Every curve here is a published parton distribution — how the proton's momentum is shared among quarks and
gluons. Pick a parton, slide the probing energy Q, and compare what the world's fitting groups get.
The shaded band is each set's 1σ uncertainty, computed with that group's own method.
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—values plotted as x·f(x) — the momentum density
How to read it. The horizontal axis is x, the share of the proton's momentum, on a log scale.
The vertical axis is x·f(x) — how much momentum that parton type carries at that x. Slide Q upward and
watch the gluon grow at small x: that is DGLAP evolution, and it is calculated, not fitted.
Where the bands from different groups fail to overlap, the groups genuinely disagree about the proton.