How to play. A collision fires tracks through the barrel layers. Charged tracks bend in the magnetic field, so each one is a smooth arc from its vertex outward. Drag across the hits of one track, or tap hits one by one and confirm. Scroll or pinch to zoom into crowded regions. Higher difficulty brings pile-up (several vertices), displaced decay vertices — sometimes out between the inner layers — low-p_T loopers that curl out and back, and noise hits that belong to no track.