 in ta titul je umoženja do kosmikrij fyziks. Ok. Čakaj, da sem tudi. Vsi nekaj delovosti in kosmikrij fyziks, sem nekaj, ki se da svoje vse vrte, ker vse je bito vse vse, pri svoj kosmikrij. Kaj je, da se prišli v konferensi, da se prišli vse, kaj je astrofizicist, in tudi, ki se prišli vse, ko se prišli vse, ki se prišli vse, ki je vsega fondamentalna fizika. In... Ok. So, these lectures you find the topic interesting, a couple of books that you may want to have a look at. There's the book by Tom Geiser, which is a very introductory-level book on cosmic rays in particle physics. There's the technical book by Schlich Geiser, and the very extensive book by Berezninski. I took the liberty in this introductory lecture that I will do today for you to borrow some of the material from Pasquale Sepico. I will acknowledge that in the going. So, would you have gone to a conference on a cosmic rays in the 20s or in the 30s? Really, that wouldn't have been called a cosmic ray conference, but more a particle physics conference, because really cosmic rays was the initial playground to make progress in particle physics. And let me take some brief historical path. So, the business started just a little less, a little more than 100 years ago with the observation by Victor S that the radiation, which was discharging electroscopes, what are electroscopes? Well, at the time, the way one measured the electric charge, you charge the apparatus, which had to finplate at the bottom, when it was charged, the finplate went apart because of electrostatic repulsion. And those kind of apparatus, I mean, you couldn't keep them charged, because there was no way of shielding from some radiation that made those kind of things get discharged. So, people were wondering what kind of radiation was that, was that some kind of radiation that was emitted from the Earth, or was some kind of radiation that was coming from above, from the heaven. So, Victor S was a brave guy, who at the time had decided to put his electroscope on a balloon and do a high altitude flight. He went up to six kilometers, and he noticed that this radiation was going up in intensity while it was going up in the atmosphere. So, after an initial deep that is sharply going up, coloster just two years later went even to higher altitudes and found this beautiful correlation. So, these results were pointing to the fact that this radiation was something that was high energy, it was penetrating the atmosphere, it was getting partially absorbed by the atmosphere itself. So, if you went up in the atmosphere, the intensity growth. So, there was no, then people started to play around, like is there any effect from measuring this radiation at night or during the day when you were facing the sun or when the sun was hidden. No, there was no such effect and then people started to suspect that these had to be some kind of extra solar stuff, some kind of cosmic stuff. So, Millikan and Cameron in 26 made a mistake, which ended up giving the name cosmic rays to this radiation because we were measuring in lakes a different altitude how this radiation was absorbed by water and they concluded in their analysis that that radiation had to be gamma rays. So, then they dubbed this radiation cosmic rays and that's what we call it today as well. Except that was wrong because it's not mainly gamma rays, it's mainly charged particles. And this was established in 28 and later in 23 by two observations. The fact that the intensity increased with latitude and then there were three different experiments, including the one by Bruno Rossi, that measured the fact that when you were at the equator this radiation of cosmic rays was coming more from the west than from the east. How is that? Well, that is explained by the fact that we are measuring this cosmic rays not in an empty environment, but we are measuring it in a background which contains a fairly large magnetic field, the Earth magnetic field, which has a dipolar structure, north pole, south pole and then, ok, you have the line field that goes like this and get parallel here at the equator. So, you can just assume that these particles are charged, you compute the Lorentz force on this, you equate to the centrifugal force and then what you see is that if these particles are less energetic than about 50 GV, there is a full shadow, which is given by the Earth itself on particles of a given charge. In particular, if these particles are positively charged, the Larmor radius that you compute is of this kind and reaches the Earth size for about 50 GV. So, if you sit here and cosmic rays are mainly protons, they will be coming freely from this direction and we will not be seen from that direction. And also, since the structure of the magnetic field is a dipole, then these forbidden trajectories are changing with the latitude and in particular they get parallel to the magnetic field lines in the north pole, so the north pole is more probable to get all particles discharging your oscilloscope. So, these are charged particles and these charged particles, as I was saying before, are absorbed in the atmosphere. So, there were observations again made by Rossi and Doje independently that discovered that these high energy particles are transversing the atmosphere and making huge air showers. So, a lot of particles that are in a bunch that comes from one initial interaction point that you can use to reconstruct the energy of this shower. We will see it in a second. Okay. And then I marked here the last milestone which was by Enrico Fermi that in 49 developed the first proposal to how to accelerate this cosmic radiation in the galaxy. We'll come back to that. So, as I told you, if you went to a particle physics conference in the 30s, that would have been a cosmic ray conference because people were really using cosmic rays as the accelerators of that time and were using this radiation that was coming for free to study how elementary particles were interacting. And in particular there was a sequence of discoveries using cosmic rays. The most famous one is the discovery by Anderson in 32 of the positron. So, he was using stuff like this which is called a cloud chamber which is a progenitor of a bubble chamber. It's something that contains a fluid that goes into two phases. So, it's compressed gas. When it is transversed by ionizing particle it changes phase. So, you see the track when you take a picture of it in a photographic plate. So, this is a picture of a positron because you have a magnetic field which is oriented outgoing from the plane and then you have a particle which is bending in this way. And I'm telling you that the track is in this way because you see it has an energy which makes up a orbit which larger radius here and then it's ionizing the mean so it's losing energy and it's getting progressively too short in the two smaller and smaller radius. So, the track is the particle that's going in here and then essentially stopped there. From the stopping you can measure what's the mass of the particle, how efficient the ionization goes. So, Anderson was able to detect the particle which is positively charged and then the mass that was consistent with the electron and that matched perfectly the prediction by Dirac in two years before that the positron had to exist. Okay, then I want to cut the story short but this is a shopping chart with the other list of phenomena that were discovered with cosmic rays. Per production in 33 what was at the time thought to be the Yukava mediator by Dr. De Muon in 37 so in 46, 45, 46 Conversi Pachini Pichoni showed that it had to be the Muon not the pion. The pion themselves were discovered hereafter and so on and so forth. Up to really the 50s when the first efficient terrestrial accelerators were devised and then gradually particle physics shifted out from being a cosmic ray field. And what about striking it back so that was my initial interest in the field so there's a renewed interest in using cosmic rays to test fundamental physics in particular to discover new particles and the new particles that we are after are possibly dar-matter particles and the idea is to identify them through the yields of their annihilation or decay and in particular if these particles, dar-matter particles are democratic and generate the same amount of matter and antimatter since antimatter is scarce in our visible universe there is some opportunity to detect primary antimatter components connected to this dar-matter annihilation. So if everything goes well at the end of these lectures I want to give you a flavor of why people got excited looking at this postron fraction or can get excited about looking at this antiproton fraction so antiproton fraction means the flux of antiprotons of the flux of proton this is the ratio of the flux of positron over electron plus positron and there is some path so to say standard which is telling you that it should go like this or like that while there is some little excess so we will try to discuss whether this is an excess and how it comes that it makes sense to think that this is a valuable tool for particle physics. So to make a step forward I really have to tell you what I am talking about so I have to introduce a little bit about detection targets and detection methods so this is the propaganda plot for cosmic rays which is put in an intensity cumulative intensity spectrum of all species of cosmic rays in a plot which has a really scary logarithmic scales there are about 32 orders of magnitude in this direction and something like 11 in this direction that is roughly speaking really the beauty of this playground in the sense that you see you have something that if you put it in this huge scale looks like a fairly good power row this is e to the minus 3 this green line that you can hardly see and you see the red dots which are the data fits more or less on this green line and they go all the way to say 100 MeV and this 100 MeV has some meaning because it corresponds roughly speaking to the energy at which the sun messes up the playground in the sense that there is this outflow of particles or plasma from the sun which really prevents you to detect to receive the cosmic radiation at energies that are lower than roughly this 100 MeV and then it goes all the way up to this 10 to the 20 electron volts which as we will discuss later is really to be considered some kind of high energy cutoff in two ways on the acceleration side maybe but mostly on the fact that again above these energies cosmic rays cannot reach you cannot do the path from the sources to you so it's a power row with two main features here so now this is getting a little bit obsolete because when we have much better measurement of this and there are features over the places but historically these are the two main one there is a softening of the spectrum here around 10 to the 15 electron volts this is called the kni and there is an hardening around 10 to the 19 electron volts around here and this is probably an imprint of the fact that you are talking about either different cosmic ray species which are more efficiently produced at different energies or you are talking about different cosmic ray population and actually it's both so when you I wanted to stress something else in this plot this plot is scary from an experimental point of view because really when you convert these flags into a number of events that you expect in your detector you go from up here which is a scale corresponding to detecting one particle if you have a one meter square of effective area per second to the kni region which is one particle per meter square per here to this region here which is one particle per kilometer cube per year so when you try to go into detail and for instance measure the different composition in the total cosmic ray flux as I was telling you cosmic rays are mostly at least a low energy protons and then there is some helium component, carbon, et cetera but note that this is not on scale because there are these rescaling factors which come paired to these fluxes so when you do want to do this exercise it's a very different exercise if you do it at one gv at 100 Tv like in this case of our 10 to the 20 electron volts so roughly up to 100 Tv or so you are able to do it really with a particle physics detector that you put on a satellite or you attach it to the international space station and wait long enough to collect statistics but when you go to this region above here above 100 Tv really you have to go back to the trick of using the atmosphere as your detector so let me sketch very briefly too so I just took two examples of particle physics detectors that have been put in space there is for instance the Fermi large area telescope and that's just a piece of material that everything that it does is to look for a gamma ray that is coming this side it is a multiple layers of silicon and this multiple layers of silicon make this gamma ray pair produced so you converted the gamma ray into an electron-positron pair and then here you have a calorie meter which measure the energy of the electron and the positron by stopping them then there is some stuff that is characteristic of particle physics detector even at accelerator and really if you want to measure cosmic rays charge cosmic rays you have to use a detector which indeed doesn't look very different from what you use at LAC in the sense that you have instruments which are put in a strong magnetic field to tell the difference between positive and negative charges you have instruments that measure with accuracy the passing of high energy species from top to bottom so they measure the velocity of the particle so they measure the charge of the particle and so on you have a calorimeter and so on so this is the other big experiment in space the alpha magnetic spectrometer which has been flying attached to the space station now since four years and the other tool is as I was telling you instead using the atmosphere so the idea is that you are looking at this shower of particles this shower of particles is not disconnected but is coming from by back tracing initial interaction point and by looking at the shower you want to retrace what is the energy of this initial incoming particle possibly what is the direction of it and then what you have to do is in this indirect measurement is really to understand what is making up your shower and there are two main cases because when this is an electromagnetic shower so this is initiated by a photon that produces and then you have an electromagnetic cascade whether you are starting with an addron like a proton and you have a cascade with addron and mesons and partial conversion into an electromagnetic cascade and so on so once you suppose you know what the atmosphere is made of and you know it it is essentially a business of implementing a dedicated Monte Carlo that knows about quantum electron dynamics and quantum chrono dynamics are these energies do we know quantum electrodynamics are these energies? well we have good reason to say yes do we know how to calibrate QCD are all energies which are of interest for cosmic ray physics well the answer is actually no because we have been able to calibrate it up to the energies that are reached at accelerators and for instance there was a great job done after the first run at the LEC but still you have to deal with extrapolations so yes so that's a good question because since so she was asking about how in sort of detail you can understand these showers where you have a single particle multi people producing other states and so on so how do you make this plot or this plot so this is a plot of a simulation for an electromagnetic shower this is a plot for a simulation for an adornic shower so since I thought that maybe in this first lecture I didn't want all of you to fell asleep following my slide I thought that this was a good point to complement this with some calculation that we can do at least at the rough level with paper and pen and I want to discuss this idler model for cascades which is something that Mr. idler developed in the fifties essentially because he didn't have a Monte Carlo to use and he was trying to find some tool that did give some feeling for why an electromagnetic shower looks in reality like this and why an adornic shower looks like that so and that's also fun because it gives me the opportunity to introduce some of the jargon that cosmic ray physicists give for granted and particle physicists don't understand so let's start with an electromagnetic shower so I'm starting with a photon this photon has an energy which I call E naught so this photon is traveling through the atmosphere and is feeling the coulomb interactions that are due to the nucleons that are sitting out there in the atmosphere so in the coulomb fields of this nucleon what you can do is to produce a pair of E plus E minus so there is some conversion probability and the conversion probability is usually given in cosmic ray physicists like what is called the Grammage lambda so there is a characteristic Grammage what is the Grammage is an integrated coulomb density along propagation so it is essentially how much material you are transversing before something happens to this particle so it's something that usually is measured in grams per centimeter cube it's really density times length so what we are measuring here is exactly this Grammage because you have a step one which is this one which is happening in some Grammage and then what will happen is that these electrons and positrons are not going to just propagate in the vacuum they are still in the atmosphere of the Earth so they as well fill these coulomb fields connected to nuclei so by brainstorming what they do is to emit a photon so that was step one this is step two which on average is going to happen at a given Grammage and then you understand what is going to happen you are going to have a step three in which this one is going to perproduce this one is going to do brainstorming perproduce brainstorming bla bla bla ok so if you are measuring the depth in the atmosphere in this Grammage language and as you call it x so the number of steps n is just what is just this total x divided by the length of each step so it's x divided by lambda ok the number of particles that I get let me call it big n at a given x is what at each step each particle is producing two particles so this is just 2 to the n ok so this is 2 to the x divided by lambda what about the energies the mean energy at a given x is what well if we are democratic and say that at each pair splitting we are sharing the initial energy splitting them into from one mother into two daughters then the mean energy is nothing but e not divided by this 2 to the x over lambda so is this going forever no this is not going forever because you see this is telling you that the energy of the mean energy of the particle is fast decreasing as you go to along this cascade and while we were considering just pair production and premstralung there is an H which I never know where to put whatever modulus the H so these were these two processes these are efficiently up to you are at the stage you are at some kind of high energy while when you deplete the energy by this proliferation of particles you end up in a regime in which collisions take over so the particle that is somewhere here in the shower what it tends to do is not going ahead with the proliferation but is losing energy continuously and at the end of the day may be stopping so there will be some characteristic energy e of collision such that when you are in the regime that e is smaller than sorry the mean e is smaller is smaller than this e collision you are going in this regime while what I call the high e is when this is larger than this e collision ok so there is going to be a maximum number of particles so this n of x is going to saturate on n max which under this democratic law I've been assuming will be just set by the fact that I've been splitting the energy initial energy into mean energy which matches this condition that stops proliferation so there will be some x max connected to this thing which is going I'm just plugging that in here and I find that x max is just lambda the log of 2 of e naught divided by e c so lambda and e naught e c sorry lambda and e c are something that is characteristic of the medium in which you are doing this shower so for the atmosphere you have to make up what is the case for nitrogen blah blah blah all the components of the atmosphere and on average you get that this characteristic this gramage is something like 35 grams per centimeter cube while this threshold energy at which you stop reproducing is around 80 MeV so what I'm seeing there just in color coding is essentially a plot in which let me put the axis like this this is increasing depth this is the point at which you had interaction the first interaction and this is the number of particles so what is happening is that along along the path that the shower is making there is some branch which is this branch here which then saturates some n max corresponding to some x mass and then what you see in the atmosphere is that the density of these electromagnetic states is going down so that's that so you see if you have an instrument that is able to map this point here then that instrument is measuring the energy of the incident particle ok so there are refinement in these that have to do with the fact that energies are not really always splitting in this democratic way that actually should take the per production and the correct brainstorm get together with collisions at the same time so there is some reshuffling but this is really QED and this we have very well under control so the electromagnetic shower we are going to get measure energy in a delightful way what about the plot on the right adronic showers yes sorry I didn't get the western sorry how easy to measure these x max you are asking yeah really you have to make a model of the atmosphere you have to know what what the atmosphere is made of ok so you have a distribution of cool on fields associated to the each individual element I mean the atmosphere is essentially ionized ok it's the cosmic rate themselves which are keeping them ionized so it's really an exercise of of no relativistik QED yes no that is that is much suppressed ok you have it but I mean you compute the probability the probability it goes like no it's not really like that no because you are not producing on shell quarks no you really have to produce two showers two adron showers then then the probability for that is is really relevant so shell adron showers matter when you start with them so adrons will have these adronica skates and plus eventually oh gosh plus eventually the electromagnetic ones so you have this proton that is propagating in the ambient field in the atmosphere and then it is splitting ok and let's make it simple let's say that these adronica skates are essentially just something that at the end of the day produces pions so you have pi plus pi minus pi zero ok these ones are going to go ahead with another adron cascade so I have adron cascade this one is doing what this one is going to go to decay so it's going to go into two photons so this one is as I said mixing up the process because this one is starting an electromagnetic cascade and then as I just replied it's a one way road in the sense that once you end up in an electromagnetic cascade it's very improbable to get a contribution from adronica skates so what you are depleting electromagnetically stays in the picture that I wrote on the other side of the blackboard so again it's going to go in steps right so again you will have a number of steps and the difference is that you are splitting energy in adrons and in an electromagnetic component so at each step suppose again to be democratic you generate the same number of pi plus, pi minus and pi zero so the adronic energy is in each step two thirds which multiplize n which multiplize the initial energy E naught the electromagnetic component is one minus this one ok and then you see that from this formulas that since from here you are not coming back in very few steps you are mostly depleting your energy again electromagnetically so this plot is telling you that there are some branches some main branches which corresponds to these adronic lines and then you attach these showers with many particles that are the electromagnetic lines so again you can do computation of what the mean energy that you are putting in the adronic part as you go down in depth and that's going to be these E naught divided by ok, I told you that I told you here that I'm democratic but what I have to model is that what is the multiplicity of this democracy let me write this multiplicity number of pi plus and pi minus that are generated each step as this new naught so then the mean energy is the mean in an event divided by the number of this the number of total particles that I'm generating so again I have a depletion of energy even in this adronic component and again I'm going to have a threshold effect and the threshold is when I break this link pi plus pi minus adronic cascade when do I break it I break it when at a given energy it will be more probable that these guys just decay into a mu plus mu minus and corresponding neutrino again it's a threshold effect because you have some typical decay energy at which this gets at energy lowers then these it gets more probable to go to this branch and this branch is again a final branch because what you generate are these energetic muons muons are very penetrating particles they just go straight through the atmosphere you need lead to stop a muon so what you generate here is just what eventually you would receive on the ground in a detector so you will have a I have to raise something let me raise this you have again a maximum number of pi plus pi minus which is just implying that formula over there is the log of e naught divided by ed divided by mu plus minus times 3 divided by 2 and then I can just connect to the number of muons that I am getting out of this adronic shower which will be equal to the number the maximum number of this pi plus minus and that is just going to be this multiplicity to the n maximum so when you put things together you get a formula which is the analogos of this formula which is that the log of the number of muons is equal to the log of e naught divided by ed and then there is a coefficient in front which is just the log of mu plus minus divided by the log of mu times 3 divided by 2 so now you have to know something about adronic showers and you need to estimate what is this ed and this number here this number here boils down to be about 0.85 again if you have a detector which somehow is able to estimate this number of muons that you get in the shower then you have a measure of the energy of the initial state so this first part you get order right when you do it in this naive way this second part is much more tricky because there is more than this step this threshold where you have to play a little bit with care then what you do is to do a Monte Carlo about what you know about QCD at the given energy so at least I hope I convey the feeling that it does make sense to look at what happens at the bottom of this shower and understand what's the energy of the incoming cosmic ray and then that's what people do in very different ways so this showers in the atmosphere they are detected for instance looking at the cone of sharing of light which is emitted in the shower so that will have some characteristic shape given an electromagnetic shower or andronic shower and then if you have a detector on earth which is essentially a large dish mirror which is pixelized and looks at the shape of this circle of light you have a detection of your event this is in particular interesting for gamma rays the energies of say few hundred GV to few tens of TV and it's the technique applied by the so-called air-charing telescopes you can look at another way this showers shine which is the so-called fluorescence light what is fluorescence light you have these photons that are floating around these photons excite nitrogen molecules in the atmosphere and these molecules excite in a slow way emitting light at a much lower frequency in the infrared so you have this powerful shower this powerful shower is making this transition on nitrogen all over around it and then you look at infrared light with dedicated again a telescope on Earth which is matching where this infrared light is coming from what's the intensity, what's the distribution if you do it if you want to be careful with at least two of these telescopes to really reconstruct a 3D shape of the shower and that's very efficient or you can do it more brute force by just putting detectors on the ground and look at what is left from this shower when it hits the ground and that will be particularly efficient if you want to track adronic escades because I was telling you the adronic escade end up with these muons so it's very hard to get rid of and then the good part is that if you equip a large area of the ground with something that can detect muons then you can reconstruct the shape of the shower on the ground so for instance there are two main experiments that do this the energy and these are the telescope array in the north hemisphere and this older and bigger brother the pieroge observatory and for instance pieroge has just some water tanks and it looks for the cherry of light of these muons when they go through the tank so actually a good way to do it is to combine everything together so pieroge as this water circle I just mentioned many of them covering an effective area which is remarkably large 3000 kilometers it has these fluorescent telescopes it has also some underground muon detector the muons can penetrate underground for some depth there is another technique of detecting this shower which is detecting the radio sound from this shower believe it or not it's a very efficient that as well and then you plug all these together with some station that monitors carefully the atmosphere because of course this number that I'm giving here is a mean number but then there could be high pressure, low pressure et cetera, et cetera so the atmosphere is continuously changing you have to continuously calibrate your Monte Carlo to the ambient condition so then this is for instance a beautiful event which is detected stereoscopically so here is the X-mass detected looking at the pressure slide and then this event is hitting the ground many tanks and based on the intensity of the radiation with respect to the distance from the central tank that is it you can estimate in a redundant way the energy the energy is estimated here you compare and you can cross calibrate so ok, this is another experiment Kazemir I don't want to go to much in detail but doing this you can also this is something I forgot you can also to tell you this process here is something that you can predict for a proton and then you can scale up for heavier elements assuming that they are some kind of bound state of proton in neutrons and that they get broken while they propagate in the atmosphere so there is some characteristic imprint of these X-max depending on whether you started the cascade with a proton or with the highest element that is conceivable to have in this primary cosmic rays which is iron I will say why this is the case in the next lecture ok, so this is your the lines are your Monte Carlos which are tuned on what we know about QCD and as I was telling you this is improved Monte Carlos after the first run at the ALAC which really changed a little bit the business and then you see this is a compilation of data from Perugio Observatory from say 10 to the 17 to 10 to the 19.5, 10 to the 20 electron volts so there is some hint of the fact that composition is varying possibly on the light side so yeah so this is a plot that I put there because up to this recalibration by the ALAC there were different experiment claiming different spectral compositions and also different energy calibrations and now all of this has gone away and for instance telescope array and doger agree very well so it's a technique which is really getting mature and trustable and I want to stress it once again it's a technique which makes us go above the energy frontier of terrestrial accelerators because if I just do a naive comparison in which I put the center of mass energy of say the ALAC in proton-proton collision is something that stands in here I converted into high energy proton colliding with atmosphere and it's putting me somewhere here 10 to the 17 electron volts but as I told you we are having events up to 10 to the 20 electron volts ok so we have cosmic rays and we know how to measure their composition and energy quite well so then what do we learn from them so the first thing one has to take into account is that you are doing this business in the atmosphere in the solar system in the galaxy this is a business in something which is very different from the vacuum you are doing something which is something in an environment which can give you many back reactions ok and this is my first slide borrowed from Cervico thank you so you have to take into account the different components polluting your cosmic ray propagation when they go out in the interstellar medium so the interstellar medium is stuff which is delute but not so irrelevantly dense it has a density of around one atom per centimeter cube which if you are integrating up to long length scales is not irrelevant what is that is mainly atomic and molecular hydrogen and then there is some heavier molecules that are in the jargon of cosmic rays called dust so dust this is not in a sense you would give it to in a cosmology course this is some some stuff which has some degree of freedom which can dissipate energy in a different way with respect to single atomic transitions that are going on in the gas so to speak so if you are talking about an object like the Milky Way is full of light ok there is this sorry this was the next one the fact that this dust is interactively with your light propagation and can obscure something about your Milky Way ok so visible light is partially absorbed infrared and radio light can go through so if you want to learn about the galaxies is better that you go to a radio telescope rather than the Hubble Space Telescope and on top of these dust and gas there is this interstellar radiation field which is a healthy population of photons in general low frequency going ultraviolet to the infrared and again we have a typical density which is not irrelevant so the order of the electron volt per centimeter cube finally you have a magnetic field that is permeating the galaxy and this has a strength of a few microgauss and this is actually what is most relevant as we will see in the next lecture for propagation of cosmic rays because prevents cosmic rays from going on straight lines ok so this is just a pie chart with a spectrum of the galaxy a different map of the galaxy a different wavelengths I put this list over here if you are not too familiar with conversion between wavelength and energies but you see it's an interplay of what is can freely propagate in the interstellar medium like for instance this radio observations and what you can really observe from Earth in the atmosphere as we just studied is playing the role of absorbing and re-emitting stuff so for instance if you are looking at visible light it's ok to have a larger telescope on Earth but you have a little bit distortion so if you want to have really clean view you have to go out of the atmosphere while for radio telescopes well there is the fortunate coincidence that the atmosphere is transparent to radio waves and then we can build this huge networks of radio telescopes to detect very different very long wavelengths this is a picture again of what I was showing before you have this larger radio emission we'll talk about that counter part which as I told you is mostly obscured the galaxy is beautifully shining in a gamma rays and then if you if you take this Fermi particle physics detector and put it in a space this doesn't quarrel with the atmosphere and you can make these beautiful plots so so then one could ask ok why am I bothering about so many different species I want to retract it all wouldn't be ok to say ok gamma rays I know how to detect let's go just for that well the point is that life is more tricky and ok you have this environment and this environment is connected to some horizons for the given species that you are considering so let me sketch for instance this one which is the gamma ray horizon I told you that in the interstellar medium there is an interstellar radiation field which is quite rich and prominent so unfortunately what can happen with very energetic gamma ray propagating in the interstellar medium is to eat this background photon and then reproduce ok it's something that good center mass and estimate that goes with scaling with this energy energy of the energy gamma ray energy of the background photon and it's this something that has kinematical threshold and also it has cross session which is very picked at this typical energy connected to the threshold so effectively what you are resonating on is a frequency which is sort of actually mapped onto the energy of the incident photon so if you have a TV photon that's absorbed by an infrared photon and that's bad because as I told you we have plenty of these infrared photons so ok I cut it out the scale but this is the density of this interstellar radiation field versus wavelength and you have these three peaks there's of course the C and B there's the far infrared the near infrared ultraviolet there doesn't matter it doesn't matter because when you look at the energetics one TV is eating this peak here and is making the propagation the absorption length of a TV photon shrink from infinity that is around 100 gV to a scale that is of the order of few hundreds megaparsec even more if you go to 100 TV photons you eat this peak and that shrinks to few megaparsec and then you have this saturation scale at this 10 to the 15 electron volts so what you have is a real gamma ray horizon which doesn't show up in the plot but is this line here so this is the given rest shift what's the energy at which you have an attenuation that goes like 1 over e and then you see if you are looking at a source that is situated at the galactic center we can hope to detect up to 10 to the 15 electron volt but if we look at the brightest extra galactic source which is Markarian 501 Markarian 501 has an attenuation length already of few tens of TV and if you go to further back in time to larger and larger distances you have a source that is happening at energies that are below the TV here is even 100 GV so you are not hoping to see what happens from the first stars looking at gamma rays those gamma rays are long absorbed before they get to you ok, just for fun but it has to particle physics something I forgot to mention here so there is an ongoing debate actually whether we have seen some anomalies about these TV gamma ray detections so there has been some int that maybe we have detected sources that are too far to be consistent with this but you don't see so that the opacity of the universe is not on this line but is slightly above it and then there are a plethora of other possible anomalies so one idea in connection to these anomalies is that particle physics is striking back and you invent some particle that has a coupling in photon that is of the same kind as axion photon coupling I believe you have had some discussion about axions yesterday so these are not darn matter axions are some axions like power states which do the job of in a magnetic field converting your photon into an axion you have a photon this photon is converted into an axion the axion doesn't interact with the interstellar radiation field so it can just propagate on a further distance than it would have been a photon and then it may also be the case that before reaching you it converts back and you are actually seeing this emitted photon like a final photon in your detector so this conversion probability depends on magnetic field I don't want to go into the details you have some conversion probability maximized for a mass scale for these axion like particles of the order of 10 to the 9 electron volts and then you can play around with the prediction that you have so instead of seeing a spectrum for an absorbed galactic nuclei that would be like that you see something that shines up at higher energy and there is some funny feature a lower energy this is in a plane in which you put the coupling and mass of your axion like particle this works in a region of the parameter space which is around here and as I was telling you is not the parameter space for the QCD axion as the armature which is this one OK let me finish with another horizon which is instead connected to protons protons again they emitted in the sources propagated in the interstellar medium and again there are these photons around which are bagging them and in particular in this case what matters since I will be talking about extra galactic sources are causing micro background photons so the interaction for this this process goes on resonance when you heat this delta resonance so the cross section is making a jump of couple of orders of magnitude and then this conversion here gets very efficient so to reach resonance with a low energy photon you really need to go to very high energy protons and then if you are talking about cause micro background the the energy of the proton has to be in this range of 10 to the 20 electron volts so this is coming back so what's happening this photon is interacting with a photon it's scattering to lower energy producing a pion the pion dissipate energy into producing gamma rays and these gamma rays are at the end of the day assort so you have so called ultra energy cosmic ray horizon which is when this goes on resonance into a loud final state you can make up the energetic that's happening around starting around 10 to the 18 electron volt getting more efficient at 10 to the 19 electron volt and saturating at 10 to the 20 electron volt so in units of this attenuation left the same quantity that was shown before for photons that you couldn't see this attenuation left is getting from being a very large irrelevant for propagation down to the 10 to the 3 mega parsec scale around 10 to the 19 electron volts and really down to 10 of mega parsec for 10 to the 20 electron volt this will depend on whether you have design energy stuff which is protons or heavy stuff like iron for comparison this is the one for gamma rays gamma rays they are always irrelevant but they are always assorted this cutoff takes the name from the three guys that discovered it in 66 Greysen s upsek in Kuzmin they predicted pretty much as soon as the cosmic background was discovered two years before and so it's the so called GZK cutoff and let me finish with the detection of the GZK cutoff so there was a lot of excitement in the early sorry late 90s because the first experiment that had actually the power to detect 10 to the 20 electron volts didn't see the cutoff so there were flood of papers written for how to explain the presence of this threshold effect based on quantum gravity blah blah blah unfortunately that cutoff is there and there is no kind of imprint from fundamental physics so iris piergeo servitor telescope array have clearly detected so there is this attenuation of the intensity that is going on at around 120 electron volts and still the game is not over because I was showing you this plot with the composition actually it was from the same data set that I'm blogging that I'm showing here and then you see when you try to model this cutoff with either protons or iron it really doesn't work very well with explaining all of it with either of the two so there is some interesting physics that is going on at this 10 to the 20 electron volts and if I can mention a very last thing this 10 to the 19 10 to the 20 electron volt window is really a fortunate coincidence in the sense that it's a hope that we have to do try energy cosmic ray astronomy so why is this open window for astronomy I told you there are magnetic fields in the interstellar medium there are magnetic fields in the galaxy and then if I want to if I estimate what the larmo radius connected to magnetic field of galactic size then that's of the order of the parsec if you have an energy of the order of 10 to the 15 electron volts but it's getting larger than the size of the galaxy when you go to this few 10 to the 19 electron volts and in the same way it's more cucumber sum because you have to do to predict somehow from first principle you haven't measured it yet but you can estimate the deflection from the galactic magnetic fields is also at the verge of being irrelevant and then as I told you it's it's this regime in which you have an attenuation length which is getting small so this means that you are going to see only nearby sources you are not going to pile up on a huge number of sources so there is some hope that in this small volume you test you are seeing a single sources which make up some isotropy in the flux so people are searching for odd spots spots in the sky in order to get a larger flux this intensity of flux is color coded here and there is one marginal odd spot in the telescope array data in a position which is consistent with Ustama in our cluster if you go to the south hemisphere so to appear there is a warm spot I would say in Centarius A this is like 4 sigma this is like 3 sigma if you try to correlate with what we know about the large scale structure in the near vicinity to us where you take some catalog of distribution of galaxies and you chop out what's in within 50 megapars you have these dots but unfortunately you see that the correlation between the dots and the odd spots or even the odd spots and the nearest galaxy cluster is not very good so we are still not doing ultra energy cosmic ray astronomy but we will have to live and see for more statistics and I think we are going to want to break