[HN Gopher] Muon g-2 doubles down with latest measurement, explo... ___________________________________________________________________ Muon g-2 doubles down with latest measurement, explores uncharted territory Author : momirlan Score : 35 points Date : 2023-08-10 19:43 UTC (3 hours ago) (HTM) web link (news.fnal.gov) (TXT) w3m dump (news.fnal.gov) | cvoss wrote: | Article doesn't indicate how close this measurement is to any | specific theoretical value, and, therefore, doesn't hint at | whether this experiment is expected to actually challenge a | standing Standard Model calculation. | | I suspect that's because | | > ... a new experimental measurement of the data that feeds into | the prediction and a new calculation based on a different | theoretical approach -- lattice gauge theory -- are in tension | with the 2020 calculation | | > Scientists of the Muon g-2 Theory Initiative aim to have a new, | improved prediction available in the next couple of years that | considers both theoretical approaches. | | Sounds like the theorists are lagging behind. | jl6 wrote: | It's probably not a problem, probably... the small discrepancy | in the theorists' predictions is expected to be narrowed and | should be back within acceptable bounds again within the next | couple of years.[0] | | [0] https://www.bbc.co.uk/news/science-environment-66407099 | sjkoelle wrote: | can some physicist explain why the calculations are so | hard... something something Taylor series? | l33tman wrote: | In essence, you need to enumerate and sum up all possible | ways everything can happen in a very little piece of the | universe that corresponds to the "reaction center" of the | experiment. All possible ways you can pop out particles | from the vacuum that interacts with your experimental | particle and pop back into the vacuum etc. It's a totally | impossible calculation to do in full, so a large chunk of | theoretical high energy physics is about finding shortcuts | to do that. | | All these ways contribute to the reaction probabilities | (which are the ones you get a number on at the end of the | day). | | The muon g-2 experiment is so interesting because there is | a vast number of pretty exotic particles and particle | pathways that have to be taken into account, and if our | understanding of any of those is wrong, or there are (even | more interestingly) particles that can pop out and | contribute that we don't even know about, it will show up | in the muon g-2 number as compared to the theoretical calc. | ketralnis wrote: | Not a physicist but basically yes. Most quantum mechanics | calculations involve the path integral[0], a sort of sum of | all possible Feynman diagrams[1] that an interaction could | involve and the more of these you involve the more accurate | you hope to be. Further the field theory that they're | calculating within is generally an effective field | theory[2] which is known to be an incomplete approximation | of the system it's modelling but "close enough" for the | energies involved. That approximation costs accuracy as you | get closer to the energy cutoff you included but you | included it because you don't know what happens above it so | it's not like you can just not do that. Depending on how | renormalization[2] is done in that effective theory the | constants involved can have their values affected. | | In this particular case you're also looking "closer" at an | interaction than you can model with the particle intuition. | For simple electron/photon scattering you can pretend that | things are particles and use that intuition to guide you | but when you start looking really close at things you can't | really do that anymore and have to work with fields and | waves in a way that belies normal physical intuition. Part | of the way that manifests is "virtual particles" popping in | and out of existence (really field interactions that aren't | really particle-like but with the virtual particle trick | can be modelled almost as if they were). These interactions | do affect the real world and the way they affect it depends | on the fields involved. If there's some unknown 4th | generation of lepton for instance a virtual one of those | could interact with the experiment, but since you didn't | know about it you didn't include it in your calculation. | | Also depending on how you're calculating things you aren't | just saying "1+2=3", you're saying "1+-theory error + | 2+-theory error = 3+-theory error" so you end up with a | range of calculations. That might manifest as "well if | there's a graviton then I need to calculate it this way but | if there isn't then I need to calculate it this way, and we | don't have experimental evidence to guide me one way or the | other". | | Lastly, QFT is quantum mechanics + special relativity and I | think it goes without saying that both of these are just | detailed, difficult fields with difficult maths involved. | So you're taking an already difficult field and since | you're trying to be accurate out to a lot of decimal places | while interacting with messy real-world experiments you're | forgoing all of the nice physics-class "massless particle | in a spherical box" approximations that normally let you | skip over all of that trouble. | | See https://indico.fnal.gov/event/7309/contributions/101335 | /atta... for some lecture notes on the base problem. | | 0: https://en.wikipedia.org/wiki/Path_integral_formulation | 1: https://en.wikipedia.org/wiki/Feynman_diagram 2: | https://en.wikipedia.org/wiki/Effective_field_theory 3: | https://en.wikipedia.org/wiki/Renormalization | omgJustTest wrote: | "The muon, like its lighter sibling the electron, acts like a | tiny magnet. The parameter known as the "g factor" indicates | how strong the magnet is and the rate of its gyration in an | externally applied magnetic field. It is this rate of gyration | that is indirectly measured in the Muon g - 2 experiment. | | The value of g is slightly larger than 2, hence the name of the | experiment. This difference from 2 (the "anomalous" part) is | caused by higher-order contributions from quantum field theory. | In measuring g - 2 with high precision and comparing its value | to the theoretical prediction, physicists will discover whether | the experiment agrees with theory. Any deviation would point to | as yet undiscovered subatomic particles that exist in | nature.[4] " | | From Wikipedia. https://en.wikipedia.org/wiki/Muon_g-2 Standard | model g factor should be 2.00233183620(86): | https://physics.aps.org/articles/v16/139 | | TLDR: Standard model says the ratio measured should be about 2 | for g, ie g-2 > 0 would indicate contributions from quantum | field theory, which are yet to be well understood and could | indicate there are more particles responsible for the excess. | [deleted] ___________________________________________________________________ (page generated 2023-08-10 23:00 UTC)