Dark matter as a cancer hazard

We comment on the paper "Dark Matter collisions with the Human Body" by K. Freese and C. Savage (Phys. Lett. B 717, 25 (2012) [arXiv:1204.1339]) and describe a dark matter model for which the results of the previous paper do not quite apply. Within this mirror dark matter model, potentially hazardous objects, mirror micrometeorites, can exist and may lead to diseases triggered by multiple mutations, such as cancer, though with very low probability.Comment: 7 pages, revtex4, some text and references added, version to be published in Physics Letters

Evading Quantum Mechanics \'{a} la Sudarshan: quantum-mechanics-free subsystem as a realization of Koopman-von Neumann mechanics

Tsang and Caves suggested the idea of a quantum-mechanics-free subsystem in 2012. We contend that Sudarshan's viewpoint on Koopman-von Neumann mechanics is realized in the quantum-mechanics-free subsystem. Since quantum-mechanics-free subsystems are being experimentally realized, Koopman-von Neumann mechanics is essentially transformed into an engineering science.Comment: 5 pages, no figure

Two-photon decay of P-wave positronium: a tutorial

A detailed exposition of two-photon decays of P-wave positronium is given to fill an existing gap in the pedagogical literature. Annihilation decay rates of P-wave positronium are negligible compared to the rates of radiative electric dipole transitions to the ground state. This circumstance makes such decays experimentally inaccessible. However the situation is different for quarkonium and the experimental and theoretical research of two-photon and two-gluon decays of P-wave quarkonia is a still flourishing field.Comment: 12 pages, 1 figure, to be published in Can. J. Phy

Majorana transformation of the Thomas-Fermi equation demystified

The Majorana transformation makes it possible to reduce the Thomas-Fermi equation to a first-order differential equation. This reduction is possible due to the special scaling property of the Thomas-Fermi equation under homology transformations. Such reductions are well known in the context of stellar astrophysics, where the use of homology-invariant variables has long proved useful. We use homology-invariant variables in the context of the Thomas-Fermi equation to demystify the origin of the otherwise mysterious Majorana transformation.Comment: 10 pages, no figure