Week 10: Lecture 46: Open problems in neutrino physics, and neutrinos as possible tools

So uh in this lecture we will discuss some open problems in nutrino physics and the possible use of nutrinos as tools to study some problems or to the possible use in even technology perhaps. Okay, so this is the plan of this lecture. Some open problems in nutrino physics. Properties of nutrinos that make them possible tools. Tommography of the earth near surface and deep interior. How we can monitor nuclear fishision and fusion reactors. Perhaps even bombs although they are not so common now. Uh people have finished with their testing. Uh also uh this the whenever the supernova event occurs uh in our galaxy or close to our galaxy then we can probe the time evolution of the neutrino production and they can tell us about uh the structure of the supernova explosion. uh we'll talk about possible futuristic technology of neutrino communication and of course what is for some of these applications we need compact neutrino sources and detectors okay so what are the open problems in nutrino physics the ones in red are the ones which we have already discussed so there are nutrinos are they dak or marona particles what the absolute mass of the neutrino and are there sterile neutrinos we have discussed this in previous uh lectures uh the ones which we'll discuss here is what is the mass ordering of the three mass states. Is it normal or inverted? In other words, we know M2 is greater than M1 but and that difference is small. Uh is M3 greater than M1 M2 or is it smaller? So if it is greater then of course it's called the normal hierarchy or normal ordering of masses and otherwise it is called the inverted hierarchy. >> [snorts] >> Is there CP violation in the neutrino sector and is it bigger than that seen in the masonic systems? Indeed, is it enough to explain perhaps together with other unification models whether it explains the baron antibaron asymmetry that is seen in the universe. Finally we come to a very futuristic and difficult problem which people knew about but there is no uh experiment which has actually measured these big bang nutrinos. They're extremely low energy about 175 micro electron volts and so on. Okay. So we'll first talk about the the open problems in nutrino physics and they can be at least the two important ones can be tackled with uh the deep underground nutrino experiment Dune which is a planned facility supposed to come up in 1930 or 32 there have been delays and so on. So the idea is that you produce nutrinos at firm lab make them propagate through the earth so you have the matter effect coming in and then you look at them 1300 kilometers away in uh in an underground mine the homestake mine in fact which was used by Ray Davis for his solar neutrino experiments and uh you have detectors in the underground facility which are huge uh four 17 kiloton liquid argon time projection chambers. Two of the singlephase variety and two of the mixedphase variety. By mixphase you mean I mean liquid and gas. And the source is these 60 to 120 GV protons. You can tune these energies when they fall on a target and the beam power is uh initially will be 1 megawatt uh and then it they plan to ramp it up to to point4. Of course, there's a lot of learning involved and that's why that might take time, but eventually they hope to get to 2.4 megawatt of beam on target. There's a reference given here. So, the liquid argon detectors are at dune consists of as I said liquid TPCs and the singlephase TPC is depicted here a schematic. You can see the size of this. Uh this is the human being of course and uh there's a cathode and then an anode where the electrons drift on either side. The drift is horizontal. Uh there is also uh you know there are these pixelated detectors at either end and also photon detectors to trigger uh on the t equal to0 and then it measures the drift time uh and so on. That is of course the way the TPC works. Uh the other uh type of detector that will be there will be a uh dualphase TPC. So the uh interaction is in the liquid but then of course as it comes out of the surface of the liquid there is a gas and there's some multiplication there and so you you you can get a slightly bigger signal and uh again these are huge. You can see this is the size of two human beings at the bottom. And uh this is of course this schematic is for a 10 kiloton liquid argon dualphase detector. Uh the drift is I said already vertical and the spatial resolution is between millimeters and centimeters. Actually I should have uh so they will actually know what kind of resolution they will get once they run this. It could be even I mean 100 micron resolution depends on how these things open. Nobody has ever built such a huge liquid argon TPC. The physics reach of Dune uh in these two uh sectors namely the mass ordering uh sensitivity is shown here in this plot and uh the earlier uh you know what do you call page had the references. So this this is taken from that reference. Uh for a five sigma sensitivity of the mass ordering uh it would take roughly 2 years. Okay. Of course if delta CP happens to be very favorable uh of the order of by 2 90° then of course you can do it in less than a year but uh this is for the uh you know for any value of delta CP. So then it would take about two years and this roughly two years 2.4 years or whatever this would uh this is five sigma is the gold standard in particle physics as we have talked about earlier. Hig boson for instance uh when the two experiments combined gave a more than five sigma result then of course the it was believed by the particle physics community. [snorts] uh the CP violation sensitivity comes about from the following the uh the probability that a muon type of neutrino so you have a muon neutrino beam of course coming from uh firm lab that it goes over to electron type of neutrino is given by these factors and an important component is the sin squared 2 theta13 which has been measured very accurately through the dab experiments and is fairly large so that's a good uh I mean start for this dune experiment uh but also in the denominator you have this delta 31 of course there is also uh delta squared here uh you have this uh these two coming about with a minus sign so delta 31 minus al then the whole squ so uh the delta cp and a switch sign when you go from new mu to new e uh appearance uh as compared to the appearance of a anti-newe starting with the anti- new mu okay so this helps in actually getting the CP as well as mass ordering sensitivity. So for the CP violation sensitivity you can see that for about 75% of delta CP uh it would take something like 13 years or so okay to have a three sigma result but uh if you want let's say only you know if it is somewhere near pi by 2 then of course you can get that number in about 7 years running. If it is like 50% of the delta CP values then you can get that in uh about 10 years. Okay. So even in the best of times or the best of uh parameters uh space of delta CP there you will still uh have only 75% of the delta CP covered. Okay. Now we'll talk about a completely different aspect here that namely uh we believe that the universe originated in a big bang uh and uh you know initially it was just a quark soup then nucleons came about and we will talk about that a little later when we talk about nuclear astrophysics but in any case we the result of this uh big bang at times such as now is that we have measured the cosmic microwave background. The photon background we have measured and with great precision. Nutrinos on the other hand which are the second most abundant. So the photon number is about 440 photons per cubic cm and the nutrino number is not very small. It's about 3/4 of that at 330 uh nutrinos per centime cube. By nutrinos I mean nutrinos of all flavors and also antiutrinos. Okay. So uh these are the second most abundant matter particles uh but not matter I would say matter and radiation uh the known ones we don't know anything about the dark matter and dark energy of course so if we don't know anything about these uh big bang nutrinos they are called cosmic relic nutrinos and weineberg in uh in I think the 50s had proposed a reaction ction inverse beta decay reaction where the electron type of neutrino interacts with trissionium to produce a electron and three helium. So this is a two body final state. So the electron is mono energetic and it is just the given by the q value of this beta dk of attrition. So that's about 18.6 kev small energy. um the estimated cross-section is about 8 10 - 45 cm squared by beta where beta is just the velocity of the neutrino uh in units of c. So if you actually calculate the flux times the cross-section that turns out to be independent of uh because then it's the flux is proportional to beta and so this uh cancels out so you get basically a saturation of sigma uh times uh the flux. So kine which we discussed earlier uses about 50 micrograms of tricia and uh they have put a bound on the flux of nutrinos uh the the so-called relic nutrinos big bang nutrinos however uh that bound is not at all uh I mean very useful unless there is somehow by some mechanism a clustering of uh uh nutrinos these relic nutrinos close to the earth or okay so only then and that clustering I mean the increase in density of nutrinos as compared to the relic neutron density average density all over the universe has to be about a million only then you can that limit would be useful uh but in any case uh if you want to really determine these nutrinos then you actually need about 100 g of tricum and so there is an audacious ious experiment as I have written here called plottomy which aims to do just that. Now how they will handle a million times stronger source than this which is already the limit of our present day technology and this experiment is actually carried out at Kalzru which is a atomic energy facility in Germany. How they would do this is unclear at the moment but anyway the experiment is uh is being thought about uh to address this question. Uh the last possibility is is there any possibility that Bose Einstein condensates of suitable atoms can be used to excite collective states of the Bose Einstein condensate or even uh elastic SC I mean uh sorry uh the inverse beta DK. Now inverse beta tk looks to be not a realistic possibility but uh maybe it is possible to excite collective states and uh what is the cross-section and so on that we one has to be uh one has to work out as yet okay now we'll discuss possible applications of nutrinos what might make them possible tools so it may be recalled that nutrinos are electrically neutral particles this is a reminder we all have uh have seen that in earlier lectures and interact with matter by the weak interaction. Of course, they also interact gravitationally, but then we are not worried about that in these set of lectures because gravitation is just a is too tiny to make any difference in nuclear and particle physics. So this makes for long mean-f free paths. So lambda new the mean where lambda new is the mean free path of the neutrino is actually proportional to E² at low energies. Low energies I mean of the order of let's say 50 MV or so and is proportional to E at high energies. high energy I mean about maybe greater than a GV or so and then of course things move there's a mix uh in between 50 MV and about 1 GB at 40 TV the mean free path is of the order of the earth's diameter so uh although the detection is difficult and involves large size detectors so partly because of the flux of atmospheric nutrinos uh the size has to be large 1 to 10 kilotons and so on. Uh but this by the way this experiment has been done and we'll see in the next couple of slides. Uh coherent nutrino scattering mediated by the Z bzon uh and evidence for which was found in 2017 in a Oakidge National Lab uh Spalatian neutron source facility uh that this coherent neutino elastic scattering about 100 times for cesium and iodine. The detector that was used was actually cesium iodide. And uh this 100 times increase uh and if you have access to a very strong source as the SNS source is then you can uh you can measure these and this is how it was indeed measured at Oakidge for the first time. But this coherent neutino scattering could be of use in making compact detectors because as I said even for a thing like cesium it's about 100 times if you go to something like lead or tungsten this would be even higher by a factor of uh 2 to four okay so let's look at the possibility of tomography of the earth near surface and deep interior nutinos travel uh that travel through the earth can through the matter effect carry information about the density of electrons in the intervening intervening earth interior. At present the only source of 5 to 10 GV in all directions uh is that of atmospheric nutrinos. If we manage to have compact uh accelerators of protons uh then perhaps we we we will be able to you know access these energies with uh man-made sources and but these atmospheric nutrinos have integrated fluxes of about 10 the 3 per meter squared per second. So that's a pretty weak source. Uh accelerators pointing neutrinos into the earth at various angles technically possible but has not been done yet. This would perhaps require another 15 or to 20 years of uh work. A compact low energy high energy nutrino source is needed. Uh laser plasma accelerators might be the future. If directed in the surface region of the earth, it might in future tell us about how earthquakes develop and perhaps lead to an early warning system for earthquakes. Now let's come to done with the biggest detector that we have as of now working detector is the ice cube detector. This is a 1 kilome cubed of ice in the south pole. Uh there should be a bracket here. There's a two pole and uh the goal of this particular ice cube experiment uh is to measure ultra high energy greater than about 100 GE cosmic nutrinos to study cataclysmic events in the universe. mainly as of now to study events uh in the galaxy but of course you you there is also a sensitivity for uh you know extra uh galactic sources. So this is a nice picture of the uh the lab in Antarctica. Uh and uh this is of course in uh summer where the sun has come up a little bit. And then this is the array where you put make holes in the ice and sink photo multiplier tubes uh modules many of them in one string and then there are many such uh strings. So that is shown here. So from the top you actually pour hot water and sink these photo multiplier modules all the way down and there are many such strings. So you look and these uh photo multipliers uh have a total coverage you can look up and down and that is the way and this is one of an example of a uh a photo uh multiplier module. So it looks in all directions and then the signals come out through a cable on top the surface where then of course this data is acquired and stored and analyzed. So for instance they have measured the energy spectrum of cosmic nutrinos. So this is the ice cube result. Uh you can see that this goes all the way down. Uh and this is like uh GV to the power of 5.5. Okay. So this is a TV is 10^ the 3 GV. So this is about u a little less than a PEV. Okay, it's a falling spectrum. It's a power loss spectrum. And the other measurements are uh shown. So this is the red measurement. Sorry. And this is these are black are these other measurements ice cube unfolding. Uh and then there is also a measurement from Amanda that is in this brown. Uh so we don't see that somewhere here. It is probably hidden somewhere here. Yeah. Okay. So one fairly early result about 7 years ago was published in nature physics letters looking at just the public data available. Okay. Uh so this is a schematic of the earth. You have the core then you have a mantle and then you have things around that. And this is in blue is the atmosphere. So the nutrinos are produced in the atmosphere that atmospheric numu events and then they travel through various sections of the or slices of the earth. Okay. So this is the detector and you could get nutrinos in all directions hitting this ice cube. So by looking at the angular dependence of these nutrinos uh the zenith angle dependence of very high energy nutrinos of the order of tens of gev they could figure out so this is the angular dependence so it's not flat as it would have been had there been no uh you know interaction between the nutrino and the earth uh so had so from this curve they extracted a electroeak measurement of the mass of the uh I mean of course as it doesn't qualify as a very good measurement of the mass of the earth is just to show the power of this method that you have used the electroeak interaction to actually measure the mass of the earth using nutrinos. So that's about 2.7 with some error bar of plus -1 into 10 24 kg and of course this agrees with the known value of the mass of the earth within here of course the errors are pretty large are about 40% plus minus Okay, you can also use anti-utrinos to monitor nuclear fishision reactors and also fusion reactors and also perhaps bombs. Uh the feasibility had had been carried out using 1 to2 ton anti-utrino detectors placed uh very close to the nuclear reactor. So the idea is that uh in a uranium based reactor of course you have lightly enriched uranium U235 but there is a lot of U38 and the thermal neutrons also get captured on U238 to produce ultimately U2 plutonium 239. This is longived. So you can monitor the plutonium 239 to uranium 235. Uh you can look at this ratio and if somebody takes away some amount of plutonium as is for instance is shown here. So this is a plot of plutonium uh content in kgs versus the time in days. So these are every 90 days you uh count and if somebody takes out plutonium then it drops and then it again grows. Okay. So uh you should be able to figure this out. Of course the anti-utino spectrum from U235 fish fragments and plutonium 239 fish fragments is given here. The blue ones are due to plutonium fish fragments. the red ones due to uranium 235 fision fragments. So you can see that the number is higher. Also the energy peak is shifted slightly. So by looking at the energy spectrum of nutrinos over a period of let's say 90 days uh with a 1 to2 ton detector you can make out whether how you can tell the composition of the fuel whether it's 23 how much is 239 plutonium how much is 235 uranium and this can be used for instance to monitor such reactors and tell if somebody has removed plutonium or not but it could also be used to remotely monitor a reactor in a very completely independent way from the way the power is measured normally. Okay. So, uh a 3 to 10 kiloton detector could monitor a reactor facility at a distance of 10 to 50 kilometers. So, suppose this is loitering near near some uh reactor facility. It can actually measure this plutonium to uranium ratio. uh such a detector would also be able to of course record a nuclear bomb blast. Uh in future fusion reactors will become a reality. At that time the secondary beta activity uh because you know neutrons are produced here they get captured and then they beta decay. These could also be detected. So this could also provide a remote monitoring of the average power level of this fusion reactor. Supernova nutrinos of course when it occurs you have a prompt neutrino burst electronutrino burst then some accretion and you have various nutrinos coming in at that time not just the uh nutrinos coming about because of electron proton capture going to neutron uh plus nutrino but also uh mixing of these neutrinos and so on and you get other flavors uh the the muon and the tow flavor and also So anti-utrinos uh nutrinos and so you can use that to study uh you know shocks in the uh way the supernova is evolving. So uh in any case the time evolution ranging right from you know something like 10 20 milliseconds to hundreds of milliseconds to seconds. So this is the cooling period and you have a afterlow and this is what actually leads uh after of course a few hours to uh the photon signal that we see that we have been seeing and this is based on a simulation by the Basel group. Okay. This is the reference given below. Okay. So supernova SN987A nutrinos were dis were uh detected and this is the Kamio experiment. This is the Irvine Michigan Brook Haven experiment and the Bakhan detector experiment. So these are the number of Newino events that were observed in each of these and so 58 and Kamoka because it's the largest detector 3 kiloton detector it found the largest number of events. So they found about 12 events uh the light curve which of course you see much later. So this is like almost uh I think it was 3 hours or something after uh the neutrino signal you see the light curve and then they have been this has been looked at by the Hubble space telescope uh but also by earlier groundbased observations. So present day detectors are much larger than the 3 kiloton kamyokande which was as I said the biggest detector. I think IMB was some one or or 800 ton detector. Uh and this was also of similar order as the IMB. Uh but present day detectors for instance the super kamoka is itself about uh 50 kiloton and the hyper k is going to be even larger than that with a fiducial volume which will be about 10 times that of super kamukande. So we expect many many more events in that and so this whole thing will just fill up if we are fortunate to have a super supernova explosion nearby. [snorts] Juno is a 20 kiloton liquid cintillator detector and uh of course this has much better energy resolution uh and also a lower threshold than the super K detector. So these Juno is already working by the way and the 50 kiloton of course super communic hyperk will come in in action in another couple of 2 three years time and so if a supernova occurs uh just now then of course you have these two detectors Juno and super kamocante but if it occurs let's say 3 years later we will also have hyperk coming in and that would be a huge improvement on the statistics okay so we have the nutrino know based communication. So this has already been done although it is at a very rudimentary level as compared to communication using electromagnetics uh photons as communication devices. Uh so what was done was nutrinos from the newi beam line were directed to the minurva detector about a kilometer away downstream and that included also about 240 m of earth. So certainly of course um electromagnetic radiation cannot penetrate 240 m of earth. The rate was very low about only.1 bit per second but the error was about 1%. So 120 GV beam uh was put on a carbon beam dump a fast extraction about a 8 microcond pulse two 10 the 13 protons every 2.2 seconds. The inflight pipel new new muse with a peak at about 3.2 2 GV and a width of about full width half max about 2.8 8 GV this results in about8 new mu events per pulse and one or zero corresponds to the presence or absence of the beam pulse okay so for practical newbased communications you need drastically better sources more intense and compact and of course compact detectors this is a reference for this so there is a need for compact neutrino sources and detectors and this is what some some thoughts on this it is clear that these two improvements are needed uh what are the possible possibilities? Laser plasma accelerators LPA are a very promising approach. I mean this sort of accelerator has already produced 9.3 GB nice beams accelerated over 30 cm only as compared to an accelerator of this energy would require something like kilometers of the normal technology accelerators. This has already been done at le uh this is the reference uh in 2024 and uh this just requires a laser of 190 m energy and a rep rate of about 5 hertz a compact detector. Oh sorry this this was for LPA for protons 1 to4 me protons sorry and here this is only 10^ the 9 protons per pulse with divergence of less than one degree uh and uh this is only low energy right now but perhaps we will soon uh maybe in the next few years we might have higher energy protons ultimately you need at least a GV proton to produce uh the neutrino beams. So these are the two references. Uh for protons this is the reference nature communications 2025 and for electrons this is a 2024 reference. Of course you also need a compact detector and this we have already talked about in the last slide that you could use coherent nutrino elastic scattering where the cross-section is about 100 times the charge current cross-section for a equal to 130. If you go to still heavier you get a higher factor here maybe you get 200 300 and so on. that will make the detector compact. Okay. So in summary, we have listed and discussed some open problems in nutrino physics. We discuss futuristic and probably not so futuristic uses of nutrinos as tools in earth tomography, nuclear reactor monitoring. All this would become a reality. Not all of this. I would say many of this much of this would become a reality if you develop compact directed and intense sources of nutrinos and compact neutrino detectors. Thank you. [bell] [music] >> [music] [music]