# Deep inelastic scattering: quention on the Hand convention for the flux.

1. Jul 12, 2012

### Einj

Hi all. I'm studying deep inelastic cross section in parton model. In particular I'm referring to Halzen-Martin's "Quarks and leptons" section 8.5, but I think this is a quite generale procedure.
In studying the deep inelastic scattering by e- on proton it cuts off the electron part and work only on the virtual photon-proton part.
In doing so it's found out that the flux term is dependent on the photon virtuality by the relation:
$$4\sqrt{{(p_ap_b)}^2-m_a^2m_b^2}=4M\sqrt{K^2-q^2}$$
where K is the photon flux (unknow because of the virtuality), q is the photon quadri-momentum and M is the proton mass.
Now, it chooses the flux factor to satisfy the Hand convention:
$$K=\nu + \frac{q^2}{2M}$$
This choise is made in order to threat the photon as in the real case. But why we have such a freedom in defining the flux factor??