Bulletin of the American Physical Society
4th Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 59, Number 10
Tuesday–Saturday, October 7–11, 2014; Waikoloa, Hawaii
Session EE: Mini-Symposium on Fundamental Symmetries (Electroweak) |
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Chair: Paul E. Reimer, Argonne National Laboratory Room: King's 1 |
Thursday, October 9, 2014 7:00PM - 7:15PM |
EE.00001: Parity-Violating and Parity-Conserving Asymmetries in $\vec{e}p$, $\vec{e}e$, and $\vec{e}N$ Scattering Damon Spayde The primary goal of the Qweak experiment at Jefferson Lab is to measure the parity-violating asymmetry $A_{PV}$ in elastic scattering of longitudinally polarized electrons from unpolarized protons at a $Q^2$ of 0.025 (GeV/c)$^2$. The proton's weak charge and the electroweak mixing angle can be extracted from $A_{PV}$. An intense (180 $\mu$A) beam of highly polarized (85\%) electrons was made incident on a 35 cm long liquid hydrogen target. A toroidal spectrometer magnet focused scattered electrons onto an azimuthally symmetric array of eight quartz Cerenkov bars. $A_{PV}$ can be determined from detector yields recorded by integrating electronics as the beam polarization was flipped. Experimental systematics were constrained via a series of additional parity-violating and parity-conserving asymmetry measurements performed with different kinematics (elastic and $N \rightarrow \Delta$), electron polarization (longitudinal and transverse), and targets (protons, electrons, aluminum, and carbon). These ancillary results contain interesting physics of their own and in many cases constitute first or highest-precision-to-date measurements. This talk will offer an overview of the various ancillary measurements, the underlying physics, and the expected precision of the final results. [Preview Abstract] |
Thursday, October 9, 2014 7:15PM - 7:30PM |
EE.00002: Analysis of the Kinematics in the Qweak Experiment Valerie Gray The Qweak experiment at Jefferson Lab aims to determine the weak charge of the proton to a precision of 4\% by parity-violating elastic electron scattering on protons in a liquid hydrogen target. After two years of data-taking, the first results from the experiment's commissioning period have been published. The weak charge of the proton is directly related to the measured asymmetry, which is proportional to the squared four-momentum transfer $Q^2$ from the incoming electron to the struck proton. The uncertainty in $Q^2$ contributes directly to the precision of the measurement of the weak charge. We used two independent sets of drift chambers to reconstruct the electron trajectory through the experiment. Horizontal drift chambers are located just after the target while vertical drift chambers are located after a magnetic field just before the final Cerenkov detectors. Monte Carlo simulation is required to deduce the scattering vertex kinematics from the observed scattered energy and momentum. A Geant4 Monte Carlo simulation of the Qweak experiment was used to determine the momentum transfer and its uncertainty. I will discuss the sources that contribute to the uncertainty in the value of the momentum transfer, and the progress towards our goal of a 0.5\% precision on $Q^2$. [Preview Abstract] |
Thursday, October 9, 2014 7:30PM - 7:45PM |
EE.00003: Parity-Violating Inelastic $\vec{e}p$ Asymmetry at 3.35 GeV James Dowd The recently completed Qweak Experiment at Jefferson Lab will make the first direct measurement of the weak charge of the proton, $Q_W^p$, via a measurement of the parity-violating asymmetry in elastic electron-proton scattering with low four-momentum transfer. To reach the precision goal of Qweak, energy dependent radiative corrections in the parity-violating asymmetry must be accounted for. The most significant of these is the $\gamma Z$ box diagram. The asymmetry arising from this diagram depends on the $\gamma Z$ interference structure functions, $F_{1,3}^{\gamma Z}$, for which there is almost no experimental data. Using the Qweak apparatus, with modifications, an ancillary measurement was taken at a higher beam energy of 3.35 GeV. The chosen kinematics allows access to inelastic scattering, where the asymmetry depends on these structure functions, allowing tests of their theoretical description. A lead wall in front of one of the eight Cerenkov detectors was added to isolate the pion background in the asymmetry measurement. Pion contamination is the largest uncertainty for this measurement. Analysis of these data will provide additional validation of the theoretical models used to predict the $\gamma Z$ box contribution to the proton's weak charge. [Preview Abstract] |
Thursday, October 9, 2014 7:45PM - 8:00PM |
EE.00004: The Measurement of a Lepton-Lepton Electroweak Reaction (MOLLER) Experiment Juliette Mammei The MOLLER experiment will measure the parity-violating asymmetry $A_{PV}$ in polarized electron-electron (M$\o$ller) scattering which arises due to the interference between the Standard Model electromagnetic and weak neutral current amplitudes. The experiment will run in Hall A of Jefferson Lab. The 11 GeV polarized electron beam with a current of 75 $\mu$A will be incident on a 1.5 m liquid hydrogen target. A two-toroid spectrometer system will focus scattered electrons (5 $< \theta_{lab} <$ 19 mrads) onto an array of 224 quartz Cherenkov detectors 28 m downstream of the target center. $A_{PV}$ at our kinematics ($Q^2$ = 0.0056 GeV$^2$) is predicted to be $\approx$ 35 parts per billion (ppb) and the statistical uncertainty of the measurement will be 0.7 ppb, resulting in a measurement of the weak charge of the electron of 2.4\% and a precision of $\pm$0.00024(stat)$\pm$0.00013(sys) of the weak mixing angle. This precision matches that of the single best determinations from high energy colliders, and is sensitive to physics beyond the Standard Model, such as multi-TeV-scale vector bosons, supersymmetry and light dark bosons among others. A summary of recent progress in the design of the apparatus and related R\&D efforts will be presented. [Preview Abstract] |
Thursday, October 9, 2014 8:00PM - 8:15PM |
EE.00005: High Luminosity Integrating Detector Development for the MOLLER Experiment at Jefferson Laboratory Michael Gericke The MOLLER collaboration is currently preparing an experiment to measure the Weak charge of the electron to a fractional accuracy of 2.3\% at very low momentum transfer, using parity violating electron scattering at $11~GeV$. At this precision, the experiment will be sensitive to new physics with a mass reach of $19~TeV$. The experiment will measure the asymmetry in the number of scattered electrons from a liquid hydrogen target, as a function of electron helicity. The asymmetry has a Standard Model predicted size of $35~ppb$ (part per billion). The measurement requires a high luminosity beam, leading to detector event rates at the level of $GHz/cm^2$. This requires either very high detector segmentation or current mode operation. The challenges we face regarding detector design include high radiation hardness, low noise and high efficiency operation, and low background sensitivity. We are currently exploring highly segmented quartz Cherenkov detectors for current mode operation. I will provide an overview of the current detector design and results from initial prototype tests performed at the MAMI facility in Mainz, Germany. This work is done in conjunction with the detector development work for the P2 experiment planned at MAMI. [Preview Abstract] |
Thursday, October 9, 2014 8:15PM - 8:30PM |
EE.00006: First Determination of the Proton's Weak Charge from the Qweak Experiment Scott MacEwan The Qweak experiment at Jefferson Lab uses parity-violating electron scattering (PVES) to make a precision measurement of the proton's weak charge $Q^p_W$. The experiment has recently reported a measurement of the asymmetry in elastic $\vec{e}-p$ scattering at low $Q^2$ = 0.0250 (GeV/c)$^2$ with a beam energy of 1.16 GeV based on approximately 1/25 of the overall data collected in the experiment (D. Androic, et al. [Qweak Collaboration], Phys. Rev. Lett. 111, 141803 (2013)). Several technical challenges were overcome to successfully measure the small asymmetry requiring a high power liquid hydrogen target, radiation hard Cerenkov detectors, and precision electron beam polarimetry. The small $Q^2$ of the measurement has made possible the first determination of the weak charge of the proton, $Q^p_W$, by incorporating earlier PVES data at higher $Q^2$ to obtain hadronic corrections. The value of $Q^p_W$ obtained this way is $Q^p_W=0.064\pm0.012$. An overview of the experimental apparatus and technical challenges will be presented alongside the details of the analysis required to extract $Q^p_W$ and its error from the measured asymmetry. [Preview Abstract] |
Thursday, October 9, 2014 8:30PM - 8:45PM |
EE.00007: Implications of the Q$_{\rm weak}$ Commissioning Result Greg Smith The commissioning results of the Q$_{\rm weak}$ experiment at Jefferson Lab, which constituted approximately 4\% of the total results obtained in that experiment, were recently published (D. Androic, {\em et al.} [Q$_{\rm weak}$ Collaboration], Phys. Rev. Lett. 111, 141803 (2013)). After a brief review of the experiment, new, unpublished results derived from that publication will be presented. The sensitivity of the fit used to extract the proton's weak charge to the choice of electromagnetic form factors, to the proton radius puzzle, and to the dipole mass used for the Q$^2$ evolution will be examined. The running of $\sin^2(\theta_W)$ and the experiment's mass reach will be discussed. The status of the ongoing effort to complete the analysis of the full experiment will also be shown. [Preview Abstract] |
Thursday, October 9, 2014 8:45PM - 9:00PM |
EE.00008: Optimizing the Kinematics, Backgrounds and Technology for a Next Generation Q$^{\mathrm{p}}$weak Measurement Roger Carlini The recent Q$^{\mathrm{p}}_{\mathrm{weak}}$ measurement at JLab was optimized to deliver the highest possible precision within the constraints of the available beam time, energies, polarized beam current, target technology, beam quality and kinematic requirements for the suppression of theoretical uncertainties. Applying what we have learned towards maximizing the figure-of-merit of a possible more precise next generation measurement suggests focusing on decreasing the beam energy (and Q$^{2}$), but only enough to sufficiently suppress theoretical correction uncertainties, while still keeping the scattering asymmetry as high as possible. This insures that the running time remains reasonable and demands on helicity correlated beam properties stay within practical limits. Of equal importance is to remain sufficiently high in electron beam energy (400 MeV to 600 MeV) that proven technologies can be employed for the most critical ancillary measurements - such as precision laser backscattering beam polarimetry. The above conditions seem at least technically feasible if given the availability of a lower energy electron beam, sufficient running time and polarized beam current while still allowing the use of much of the previous generation Qweak instrumentation and methodology. [Preview Abstract] |
Thursday, October 9, 2014 9:00PM - 9:15PM |
EE.00009: Hadronic parity violation in three-nucleon systems Matthias Schindler Parity-violating interactions between nucleons are the manifestation of an interplay of strong and weak interactions between quarks in the nucleons. Because of the short range of the weak interactions, these parity-violating forces provide a unique probe of low-energy strong interactions. Theoretical calculations in three-nucleon systems based on effective field theory methods will be presented. In addition to a number of parity-violating observables in nucleon-deuteron scattering, I will also discuss the role that possible parity-violating three-nucleon interactions may play, which would severely complicate a consistent theoretical understanding of hadronic parity violation in few-nucleon systems. [Preview Abstract] |
Thursday, October 9, 2014 9:15PM - 9:30PM |
EE.00010: Muon $g-2$ at Fermilab: Challenging the Standard Model and searching for New Physics Brendan Kiburg The Muon $g-2$ experiment at Fermilab will measure the muon's anomalous magnetic moment, $a_{\mu}$, to 140 parts-per-billion. Modern electroweak and QCD calculations for $a_{\mu}$ differ from the Brookhaven E821 experimental result by 3.6 $\sigma$. To test this discrepancy the Muon $g-2$ experiment will implement several upgrades to the E821 approach and collect 20 times as many muons. This talk will outline possible sources that could be responsible for this hint of new physics. A brief overview of the experimental status will be described. The upgrades associated with the ongoing storage ring magnet reassembly and the production and measurement of the highly uniform magnetic field will be highlighted. [Preview Abstract] |
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