Session C1: Particle and Nuclear Physics

A Search for Physics beyond the Standard Model: Update on the Qweak Experiment

The Qweak experiment aims to make a precision measurement of the proton's weak charge ($Q^{P}_{weak})$ using parity-violating elastic electron-proton scattering. The goal, a $\pm $4{\%} measurement of the proton's weak charge ($Q^{P}_{weak}$ \textit{= 1 - 4sin}$^{2}$\textit{$\theta $}$_{W}$ ), will allow for a $\pm $0.3{\%} determination of \textit{sin}$^{2}$\textit{$\theta $}$_{W}$ at low momentum transfers ($Q^{2}$\textit{ $\sim $ 0.026 GeV}$^{2}/c^{2})$, and provide a measure of the running of \textit{sin}$^{2}$\textit{$\theta $}$_{W}$ . Furthermore, since $Q^{P}_{weak}$ is well-determined in the Standard Model, this experiment will be a sensitive test for physics beyond the Standard Model. The experiment, which scatters longitudinally polarized electrons off a 0.35m long liquid hydrogen target, is currently running in Hall C at the Thomas Jefferson National Accelerator Facility. At this time, only a fraction of the data required to reach the desired statistical uncertainties has been collected, with the bulk of the data-taking scheduled for late-2011 and 2012. The design and present status of the Qweak experiment will be summarized in this talk.   

The New Pi-e-nu Experiments

Electron-muon universality in weak interaction has been tested at the 0.16\% level by comparing the branching ratios of two pion decay modes, $\pi^+\rightarrow e^+ + \nu_e$ and $\pi^+\rightarrow \mu^+ + \nu_\mu$; $R^{e/\mu} = \Gamma(\pi^+\rightarrow e^+ + \nu_e)/\Gamma(\pi^+\rightarrow \mu^+ + \nu_\mu) = (1.231\pm 0.004)\times 10^{-4}$. The smallness of the value $R^{e/\mu}$ is owing to the helicity suppression in the $\pi^+ \rightarrow e^+ \nu_e$ decay, and because of this, the measurement of $R^{e/\mu}$ is sensitive to pseudo-scaler type of couplings, which arise in many extensions of the standard model of particle physics. In addtion, the effect of the pseudo-scaler interaction comes as an interference term with the dominant axial-vector term, thus the contribution from the pseudo-scaler interaction is proportional to $1/\Lambda^2$ where $\Lambda$ is a mass scale of the new interaction. The measurement of a 0.1\% level of $R^{e/\mu}$ corresponds to 1000~TeV of the mass scale $\Lambda$. New experiments aiming to measure $R^{e/\mu}$ to an accuracy at a level of 0.05\% are ongoing in both TRIUMF and PSI. The details of the experiment at TRIUMF (PIENU) will be explained in this talk. The experiment at PSI (PEN) will be also explained in comparison with PIENU.   

Casimir-type effects in QCD as a source of Dark Energy

I discuss a Casimir-like behaviour in the $\theta$-dependent part of the energy in a ``deformed'' QCD. Defining the system on a manifold of size $L$, the energy takes the form $E = A\left[1+\frac{B}{L} + {\cal O}(L^{- 2})\right]$, despite the presence of a mass gap. In contrast, one would naively expect the form $E = A[1+Be^{-mL}]$ originating from any physical massive degrees of freedom. I explain how this form comes instead from a non-dispersive ``contact'' term which does not originate from any propagating degrees of freedom, so that the naive argument is not applicable. I then present some explicit results in a ``deformed'' QCD, which while weakly coupled and under full theoretical control still exhibits interesting properties of true QCD such as confinement, a mass gap, and non-trivial $\theta$-dependence. If the Dark Energy is defined as a mismatch between the energies of the system defined in a bounded system and in the Minkowski vacuum, then the discussed effect gives a Dark Energy estimated at $\Delta E \sim H\Lambda_{\mathrm{QCD}}^3 \sim (10^{-3}eV)^4$, which is astonishingly close to the observed value.   

Quantum Number Density Asymmetries Within QCD Jets Correlated with Lambda Polarization

The observation of jets in a variety of hard-scattering processes has allowed the quantitative study of perturbative chromodynamics (PQCD) by comparing detailed theoretical predictions with a wide range of experimental data. This paper examines how some important, nonperturbative, facets of QCD involving the internal dynamical structure of jets can be studied by measuring the spin orientation of Lambda hyperons produced in these jets. The measurement of the transverse polarization for an individual Lambda within a QCD jet permits the definition of spin-directed asymmetries for quantum number densities in rapidity space (such as charge, strangeness and baryon number densities) involving neighboring hadrons in the jet. These asymmetries can only be generated by soft, nonperturbative dynamical mechanisms and such measurements can provide insight not otherwise accessible into the color rearrangement that occurs during the hadronization stage of the fragmentation process.   

A High-Intensity Source of $^6$He Atoms for Fundamental Research

We have designed and built a lithium target station for the production of $^6$He atoms for fundamental research. The system relies on the reaction $^7$Li($^2$H,$^3$He)$^6$He using a deuteron beam provided by the tandem Van de Graaff accelerator available at the Center for Experimental Nuclear Physics and Astrophysics of the University of Washington reaching a maximum intensity and energy of 10$\mu$A and 18 MeV. The extracted intensity of gaseous $^6$He atoms was measured to be $\sim$10$^9$ atoms/s in a low-background experimental area. This represents the highest intensity of $^6$He currently available in the world. This presentation will discuss the details of the target design and give a brief overview of the experimental program.   

Unitary Mixing Matrix Parameterizations

Unitary mixing matrices, such as the quark and lepton mixing matrices, have unphysical degrees of freedom; one can multiply rows and columns by arbitrary complex phases. In elementary particles, standard practice is to choose phases and parameterize what's left of the unitary matrix. We show that one can use an ``unbiased state'' to define the complex phases. This defines a natural parameterization of unitary mixing matrices through a Lie subgroup of $U(n)$ that contains no phase information and is isomorphic to $U(n-1)$. We show that Jarlskog invariants and $J_{CP}$ are related to Berry-Pancharatnam or quantum phases. For $3\times 3$ matrices, we solve the relationship between the Lie subalgebra and Lie subgroup in closed form. This defines a new parameterization that is close to the standard $3\times 3$ parameterization used for the CKM matrix, but treats the (12), (23), and (13) permutations equally. The new fourth parameter gives the (123) and (132) permutations. We provide two more parameterizations for these matrices, one of which brings the tribimaximal form for the MNS matrix into a particularly simple form.   

Measurement of Neutron Capture Cross Sections of Selenium Isotopes

There have been numerous measurements of the neutron capture cross sections of the stable Se isotopes, most dating from at least 40 years ago. The various results for individual isotopes are often in poor agreement with one another, but as yet there has been no attempt at a systematic measurement of the capture cross sections leading to all seven radioisotopes formed from capture by natural Se, which range in halflife from 17 s to 120 d. Using cadmium-shielded and unshielded irradiations of natural Se in various irradiation sites in OSU's TRIGA reactor, we have determined the thermal cross sections and resonance integrals for captures leading to $^{75,77m,79m,81g,81m,83g,83m}$Se.   

Detailed studies of survival probabilities in hot fusion reactions

In the synthesis of new heavy nuclei by hot fusion reactions, the cross section for producing a new heavy nucleus, $\sigma_{EVR}$, can be represented as $\sigma_{EVR} (E_{C.M.})=\sum^{J_{max}}_{J=0}\sigma_{CN}(E_{c.m.},J) \cdot W_{sur}(E_{c.m.},J)$ where $\sigma_{CN}$ is the complete fusion cross section and W$_{sur}$ is the survival probability of the completely fused system. The survival probability is essentially the probability of de-exciting by neutron emission instead of fission. In this work, we are attempting to measure the survival probability, W$_{sur}$, for the first chance fission of excited Hs nuclei. We form the Hs nuclei using the $^{25,26}$Mg + $^{248}$Cm reaction. We measure the neutrons associated with pre-fission emission and those emitted by the fission fragments after fission using six BC501 neutron detectors and six silicon fission fragment detectors. We measure the angular distribution of the emitted neutrons and are able to separate the pre- and post-fission neutrons.   

High Precision Determination of the $^6$He Half-life

We performed a high precision measurement of the half-life of $^6$He. The motivation for this experiment lay not only in resolving a long-standing discrepancy between the previous most precise measured values of $806.7 \pm 1.5$~ms and $798.1 \pm 1$~ms, but also in serving as a solid basis for the extraction of the axial coupling constant $g_A$ by comparing the result to ab initio calculations. The measurement took place at the tandem accelerator of the Center for Experimental Nuclear Physics and Astrophysics of the University of Washington where we used the $^7$Li(d,$^3$He) reaction to produce $^6$He. We directed a 10~$\mu$A, 18~MeV deuteron beam onto a liquid lithium target station that delivers $\sim$$10^9$ atoms/s to a low-background experimental area where the amount of other short-lived isotopes, particularly 8Li, is greatly reduced. Here we present the design of the lithium target station, the $^6$He half-life experiment setup and the half-life determination process.   

Neutron Capture by Cadmium: Thermal Cross Sections and Resonance Integrals of $^{106,108,110,112,114,116}$Cd

The neutron capture cross sections of the stable, even-mass Cd isotopes (A = 106, 108, 110, 112, 114, and 116) have been previously measured in sources of natural abundance or low enrichment, often making the results uncertain owing to the large absorption cross section of naturally occurring $^{113}$Cd. Ambiguities in values of the isomeric branching ratios have also contributed to uncertainties in previous results. We have remeasured the Cd neutron capture cross sections using samples of greater than 90{\%} isotopic enrichment irradiated in the OSU TRIGA reactor. Gamma-ray emission spectra were analyzed to determine the effective resonance integrals and thermal cross sections leading to eight radioactive ground and isomeric states in the Cd isotopes.   

Project 8: Exploring a novel technique for the measurement of neutrino mass

Project 8 is a neutrino mass measurement experiment. It uses radio frequency techniques to observe the cyclotron radiation from beta-decay electrons trapped in a magnetic bottle. From the electrons' cyclotron frequencies, the beta-decay energy spectrum, and thus the mass of the electron neutrino may be observed. This nondestructive energy measurement technique may provide improved sensitivity to the neutrino mass over previous experiments. Currently, Project 8 is seeking to demonstrate an energy measurement made on single electrons. Recent progress and prototype status will be discussed.   

Characterization of a New Lead Slowing Down Spectrometer

Pacific Northwest National Laboratory is studying the application of Lead Slowing Down Spectrometry (LSDS) to measure the mass of actinides in used nuclear fuel. LSDS has been used for decades to make cross-section measurements on relatively small isotopic samples of well know masses. In LSDS, a pulse of fast neutrons is injected into a large lead stack ($\sim $ 1m$^{3})$. The neutrons quickly down-scatter to the point at which elastic scattering dominates. At this point, the energy of the neutron and the time the neutron has been in the lead become correlated. By measuring this elapsed time, it is possible to measure interactions of the neutrons with the fuel in the 0.1 to 1,000 eV range. Many of the actinides have strong resonances in this region, making it possible, through careful measurements and analysis, to extract isotopic masses from LSDS measurements. This paper will present results of the effort to construct and characterize a new lead slowing down spectrometer. To characterize the spectrometer, a series of (n, $\gamma )$ experiments were conducted to measure the correlation between the time after the neutrons enter the lead and the energy of the interaction. Results from these measurements as well as plans for future development of the spectrometer will be discussed.   

Resolution of the EPR Paradox for Fermion Spin Correlations

The EPR paradox addresses the question of whether a physical system can have a definite state independent of its measurement. Bell's Theorem places limits on correlations between local measurements of particles whose properties are established prior to measurement. Experimental violation of Bell's theorem has been regarded as evidence against the existence of a definite state prior to measurement. We model fermions as having a spatial distribution of spin values, so that a Stern-Gerlach device samples the spin distribution differently at different orientations. The computed correlations agree with quantum mechanical predictions and experimental observations. Bell's Theorem is not applicable because for any sampling of angles, different points on the sphere have different density of states.   

The Electroweak Force may be a result of a Horizon of a Curved Universe

This author has previously written that the CMBR may be the result of looking at the Horizon of a curved sphere-like Universe (sphere-like in 3 space dimensions, with one linear time dimension perpendicular to each point on the Sphere). When one looks at the Horizon of a two dimensional surface, such as the Earth, the Horizon is a one dimensional curved line, because objects at a distance, not only shrink, but tilt back away from the observer. In space-time something more dramatic happens, the tilt is in the time dimension so the horizon has a velocity and an acceleration, away from the observer no matter where the observer is located on the surface (with perhaps the exception of black and white holes). It can be seen that this sort of Horizon breaks CPT invariance in that time can only have one direction at the Horizon, since it has to tilt back away from the observer. It may be that this breaking gives rise to masses in particles that would not normally be there if the Universe where flat. These masses would be Universal and would appear to permeate all of Space-Time just as the CMBR does, in essence Curvature gives Energy, but in an unusual way. A way of testing for this phenomena is to look at different sizes of Black Holes, and their Horizons. Different curvatures should change the energies of the Higgs particle, and perhaps the strength of the Electroweak Force.