2,140 research outputs found

    Measurement of the Permanent Electric Dipole Moment of the 129^{129}Xe Atom

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    We report on a new measurement of the CP-violating permanent Electric Dipole Moment (EDM) of the neutral 129^{129}Xe atom. Our experimental approach is based on the detection of the free precession of co-located nuclear spin-polarized 3^3He and 129^{129}Xe samples. The EDM measurement sensitivity benefits strongly from long spin coherence times of several hours achieved in diluted gases and homogeneous weak magnetic fields of about 400~nT. A finite EDM is indicated by a change in the precession frequency, as an electric field is periodically reversed with respect to the magnetic guiding field. Our result, (−4.7±6.4)⋅10−28\left(-4.7\pm6.4\right)\cdot 10^{-28} ecm, is consistent with zero and is used to place a new upper limit on the 129^{129}Xe EDM: ∣dXe∣<1.5⋅10−27|d_\text{Xe}|<1.5 \cdot 10^{-27} ecm (95% C.L.). We also discuss the implications of this result for various CP-violating observables as they relate to theories of physics beyond the standard model

    HAWKS: Evolving Challenging Benchmark Sets for Cluster Analysis

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    Comprehensive benchmarking of clustering algorithms is rendered difficult by two key factors: (i) the elusiveness of a unique mathematical definition of this unsupervised learning approach and (ii) dependencies between the generating models or clustering criteria adopted by some clustering algorithms and indices for internal cluster validation. Consequently, there is no consensus regarding the best practice for rigorous benchmarking, and whether this is possible at all outside the context of a given application. Here, we argue that synthetic datasets must continue to play an important role in the evaluation of clustering algorithms, but that this necessitates constructing benchmarks that appropriately cover the diverse set of properties that impact clustering algorithm performance. Through our framework, HAWKS, we demonstrate the important role evolutionary algorithms play to support flexible generation of such benchmarks, allowing simple modification and extension. We illustrate two possible uses of our framework: (i) the evolution of benchmark data consistent with a set of hand-derived properties and (ii) the generation of datasets that tease out performance differences between a given pair of algorithms. Our work has implications for the design of clustering benchmarks that sufficiently challenge a broad range of algorithms, and for furthering insight into the strengths and weaknesses of specific approaches

    New method to study ion-molecule reactions at low temperatures and application to the H2+_2^+ + H2_2 →\rightarrow H3+_3^+ + H reaction

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    Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the H2++H2→H3++H{\rm H}_2^+ + {\rm H}_2\rightarrow {\rm H}_3^+ + {\rm H} reaction with the H2+{\rm H}_2^+ ion prepared in a single rovibrational state at collision energies in the range Ecol/kB=5E_{\rm col}/k_{\rm B} = 5-60 K. To reach such low collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H2_2 molecules and the second is a supersonic beam of H2_2 molecules excited to Rydberg-Stark states of principal quantum number nn selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10∘^\circ. To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state H2{\rm H}_2 beam and by adapting the electric-potential functions to the electrodes of the deflector. The collision energy is varied down to below Ecol/kB=10E_{\rm col}/k_{\rm B}= 10 K, i.e., below Ecol≈1E_{\rm col}\approx 1 meV, with an energy resolution of 100 ÎŒ\mueV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical-Langevin-capture model in the collision-energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studyingComment: 39 pages, 10 figure

    Precise Measurement of Magnetic Field Gradients from Free Spin Precession Signals of 3^{3}He and 129^{129}Xe Magnetometers

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    We report on precise measurements of magnetic field gradients extracted from transverse relaxation rates of precessing spin samples. The experimental approach is based on the free precession of gaseous, nuclear spin polarized 3^3He and 129^{129}Xe atoms in a spherical cell inside a magnetic guiding field of about 400 nT using LTC_C SQUIDs as low-noise magnetic flux detectors. The transverse relaxation rates of both spin species are simultaneously monitored as magnetic field gradients are varied. For transverse relaxation times reaching 100 h, the residual longitudinal field gradient across the spin sample could be deduced to be∣∇⃗Bz∣=(5.6±0.4)|\vec{\nabla}B_z|=(5.6 \pm 0.4) pT/cm. The method takes advantage of the high signal-to-noise ratio with which the decaying spin precession signal can be monitored that finally leads to the exceptional accuracy to determine magnetic field gradients at the sub pT/cm scale
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