Session D15: Electroweak Interactions

What if $G_E^s $ is Zero? Implications for $G_M^s $and $G_A^s $

Because strange quarks are the lightest quarks present in nucleons via only vacuum fluctuations, studying their activities in nucleons gives us insight to the vacuum's effects on nucleon properties. These contributions can be accessed through electroweak interactions---in particular through parity-violating \textit{eN} and \textit{$\nu $N} elastic scattering. Recent data from parity-violating \textit{eN} elastic scattering (HAPPEX, PVA4) suggests that the strange contribution to the proton electric form factor, $G_E^s $, may be nearly zero in the range 0 $<$ Q$^{2 }<$ 1 GeV$^{2}$. We assume that $G_E^s $ is small and use existing \textit{$\nu $N} data to explore the consequences for $G_M^s $ and $G_A^s $.   

A New Precision Measurement of the Lifetime of $^{19}$Ne

The mixed $\frac{1}{2}^+\rightarrow\frac{1}{2}^+$ decay of $^{19}$Ne is an important system for studies of the weak interaction. A measurement of the lifetime of this decay at the 10$^{-4}$ level combined with the measured value of the $\beta$-asymmetry enables a determination of V$_{ud}$ that rivals the precision obtained from $0^+\rightarrow0^+$ superallowed Fermi beta decays. The lifetime is currently known to a precision of about 0.08\%, and by utilizing the unique capabilities of the Trapped Radioactive Isotopes: $\mu$icro-laboratories for fundamental Physics (TRI$\mu$P) facility at the Kernfysich Versneller Instituut (KVI), we can improve this precision by up to a factor of three. We describe recent progress towards a high-precision lifetime measurement and present preliminary results.   

Improved $\beta$ Decay Branching Ratios

The work we report here aims at increasing the precision possible in the measurement of branching ratios for superallowed $\beta ^+$decays. Such highly accurate values are essential in generating precise $ft$-values for $0^+\to 0^+$decays, which can then be used to test the Standard Model \textit{via} the unitarity of the Cabibbo-Kobayashi-Maskawa matrix [1]. The required precision is $\sim $0.1{\%} or better. While this limit was already achieved in the case of ${ }^{34}Ar$ [2], it would have been very difficult, if not impossible, to achieve it for other $\beta ^+$-decays without an upgrade to our acquisition and data-reduction systems. We have thus improved the controls over all the key elements in our experimental set-up: we now have direct control over the dead-time for the singles and coincidence channels and $<$0.1 mm control over the source-detector distance. In addition, we have extensively studied the efficiency of the $\beta $-detector with source-measurements tested against various Monte Carlo programs [3]. We have tested our new acquisition set-up on ${ }^{60}Co$ and ${ }^{22}Na$ ($\beta ^-$ and $\beta ^+$ emitters respectively) to validate our new methods. Preliminary results on the two sources are statistically consistent with the expected values. An ${ }^{34}Ar$ decay experiment using the new experimental configuration has already been performed and is currently analyzed. [1] J.C. Hardy and I.S. Towner, PRC \textbf{71}, 055501 (2005) [2] V. Iacob \textit{et al.}, BAPS \textbf{52}(3)B16; BAPS \textbf{52}(9)HF3 [3] V. Golovko \textit{et al.}, BAPS \textbf{52}(9)DH4; this BAPS   

MuCap: From first results to final precision on determining $g_P$

The MuCap collaboration recently reported the muon capture rate from the hyperfine singlet ground state of the $\mu p$ atom to be $\Lambda_S=725\pm17.4$ s$^{-1}$. The extracted nucleon induced pseudoscalar form factor, $g_P$, is $7.3\pm 1.1$. Subsequent runs contain 10 times more data and significant improvements have been made to the experimental apparatus. The aim is to reduce the final uncertainty of $\Lambda_S$ by a factor of three. The isotopic and chemical impurities in the protium target have been greatly reduced and an electrostatic kicker has been used to increase the effective data rate threefold. The experiment was upgraded with neutron detectors and full analog recording using custom built FADC modules for several critical detectors. I will present a description of the experimental upgrades and the status of the analysis, which with new data will reduce the overall uncertainty of $g_P$ to 7\%.   

Nuclear Transparency of Kaons (K+)

Quantum Chromo Dynamics (QCD) is the fundamental theory of the strong force. The transition from nucleons and mesons to the quarks and gluons of QCD can be studied by looking for the onset of phenomena predicted by QCD, such as Color Transparency (CT). CT is the disappearance of final (initial) state interactions for hadrons produced in exclusive processes at high momentum transfers. An experiment to measure the transparency of pions, in search of CT was completed in Dec 2004 at JLab in Hall C. The same set of data also has a considerable sample of kaons that can be used to study the transparency of kaons. Kaon transparency via electro-production has not been studied before and will provide useful information regarding the nature of the transition from quarks to hadrons. In addition, this data will help us investigate the anomalous strangeness transparency reported for kaon-nucleus scattering data. We will extract the kaon transparency by comparing the electro-production of kaons from various nuclear targets to electro-production from hydrogen which is similar to the technique used to measure pion transparency. Preliminary results from this analysis will be presented.   

Preliminary results from Jefferson Lab HKS experiment

Jefferson Lab hypernuclear program aims to obtain high resolution hypernuclear spectroscopy in a wide mass region by utilizing high precision electron beam. The second experiment in the program, JLab HKS experiment, which was carried out in 2005, employed an on-target Splitter magnet to detect both scattered e' and K$^+$ at very forward angles in order to increase hypernuclear yield. The preliminary results from this experiment has demonstrated the ability of this experimental program to obtain high resolution, high statistics spectroscopy. A specially designed calibration procedure for the spectrometer system has enabled us to further improve the energy resolution of the spectra. In this talk, I will present the current updated spectra of $^{12}_\Lambda$B, $^{28}_\Lambda$Al and $^{7}_\Lambda$He. The experimental setup and spectrometer calibration procedure will also be described.   

Finite-$Q^2$ Corrections to Parity-Violating DIS

Parity-violating deep inelastic scattering (PVDIS) has been proposed as an important new tool to extract the flavor and isospin dependence of parton distributions in the nucleon. We discuss finite-$Q^2$ effects in PVDIS asymmetries arising from subleading kinematical corrections and longitudinal contributions to the $\gamma Z$ interference. For the proton, these need to be accounted for when extracting the $d/u$ ratio at large $x$. For the deuteron, the finite-$Q^2$ corrections can distort the effects of charge symmetry violation in parton distributions, or signals for physics beyond the standard model. We further explore the dependence of PVDIS asymmetries for polarized targets on the $u$ and $d$ helicity distributions at large $x$.