Almost sure upper bound for sums of random multiplicative functions and critical chaos

Let $f$ be a Steinhaus or Rademacher random multiplicative function. We use methods from the theory of critical chaos to improve on the best known upper bound for partial sums of random multiplicative functions. In particular, our results imply that for any $\epsilon >0$, almost surely $$\mid \sum _{n\le x}f(n)\mid {\ll }_{f,\epsilon }\sqrt{x}({\log }_{2}x{)}^{1\mathrm{/}4}({\log }_{3}x{)}^{1+\epsilon }\mathrm{.}$$ This proves in a strong form a conjecture of Harper on large fluctuations of partial sums of random multiplicative functions, and determines the exact corresponding logarithmic exponent.