A proton did not scatter like a smooth smear
In the late 1960s, the SLAC–MIT collaboration fired energetic electrons into hydrogen and measured those that emerged after losing substantial energy. At large momentum transfer, the rate remained surprisingly high. The proton behaved as if it contained compact charged objects that an electron could strike. The data gave the quark picture a new kind of support, one obtained by probing inside matter.
An electron is useful because its electromagnetic interaction was already well understood. Send it toward a proton and measure how its direction and energy change. A gentle encounter can leave the proton intact, revealing its overall charge distribution. A harder collision can break it into a hadronic final state. The latter process is inelastic; deep inelastic means the transferred momentum and final-state mass are large enough to look past individual low-energy proton excitations.
The spectrometers measured the electron after the collision
SLAC’s long linear accelerator supplied the incident electron beam. Magnetic spectrometers selected outgoing electrons at known angles and measured their momenta. The famous 1969 papers discussed measurements at six and ten degrees. Those small laboratory angles still corresponded to substantial momentum transfer because the incoming beam energy was so high.
The electron’s incoming and outgoing energies determine how much energy it gives to the proton. Its deflection supplies information about the momentum transfer. Together these quantities describe the collision without requiring every hadronic fragment to be reconstructed. This inclusive method made it possible to measure a stable scattering pattern across many final states.
A smooth extended charge distribution normally suppresses a large-angle, high-resolution response. Contributions from different parts of an extended object partly cancel when the probe resolves distances shorter than the object’s size. The observed hard-scattering yield was therefore a clue that some charge resided in much smaller structures. The electron could couple to one compact constituent instead of seeing only the proton as a whole.
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Energy transfer and momentum transfer ask different questions
Energy transfer and momentum transfer play different roles. A collision can give the proton a large amount of energy without resolving the same distance scale as another collision with a larger deflection. Measuring both the outgoing energy and angle separates these possibilities. It also distinguishes the elastic response, in which a proton remains intact, from broad inelastic production. The apparatus compared that response across a range of energies and angles.
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Changing the magnification exposed an unexpected regularity
The team summarized the proton’s response with structure functions. A structure function is a quantity extracted from measured scattering rates that describes how the target responds to energy and momentum transfer. It is not a photograph of a quark. Its usefulness is that data taken with different electron energies and angles can be compared in common variables.
One variable, now usually called Bjorken x, combines the momentum transfer and transferred energy. In a simple fast-moving proton model, it is related to the fraction of the proton’s longitudinal momentum carried by the struck constituent. Early data showed that an appropriate structure-function combination depended mainly on this variable over a substantial range, with much weaker dependence on the separate probing scale. This approximate pattern is called scaling.
The original analysis was explicit about limitations. The early kinematic range did not separate the two structure functions independently, so extracting one required assumptions about the relative responses. That qualification does not erase the striking regularity. It tells a reader which part came straight from measured rates and which part entered through their interpretation.
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The constituents gained a dynamics as well as a name
The compact constituents became known as partons in the scattering description and were connected to quarks through many subsequent tests. Quarks supplied the charge and flavour pattern already used in hadron classification. Gluons, which carry the strong interaction, also share the proton’s momentum. Quantum chromodynamics predicts that structure functions change slowly with the probing scale as quarks radiate gluons and gluons split.
Jerome Friedman, Henry Kendall and Richard Taylor received the 1990 Nobel Prize for pioneering the deep-inelastic investigations. The central observation dates from the 1960s, with the landmark papers in 1969. Its appeal remains physical and direct: a large object answered a hard probe as though it contained smaller targets, and its response repeated across changing experimental conditions.