What goes on inside a proton?

[n2doc]

by Jon Butterworth

I spent part of last week at a meeting in the Pyrenees, northern Spain, discussing the internal structure of the proton. The meeting was in the small village of Benasque, hosted by Juan Rojo at the Centro de Ciencias de Benasque Pedro Pascual.

The proton (my favourite particle) is the nucleus of a hydrogren atom, is useful for curing some cancers, and is the particle collided by the Large Hadron Collider as it extends the frontiers of our knowledge of the structure of matter. Protons are made of two up-quarks and a down-quark, but there is quite a lot more than that to discover about their internal structure.

Most what we know about proton structure comes from the scattering of electrons off protons. Electrons and quarks are both electrically charged, so they repel or attract each other, depending whether the quark in question is positively or negatively charged (the electron has negative charge). By measuring how often, and with what energy and angle, electrons scatter off protons, we can work out several things. Firstly, we can tell the proton is not fundamental; it has something inside it. Consider this sketch:


This is a rough sketch of what we might see if the proton was fundamental - infinitely small and with no internal strucure. The vertical axis (labelled σ, “sigma”) is proportional to the number of times an electron-proton scattering happens, and the horizontal axis (x) is the fraction of the proton which the electron “saw”. If the proton is fundamental, this is always one, hence the spike at unity.

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http://www.theguardian.com/science/life-and-physics/2015/feb/21/what-goes-on-inside-a-proton