bar so small if it's the sum of two unrelated stuff so basically why are these two things canceling exactly? This is called the strong CP problem and one way to solve it that probably is the most elegant way to solve it is to introduce a new global symmetry which is called the Pechey-Quinn symmetry that gets spontaneously broken at some high energy scale and as a result of this spontaneous symmetry breaking a pseudo-nambu-golson boson appears that in principle is mass less but then this Pechey-Quinn symmetry gets a specificity broken by instant tone effects and so this particle which is called the axion gets a mass that is suppressed by the energy scale of this Pechey-Quinn energy. So this particle mixes with pions and therefore can mix also with visible photons and then we get an effective Lagrangian that couples axions to two photons and the coupling constant between axions and photons is given by this G a gamma gamma which is proportional to the energy scale of the Pechey-Quinn symmetry so also is really suppressed because we think that this scale should be higher than the electro-wicks scale and therefore we are end up if we want to solve the strong CP problem in this way we ended up with a massive boson that is extremely weakly coupled to the particles of the standard model therefore we cannot look for this particle by colliding protons and so on but we need to look for really precise experiments. I will tell you later how can we look for this particle. So the equations of motion between axions and photons are written down there. If you separate the photon into a polarization sorry I should tell you I should stress that as you can see the coupling between axions and photons is given by the product E dot B E being the electric field and B being the magnetic field therefore one could think that you need two photons in order to produce an axion. So there is a process called a Primakov process which actually can convert a photon into an axion and this is what I will tell you in this next slide. So as you can see in this image we have for instance a region where an external magnetic field exists and we can shine some photon into this region. If you split this photon that you are sending into polarization that is parallel to the applied magnetic field and a polarization that is transverse to this applied magnetic field you will get the equations of motion that I am showing up here. So as you can see the component of the photon that is perpendicular to the applied magnetic field has the equation of motion of always so a decoupled photon but the parallel component gets mixed with the axion and this has some physical consequences because of the mixing of the parallel component of the photon with the axion. This component will travel a different speed basically so then when you combine the two after passing to this magnetic region you will find that the observed light gets an extra phase. So this translates into an electricity of the beam and basically this magnetic media acts like a birefringent media to the photons. So this would be the first way to look for these particles mainly to shine light into this magnetic region. After the axion was proposed, very long ago, like more than 30 years ago, appear what is called the PV last puzzle. So in 2007 there was an experiment that was looking for measuring the vacuum birefringent of QED, the prediction, and they observed a positive signal of this birefringent vacuum but the observation was not compatible with the QED expectation. Therefore the axion appeared as an alternative to explain this observation but the QCD axion, the axion that was invented to solve the strong CP problem could not explain this signal. Therefore people started to develop some theory and they realized that actually for instance from string theory you will get plenty of axion-like particles, so particles that are quite similar to the QCD axion in the sense of the couplings, the mass, etc. but they don't solve the strong CP problem. And people also looked into hidden photons that was an old idea of Bob Holden, of hypothetical particles that are vector bosons that also mix with visible photons so they both could actually explain this PV last puzzle. A year later, so in 2008, it was found that the PV last result was actually due to an artifact of the apparatus and the signal was actually not physical. So even though the PV last experiment at the end was not finding any new physics, it actually was good in the sense of the theoretical development of new ideas and axion-like particles actually came strongly the same with hidden photons and so people started to actually look for these particles. This would be the bottom-up motivation and now I will tell you about the top-down motivation is that if you want to look for physics beyond the standard model, perhaps you should rethink the foundations. And one alternative for instance would be to look for string theory and you will find that string theory no matter what compactification do you use to reach the low energy Lagrangian will lead to at least one axion-like particle. So actually if string theory is an alternative for the physics beyond the standard model then you will be full of these axion-like particles and also you will be full of these extra hidden U1 gauge bosons that could emerge from a hidden sector that it's coupled at high energies and when you compactify to reach the low energy effective theory it will appear as a mixing between hidden photons and photons. I will tell you in the next slide. So these are the hidden sector photons. They are a valiant gauge bosons from a U1 sector that is weakly coupled to our visible standard model. This was an old idea of Bob Holdum in the 85 but now people has looked into string theory models in order to generate this low energy Lagrangian. And mainly as you can see here we have the usual Maxwell Lagrangian and then I have a Maxwell Lagrangian-like term for this hidden photon. The third term corresponds to the mixing between the photon and the hidden photon. So X mu nu is the field strength of the hidden photon. And this chi parameter is the so-called kinetic mixing parameter that is generated at loop level via heavy messenger and is predicted to be small, at least smaller than one. And the fourth term in the Lagrangian is a mass term for the hidden photon and this mass term can be generated either by a hidden Higgs mechanism or by a Stuckelberg mechanism. And if the hidden photon doesn't have a mass then it's unobservable because I will show you later that you can actually make a rotation of the fields and if there is no mass for the hidden photon then it's unobservable. So the kinetic mixing this term that I showed you before can be eliminated if you for instance make this transformation of the fields to this tilde fields. And as you can see if I make this transformation the mixing goes to the mass part of the Lagrangian. And if you recall this is the same that happens with the with neutrinos. So actually there is here a mass matrix that is non-diagonal and this will lead to oscillations between hidden photons and invisible photons. And this mixing is does not need of an external magnetic field as axions do because axions are spin zero also so you need to have an external magnetic field in order to get the mixing with photons. Now I will tell you a bit about how can they be called dark matter candidates. So at the energy of this spontaneous symmetry breaking of the global U1 symmetry that in the case of QCD axions will be the Peshay queen symmetry. This Nambu-Golson boson will emerge and the same for the hidden photon will appear this massive vector. And you can check that in an expanding universe in a Friedmann-Robertson Walker metric you will find the equation of motion that I wrote here which is quite similar to the Inflaton one. And this equation is valid to axion, axion-like particles and to the spatial modes of the hidden of the hidden photon. Here H is the Hubble parameter and I am assuming here that the field is homogenized after inflation so therefore it's only time dependent and it's homogeneous in space. So at very high energies when the term tree H is much bigger than the mass of the particle as you can see from this equation the field is frozen so mainly the solution of this equation is a constant so the field is frozen. But at some time the mass of the particle will equate the parameter tree H and in this moment the solution of the equation is an oscillation so the field will start to oscillate and this can be interpreted as the potential of the field changes. It tilts because of the mass and therefore the field can roll down the potential, hit his vacuum expectation value and start to oscillate around this value. And this is where the particles are produced and the initial amplitude of this field is then set by inflation. So if you solve this equation that I showed you before you will find the third equation here in the screen and as you can see the amplitude of the field is time dependent and is labeled here by this calligraphic A and if you compute the energy density of these particles you will see that goes as one half than the mass square times the amplitude square plus I put some dot here because these are derivatives of A which as you can see the derivative of A goes as the Hubble parameter so it's much less than the than the mass so I neglect these other terms. The same I can compute the pressure of the particles and is given there so when you compute the average pressure and the average energy density you find the equation of motion of matter so basically dust and therefore these particles are born with non relativistic so they always have the equation the state equation omega equals zero so they are born relativistic they are never in in equilibrium with the rest of the universe and therefore they are born cold and they will be bi-level dark matter candidates if there is a sufficient dark matter production if they are long lived and if well if basically they don't decay so if the the lifetime is bigger than the than the age of the universe and as I told you this mechanism works for action action like particles and hidden photons to each spatial mode and this is the parameter space where actions can live so in the vertical axis I'm showing you the coupling constant to two photons and the horizontal axis is the mass of the action here this plot is valid for action and action like particles so the QCD actions live in this in this line this dashed line that appears here that says axiom models and the rest of the parameter space is where action like particles can live so all the the colored regions are exclusion from observations mainly astrophysical except except the pinkish and red region which is the parameter space where axioms and action like particles can be called dark matter candidates so as you can see not so many experiments are currently exploring this region where they can be dark matter candidates we have only these alloscope searches that I will tell you my think a bit later about it but mainly this is the status of the action like particles today this is the same plot but for hidden photons so again I'm showing you this kinetic mixing parameter that couples hidden photons to visible photons and I'm showing you the mass of the particle and again all the colors regions are constraints from observations and everything below this red line this pinkish region is the parameter space where they can be called dark matter so now some detection techniques so because of this oscillation between axioms or hidden photons and invisible photons you can design an experiment called light shining through a world so mainly consists into shining photons and send them into this wall that is the black rectangle that appears there and this wall is opaque to photons so mainly will stop all photons but if in the way of traveling the photon oscillated into a wisp so either an axion or an hidden photon this new particle can pass through the wall and therefore can be appear into the other side and be re-converted into a photon and then you put a detector and you can then wait for a photon to appear in this detector yeah this would be the most easiest way to to look for this particle this experiment has been done actually at DC and at CERN and many other places but the problem is that the probability of of CERN a photon into this detector is really small because you need to multiply the probability that a photon will convert into let's say an axion in the first region and then the probability that this axion in the second region will re-convert into a photon so therefore to to make this experiment you need a very powerful laser for instance that can shine many photons per second and you need to have a really good detector because you need to count single photons basically so this is what I meant at the beginning with that you can look for these particles by using high precision experiments and as you can see also well in the case of axions you need a strong magnetic field to enhance the conversion and also you can use the sun as your source of photons and these experiments are called helioscopes so basically you you know the the predicted flux of of photons that should reach the earth and then well you can constrain these particles either by by the lifetime for instance of the sun which actually comes places a huge constrain to these particles and the other one will be to perform a light shining through a wall experiment but using the sun as a source of photons and this third idea is something that we are working currently with Benjamin Koch of the Universidad Católica de Chile with Jair Jacquel from Heidelberg and Javier Redondo from Zaragoza which consists in to look for hidden photons using this setup I will tell you what it means so that this yellow cylinder is a solenoid that will produce a constant magnetic field for instance in the seat direction and then you put a superconducting coat a superconducting shielding into this solenoid there will be a small vacuum in between and if if the hidden photon does not exist or does not mix with the with photons if I put a magnetometer outside the superconducting shielding I should observe zero magnetic flux but if a photon can oscillate into a hidden photon actually the hidden photon can escape the superconducting shielding and therefore if I put a magnetometer outside the shielding I could observe a magnetic flux different from zero of course this magnetic flux will be really really small so I need a very powerful magnetometer currently the most sensitive magnetometers are squid's that can detect around 10 to the minus 18 tesla so this would be the idea this is the graph or the plot that shows the behavior of the magnetic field without hidden photon so from zero to one it is the the solenoid region then from one to five I put this vacuum region and then the magnetic field inside the superconductor is exponentially suppressed and therefore if I'm choosing of course the right superconducting shielding I should not observe anything after after the shielding and this would be the same plot but assuming that axioms mix I'm sorry photons can mix with hidden photons therefore I can I can have a magnetic flux outside the shielding and this could be detected another idea that we were working on to detect hidden photon dark matter in this case we did this work in collaboration with Babet Dobrik from Desi she's an experimentalist so she helped us a lot into into develop this this idea that it's based in a similar idea for axioms and it worked like this so if the hidden photon is the cold dark matter of the universe and it oscillates in in time as I show you in this equation before and has a constant amplitude that does not depend on the on the space yeah if I put this expression for the hidden photon into the equations of motion assuming so this kinetic mixing with the with the normal photon you will see that the Maxwell equation changes and I have a source term for a magnetic field so and would be this minus chi and the derivative of the hidden photon field with respect to time and this term therefore behaves like a like a displacement current yeah so I would have this this condensate of hidden photons that permeates the whole universe and therefore should be a tiny Lorentz invariance violation because as you can see this can generate a magnetic field that will have a fixed direction in space and I would like to then try to measure that magnetic field and so it works like this so because the the dark matter the hidden photon dark matter has this amplitude uh oscillating in time with frequency equal to the mass of the particle you can see that uh part of the energy of the of the cold dark matter will be invested into the creation of a visible electric field that you can compute by taking the derivative of the field with respect to time and and therefore this electric field that again will permeate the universe can be a source of a magnetic field because oscillates in time and the idea is then to enhance this signal using an an LC circuit so a circuit with a capacitor and an inductor and this would be the sketch for that so mainly you have here this loop so with the loop one expect to catch up the the magnetic flux the the change in the magnetic flux of of the dark matter and if there is a change in flux through this loop we know that a current will be generated in the in the loop and this current can be can be amplified by this LC circuit basically if you tune the the product L times C to the mass of the particle and so then this this current will get amplified and you can put a again a magnetometer in the inductor of the circuit and try to measure some some very small uh flux magnetic flux in the another I think we cannot hear him okay now okay so no would you run the experiment with a different magnetic field or it's not necessary I'm sorry I missed you at the beginning you said that the experiment what sir and if you need to run the experiment with a different magnetic field to see the the the magnetic dependence of the okay okay so to answer that again let me go for instance to this light shining through a wall so let's assume that I detect some photons uh after the world then I wouldn't know if this was because of an axiom or because of a hidden photon but then I could for instance repeat the experiment but with the magnetic field switch off because of course as you can you can imagine um the experiment is done always with the magnetic field on and um and you can use it to axioms and hidden photons at the same time basically you can constrain both but if you observe something then you should turn off the magnetic field or changing it making it stronger and then lower and see if the signal depends on that or not and then you could discriminate between a photon and an an axiom in this in this kind of experiment and in the one that I show you for instance in the superconducting box this is only uh possible for um for hidden photons yeah because even if I produce an axiom I don't have a magnetic field to re-convert into a photon so for instance this experiment would be only sensitive to hidden photons and not to axioms okay thank you very much Paula uh do we have any other question yes I have a very short question not just just regarding to the wisp re-conversion or wisp production because in most of the cases dependent of the of the strength of the magnetic field but I was wondering if it is possible to I don't know by be a higher operator or something like that to be dependent to a strong gradient of magnetic field instead of the absolute value of the magnetic field field itself so you mean for instance a magnetic field that um depends somehow in in space or in time exactly it could be a strong gradient for instance like a needle of magnetic field or something like that yes this is a very interesting question because actually yes uh it is it is possible to to enhance the probability by uh by tuning for instance the either the time dependence or the or the spatial dependence of the magnetic field to the mass of the of the particle so you could think in in this oscillation between a photon and a wisp like a two-level system if you recall a two-level system for instance um like the spin of electron spin for instance you can have this uh this uh ravi resonance I don't know if you recall about this effect but mainly tells you that uh you can tune the the external um input that you are putting with the frequency of of uh the natural frequency of the two-level system so actually there are works where people is considering um at for instance a spatial dependent uh magnetic field and uh if you if you can tune this spatial dependence um the of course should be um periodic the the spatial dependence of the magnetic field so if you are thinking in a needle this should be several needles equally spaced in in space and um and then you will find uh an enhancement in the probability I don't know why probably it's hard to do this experimentally this I'm not sure because currently there are no experiments exploiting this but uh but there are works that show that you can enhance the probability of conversion by uh by using this uh this space oscillating magnetic fields and actually we are also looking now with uh with a colleague from here with Jorge Gamboa what will be the effect of having time dependent oscillating time dependent electric fields magnetic fields sorry okay good thank you okay thank you paola is there any other question if not uh we will thank you very much paola again uh for this very nice seminar it was very interesting yeah thank you and I think people get very interested especially in colombia in in this project and well and just to say that well we will have a new webinar in two weeks and we hope to have many of you um attending to this okay thank you very much and goodbye thank you bye bye