when a theory makes a prediction that disagrees with an experimental test sometimes it means we should throw that theory away but what if that theory has otherwise produced the most successful predictions in all of physics then that little glitch may be pointing the way to laser physics deeper than we've yet imagined well fermilab's muon g minus 2 experiment has been chasing what might be the most promising glitch of all and they've just announced their results [Music] the standard model of particle physics describes the elementary building blocks of nature with incredible success at some level it's right in a very fundamental way but it's not the whole picture for one thing it doesn't explain gravity in fact it doesn't play nice with einstein's entire theory of gravity which itself is clearly right in its own way our search for a theory of everything which will bring these theories together is perhaps the next great question physics to find the path forward we need to find glitches in these theories loose threads that might lead us to deeper layers of physics we don't have many leads these theories are woven pretty tight but there is one glitch one stray thread that is just begging to be tugged and that's the anomalous magnetic dipole moment of the muon and the scientists at fermilab have just tugged it hard with the muon g minus 2 experiment today we're going to see what's been unraveled and what might lie beneath but first what is all this technobabble what's an anomalous magnetic dipole moment what is a muon what is g minus 2 let's take them one at a time we've actually talked about the anomalous magnetic dipole moment before in terms of the electron that's an episode well worth checking out for a deeper dive but briefly of all the incredible successes of the stanton model this seems the most miraculous it comes from the part of the stanton model that describes how particles with electric charge interact via the electromagnetic force quantum electrodynamics one of the interactions that qed describes is how a charged particle will tend to rotate to a line with a magnetic field the strength of that interaction is defined by something called the g factor for the particle that's the g in our g minus two and we'll come back to that later qed predicts a value for the electron's g-factor that matches experimental measurements to one part in a billion by far the most accurate prediction in all of physics if this works so well for electrons surely it works for other particles too well actually not so much the muon is a close cousin to the electron identical in all properties besides its heavier mass starting 20 years ago experimental measurements of the muon g factor did not agree with the qed calculation and that's not because qed is just wrong rather it tells us that the calculations have missed something and that's something maybe physics beyond the standard model let's start by talking about quantum spin we're actually going to do a deep dive into this topic soon so today we'll just cover what we need to get by every particle with electric charge also has quantum spin this isn't the same thing as simple rotation the particles with quantum spin do generate a magnetic field same as if you send an electric charge around a looped wire or have electrical currents in earth's spinning core the result is a dipole magnetic field with a north and a south pole place an object with such a field inside a second magnetic field and the object will tend to rotate to align with that field the strength of that rotational pull or the torque is defined by the object's dipole moment for a rotating charge that depends on the object's rate of rotation or angular momentum its charge and its mass here's the equation for the classical dipole moment for a non-quantum rotating charge yes this will be on the test and it's also useful for the next thing i'm about to say an electron also has a dipole field and a dipole moment which depends on the electron spin charge and mass but the electron dipole moment is different from the classical one by this factor g for the electron g is around two so the electron responds to a magnetic field twice as strongly compared to what you'd expect for an equivalent classical rotating charge quantum electrodynamics tells us exactly what this difference is to understand this let's look at the qed picture of the world in this theory electromagnetic interactions result from charged particles communicating by exchanging virtual photons in qed you figure out the strength of an interaction by counting up all the ways that this interaction could occur for example a pair of electrons could repel each other by exchanging one virtual photon or two virtual photons or three etc or those virtual photons could do something weird like momentarily becoming an electron positron pair we depict these interactions in findment diagrams each feynman diagram represents one family of ways that the interaction could proceed and the sum of all possible firemen diagrams gives you the interaction strength for a deeper dive into fimin diagrams virtual particles and quantum electrodynamics we have you covered episode list in the description we can represent an electron interacting with a magnetic field with the simplest possible feynman diagram really not even a fulfillment diagram we have an electron being deflected by a single photon from that field if you calculate the g factor from just this simplest case you get a value of exactly two paul dirac first calculated g equals two from his eponymous equation but there are other ways this interaction can happen the next simplest is for the electron to emit a virtual photon just prior to absorbing the magnetic field photon and then reabsorbing that virtual photon same particles in and same particles out but a slightly more complicated sequence of events adding this interaction allowed julian schwinger to calculate a slightly higher value of g equals 2.0011614 over time more and more complicated interactions were added each layer of complication involved many more firemen diagrams but also added less and less to the interaction the latest calculations rely on powerful computers to add many thousands of finding diagrams and get us our g factor to 12 significant figures and a latest calculated electron g factor of 2.001159652181643 by the way the anomalous in the anomalous magnetic dipole moment refers to that little bit extra after the two so the g minus two in fermilab's experiment name refers to that leftover bit measure that leftover bit and you're testing the subtlest interactions of the particle for the electron the current measurement is precise to one part in a billion and to that degree it perfectly agrees with the theoretical number an obvious next step is to do the same for other particles the electron is the lightest and most common of the lepton family it has two heavier cousins the muon and the tau particle muons are nice because they're easily produced in radioactive decay they live only a few microseconds but that's long enough to work with them during their brief existence they're very similar to electrons they have the same exact charge interact with the same forces and have the same quantum spin they have a different g factor because there are slightly different ways that the muon can interact with the quantum fields so you add up all of the firemen diagrams for all possible electromagnetic interactions of the muon but you don't stop there the quantum vacuum is seething with an incredible variety of possible virtual particles there can be very subtle interactions that involve the other forces weak strong and even the higgs field all of these tweak the muon's g-factor by a tiny degree and when we include every possibility encompassed by the standard model of particle physics we get a g factor that's ever so slightly off the experimental results so why would we get the wrong value for the muon but not for the electron well i can tell you what the physicists hope is the reason the muon is 200 times more massive than the electron the probability of interaction between a particle and some massive virtual particle is proportional to mass squared so the muon is 40 000 times more likely to be perturbed in this way it's 40 000 times more likely than the electron to encounter say a virtual higgs boson or a virtual proton or other hadron and it's 40 000 times more likely to encounter any completely unknown virtual particles that might be hiding out there accounting for all of the known particles still gives a muon g factor that's off so the rising hope is that an as yet unknown particle is at work here so finally we get to the fermilab muon g minus 2 experiment prior to this experiment various labs over the past 20 years have refined the muon g factor measurement until now it had been measured precisely enough to claim a 3.7 sigma difference compared to theory that was at brookhaven national laboratories in 2001. there's roughly a 1 in 10 000 chance that random fluctuations could lead to that degree of difference just by chance physicists however prefer a five sigma signal before declaring a discovery for five sigma there's only one in 3.5 million chance of random noise resulting in the same signal the muon g minus 2 experiment at fermilab hopes to push closer to this level of confidence and is designed to achieve four times the sensitivity of the brookhaven experiment at the fermilab experiment physicists send muons flying at nearly the speed of light around a 50-foot diameter magnetic tube the muons interact with the magnetic field and their own magnetic dipole axes rotate like a top just before it falls we call this lamour precession and its frequency depends on the g-factor the frequency of the procession also governs the energy of the particles that these muons decay into so by measuring the energies of those particles positrons in particular the researchers can determine the precession rate and so measure the g factor so what did they find before i get to that you might remember around five years ago physicists at the large hadron collider thought they detected a new particle based on a slight bump at a particular energy of the decay products that was a three point something sigma detection more data was collected and the bump went away it really was just a random statistical fluctuation so has the muon g factor deviation gone away no no it hasn't the muon g-2 team literally just announced their latest result and the confidence is now at 4.2 sigma so not yet a slam dunk detection but definitely moving in the right direction and with more confidence than the lhc bump ever achieved the chance of randomly getting a 4.2 fluctuation is just a little over one in a hundred thousand the helloed five sigma confidence will take time and many more muons unless of course this was a systematic error some unknown factor influencing the measurement that is not a new particle the g minus 2 team worked very hard to rule anything like that out but the only way to be sure is to repeat the measurement with a new independent experiment but if all that goes well then this may be a faint glimmer of light in the current theoretical wasteland you can bet that physicists will be chasing down that glimmer with great enthusiasm in fact i expect a flurry of theoretical papers in the very near future perhaps this time some of them will be right so there you have it that's how we appear beneath the hood of reality we scratch our heads and scroll on chalkboards for about a hundred years then we build a giant magnet and watch the muons dance and that dance may just have revealed to us the next step on our path to a more complete understanding of our quantum space-time [Music] you