NEWS: What's up with Muons? - Sixty Symbols

yeah a lot of excitement brady a lot of excitement because particle physics is still in the game it hasn't died it's amazing isn't it it's it's a bit like waiting for buses i was thinking about this the other day you know you're waiting for a bus and then all of a sudden three come along at once yeah we were all worried there was there was no new particle physics to be found there was no new particles we'd found everything that was going to be found we're waiting for a new particle and then all of a sudden the bbc announces at least two there are sort of tantalising hints of some really exciting new physics um from two experiments recent sort of announcements from two experiments one of them's at cern it's um the lhcb experiment and you might recall we've been to the lhc and we saw the atlas detector and we saw the cms detector they're called general purpose detectors these huge fantastic pieces of kit which discovered the higgs there are two more key detectors on the lhc ring one is called alice and but there's another one lhcb so the real excitement the real excitement is coming from the experiment coming from fermilab where they've been looking at the magnetic moment of the of the muon lhcb is is a high precision detector that's looking for very fine events that might not occur very often but by searching in particular for particular types of decay then they've got a way of testing the standard model of particle physics very accurately and it's through there that this potential new discovery has emerged what they did was they if you remember at the lhc they fire protons around in opposite directions and then they collide them and they collide them here in in the lhcb detector they're looking for the decay of a particular type of meson a meson is a combination of two quarks a quark and an antiquark and they're looking for a meson called a b meson which has got a b quark in it that we've got these three generations of of quarks and the b meson also called the bottom of beauty is the b quark is often associated with the top quark you and i are made up of the up and down quarks in fact everywhere around us here all that we're re everything around here is made up of up and down quarks so these b quarks are very rare and they decay rapidly they form in these mesons and they decay such that when they decay they decay to another quark which is called an s quark this is what the particular route they're looking for a strange quark and as they do so they decay producing two leptons what are the electrons are what you and i are made of electron is an example of an electron of a lepton but there's another key lepton called the muon and the muon is is going to be the star of the show in particular the number of times the b quark decays either into electrons and a positron or muons and it's anti-particle and the the key thing that it's trying to measure here is the relative distribution of these like how many times does this process the decay of this b meson go into the electrons compared to how many times does it go into the muons and there's a very important principle from the standard model called lepton universality and just this idea of universality suggests that there's a common theme between the various leptons and in fact the belief is that you should produce electrons just as often as you produce the muons so that if you can detect enough of these decays then you'll find half of them the time they're electrons and half of the time they're muons well about uh a decade or so ago there were experiments which suggested this wasn't quite working out that actually they were producing muons at a a slower rate than they were producing the electrons whereas you would have expected them to be at the same rate and so what they have done this quite remarkable team of working at the lhcb is they've been assembling data from 2012 in fact i think maybe even earlier but certainly 2012 onwards looking at all the decays of these b mesons into what are called k masons but basically it's the bottom quark decay or the beauty quark decaying to the strange and looking at the output in terms of number of electrons and number of muons you would expect in sound processes that you would decay just as equally in terms into electrons as you would into muons there's no there's no preference there because the couplings are the same everything's just working at the same strength but that's not what seems to be happening there seems to be a preference for for electrons the ratio of the number of times you decay to the muon divided by the number of times you decay to the electron isn't one but it's about 0.8 so it's decaying to the muon far less often than it's decaying to the electron and an interpretation of that is that maybe there's another particle playing a role in this that we haven't detected that's played no major role in anything else we've seen but it's actually stopping the muon somehow from being from from it decaying to the muon it's allowing the electron route to go through but somehow it's stopping the muon and so it's there's been a lot of work on this type of thing for many years and there's a number of particles out there which have been suggested as possible ways of explaining this and probably the most common one that people have thought of is called a leptoquark leptons and quarks we usually think of as totally separate particles but a lepto kind of is a new particle which brings them together and and it has properties of both leptons and quarks so it can feel the strong force as well as just feeling the the the weak force that the an electromagnetic force that that the quarks the electrons feel and so i think there's going to be a lot of interest in the possibility that this process is now taking place so there's this new particle existing for a very short period of time that then decays into quarks and leptons and it's in such a way that it's preventing the b the beauty quark from decaying into the muon as often as it decays into the electron why does it have to be a particle why can't it just be that nature favors the electron root or something like that it's a very good question and in fact the standard model seems to suggest that it should go evenly in both directions so if it's favoring one clearly favoring one over the other we're assuming that the experiment is is correct and is picking up all the new ones that are being created then it's hard to do it within the standard model itself so now so then what do you do and it's just that the easiest thing to try and come up with is another particle one thing i should mention is perhaps it gets a little bit technical but it's a boson we have our fermions the stuff that you and i are made of the matter in the universe is made up of fermions things with spin half but these are effectively bosons which are force carriers so the photon has a spin one the graviton has a spin too and this is this has got a an integer spin it's a boson and so it actually also acts as a force so it could be that this is in the news you might have heard you know they talk about a fifth force and this is acting as that fifth force but it's hard to reconcile it simply within the standard model and so you have to come up with something and if you've got your favorite candidate that you can think of that might be some fluid or then bring it to the table in fact there are many candidates i mentioned the leptor quarks some people think of a particle called a z-prime particle this is just the z-particle but very massive said particle and then there's another called vector vector quarks which are sort of there's a whole series of them so the second boss that came brown is also quite remarkable i mean this is the g minus two muon experiment so what the muon is is like the the electrons heavier cousin it's essentially very like the electron in every sense and but it's heavier it's about 200 times heavier than the than the electron and just like the electron it has what's called a magnetic moment this basically just is is is it's a little bit of sort of intrinsic magnetism before we describe it let's just let me just give you a little bit of background of this experiment which is a ring basically which is 50 feet across 15 meters it was initially performed at the brookhaven national laboratory on long island and then once they'd finished and done its experiment they decided the people involved in it decided they would like to upgrade it and also take it to somewhere where there's a better source of new ones but to get it from long island to fermilab which is in batavia illinois rather than going over cross land it went out down the north atlantic up through down to sort of new orleans up through the mississippi river into the illinois river and then got put on around chicago area got put on a huge huge lorry and got taken up the highway to fermilab what this experiment does is again it looks at the muon if you didn't have sort of you know all the corrections coming from quantum field theory things like that then the magnetic moment of the of the muon like the electron it would be two it would just be the the number two that's related to the fact that it that it uh it has spin a half but the answer is not exactly two okay it's slightly bigger than two the magnetic moment of the of the of the moon is slightly bigger than two and the reason is the reason it's slightly bigger is it comes from sort of quantum corrections to that number so what happens is is the muon it can sort of shapeshift and the mu one is a particle that's got a charge and so it's got a dipole which means it acts like a little dipole magnet a bar magnet and if you if you move that muon in a magnet in a circle in a magnetic field it will begin to wobble a bit like a spinning top you know if you push a spinning top down and let it go around it just it doesn't keep at the same height it sort of wobbles as it goes around and you can measure the frequency of this wobble now the frequency of this wobble tells you about the strength of the magnet that the dipole is it's called the magnetic moment of the dipole so this is what's being measured okay and the the strength of this dipole it can give off a a photon and then re-absorb it and this will create a correction to to the you know the predictive value for the for the magnetic moment that photon itself can sort of shapeshift into an electron positron pair that can then be created and annihilate become another photon which gets reabsorbed there are all these sort of quantum corrections this cloud of quantum corrections that can affect the value for the uh for the magnetic moment then people sort of naturally said well it's not just electrons and quantum electrodynamics that could be important maybe w and z bosons might be important maybe they're popping in and out of existence as well and so they started doing calculations based on the what's called the vacuum fluctuations of these particles and they modified this number it still was two 2.00223 but you had more digits added on and everything was fine until you reach the seventh digit the seventh decimal place because at that point you have to start including the effects of quantum chromodynamics the quarks that can exist and they they're formed by the electron or the muon it could be any of these electrons and the muon being much heavier is one that is worth probing because it can create these extra particles around it in the vacuum fluctuations the muon can lead to it a photon will emerge a virtual photon then decay and then from it comes a quark antiquark pair which then reconnects to a photon and then it gets reabsorbed by the mu and that's the process but this quark antiquark pair you have to be able to account for and that's where the problem is what we found in 2001 was that the theoretical expectation for the for the magnetic moment was different to the to the observed prediction they disagreed at the eighth decimal place okay so the eighth decimal place when you really start to probe these rare processes the numbers differed and so people at the time got very excited about this but of course it wasn't statistically significant then i think it was around a sort of three sigma results okay and then you know not quite enough to classes as a genuine discovery in particle physics but enough to get people excited so the theory now is to try and calculate that and there are two approaches to these calculations for the quarks they they're called the virtual hadron processors and they differ now one of them differs in a way which is consistent with the observation of this experiment which i'll now describe and the other differs in a way which leaves it inconsistent so their value for g is inc is different to the experimental value by 4.2 sigma and if you recall five sigma difference between a theory and an experimental value would be a discovery 4.2 is pretty good it's getting close isn't it the experiment is this 50 feet or 15 meters for our european colleagues ring so fermilab they can produce high-energy muons they fire them into the ring and they go around in a big magnetic field and if you recall i said that because they've got a magnetic dipole they begin to wobble a muon's heavier 200 times heavier than a than an electron otherwise pretty much identical it doesn't want to stay there it lives for about a millionth of a second by which time it's circled around about a few hundred times but it then decays to an electron it produces an electron and the electron gets detected in the detectors surrounding the ring and the muons are carrying on going around and decaying and by the by looking at the energy of the electron that comes out and the direction that it comes into the detector you know about the wobble and the wobble if you recall the wobble of its sort of parent muon yes exactly and that wobble tells me directly about the muon magnetic moment this g that we're trying to get it's very hard to calculate these these sort of hydronic contributions to the magnetic moment of the muon it's very difficult to calculate but people got better at doing the calculations they started to zoom in on an agreed value for for the sort of standard prediction and then of course we needed a better experiment which could really pin down what the observational result was and they've done this now for they've been doing this experiment for the past i don't know five five six years and they've begun to they've now started releasing their data they've released the first six percent of their data the key thing they're trying to do of course is control the errors reduce the systematics make sure they understand and i sadly i can't do that justice but that's a remarkable achievement that they've done so the theory seems to have tightened up a bit but also the experiment is as has now tightened up as well and now we've got over a four sigma result which again is still not quite enough for it to clusters of particle physics discovery but it's really starting to get interesting now and it does seem that it's sensitive to new physics it was all done blind so that when they released the data all of the teams saw it for the first time there was a key thing a key frequency clock which was they didn't know about and that they were given that information that then allowed them to plug it into a key formula to get what g is and they found the number g that they got was the same as the number that they they'd got at the brooklyn national laboratory and if you remember what i've just said that is 4.2 sigma away from the theoretical prediction that came from this international group of of uh physicists who were working on the theory behind it and so there was this remarkable result that's emerged that suggests if it holds that the theory is based on standard model physics the experiment is the experiment you just do the measurement and it's once again suggesting that there's something else happening that the muons are being tilted a bit more because the actually the theory result is slow is smaller the value of g from theory is smaller than the value of g from the experiment and so it means the magnetic effect is larger in the experiment so whatever is causing this change if it is new physics is causing the muon to have a slightly larger internal magnetic field than it otherwise would and leptor quarks and other possible candidates are out there let me see if i've understood this correctly it sounds to me like in in nature in physics there are particles popping into and out of existence spontaneously all the time like a sort of a soup yes and one way to detect that soup to measure what's in it what vegetables and carrots and sprouts and things are in it is to look at how that soup affects muons and so they've been doing really really good experiments with muons to see how they're being affected as they travel through this soup to get a better and better idea of what's in the soup perfect and now that they're getting these measurements better and better there's a new vegetable that they've never seen before absolutely there's this quantum soup that surrounds all the particles now you can imagine taking the particle out of the soup right and that's what we do with our first approximation in our calculations we take the particle out of the suit we do the calculation we make a prediction okay but then we start putting the particle in the soup right and we see that the soup starts to affect our results and absolutely you're right if there's some sort of i don't know alpha spaghetti spaghetti floating around in there or something it's going to affect your results and muons indeed because they're heavier they're more sensitive to that unexpected unknown physics that's that's that could be out there then say the electron would have been this is a really interesting time first of all it's worth pointing out at the beginning i mentioned this high precision experiments these deviations between the experiment and the theoretical predictions are in the eighth decimal place of the calculation and also there is another group i said of these two theory groups approaching this problem the theory group that the experimental group have aligned with in in the sense of comparing their results directly with have had to incorporate experimental data into their analysis not not the experiment of the g minus two but other experimental data to deal with this hadron question the vacuum polarization of the hadrons the and they're the ones that have come out with the very low number 4.2 sigma away we maybe shouldn't get ahead of ourselves it might not be a new particle right so what it could be is it could be that the theory prediction is still not right and it's a really unusual state of affairs now we actually have there's been a rival paper which is claims to have done the the theory calculation in a different way and gets a result that's more in line with just with standard model physics nothing fancy but which is more in line with the observed volume there is another group of people a smaller group but that have just simply used qcd on a supercomputer they've just looked at lattice q euclidean lattice qcd it's called worked on a supercomputer no experimental input using the standard model and they have come out with the result which is actually compatible with the experiment okay and so we are in this amazing situation where we have the two theory approaches this this shows you how we that science is not a single track to get to somewhere these two theory approaches both trying to solve a problem are coming in with numbers which are inconsistent with each other one of them is consistent with the experiment which was consistent with the original experiment so it suggests the experiment is okay right if these two are consistent 20 years apart sort of accepted result the one that we're saying is different to the to the observation what they do is they they take a sort of data-driven approach they're worried about the effect of these hadrons in the quantum soup right so those are very difficult to calculate hadrons they're strongly coupled they're all tied in with gluons in this sort of gluey mesh it's difficult quarks are a pain to deal with right this is difficult so rather than actually try to calculate that because it's too hard they take input from other experiments or this so it's a data-driven approach say well given what that experiment's showing this probably suggests that this value of this thing is about this therefore we'll calculate the rest then they get an answer for the magnetic moment of the muon that doesn't agree with experiments but the other guys this bmw collaboration they're called right nothing to do with the cars named after budapest marseille and uh vuter pal i think the name of your city is so this this bmw correlation what they've done is they've shoved everything into a computer a really fancy spanking super computer that literally puts all those quarks on and hadrons on a giant lattice and does the brute force calculation right really really sophisticated something that you could never have done decades ago really powerful calculation and they've done and so they've done this calculation they're getting something more in line why don't we just think that the first group are the one that got it right that seems like the more obvious solution yes i mean it and and and quite a few people think this will probably may turn out to be correct which would be in some ways sad if you're really looking for new physics which we know has to be there somewhere we still don't know why they we're certain about matter and not antimatter we don't know what the dark matter is we need some new physics it's entirely possible that there's some systematics that aren't quite right in the in the lattice calculation in the bmw calculation but maybe not it's a really really impressive calculation that those guys have done or maybe the data-driven calculation isn't right but even if the data-driven calculation isn't right there's something going on here because they're using data so it's like is there something we're not understanding about that part of the data and how we feed it into this calculation it still suggests smells that there's something we're not understanding about fundamental physics here so either way whether the whoever's right about the the prediction here there's something going on that we don't understand it might not be the magnetic moment experiment that that's that's we're missing it might be something else i think the fact that we're seeing something at the lhcb this muon this decay of the beauty quark to muons far less than than it goes to electrons and then this g minus two also involving the muons is suggesting there's something going on the muons the heavier leptons are being affected by this by something and so i'd i'd like to waver on the side of saying there's something there i think it's a new physics i i really do and so there is i mean look there are reasons for other reasons to expect new physics to be just around the corner you know we talk about why is the higgs particle so light this is a mystery that we don't understand why is hex particles so light new physics can explain it you know there's things like what is dark matter you know we know dark matter's out there we need a particle to explain it where is it okay we there's there's another sort of quirk which is that the the um we like to think that all the different coupling strengths of particle physics will come together and unify at some high energies and you need new physics to make that happen so this this beautiful elegance so there's lots of reasons to expect that new physics is just around the corner it's why we built the lhc for example right so the fact that they had now and this is why everyone was freaking out because it was like it didn't seem like there was any no experiment was showing anything up except for this this is finally pushing us in that direction that maybe there is something there you just have to look in the right place and that's really exciting the day after the day after this announcement of g minus 2 30 papers appeared on the archive another 13 today that paper the prl that has appeared already has 36 citations after two days it's just kind of through the roof now of course there'll be a thousand papers i'm exaggerating but there'll be a lot of papers come out in the next few days people ambulance chasing explaining what explains this this this new new phenomena i mean we were i was chatting to some collaborators yesterday right and we weren't thinking about the the muon experiment you know the neon experiments at all we were just thinking about trying to explain dark energy and string theory and then we had this particle in it that i was a bit worried about how light this particle was and what what it might affect and we did some digging while we were on teams obviously everyone's on teams aren't they and we actually came across a constraint based on neuromagnetic moments and i was like well this could be a you know sort of joking right but you never know these two announcements have obviously made a real celebrity of the muon which which is a particle that doesn't get a lot of love i don't know much about muons i like the new one why are you even harsh on the view of i think it's great you make it sound like they're really unstable do they are there muons in this room on a permanent basis or are they kind of uh sort of a transient visitor they'll be if they're there they'll just be trans yeah they're certainly not here sort of on a permanent basis and and they they were first detected by cosmic rays you know the cosmic rays is this we need to do something on cosmic rays by the way but the cosmic rays the highest energy particles we've got hit the atmosphere and and produce all fundamental particles that that that are consistent with the e equals m c squared and the muon is one of them it decay it decays in about a millionth of a second so if they're popping up here they're decaying very rapidly do you get jealous that the experimentalists seem to get all the headlines and the big moments and the big stories i'll behave i mean like a lot the superstars of physics are all theorists right of course they are that's that's without question so so now i mean come on einstein new and all these superstars they're all theories certainly experimentalists are just the lumpers don't say that don't forget allows you then to read off where the particles have come from the particles aren't affected they then move out to a second set of detectors called the calorimeters and the calorimeters actually stop the particles

Channel: Sixty Symbols

Views: 269,277

Rating: 4.9584827 out of 5

Keywords: sixtysymbols, muons, muon g-2, lhc, particle phsyics

Id: kBzn4o4z5Bk

Length: 27min 36sec (1656 seconds)

Published: Mon Apr 12 2021

Reddit Comments

[u/Shaneypants]

Very nice overview of the situation. I had seen the BMW paper posted here and I read the abstract but it was nice to hear the takes of some particle physicists, as it is not my field and I had no idea how seriously it is being taken.

[u/Hippie_Eater]

Seeing Ed Copeland get excited feels really wholesome.

[u/Baxterftw]

2 of my favorite speakers on the channel

Timely episode and Brady asking excellent questions as usual

[u/CarsCarsCars1995]

B1!

[u/wonderingdrew]

I'm sure I'm missing something obvious but when the B meson decays and the result is a Kaon and either election and positron or muon and antimuon is there no problem balancing the equation given the different mass of the electron and muon?

Like in terms of mass, should kaon + electron + positron = kaon + muon + antimuon when

208 electrons = 1 muon ?

[u/Tectix]

I watched this and some of it went over my head. I got the basics, and I’ve watched every other SS video on particle physics, but I crave more. Anyone have a recommendation outside of the official Fermilab channel?

[u/MrHall]

https://theconversation.com/proof-of-new-physics-from-the-muons-magnetic-moment-maybe-not-according-to-a-new-theoretical-calculation-157829

has anyone talked about the result along with these predictions?

it's weird to see this in the conversation and not mentioned anywhere else, but it looks reputable, published in nature.

EDIT: ok I just got to the bit where they address this exact paper!

[u/EstablishmentNo8714]

Thats mean you can download everything!

[u/petezilla]

I've been watching everything I can about this for days, and this is the video that tied it altogether for me. Everywhere else is really just emphasizing the Fermilab results (& re-explaining muons/anomalous magnetic dipole moment), couldn't find a good explanation of the BMW, and I hadn't even heard of the LHCb findings at all till now. Glad to have found a new channel to watch