= Antiproton
{title2=$\bar p$}
{wiki}
The <antiproton> is the <antiparticle> of the <proton>: it has the same <rest mass> and <spin> $1/2$, with opposite <electric charge> and <baryon number>. A negative <electric charge> alone cannot identify an <antiproton>, because a negative <pion> also bends in that direction in a <magnetic field>. Combining <magnetic rigidity> with <velocity> determines a <rest mass>; agreement with the <proton> <rest mass> then makes the identification much more discriminating.
The <mode expansion of a Dirac field> explains how independent particle and <antiparticle> excitations enter a relativistic field description. It supplies a theoretical language for the <antiproton>, while the <Chamberlain-Segrè antiproton identification> supplies an experimental test. The <proton> is a composite <baryon>, so a free <Dirac field> is not a complete account of its internal dynamics or of <antiproton> production in a target.
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