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Register a new userParity Violation with Electrons and Hadrons
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E. J. Beise
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A key question in understanding the structure of nucleons involves the role of sea quarks in their ground state electromagnetic properties such as charge and magnetism. Parity-violating electron scattering, when combined with determination of nucleon electromagnetic form factors from parity-conserving e-N scattering, provides another degree of freedom to separately determine the up, down and strange quark contributions to nucleon electromagnetic structure. Strange quarks are unique in that they are exclusively in the nucleons sea. A program of experiments using parity violating electron scattering has been underway for approximately a decade, and results are beginning to emerge. This paper is a brief overview of the various experiments and their results to date along with a short-term outlook of what can be anticipated from experiments in the next few years.
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The parity-violating electron scattering community has made tremendous progress over the last twenty five years in their ability to measure tiny asymmetries of order 100 parts per billion (ppb) with beam-related corrections and systematic errors of a few ppb. Future experiments are planned for about an order of magnitude smaller asymmetries and with higher rates in the detectors. These new experiments pose new challenges for the beam instrumentation and for the strategy for setting up the beam. In this contribution to PAVI14 I discuss several of these challenges and demands, with a focus on developments at Jefferson Lab.
The history and phenomenology of hadronic parity nonconservation (PNC) is reviewed. We discuss the current status of the experimental tests and theory. We describe a re-analysis of the asymmetry for polarized proton-proton scattering that, when combined with other experimental constraints and with a recent lattice QCD calculation of the weak pion-nucleon coupling, reveals a much more consistent pattern of PNC couplings. In particular, isoscalar coupling strengths are similar to but somewhat larger than the best value estimate of Donoghue, Desplanques, and Holstein, while both lattice QCD and experiment indicate a suppressed parity-nonconserving pion-nucleon coupling. We discuss the relationship between meson-exchange models of hadronic PNC and formulations based on effective theory, stressing their general compatibility as well as the challenge presented to theory by experiment, as several of the most precise measurements involve significant momentum scales. Future directions are proposed.
We used a torsion pendulum containing $approx 10^{23}$ polarized electrons to search new interactions that couple to electron spin. We limit CP-violating interactions between the pendulums electrons and unpolarized matter in the earth or the sun, test for rotation and boost-dependent preferred-frame effects using the earths rotation and velocity with respect to the entire cosmos, and search for exotic velocity-dependent potentials between polarized electrons and unpolarized matter in the sun and moon. Finally, we find that the gravitational mass of an electron spinning toward the galactic center differs by less than about 1 part in $10^{21}$ from an electron spinning in the opposite direction. As a byproduct of this work, the density of polarized electrons in Sm$ $Co$_5$ was measured to be $(4.19pm 0.19)times 10^{22} {rm cm}^{-3}$ at a field of 9.6 kG.
Measurements of $CP$-violating observables in $B$ meson decays can be used to determine the angles of the Unitarity Triangle and hence probe for manifestations of New Physics beyond the Cabibbo-Kobayashi-Maskawa Standard Model paradigm. Of particular interest are precise measurements of the angles $gamma$ and $beta$. Also of great importance are studies of $CP$-violation involving $B_s^0$ mesons, in particular the phase $phi_s$, which is a golden observable in flavour physics at the LHC. Complementary to these studies is the continuing search for direct and indirect $CP$-violation in the charm system, where the experimental precision is now at the $10^{-3}$ level. I will present new and recent results in these topics, and in $CP$-violation searches in baryon decays, with specific emphasis on the measurement programme at the LHC.
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