A highlight from Big Physics News: The Muon g-2 Experiment, Explained

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This is scientific american sixty seconds science. Podcast i'm clara moskowitz. There are probably many more particles out there in the universe than the ones we know about and today physicists. Got a hint about where they might be hiding. The finding comes from an experiment at fermilab called mu on g. minus two which looks particles called nuance that are heavier cousins of electrons. It turns out there spins wobble more than the standard laws of physics. Say they should here to tell us all about. It is david herzog of the university of washington. One of the physicists. On the experiment by the way this segment is on the longer side dear listener but hey this is complicated physics david. Thanks for being here. Thanks claire this is a really exciting time for us okay. let's get grounded. Why are new important. Well since the discovery of the mule on its played actually a rather unique in versatile role in subatomic physics topics that people use meals for range from fundamental constants of nature basic cemeteries week nuclear nuclear interactions and for us we care about the most is standard model tests and searches for new physics. That's what we're gonna do with now. The mmu on is an unstable particle. It only lives for about two microseconds. But that's sufficiently long to precisely study its properties and yet it's actually sufficiently short so that we have enough decays that we can also study a lot of tremendous information or in the decay processes now work of nature which we parody non conservation or or space non conservation. Milan's born what we call fully polarized they have spins in a direction that we go like top stew and when they decay. We say they are self analyzing. We can figure out which way were spitting when they decayed and these. Two attributes are essential for the experiment. That i'm going to talk to you about so tennis by iran this experiment in the first place. Why look at new on spin. Well when we measure the rate that the new on spin wobbles are i use the word processes in a magnetic field. We learned directly about its own magnetism which we call the attack moment. But you might ask. What do we care about that for while the laws of physics predict this magnetism very very precisely and the loss if we think we know them completely intern. Telus the rate of that wobbling that we should expect in the magnet so measuring the rate we can learn. The laws of physics are missing anything. Tell us about the set up of this experiment in basic terms. How did it work. It is a very very complicated setup player but let me just try to break it down simply we should. Big batches of meal wants into a large fourteen meter diameter superconducting magnet. All of them we shooting with their spins lined up in the direction. They're going like headlights on a car. When the moon's begin to circulate run around the magnetic ring they sort of act like race cars going around a circular track so as they go round and around and around it turns out that the direction that they're spins point no longer stays kind of lined up with the way they were when they were injected and every twenty nine times around the track the spin direction actually makes an extra full her so this difference is what we measure. We measure the difference between the spin direction and the direction. The ones were going that signal then is all tied up in the comment. I made about parody violation earlier in self analyzing spin. I said we record the products of the mu on decays win. They spin around like that and we collect them into a spectrum. That ends up with the kind of modulation exactly at that lapping frequency. So the frequency is the ticket. How fast spin goes around faster than one runs around. And what did you find what we found that. That wobble frequency was faster than the prediction. And we've found also. Interestingly enough that the saying kind of level faster than what had been measured twenty years ago at brookhaven national lab. So we confirm this this value. That was out there for about twenty years. That people were kind of like is that right. So what does this mean. And why is it so exciting. It's truly exciting. Because the significance of the difference now between the prediction and the experiments is so high that it looks like it might be revealing something. Twenty years ago it was just a soft difference between the prediction but we have a higher precision experiment. Now we can buy that with the measurement from twenty years ago. The precision is pretty high and were really at the level where people begin to think that starts to look like a discovery so at the very beginning clarify what you said was perhaps there are more particles in the universe than the ones we know about in its those particles which would cause this spin to go faster than prediction is such a complicated experiment. Republishing four papers that once probably one hundred pages in the journals to explain the whole thing. I've been doing this for about thirty years. So you also realize we do this blinded right way so. When did you find out just about a month ago. So you know just enough to put in the numbers into the final plots to right the beginning at ends of the paper ninety percent of the papers written before we know the result and basically a hundred seventy people sitting on zoom meeting. That i'll have to be satisfied in votes. Then we reveal these secret envelopes and then we decode the clock frequency and suddenly we see the results. it's really unnerving but it absolutely means we are not biased as We're going to get because we have no way to change the number after the week type in the secret code. And what did you feel when you saw that number to be honest. We all screamed in excitement but maybe for different reasons for me having been involved in this so long ago also being involved in the previous experiment i was extremely happy that we were verifying that. The previous experiment was correct but then the second emotion comes along that the two of them together. Now push the difference to what's actually called four point two standard deviations between since about one forty thousand chances so being a fluke and that really is exciting. Because we're all looking for new physics. That's amazing so we might actually be seeing the work of particles that we never knew about before it sure does but we do have more work to go just sitting on the smallest pile of the data so far terms of the results. We have a lot more data that we're taking as we speak and only then when we analyze all of it might we actually know you know the final truth to this but the other thing that this makes it kind of interesting is from all of the students and post. Docs in young people still working on this with so much more data to go basically. We're not quite over the line of what they call discovery at five standard deviations though this is very motivating for us to finish the job since we have so much more data that we can look at our sort of fell in the right place to keep it. Keep it kind of cool. Well then i'm going to stay tuned. Thank you so much david. Bet lose enjoyable for scientific american. Sixty seconds science podcast. I'm clara

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