The effect of ionising radiation on poly(methyl methacrylate) used in intraocular lenses

Gamma radiation is increasingly being used to sterilise intraocular lenses (IOLs) made from poly(methyl methacrylate) (PMMA). In this study, samples of PMMA used in the fabrication of IOLs were exposed to irradiation doses typically used for their sterilisation. The effect of this treatment on the polymer was analysed by size-exclusion chromatography (SEC), UV-visible and infrared spectroscopy and scanning electron microscopy (SEM). The PMMA was found to have undergone chain scission, decarboxylation and colour change following the irradiation.     

Natural electroweak symmetry breaking in generalised mirror matter models

It has recently been pointed out that the mirror or twin Higgs model is more technically natural than the standard model, thus alleviating the “little” hierarchy problem. In this Letter we generalise the analysis to models with an arbitrary number of isomorphic standard model sectors, and demonstrate that technical naturalness increases with the number of additional sectors. We consider two kinds of models. The first has N standard model sectors symmetric under arbitrary permutations thereof. The second has p left-chiral standard model sectors and p right-chiral or mirror standard model sectors, with p  -fold permutation symmetries within both and a discrete parity transformation interchanging left and right. In both kinds of models the lightest scalar has an invisible width fraction 1/N$1/N$, which will provide an important means of experimentally testing this class of models.     

A simple explanation of the PVLAS anomaly in spontaneously broken mirror models

The PVLAS anomaly can be explained if there exist millicharged particles of mass ?0.1 eV$?0.1\text{eV}$ and electric charge ?∼¹⁰⁻⁶e$\mathrm{?}\sim {10}^{\mathrm{-6}}e$. We point out that such particles occur naturally in spontaneously broken mirror models. We argue that this interpretation of the PVLAS anomaly is not in conflict with astrophysical constraints due to the self interactions of the millicharged particles which lead them to be trapped within stars. This conclusion also holds for a generic paraphoton model.     

Experimental observation of the tilting mode of an array of vortices in a dilute Bose-Einstein condensate

We have measured the precession frequency of a vortex lattice in a Bose-Einstein condensate of Rb87 atoms. The observed mode corresponds to a collective motion in which all the vortices in the array are tilted by a small angle with respect to the z axis (the symmetry axis of the trapping potential) and synchronously rotate about this axis. This motion corresponds to excitation of a Kelvin wave along the core of each vortex and we have verified that it has the handedness expected for such helical waves, i.e., precession in the opposite sense to the rotational flow around the vortices.