Bulletin of the American Physical Society
2006 73rd Annual Meeting of the Southeastern Section of the APS
Thursday–Saturday, November 9–11, 2006; Williamsburg, Virginia
Session DC: Nuclear Physics II |
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Chair: Mark Pitt, Virginia Tech Room: Williamsburg Hospitality House Yorktown |
Thursday, November 9, 2006 2:00PM - 2:12PM |
DC.00001: Measuring $G_{E}^n$ at High Momentum Transfer Robert Feuerbach A precision measurement of the electric form-factor of the neutron, $G_E^n$, at $Q^2$ up to 3.5 $\mathrm{GeV}^2$ was recently completed in Hall A at the Thomas Jefferson National Accelerator Facility(Jefferson Lab). The ratio of the electric to magnetic form-factors of the neutron, $G_E^n/G_M^n$, was measured through the beam-target asymmetry $A_\perp$ of electrons quasi-elastically scattered off neutrons in the reaction ${}^{3}\overrightarrow{He}(\overrightarrow{e},e' n)$. The experiment took advantage of recent developments of the electron beam and target, as well as two detectors new to Jefferson Lab. The measurement used the accelerator's 100\% duty-cycle high-polarization (typically 84\%) electron beam and a new, hybrid optically-pumped polarized ${}^{3}\overrightarrow{He}$ target which achieved in-beam polarizations in excess of 50\%. A medium acceptance (80msr) open-geometry magnetic spectrometer (BigBite) detected the scattered electron, while a newly contructed neutron detector observed the released neutron. An overview of the experiment and the experimental motivation will be discussed, in particular the large range of predictions from modern calculations for $G_E^n$ at this relatively high $Q^2$. Finally, the analysis progress and preliminary results will be presented. [Preview Abstract] |
Thursday, November 9, 2006 2:12PM - 2:24PM |
DC.00002: Measuring the proton's electromagnetic form factors to high Q$^{2}$ via recoil polarimetry in Hall C at Jefferson Lab Andrew Puckett The electromagnetic form factors of the nucleon, as measured in elastic electron-nucleon scattering, are of crucial interest to a wide variety of areas of current research in both theoretical and experimental nuclear and subnuclear physics. In addition to being an important component of an understanding of nucleon structure in terms of QCD (e.g. the connection between elastic form factors and Generalized Parton Distributions), empirical knowledge of the elastic form factors is an important input for the interpretation of many other experiments, particularly quasi-elastic electron and neutrino scattering. Experiment E04-108 in Jefferson Lab's Hall C will measure the ratio of the electric and magnetic form factors of the proton to high Q$^{2}$ by measuring the components of the transferred polarization in the $^{1}H(\vec{e},e'\vec{p})$ reaction, and is \emph{tentatively} scheduled to take data in the latter part of 2007. This paper will discuss the current status of preparations for the experiment, in particular the new equipment necessary to perform this measurement in Hall C. BigCal, a large solid-angle electromagnetic calorimeter consisting of 1744 lead-glass blocks, will detect the scattered electron. The Focal Plane Polarimeter for the Hall C High Momentum Spectrometer, consisting of a series of two blocks of CH$_{2}$ analyzer, each followed by a gas drift chamber, will measure the polarization of the scattered proton. [Preview Abstract] |
Thursday, November 9, 2006 2:24PM - 2:36PM |
DC.00003: Qweak: A Precision Measurement of the Proton's Weak Charge Klaus Grimm The $Q_{weak}$ experiment at Jefferson Lab aims to make a 4\% measurement of the parity-violating asymmetry in elastic scattering at very low $Q^2$ of a longitudinally polarized electron beam on a proton target. The experiment will measure the weak charge of the proton, and thus the weak mixing angle at a low energy scale, providing a precision test of the Standard Model. Because the value of the weak mixing angle is approximately 1/4, the weak charge of the proton $Q^p_w=1-4\sin^2\theta_W$ is suppressed in the Standard Model, making it especially sensitive to the value of the mixing angle and also to possible new physics. The experiment will be a 2200 hour measurement, employing: an 80\% polarized, 180\,$\mu$A, 1.2\,GeV electron beam; a 35\,cm liquid hydrogen target; and a toroidal magnet to focus electrons scattered at $8^\circ \pm 2^\circ$, a small forward angle corresponding to $Q^2 = 0.03~{\rm (GeV/c)^2}$. With these kinematics the systematic uncertainties from hadronic processes are strongly suppressed. To obtain the necessary statistics the experiment must run at an event rate of over 6 GHz. This requires current mode detection of the scattered electrons, which will be achieved with synthetic quartz \v{C}erenkov detectors. A tracking system will be used in a low-rate counting mode to determine the average $Q^2$ and the dilution factor of background events. The theoretical context of the experiment and the status of its design are discussed. [Preview Abstract] |
Thursday, November 9, 2006 2:36PM - 2:48PM |
DC.00004: Reducing Backgrounds in a High Precision Measurement of the Proton's Weak Charge at Jefferson Lab K.E. Myers The \mbox{$Q_{weak}$} collaboration at Jefferson Lab is developing an experiment which will precisely measure the parity violating asymmetry in elastic electron-proton scattering. This asymmetry is proportional to the proton's weak charge -- the basic property which determines the proton's response to the weak interaction. The Standard Model makes a firm prediction of the proton's weak charge, $Q_w^p=1-4\sin^{2}\theta_{W}$, based on how the weak mixing angle $\sin^{2}\theta_{W}$ evolves from the $Z^{0}$ pole down to low energy scales. The ultimate goal of the experiment is to determine \mbox{$Q_w^p$} with 4\% combined statistical and systematic uncertainties, which in turn leads to a 0.3\% measurement of $\sin^{2}\theta_{W}$. Achieving this degree of precision requires an in-depth study of the backgrounds generated and methods to reduce them. Using a GEANT3 based simulation we have identified the acceptance defining collimator, the apertures of the shielding hut, and the beamline as sources of different types of backgrounds. Quantitative estimates of these backgrounds will be presented. The possible effects of these backgrounds on the goals of the experiment and methods to reduce them without producing new backgrounds will also be discussed. [Preview Abstract] |
Thursday, November 9, 2006 2:48PM - 3:00PM |
DC.00005: Measurement of $Q^{2}$ for the $Q_{weak}$ Experiment Juliette Mammei The $Q_{weak}$ collaboration proposes to make a precise measurement of the parity violating elastic electron-proton scattering asymmetry at a $Q^{2}$ of 0.03 $\rm (GeV/c)^2$. A 2200 hour measurement with a 1.165 GeV, 85$\%$ polarized, 180$\ \mu$A beam on a 35 cm LH$_{2}$ target will determine the proton's weak charge with $\simeq$ 4$\%$ error. In the absence of physics beyond the Standard Model, this will provide a $\simeq$ 0.3$\%$ measurement of sin$^{2}\theta_w$, the weak mixing angle. In order to achieve such high precision, $\langle Q^{2} \rangle$ must be determined to $\pm$0.5$\%$. A precision collimator will define the $Q^{2}$ acceptance of the experiment. A dedicated tracking system will determine the difference between the actual $Q^{2}$ acceptance of the experiment, which includes energy loss and radiative effects, and the simple geometrical acceptance as defined by the collimator. The tracking system will operate at reduced beam current to determine, on an event-by-event basis, the scattered electron angle and energy, interaction vertex, and the angle and position of the electron at the entrance to the main detector. This will allow the determination of $\langle Q^{2} \rangle$ and the efficiency map of the main {\v C}erenkov detector. The tracking system will also determine the ``dilution factor'', the contribution of non-elastic events from the target and backgrounds in the hall. This talk will present an overview of the collimator and tracking system design. [Preview Abstract] |
Thursday, November 9, 2006 3:00PM - 3:12PM |
DC.00006: Results on the Spin Structure of 3He and the Neutron at Low Q$^2$ Timothy Holmstrom For the past few decades there has been a strong interest in understanding the nucleon spin structure. Sum rules, including the Gerasimov-Drell-Hearn (GDH), and moments of the nucleon spin structure functions are powerful tools for understanding nucleon structure. The goal of Jefferson Lab experiment E97-110 is to perform a precise measurement of the Q$^2$ dependence of the generalized GDH integral and the moments of the 3He and neutron spin structure functions between 0.02 and 0.3 (GeV/c)$^2$ using the Hall A polarized $^3$He target. This Q$^2$ range will allow us to test the dynamics of Chiral Perturbation Theory, and test the GDH sum rule by extrapolating to the real photon point for $^3$He and the neutron. The measurement will also contribute to the understanding of nucleon resonances. The status of the data analysis will be discussed and some preliminary results will be shown. [Preview Abstract] |
Thursday, November 9, 2006 3:12PM - 3:24PM |
DC.00007: Measurement of Single Target-Spin Asymmetry in Semi-Inclusive Pion Electroproduction on a Transversely Polarized $^3$He Target Xiaofeng Zhu The study of transverse spin distributions and transverse spin phenomena is at the frontier of recent research activities to understand the nucleon spin structure and QCD. We plan to measure the target single spin asymmetry in the semi-inclusive deep-inelastic $\vec n (e,e'\pi^{-})X$ and $\vec n (e,e'\pi^{+})X$ reaction with a transversely polarized $^3$He target as an effective polarized neutron target. The transverse single spin asymmetry on the ``neutron'' as a function of the Collins angle and Sivers angle will be studied in the $x$ range of 0.13 to 0.41, providing a separation between the two competing mechanisms: the chiral-even Sivers effect and the chiral-odd Collins effect, a crucial step towards extracting the quark transversity distribution. It will be the first experiment on ``neutron'' transversity, complementary to the HERMES measurement on proton and the COMPASS measurement on deuteron, providing constraints on the transversity distributions and Sivers functions for both u-quark and d-quark in the valence quark region. The tentative schedule for this experiment is fall of 2007. [Preview Abstract] |
Thursday, November 9, 2006 3:24PM - 3:36PM |
DC.00008: Polarized Electron Beams for the Jefferson Lab Nuclear Physics Program Joseph Grames Almost eighty percent of the present physics program at Jefferson Lab requires polarized electron beams to probe nuclear structure. The accelerator can provide beam to three experimental halls simultaneously from the same 100 kV DC electron gun and GaAs photocathode. Multiple hall operation is a key design feature of the lab that maximizes the physics output. However, multiple hall operation also imposes restrictions on users. For example, only specific beam energies can transfer the full component of longitudinal polarization from the source, as high at 85{\%}, to multiple halls simultaneously. This talk describes the details of polarized beam delivery to experimental halls and the factors that affect beam quality, particularly those factors relevant for conducting parity violation experiments. These details will be described in context of the progress of GaAs polarized electron sources. In addition, the state-of-the-art and future prospects for higher current and beam polarization will be discussed. [Preview Abstract] |
Thursday, November 9, 2006 3:36PM - 3:48PM |
DC.00009: Spin-Polarizing $^{3}$He at 8 atm with a frequency narrowed diode laser* C.W. Arnold, T.V. Daniels, A.H. Couture, T.B. Clegg In support of measurements of spin-correlation parameters in low-energy p + $^{3}$He elastic scattering, we have been working to improve our polarizer systems [1]. Polarized $^{3}$He gas is commonly produced by spin-exchange with optically pumped Rb vapor. It has been shown [2], [3] that a frequency narrowed diode laser array is a relatively inexpensive way to improve polarization. Thus, we have developed such a system which employs a nominal 50 W Quintessence 25-diode laser array. In non-narrowed mode, it operates at 790 to 795 nm with a linewidth of $\sim $2nm. We have seen up to 32 W output when this array is placed in an external optical cavity with a Littrow mounted grating to narrow the linewidth to $\sim $0.3 nm. Using this system, we have achieved over 30{\%} polarization in a degaussed Pyrex cell filled to 8 ATM with a 60:1 ratio of $^{3}$He to N$_{2}$. This represents a 20{\%} improvement over values obtained with our former non-narrowed 60W Optopower laser system. The improvement is thought to arise from an increase spectral power placed into the Rb D1 absorption line, and a decrease in the amount of unwanted pumping on the D2 line. Spectra and operating conditions will be discussed. *Work supported by US DOE Grant No. DE-FG02-97ER41041 [1] T. Katabuchi et al, Rev. Sci. Instrum. 76, 033503, (2005) [2] I.A. Nelson et al, Appl Phys Lett., Vol 76 No. 11, 1356 (2000) [3] B.Chann et al, J. Appl. Phys. Vol 94, No. 10, 6980 (2003) [Preview Abstract] |
Thursday, November 9, 2006 3:48PM - 4:00PM |
DC.00010: Hybrid K-Rb Spin Exchange Optical Pumping Cells for the Polarization of $^{3}$He Alex Couture, Tim Daniels, Charles Arnold, Tom Clegg We are transitioning from polarizing $^{3}$He using optical pumping cells charged with pure Rb to using a mixture of Rb and K, lean in Rb. The reason for this is the spin exchange efficiency between K and $^{3}$He is an order of magnitude greater than that of Rb and $^{3}$He. Also the spin exchange cross section between Rb and K is very large, which leads to a very fast rate of polarization transfer from Rb to K. Thus by optically pumping using a standard 795 nm Rb laser on a hybrid K-Rb cell, we can obtain significant improvements in spin-up time as well as improvements in overall polarization.[1] We produce hybrid pumping cells at TUNL using a filling station consisting of an oven and a turbo pumping station to bake out and pump away any impurities in the cells. The alkali metals are introduced into the pumping cells from a Y-shaped manifold with a separate retort for each alkali. We are able to determine the ratio of K to Rb in the vapor using white light absorption spectroscopy. Light from a halogen light bulb is incident upon the heated cell and enters a spectrometer beyond. We examine the relative sizes of the D1 and D2 absorption lines for the two alkali metals. We will have data comparing hybrid cells to pure Rb cells, GE-180 cells to Pyrex, and are working to obtain comparative performance data for spectrally unnarrowed and narrowed lasers. Our latest results will be reported. [1] E. Babcock, et al. (2003) Phys. Rev. Letter Vol. 91, Num.12, 123003 [Preview Abstract] |
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