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Unexpected instabilities, some involving the beam's interaction with its surroundings, have kept performance below design.","posed_since":"","precise":"In crab-waist schemes with Piwinski angle $\\phi >> 1$, strong-strong simulations predict coherent X-Z instabilities and interplay with longitudinal and transverse impedance. Determine the stability boundary in $(\\xi_x,\\xi_y,\\nu_s,\\mathrm{impedance})$ and whether it explains the luminosity shortfall of SuperKEKB relative to its design value of $8 \\times 10^{35}\\ \\mathrm{cm}^{-2}\\ \\mathrm{s}^{-1}$ (later target about $6.5 \\times 10^{35}$).","problem_ref":"41c19eed84fa3cbb2061dba471aada01e22074a80a41760fba17798f9c749435","references":"","settled_by":"Strong-strong simulations including impedance that reproduce measured SuperKEKB blow-up and luminosity, and machine experiments that confirm the predicted cures.","status_note":"SuperKEKB set a world-record peak of about $5.1 \\times 10^{34}\\,\\mathrm{cm}^{-2}\\,\\mathrm{s}^{-1}$ in December 2024, roughly 6 to 8 percent of its design and later target values; vertical blow-up, a vertical instability linked to collimator impedance and sudden beam losses are the identified limits.","title":"Beam-beam instabilities in large Piwinski angle crab-waist 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