space-like virtualities of partons

Kyong

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.

Attached Files

list.txt (9.3 KB, 7 views)

mfb

All particles 4-43 are virtual. Why do you expect a positive (real) mass there?
All particles after 44 look real.