Physical interpretation of pp-waves with axial torsion

We consider generalised pp-waves with purely axial torsion, which we previously showed to be new vacuum solutions of quadratic metric-affine gravity. Our analysis shows that classical pp-waves of parallel Ricci curvature should not be viewed on their own. They are a particular representation of a wider class of solutions, namely generalised pp-waves of parallel Ricci curvature. We compare our pp-waves with purely axial torsion to solutions of Einstein-Weyl theory, the classical model describing the interaction of gravitational and massless neutrino fields.Comment: 5 pages. Proceedings of the MG14 Meeting on General Relativity, University of Rome "La Sapienza", Italy, 12 - 18 July 2015. Edited by: Massimo Bianchi (Universit\`a degli Studi di Roma "Tor Vergata", Italy), Robert T Jantzen (Villanova University, USA), Remo Ruffini (International Center for Relativistic Astrophysics Network (ICRANet), Italy and University of Rome "La Sapienza", Italy