as I don't know too much about experiments I have the following question concerning the evidence about the quark constituents in neutrons.

After a quick google investigation I found some sentences like "In 1968, at the Stanford Linear Accelerator Center (SLAC), electrons were fired at protons and neutrons in atomic nuclei." (Wikipedia article on deep inelastic scattering). However, the referred sources are always only dealing with deep inelastic proton scattering experiments.

Hence, the following question: Is there any evidence (any experiment) that tests whether neutrons are composed the way the standard model describes it? (ie neutron = {up, down, down} etc)

Thanks in advance for your insights!!

Best, Alex

  • 1
    $\begingroup$ It's hard to handle neutrons in a collider, so the neutrons are generally studied with deep inelastic scattering from deuterium and then subracting off the proton scattering. This isn't as clean a measurement as scattering from protons, but it works and a Google for something like neutron parton distribution function should find you plenty of references. $\endgroup$ Sep 23, 2015 at 11:13
  • $\begingroup$ Very good question. Yes, such experiments were done. Se e.g. "Measurement of elastic electron-neutron scattering and inelastic electron-deuteron scattering cross sections at high momentum transfer", S.Rock et al. SLAC-PUB-5239 November 1991. I am not a theoretician, so I can't interpret the results for you. Probably a better read is this thesis "Electron Scattering from an Almost Free Neutron in Deuterium" by Nathan Kidd Baillie from 1991. The thesis contains an overview of the theory and methodology used. I am looking forward to a real answer, though. $\endgroup$
    – CuriousOne
    Sep 23, 2015 at 11:13
  • $\begingroup$ Thanks a lot for that hin! I'll have a look at those papers :) $\endgroup$
    – sani
    Sep 24, 2015 at 12:52

1 Answer 1


I imagine that the simplest experiment you could do to show the non-specialist would be measuring the pion charge ratios near the pion-production threshold on the first few light targets ($^1\mathrm{H}$, $^2\mathrm{H}$, $^3\mathrm{He}$ $^4\mathrm{He}$).

That is we're looking at the reactions $$\begin{align} e^- + {}^Z\!A &\longrightarrow e^- + {}^{Z-1}\!A + \pi^+ \\ e^- + {}^Z\!A &\longrightarrow e^- + {}^{Z+1}\!A + \pi^- \,, \end{align}$$ with several different targets. The measurement should be as exclusive as possible.

At low momentum transfer the process are dominated by the valence (as opposed to sea) quark content, so the rates of $\pi^+$ ($\pi^-$) production are proportional to the number of up (down) quarks in the valence content of the nucleus.

We'd expect to see the ratio $R = \frac{\sigma_{\pi^+}}{\sigma_{\pi^-}}$ near 2 on protons, near 1 on deuterium and helium-4, and near 4/5 on Helium-3.

  • $\begingroup$ I guess that is basically what they do in the paper suggested by CuriousOne. Thanks ! $\endgroup$
    – sani
    Sep 24, 2015 at 14:10

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