Particle theory

Abstract:

A recently reported anomaly in the time structure of signals in the KARMEN neutrino detector suggests the decay of a new particle $x$, produced in $\pi^+ \to \mu^+ x$ with mass $m_x=33.9$ MeV. We discuss the constraints and difficulties in interpreting $x$ as a neutrino. We show that a mainly-sterile neutrino scenario is compatible with all laboratory constraints, within narrow limits on the mixing parameters, although there are problems with astrophysical and cosmological constraints. This scenario predicts that appreciable numbers of other $x$-decay events with different origins and time structures should also be observable in the KARMEN detector. Such $x$-decay events should also be found in the LSND experiment and may be relevant to the search for $\bar\nu_\mu\to\bar\nu_e$ oscillations.

Abstract:

We report on a high statistics numerical study of the crystalline random surface model with extrinsic curvature on lattices of up to $64^2$ points. The critical exponents at the crumpling transition are determined by a number of methods all of which are shown to agree within estimated errors. The correlation length exponent is found to be $\nu=0.71(5)$ from the tangent-tangent correlation function whereas we find $\nu=0.73(6)$ by assuming finite size scaling of the specific heat peak and hyperscaling. These results imply a specific heat exponent $\alpha=0.58(10)$; this is a good fit to the specific heat on a $64^2$ lattice with a $\chi^2$ per degree of freedom of 1.7 although the best direct fit to the specific heat data yields a much lower value of $\alpha$. Our measurements of the normal-normal correlation functions suggest that the model in the crumpled phase is described by an effective field theory which deviates from a free field theory only by super-renormalizable interactions.