Laser spectroscopy of the antiprotonic helium atom–molecule {\bar pHe}^+

The metastable antiprotonic helium atom–molecule (in short, atomcule), , which was discovered in 1991 as being the cause of the anomalous longevity of antiprotons with an overall lifetime of about 3 μs, now provides an exotic playground for laser spectroscopy, helped by the existence of numerous long lived states with 1–2 μs lifetimes. This three body system is characterized by a large angular momentum l ∼ 38 of coupled with the electron in the 1s ground orbital, and looks like an exotic hydrogen isotope with many different “nuclear states” as well as an exotic molecule with two centers (He²⁺ and ). So far, a number of laser resonance transitions have been identified. The present talk will cover the following topics: (1) high precision spectroscopy in comparison with three body theories involving the relativistic effect and QED, and (2) hyperfine structure of ;laser-microwave triple resonance. This revised version was published online in August 2006 with corrections to the Cover Date.     

High resolution X ray spectroscopy in light antiprotonic atoms

At the LEAR facility, CERN, antiprotonic Lα transitions in light elements have been investigated with a focussing crystal spectrometer. The high resolution of the experiment allowed for the first time to resolve in H the 2³P₀ state from the close lying states 2³P₂, 2¹P₁, and 2³P₁. In D the corresponding transitions were found to be more than an order of magnitude broader. To a large extent the results for H support the meson exchange model. This revised version was published online in August 2006 with corrections to the Cover Date.     

The nature of the {\bar p} metastability in helium

A new interpretation of the nature of the metastable states of antiprotons in helium is proposed. The first step is the formation of an ion ( ), which shall be called “ioncule”. After its decay a neutral “atomcule” ( ) is formed which was observed in the experiments on metastable antiprotonic helium. This model explains the experiments in a natural way, in particular the quenching by atomic and molecular admixtures and the pressure dependence of quenching in pure helium. At high helium densities the ioncule creates a vacuum bubble in helium. It is shown that the annihilation of slowed-down can be suppressed at helium pressures of ∼ 10 000 bar. This revised version was published online in August 2006 with corrections to the Cover Date.     

High-precision laser spectroscopy of antiprotonic helium atomcules and effects of collisions at high-density conditions

We have performed laser spectroscpy of metastable antiprotonic helium atoms (or “atomcules”) ( He⁺) and have observed a density dependence of the resonance vacuum wavelengths for the known transitions (n,l)=(39,35)→(38,34) and (37,34)→(36,33). They showed linear red-shifts of 0.61±0.01 GHz and 0.22±0.02 GHz per 1 g℞, respectively. With the shift parameters above, the transition vacuum wavelengths were extrapolated to zero-density limits, yielding λ₀ = 597.2570± 0.0003 nm and λ₀ = 470.7220±0.0006 nm, respectively. These values were compared with the result of recent theoretical calculations on the energy of the Coulombic three-body system, including relativistic corrections and the Lamb shift. The agreements between our experimental values and the calculations have become as good as 2×10^-6. This sets a severe constraint on the antiproton charge ( ) and mass ( ) with |Q _p - |/e < 5 × 10^-7 and |M _p - |/M _p < 5 × 10^-7, under a more precisely known constraint on the charge-to-mass ratio. Thus we have opened a new possibility of measuring fundamental constants of the antiproton. This revised version was published online in August 2006 with corrections to the Cover Date.     

Theoretical analysis of collisional shift and broadening of antiprotonic helium spectral lines

Effect of medium density on shift and broadening of ( ) spectral lines is considered in the framework of binary-collision approximation with an effective ( )–He potential including the long-range attraction and short-range repulsion. Quantum calculations of the scattering phase shifts at reasonable values of the potential parameters allow to explain the main features of the observed density effect on the shape of ( ) spectral lines in low-temperature helium. This revised version was published online in August 2006 with corrections to the Cover Date.     

High precision X-ray spectroscopy in hydrogen-like fermionic and bosonic atomic systems

Some time after its formation an exotic atom may be considered a hydrogen-like system consisting of a nucleus and an exotic particle in a bound state. In this situation it is an ideal tool to study cascade properties, while for the innermost orbits it can be used to probe the interaction with the nucleus. From an extended series of experiments using high resolution X-ray spectroscopy for both aspects typical examples are reported and preliminary results are given: 1. To determine the complex scattering length in H the hyperfine transitions have been measured. 2. To determine the pion mass the 5 4 transitions in N have been studied. In all cases a major contribution to the uncertainty originates from the calibration. Therefore a new method is proposed that will establish a universal set of high precision calibration lines for pionic, muonic and electronic systems. This revised version was published online in July 2006 with corrections to the Cover Date.     

High-Precision Spectroscopy of Antiprotonic Helium – First Results from the AD of CERN

New results of the laser and microwave spectroscopy of antiprotonic helium “atomcules” obtained in the first year of operation of the Antiproton Decelerator (AD) facility of CERN are presented. They include the discovery of three new resonant transitions and the determination of the zero-density wavelength of six transitions with an accuracy of 130 ppb in the best case. Auger rates of those states were also determined, and two of them were found to be several orders of magnitude larger than expected from a simple estimate based on the multipolarity Δl, i.e., the jump in angular momentum required for the antiproton to reach the next lower-lying state of ionized He⁺⁺. Furthermore, a first signal of a two-laser microwave triple resonance to measure the hyperfine splitting in antiprotonic helium was observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Antiproton–hydrogen and antihydrogen–hydrogen annihilation at sub‐Kelvin temperatures

The main properties of the interaction of ultra low‐energy antiprotons (E⩽10^-6 a.u.) and antihydrogen with atomic hydrogen are established. Elastic and inelastic cross‐sections and Protonium formation spectrum are calculated within quantum mechanical coupled channels approach. The results of calculations differ strongly from predictions made by extrapolation of semiclassical models to the energy domain of interest. It is shown that the main feature of the observables behaviour is determined by existence of a family of H, H near‐threshold metastable states. This revised version was published online in July 2006 with corrections to the Cover Date.     

Energy dependence of the charge exchange reaction from muonic hydrogen to oxygen

The charge exchange reaction of negative muons from the atom to oxygen has been measured in gaseous mixtures of H₂ + O₂. The measurements were performed at three different relative oxygen concentrations ranging from 0.2% to 0.8% and total pressures 3.5–15 bar. A mean transfer rate of , describing the transfer from the ground state of thermalized atoms to oxygen, was determined. In order to investigate the energy dependence of the transfer rate, Monte Carlo simulations of the thermalization and the muon transfer were carried out. The comparison of measured and simulated time spectra yielded an epithermal transfer rate =3.9 10¹¹ s^-1 in the energy interval 0.12–0.22 eV. The analysis with the model of Two components shows that all measured time spectra can be reproduced with the same set of parameters.     

Auger decay rates of antiprotonic helium

Auger decay rates of the metastable antiprotonic helium ^3,4He e are calculated. The variational method and solution of coupled differential equations are combined to determine the initial metastable state wave function. Besides metastable states, the calculation reveals specific short-lived states of the antiprotonic helium with an essentially different structure of the wave function. An effect of mixture of the wave functions is taken into account to calculate the decay rate for a few metastable states, which are close in energy to the short-lived ones. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of Wave-Function Mixing in Antiprotonic Helium

The effect of wave-function mixing in antiprotonic helium is discussed. This effect manifests itself in a considerable change of the Auger decay rate of long-lived states of ^3,4He e. Properties of short-lived states which influence the decay rate of long-lived ones are described. It is proposed to observe these short-lived states in a direct experiment. This revised version was published online in August 2006 with corrections to the Cover Date.     

Low Temperature Hydrogen Antihydrogen Interactions

In view of current interest in the trapping of antihydrogen ( ) atoms at low temperatures [1–3], we have carried out a full four-body variational calculation to determine s-wave elastic phase shifts for hydrogen antihydrogen scattering, using the Kohn Variational Principle. Terms outside the Born–Oppenheimer approximation have been taken into account using the formalism of Kołos and Wolniewicz [4]. As far as we are aware, this is the first time that these terms have been included in an H scattering calculation. This is a continuation of earlier work on H– interactions [5–7]. Preliminary results differ substantially from those calculated using the Born–Oppenheimer approximation [8–10]. A method is outlined for reducing this discrepancy and taking the rearrangement channel into account. This revised version was published online in August 2006 with corrections to the Cover Date.     

Present and future possibilities of antiproton production from fixed targets at CERN

The CERN Antiproton Accumulator Complex, AAC, stores 3.5 GeV/c antiprotons produced in the collisions between an external 26 GeV/c proton beam and a fixed target. The present operational yield is , which, for a beam of 10¹³ protons every 2.4 s, corresponds to a maximum production rate of . Accumulation rates are from one third to one half of the production rates due to losses during the collection and stacking processes. This performance can be improved somewhat by collecting antiprotons into a larger solid angle using a more powerful collector lens, but the parameters of the AA Complex restrict this potential gain to a factor of only 1.5. Two future improvements: increased proton flux and antiproton focusing within the target can extend the production rate to , but these developments will require more sophisticated targets at the limits of known technologies.     

Antihydrogen synthesis: A progress report

The main suggested routes for atomic antihydrogen ( ) formation will be reviewed in a pedestrian fashion. These are the following reactions: , and (iii) . The present status of the projects related to these reactions, as well as the projects' experimental scope, will also be discussed or referred to. Some speculations will be made regarding the physics that can be yielded by an antihydrogen probe.     

The Standard Model in strong fields: electroweak radiative corrections for highly charged ions

Electroweak radiative corrections to the matrix element are calculated for highly charged hydrogen like ions. The operator represents the parity nonconserving relativistic effective atomic Hamiltonian at the tree level. The deviation of these calculations from the calculations valid for the neutral atoms demonstrates the effect of the strong field. This revised version was published online in August 2006 with corrections to the Cover Date.     

BV Structure of the Cohomology of Nilpotent Subalgebras and the Geometry of(W) Strings

Given a simple, simply laced, complex Lie algebra corresponding to the Lie group G, let be thesubalgebra generated by the positive roots. In this Letter we construct aBV algebra whose underlying graded commutative algebra is given by the cohomology, with respect to , of the algebra of regular functions on G with values in . We conjecture that describes the algebra of allphysical (i.e., BRST invariant) operators of the noncritical string. The conjecture is verified in the two explicitly known cases, ₂ (the Virasoro string) and ₃ (the string).     

The COMPLIS experiment on neutron deficient Au, Pt and Ir isotopes

The COMPLIS (COllaboration for spectroscopic Measurements using a Pulsed Laser Ion Source) experiment is carried out at the ISOLDE Booster facility at CERN. The aim is to measure the hyperfine structure and the isotope shift of a series of radioactive laser desorbed atoms by multiple step laser ionization. This technique is especially suitable for refractory elements not directly available as beams from the isotope separator. By collecting a primary radioactive beam, it is possible to delay the laser desorption as long as necessary to obtain the particular daughter of interest. Laser spectroscopy measurements have been carried out on very neutron deficient gold, platinum and iridium isotopes. Magnetic moments μ_I, spectroscopic quadrupole moments and changes of the nuclear mean square charge radius along each isotopes series have been extracted. From some results, a detailed comparison with the predictions of the models is presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the Product of Real Spectral Triples

The product of two real spectral triples and , the first of which is necessarily even, was defined by A.Connes as given by and, in the even-even case, by . Generically it is assumed that the real structure obeys the relations , , , where the -sign table depends on the dimension n modulo 8 of the spectral triple. If both spectral triples obey Connes' >-sign table, it is seen that their product, defined in the straightforward way above, does not necessarily obey this -sign table. In this Letter, we propose an alternative definition of the product real structure such that the -sign table is also satisfied by the product.     

NMR dynamics in disordered magnets

The excitation dynamics of site diluted magnets can be described at low energies (long length scales) by magnons, and above a crossover frequency, ω_c, (short length scales) by fractons. The density of fracton states is given by , where is the fracton dimensionality. Dilution gives rise to a characteristic length ξ∝(p−p _c)^ν, wherep _c is the critical concentration for (magnetic) percolation. The crossover frequency ω_c is proportional to ξ^-1[1+(θ/2)], where θ is the rate at which the diffusion constant decays with distance for diffusion on an equivalent network. A fractal dimensionD describes the density of magnetic sites on the infinite network, and . For percolating networks, for all dimensions ≥2. Neutron scattering structure factor measurements by Uemura and Birgeneau compare well with calculations using fracton concepts. Magnons are extended at low energies, while the fracton states are geometrically localized, with a wave function envelope proportional to exp . Here, is the fracton length scale at frequency ω. The exponentd _ϕ lies between 1 andd _min, the chemical length index (of the order of 1.6 in three dimensions). The localization of the magnetic excitations causes a spread in the NMR relaxation rates. A given nuclear moment will experience only a limited set of fracton excitations, resulting in an overall non-exponential decay of the NMR relaxation signal. When strong cross-relaxation is present, the relaxation will be exponential, but the temperature dependence will be strongly altered from the concentrated result.