Two identical point charges, each with charge \(+Q\), are fixed in place on the \(y\)-axis at \(y = +a\) and \(y = -a\). A particle of mass \(m\) and negative charge \(-q\) (where \(q > 0\)) is constrained to move along the \(x\)-axis and is released from rest at a small displacement, executing bounded oscillations about the origin. Assuming the electric potential energy is defined to be zero infinitely far from the fixed charges, which of the following graphs best represents the electric potential energy \(U\) of the system as a function of the particle's position \(x\)?