Collision Exclusion in the de Bruijn-Newman Heat Flow

For the de Bruijn–Newman heat-flow family H_t, we prove a positive-time collision-exclusion theorem: H_t has no multiple real zero for

0.172422 ≤ t ≤ 0.2. Together with the published bound Λ ≤ 0.2, this yields

Λ < 0.172422.

The method separates the globally-real anchor from the descent mechanism. A positive-stall theorem reduces any obstruction to further descent to a finite real multiple zero. Inside a moving separator, the backward heat equation fixes the local orientation of collisions through a Hermite normal form. Outside the separator, a mollify-first amplitude-free double-zero resultant is controlled by coefficientwise positivity, time monotonicity, and a directed fall/jump induction across every Riemann–Siegel index transition, until an exact-phase freezing theorem takes over at tlog(x/4π) = 5.17. The proving numerics use outward-rounded MPFR interval arithmetic, with independent higher-precision nesting checks on the most sensitive layers. A complete replay archive accompanies the paper. The result is an upper bound for Λ and is not a proof of the Riemann hypothesis.