Do virtual electron-positron pairs have mass?

Do virtual electron-positron pairs have mass?

Virtual particles are within an integral depicted by a Feynman diagram


Only lines entering or leaving the diagram represent observable particles. Here two electrons enter, exchange a photon, and then exit. The time and space axes are usually not indicated. The vertical direction indicates the progress of time upward, but the horizontal spacing does not give the distance between the particles.

The virtual lines are described by a fourvector, and the "length" of a four vector is the mass for a free particle.In an integral this mass is variable within the limits of integration.

you ask:

When a photon produces an electron-positron pair, do both these particles have mass? Why or why not?

A photon has zero mass, and conservation of energy and momentum do not allow the decay into massive particles.