(a) Invariant-mass distribution of unlike-sign (US) kaon pairs (black), with the scaled like-sign (LS) pair distribution (red) used to approximate the combinatorial background (BG). (b) Background subtracted invariant-mass distribution of unlike-sign kaon pairs (black). The $\phi$(1020) mass peak is fit (blue dashed line) by a sum of a Voigt function and a first-order polynomial, which accounts for the residual background. This linear fit of the residual background is shown as a red line. Statistical uncertainties are plotted as bars, but are too small to be visible here.