New Probes of the Ultra-Light Frontier

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This award funds the research activities of Professor Surjeet Rajendran at the University of California, Berkeley. Physical laws often lead to new particles through which their effects are manifested on the natural world. For example, we experience the electric force because of the existence of particles like the electron and the proton that carry electric charge, with the force between them carried by the photon. Searches for particles is thus a fruitful way to discover new fundamental forces. It is important to search for such fundamental forces since they have the potential to significantly enhance our understanding of the origins of the laws of nature. They may also potentially enable us to interact with the natural world in dramatically new ways, potentially permitting advances as remarkable as the effects of electromagnetism on our daily lives. Many models of fundamental physics predict the existence of particles that have very small masses and also have very weak interactions with our world. In some cases, these particles can also be the dark matter of the universe. Their weak interactions can be overcome either with large systems or through precision technology. This project develops methods that use large astrophysical objects such as millisecond pulsars as well as precision laboratory techniques to significantly expand our reach into the parameter space of such particles.Professor Rajendran will develop new experimental approaches to detect ultra-light particles that interact very weakly with the standard model such as axions and dark photons. Such particles naturally emerge in many frameworks of physics beyond the standard model, and may even be the dark matter of the universe. The methods developed by Professor Rajendran include the use of the existence of the super-radiant instability of rotating systems to argue that the existence of certain kinds of light particles would cause the rapid spin-down of millisecond pulsars. Observations of such pulsars can therefore constrain these particles. Professor Rajendran will point out qualitatively new effects of dark photons emerging from their longitudinal modes. These effects have been overlooked in the literature and by incorporating them, he will show how existing experiments can parametrically extend their reach into the parameter space of such models. These gains also extend to the case where such dark photons constitute the dark matter of the Universe and he will invent techniques tailored to detect such dark matter. Professor Rajendran will also show how current experimental methodologies that have been developed to search for fundamental sources of CPT violation or a cosmologically preferred direction can also be used to search for certain kinds of light dark matter. The dark matter signal can be qualitatively different from the signals typically expected in these experiments and may permit ways to overcome the systematic limitations of such experiments. With the increasing costs of collider experiments, the future of particle physics may lie as much in non-collider experiments that can probe such hidden sectors that lie far in the ultra-violet.