Session 1WB: Expanding the Standard Model: Dark Photon and CP Violation

Dark Forces, Dark Matter, and the GeV-Scale Discovery Frontier

The search for new forces mediated by sub-GeV particles with very weak coupling to matter (``dark forces'') is an emerging frontier in fundamental physics with well-motivated connections to dark matter. These forces remain very weakly constrained, but a wide variety of recent and upcoming experiments are greatly extending sensitivity to them. I will present the theoretical motivations for dark forces and their possible connections to dark matter and the anomalous magnetic moment of the muon. I will also discuss strategies, results, and prospects for searches at high-energy colliders, flavor factories, and dedicated fixed-target experiments. I will focus primarily on the program of searches for dark forces at Jefferson Laboratory.   

The DarkLight Experiment at the JLab FEL

DarkLight will study the production of gauge bosons associated with Dark Forces theories in the scattering of 100 MeV electrons on proton a target. DarkLight is a spectrometer to measure all the final state particles in $e^-+p\rightarrow e^-+p+e^-+e^+$. QED allows this process and the invariant mass distribution of the $e^+e^-$ pair is a continuum from nearly zero to nearly the electron beam energy. Dark Forces theories, which allow the dark matter mass scale to be over 1 TeV, predict a gauge boson $A^{\prime}$ in the mass range of 10-1,000 MeV and decays to an electron-positron pair with an invariant mass of $m_{A^{\prime}}$. We aim to search for this process using the 100 MeV, 10 mA electron beam at the JLab Free Electron Laser impinging on a hydrogen target with a 10$^{19}$cm$^{-2}$ density. The resulting luminosity of 6$\times$10$^{35}$/cm$^2$-s gives the experiment enough sensitivity to probe $A^{\prime}$ couplings of 10$^{-9}\alpha$. DarkLight is unique in its design to detect all four particles in the final state. The leptons will be measured in a large high-rate TPC and a silicon sensor will measure the protons. A 0.5 T solenoidal magnetic field provides the momentum resolution and focuses the copious M\o ller scattering background down the beam line, away from the detectors. A first beam test has shown the FEL beam is compatible with the target design and that the hall backgrounds are manageable. The experiment has been approved by Jefferson Lab for first running in 2017.   

The HPS Experiment: Searching for Dark Photons at Jefferson Lab

Recently, there has been much interest in new physics models with hidden sectors with massive U(1) gauge bosons, called heavy photons or dark photons. Heavy photons are expected on general theoretical grounds, and astrophysical evidence suggesting they might mediate dark matter annihilations and/or interactions with ordinary matter has motivated the search for a heavy photon in the mass range $m_{A'} \sim$ 20 to 1000 MeV/c$^2$. Heavy photons couple to ordinary photons through kinetic mixing, which induces their weak coupling to electrons, so they are radiated in electron scattering and can subsequently decay into narrow $e^+e^-$ resonances which can be observed above the QED background. The Heavy Photon Search (HPS) experiment at Jefferson Lab will search for the $e^+e^-$ or $\mu^+\mu^-$ heavy photon decay products using a forward spectrometer that includes silicon microstrip detectors for vertexing and tracking, a PbWO$_4$ electromagnetic calorimeter for fast triggering and electron identification, and a muon detector for muon identification. HPS will explore a large, unexplored domain in the mass/coupling plane with great sensitivity. This talk will review the experiment, with results from the recent test run and plans for future construction and data taking.   

Nuclear Physics Tests of CP

The search for yet unseen violation of CP symmetry both within and beyond the Standard Model is at the forefront of nuclear physics tests of fundamental symmetries and studies of neutrino properties. I this talk, I review the theoretical expectations and implications of searches for permanent electric dipole moments of neutral atoms, nucleons and nuclei as well as tests of CP symmetry in neutrino interactions.   

Current searches for a neutron electric dipole moment

The search for particle Electric dipole moments is generally accepted as the most promising place to search for physics beyond the Standard Model. One reason for this is that the standard model backgrund is so small. Supersymmetric theories, for example, are already constrained by existing EDM limits. In the fifty year history of these experiments numerous models have been falsified. Searches for hadronic sector electric dipole moments are particularly sensitive to CP-violation that might contribute to generation of a matter/anti-matter asymmetry at the electroweak symmetry breaking transition. At the moment there are about six serious projects to reduce the upper limit on a possible non-zero neutron electric dipole moment. After a brief survey of these experiments attention will be focused on a new cryogenic experiment [1] at the ORNL SNS and a room temperature experiment [2] at the Munich FRM II reactor. All modern experiments use trapped ultra - cold neutrons, which virtually eliminates the v xE systematic which plagued earlier beam experiments but is subject to a new variant of that related to the geometric phase.\\[4pt] [1] R. Golub and Steve K. Lamoreaux, Phys. Rep. 237 (1994) 1.\\[0pt] [2] I. Altarev, et al., Nuovo Cim. C035N04 (2012) 122.   

CP and the Search for Neutrinoless Double Beta Decay

Neutrinoless double beta decay can only occur if lepton number is violated and if the neutrino is its own anti-particle; therefore observation of this decay will provide a new window to physics beyond the Standard Model. The implications of neutrinoless double beta decay in theories of CP violation will be discussed.   

CP violation in the neutrino sector

This talk will cover the status of searches for CP violation in neutrinos and prospects for future measurements, including long-baseline beams, atmospheric neutrinos and pion decay-at-rest experiments.