Published papers

We investigate, using a 1/N expansion, the behavior of a parameter in the scalar-fermion sector of the standard model that shows perturbative nondecoupling as the fermion becomes heavy. This low-energy parameter is related to the S parameter defined through W3-B mixing. We obtain the leading 1/N contribution to this parameter that, if expanded perturbatively, collapses to its constant one-loop result. Nonperturbatively, we find that as the mass of the fermion approaches the triviality scale the behavior of the parameter is nonuniversal and shows nondecoupling. © 1993 The American Physical Society.

© 2017 American Physical Society. Through the study of the power spectra of a monochromatic light beam reflected by metallic mirrors, fluctuations in their reflectance are observed. The power spectra were obtained down to a factor 10-6 below the standard quantum limit, with a dynamic range of 105 in frequency and power, using methods we developed. The properties of the spectra are investigated, and their dependence on the material is analyzed. The physics underlying the phenomenon is also discussed. These fluctuations provide a window into the degrees of freedom responsible for the reflection process in metals.

Fluctuations in light absorption by atoms are observed by applying laser light on rubidium atoms and measuring the transmitted light intensity fluctuations. These fluctuations are spontaneous noise, generic to photon atom interactions. By making use of the sub-shot noise random signal detection technology, we have measured the spectra at sub-shot noise levels to reveal their rich nature, whose whole structure had previously been unobserved. The effects of atoms transiting the laser beam, Rabi flopping in the optical transitions and Larmor precession of the magnetic moment are observed in the spectra. The properties of the fluctuations reflect not only the quantum behavior of atoms but also that of light. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2013.

© 2016 American Physical Society. Using resonant light absorption, random-walk motions of rubidium atoms in nitrogen buffer gas are observed directly. The transmitted light intensity through atomic vapor is measured, and its spectrum is obtained, down to orders of magnitude below the shot-noise level to detect fluctuations caused by atomic motions. To understand the measured spectra, the spectrum for atoms performing random walks in a Gaussian light beam is computed, and its analytical form is obtained. The spectrum has 1/f2 (f is frequency) behavior at higher frequencies, crossing over to a different, but well-defined, behavior at lower frequencies. The properties of this theoretical spectrum agree excellently with the measured spectrum. This understanding also enables us to obtain the diffusion constant, the photon cross section of atoms in buffer gas, and the atomic number density from a single spectral measurement. We further discuss other possible applications of our experimental method and analysis.

Various asymmetric orbifold models based on chiral shifts and chiral reflections are investigated. Special attention is devoted to the consistency of the models with two fundamental principles for asymmetric orbifolds: modular invariance and the existence of a proper Hilbert space formulation for states and operators. The interplay between these two principles is non-trivial. It is shown, for example, that their simultaneous requirement forces the order of a chiral reflection to be 4, instead of the naive 2. A careful explicit construction is given of the associated one-loop partition functions. At higher loops, the partition functions of asymmetric orbifolds are built from the chiral blocks of associated symmetric orbifolds, whose pairings are determined by degenerations to one-loop. © 2004 Elsevier B.V. All rights reserved.