Physics Quiz: Standard Model Answers

by Eric Drexler on 2014/04/19

Last week’s Physics Quiz asked about errors in Wikipedia’s current diagram of the Standard Model.

Here’s the diagram with [corrected] corrections; answers follow:

  1. What do the arcs represent?
    • The Standard Model interactions between particles: the boson-mediated strong, weak, and electromagnetic forces.
  2. Which of the arcs is incorrect? [And a question that got lost: Where is an arc is missing?]
    • The arc linking charged and uncharged leptons is spurious: It does not represent a force.
    • The arc representing the Higgs-neutrino interaction was omitted (see also #4).
  3. Extra credit, Wikipedia history department:
      How did one correction lead to both errors?
    • Neutrinos had been lumped together with the charged leptons, wrongly suggesting that all leptons interact with photons; when they were split off in the current diagram, the two classes of leptons were linked by the spurious arc, while neutrinos lost their link to the Higgs.
  4. Where is a[n] second arc missing?
    • The Z boson interacts with other Z bosons via the weak force, paralleling interactions between W bosons; this arc is missing.

Note that the Z boson is, from a practical point of view, a leading candidate for the most useless particle in the Standard Model. They’re crucial for neutrino-neutrino scattering, of course, as if anyone really cared.

Updated 25 April, to reflect the genuinely puzzling absence of Higgs-neutrino coupling in the Standard Model. My thanks to Kevin (see comments).

{ 4 comments… read them below or add one }

Loki April 20, 2014 at 8:14 pm UTC

Someone cares.

Kevin April 21, 2014 at 1:49 am UTC

Actually, in the Standard Model, neutrinos do not couple to the Higgs. The Higgs Yukawa interaction couples left- and right-handed fermions, and right-handed neutrinos do not exist in the Standard Model. This is why neutrino oscillations and masses are technically BSM physics, with the debate about the mass mechanism still unresolved (Dirac or Majorana).

Also, the function of the Z boson depends on which practical point of view you take. It’s great for sanity checks when analyzing collider data. Reconstructing two electrons or two muons to look for a peak at the Z mass is one of the first things a typical grad student learns.

Eric Drexler April 25, 2014 at 1:12 pm UTC

Kevin —

Thank you! Updated to reflect puzzlement: What is the origin of neutrino mass, if “massless neutrinos go hand in hand with the absence of right-handed neutrinos in the Standard Model”? [here, and in an article in Physics World. pdf]

This would seem to call for another mode of coupling to the Higgs, which I see has been suggested in the literature.

(Z boson are, of course, important in particle physics, as is everything that has a detectable effect, and actual utility in of the sort that you describe goes a bit beyond this.)

Carbonoid April 26, 2014 at 11:04 pm UTC

Eric, I hope you don’t mind me bringing this up here, regarding subatomic particles. In Engines of Creation you touched on the concept of machines made of parts smaller than atoms, and you stated that, based on known physics, the atom is the smallest stable structure, at normal temperatures and pressures.

I have read alot of speculation about “Pico and Femtotechnologies” on the web, including one man claiming it is possible to make rods, sheets, and wires from pure atomic nucleus matter. Do you think, based on what we know, the atom is still the only stable structure for actual machine and material fabrication?

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