Mathisson's helical motions for a spinning particle --- are they unphysical?

Our group coordinated the "Numerical Relativity and High Energy Physics" IRSES network (2012-2015). Here is a list of the global network meetings organized:

First Meeting:
9-13 July 2012, Aveiro, Portugal

Second Meeting:
11-14 March 2013, Lisbon, Portugal

Third Meeting:
6-10 January 2014, Mississippi, USA

Fourth Meeting:
7-10 July 2015, Rome, Italy

Fifth Meeting:
28 Sep-2 Oct 2015, Belém, Brazil