Metastable Antiprotonic Helium Atoms Research Articles

Transitions between States of the Hyperfine Structure of Antiprotonic $${}^{\mathbf{4}}$$He in Collisions with Atoms of the Medium: Interaction Ab Initio

Collisions of the metastable antiprotonic helium atoms with the atoms of the medium cause, among other processes, transitions between hyperfine structure (HFS) states, as well as shifts and broadening of microwave M1 spectral lines. To obtain the interaction potential matrix ( $$\bar{p}$$ $$\bar{He}^{+}){-}\textrm{He}$$ , the potential energy surface (PES) is calculated by the unrestricted Hartree–Fock method with consideration of the electron correlations within the second-order of perturbation theory (MP2). With this potential, the system of equations of close coupling of the HFS channels is numerically solved; the cross sections and transition rates, shifts, and broadenings of M1 spectral lines are calculated; they are then used to solve the kinetic equation that determines the time evolution of the HFS state density matrix. The results of these calculations are compared with the experimental data and with the results of model calculations.

Method for laser spectroscopy of metastable pionic helium atoms

The PiHe collaboration is currently attempting to carry out laser spectroscopy of metastable pionic helium atoms using the high-intensity π− beam of the ring cyclotron facility of the Paul Scherrer Institute. These atoms are heretofore hypothetical three-body Coulomb systems each composed of a helium nucleus, a π− occupying a Rydberg state, and an electron occupying the 1s ground state. We briefly review the proposed method by which we intend to detect the laser spectroscopic signal. This complements our experiments on metastable antiprotonic helium atoms at CERN.

Effects of impurity molecules on the lifetime of antiprotonic helium atoms

Quenching of metastable antiprotonic helium atoms in collisions with hydrogen and deuterium molecules has been studied using laser spectroscopy at CERN’s antiproton decelerator. The temperature dependence of the quenching cross sections of the antiprotonic states ( n, l)=(37,34), (38,35) and (38,37) has been investigated and a deviation from the Arrhenius law was found at low temperatures. In case of the state (38,37) with deuterium, detailed measurements revealed that the quenching cross section levels off at low temperatures indicating a strong quantum tunneling effect.

Analog Cherenkov detectors used in laser spectroscopy experiments on antiprotonic helium

We describe some acrylic Cherenkov detectors read out by gateable fine-mesh photomultipliers, used in laser spectroscopy experiments of metastable antiprotonic helium ( p ̄ He + ) atoms carried out at the LEAR and AD facilities at CERN. The atoms were produced by stopping pulsed antiproton beams in a helium target. Charged particles emerging from the antiproton annihilations produced Cherenkov light in the detector, the time envelope of which consisted of a strong flash from the promptly-annihilating antiprotons, followed by a much longer but less intense tail from the delayed annihilations of the metastable atoms . The photomultiplier was turned off during the initial light flash by reversing the electric potential on its dynodes, thus allowing only the delayed annihilations to be recorded as an analog pulse. The atoms were irradiated with a laser pulse tuned to the characteristic wavelength which stimulated antiproton transitions from a metastable state to a state with a short lifetime against annihilation. The resonance condition between the laser beam and the atom was thus revealed as a sharp spike superimposed on the Cherenkov light pulse. We tested Cherenkov radiators of various sizes and different types of photomultipliers in order to (i) suppress the spurious afterpulsing in the photomultiplier signal caused by the strong flash of Cherenkov light during the prompt annihilation, (ii) evaluate the background caused by π + →μ + →e + decay, and (iii) optimize the linearity, dynamic range, and response time of the detector.

Quenching of metastable antiprotonic helium atoms in collisions with deuterium molecules

Quenching of metastable antiprotonic helium atoms in collisions with deuterium molecules has been studied using laser spectroscopy at CERN's new Antiproton Decelerator facility. The quenching cross-sections of the states (n, l )= (39, 36), (39, 37), and (39, 38) were determined from the decay rates of the states which were observed using the “deuterium-assisted inverse resonance” (DAIR) method. The results revealed a similar (n, l )-dependence of the quenching cross-sections as in the case of hydrogen but the values were smaller by a factor of ∼1.5.

Sub-ppm laser spectroscopy of antiprotonic helium and a CPT-violation limit on the antiprotonic charge and mass.

Six laser-resonant transitions have been detected in metastable antiprotonic helium atoms produced at the CERN Antiproton Decelerator. They include UV transitions from the last metastable states in the v = n-l-1 = 0 and 1 cascades. Zero-density frequencies were obtained from measured pressure shifts with fractional precisions between 1.3 x 10(-7) and 1.6 x 10(-6). By comparing these with QED calculations and the antiproton cyclotron frequency, we deduce that the antiproton and proton charges and masses agree to within 6 x 10(-8) with a confidence level of 90%.

Latest news from PS205 High-precision laser spectroscopy of antiprotonic helium atoms

We have performed laser spectroscpy of metastable antiprotonic helium atoms (or “atomcules”) ( p 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/ℓ, respectively. With the shift parameters above, the transition vacuum wavelengths were extrapolated to zero-density limits, yielding λ 0 = 597.2570 ± 0.0003 nm and λ 0 = 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 (−Q p ) and mass (M p ) that |Q p−Q p |/e < 5 × 10 −7 |M p−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.

Variational expansion for antiprotonic helium atoms

A very accurate variational expansion is suggested for calculation of nonrelativistic energies of the metastable antiprotonic helium atoms ${\mathrm{He}}^{+}\overline{p}.$ This expansion reflects the dual atomic-molecular nature of the system. Convergence of the results as a function of the increasing sets of basis functions shows an accuracy better than ${10}^{\ensuremath{-}10}\mathrm{a}.\mathrm{u}.,$ two orders of magnitude better than in our previous calculations.

Laser measurements of the density shifts of resonance lines in antiprotonic helium atoms and stringent constraint on the antiproton charge and mass

We have performed laser spectroscopy of metastable antiprotonic helium atoms $(\overline{p}{\mathrm{He}}^{+})$ formed in helium media of 0.2--8.0 bars at 5.8--6.3 K and have observed a density dependence of the resonance vacuum wavelengths for the known transitions $(n,l)=(39,35)\ensuremath{\rightarrow}(38,34)$ and $(37,34)\ensuremath{\rightarrow}(36,33).$ They showed linear redshifts of $0.61\ifmmode\pm\else\textpm\fi{}0.01$ GHz and $0.22\ifmmode\pm\else\textpm\fi{}0.02$ GHz per 1 g/l, respectively. With the shift parameters above, the transition vacuum wavelengths were extrapolated to zero-density limits, yielding ${\ensuremath{\lambda}}_{0}=597.2570\ifmmode\pm\else\textpm\fi{}0.0003$ nm and ${\ensuremath{\lambda}}_{0}=470.7220\ifmmode\pm\else\textpm\fi{}0.0006$ nm, respectively. These values, with a 0.5-ppm precision, 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 agreement between our experimental values and the calculations has become as good as $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}.$ This excellent agreement in turn provides a precise value of the antiproton Rydberg constant that surpasses the currently known precision and sets a severe constraint on the antiproton charge $(\ensuremath{-}{Q}_{\overline{p}})$ and the mass ${(M}_{\overline{p}})$ that both $|{Q}_{\mathit{p}}\ensuremath{-}{Q}_{\overline{p}}|/e$ and $|{M}_{\mathit{p}}\ensuremath{-}{M}_{\overline{p}}|{/M}_{p}$ be less than $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7},$ when a more precisely known constraint on the charge-to-mass ratio is combined. Thus we have opened a possibility of determining fundamental constants of the antiproton.

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”) (\({\bar p}\)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 (\(- Q_{\bar p} \)) and mass (\(M_{\bar p} \)) with |Q p - \(Q_{\bar p} \)|/e < 5 × 10-7 and |M p - \(M_{\bar p} \)|/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.

Influence of oxygen admixtures on the lifetime of metastable antiprotonic helium atoms

Two experiments have been performed at the CERN Low Energy Antiproton Ring (LEAR) facility to study the lifetime shortening of metastable antiprotonic helium atoms by the presence of ppm admixtures of oxygen. The change in shape and lifetime of the delayed annihilation time spectra can be reproduced fairly well by fitting a simple four-level model to the data and assuming equal quenching cross sections for all metastable levels. From this we deduce a quenching cross section of $(1.35\ifmmode\pm\else\textpm\fi{}0.03)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15} {\mathrm{cm}}^{2}$ at a temperature of 100 K. An analysis of data from a previous experiment gives a value for the quenching cross section at room temperature of $(1.34\ifmmode\pm\else\textpm\fi{}0.07)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15} {\mathrm{cm}}^{2}.$ Resonant laser deexcitation of the levels $(n,l)=(37,34)$ and $(39,35)$ (the lowest metastable levels of the cascades $\ensuremath{\upsilon}=n\ensuremath{-}l\ensuremath{-}1=2$ and $3,$ respectively) was used to examine the time development of the population of these levels. Quenching cross sections of $(1.32\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15} {\mathrm{cm}}^{2}$ and $(1.90\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15} {\mathrm{cm}}^{2}$ were deduced for the $\ensuremath{\upsilon}=2$ and $\ensuremath{\upsilon}=3$ cascades from these studies at 100 K.

Laser spectroscopic studies of state-dependent collisional quenching of the lifetimes of metastable antiprotonic helium atoms

A laser spectroscopic method was developed to directly observe the lifetimes of individual states in metastable antiprotonic helium atoms $(\overline{p}{}^{4}{\mathrm{He}}^{+}).$ Collisional effects with surrounding helium atoms were studied in cryogenic gas targets at pressures between 0.1 and 9.0 bars and at temperatures between 5.5 and 7.0 K. The metastable state $(n,\mathcal{l})=(39,35)$ showed a lifetime of 1.5 $\ensuremath{\mu}\mathrm{s}$, regardless of density, whereas the lifetime of the energetically lower-lying state $(37,34)$ was found to shorten from 1.3 to 0.1 $\ensuremath{\mu}$s with increasing density. The lifetimes of the states $(38,34)$ and $(36,33)$ were determined to be 10 ns and 5 ns, respectively, showing the effects of the internal Auger process. When extrapolated to zero density, the measured lifetimes agreed with the results of calculations for the radiative and Auger widths. Using the fact that the state $(37,34)$ becomes short lived at high density, the resonance $(38,35)\ensuremath{\rightarrow}(37,34)$ at 529.62 nm was detected.

Instrumentation for laser-induced annihilation spectroscopy of metastable antiprotonic helium atoms

We have established a technique for laser resonance spectroscopy of metastable antiprotonic helium atoms (p̄He +) by successfully observing laser-induced annihilation. In this paper, we describe the full instrumentation of the experiment in detail, namely, the particle detectors, the laser system, the cryostat and the helium gas target, the data acquisition and analysis method. For an effective laser triggering selective to 3% metastable antiprotons, a highly efficient detection system for antiproton annihilation was required, with a suppressed inefficiency of less than a percent. Guided by simulations, we designed a system of seven lead-scintillator sandwich counters to detect the charged pions and π 0-decay gamma rays from antiproton annihilation, and achieved 99.7±0.1% efficiency for the detection of annihilation. We employed two identical sets of excimer-pumped dye laser systems with 2–5 mJ output energy per pulse, which were triggered by every metastable antiproton formed in a helium gas target of 0.3–1.0 bar at 4.5–10 k. The resonant deexcitation of the metastable states was detected as a spike-like forced annihilation in the time spectrum, thus revealing the level structure of this exotic atom. This powerful technique enabled us to study also the lifetimes and populations of specific metastable states, by changing the timing and the wavelengths of the laser pulses.

Isotopic effect in the decay of antiprotonic helium

An important decay channel of recently discovered metastable antiprotonic helium atoms is the emission of an electron, i.e., the Auger transition. The rate of this process for a number of states of the 3He\barpe, 4He\barpe and 6He\barpe systems is calculated and a substantial isotopic effect is found. While the reduced mass of heavy particles increases slightly with increasing nucleus mass, the Auger transition rate decreases by about a factor of three. The explanation of this strong amplification is given and the role of this effect in experiments is discussed.

Metastable antiprotonic helium atoms: Past, present and future

This review briefly summarises the experimental studies carried out on metastable antiprotonic helium over the last few years, and points out a possible way ahead, assuming that very low-energy antiprotons continue to be available. Very recently, an abundance of new results has been found by the Japanese-European collaboration working in these exotic atoms at LEAR, the Low Energy Antiproton Ring at CERN. Together with tremendous progress in several of the techniques needed for antihydrogen synthesis, this has provided new hope that some way will be found to continue this fruitful line of research after the closure of this excellent machine.

Analog measurement of delayed antiproton annihilation time spectra in a high intensity pulsed antiproton beam

Abstract An analog detection system has been developed to measure delayed antiproton annihilation time spectra for laser resonance spectroscopy of metastable antiprotonic helium atoms using the high-intensity pulsed beam of antiprotons from LEAR at CERN.

Laser-induced resonant transition at 470.724 nm in the v=n-l-1=2 cascade of metastable antiprotonic helium atoms.

A laser-induced resonant transition in metastable antiprotonic helium atoms has been found at a wavelength of 470.724{plus_minus}0.002 nm and assigned to the transition ({ital n},{ital l})=(37,34){r_arrow}(36,33). From the time evolution of the laser resonance intensity, the decay rate of the (37,34) state was determined to be 1.17{plus_minus}0.09 {mu}s{sup {minus}1}, about 60% larger than the theoretical radiative decay rate. The sum of the initial populations in the {ital v}={ital n}{minus}{ital l}{minus}1=2 decay chain was 23{plus_minus}4% of the total number of metastable atoms formed.

Phase and density dependence of the delayed annihilation of metastable antiprotonic helium atoms in gas, liquid, and solid helium.

Systematic studies of the phase and density dependence of the delayed annihilation time spectra of antiprotons (DATS) in gaseous, liquid, and solid helium have been performed using the low energy antiproton ring at CERN. The results show a small but significant dependence of DATS on the phase and density of the helium medium. The average lifetime of antiprotons ([ital T][sub [ital a][ital v]]) in gas at 10--30 K was found to be 4.03[plus minus]0.02 [mu]s. In liquid helium [ital T][sub [ital a][ital v]] decreases with density from 3.0 [mu]s at 1 bar to 2.5 [mu]s at 60 bars, while in solid helium its value is 2.14[plus minus]0.03 [mu]s, 20% shorter than that in liquid helium of the same density. There is no change of DATS betweeen normal 1-bar liquid and superfluid helium. An isotope effect between [sup 4]He and [sup 3]He similar to the one previously observed in the gas phase has also been found for liquid helium. All the data can be fitted fairly well with a simple three-level chain decay model with describes the general structure of DATS, characterized by downaward curving behavior on a logarithmic scale at later times and by the presence of a short-lived component.more » No delayed annihilation was observed in lithium.« less

Exotic states of hadronic atoms

Recenly discovered metastable antiprotonic helium atoms are described.

Laser studies of the decay chain of metastable antiprotonic helium atoms.

Laser studies of metastable antiprotonic helium atoms, which we recently initiated by observing a sharp increase of the antiproton annihilation rate induced by laser-stimulated resonant transitions, have been extended. With a single laser tuned to the resonance already found at 597.26 nm, we have now established the time dependence of the upper state population. With two lasers ignited at variable time separation, we also studied the feeding of the upper state from higher atomic levels. The initial populations and level lifetimes of excited exotic atoms were determined.