# Space-like virtualities of partons

## Main Question or Discussion Point

I used PYTHIA (an event generator) to have parton showers from the process e+ e- → γ*/Z0→q \bar{q}→parton showers (PYEEVT).
I realized that some partons (gluons or quarks) produced in this process have imaginary masses (Q2=E2-px2-py2-pz2<0), i.e. space-like virtualities.
As I know, space-like partons usually come from initail state radiation. However, in case of the above process, there is no initial state radiation.
I don't believe that PYTHIA was wrong because PYTHIA has been corrected and polished much by many people for about thirty years(?).
Can anybody explain me why the space-like virtualities happened and what they mean?
I attached a text file for an event list.

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