Week 8: Lecture 39: Does the proton decay?

So in this lecture we will talk about uh proton decay. So it's a question that uh is still being posed by uh groups uh all over the world. namely does the proton decay. So we'll look at why we should look for proton decay. Uh how do we look for proton decay and indeed nucleon decay because you can also look for uh nucleon uh neutron embedded in the nucleus uh and its decay uh and what are the present bounds and what are the planned experiments ongoing experiments as well. So proton decay would signify beron number uh conservation is violated. So uh there is a berion number non-conservation. Okay. [snorts] Now proton is the lightest uh baron. uh uh the neutron is slightly heavier than that and so are the other uh varian like the delta lambda and so on and uh Sakurov when he was discussing CP violation in the early universe uh one of the things that he mentioned was that you also need uh proton decay and this was a hypothesis uh this was in 1967. Now there is a prediction of all these models that uh would like to unify the strong and electroeak interactions namely the grand unification theories called guts for short uh and they put quarks and lepttons the fundamental particles that we know in a bigger family so there is mixing between them and that leads to uh beron number non-conservation if CP violation is there then of Of course you also get matter antimatter asymmetry in the early universe. So suppose there is a matter anti- matters asymmetry like one part in 10 the 10 in the very early universe just after the big bang then what happens is that 10 minus1 matter antimatter particles annihilate produce radiation but that one uh odd particle which is produced by this very tiny asymmetry that survives and that is what we see in the early universe. So that was the uh theory of Andre Sakarov and then he laid down several conditions which have to be satisfied in order to get this uh matter antimatter asymmetry in the early universe. >> [snorts] >> Now as I said already this also uh one of the things that is a consequence is that proton decay is almost inevitable uh in both in these gut theories and uh the also one should also say that the beron number conservation is a is not a symmetry which is protected like let's say the electric charge conservation which is because of a local gauge invariance. in the electromagnetic field. So uh it it could very well be that uh berion number is not conserved and so then it is uh uh of course uh necessary that you look for beron number non-conservation. The early uh models which unified uh electroeak and strong interactions were due to Jokesh Pati he's a Indian origin scientist uh and Abdul Salam in 1973 and independently by Howard Georgie and Sheldon Glashaw in 1974. So the predicted modes are several of course but some of the more prominent modes for these models is the electron and pion uh pi 0 then the neutrino plus k plus for proton decay. So this is uh E+ it should be uh this should be E+ron and a pi 0 uh outgoing channel. So these are the people involved who proposed these gut theories. Of course there have been several now uh since then but these were the early ones. Uh so this is Jokesh Pati, this is Abdul Salam, this is Georgie and this is Sheldon Glacial. Of course Abdul Salam and Glacaw got a Nobel Prize. They shared the Nobel Prize with Weineberg uh for the electroeak unification. So the the first detector to get off the ground, first large detector I would say. There were experiments which looked for proton decay earlier to the colar gold field experiment. But the first large detector to get into action was the one in Colar gold fields in southern India. And uh of course there is a history to the activities going on in Colar gold. was started by homie Baba uh in 1951. Uh basically Professor Shriikantan was asked to go and measure the muons at various depths. So what is shown here is the entrance to the Champion Reef mine which was the deepest mine. I think it went down to something like 3.2 km deep at its deepest. So uh at this lab when the uh muon fluxes were measured as a function of depth it was realized that if you go sufficiently deep uh and also look at sideward events then that is a good place to look for atmospheric nutrinos and indeed the uh the atmospheric nutrino detection was done in 1965 by a TFR Osaka City University and Durham University collaboration. So these are the uh this is the paper which was published in physics letters. Interestingly the publication came out on the 15th of August 1965. So what they this is one event which is shown here. uh they had several uh detectors and there was a track corresponding to nutrino interacting with the rock and then producing muons which were tracked in this uh detector. [snorts] So a photograph of that detector is shown here. Uh so there were actually several detectors three to four kinds of detectors. uh a an experiment was also done in the South African gold mine at similar depths uh by a group led by Fred Rehes who if you remember actually discovered the electron neutrino in a reactor experiment in uh the mid50s. So he also uh uh carried out this experiment and discovered atmospheric nutrinos. The paper was published about two weeks later than this paper. So these are samples of nutrino events uh front view and the side view. What they realized when the TFR Osaka Darham group measured these uh muons uh they they found that there is a atmospheric neutrino muon flux which goes like this with zenith angle and at larger zenith angles. So which means it traverses a larger uh amount of earth before it reaches this detector. Then it kind of uh flattens out and this flattening can only be explained if there are now nutrino events here and not atmospheric muons because the atmospheric muons would have gone down this way. So for instance they would be very small uh in flux as compared to the event rates that were uh found in the experiment. So in any case uh what was realized was that if you go to those depths uh more than about uh uh 2 kilometers or so at uh kgf that is a good place to put a detector to look for uh proton decay. Before I go there, let me also point out that there were other searches which were carried out before and uh they are very nicely summarized in this annual reviews of nuclear and particle science uh review article by Don Perkins in 1984 where he talks about the proton decay experiments. So the radiochemical searches were the first ones and uh they were I think they they were done even in the 50s and 60s okay before some of these big detectors got off the ground and one of the ways of looking for it was for protonuc or proton or nucleon decay was to look for daughter products. When a nucleon in a nucleus decays then it can cause for instance if it is a heavy enough nucleus it can cause fishision and for instance thorium 232 has a very long halflife of the order of uh uh 10 to the 10 billion years or more uh sorry 10 to the 10 years which means more than 10 billion years and uh they looked for fishision which can happen if a nucleon in this 232 thorium decays uh Then you get fishision and you can see tracks in mica caused by the headrons that come out from the nucleon decay causing spellation reactions because the amount of energy released is large. Uh so you can either look for fishision or for tracks in mica caused by nucleon decay. So these are two different kinds of experiments but both uh either radiochemical or track detector experiments. So the lower limits uh the best ones were for the fish part where were I think in something like 2 10 27 years should be years here. Uh so uh this is the kind of limit that limits that were put using radiochemical or track detector uh experiments. These are kind of offline experiments. They are not realtime experiments. So you expose something and then look at these tracks and then infer a lifetime you know you know the uh number of nucleons that there are and you have seen uh how much is the background and basically it was background that they saw but you can put a bound uh a lower limit on the lifetime direct or realtime searches were also carried out later and as I told you the the first one to get off the ground was the KGF experiment and that used layers of iron and active detectors. So the first detector was a 140 ton detector u uh at KGF and that used proportional counters 10 cm by 10 cm square counters long counters of the order of several meters in length and they were crisscross. So one layer had let's say horizontal uh oriented detectors and then the other layer subsequent layer had detectors which are also horizontal but at 90° to the earlier detectors. So you could actually get X and Y information not in the same detector but in subsequent layers. So you could track the particles charged particles that were produced if uh the proton decay. uh other detectors were used in new sex uh which is streamer counters. Similarly flash tubes were used in fridges and the of course these came online a little after the KGF experiment. So the KGF was the first one to put uh uh lower bounds on the uh lifetime of the proton. Uh later there were also much bigger detectors. uh the Kamioa detector was a 3 kiloton uh water churn and cough detector. Similarly, Irvine Michigan was I think something like a 2 kiloton or I mean they were in the same ballpark uh both being water and cough detectors. These early experiments gave lower bounds on the partial lifetimes uh assuming of course uh likely decay channels because of course if uh the proton decayed into all nutrinos then these uh bounds don't hold because then the nutrinos would not be detected uh in the detectors that were used. Uh but if you look at the uh predictions of some of these theories and uh in any case you look at uh charged particles then you can put bounds on uh some of these branch uh lifetime times the branching ratio and these came out to be of the order of 10 ^ of 31 years or so. So no evidence for this was found although there were candidate events maybe a one or two candidate events but they could have been due to background. So the conservative uh approach said that the there is only a lower bound to the proton decay lifetime. So this was the KGF phase 1 nucleon decay detector. That was as I said a 140 ton uh iron uh proportional counter iron proportional counters kind of uh sandwich detector. And this is some of the early electronics that was used. Uh and this is a picture taken when Professor Abdul Salam visited the underground laboratory. Uh this is professor Shriantton who started the first experiments at uh uh the Kolar gold fields. Uh this is professor Mandal and this is professor Narimum and so on and several other collaborators. uh uh the KGF uh collaboration of course upgraded their experiment and they ran it for about 8 years or so uh in the phase two of this uh experiment and that was about a uh 350 ton or so detector. So this is a picture shown when the detector was being assembled and this is a picture of the uh collaboration part of the collaboration not all the members are there and this is another view of the same detector. So in the uh 1982 Paris conference Perkins summarized the situation uh in the following way. uh nucleon decay if it is ever discovered will have to be based on unimpeachable evidence from several independent experiments using different techniques and in that year of course he said we are a long long way from such a goal. uh present experiments of course put much more stringent bounds and uh the most stringent bound as of now uh for the channels that have been looked at comes from the uh super kamioe water and detector which is a which is a total mass of 50 kiloton but a fiducial mass that means the uh the outer part of the detector acts like a shield and so you look at only events that are sort of inside this 50 kilot detector in inner volume. So the fiducial mass is less than half of its total mass. And uh if you use that then the uh toao is greater than the branching ratio times 2.4 10 34 years at 90% confidence level. So toao by branching ratio is something that is quoted by all these big detectors. Uh so other DK channels of course have their own uh bounds uh depending on the branching ratio that is assumed. And so the other uh branch branching that is looked at is instead of p going to a e plus pi0 you can look for a mu plus pi0 or a neutrino and a k plus mezison and so on and you can put bounds on that and indeed they have been put by all these uh groups. Okay. Now we'll come to another interesting possibility. uh it may not be the most popular possibility that has been looked at but nevertheless uh the question that some people have asked is is the exponential decay law valid for very short times as compared to the lifetime that you are uh looking at. uh for instance the exponential decay law has only been tested for t by toao where t is the observation time uh and uh toao is the lifetime uh for greater than 10us 10 uh so this is a paper by Norman and company in 1995 they looked at potassium 40 decay uh and uh So you can produce potassium 40 by uh irradiating potassium 39. But you can also look at uh the long uh something that has existed since the beginning when the earth was formed and you can look for possible differences in their decay rates. You know what is the potassium 40 that is there in either of these samples or even a mixture doesn't matter. As long as you know what is fresh and what is old, you can look for any differences in the uh decay rates and uh such a technique was used to put a bound on decay at short time. So for t by to greater than 10 the minus 10 indeed the potassium 40 shows exponential decay. [snorts] Uh more stringent tests of course could be performed using longer even longer lived uh uh nuclei such as 209 bismouth which in principle decays to alpha plus lead uh sorry that uh 81 thallium 205 or for instance uh zenon 136 under goes uh double beta decay with two neutrinos and two betas and that has a halflife uh that is a lifetime of about 3 10 21 years or for instance 128 to which has again a uh 2 nutrino or 2 beta decay halfife of 3 into 10 24 years. So if you can somehow freshly produce this in let's say a year or few years uh and then look for its decay and look see if it is different from the old uh bisma decay then you can look for possible differences uh in t by toao which is of the order of 10 the -19 -21 or 10 the minus4 and this would be relevant for proton decay because you're looking at protons uh decaying with half lives of the order of 10 to the 31 32 years and so on and the earth uh the lifetime of the earth is of the order of uh a few billion years. So a few billion by 10 the 33 is of this order 10 the minus 23 24. So if you can test the exponential decay law on these time scales uh using these nuclei then perhaps you can say whether you can have confidence that the exponential decay law is still valid. And why is all this uh you know why is it that people worry about such a exponential decay law at short times. There is something called the kaline theorem is published in ' 68 and there is also a reference in physics letters. uh the English version uh again written by Khaline it it this theorem says that the decay rate of an unstable state using very general assumptions of positivity of energy and so on of a free quantum system uh that is zero ex identically zero at t equal to0 now of course the theorem doesn't say on what time scale it comes back to the exponential decay law or how it varies in between. Uh however, this so-called quantum xeno effect was discussed by Mishra and Sudaran who concluded that uh the slowing down of the evolution of a quantum state in the limit that the state is observed continuously. Uh so you you this called the quantum xeno effect. If you keep observing a quantal system, it will not decay to first order. Okay. And this is you can keep looking at it and this was actually observed experimentally by itano uh in 1987 experiment on an atomic system in an atomic system. So the argument goes like this. If I look at the time evolution of the state vector of a quantal object and h is the Hamiltonian then for small t you can expand and s of t is just 1 minus i is t uh plus second order terms operating on the state vector at zero time. Now if you take uh the probability of survival then you just sandwich this s of t s of 0 and take the square you get a 1 minus delta h where delta h is defined like this delta h squ is equal to the expectation value of h squ minus expectation value of the hamiltonian squared uh and the difference between the two uh then you can see that if you take the derivative of tp dp then this of of course is a constant so that vanishes you get a t dependent term and if you take the limit that t is equal to zero then of course dp by dt is zero. So probing a system at sufficiently small intervals prevents the state from decaying and this is the calfen reference in physics letters uh 1982. So Fleming uh uh this publication is in 1983 proposed that the non-observation of proton decay could be just a consequence of the culfin theorem. Uh okay so if if you don't observe proton decay it could mean that maybe the exponential decay law is not valid at such short times. Of course, uh, Sudarian argued that this is not so that in a in a nucleus or even in a perhaps a free proton, there are things happening inside that, uh, are akin to observing something. And so, uh, you know, the the you will still see exponential decay on time scales, uh, much larger than 10us 24 seconds. However, uh I think this argument of Lemming has not yet been refuted. So, as I already pointed out, the big unknown is the time scale over which the decay rate reaches that of the exponential decay. So, uh if you were to plot let's say uh uh lambda as a function of time versus time, then it is zero at t equal to0. But how does it reach this uh uh lambda infinity? Okay, this time scale uh is not known and uh if it is extremely short 10us 24 seconds then of course it will be it will follow the exponential decay law. But if this happens to be of the order of let's say uh a billion years or 10 billion years uh then uh you might not observe this if uh [snorts] this falls within the calfen regime of close to zero time. Anyway, this is an at least an intriguing possibility. Future prospects for proton decay searches uh are some of them are listed here. There is already an experiment called Juno which is looking at reactors electronutrinos to look for the mass hierarchy to look for the mass ordering of the three mass states. Uh they are operating at 20 kiloton a huge liquid cintilator. Uh it's 20 times bigger than kamland as we will see in a later lecture or you know borexino which is only a 300 ton liquid centilator. So this is the biggest liquid cintillator uh experiment uh in the world and uh this could also address uh proton decay. Uh so this is one experiment that is already ordering. This is on their menu. Uh not just looking at uh uh nutrino mass ordering but also looking at other exotic things. They will also look at solar neutrinos uh proton decay etc. There's a whole menu which they have. [snorts] Similarly, hyper chamocioande is a detector which will have roughly 10 times the fiducial mass the active mass so to speak uh as compared to its predecessor the super kamio detector which had a fiducial mass of about 22 kilotons. So this will be about 10 times bigger and this will commence uh data taking in 2027. This is in Japan uh in the Kamoka range of mountains. [snorts] Uh there is a experiment which is coming up in the US. Unfortunately, it has got delayed but uh it is based on uh ultimately what they hope to have is four 17 kiloton liquid argon time projection chamber detectors and that is also designed not for proton decay but to address the neutrino mass hierarchy. uh and uh they will do it uh once it gets going they will have uh evidence for the nutrino mass hierarchy at more than five sigma level of uh which is the gold standard in particle physics in just a year but it is it has a higher goal namely to look for CP violation in uh in nutrino sector of course because it is big and because it is a modern version of a cloud chamber it can also address proton decay and uh certainly it is a very high resolution spatial resolution detector and it can look at some of the decay modes that other detectors cannot look at. So uh this is a picture of the Juno detector in China. This is a picture of uh what HyperK would look like uh when it comes up in 2027. And this is the dune detector which will come up which is coming up in uh the homestake mines in the US. You can see that this is a huge detector uh almost like uh you know 70 m long and about 20 m wide 20 m in height. Uh what is shown for comparison is uh the blue whale. Okay. So it's just a it's just a small entity as compared to this huge detector. Okay. So in summary we have discussed uh the proton decay uh searches the early bounds on proton decay uh some things about the KGF experiments then the the lowest bounds that have been got by super kamio and an intriguing possibility related to the quantum xeno paradox and the kfine theorem and also what future experiments are and what they might tell us about whether proton decay is there or not there at the level that at the level of sensitivity of these experiments. Thank you. [music] >> [music]