Improved Measurements of Muonic Helium Ground-State Hyperfine Structure at a Near-Zero Magnetic Field.

Muonic helium atom hyperfine structure (HFS) measurements are a sensitive tool to test the three-body atomic system and bound-state quantum electrodynamics theory, and determine fundamental constants of the negative muon magnetic moment and mass. The world's most intense pulsed negative muon beam at the Muon Science Facility of the Japan Proton Accelerator Research Complex allows improvement of previous measurements and testing further CPT invariance by comparing the magnetic moments and masses of positive and negative muons (second-generation leptons). We report new ground-state HFS measurements of muonic helium-4 atoms at a near-zero magnetic field, performed for the first time using a small admixture of CH_{4} as an electron donor to form neutral muonic helium atoms efficiently. Our analysis gives Δν=4464.980(20) MHz (4.5 ppm), which is more precise than both previous measurements at weak and high fields. The muonium ground-state HFS was also measured under the same conditions to investigate the isotopic effect on the frequency shift due to the gas density dependence in He with CH_{4} admixture and compared with previous studies. Muonium and muonic helium can be regarded as light and heavy hydrogen isotopes with an isotopic mass ratio of 36. No isotopic effect was observed within the current experimental precision.

First observation of a (1,0) mode frequency shift of an electron plasma at antiproton beam injection.

The frequency shift of the center-of-mass oscillation, known as the (1,0) mode, of a trapped electron plasma and, furthermore, its time evolution were observed during the cooling of an injected antiproton beam for the first time. Here, antiprotons mixed with the electrons did not follow faster electron oscillations but contributed to the modification of the effective potential. The time evolution of the plasma temperature, deduced from the frequency shift of the excited (3,0) mode, suggested that there was an abnormal energy deposition of the antiproton beam in the electron plasma before thermalization.

Target structure induced suppression of the ionization cross section for very low energy antiproton-hydrogen collisions.

Low energy antiprotons have been used previously to give benchmark data for theories of atomic collisions. Here we present measurements of the cross section for single, nondissociative ionization of molecular hydrogen for impact of antiprotons with kinetic energies in the range 2-11 keV, i.e., in the velocity interval of 0.3-0.65 a.u. We find a cross section which is proportional to the projectile velocity, which is quite unlike the behavior of corresponding atomic cross sections, and which has never previously been observed experimentally.

Synthesis of cold antihydrogen in a cusp trap.

We report here the first successful synthesis of cold antihydrogen atoms employing a cusp trap, which consists of a superconducting anti-Helmholtz coil and a stack of multiple ring electrodes. This success opens a new path to make a stringent test of the CPT symmetry via high precision microwave spectroscopy of ground-state hyperfine transitions of antihydrogen atoms.

A source of antihydrogen for in-flight hyperfine spectroscopy.

Antihydrogen, a positron bound to an antiproton, is the simplest antiatom. Its counterpart-hydrogen--is one of the most precisely investigated and best understood systems in physics research. High-resolution comparisons of both systems provide sensitive tests of CPT symmetry, which is the most fundamental symmetry in the Standard Model of elementary particle physics. Any measured difference would point to CPT violation and thus to new physics. Here we report the development of an antihydrogen source using a cusp trap for in-flight spectroscopy. A total of 80 antihydrogen atoms are unambiguously detected 2.7 m downstream of the production region, where perturbing residual magnetic fields are small. This is a major step towards precision spectroscopy of the ground-state hyperfine splitting of antihydrogen using Rabi-like beam spectroscopy.

Radial compression of an antiproton cloud for production of intense antiproton beams.

We report here the radial compression of a large number of antiprotons ( approximately 5 x 10(5)) in a strong magnetic field under ultrahigh vacuum conditions by applying a rotating electric field. Compression without any resonant structures was demonstrated for a range of frequencies from the sideband frequency of 200 kHz to more than 1000 kHz. The radial compression achieved is a key technique for synthesizing and manipulating antihydrogen atoms and antiprotonic atoms.

Ionization of helium and argon by very slow antiproton impact.

The total cross sections for single ionization of helium and single and double ionization of argon by antiproton impact have been measured in the kinetic energy range from 3 to 25 keV using a new technique for the creation of intense slow antiproton beams. The new data provide benchmark results for the development of advanced descriptions of atomic collisions and we show that they can be used to judge, for the first time, the validity of the many recent theories.

Determination of the antiproton-to-electron mass ratio by precision laser spectroscopy of pHe+.

A femtosecond optical frequency comb and continuous-wave pulse-amplified laser were used to measure 12 transition frequencies of antiprotonic helium to fractional precisions of (9-16)x10(-9). One of these is between two states having microsecond-scale lifetimes hitherto unaccessible to our precision laser spectroscopy method. Comparisons with three-body QED calculations yielded an antiproton-to-electron mass ratio of Mp/me=1836.152674(5).

Observation of cold, long-lived antiprotonic helium ions.

Cold, two-body antiprotonic helium ions p 4He2+ and p 3He2+ with 100-ns-scale lifetimes, occupying circular states with the quantum numbers ni=28-32 and li=ni-1 have been observed. They were produced by cooling three-body antiprotonic helium atoms in an ultra-low-density helium target at temperature T approximately 10 K by atomic collisions, and then removing their electrons by inducing a laser transition to an autoionizing state. The lifetimes of p 3He2+ against annihilation induced by collisions were shorter than those of p 4He2+, and decreased for larger-ni states.

Confinement of a large number of antiprotons and production of an ultraslow antiproton beam.

We have used a radio frequency quadrupole decelerator to decelerate antiprotons emerging from the CERN Antiproton Decelerator from MeV- to keV-scale energy, and collected five decelerated pulses in a multiring trap. Some 5 x 10(6) antiprotons were stacked in this way. Cooling of the trapped antiprotons by a simultaneously trapped electron plasma was studied nondestructively via shifts in plasma mode frequencies. We have also demonstrated the first step in extracting a 10-500 eV antiproton beam from the trap.

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%.

Direct measurement of transition frequencies in isolated pHe+ atoms, and new CPT-violation limits on the antiproton charge and mass.

A radio frequency quadrupole decelerator and achromatic momentum analyzer were used to decelerate antiprotons and produce p4He+ and p3He+ atoms in ultra-low-density targets, where collision-induced shifts of the atomic transition frequencies were negligible. The frequencies at near-vacuo conditions were measured by laser spectroscopy to fractional precisions of (6-19) x 10(-8). By comparing these with QED calculations and the antiproton cyclotron frequency, we set a new limit of 1 x 10(-8) on possible differences between the antiproton and proton charges and masses.

Hyperfine structure of antiprotonic helium revealed by a laser-microwave-laser resonance method.

Using a newly developed laser-microwave-laser resonance method, we observed a pair of microwave transitions between hyperfine levels of the (n,L)=(37,35) state of antiprotonic helium. This experiment confirms the quadruplet hyperfine structure arising from the interaction of the antiproton orbital angular momentum, the electron spin and the antiproton spin as predicted by Bakalov and Korobov. The measured frequencies of nu(+)(HF)=12.895 96+/-0.000 34 GHz and nu(-)(HF)=12.924 67+/-0.000 29 GHz agree with recent theoretical calculations on a level of 6x10(-5).

Primary populations of metastable antiprotonic (4)He and (3)He atoms.

Initial distributions of metastable antiprotonic (4)He and (3)He atoms over principal (n) and angular momentum (l) quantum numbers have been deduced using laser spectroscopy experiments. The regions n = 37-40 and n = 35-38 in the two atoms account for almost all of the observed fractions [(3.0 +/- 0.1)% and (2.4 +/- 0.1)%] of antiprotons captured into metastable states.