New fundamental particles at high energy scales that have not been reached by the Large Hadron Collider (LHC) could explain the observed matter-antimatter asymmetry that cannot be understood by the Standard Model of particle physics. These hypothetical particles, if they exist, will introduce a tiny electric dipole moment on the electron (eEDM), which can be probed by extremely sensitive measurement of the electron spin precession in a huge intra-molecular electric field. Previous eEDM measurements using relatively warm molecules are all consistent with zero and are mainly limited by spin coherence and interogation time. Here we propose to measure the eEDM using ultracold molecular beams. We will apply polarization gradient cooling in the two transverse directions to reduce the temperature to below 50 microkelvin. This will significantly increase the number of molecules that can be detected in the forward direction and improve the coherence. We will also develop a new deceleration technique for molecules, called Zeeman-Sisyphus deceleration, to reduce the forward velocity of the molecules to below 30 m/s that allows around 0.1 s interogation time, 100 times longer than the longest in the current beam experiments, in the measurement apparatus. Together with other improvements on molecule production, initial state preparation, magnetic field shielding and control, and spin state detection, we expect that the eEDM can be measured at least 10 or 100 times more precisely than the state-of-the-art level. This will be a search for new particles responsible for matter-antimatter asymmetry and a test of new physics beyond the Standard Model up to a few 100 TeV. This energy range extends far beyond the kinematic reach of any existing and near-future particle colliders and lies around the favoured mass range of many supersymmetric models.
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