I wished to clarify the interpretation of parton density plot1.
Are the following interpretation correct?
- First fix $Q^2$ for an experiment.
- Then the area under each curve $\int_0^1 xf(x)dx$ describes fraction of the total momentum of proton or nucleon carried by a particular parton. (And I guess this probability or fraction is valid only for that specific value of $Q^2$)
Now, what I am confused with is that since we fix $Q^2$ for a particular plot, doesn't this also fix the fraction of momentum carried by a parton? I mean to say that at fixed $Q^2$, if gluon carries 50% of the proton momentum , its x value should be 0.5 only. Then why does we get an entire range of values of x?
Also, in the plot, at around x = 0.1 or 0.2 in the plot, we see that number of up quarks are almost twice of down quarks(assuming that they refer to valence quarks). So should I interpret this x as the value where most of the experiments predict the actual composition of proton for that particular Q^2?