[HN Gopher] Muon g-2 doubles down with latest measurement, explo...
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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]
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