Title of seminar: (Cold) Molecules as a tool to probe fundamental physics
Abstract:
Modern physics is struck with a paradox: there are very good reasons to believe that our current understanding of Nature is incomplete, yet it is exceedingly difficult to conduct an experiment which contradicts our best theories. This tension is particularly palpable since the Large Hadron Collider explored a wide range of energies without a clear detection of new physics, thereby discarding many extensions Beyond the Standard Model (BSM). Searching for new physics in the high energy domain now demands extremely large investments to build bigger and brighter instruments. Another approach, complementary but very powerful, is to perform small scale, low-energy experiments but with very high precision1. In the last 20 years in particular, this led the community to look for new physics in much larger objects than elementary particle: molecules. Many proposals and experiments have demonstrated the potential of molecules to test our fundamental understanding of Nature: from probing variations of fundamental constants like the electron mass over the proton mass or the fine-structure constant , or different models for dark matter, to testing the symmetrisation postulate of Quantum Mechanics or Bose-Einstein statistics. Especially notable are experiments currently responsible for the most stringent limit on the value of the electron electric dipole moment (which constraints strongly potential BSM theories).
However, the field is still in its infancy: the molecules that are currently used to explore new physics are composed of 2, 3 or 4 atoms at most, even though larger molecules offer qualitatively distinct opportunities in fundamental physics. The abundance of nearly degenerate rovibrational levels in the larger, polyatomic molecules (with more than 10 atoms) enhances their sensitivity to variations of fundamental constants or to potential ultralight bosonic dark matter. They can also provide a new system for implementing quantum computing, using either rotational or vibrational levels, and taking advantage of robust systematic error rejection schemes. Having more than 3 atoms also opens up the possibility to use molecular chirality to probe parity-violating interactions. Indeed, the energy levels of two enantiomers (the mirror-images of chiral molecules) are predicted to be slightly different, because of the parity-violation inherent to the weak interaction, or as a result of their interaction with dark-matter fields. Thus, a measurement of this symmetry-breaking energy difference ΔEPV is a sensitive probe of the Standard Model and of physics BSM.
However, as the number of atoms in a molecule goes up, the number of energy levels increases very rapidly, as well as the couplings between these energy levels. These couplings limit the applicability of the usual cooling mechanisms because they induce population losses towards many different states. Couplings between different vibrational modes (denoted in the literature as Intramolecular Vibrational Redistribution (IVR)) also blur out spectroscopic resolution by what can be viewed as a loss of coherence due to non-radiative dynamic energy distribution in the molecule.
In this talk I will review the current use, the potential ahead and the obstacles on the way of using molecules for tests of fundamental physics, focussing on our on-going attempt to measure ΔEPV in chiral molecules at the Laboratoire de Physique des Lasers.