Strong axial anisotropy of the magnetic penetration length in superconducting UPt3

We report muon spin rotation measurements of the temperature dependence and anisotropy of the magnetic field penetration lengths in the heavy fermion superconductor UPt3. We observe a strong axial anisotropy. At 0.05 K we obtain for the penetration length parallel and perpendicular to the c axis lambda(c) = 4260 (150) Angstrom and lambda(a) = 6040 (130) Angstrom, respectively. lambda(a)(-2)(T) at low temperatures excludes a superconducting order parameter in the B phase with only a line of nodes in the equatorial plane of the Fermi surface. The combined analysis of lambda(c)(-2)(T) and lambda(a)(-2)(T) measured in the B phase favours an hybrid order parameter with point nodes at the poles and a line of nodes at the equatorial plane. The A phase is characterized by a larger density of nodes than the B phase.