Pions are the lightest mesons, with mass some 270 times that of the electron. Charged pions decay typically into a muon and a neutrino or antineutrino. This makes pion beams useful for producing beams of neutrinos, which physicists use to study those elusive particles. In a medical application during the late 20th century, accelerator centers installed “biomedical beam lines” to test pions for cancer therapy. In these experiments, pions attached themselves to atomic nuclei within cancer cells. The nuclei would literally explode, delivering a “pion star” of cancer-killing nuclear debris. Unfortunately, results were not as encouraging as hoped, and enthusiasm for this technique has waned.

In the cancer-treatment experiments described in the passage, for which pions is it energetically easiest to be captured by a nucleus?
a. π⁺
b. π⁰
c. π^-
d. Energetically, capture is equally likely for all three pions.