Past exam of the mathematics course of the University of Cambridge / 2004 / iii / Paper 10 / 3 / Solution

A primitive ideal is the kernel of an algebraically irreducible unital representation, equivalently the annihilator of a module for a nonzero simple module. Irreducibility here excludes all proper nonzero invariant linear subspaces, not just closed ones. In a Banach algebra the simple modules can be represented as $A/L$ with $L$ a maximal left ideal. Such an ideal is closed: if its closure were all of $A$, it would contain an element close enough to $1$ to be invertible by the Neumann series, contradicting properness. Maximality then makes its closure itself. Thus the quotient carries a Banach-space norm and a bounded left action.

Let $P$ be a maximal proper two-sided ideal. The quotient ring $R=A/P$ is nonzero, unital and simple as a ring. By the Zorn lemma, choose a maximal left ideal $L$ of $R$. The simple module $R/L$ has an annihilator which is a proper two-sided ideal of $R$. Simplicity forces that annihilator to be zero. Its pullback to $A$ is exactly $P$, proving every maximal proper two-sided ideal is primitive. If one wants the Banach representation explicitly, realize the same module as the quotient of $A$ by the pullback maximal left ideal, which is closed by the preceding argument.