Session KJ: Electroweak Interactions

The TRIUMF TWIST experiment

The TWIST experiment at TRIUMF has been conducted by a Canadian-USA-Russia collaboration which has measured the parameters describing the decay at rest of polarized muons, with the goal of placing constraints on physics beyond the Standard Model. The original objective of an order of magnitude improvement on the precision with which the parameters rho, delta and Xsi,-which describe the positron momentum and emission angle spectra,-are determined is within reach and final analysis of the systematic uncertainties on the new measurements are almost complete. I will present an update on this important high precision experiment and place it in the context of several recent other muon decay measurements.   

Status of the MiniCLEAN dark matter experiment

MiniCLEAN utilizes over 400 kg of liquid cryogen to detect nuclear recoils from WIMP dark matter with a projected sensitivity of 2$\times 10^{-45}$ cm$^{2}$ for a mass of 100 GeV. The liquid cryogen is interchangeable between argon and neon to study the A$^{2}$ dependence of the potential signal and examine backgrounds. MiniCLEAN utilizes a unique modular design with spherical geometry to maximize the light yield using cold photomultiplier tubes in a single-phase detector. Pulse shape discrimination techniques are used to separate nuclear recoil signals from electron recoil backgrounds. Particular attention is being paid to mitigating the backgrounds from contamination of surfaces by radon daughters during assembly. The design and assembly status of the experiment will be discussed. The projected timeline and future plans for staging the experiment at SNOLAB in Sudbury, Canada will be presented.   

Optical Tests in support of the MiniCLEAN Dark Matter Search

The MiniCLEAN experiment will search for WIMP dark matter with a WIMP-nucleon cross section sensitivity of $2 \times 10^{-45}\mbox{cm}^{2}$. The detector has a fiducial volume of over 150 kg of liquid argon with the capability to be changed to liquid neon for background studies and R\&D for a future detector. The MiniCLEAN experiment will be located at SNOLAB in Sudbury, Canada in early 2010. MiniCLEAN will use position reconstruction and the time structure of scintillation light pulses to distinguish signals from backgrounds on an event-by-event basis. The DEAP/CLEAN collaboration has undertaken a suite of R\&D projects to characterize the optical train of the experiment: from creation of Extreme Ultraviolet (EUV) scintillation light in the liquid cryogen, to the down-scatter of of EUV to visible light by wavelength shifting films, and the collection visible light by photomultiplier tubes operating at low-temperatures. We present these efforts in the context of previous measurements as well as outline our current experimental program and its future direction in support of MiniCLEAN.   

Measurement of Q-Weak Detector Sensitivities

The Q-Weak experiment at Jefferson Laboratory will provide a 4{\%} measurement of the proton's weak charge (Q$^{p}_{w})$ using parity-violating electron scattering from a liquid Hydrogen target. The scattering rates into the detectors depend significantly on five electron beam parameters at the target: transverse position x and y, angle x' and y', and incident energy, E. Small helicity-correlated variations in these parameters produce false asymmetries which are enhanced by various broken symmetries in the apparatus. While great care is being taken to suppress or eliminate helicity-correlated changes in beam parameters at the polarized source, we will measure the detector sensitivities (dA$_{f}$/dx$_{i}$ (i=1..5)) for first order offline correction of beam false asymmetries. To directly measure the detector sensitivities, we will modulate the beam in (x,x',y,y') using pairs of electromagnets, and in E using an SRF cavity. We estimate a 10{\%} measurement of the 5 beam sensitivities will be possible each day using only 1{\%} of our total beam time. I will discuss simulations of predicted detector sensitivities, as well as our group's work on the design and implementation of a robust beam modulation system.   

The Qweak \v{C}erenkov Detector

The Qweak experiment at Jefferson Laboratory will make a determination of the proton's weak charge $\mathrm{Q_W^P = 1-4 \sin^2\theta_W}$ with approximately 4$\%$ combined statistical and systematic errors. This will enable us to extract the weak mixing angle $\mathrm{\sin^2\theta_W}$ at $\mathrm{Q^2 = 0.03 (GeV/c)^2}$ to approximately 0.3$\%$ testing the Standard Model prediction. The key apparatus includes a liquid hydrogen target, a toroidal magnetic spectrometer and a set of eight \v{C}erenkov detectors. The proton's weak charge is determined by measuring the parity violating e-p scattering asymmetry $\mathrm{A_{PV}}$. The proposed experimental precision and statistical uncertainty demand a high performance \v{C}erenkov detector system working in integration mode. These \v{C}erenkov detectors are made of fused silica, allowing us to handle the high rate of 800 MHz per detector for a running time of 2500 hours without significant radiation damage. In this talk, I shall introduce the Monte Carlo simulation, the design, the construction of this detector system, as well as detector performance tests.   

Track Reconstruction and Extrapolation In Qweak Experiment

The Qweak experiment at Jefferson Laboratory is designed to precisely determine the proton's weak charge ($\mathrm{Q^p_W}$) and thus the weak mixing angle ($\mathrm{\sin^2\theta_W}$) by measuring the parity violating asymmetry in elastic electron-proton scattering at low momentum transfer $\mathrm{Q^2 = 0.03\ (GeV/c)^2}$. The Qweak experimental result will enable a precise test of the firm Standard Model prediction of $\mathrm{Q^p_W}$, and hence will probe new physics. The experiment will be operated in a high-current (integration) mode for the parity measurement, and in a low-current (counting) mode for Q$^2$ determination. To reach the proposed experimental precision, the average Q$^2$ needs to be determined to 0.7$\%$ requiring individual tracks to be reconstructed with high efficiency. A set of high resolution tracking detectors were designed for this purpose. The hit information for each detector will then be fed into the tracking software for reconstructing the trajectories and extracting the Q$^2$. The tracking detectors however, are only operable in low-current mode. Therefore a tracking \v{C}erenkov detector, the ``focal-plane scanner,'' was designed for further extrapolating the Q$^2$ from low current to high current. A brief introduction on the Qweak tracking mechanism will be presented, along with some detailed track reconstruction strategies and the Q$^2$ extrapolation method.   

Systematic Studies with the Qweak Tracking System

Qweak is an upcoming experiment at the Thomas Jefferson National Accelerator Lab that will use parity-violating elastic electron-proton scattering to measure the weak charge of the proton (Q$^{P}_{weak}$). This experiment will be a sensitive test for physics beyond the standard model, as Q$^{P}_{weak}$ is well predicted in the Standard Model. Longitudinally polarized electrons will scatter off a liquid hydrogen target and pass through a toroidal-field magnetic spectrometer. In order to perform a 4\% measurement of Q$^{P}_{weak}$, we will need to measure the momentum transfer (Q$^{2}$) to 0.5\%. The Q$^{2}$ will be measured using a tracking system consisting of two gas electron multipliers (GEM), four horizontal drift chambers (HDC), and four vertical drift chambers (VDC). In this talk I will outline the design and status of each tracking device and discuss the details of the Q$^{2}$ measurement, as well as several systematic studies that will be performed with this tracking system.   

Target Density Fluctuation Studies for the Qweak Experiment

The $Q_{weak}$ experiment will measure the proton's neutral weak charge using parity-violating electron scattering. Target density fluctuations are an important issue for liquid hydrogen targets in parity-violating electron scattering experiments because they contribute to the statistical error. A proposed technique for reducing their importance relative to counting statistics is to increase the data-taking frequency. To determine how this technique may benefit the $Q_{weak}$ experiment, a study was done using a standard Jefferson Lab Hall C liquid hydrogen target. The purpose of the study was to determine how target density fluctuations depend on data-taking frequency. \v Cerenkov detectors mounted at small scattering angles were used to detect scattered electrons from carbon and liquid hydrogen targets at 30, 250, and 1000 Hz data-taking frequencies for beam currents in the 10 - 80 $\mu$A range. The data on the carbon target was used to understand sources of random noise other than target density fluctuations. The study resulted in an empirical determination of the dependence of the target density fluctuation amplitude on data-taking frequency.   

Ultra-High Precision Half-Life Measurement for the Superallowed $\beta^+$ Emitter $^{26}$Al$^m$

The calculated nuclear structure dependent correction for $^{26}$Al$^m$ ($\delta_{C}-\delta_{NS} = 0.305(27)\%$ [1]) is smaller by nearly a factor of two than the other twelve precision superallowed cases, making it an ideal case to pursue a reduction in the experimental errors contributing to the $\mathcal{F}${\em t} value. An ultra-high precision half-life measurement for the superallowed $\beta^+$ emitter $^{26}$Al$^m$ has been made at the Isotope Separator and Accelerator (ISAC) facility at TRIUMF in Vancouver, Canada. A beam of $\sim 10^5$ $^{26}$Al$^m$/s was delivered in October 2007 and its decay was observed using a 4$\pi$ continuous gas flow proportional counter as part of an ongoing experimental program in superallowed Fermi $\beta$ decay studies. With a statistical precision of $\sim0.008\%$, the present work represents the single most precise measurement of any superallowed half-life to date. \\[4pt] [1] I.S.~Towner and J.C.~Hardy, Phys. Rev. C {\bf 79}, 055502 (2009).   

High Precision Measurement of the $^{19}$Ne Lifetime

Recently, a rigorous review of the T=$\frac{1}{2}$ mirror transitions has identified several systems which can contribute to high precision tests exploring deviations from the Standard Model's description of the electroweak interaction. Arguably, one of the best candidates is the $\beta^+$ decay of $^{19}$Ne to $^{19}$F. In this system, the main contribution to the uncertainty of extracted Standard Model parameters is due to the measured value of the lifetime of the decay. In March 2009, a high precision measurement of the lifetime of $^{19}$Ne was made by a collaboration between the Triangle Universities Nuclear Laboratory (TUNL) and the Kernfysisch Versneller Instituut (KVI) at the Trapped Radioactive Isotopes: Microlaboratories for Fundamental Physics (Tri$\mu$p) facility. An overview of the experiment and preliminary results will be presented.   

Confirmation of the Precise Half Life of $^{26}$Si

Precise \textit{ft}-values (with uncertainties below 0.1{\%}) for superallowed $0^+\to 0^+ \quad \beta $ transitions provide a demanding test of the Standard Model \textit{via} the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. Our preliminary report of such a measurement for the half-life of $^{26}$Si [1], was consistent with the previously accepted value but turned out to be higher than a subsequent result published in 2008 [2]. This prompted us to repeat the measurement described in [1] with increased statistics and with a strong focus on all experimental details that could have generated a biased result. We collected more than 200 million $^{26}$Si nuclei in 60 separate runs, which differed from one another in their discriminator threshold, detector bias or dominant dead-time setting. We repeatedly verified and confirmed the stability of the source purity and detector response function. The analysis of these separate runs shows no systematic bias with these parameters and confirms our initial result [1]. The discrepancy between our result and that of reference [2] can be accounted for by the latter's neglect [3] of the difference in beta-detection efficiencies between the parent and daughter decays. Our preliminary result is 2245(3) ms, with the final analysis expected to yield an uncertainty of 0.05{\%} or better. [1] V. Iacob \textit{et al.}, Bulletin APS 53, (12) DNP-Meeting 2008 [2] I. Matea \textit{et al.}, EPJ A37, 151 (2008) [3] B. Blank, private communication   

Upgrade of a Right-Handed Current Search Using Highly-Polarized, Laser-Cooled \textsuperscript{37}K

The TRIUMF Neutral Atom Trap facility is able to provide $^{37}$K that is laser-cooled and highly polarized. This will be used to search for new physics via studies of the angular distributions of its decay products. The first planned experiment will measure the $\beta$ asymmetry parameter, $A_{\beta}$, using back-to-back detectors placed along the polarization axis. One key aspect of the upgraded system is the addition of a shake-off electron detector which, used in coincidence with the $\beta$ detectors, will minimize the background from untrapped depolarized atoms. The other major improvement is the conversion of our traditional magneto-optical trap (MOT) to an AC-MOT~[1] to allow us to switch between trapping and polarizing much more quickly. Our goal is to measure $A_{\beta}$ to $\la 1.0\%$ of its value and to estimate systematics as we work towards a $\approx0.1\%$ experiment. An overview of the experiment and our expected sensitivity to physics beyond the Standard Model based on GEANT4 simulations will be presented. \\[4pt] [1] M. Harvey and A. Murray PRL \textbf{101}, 173201 (2008).