Particle Physics Discoveries that Disappeared

today we'll talk about data anomalies but first i want to tell you something about our sponsor datacamp datacamp is an online learning platform with many interactive courses where you can build up your skills for understanding analyzing and working with data you don't need to bring any prior knowledge and data camp is super easy to use you don't need special software you just sign up and do the courses on your browser a great course to get you started for example is data science for everyone which doesn't require any coding skills you can also learn the basics of artificial intelligence in their course machine learning for everyone and if you bring prior knowledge already you can build up for example your python or our skills or learn how to work with sql subscriptions for unlimited access start at 25 dollars a month you can check out the first chapter of any course for free if you use my link in the description below now let's talk about those data anomalies i get asked a lot what i think about this or that report of an anomaly in particle physics like the beam as an anomaly at the large hadron collider which made headlines last month or the mu and g minus 2 that was just all over the news but i thought instead of just giving you my opinion which you may or may not trust i will instead give you some background to gauge the relevance of such headlines yourself why are there so many anomalies in particle physics and how seriously should you take them that's what we'll talk about today the higgs boson was discovered in 1984. i'm serious the crystal ball experiment at desi in germany saw a particle that fit the expectation already in 1984. it made it into the new york times with the headline physicists report mystery particle but the supposed mystery particle turned out to be a data fluctuation the higgs boson was actually only discovered in 2012 at the large hadron collider at cern and 1984 was quite a year because also supersymmetry was observed and then disappeared again how can this happen particle physicists calculate what they expect to see in an experiment using the best theory they have at the time currently that's the standard model of particle physics in 1984 that would have been the standard model minus the particles which hadn't been discovered but the theory alone doesn't tell you what to expect in a measurement for this you also have to take into account how the experiment is set up for example what beam and what luminosity and how the detector works and how sensitive it is this together theory setup detector gives you an expectation for your measurement what you are then looking for are deviations from that expectation such deviations would be evidence for something new here's the problem these expectations are always probabilistic they don't tell you exactly what you will see they only tell you a distribution over possible outcomes that's partly due to quantum indeterminism but partly just classical uncertainty therefore it's possible that you see a signal when there isn't one as an example suppose i randomly distribute 100 points on this square if i divide the square into four pieces of equal size i expect about 25 points in each square and indeed that turns out to be about correct for this random distribution here is another random distribution looks reasonable now let's do this a million times no actually let's not do this i let my computer do this a million times and here's one of the outcomes wow that doesn't look random it looks like something's attracting the points to that one square maybe it's new physics no there's no new physics going on keep in mind this distribution was randomly created there's no signal here it's all noise it's just that every once in a while noise happens to look like a signal this is why particle physicists like scientists in all other disciplines give a confidence level to their observation that tells you how confident they are that the observation was not a statistical fluctuation they do this by calculating the probability that the supposed signal could have been created purely by chance if fluctuations create a signature like what you are looking for one in 20 times then the confidence level is 95 if fluctuations created one in 100 times the confidence level is 99 and so on loosely speaking the higher the confidence level the more remarkable the signal but exactly at which confidence level do you declare discovery is convention since the mid-1990s particle physicists have used for discovery a confidence level of 99.99994 that's about one in a million chance for the signal to have been a random fluctuation it's also frequently referred to as five sigma where sigma is one standard deviation but of course deviations from the expectation attract a tension already below the discovery threshold here is a little more history quarks for all we currently know are elementary particles meaning we haven't seen a substructure but a lot of physicists have speculated that quarks might be made up of even smaller things these smaller particles often called prions they were found in 1996. the new york times reported tiniest nuclear building block may not be the quark the significance of the signal was about three signa that's about a one in a thousand chance for it to be coincidence and about the same as the current b meson anomaly but the supposed quark substructure was a statistical fluctuation the same year the higgs was discovered again this time at the large electron positron collider at cern it was an excess of higgs-like events that made it to almost four sigma which is a one and a sixteen thousand chance to be a random fluctuation guess what that signal vanished too then in 2003 supersymmetry was discovered again this time in form of a supposed spottum quark that's the hypothetical supersymmetric partner particle of the bottom quark that signal too was at about three sigma but then disappeared and in 2015 we saw the diphoton anomaly that made it above four sigma and disappeared again there have even been some six sigma signals that disappeared again though these had no known interpretation in terms of new physics for example in 1998 the tevetronet formula measured some events they dubbed super jets at six sigma they were never seen again in 2004 hera at daisy saw pentaquarks that are particles made of five quarks with six sigma significance but that signal also disappeared and then there's the immune g minus 2 anomaly that recently increased from 3.7 to 4.2 sigma but it hasn't crossed the discovery threshold of course not all discoveries that disappeared in particle physics were due to fluctuations for example in 1984 the ua1 experiment at cern saw 11 particle decays of a certain type when they expected only 3.5 the signature fit to that expected for the top quark the physicists were quite optimistic they had found the top quark and this news too made it into the new york times turned out though they had misestimated the expected number of such events really there was nothing out of the ordinary the top quark wasn't actually discovered until 1995. a similar thing happened in 2011 when the cdf collaboration at formula saw an excess of events at about four sigma these were not fluctuations but they required better understanding of the background and then of course there are possible issues with the data analysis for example there are various tricks you can play to increase the supposed significance this basically doesn't happen in collaboration papers but you sometimes see individual researchers that use very creative methods of analysis and then there may be systematic problems with the detection triggers of filters and so on in summary possible reasons why a discovery might disappear are a fluctuations b miscalculations c analysis screw-ups and d systematics the most frequent one just by looking at the history are fluctuations and why are there so many fluctuations in particle physics it's because they have a lot of data and the more data you have the more likely you are to find a fluctuation that looks like a signal that by the way is why particle physicists introduce the five sigma standard in the first place because otherwise they'd constantly have discoveries that disappear so what's with that bmw anomaly at the lhc that recently made headlines it's actually been around since 2015 but recently new analysis came out and so it was in the news again it's currently lingering at 3.1 sigma as we saw signals of that strength go away all the time but it's interesting that this one stuck around instead of going away that makes me think it's either a systematic problem or indeed a real signal thank you for watching and special thanks to our tier 4 supporters on patreon your help is greatly appreciated and you too can help us to carry on with this channel go check out our patreon page and don't forget to subscribe see you next week you

Channel: Sabine Hossenfelder

Views: 201,537

Keywords: physics, data, data anomalies, particle physics, particle physics anomalies, muon g-2, b-meson anomaly, large hadron collider, 5 sigma, why do discoveries disappear, what are data fluctuations, confidence level, higgs boson, particle physics discoveries, supersymmetry, what are data anomalies, what is a data anomaly, hossenfelder, science without the gobbledygook, statistical significance

Id: kGggYZTK1ik

Length: 10min 50sec (650 seconds)

Published: Sat Apr 24 2021

Reddit Comments

[u/Langdon_St_Ives]

Ex-Physicist here, I can confidently say I understood everything, and think it was well-presented. Coincidentally I just discovered her channel a few days ago, only saw two or three other videos but they were all awesome so I subscribed.

So what’s your question about this one? AMA ;-)