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Current News

18.07.17: Most precise measurement of the proton mass

One of the fundamental constants, which is crucial to develop precise quantitative understanding of nature and its symmetries is the mass of the proton mp. Its value is among others required for the precise comparison of the proton and antiproton mass, in order to perform a precise test of the fundamental Charge, Parity, and Time Reversal (CPT) invariance.
Mass measurements are generally based on the comparison of the mass of interest with a precisely known reference mass. The cyclotron frequency νc of an stored ion in a Penning trap is proportional to its charge-to-mass ratio q/m: νc = 1/(2π) (q/m) B. Thus, precision Pennig-trap mass measurements of the proton are performed by comparing the cyclotron frequency of the proton with the cyclotron frequency of a reference ion in the same homogeneous magnetic field B.

In a recently in Physical Review Letters published article F. Heiße et al. report on a high-precision measurement of the proton mass mp in atomic mass units, which was based on cyclotron frequency comparisons of protons and highly charged carbon (12C6+) ions. The mass of 12C6+ can be related to the mass of 12C resp. the atomic mass unit by correcting for the mass of the missing six electrons and their respective binding energies.
The measurements have been carried out in a highly optimized, purpose-built cryogenic (4 K) Penning-trap setup, dedicated to mass measurements on light ions. The superconducting magnet and the liquid helium cryostat of the preceding Mainz g-factor experiment for highly charged ions could be re-used. The trap section as well as the cryogenic electronics and detection circuitry have been newly developed. The new trap tower includes two separate storage traps and the measurement trap (MT).
The ions are produced in-situ using a miniature electron beam ion source (EBIS). By shuttling the ions between the two storage traps and the measurement trap, the time between successive measurements in the MT is minimized. For the first time two independent superconducting detection circuits for the proton and for the carbon ion were implemented, which needed to fit very precisely. They allowed for measurements at the identical electrostatic and magnetic field configurations.

The cyclotron frequency νc is determined by the three particle's motional eigenfrequencies in the Penning trap using the invariance theorem: νc = (ν+2 + νz2 + ν-2)1/2. Of the three independent eigenmotions, the tank circuit can only detect the axial motion directly. In order to determine the axial frequencies of the stored ions, the image current the ions induce on the trap electrodes when oscillating with the axial eigenfrequency νz of about 525 kHz for 12C6+ and 740 kHz for the proton, respectively, was measured. The axial frequency νz was determined from a dip spectrum, the modified cyclotron frequency ν+ (and similarly the magnetron frequency ν-) was measured with a "double-dip" method. Here the modified cyclotron motion was coupled to the axial motion via radio frequency drives on the motional sideband at ν+ - νz. This leads to the so called “double-dip”.
Since the uncertainty of ν+ is dominant in the invariance relation, the about an order of magnitude more precise phase-sensitive PnA (Pulse and Amplify) technique was also used for determination of ν+ with highest precision and very low kinetic energy of the ion, and thus low systematic frequency shifts.

Taking into account the statistical uncertainties (1st bracket) as well as the systematic shifts and their uncertainties (2nd bracket) of the PnA data, the ratio νc(12C6+)/νc(p) = 0.503 776 367 662 4(77)(146) was obtained. Here the statistical and systematic uncertainties are listed separately. The by a factor of around four less precise result from the double-dip data was in excellent agreement with the PnA result.
From this cyclotron frequency ratio the proton mass in atomic mass units mp = νc(12C6+)/νc(p)·m(12C6+)/6 = 1.007 276 466 583(15)(29) u was calculated. The value of mp has a relative precision of 32 parts-per-trillion (ppt), which is a factor of three times more precise than the current CODATA (Committee on Data for Science and Technology) value. Moreover, it shows a deviation from the literature value by more than three standard deviations.

In addition, using the independently measured electron mass me, the new value of mp yielded a factor of two more precise proton-electron-mass ratio mp/me = 1 836.152 673 346(81).

More information can be found in the Physical Review Letters article ... >

Further information also in the press release of the MPIK external Link and the Synopsis external Link of Michael Schirber.

Further press releases:

17.07.17: First photodetachment spectroscopy on a fast OH- beam in the CSR

Molecular anions play an important role in chemical reactions in interstellar space and planetary atmospheres and were, for example, observed in the ionosphere of Saturn's moon Titan. Ion traps provide radiation shielding and often extremely low pressures for laboratory experiments with ions at rest. For molecular ions, several studies recently focused on their cooling in laser fields or in collisions with other particles. Investigations where ions are kept in isolation and studied over long time have been rare. Recently constructed cryogenic storage rings for fast ion beams have opened up such studies since ions can be stored for very long durations and individually detected through their fast motion. For anions, in which a neutral atom or molecule binds an excess electron, the photodetachment of this electron offers sensitive single-ion detection.

At MPIK Heidelberg, the novel ultracold storage ring CSR for experiments under conditions as they occur in space was inaugurated in May 2016. It allows for experiments with negative ions with lifetimes of minutes or hours, e. g. small molecular anions like OH-, in order to investigate their interactions in the lowest rotational states. Those studies are crucial for the formation of molecules in interstellar space and for low-temperature plasma chemistry in general.

In a recent article published in Physical Review Letters, C. Meyer et al. report on a state-selective photodetachment experiment on OH- molecular anions in the cryogenic storage ring CSR at MPIK Heidelberg. In this experiment a beam of OH- anions from a Cs sputter ion source was accelerated to 60 keV and injected into the ultracold storage ring. About 107 ions were stored at an ambient temperature near 6 K (-267 °C) and extremely high vacuum (below 10-14 mbar). The ions could be stored for up to 1200 s (20 min). This allowed to observe the spontaneous decay of low-lying excited rotational levels of OH- and to follow the in-vacuo rotational relaxation over times long compared to the natural lifetime of the first excited rotational level.
Frequency and time dependent photodetachment spectroscopy was performed by overlapping nearly collinear laser beams with the ion beam. The fast neutral particles, produced by photodetachment at a steady small rate, were counted with a large microchannel plate detector about 3 m downstream from the interaction region.

An effective radiative temperature Tr = 15.1(1) K with about 90% of all ions in the rotational ground state was obtained.
The Einstein coefficients AJ, the natural lifetimes τJ = (AJ)-1 and the corresponding transition dipole moments were determined for the lowest OH- rotational states (J = 1,2,3). The natural lifetime of the first excited rotational level J = 1 was found to be about 193 s. Such direct in-vacuo lifetime measurements on low-lying, purely rotationally excited states in small molecules have not been reported previously. The electric dipole moment was measured with 1.5% uncertainty. It differs significantly from the theoretical values available.
At photon energies close to the electron binding energy, the photodetachment cross section is a powerful probe for the internal states of the anion and the neutral daughter molecule as well as for the interaction of the outgoing low-energy electron with the neutral molecule. Thus, the relative photodetachment cross sections were additionally determined over a sample of near-threshold energies for individual rotational levels of OH-.

The photodetachment spectroscopy in the CSR allows precise laboratory measurements of natural lifetimes and line intensities for extremely slow, purely rotational transitions in molecular ions. To date, line strengths for ionic rotational transitions are generally obtained from calculated molecular dipole moments. Thus, rotational lifetimes from such measurements add a further, so far unavailable experimental benchmark for quantum chemical calculations.
Moreover, in the future, the single-level sensitivity of time-dependent near-threshold photodetachment spectroscopy in the CSR can be used to probe rotational population changes by in-ring molecular collisions.

Please read more in the Physical Review Letters article ... >

Further information also in the press release of the MPIK external Link and the focus story external Link on the article.

23.06.17: Review article on the direct neutrino mass experiment ECHo

Already in 1930, a new neutral particle was postulated by Wolfgang Pauli to explain the β-decay, which was named "neutrino" by Enrico Fermi in 1933. In 1956 the existence of neutrinos was experimentally proved. Neutrinos were long believed to be massless, but experiments between 1997 and 2002 showed that the neutrinos change their flavour (electron, muon, or tau neutrino) periodically. According to theory, this so called neutrino oscillation can only occur if neutrinos have mass.
In neutrino oscillation experiments, only the squared mass difference can be extracted but no information about the absolute neutrino mass.
The determination of the neutrino masses is one of the most difficult challenges in the investigation of the fundamental properties of elementary particles. Thus, although hundred trillion neutrinos are traversing every human per second, their tiny mass is still unknown. So far, only upper limits of the neutrino mass could be determined, confirming it to be tiny but not zero as predicted by the current Standard Model of particle physics. Thus, it is incomplete and an extension is needed to understand the structure of the universe, the hierarchy of masses and to achieve a grand unification.
Precision measurements of the kinematics of weak interactions represent the only model independent approach to determine the absolute scale of neutrino masses. The KArlsruhe TRItium Neutrino (KATRIN) experiment external Link has been designed to measure the mass of the electron antineutrino directly with a sensitivity of 0.2 eV. In the Electron Capture 163-Ho experiment (ECHo) external Link, the ECHo collaboration external Link aims at extracting the neutrino mass from measurements of the energy emitted in the electron capture (EC) decay of the artificial holmium-163 isotope to the stable stable dysprosium-163.

In a recent review article published in the "European Physical Journal Special Topics" the ECHo collaboration discusses the motivations, expectations as well as the first two phases of the ECHo experiment. It has been conceived to reach sub-eV sensitivity on the electron neutrino mass by the analysis of the endpoint region of the calorimetrically measured 163Ho electron capture spectrum. In order to achieve this sub-eV sensitivity, a large number of cutting edge technologies have to be developed, e.g. the production of large sample of high purity 163Ho as well as the development of large metallic magnetic calorimeters (MMCs) arrays and a precise parameterization of the 163Ho spectrum.

The ECHo experiment will be divided into several phases. The first phase, the present medium scale experiment ECHo-1k, is a three-year project, started in 2015, which will allow to reach a sensitivity below 10 eV for the electron neutrino mass, employing about 1000 Bq (1 Kilobecquerel) of highly radiochemically pure 163Ho, which will be divided into an array of about 100 low temperature MMCs detectors.
Already in the first phase of ECHo-1k very interesting results have been achieved, e.g. the discovery of spectral structures due to higher order excited states in 163Dy. Furthermore, our working group at MPIK participated in the first direct high-precision determination of the atomic mass difference of 163Ho and 163Dy with the Penning-trap mass spectrometer SHIPTRAP external Link using the phase-imaging ion-cyclotron-resonance technique (PI-ICR). The measurement yielded a QEC-value of 2833 eV with an uncertainty of only a few tens of eV, providing confidence that a sensitivity below 10 eV for the neutrino mass can be reached in ECHo-1k.

The second phase, ECHo-1M, is under study and will start at the end of ECHo-1k. ECHo-1M will be characterized by a 163Ho activity of 1 MBq, embedded in large MMCs arrays and will allow to reach a sensitivity on the electron neutrino mass below 1 eV. In particular, this aimed sub-eV sensitivity requires a direct measurement of the QEC-value of the EC in 163Ho with an unprecedentedly low uncertainty of approximately 1 eV. This will become possible by improving the accuracy of the decay energy value (QEC) by an order of magnitude using the Penning-trap mass spectrometer PENTATRAP, which is currently being built by our working group at MPIK Heidelberg.

Please read more in the review article ... >

24.04.17: Review article on new methods and highlights of collinear laser spectroscopy at ISOLDE

In a recent review article published in "Journal of Physics G" R. Neugart et al. report on new methods and highlights of collinear laser spectroscopy at the on-line isotope mass separator ISOLDE external Link at CERN, Geneva.

Collinear laser spectroscopy at ISOLDE has a successful history of more than three and a half decades. The COLLAPS external Link (COLlinear LAser SPectroscopy) setup has been introduced at ISOLDE for the investigation of fundamental properties of exotic nuclei such as nuclear ground state spins, electromagnetic moments and charge radii. In 1980 the first COLLAPS setup was completed and first successful experiments were performed on the neutron-rich barium isotopes followed by numerous studies of further isotopic chains until 1990, mainly in the heavier-mass region.
In 1992 a second period of COLLAPS operation started at the new ISOLDE location at the Proton Synchrotron Booster external Link (PSB) of CERN. During this period highly sensitive experimental techniques were developed which exploited the properties of special classes of atomic systems.

The classical collinear spectroscopy at the COLLAPS beam line is based on a single resonant excitation by a continuous wave (cw) laser and the fluorescence decay from the excited level is used as a straightforward detection method. In order to improve the sensitivity of COLLAPS an rf quadrupole cooler and ion buncher (RFQCB), the ion cooler–buncher ISCOOL external Link, was installed at ISOLDE to deliver radioactive beams of improved quality which allows a suppression of background by several orders of magnitude. Since the fall of 2008, nearly all optical detection experiments at COLLAPS have been performed using this new method on bunched beams from ISCOOL.

The combination of collinear laser spectroscopy with the principle of laser resonance ionisation is used in the new CRIS external Link (Collinear Resonance Ionisation Spectroscopy) setup at ISOLDE. The collinear resonance ionisation method uses a second (or third) laser to further excite the level populated in the first high-resolution step to the continuum and thus ionise the atoms. This is a very selective process which allows the detection of optical resonance by counting ions very efficiently and virtually without any background. Since 2012, the novel CRIS technique is used with promising results for measurements on the francium isotope chain.

The introduction of ISCOOL and the new CRIS method at ISOLDE yielded a breakthrough in sensitivity for nuclides in wide mass ranges. The determination of nuclear ground state properties along isotopic chains from collinear laser spectroscopy is now indispensable in nuclear physics research with radioactive beams.

Please read more details in the review article on the experimental principle of collinear laser spectroscopy, the new methods and highlights since 2000 as well as the main future directions at ISOLDE, e. g. an proposed electrostatic ion beam trap (EIBT) for ultra-sensitive collinear laser spectroscopy of radionuclides and the proposed installation of the heavy ion storage ring TSR external Link from MPIK Heidelberg, that provides excellent opportunities to study challenging cases which could not be addressed at ISOLDE so far ... >

21.03.17: First observation of individual spin quantum transitions of a single antiproton

In the past years, spectroscopy experiments that were based on the observation of quantum phenomena in electron/positron systems enabled sensitive measurements with highest resolution and e.g. led to the development of first optical frequency standards, to precise measurements of the Planck constant, to the most precise measurement of the fine-structure constant, and to a stringent test of the fundamental charge-parity-time (CPT) invariance. In 2012 quantum transitions of a pure antimatter system (antihydrogen) have been observed for the first time.
In order to make comparable observations in the proton/antiproton system, a considerably higher experimental sensitivity is needed. Thus, the application of quantum-transition based spectroscopy schemes is more challenging compared to the electron/positron system. The recent observation of individual spin transitions of a single trapped proton led to a high-precision measurement of the proton magnetic moment with 3.3·10-9 relative precision in 2015.

In a recently in Physics Letters B published article C. Smorra et al. report on the first non-destructive detection of individual spin transitions of a single antiproton. The experiment was carried out with the BASE external Link Penning-trap system located at the antiproton decelerator facility (AD) external Link of CERN, Geneva.
The BASE Penning-trap system consists of four Penning traps in the horizontal bore of a superconducting magnet. A reservoir trap (RT) serves as interface between the AD and the measurement traps and supplies single particles from the reservoir into the other traps when needed. The precision trap (PT) and the cooling trap (CT) are required for the precision frequency measurements, and efficient cooling of the modified cyclotron mode, respectively. The observation of individual spin quantum transitions was achieved by using the continuous Stern–Gerlach effect in the 5-pole analysis trap (AT) with a superimposed magnetic bottle of Bz = 272(15) mT/mm2 generated by the superconducting magnet with field strength B0 = 1.945 T. This couples the spin-magnetic moment of the stored antiproton to its axial oscillation frequency νz.
In the described antiproton experiment, the axial frequency fluctuation of the particle in the magnetic bottle was at 48.1mHz for 96s averaging time. Under these conditions, it could be demonstrated that 92.1% of the spin states detected in the measurement sequence are identified correctly.

To measure the antiproton g-factor with very high precision, the BASE collaboration external Link aims at the application of the challenging double Penning-trap technique, which was successfully demonstrated with a single proton for the first time in 2013 at the University of Mainz. Spin-state initialization with >99.9% fidelity and an average initialization time of 24 minutes were demonstrated in the BASE antiproton experiment. This enables an antiproton double-trap g-factor measurement with high contrast and will allow to reach the intended relative precision on the part-per-billion level (10-9). This high precision will allow for one of the most stringent tests of charge-parity-time invariance in the baryon sector.

Please read more in the article ... >

18.01.17: Most precise measurement of the g-factor of the antiproton by the BASE collaboration

The observable universe shows no matter-antimatter symmetry but is matter-dominated. However, in particle physics every particle is produced with its corresponding antiparticle and such particle-antiparticle pairs annihilate each other. To explain the evident matter-antimatter imbalance on cosmological scales scientists assume that at the origin of the universe matter particles outnumbered antimatter particles. Therefore, all antimatter particles were destroyed, leaving behind only matter. According to the fundamental Charge, Parity, and Time Reversal (CPT) symmetry of the Standard Model of particle physics the fundamental properties of a particle and its antiparticle are exactly equal. Therefore, the highly precise comparision of the properties of a particle and its antiparticle provides a test of the CPT symmetry and contributes to a better understanding of the matter-antimatter imbalance of our universe. This inspires physicists to invent novel techniques and experimental setups to find minutest differences between the properties of matter and antimatter.

In a recently in "Nature Communications" published article H. Nagahama et al. report on high-precision measurements of the magnetic moment of the antiproton at the Baryon Antibaryon Symmetry Experiment (BASE) external Link at the Antiproton Decelerator (AD) external Link of CERN, Geneva. The experiment aimed at performing a stringent test of the CPT symmetry by comparing the magnetic moments of the proton and the antiproton with high precision. The used spin-flip technique has already been successfully applied to electrons and positrons, however, its application to measure the magnetic moments of the proton/antiproton is much more challenging, since their magnetic moments are 660 times smaller. Thus, the detection of single antiproton spin transitions required a specially designed ultra-strong magnetic bottle with an inhomogeneity of 2.88·105 Tm-2. This is more than 1000 times stronger than the inhomogeneous magnetic field used in the electron/positron experiments.

Within BASE the proton/antiproton g-factors are determined by measuring the respective ratio of the spin-precession frequency νL (Larmor frequency) to the cyclotron frequency νc. The spin precession frequency is measured by non-destructive detection of spin quantum transitions using the continuous Stern-Gerlach effect, and the cyclotron frequency is determined from the particle's motional eigenfrequencies in the Penning trap using the invariance theorem: νc2 = ν+2 + νz2 + ν-2. The modified cyclotron frequency ν+, the axial frequency νz, and the magnetron frequency ν- are measured via image current detection.
The BASE experiment uses a cryogenic Three-Penning trap system, which is mounted in the horizontal bore of a superconducting magnet with field strength B=1.945 T. A cloud of antiprotons is stored in the reservoir trap (RT), which supplies single particles to the co-magnetometer trap (CT) and the analysis trap (AT) when required. The CT is used for continuous magnetic field measurements. The AT is the trap with the strong superimposed magnetic bottle, which is used to measure the cyclotron frequency and the Larmor frequency.

Six direct g-factor measurements of single antiprotons have been performed with a fractional precision of 0.8 p.p.m (10-6). The new value of 2,7928465(23) is 6 times more precise than the value of the previous best measurement by the ATRAP collaboration external Link at CERN in 2013. It is in agreement with the most precise proton g-factor value of 2,792847350(9) measured at University of Mainz in 2014 and therefore agrees with the fundamental charge, parity, time (CPT) invariance of the Standard Model of particle physics.
The 1000 times more precise proton measurements at Mainz are based on the application of the challenging double Penning-trap technique. The BASE collaboration external Link plans the implementation of this method to further improve the precision in antiproton magnetic moment measurements to the level of a few parts per billion (10-9).

Please read more in the "Nature Communications" article ... >

Further information also in the press releases of the MPIK external Link, CERN external Link, RIKEN external Link and the Johannes Gutenberg University Mainz external Link.

Further press releases:

10.01.17: High-precision laser spectroscopy of Be+ for nuclear structure investigations and many-body QED tests

The investigation of the atomic structure by high-precision laser spectroscopy is of great relevance for the understanding of fundamental physics problems. It e.g. gives detailed insight into ground state properties of short-lived nuclei as the nuclear charge radii which allows to test and improve nuclear structure models. Moreover, the combination of laser spectroscopy with theoretical calculations is an indispensable tool to explore many-body quantum electrodynamics in weak and strong fields.

In a recently in Applied Physics B published article A. Krieger et al.. report on high-precision measurements on stable and radioactive beryllium isotopes at the COLLAPS external Link collinear spectroscopy beam line at CERN-ISOLDE external Link in Geneva. These investigations aimed to obtain more detailed information on the nuclear structure and to provide an important test for bound-state QED calculations in three-electron systems.
For this purpose, the technique of conventional collinear laser spectroscopy was further developed and combined with a frequency comb to provide high-precision measurements of the transition frequencies of the D1 and D2 transitions 2s2S1/2 → 2p2P1/2,3/2 in Be+. To overcome the accuracy limitations of conventional collinear laser spectroscopy due to the uncertainty in ion acceleration voltage determination the quasi-simultaneous excitation by a collinear and an anticollinear laser beam was introduced. Thus, two frequency-stabilized dye laser systems were required which delivered UV beams that superposed the beryllium ion beam in opposite directions.
The isotope shifts were obtained in two beam times as difference of the transition frequency of the Be isotope of interest and the stable reference isotope 9Be. The use of a frequency comb enabled the determination of the laser frequencies with the required relative accuracy better than 10−9. This yielded the isotope shifts with an accuracy better than 10−5.
From its isotope shift the corresponding change in the mean-square nuclear charge radius of the isotope can be derived. Hence, the accurate isotope shifts led to a precise determination of the nuclear charge radii along the beryllium isotopic chain 7,10−12Be relative to the stable isotope 9Be.

Additionally, the 2p fine-structure splitting as a function of the atomic number along the beryllium isotopic chain was extracted from the precise transition frequencies. Only recently the fine-structure splitting in three-electron atoms became calculable with high precision. Thus, the measurements also allowed a test of such high-precision bound-state QED calculations. The experimental results confirmed first calculations for the Z=4 three-electron system of Be+. The measured mass dependence of the fine-structure splitting of 7,10−12Be also provided a check of the so-called splitting isotope shift (SIS) which can be calculated theoretically to very high accuracy.

Please read more in the article ... >