Improved Limits on Spin-Dependent WIMP-Proton Interactions from a Two Liter CF3I Bubble Chamber

Data from the operation of a bubble chamber filled with 3.5 kg of CF₃I in a shallow underground site are reported. An analysis of ultrasound signals accompanying bubble nucleations confirms that alpha decays generate a significantly louder acoustic emission than single nuclear recoils, leading to an efficient background discrimination. Three dark matter candidate events were observed during an effective exposure of 28.1kgday, consistent with a neutron background. This observation provides strong direct detection constraints on weakly interacting massive particle (WIMP)-proton spin-dependent scattering for WIMP masses >20GeV/c².