Antiproton synthesis For a pion minus hitting a stationary proton, what are the other particles if an antiproton is to be created among them? A positive pion is possible but the total rest mass energy of the final state is comparable to the initial particles.
 A: I'm not sure you have spelled out the conditions of your problem.
Does
$$
\pi^- + p \to n + \bar p + p , 
$$
that is,
$$
d\bar u + uud \to ddu +\bar u \bar u \bar d + uud 
$$
meet them? To conserve baryon number, this is your most economical option.
A: Begin with the incomplete reaction
$$p + \pi^- \to \bar{p}\ +\ ?$$
Then write the hadrons as their quark-composites.
$$uud + \bar{u}d \to \bar{u}\bar{u}\bar{d}\ +\ ?$$
By counting the quarks and anti-quarks you find,
3 $u$ and 3 $d$ quarks need to be added on the right side.
There are many ways to do this.
Just play around with baryons (3 quarks) and mesons (quark and anti-quark).
Some examples are:
$$\begin{align}
uud + \bar{u}d &\to \bar{u}\bar{u}\bar{d} + uud + udd \\
uud + \bar{u}d &\to \bar{u}\bar{u}\bar{d} + uuu + ddd \\
uud + \bar{u}d &\to \bar{u}\bar{u}\bar{d} + uuu + udd + \bar{u}d \\
uud + \bar{u}d &\to \bar{u}\bar{u}\bar{d} + uud + ddd + u\bar{d} \\
uud + \bar{u}d &\to \bar{u}\bar{u}\bar{d} + uus + udd + d\bar{s} \\
uud + \bar{u}d &\to \bar{u}\bar{u}\bar{d} + uud + uds + d\bar{s}
\end{align}$$
Translating the quark-composites back to baryons and mesons you get:
$$\begin{align}
p + \pi^- &\to \bar{p} + p + n  \\
p + \pi^- &\to \bar{p} + \Delta^{++} + \Delta^- \\
p + \pi^- &\to \bar{p} + \Delta^{++} + n + \pi^- \\
p + \pi^- &\to \bar{p} + p + \Delta^- + \pi^+ \\
p + \pi^- &\to \bar{p} + \Sigma^+ + n + K^0 \\
p + \pi^- &\to \bar{p} + p + \Lambda^0 + K^0
\end{align}$$
Which of of these reactions actually happen,
depends on how much energy the incident $\pi^-$ brings in.
