False Asymmetries Research Articles

Probing Axial Symmetry Breaking in the Galaxy with Gaia Data Release 2

Abstract We study a set of solar neighborhood (d < 3 kpc) stars from Gaia Data Release 2 to determine azimuthal star count differences, i.e., left and right of the line from the Galactic Center (GC) through the Sun—and compare these differences north and south. In this companion paper to Gardner et al., we delineate our procedures to remove false asymmetries from sampling effects, incompleteness, and/or interloper populations, as this is crucial to tests of axisymmetry. Particularly, we have taken care to make appropriate selections of magnitude, color, in-plane Galactocentric radius, and Galactic and . We find that requiring parallax determinations of high precision induces sampling biases, so that we eschew such requirements and exclude, e.g., regions around the lines of sight to the Magellanic Clouds, along with their mirror-image lines of sight, to ensure well-matched data sets. After making conservative cuts, we demonstrate the existence of azimuthal asymmetries and find differences in those, north and south. These asymmetries give key insights into the nature and origins of the perturbations on Galactic matter, allowing us to assess the relative influence of the Magellanic Clouds (LMC and SMC), the Galactic bar, and other masses on the Galactic mass distribution, as described in Gardner et al. The asymmetry’s radial dependence reveals variations that we attribute to the Galactic bar, and it changes sign at a radius of (0.95 ± 0.03)R 0, with R 0 the Sun–GC distance, to give us the first direct assessment of the outer Lindblad resonant radius.

A cylindrically symmetric "micro-Mott" electron polarimeter.

A small, novel, cylindrically symmetric Mott electron polarimeter is described. The effective Sherman function, Seff, or analyzing power, for 20 kV Au target bias with a 1.3 keV energy loss window is 0.16 ± 0.01, where uncertainty in the measurement is due primarily to uncertainty in the incident electron polarization. For an energy loss window of 0.5 keV, Seff reaches its maximum value of 0.24 ± 0.02. The device's maximum efficiency, I/Io, defined as the detected count rate divided by the incident particle rate, is 3.7 ± 0.2 × 10(-4) at 20 keV. The figure-of-merit of the device, η, is defined as Seff (2)IIo and equals 9.0 ± 1.6 × 10(-6). Potential sources of false asymmetries due to detector electronic asymmetry and beam misalignment have been investigated. The new polarimeter's performance is compared to published results for similar compact retarding-field Mott polarimeters, and it is concluded that this device has a relatively large Seff and low efficiency. SIMION(®) electron trajectory simulations and Sherman function calculations are presented to explain the differences in performance between this device and previous designs. This design has an Seff that is insensitive to spatial beam fluctuations and, for an energy loss window >0.5 keV, negligible background due to spurious ion and X-ray production at the target.

The Qweak experimental apparatus

The Jefferson Lab Qweak experiment determined the weak charge of the proton by measuring the parity-violating elastic scattering asymmetry of longitudinally polarized electrons from an unpolarized liquid hydrogen target at small momentum transfer. A custom apparatus was designed for this experiment to meet the technical challenges presented by the smallest and most precise e→p asymmetry ever measured. Technical milestones were achieved at Jefferson Lab in target power, beam current, beam helicity reversal rate, polarimetry, detected rates, and control of helicity-correlated beam properties. The experiment employed 180μA of 89% longitudinally polarized electrons whose helicity was reversed 960 times per second. The electrons were accelerated to 1.16GeV and directed to a beamline with extensive instrumentation to measure helicity-correlated beam properties that can induce false asymmetries. Møller and Compton polarimetry were used to measure the electron beam polarization to better than 1%. The electron beam was incident on a 34.4cm liquid hydrogen target. After passing through a triple collimator system, scattered electrons between 5.8° and 11.6° were bent in the toroidal magnetic field of a resistive copper-coil magnet. The electrons inside this acceptance were focused onto eight fused silica Cherenkov detectors arrayed symmetrically around the beam axis. A total scattered electron rate of about 7GHz was incident on the detector array. The detectors were read out in integrating mode by custom-built low-noise pre-amplifiers and 18-bit sampling ADC modules. The momentum transfer Q2=0.025GeV2 was determined using dedicated low-current (~100pA) measurements with a set of drift chambers before (and a set of drift chambers and trigger scintillation counters after) the toroidal magnet.

Measurement of the weak nucleon-nucleon interaction by polarized cold neutron capture on protons

The NPDGamma Experiment at the Spallation Neutron Source at Oak Ridge National Laboratory is measuring the parity-odd correlation between the neutron spin and the direction of the emitted photon in the capture of polarized cold neutrons on protons. A parity violating asymmetry from this process is directly related to the strength of the hadronic weak interaction between nucleons. The experiment was run first with heavier nuclear targets to check systematic e ects, false asymmetries, and backgrounds. Since early 2012 the experiment has been collecting data with a 16-liter liquid parahydrogen target. Data taking will continue through 2013 until statistics for a 10 8 asymmetry mea- surement are expected. The experiment performance will be discussed as well as the status of the asymmetry measurements.

A simulation package to study the feasibility of an experiment to search for parity violation in [formula omitted

We combine a simulation program for Laser Compton Scattering (LCS) with another for deuteron photodisintegration to simulate an experiment to search for parity violation in the d(γ→,n)p reaction using a LCS source. We extract the differential cross-section for d(γ→,n)p near threshold from fits to all relevant available d(γ→,n)p experimental data. We use this information along with theoretical estimates of parity violation in this reaction to calculate neutron yields and PV asymmetries in d(γ→,n)p near threshold using the weight method. This work lays a foundation for us to study the feasibility of this experiment, the optimization of experimental conditions, and systematic effects and false asymmetries in detail.

Helicity correlated asymmetries caused by optical imperfections

False asymmetries in electron scattering experiments utilizing photoelectron sources are generated by imperfections of the light polarization optics. The false asymmetries are amplified by an analyzing power of the photocathodes. We have demonstrated compensation techniques which allow to approach zero intensity asymmetry with good stability. Even in the absence of analyzing powers fluctuations may be generated by more subtle effects like varying interference patterns of the exciting laser beam.

A luminosity monitor for the A4 parity violation experiment at MAMI

A water Cherenkov luminosity monitor system with associated electronics has been developed for the A4 parity violation experiment at MAMI. The detector system measures the luminosity of the hydrogen target hit by the MAMI electron beam and monitors the stability of the liquid hydrogen target. Both are required for the precise study of the count rate asymmetries in the scattering of longitudinally polarized electrons on unpolarized protons. Any helicity correlated fluctuation of the target density leads to false asymmetries. The performance of the luminosity monitor, investigated in about 2000 h with electron beam, and the results of its application in the A4 experiment are presented.

G0 beam quality and multiple linear regression corrections

The G0 experiment measures parity-violating elastic e-p scattering asymmetries to probe the strange quark content of the nucleon. The goal is to measure the asymmetries with an overall uncertainty of 5% of the measured asymmetries which will be of order 10−5 to 10−6. In order to achieve the above precision, systematic effects which can induce false asymmetries must be controlled. One such systematic error is helicity-correlated changes in beam parameters, which, coupled with the sensitivity of the G0 spectrometer to such beam variations, can induce false asymmetries. Beam parameters monitored for helicity-correlation are beam current, beam position, beam angle, and beam energy. A feedback loop was successfully used to reduce the helicity-correlation in beam current and beam position. The sensitivity of the G0 spectrometer to fluctuations in beam parameters has also been measured, and the false asymmetries have been determined to be of order 10−8. This contribution will address the sensitivity of the G0 spectrometer to changes in beam conditions, the performance of the feedback loops as well as the resulting parity quality of the G0 beam, and the resulting false asymmetry.

Beam optics for electron scattering parity-violation experiments

Parity-violating electron scattering experiments at intermediate energies measure asymmetries in the 10−6 – 10−5 range and therefore require stringent control of false asymmetries. One of the primary sources of such asymmetries is the combined effect of helicity-correlated changes in a certain beam property, accompanied by a change in the detector response. Careful control of the beam, including the optical properties of the acceleration and transport system, is required in order to reduce these false asymmetries to a manageable level. Developments in beam optics associated with the HAPPEX and G0 experiments at Jefferson Lab are presented.

A current mode detector array for [formula omitted]-ray asymmetry measurements

We have built a CsI(Tl) γ -ray detector array for the NPDGamma experiment to search for a small parity-violating directional asymmetry in the angular distribution of 2.2 MeV γ -rays from the capture of polarized cold neutrons by protons with a sensitivity of several ppb. The weak pion–nucleon coupling constant can be determined from this asymmetry. The small size of the asymmetry requires a high cold neutron flux, control of systematic errors at the ppb level, and the use of current mode γ -ray detection with vacuum photodiodes and low-noise solid-state preamplifiers. The average detector photoelectron yield was determined to be 1300 photoelectrons per MeV. The RMS width seen in the measurement is therefore dominated by the fluctuations in the number of γ -rays absorbed in the detector (counting statistics) rather than the intrinsic detector noise. The detectors were tested for noise performance, sensitivity to magnetic fields, pedestal stability and cosmic background. False asymmetries due to gain changes and electronic pickup in the detector system were measured to be consistent with zero to an accuracy of 10 - 9 in a few hours. We report on the design, operating criteria, and the results of measurements performed to test the detector array.

Measuring CP asymmetries in the decay of Bd0 mesons produced in proton-proton interactions

Apart from statistics, which is a major consideration in the study of CP violation, certain systematic effects occur which give rise to false asymmetries. One of these arises because of the difference in production cross section of, for example, B0 and B0 mesons, because of the unequal associated production of baryon and antibaryon containing respectively a b and b quark, despite the overall equality in production of b and b quarks. Unlike K0 decays, where the long-lived (KL) mass eigenstate is self-selecting, no certain identification of the B0(B0) CP eigenstate is possible from the single decay, and it is necessary to tag the B0 via the decay of an accompanying B particle. A second, false, asymmetry may then arise from misidentified tags, in addition to a dilution of the signal which most certainly does occur. The authors concentrate on these two systematic effects, illustrated for the very important decay Bd0 to psi KS, where the psi is identified from its decay into two leptons ( mu + mu - and e+e-), and the KS from its decay into pi + pi -.

Neutron-proton analyzing power measurements from 375 to 775 MeV.

As part of an experimental study of the nucleon-nucleon interaction at medium energy, the free neutron-proton analyzing power A„{0„, T„) has been measured at nine incident neutron energies in the range 375 ~ T„»775 MeV and for neutron c.m. angles in the range 57' ~ 0„*~ 159'. Unpolarized neutrons with a broad continuum of energies, produced by interaction of an 800 MeV proton beam with a beryllium target, were scattered from a polarized proton target. At each angle, for the whole energy region, the scattered neutron and conjugate recoil proton were detected in coincidence. A previously unseen minimum is observed in the energy dependence of A„(0*— 100') near 625 MeV. I. INTRODUCTION Understanding of the isoscalar (I=O) part of the nuclear force at medium energies requires comprehensive measurements of neutron-proton (n-p ) scattering observables, including those involving the spins and relative orbital angular momenta of the nucleons. Experimental activity in this field began more than 20 years ago, ' although it was recognized-much earlier that information on the- noncentral nature of the nuclear force must be obtained through measurements of spin dependence. Nucleon-nucleon (N S) phase--shift analyses (PSA's) combine the proton-proton (p-p ) and n-p data in such a way that the I = 1 phases are determined by the generally more accurate p-p data; the I=0 phases can only be determined after n -p experiments have been done. The analyzing power is directly sensitive to those phase shifts which depend on spin-orbit terms. Analyzing power measurements in the medium-energy region will contribute to an understanding of the energy dependence of spin-orbit forces in the I =0 D and 6 waves. Measurements of the analyzing power are also of particular importance because the measurements of other spin observables almost always hinge upon knowledge of the analyzing power. Most of the previous n-p*' data in the 500— 800 MeV region were actually p-n data obtained by quasi-free proton-deuteron scattering. The experiment reported here is a measurement of the free n panalyzing -power A„(0„*,T„) for neutron c.m. angles 57'~ 8„*~ 159 and laboratory energies 375 ~ T„~775 MeV. In this experiment a neutron beam of large energy spread was collimated onto a polarized proton target, so that free n -p analyzing power data for a given angular range of about 16 deg in the center of mass were accumulated simultaneously for the wide region of neutron energies between 375 and 775 MeV. Since normalization errors caused by uncertainties in the target polarization were the same for all energies at each angle setting, the shape of the energy dependence of A„(O„*,T„) at each angle was reliably determined in this experiment, since energy-dependent false asymmetries were considered to be negligible.

Scanning electron microscopy with polarization analysis studies of Ni-Fe magnetic memory elements

The authors describe the use of scanning electron microscopy with polarization analysis (SEMPA) to image the magnetic structure of Permalloy magnetic memory elements quantitatively. Various methods of determining the absolute magnitude and direction of the magnetization vector are described. The magnetic domain structures are observed as a function of ion milling time. During ion milling the surface composition is monitored by Auger analysis. Experimental results are presented for Permalloy memory elements. Two methods of determining and eliminating spurious apparatus asymmetries in SEMPA measurements were used. The first relied on assumptions about the symmetry of the domain pattern to establish the polarization zero. In general, this produced accurate maps of the magnetization direction as long as false asymmetries due to surface topography were not important. The second method used the spin-independent scattering from a graphite target as a reference. This method eliminated the instrumental and topographic asymmetry and is preferred for absolute, quantitative magnetization measurements. Both methods permit the angle of the magnetization vector to be measured to within +or-10 degrees .< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The new laval facility for polarization measurements

Abstract A new polarimetry facility has been set up. The four polarimeters used in different geometries are equipped with silicon or carbon analyzers. Extreme care has been taken to cancel out false asymmetries from apparatus misalignments. To that purpose, an automatic beam centering system has been built, two polarimeters are always run simultaneously on both sides of the beam at symmetric angles and geometric means are performed on data reduction. The installation is described in detail and data analysis and calibration runs are presented and discussed.

Measurement of vector and tensor analyzing powers in the [formula omitted] three-body break-up

The vector analyzing power A y and the two tensor analyzing powers A yy and A xx have been measured for the three-body break-up reactions d + 3 He → p + 3 He + n and d + 3 He → p + t + p . Two of the outgoing particles were identified and detected in coincidence at θ lab = ±30°. The deuteron bombarding energy was E d = 15 MeV. The measurements are backed up by a series of tests in order to prove the absence of false asymmetries.

Me´lange de parite´dans la transition de 482 keV DU 181Ta

The parity-violating component of the 482 keV gamma transition of 181Ta has been investigated by measuring its degree of circular polarization. Two Compton forward scattering analysers were used and the scattered gamma rays were detected by eight lead-loaded plastic scintillators, each one connected to an independent energy selection and counting system. Because of the small asymmetry expected a careful investigation of the various sources of systematic errors was made. The possible causes of such systematic errors were minimized or eliminated. False asymmetries of the polarimeters and detectors were measured using the 498 keV gamma transition from a 103Ru source. A calculation of the asymmetry due to bremsstrahlung shows that it is negligible, due to the energy selection. The efficiency of the polarization analysers was calculated and experimentally measured by beta-gamma circular polarization correlation on 198Au. Our result for the polarization of the 482 keV transition of 181 Ta is: {ce:inline-formula}P = −(0.21±0.11)·10 −4{/ce:inline-formula}.Comparisons with theoretical estimates and previous experiments are made.

On optimizing the signal-to-noise ratio of two-state experiments

It is shown that in a two-state experiment of total duration T , false asymmetries due to drifts with frequencies smaller than ≈ 1 T can largely be eliminated by switching the state of the experiment in a sequence of unequal time intervals with average duration t . In this frequency range this technique is greatly superior to the conventional method of switching at equal intervals t , while both methods are about equally insensitive to drifts with frequencies larger than ≈ 1 T . This result is applied to the design of an experiment in which asymmetries on the order of 10 −7 to 10 −8 are measured. The optimum switching pattern consisting of unequal time intervals is calculated, and the conditions for its use are discussed. Comparison with a conventional switching pattern shows that effects of slow drifts can be lowered by several orders of magnitude in most cases of practical interest.

Experiments on Parity Nonconservation in Nuclear Forces. I. Gamma Transitions inTa181andLu175

We report measurements of the circular polarization of $\ensuremath{\gamma}$ rays from unpolarized sources of ${\mathrm{Hf}}^{181}$ and ${\mathrm{Yb}}^{475}$ of 100-1000-Ci strength, using a Compton polarimeter in a forward-scattering geometry. Our technique consisted of integrating the total detector current and digitizing the charge. Several aspects of this novel method are discussed, such as the role of the switching pattern in optimizing the signal-to-noise ratio. All possible causes of false asymmetries are investigated. Control experiments are carried out with ${\mathrm{Ru}}^{103}$ sources. The contribution to the observed circular polarization due to bremsstrahlung has been calculated, as well as determined experimentally. Our results for the circular polarization are ${P}_{\ensuremath{\gamma}}=(\ensuremath{-}3.9\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ for the 482-keV transition in ${\mathrm{Ta}}^{181}$, and ${P}_{\ensuremath{\gamma}}=(6.3\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ for the 396-keV transition in ${\mathrm{Lu}}^{175}$. These findings confirm the existence of a parity-nonconserving term in the nuclear Hamiltonian.

Polarization of protons elastically scattered from deuterons

The polarization of protons elastically scattered from deuterons has been measured by a double scattering experiment. Partially polarized protons of energy 1.50, 2.02 and 2.52 MeV were obtained by scattering a 4.1 MeV proton beam from carbon and by subsequent energy degradation in suitable foils. The scattering chamber was filled with a mixture of deuterium, helium and xenon, and the proton groups corresponding to elastic scattering from the individual gases were resolved in two symmetrically located semi-conductor detectors. The simultaneous measurement of right-left asymmetries for xenon, helium and deuterium permitted a continuous monitoring of false asymmetries, proton beam polarization and the p-d polarization, respectively. The measurements yield positive polarizations, relatively independent of energy, for p-d scattering at angles (c.m.) between 45° and 120° with maxima of about (8±2)% near 90°, and near zero values at 45° and 120°. These results are consistent with recent n-d polarization experiments of comparable accuracy.

Proton-proton polarization between 27 MeV and 100 MeV

The polarization in p-p scattering has been measured at various energies between 27 MeV and 100 MeV in a counter experiment using the 150 MeV, 48% polarized proton beam of the Harwell synchrocyclotron incident on a liquid hydrogen target. The beam energy was reduced by suitable absorbers, and the mean energy resolution of each measurement were defined by absorbers in an anti-coincidence range counter telescope. A solenoid was used to rotate the polarization of the beam through ± 180° in order to eliminate geometrical false asymmetries. It was found possible to represent the energy dependence of the polarization by a simple power law P = aE b up to about 70 MeV.