D. Barna

Two-photon laser spectroscopy of antiprotonic helium and the antiproton-to-electron mass ratio

Physical laws are believed to be invariant under the combined transformations of charge, parity and time reversal (CPT symmetry). This implies that an antimatter particle has exactly the same mass and absolute value of charge as its particle counterpart. Metastable antiprotonic helium (He+) is a three-body atom consisting of a normal helium nucleus, an electron in its ground state and an antiproton () occupying a Rydberg state with high principal and angular momentum quantum numbers, respectively n and l, such that nu2009≈u2009lu2009+u20091u2009≈u200938. These atoms are amenable to precision laser spectroscopy, the results of which can in principle be used to determine the antiproton-to-electron mass ratio and to constrain the equality between the antiproton and proton charges and masses. Here we report two-photon spectroscopy of antiprotonic helium, in which 3He+ and 4He+ isotopes are irradiated by two counter-propagating laser beams. This excites nonlinear, two-photon transitions of the antiproton of the type (n, l)u2009→u2009(nu2009−u20092, lu2009−u20092) at deep-ultraviolet wavelengths (λ = 139.8, 193.0 and 197.0u2009nm), which partly cancel the Doppler broadening of the laser resonance caused by the thermal motion of the atoms. The resulting narrow spectral lines allowed us to measure three transition frequencies with fractional precisions of 2.3–5 parts in 109. By comparing the results with three-body quantum electrodynamics calculations, we derived an antiproton-to-electron mass ratio of 1,836.1526736(23), where the parenthetical error represents one standard deviation. This agrees with the proton-to-electron value known to a similar precision.

Antiproton magnetic moment determined from the HFS of pHe

Abstract We report a determination of the antiproton magnetic moment, measured in a three-body system, independent of previous experiments. We present results from a systematic study of the hyperfine (HF) structure of antiprotonic helium where we have achieved a precision more than a factor of 10 better than our first measurement. A comparison between the experimental results and three-body quantum electrodynamic (QED) calculations leads to a new value for the antiproton magnetic moment μ s p ¯ = − 2.7862 ( 83 ) μ N , which agrees with the magnetic moment of the proton within 2.9 × 10 − 3 .

Buffer-gas cooling of antiprotonic helium to 1.5 to 1.7 K, and antiproton-to–electron mass ratio

Exotic molecule tests fundamental symmetry Spectroscopy of exotic molecules can offer insight into fundamental physics. Hori et al. studied the transition frequencies of an unusual helium atom in which one of the two electrons was substituted by an antiproton, the negatively charged antiparticle partner of the proton (see the Perspective by Ubachs). The antiprotonic helium was cooled down to low temperatures to allow the frequencies to be measured with high precision. The extracted mass of the antiproton (relative to the electron mass) was in good agreement with previous measurements of the proton mass. This finding is in keeping with the implications of the combined charge, parity, and time-reversal symmetry of physical laws. Science, this issue p. 610; see also p. 546 Spectroscopy of a cold exotic molecule yields a precise value of the antiproton mass relative to the mass of the electron. Charge, parity, and time reversal (CPT) symmetry implies that a particle and its antiparticle have the same mass. The antiproton-to-electron mass ratio Mp¯/me can be precisely determined from the single-photon transition frequencies of antiprotonic helium. We measured 13 such frequencies with laser spectroscopy to a fractional precision of 2.5 × 10−9 to 16 × 10−9. About 2 × 109 antiprotonic helium atoms were cooled to temperatures between 1.5 and 1.7 kelvin by using buffer-gas cooling in cryogenic low-pressure helium gas; the narrow thermal distribution led to the observation of sharp spectral lines of small thermal Doppler width. The deviation between the experimental frequencies and the results of three-body quantum electrodynamics calculations was reduced by a factor of 1.4 to 10 compared with previous single-photon experiments. From this, Mp¯/me was determined as 1836.1526734(15), which agrees with a recent proton-to-electron experimental value within 8 × 10−10.

Improved study of the antiprotonic helium hyperfine structure

We report the initial results from a systematic study of the hyperfine (HF) structure of antiprotonic helium (n, l) = (37, 35) carried out at the antiproton decelerator (AD) at CERN. We performed laser–microwave–laser resonance spectroscopy using a continuous wave (CW) pulse-amplified laser system and microwave cavity to measure the HF transition frequencies. Improvements in the spectral linewidth and stability of our laser system have increased the precision of these measurements by a factor of 5 and reduced the linewidth by a factor of 3 compared to our previous results. A comparison of the experimentally measured transition frequencies with three-body QED calculations can be used to determine the antiproton spin magnetic moment, leading towards a test of CPT invariance.

Production of ultra‐slow antiproton beams

We have recently succeeded in decelerating and confining millions of antiprotons, 50 times more efficiently than conventional methods, in an electromagnetic trap. These antiprotons were cooled by preloaded electron plasma to an energy below an electronvolt. They were then extracted out of the magnetic field of 2.5 T and transported typically at 250 eV along a beamline, designed for efficient transport at 10–1000 eV. This unique beam from our apparatus named MUSASHI opens up a new field of atomic and nuclear physics probed by ultra‐slow antiprotons.In this paper, the whole experimental setup and procedure will be overviewed: deceleration, capture, cooling and extraction of antiprotons will be discussed in detail, including technical description of diagnostic devices.

Observation of the 1154.9 nm transition of antiprotonic helium

The resonance transition (n, l) = (40, 36) ? (41, 35) of the antiprotonic helium (He+) isotope at a wavelength of 1154.9?nm was detected by laser spectroscopy. The population of He+ occupying the resonance parent state (40, 36) was found to decay at a rate of 0.45 ? 0.04 ?s?1, which agreed with the theoretical radiative rate of this state. This implied that very few long-lived He+ are formed in the higher-lying states with principal quantum number n ? 41, in agreement with the results of previous experiments.

First observation of two hyperfine transitions in antiprotonic 3He

We report on the first experimental results for microwave spectroscopy of the hyperfine structure of p¯3He+. Due to the helium nuclear spin, p¯3He+ has a more complex hyperfine structure than p¯4He+, which has already been studied before. Thus a comparison between theoretical calculations and the experimental results will provide a more stringent test of the three-body quantum electrodynamics (QED) theory. Two out of four super-super-hyperfine (SSHF) transition lines of the (n,L)=(36,34) state were observed. The measured frequencies of the individual transitions are 11.12559(14) GHz and 11.15839(18) GHz, less than 1 MHz higher than the current theoretical values, but still within their estimated errors. Although the experimental uncertainty for the difference of these frequencies is still very large as compared to that of theory, its measured value agrees with theoretical calculations. This difference is crucial to be determined because it is proportional to the magnetic moment of the antiproton.

Microwave spectroscopic study of the hyperfine structure of antiprotonic 3He

In this work, we describe the latest results for the measurements of the hyperfine structure of antiprotonic 3 He. Two out of four measurable super–super-hyperfine (SSHF) transition lines of the (n,L) = (36, 34) state of antiprotonic 3 He were observed. The measured frequencies of the individual transitions are 11.125 48(08) GHz and 11.157 93(13) GHz, with the increased precisions of about 43% and 25%, respectively, compared to our first measurements with antiprotonic 3 He (Friedreich et al 2011 Phys. Lett. B 700 1–6). They are less than 0.5 MHz higher with respect to the most recent theoretical values, still within their estimated errors. Although the experimental uncertainty for the difference of 0.032 45(15) GHz between these frequencies is large as compared to that of theory, its measured value also agrees with theoretical calculations. The rates for collisions between antiprotonic helium and helium atoms have been assessed through comparison with simulations, resulting in an elastic collision rate of γe = 3.41 ± 0.62 MHz and an inelastic collision rate of γi = 0.51 ± 0.07 MHz. (Some figures may appear in colour only in the online journal)

Control of plasmas for production of ultraslow antiproton beams

To produce ultraslow antiproton beams, decelerated antiprotons were captured in an electro‐magnetic trap and cooled by collisions with preloaded electrons. This electron cooling feature was nondestructively monitored by measurement of electrostatic oscillations of the confined electron plasma. The radial distribution of the plasma was controlled for efficient cooling and extraction by utilizing rotating wall field technique.

Extraction of ultra-slow antiproton beams and their physics application

The Trap group of ASACUSA collaboration has decelerated and confined millions of cooled antiprotons in an electromagnetic trap, 50 times more efficiently than conventional methods. They were then extracted out of the magnetic field of 2.5 T and transported at 10–500 eV. This unique ultra‐slow antiproton beam from our apparatus named MUSASHI is expected to open up a new field of atomic and nuclear physics.