Particle Physics: Question 8

Worked solution

Step 1: Identify the charge of each constituent

The $\pi^+$ meson is made of an up quark and a down antiquark. An antiquark carries the opposite charge to its corresponding quark, so: $\text{up quark} = +\frac23e, \qquad \text{down antiquark} = -\left(-\frac13e\right) = +\frac13e$

Step 2: Sum the two charges

$Q = \left(+\frac23e\right) + \left(+\frac13e\right) = \left(\frac23+\frac13\right)e = \frac33e = +1e$

Step 3: Why the other options are wrong

• Option A ($-1e$) would result from treating both constituents as negatively charged, ignoring that this is a quark-antiquark pair, not two quarks of the same sign.
• Option B ($0$) and option C ($+\frac13e$) would both result from mistakenly using the down quark’s charge ($-\frac13e$) instead of the down antiquark’s charge ($+\frac13e$), option B from then further miscombining the signs, and option C from adding $+\frac23e$ and $-\frac13e$ directly.

Final answer

• The total charge of the $\pi^+$ meson is $\boxed{+1e}$, option D.