system
Given the following post, respond as if you're commenting on Reddit:
human
Why does beta plus (β+) decay happen in proton-proton chain reactions; why don't the two protons just form helium instead of deuterium?
gpt
Helium-2 is unbound, so it decays on extremely short timescales. The only way to combine two protons into a bound system is to rely on the weak force to change one of the protons into a neutron, producing a deuteron.

This is not a beta decay, it's a nuclear reaction which involves the weak force. That's why the probability of it occurring is so small, and why it forms a bottleneck for the entire pp-chain.