High Energy Physics

I will report on some work in progress with Dan Freed and Greg Moore. In an orientifold background, D-brane charge takes values in a certain twisted version of KR Theory. Moreover, there is a nontrivial background charge (\'tadpole\'). Up \'til now, this background charge has only been calculated rationally -- i.e., ignoring torsion. We derive a formula for it, over the integers. Only after \'inverting 2\', does the charge localize to the fixed point sets of the orientifold action, and we can give a compact formula for it. This reproduces the previously known rational results, but contains new information.

Amanda Peet received her Ph.D. at Stanford University and currently is Associate Professor at the University of Toronto, her “intellectual home base.” She is also an Affiliate Member of Perimeter Institute. Amanda's goal is to understand the fundamental dynamics of all forces and particles seen so far in Nature, especially gravity. Broadly: She studies the quantum dynamics of interactions between gravity and matter using string theory, with applications to black holes and cosmology, and links to gauge theory and particle physics. Past work has focused on the black hole information paradox, black hole entropy, D-brane models of black holes, duality, holography, building of new geometries, spacetime singularity resolution, and cosmology. Amanda continues to develop these interests, as well as develop others as new particle accelerator data from LHC and cosmological data (further) influence the field.

New low frequency radio telescopes currently being built open up the possibility of observing the 21 cm radiation before the Epoch of Reionization in the future, in particular at redshifts 200 ≥ z ≥ 30, also known as the dark ages. At these high redshifts, Cosmic Microwave Back-ground (CMB) radiation is absorbed by neutral hydrogen at its 21 cm hyperfine transition. This redshifted 21 cm signal thus carries information about the state of the early Universe and can be used to test fundamental physics. We study the constraints these observations can put on the variation of fundamental constants and on fundamental mass scales. We show that the 21 cm radiation is very sensitive to the variations in the fine structure constant and can in principle place constraints comparable to or better than the other astrophysical experiments. Cosmic strings, if they exist, contribute to the anisotropies in the primordial gas leaving an imprint on the 21 cm radiation. They can tell us about the fundamental mass scales involved in the theories beyond the standard model. We show that the 21 cm radiation can potentially probe cosmic strings of tension ~10−12 asumming intercommutation probability of 1. Making such observations will require radio telescopes of collecting area 10 − 106 km2 compared to ~ 1 km2 of current telescopes.

Precision tests of Local Position Invariance (LPI) involve many different methods in atomic, nuclear and gravitational physics, astrophysics and cosmology, and many different epochs and environments. We present some methods for comparing or combining different methods, either in a model-independent way or within simple scalar field models of variation. We focus on which methods are most sensitive to cosmologically recent time variation, and also on tests of spatial variation within the Solar System.

I will describe a method of understanding how the nuclear binding energies depend on the masses of the light quarks. This is useful in applications ranging from anthropic constraints to equivalence principle tests and bounds on the time variation on the quark masses.

To date, optical clocks based on singly trapped ions1) and ultracold neutral atoms trapped in the Stark-shift-free optical lattices2) are regarded as promising candidates for future atomic clocks. So far “optical lattice clocks” have been evaluated with uncertainty of 1×10-15 (ref. 3)) limited by that of Cs atomic clocks. Frequency comparison between highly-stable and accurate optical lattice clocks is, therefore, crucial for their further evaluation. Looking toward fractional uncertainties of 10-16 and below, collisional frequency shift, Black body radiation (BBR) shift, and hyperpolarizability effects, all of which depend on interrogated atomic elements and experimental configurations, are becoming major concerns. In this talk, we discuss optimal lattice geometries in view of the quantum statistics and related spins of interrogated atoms. This leads to two promising configurations for the lattice clock: One-dimensional (1D) lattice loaded with spin-polarized fermions4) and 3D lattice loaded with bosons. We present frequency comparison of these two optical lattice clocks using fermionic 87Sr and bosonic 88Sr. Such lattice clock comparison will offer an important step to ascertain the clocks’ uncertainty beyond the Cs limit of 1×10-15. As for the latter two issues, the BBR and the lattice laser related uncertainties, we discuss prospects for a cryogenic clock, a “blue-detuned” magic wavelength, and a Hg based optical lattice clock5). References: 1) T. Rosenband et al., Science 319 (2008) 1808. 2) H. Katori, M. Takamoto, V. G. Pal\'chikov and V. D. Ovsiannikov, Phys. Rev. Lett. 91 (2003) 173005. 3) S. Blatt et al., Phys. Rev. Lett. 100 (2008) 140801. 4) M. Takamoto et al., J. Phys. Soc. Jpn. 75 (2006) 104302

We propose new experiments with high sensitivity to a possible variation of the electron-to-proton mass ratio µ me/mp. We consider a nearly degenerate pair of molecular vibrational levels, each associated with a different electronic potential. With respect to a change in µ, the change in the splitting between such levels can be large both on an absolute scale and relative to the splitting. We demonstrate the existence of such pairs of states in Cs2, where the narrow spectral lines achievable with ultracold molecules make the system promising for future searches for small variations in µ.

High precision measurements in atomic and molecular systems have reached unprecedented accuracy owing to the state-of-the-art quantum control of both light and matter. We have recently completed an evaluation of the uncertainty of our 87Sr optical lattice clock at the 1x10e-16 fractional level, surpassing the best current evaluations of Cs primary standards. By analyzing worldwide measurements of the absolute frequency of the clock transitions in Sr, we constrain temporal variations of fundamental physical constants as well as their possible couplings to the gravitational potential. We will report the latest results on our 87Sr optical atomic clock, as well as the use of the Sr system to constrain variations of the fine-structure constant.