The physics anomaly no one talks about: What's up with those neutrinos?

in the past months we've talked a lot about topics that receive more attention than they deserve today i want to talk about a topic that doesn't receive the attention it deserves that's a 20 years old anomaly in neutrino physics which has been above the discovery threshold since 2018 but chances are you've never even heard of it so what are neutrinos what's going on with them and what does it mean that's what we'll talk about today i really don't understand why some science results make headlines and others don't for example we've seen loads of headlines about the anomaly in the measurement of the mu and g minus 2 and the laptop anomaly at the large hadron collider in both of these cases the observations don't agree with the predictions but neither is statistically significant enough to count as a new discovery and in both cases there are reasons to doubt it's actually new physics but in 2018 the mini boon neutrino experiment at formula confirmed an early anomaly from an experiment called lsnd at the los alamos national laboratory the statistical significance of that anomaly is now at six sigma and in this case it's really difficult to find an explanation that does not involve new physics so why didn't this make big headlines i don't know maybe people just don't like neutrinos but there are lots of reasons to like neutrinos neutrinos are elementary particles in the standard model of particle physics that they are elementary means they aren't made of anything else at least not for all we currently know in the standard model we have three neutrinos each of them is a partner particle of a lepton the leptons are the electron muon and tau so we have an electron neutrino a muon neutrino and a tau neutrino physicists call the types of neutrinos the neutrino flavor the standard model neutrinos each have a flavor half spin one half and no electric charge so far so boring but neutrinos are decidedly weird for a number of reasons first they are the only particles that interact only with the weak nuclear force or the other particles we know either interact with the electromagnetic force or the strong nuclear force or both and the weak nuclear force is weak which is why neutrinos rarely interact with anything at all they mostly just pass through matter without leaving a trace this is why they are often called ghostly while you've listened to this sentence about 10 to the 50 neutrinos have passed through you this isn't the only reason neutrinos are wet what's even weirder is that the three types of neutrino flavors mix into each other that means if you start with say only electron neutrinos they'll convert into new neutrinos as they travel and then they'll convert back into electron neutrinos so depending on what distance from a source you make a measurement you get more electron neutrinos or more immune neutrinos crazy but it's true we have a lot of evidence that this actually happens and indeed a nobel prize was awarded for this in 2011. now to be fair neutrino mixing in and by itself isn't all that weird indeed quarks also do this mixing it's just that they don't mix as much that neutrinos mix is weird because neutrinos can only mix if they have muscles but we don't know how they get masses you see the way that other elementary particles get masses is that they couple to the higgs boson but the way this works is that we need a left-handed and the right-handed version of the particle and the higgs needs to couple to both of them together that works for all particles except the neutrinos because no one has ever seen a right-handed neutrino we only ever measure left-handed ones so the neutrinos mix which means they must have masses but we don't know how they get these masses there are two ways to fix this problem either the right-handed neutrinos exist but they are very heavy so we haven't seen them yet because creating them would take a lot of energy or the neutrinos are different from all the other spin one half particles in that their left and right handed versions are just the same this is called a majorana particle but either way something is missing from our understanding of neutrinos and the weirdest bit is the anomaly that i mentioned as i said we have three flavors of neutrinos and these mix into each other as they travel this has been confirmed by a large number of observations on neutrinos from different sources there are natural sources like the sun and neutrinos that are created in the upper atmosphere when cosmic rays hit and then there are neutrinos from man-made sources particle accelerators and nuclear power plants in all of these cases you know how many neutrinos are created of which type at what energy and then after some distance you measure them and see what you get what physicists then do is that they try to find parameters for the neutrino mixing that fit to all the data this is called a global fit and you can look up the current status online the parameters you need to fit are the differences in masses which determines the wavelength of the mixing and the mixing angles that determine how much the neutrinos mix by 2005 or so physicists had pretty much pinned down all the parameters except there was one experiment which didn't make sense that was the liquid scintillator neutrino detector lsnd for short which ran from 1993 to 98. the lsnd data just wouldn't fit together with all the other data it's normally just excluded from the global fit in this figure you see the lsd results from back then the red and green is what you expect the dots with the crosses are the data the blue is the fit to the data this excess has a statistical significance of 3.8 sigma as a quick reminder one sigma is a standard deviation the more sigmas away from the expectation the data is the less likely the deviation is to have come about coincidentally so the more sigma the more impressive the anomaly in particle physics the discovery threshold is 5 sigma the 3.8 sigma of the lsnd anomaly wasn't enough to get excited but too much to just ignore 15 years ago i worked on neutrino mixing for a while and in my impression back then most physicists thought the lsnd data was just wrong and it did not be reproduced that's because this experiment was a little different from the others for several reasons they detected only anti-neutrinos created by a particle accelerator and the experiment had a very short baseline of only 30 meters shorter than all the other experiments still a new experiment was commissioned to check on this this was the mini boone experiment at fermilab that's the mini booster neutrino experiment and it's been running since 2003. as you can tell by then the trend of cooking up funky acronyms had taken hold in physics mini boon is basically a big tank full of mineral oil surrounded with photodetectors which you see in this photo the tank waits for neutrinos from the nearby booster accelerator which you see in this photo for the first data analysis in 2007 miniboom didn't have a lot of data and the results seemed to disagree with lsnd this was what everyone expected look at this headline from 2007 for example but then in 2018 with more data mini boone confirmed the rsnd result yes you heard that right they confirmed it with 4.7 sigma with a combined significance of 6 sigma what does that mean you can't fit this observation by tweaking the other neutrino mixing parameters there just aren't sufficiently many parameters to tweak the observations are just incompatible with the standard model so you have to introduce something new some ideas that physicists have put forward are symmetry violations or new neutrino interactions that aren't in the standard model there's also of course still the possibility that physicists misunderstand something about the experiment itself but given that this is an independent reproduction of an earlier experiment i find this unlikely the most popular idea which is also the easiest is what's called sterile neutrinos a sterile neutrino is one that doesn't have a lepton associated with it and it doesn't have a flavor so we wouldn't have seen it produced in particle collisions sterile neutrinos can however still mix into the other neutrinos indeed that would be the only way sterile neutrinos could interact with the standard model particles and so the only way we can measure them one steroid neutrino alone doesn't explain the mini boon ls and d data though you need at least two or more or something else in addition interestingly enough stereo neutrinos could also make up dark matter when will we find out indeed seeing that the result is from 2018 why don't we know already well it's because neutrinos interact very rarely this means it takes a really long time to detect sufficiently many of them to come to any conclusions just to give you an idea the mini boon experiment collected data from 2002 to 2017. during that time they saw an excess of about 500 events 500 events in 15 years so i think we're on to something here but glaciers now move faster than particle physics this isn't a mystery that will resolve quickly but i'll keep you up to date so don't forget to subscribe this video was sponsored by brilliant neutrino oscillations might sound somewhat mysterious but really it's just linear algebra and complex numbers unitary matrices and eigenvectors and so on if you want to understand this in more depth brilliant is a great starting point brilliant is a website and app that offers interactive courses on a large variety of topics in science and mathematics they have for example a course on linear algebra and also one on complex numbers and all their courses will challenge you with questions so you can check your understanding along the way to support this channel and learn more about brilliant go to brilliant.org sabine and sign up for free the first 200 subscribers using this link will get 20 of the annual premium subscription thanks for watching see you next week

Channel: Sabine Hossenfelder

Views: 609,683

Keywords: physics, particle physics, neutrinos, standard model, standard model of particle physics, neutrino masses, anomaly in particle physics, lsnd experiment, miniboone, neutrino oscillation, sterile neutrinos, physics beyond the standard model, new physics, science, education, fermilab, hossenfelder, science without the gobbledygook

Id: p118YbxFtGg

Length: 11min 53sec (713 seconds)

Published: Sat Sep 18 2021