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Penning trap/electron radius

  1. Mar 20, 2009 #1
    how is a penning trap used to determine the upper limit of the electron radius. google hasnt been much help.

    I found this but I dont understand it at all.


    What can be learned from the more accurate electron g? The first result beyond g itself is the fine structure constant, α = e2/4πε0hbarc – the fundamental measure of the strength of the electromagnetic interaction, and also a crucial ingredient in our system of fundamental constants. A Dirac point particle has g = 2. QED predicts that vacuum fluctuations and polarization slightly increase this value.

    The third use of the measured g is in probing the internal structure of the electron – limiting the electron to constituents with a mass m* > m/√(δg/2) = 130 GeV/c2, corresponding to an electron radius R <1 × 10–18 m. If this test was limited only by our experimental uncertainty in g, then we could set a limit m* > 600 GeV. This is not as stringent as the related limit set by LEP, which probes for a contact interaction at 10.3 TeV. However, the limit is obtained quite differently, and is somewhat remarkable for an experiment carried out at 100 mK.
    Last edited: Mar 20, 2009
  2. jcsd
  3. Mar 20, 2009 #2

    Wikipedia cites the abstract in reference 2.
    QED Penning trap upper limit for the electron particle radius:
    [tex]r_e \leq 10^{-22} \; \text{m}[/tex]

    [tex]\frac{1}{2} g = \frac{v_s}{v_c} = 1.001 159 652 188(4)[/tex]

    http://en.wikipedia.org/wiki/Penning_trap" [Broken]
    http://en.wikipedia.org/wiki/Stern%E2%80%93Gerlach_experiment" [Broken]
    Last edited by a moderator: May 4, 2017
  4. Mar 20, 2009 #3
    thank you


    The classical notion of an atomic particle at rest in free space is discussed, and shown to be approximable by zero-point confinement of the particle in a laboratory trap. An important tool for cooling the particle, and in the case of an electron, for obtaining directly the difference of spin and cyclotron frequencies vs, vc, is side band excitation. The quantum numbers of the geonium "atom", an electron in a Penning trap, have been continuously monitored in a non-destructive way by the new "continuous" Stern-Gerlach effect. In this way the g-factors of electron and positron have been determined to unprecedented precision,

    ½g ≡ vs/vc ≡ 1.001 159 652 188(4),

    providing the most severe tests of QED and of the CPT symmetry theorem, for charged elementary particles. From the close agreement of experimental and theoretical g-values a new, 10^4 × smaller, value for the electron radius, Rg < 10^-20 cm, may be extracted.
  5. Mar 20, 2009 #4


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    learning more about the anomalous magnetic moments of electrons, and muons, can lead one to find corrections and contributions from non-Standard Model particles/interactions.

    See for instance g-2 experiment home page:

    If you want more information:

    Similar measurements on rare decays such as pi0-> e+e- can reveal if there is contribution from non-SM particles/interactions.
  6. Mar 20, 2009 #5
    Geonium atom...

    http://en.wikipedia.org/wiki/Geonium_atom" [Broken]
    Last edited by a moderator: May 4, 2017
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