Behavior of many ions in a Penning trap and results of the WITCH experiment

Precision measurements of the beta−neutrino angular correlation in nuclear beta-decay provide a unique window into the physics beyond the Standard model. The WITCH (Weak Interaction Trap for CHarged particles) experiment aims to measure this correlation, a(beta-nu), in order to impose a more stringent constraint on the exotic scalar current admixture in the beta-decay Hamiltonian. The apparatus is situated at CERN/ISOLDE laboratory and consists of a unique combination of a retardation spectrometer and two Penning traps, with one of them serving as a scattering-free source. This configuration is suited for a precise measurement of the energy spectrum of 35Ar recoiled daughter ions. The shape of the spectrum then allows a determination of a(beta-nu) and consequently of the presence or absence of a scalar current. Radioactive 35Ar ions are created at ISOLDE by impinging 1.2 GeV protons on the target material. After being separated by a magnetic separator and bunched by REXTRAP, a high-capacity Penning trap, they are delivered to the WITCH beam line with an energy of 30 keV per ion. A pulsed drift tube then reduces this energy to about 100 eV, enabling their capture in the cooler trap where they undergo buffer gas cooling and centering processes before being transferred to the decay trap. After decay, the energy spectrum of recoil 35Cl daughter ions is probed in the retardation spectrometer before they are finally counted by an MCP detector. Retardation spectra for various values of a(beta-nu) are simulated by SimWITCH, a Monte Carlo simulation software that tracks the recoil ions from a cloud in the decay trap through the spectrometer. The experimental spectrum is then fitted to simulated spectra and the value of a(beta-nu) can be extracted. The properties of the ion cloud from injection into the cooler trap to transfer into the decay trap are obtained with Simbuca, a simulation software package that exploits native GPU parallelism for fast ab initio simulation of ion cloud dynamics in a Penning trap, with realistic electric and magnetic field maps. Furthermore, as the main focus of this work, systematic effects arising in large ion clouds (of 105 − 106 ions) are studied experimentally with stable 39K ions as well as computationally with Simbuca and comparatively analyzed and presented. Specifically, the influence of space-charge and buffer gas on cyclotron cooling resonances is investigated. Experimental ion cyclotron resonances are compared with ab initio Coulomb simulations and found to be in agreement, showing an increase of central values and FWHMs with increasing space-charge and buffer gas pressure. The ability to accurately simulate the behavior of large ion clouds in specific experimental conditions is of special interest for the design and optimization of high-capacity Penning traps and their operation as mass separators. Another important systematic effect of the WITCH experiment, the magnetron eigenmotion of the ion cloud around the trap center, is experimentally studied under increasing space-charge conditions. In addition, the helium buffer gas pressure in the Penning trap is determined by comparing experimental cooling rates with simulations. In June 2011, an experiment resulting in a first determination of a(beta-nu) with WITCH was performed, albeit with low statistics and with no systematic effects considered. The November 2012 experiment yielded a much larger data set. Combined with a significantly improved data-acquisition system, this enables us to uncover several important systematic effects and account for their influence. These include energy-dependent efficiency of the main MCP, two separate radioactive components stemming from the decay of overshot 35Ar ions implanted into the main MCP detector and from the beta-particles originating in the decay trap, and an effect of the magnetron motion of the entire ion cloud around the trap center. The data is then fitted with a function tailored to account for these systematic effects. However, another background component difficult to account for was found to significantly distort the low energy part of the recoil spectrum and hamper the extraction of a(beta-nu). Using Monte Carlo simulations of the spectrum, it is found that this component is most likely correlated with the amount of radioactive ions in the decay trap and is composed of low-energetic Gaussian-distributed rest gas or buffer gas ions. An overview of measures needed to characterize and reduce this background are given. Finally, constraints related to availability of radioactive beams at ISOLDE leading to the discontinuation of the project are discussed

Measurement of the beta-asymmetry parameter of Cu-67 in search for tensor-type currents in the weak interaction

The experimental value, ˜A = 0.587(14), is in agreement with the standard model value of 0.5991(2) and is interpreted in terms of physics beyond the standard model. The limits obtained on possible tensor-type charged currents in the weak interaction Hamiltonian are −0.045 < (C_T + C'_T)/CA < 0.159 (90% C.L.). The obtained limits are comparable to limits from other correlation measurements in nuclear β decay and contribute to further constraining tensor coupling constants.status: publishe

First β-ν correlation measurement from the recoil-energy spectrum of Penning trapped 35Ar ions

We demonstrate a novel method to search for physics beyond the standard model by determining the β-ν angular correlation from the recoil-ion energy distribution after β decay of ions stored in a Penning trap. This recoil-ion energy distribution is measured with a retardation spectrometer. The unique combination of the spectrometer with a Penning trap provides a number of advantages, e.g., a high recoil-ion count rate and low sensitivity to the initial position and velocity distribution of the ions and completely different sources of systematic errors compared to other state-of-the-art experiments. Results of a first measurement with the isotope 35Ar are presented. Although currently at limited precision, we show that a statistical precision of about 0.5% is achievable with this unique method, thereby opening up the possibility of contributing to state-of-the-art searches for exotic currents in weak interactions.status: publishe

Using GPU parallelization to perform realistic simulations of the LPCTrap experiments

The LPCTrap setup is a sensitive tool to measure the β − ν angular correlation coefficient, aβν, which can yield the mixing ratio ρ of a β decay transition. The latter enables the extraction of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element Vud. In such a measurement, the most relevant observable is the energy distribution of the recoiling daughter nuclei following the nuclear β decay, which is obtained using a time-of-flight technique. In order to maximize the precision, one can reduce the systematic errors through a thorough simulation of the whole set-up, especially with a correct model of the trapped ion cloud. This paper presents such a simulation package and focuses on the ion cloud features; particular attention is therefore paid to realistic descriptions of trapping field dynamics, buffer gas cooling and the N-body space charge effects.status: publishe

Space-charge effects in Penning ion traps

The influence of space-charge on ion cyclotron resonances and magnetron eigen frequency in a gas-filled Penning ion trap has been investigated. Off-line measurements with 39K using the cooling trap of the WITCH retardation spectrometer-based setup at ISOLDE/CERN were performed. Experimental ion cyclotron resonances were compared with ab initio Coulomb simulations and found to be in agreement. As an important systematic effect of the WITCH experiment,the magnetron eigen frequency of the ion cloud was studied under increasing space-charge conditions. Finally, the helium buffer gas pressure in the Penning trap was determined by comparing experimental cooling rates with simulations.publisher: Elsevier articletitle: Space-charge effects in Penning ion traps journaltitle: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment articlelink: http://dx.doi.org/10.1016/j.nima.2015.02.057 content_type: article copyright: Copyright © 2015 CERN for the benefit of the Authors. Published by Elsevier B.V.status: publishe

Search for a scalar component in the weak interaction

The WITCH project (Weak Interaction Trap for CHarged particles) at the isotope separator ISOLDE at CERN is trying to probe the structure of the weak interaction in specific low energy beta–decays in order to look for possible scalar or tensor components or at least significantly improve the current experimental limits. This worldwide unique experimental setup consisting of a combination of two Penning ion traps and a retardation spectrometer allows to catch, trap and cool the radioactive nuclei provided by the ISOLDE separator, form a cooled and scattering-free radioactive source of beta–decaying nuclei and let these nuclei decay at rest. The precise measurement of the shape of the energy spectrum of the recoiling nuclei, the shape of which is very sensitive to the character of the weak interaction, enables searching for a possible admixture of a scalar/tensor component in the dominant vector/axial vector mode.status: publishe

Search for a scalar component in the weak interaction

Weak interactions are described by the Standard Model which uses the basic assumption of a pure “V(ector)-A(xial vector)” character for the interaction. However, after more than half a century of model development and experimental testing of its fundamental ingredients, experimental limits for possible admixtures of scalar and/or tensor interactions are still as high as 7%. The WITCH project (Weak Interaction Trap for CHarged particles) at the isotope separator ISOLDE at CERN is trying to probe the structure of the weak interaction in specific low energy $\beta$–decays in order to look for possible scalar or tensor components or at least significantly improve the current experimental limits. This worldwide unique experimental setup consisting of a combination of two Penning ion traps and a retardation spectrometer allows to catch, trap and cool the radioactive nuclei provided by the ISOLDE separator, form a cooled and scattering-free radioactive source of $\beta$–decaying nuclei and let these nuclei decay at rest. The precise measurement of the shape of the energy spectrum of the recoiling nuclei, the shape of which is very sensitive to the character of the weak interaction, enables searching for a possible admixture of a scalar/tensor component in the dominant vector/axial vector mode. First online measurements with the isotope $^{35}$Ar were performed in 2011 and 2012. The current status of the experiment, the data analysis and results as well as extensive simulations will be presented and discussed