LIONTRAP mass measurements of the deuteron and the HD+ molecular ion with record precision
The electron, the proton and the neutron are fundamental building blocks of matter. Their masses and the masses of their simplest
combinations forming the lightest atomic nuclei provide quantitative links between a multitude of observables in atoms and molecules.
E.g., the mass of the proton mp is essential in hydrogen and muonic hydrogen spectroscopy, as it is required for the determination of the Rydberg constant or the proton radius with high precision. Furthermore, the proton-electron mass ratio mp/me is a central parameter in various atomic physics experiments. The mass of the deuteron (atomic nucleus consisting of one proton and one neutron) in combination with its measured nuclear binding energy enables the mass of the neutron to be determined with high precision. The highly precise mass difference of triton (atomic nucleus consisting of one proton and two neutrons) and helion (atomic nucleus of 3He, consisting of two protons and one neutron) is of high interest for the KATRIN (KArlsruhe TRItium Neutrino) experiment.
At present, the most precise values for these fundamental parameters come from Penning-trap mass spectrometry, which achieves relative mass uncertainties in the range of 10−11. However, the most precisely measured light ion masses from spectrometers around the world show inconsistencies, which can be expressed in terms of the term Δ=mp+md-mHe. This is the so-called "the light ion mass puzzle". This value can either be derived from a mass ratio measurement of 3He+ and HD+, or from the mass measurements of proton, deuteron and helion in atomic mass units. Both values differ by five standard deviations. This suggests that the uncertainty of these important mass values may have been underestimated.
In a recent article published in "Nature", S. Rau et al. present the results from absolute mass measurements of the deuteron and
the HD+ molecular ion against 12C as a mass reference. The measurements were performed using
LIONTRAP (Light ION TRAP),
a cryogenic Penning-trap mass spectrometer aiming for most precise mass measurements on various light ions, situated at the
Johannes Gutenberg University in Mainz, Germany.
In a Penning trap, the confinement of a single charged particle is achieved by the use of a strong, homogeneous magnetic field that keeps the ion radially confined and an electric quadrupole potential that stores it axially. This yields a motion of the stored ion which can be divided into three harmonic eigenmotions: the modified cyclotron motion with the modified cyclotron frequency ν+, the very slow magnetron movement with the magnetron frequency ν- and the oscillation in axial direction with the axial frequency νz. The actual cyclotron frequency νc=(1/2π)·(q/m)·B of a stored ion with charge q and mass m can be calculated from the invariance theorem: νc=(ν+2+νz2+ν-2)1/2
In Penning-trap mass ratio measurements, the cyclotron frequency νc of the ion under investigation is compared to the cyclotron frequency νcref of a reference ion in the same magnetic field B: m=(q/qref)·(νcref/νc)·mref
Thus, the mass determinations are based on the high-precision measurement of the cyclotron frequency ratio νcref/νc. Since the atomic mass unit u is one twelfth of the mass of a 12C atom, 12C was used as mass reference. To measure the mass of deuteron md with LIONTRAP, 12C6+ was used as reference ion. For the mass measurement of the HD+ molecular ion, 12C4+ was used as reference ion. Ideally, the frequency ratio νcref/νc has to be measured simultaneously at the same position inside the trap in order to nullify the magnetic field fluctuation. At LIONTRAP this ideal measuring principle is approached by using a precision trap (PT) and two separate storage traps (ST-I, ST-II). By shuttling the ion under investigation (deuteron or HD+) and reference ion (12C6+ or 12C4+) between the storage traps and the PT, the time between successive cyclotron frequency measurements is minimized. Furthermore, the identical electrical field configurations for both ions guarantee the identical position of the ions in the PT and therewith the same magnetic field for the νcref/νc measurement.
At LIONTRAP, the axial frequency νz can be determined by detecting the image currents induced by the axial movement of the ion of interest on the trap electrodes (so-called "dip" spectrum). The modified cyclotron frequency ν+ and the magnetron frequency ν- can be measured by sideband coupling. To this end, the axial and the radial modes are coupled via radiofrequency excitations using radio frequency drives on the motional sidebands at ν+-νz and νz+ν- (so-called "double-dip" technique). Since ν+>>νz>>ν-, the modified cyclotron frequency ν+ has the highest significance and thus was measured with highest precision using the fast phase-sensitive measurement method PnA (Pulse and Amplify).
The LIONTRAP mass value for the deuteron md=2.013553212535(17) u supersedes the precision of the current CODATA (Committee on Data
for Science and Technology) literature value by a factor of 2.4 and deviates from this by 4.8 standard deviations. With a relative
uncertainty of 8 parts per trillion (ppt) this is the most precise mass value measured directly in atomic mass units. The directly
measured mass of the HD+ molecular ion, m(HD+)=3.021378241561(61) u, allowed for a rigorous consistency check of the measurements
of the deuteron mass in the present experiment and the proton mass (see our
news of 18.07.17). The mass value derived from the masses
of proton and deuteron m(HD+)p+d=3.021378241576(37) u agrees with the directly measured one on a one sigma level. This striking
agreement among the LIONTRAP measurements with different masses and systematics substantiates the applied measurement methods.
Furthermore, the deuteron-to-proton mass ratio md/mp(LIONTRAP)=1.999007501228(59) agrees with a direct measurement using H2+ molecular ions recently reported by the Florida State University [Phys. Rev. Lett. 124, 013001 (2020) ] on a one sigma level.
The agreement between the measurements at LIONTRAP and the Florida State University opens up the possibility of a least squares adjustment, reducing the uncertainty of the proton mass mp by a factor of three compared to the current CODATA value.
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Laser spectroscopic and theoretical nuclear structure investigation of magic tin
An international research project with the participation of scientists of the TU Darmstadt, the MPIK Heidelberg, the Friedrich–Alexander University Erlangen–Nürnberg and the Johannes Gutenberg University Mainz performed high-precision measurements in the long isotopic chain of magic tin and determined deviations from the spherical nuclear shape. The laser spectroscopic experiments on the partly very short-lived tin isotopes were conducted at the radioactive ion beam facility ISOLDE of CERN in Geneva.
The so-called quadrupole moment Q is a quantitative measure for the nuclear deformation. The recent investigations
on tin isotopes revealed that Q does not show a linear trend along the isotopic chain, but a nearly perfect
The researchers compared this experimental result with the predictions of the Fayans density functional, which recently could reproduce the trend of charge radii of cadmium isotopes (see our news of 04.09.18). This nuclear model roughly describes the general trend of nuclear deformation of the tin isotopes, but it substantially differs in the details. Thus, the new data can contribute to further improve our understanding of the nuclear forces.
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In June 2020, the article was selected for the Home page of Communications Physics .
PENTATRAP detects extremely long-lived metastable state in highly charged ions
Every time measurement using a clock is based on counting periodic processes, e.g. the pendulum oscillation of a pendulum clock. If the number of swings per second is known (or defined), the counting of the number of oscillations allows to determine the elapsed time in seconds. Today, the definition of time is based on counting the oscillations of a transition in an atom with frequencies in the microwave (gigahertz) range. The current international time standard using cesium atomic clocks reaches a fractional precision of 10−16. Thus, advanced atomic clocks gain or lose only a second in over 100 million years.
Optical atomic clocks are the next generation of atomic clocks and have clock frequencies in the optical spectral (terahertz) range. Optical clocks
achieve fractional precisions of 10−18 and below using ensembles of atoms in optical lattices or individual ions in radiofrequency traps.
No electronics is fast enough to measure the terahertz frequencies of an optical clock. Nevertheless, the use of frequency combs allows the construction of optical clocks. Frequency combs work like a "frequency gear". They transform the optical terahertz frequencies into electronically measurable gigahertz frequencies. Optical atomic clocks are used as frequency standards and in searches for possible variations of fundamental constants, dark matter detection, and physics beyond the Standard Model.
Promising candidates for a new generation of clocks are highly charged ions (HCIs, see e.g. recent quantum logic spectroscopy with 40Ar13+ [Nature 578, 60-65 (2020) ]) and nuclear transitions (e.g. the 229Th nuclear clock transition [Nature 573, 243-246 (2019) ]). They are largely insensitive to external perturbations and reach wavelengths beyond the optical range, now becoming accessible to frequency combs.
However, the development of novel HCI clocks depends on the identification of suitable clock transitions in HCIs. The transition detection is hindered by difficulties when using conventional spectroscopy methods and by insufficiently accurate atomic structure calculations.
In a recent article published in "Nature", R. X. Schüssler et al. report on the detection of an extremely long-lived metastable state in highly charged ions
(187Re29+) using a unique approach to determine the electronic excitation energy. It was determined by measuring the mass difference of the ground and the
excited state of the 187Re29+ ion. The experiment was performed with the novel five-trap Penning trap-system
PENTATRAP at the Max Planck Institute for Nuclear
Physics (MPIK) in Heidelberg, Germany. It was the first direct, non-destructive determination of an excitation energy in HCIs using high-precision
Penning-trap mass spectrometry.
The mass determination with PENTATRAP is performed by measuring the free cyclotron frequency νc=1/(2π)·qB/m of an HCI inside a Penning trap. The cyclotron frequency νc can be determined from the three trap frequencies ν- (magnetron frequency), νz (axial frequency) and ν+ (modified cyclotron frequency) of the HCI motion using the invariance theorem: νc2 = ν-2 + νz2 + ν+2.
The unique multi-trap configuration of PENTATRAP consists of five identical cylindrical traps for simultaneous mass measurements. This significantly reduces the uncertainty in the cyclotron frequency-ratio (i.e. mass-ratio) determination. The cyclotron frequency ratio R of 187Re29+ in the ground state to the metastable state could be determined with an unprecedented precision of δR = 1·10−11. The calculated energy of the metastable state of 187Re29+ with respect to the ground state is ΔERe = 202.2(17) eV.
The experimentally determined excitation energy of the long-lived metastable state was compared to advanced theoretical calculations. These were
obtained using Multi-configuration Dirac-Hartree Fock approaches in two different implementations and by means of a
configuration-interaction (Quanty) calculation.
The theoretical values and experimental results agree very well. The calculations also helped to identify the observed metastable state [Kr]4d9 4f 3H5 of the 187Re29+ ion.
With a lifetime of about 130 days, the potential soft x-ray frequency reference at ν = 4.86·1016 Hz has a linewidth of only Δν ≈ 5·10−8 Hz, and one of the highest electronic quality factors (Q = ν/Δν ≈ 1024) ever seen in an experiment. This low uncertainty enables searching for more HCI soft x-ray clock transitions using PENTATRAP.
The PENTATRAP project receives funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme under grant agreement No 832848 - FunI .
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PENTATRAP significantly improves the measurement accuracy for heavy highly charged ions
Many areas of fundamental physics require the highly precise knowledge of mass differences or mass ratios of a variety of
nuclides, e.g. neutrino physics, the test of special relativity and bound-state quantum electrodynamics (BS-QED),
ion clocks and the search for Dark Matter via high-resolution isotope shift measurements. In order to satisfy these requirements,
the novel Penning-trap experiment PENTATRAP has been set up
at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg.
PENTATRAP is based on high-precision Penning-trap mass spectrometry (PTMS). A unique feature of the PENTATRAP mass spectrometer
is its multi-trap configuration which consists of five identical cylindrical traps. Such a multi-trap configuration significantly
reduces the uncertainty in the mass-ratio determination. PENTATRAP offers a realistic opportunity to pursue a mass-ratio measurement
on heavy (A>100) highly charged ions with a fractional uncertainty δm/m below 10-11.
The mass determination of an ion in a Penning trap is done by measuring its free cyclotron frequency νc = 1/(2π) · qB/m, where q/m is the charge-to-mass ratio of the ion and B the magnetic field. The cyclotron frequency νc can be determined from the three independent harmonic oscillations ν- (magnetron frequency), νz (axial frequency) and ν+ (modified cyclotron frequency) of the ion motion in the Penning trap using the invariance theorem: νc2 = ν-2 + νz2 + ν+2.
In a recent article published in Physical Review Letters A. Rischka et al. present the first mass measurements carried out
with PENTATRAP on stable isotopes of xenon (Z=54) with relative uncertainties close to 1·10-11.
The mass differences of the five pairs of stable xenon isotopes, 134Xe-132Xe,
129Xe-128Xe and 128Xe-126Xe, were determined by measuring the free-cyclotron
frequency ratios of the corresponding xenon ions in a charge
state of 17+. The high precision was achieved by the use of highly charged ions (q=+17), the long storage time of a single
Xe ion and the application of the fast phase-sensitive frequency-measurement technique PnP (Pulse-and-Phase).
Furthermore, the cyclotron and the axial frequencies were measured simultaneously for first time in this experiment, which
substantially reduced the uncertainty in the determination of the free cyclotron frequency.
The demonstrated ability of PENTATRAP to perform high-precision measurements on an isotope chain of the same element will be applied on isotope chains such as Ca, Sr and Yb, which are of very high importance to laser-spectroscopic experiments for Dark Matter searches.
Moreover, the scientists determined the binding energy of the 37th electron in xenon by measuring the ratio of the
free cyclotron frequencies of 131Xe18+ and 131Xe17+ ions:
The experimental result was compared with two theory values obtained by using two independent different implementations of the multiconfiguration Dirac-Hartree-Fock method. The measured value of 432.4(1.3)(3.4) eV agrees within one-sigma uncertainty with the theoretical values 432.4(3.0) eV and 435.1(1.0) eV.
The comparison is the result of the great collaboration between the theory division of Christoph H. Keitel and our division.
This "proof-of-principle" measurement with PENTATRAP opens the door to future measurements of electron binding energies in very highly charged heavy ions (e.g. in hydrogen-like xenon ions) with an uncertainty of an eV required to perform stringent tests of bound-state QED in strong electromagnetic fields.
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The article has been selected as "Editors' Suggestion".
First experimental and theoretical investigation of the N=82 shell closure below Z=50
In atomic nuclei, the protons and neutrons occupy quantum levels that are separated by energy gaps leading to the simple nuclear shell
model which is comparable to Bohr's model of the atom. According to the shell model, each shell can be filled with a certain maximal number
of protons or neutrons. These so-called "magic" numbers (Z, N=8,20,28,50,82 and N=126) of protons and neutrons are associated with large
energy gaps in the effective single-particle spectrum of the nuclear mean field, revealing shell closures.
The magic numbers are intimately connected to the nuclear interaction and thus represent essential benchmarks for nuclear models. In order to test new, improved theoretical models, in recent years, numerous high-precision experiments have been performed for nuclear-structure studies of isotopes and isotopic chains in the vicinity of shell closures.
New, but weaker shell closures have been found by experiments with light exotic nuclei. E.g. a shell closure at N=32 by precision mass measurements of the exotic calcium isotopes 53Ca and 54Ca (see our news of 19.06.13) and a shell closure at N=34. Significantly less is known for heavier nuclei, in particular for the magic N=82. Recently, the doubly-magic nature of 132Sn (P=50, N=82) was reconfirmed by high-precision laser spectroscopic measurements (see our news of 16.05.19).
Theoretical predictions for the N=82 shell gap rely on precise experimental data for nuclides with Z<50 and N≈82. In recent years, the neutron-rich cadmium isotopes (Z=48) have been investigated by decay-spectroscopy, high-resolution laser spectroscopy (see our news of 07.05.13) and precision mass measurements (see our news of 04.12.15). However, the energies of the low-lying isomers in 129Cd and the N=82 two-neutron shell gap remain unknown.
In a recent article published in Physical Review Letters V. Manea, J. Karthein et al. present the first direct determination of the N=82 shell gap for Z<50 with mass measurements of exotic neutron-rich cadmium isotopes and isomers between 124Cd and 132Cd. The experiments were performed with the ISOLTRAP spectrometer at ISOLDE /CERN , Geneva, exploiting all mass-measurement techniques of ISOLTRAP. Depending on the half-life and production yield of the ion of interest, the mass determination at ISOLTRAP can be performed either by the time-of-flight ion cyclotron resonance technique (ToF-ICR) using the precision Penning trap or by performing time-of-flight mass spectrometry with the multireflection time-of-flight mass separator (MR-ToF MS, see, for example, our news of 19.06.13).
The masses of 131,132Cd were determined with the MR-ToF MS. The masses of the other studied cadmium isotopes were determined with the
precision Penning trap by measuring their cyclotron frequency (as singly charged ions) in the trap.
The Penning-trap measurements of 124,126,128,131Cd were performed with the ToF-ICR method, including Ramsey-type excitations. For 127,129Cd the ion beam was a mixture of ground and isomeric state (J = 3/2+ and J = 11/2-). In 2015, these two states could not be separated by a long-excitation ToF-ICR measurement due to the short half-lives. In the present work, the recently developed phase-imaging ion-cyclotron-resonance (PI-ICR) method was used instead (see our news of 19.02.13 and 11.02.14) to separate the 11/2- and 3/2+ states of 129Cd. The ordering of the low-lying isomers in 129Cd and their energies were determined, establishing the inversion of the 11/2- and 3/2+ states in 129Cd. It shows that the h11/2 neutron orbital is key for the evolution of the N=82 shell gap towards Z=40.
The new mass of 132Cd allowed addressing a broader range of nuclear models via determination of the N=82 two-neutron shell gap Δ2n(Z, N) = S2n(Z, N) - S2n (Z, N + 2) (S2n: two-neutron separation energy). This quantity involves only even nuclei and 132Cd (Z=48, N=84) yields the first Δ2n(Z, N) value below the doubly magic 132Sn (Z=50, N=82). The new data reveal a peak of Δ2n(Z, N) at the proton magic number Z=50 and thus confirm the phenomenon of "mutually enhanced magicity" at 132Sn (i.e. increased stability associated with neutron magic number when the proton number is magic, and vice versa).
The measured data were interpreted in comparison to the large-scale shell model and to new calculations made with a beyond-mean-field (BMF) approach, as well as the ab initio valence-space in-medium similarity renormalization group (VS-IMSRG). The BMF model reproduces the effect, but underestimates the size of the drop of Δ2n(Z, N) below Z<50. The VS-IMSRG approach shows an off-set to experiment, but describes the peak at the proton magic number Z=50 qualitatively.
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The KATRIN experiment narrows the mass of neutrinos down further
In 1930 Wolfgang Pauli postulated a new neutral particle to explain the β-decay. It was named "neutrino" by Enrico Fermi in 1933 and in
1956 the existence of neutrinos was experimentally proved. Apart from photons, neutrinos are the most abundant elementary particles in
the universe. About hundred trillion neutrinos are traversing every human per second.
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.
So far, only upper limits of the neutrino mass could be determined, confirming it to be tiny but not zero, contrary to the prediction of the current Standard Model of particle physics. Thus, neutrinos and their small non-zero masses play a prominent role in the evolution of large-scale structures in the cosmos, as well as in the world of elementary particles, where their small mass scale points to new physics beyond known theories.
Precision measurements of the kinematics of weak interactions represent the only model independent approach to determine the absolute scale of neutrino masses. The β-decay of 3H (tritium) to 3He is the most promising candidate for the direct mass determination of the neutrino. A neutrino mass larger than zero modifies the shape of the energy spectrum of the emitted β-electron near the high-energy endpoint. The KArlsruhe TRItium Neutrino (KATRIN) experiment located at the Karlsruhe Institute of Technology (KIT) has been designed to measure the mass of the electron antineutrino directly with an unprecedented sensitivity of 0.2 eV/c2.
In a recent article published in Physical Review Letters the KATRIN collaboration, formed by 20 institutions from 7 countries, presents the results of a first four-week science run in spring 2019. During this measurement campaign, high statistics energy spectra of electrons from tritium β-decay were collected. Subsequently, three international analysis teams worked separately on extracting the first KATRIN neutrino mass result. No team had the complete information to prematurely deduce the neutrino mass result before completion of the final analysis step. To coordinate their final steps, the analysts met for a week-long workshop at KIT in mid-July 2019. Their analysis programs independently yielded identical results. They limit the absolute mass scale of neutrinos to a value of less than 1.1 electron-volt (eV)/c2 at 90% confidence. Thus, a half of millions of the neutrinos weigh less than one electron, the second lightest elementary particle.
With the now established world-leading upper limit on the neutrino mass, the KATRIN experiment has taken its first successful step in elucidating unknown properties of neutrinos. The KATRIN collaboration looks forward to further significant improvements of the neutrino mass sensitivity and in the search for novel effects beyond the Standard Model of Particle Physics.
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The article has been selected for a Viewpoint in Physics. Please read also the viewpoint on the article by R. Brugnera.
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First experimental constraint on the coupling strength of antimatter with dark matter candidate
Humans have always been fascinated by the observation of the starry sky und tried to find regularities and fundamental laws in order to understand and predict the observable phenomena. It was not until the 20th century, however, that scientists realized that the stars and the other visible astronomical objects are made of ordinary matter and thus obey the already well-known laws of physics. In the early 1930s, the astronomers Jan Hendrik Oort and Fritz Zwicky postulated the existence of unvisible ("dark") ordinary matter to explain several new astronomical observations. But only from the 1960s onwards, it became clear by analyses of the dynamics of galaxies, that there has to be about five times more so-called "dark matter" in our Universe in addition to the well-known visible "baryonic" matter. Moreover, to explain the observed accelerated expansion of the Universe, since the 1990s the physicists assume that about 70% of the energy density of the Universe consists of so-called "dark energy".
The microscopic properties of "dark matter" and "dark energy" remain unknown. Many physicists assume that dark matter particles are
electrically neutral and are only weakly interacting with ordinary matter. One dark-matter candidate are the so-called axions, which
are light and spinless bosons that were already postulated as hypothetical particles in 1977. Originally, to explain the experimentally
observed complete symmetry of the strong interaction, which seems unnatural considering the symmetry violation involved in the weak
Neither axions nor other dark-matter candidate particles could be experimentally detected so far. Not only the nature of dark matter and dark energy is not yet understood. Even the matter-antimatter imbalance of the ordinary visible matter, that accounts for about 5% of the total energy density of the Universe, still lacks any consistent explanation.
At present, we have no direct information on the strength of the interaction between axions and antimatter. According to the fundamental and so far always experimentally confirmed charge-, parity- and time-reversal (CPT) invariance of the Standard Model, particles and antiparticles have equal coupling strengths. For example, if antiprotons couple stronger to axions or axion-like particles (ALPs) than protons, such an asymmetric CPT-odd (i.e. charge-reversal) coupling could provide a link between dark matter and the matter-antimatter imbalance in the Universe.
In a recent article published in "Nature" C. Smorra, the BASE collaboration and the Budker group from the Helmholtz Institute Mainz report on the direct experimental search for interactions between antimatter and dark matter. The measurement was performed at the Antiproton Decelerator (AD) of CERN, Geneva, utilizing a novel two-particle spectroscopy method in an advanced cryogenic multi-Penning trap system. More information about the used novel spectroscopy method can be found in our news of 18.10.17 and the corresponding "Nature" article (Nature 550, 371–374 (2017) ). The result reported in the present article searches for periodic changes of the antiproton spin precession frequency as signature for the axion-antiproton interaction. The measurements allowed setting considerable first constraints on the possible strength of the interaction between ultralight axion-like particles with antiprotons, which are five orders of magnitude more sensitive than astrophysical limits.
According to theory, the interaction of fermions, e.g. antiprotons, with axions causes a spin precession analogous to the known precession
of the particle spin in an external magnetic field (Larmor precession). These spin-precession effects from ultralight axions have a
characteristic frequency governed by the mass of the underlying particle. Thus, a potential coupling of antiprotons and axions can be
detected by a Fourier analysis of the spin-flip resonance data acquired with a single antiproton in a Penning trap. The researchers found
no significant indication of a periodic interaction of the antiproton spin with the axion field.
The performed data analysis constrained the axion-antiproton interaction parameter to values greater than 0.6 GeV.
In the future, improved limits will be provided by more precise measurements of the antiproton spin precession frequency, and similar searches for interaction effects with axions can be performed for other antiparticles, such as positrons and anti-muons.
Further information also in the press release of the MPIK .
Further press releases:
- Press Release Johannes Gutenberg University Mainz (idw )
- BASE News
- CERN News
- EurekAlert! Science News: University Mainz | RIKEN
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