Proton Structure and Parton Distribution Function I recently read a question about whether photons, electrons, neutrinos... are a part of the proton since there are these sea-quarks he learned about. And the answers were confusing to me because the most of them stated that 

Since the sea-quarks and also the electrons and positrons are virual particles there are no real positrons or electrons inside the proton, which could be observed.

My question is, what about the deep inelastic scattering? Isnt there a chance to scatter on a strange quark for example when colliding a proton and a electron, since a part of the four-momentum of the proton is carried by a strange quark? And isnt that a observation of a sea-quark ?
 A: 
My question is, what about the deep inelastic scattering? Isnt there a chance to scatter on a strange quark for example when colliding a proton and a electron, since a part of the four-momentum of the proton is carried by a strange quark? And isnt that a observation of a sea-quark ?

Well, it depends on your definition of observation. It is necessary to include strange etc virtual quarks in the proton structure, in order to be able to fit  the distributions of deep inelastic scattering, and in that sense one "observes" that virtual sea quarks are necessary . BUT a sea quark,gluon,valence is off mass shell, which means cannot be individually measured, but is part of the mathematics of the interaction studied, affecting the probability of scatter and its influence seen in the fit of the parton distribution collectively.
This is in contrast with, for example,measuring the probability of creation of lamda in a scatter. Individual lamdas are experimentally meassured to make the distribution.
It is usual in physics to accept as observable particles those whose four vector length is at the mass of the particle named. Virtual particles carry the quantum numbers of the  name , but not the mass.
