The GZK bound and strong neutrino-nucleon interactions above 1019 eV: a progress report

Cosmic ray events above 1019 eV have posed a fundamental problem for more than thirty years. Recent measurements indicate that these events do not show the features predicted by the GZK bound. The events may, in addition, display angular correlations with points sources. If these observations are confirmed for point sources further than 50–100 Mpc, then strong interactions for the ultra-high energy neutrino are indicated. Recent work on extra space-time dimensions provides a context for massive spin-2 exchanges which are capable of generating cross sections in the 1–100 mb range indicated by data. Some recent controversies on the applicability of extra-dimension physics are discussed.