Direct Measurement of Transition Frequencies in Isolatedp¯He+Atoms, and NewCPT-Viola

Abstract

A radio frequency quadrupole decelerator and achromatic momentum analyzer were used to decelerate antiprotons and produce p 4 He and p 3 He 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 10 ÿ8 . By comparing these with QED calculations and the antiproton cyclotron frequency, we set a new limit of 1 10 ÿ8 on possible differences between the antiproton and proton charges and masses. DOI: 10.1103/PhysRevLett.91.123401 PACS numbers: 36.10.-k, 14.20.Dh, 32.70.Jz Recently, we measured some transition frequencies in antiprotonic 4 He atoms [1] (p 4 He e ÿ ÿ p ÿ 4 He 2 ) to sub-ppm precision [2] at the Antiproton Decelerator (AD) of CERN. By comparing these with three-body QED calculations and using the measured value of the antiproton cyclotron frequency [3], a 60-ppb limit was obtained for the possible differences p between the antiproton and proton charges and masses. Any such difference, however small, would imply CPT violation, i.e., that physical laws are not perfectly invariant under a combined transformation of charge conjugation, parity, and time reversal [4]. The precision of the experiment [2] was limited by the high density of the cryogenic helium targets (atomic density 10 21 cm ÿ3 , corresponding to pressures p 10-200 bars at room temperature) used to stop the 5.3-MeV antiproton beam and produce these atoms. The p 4 He underwent many collisions with helium atoms, resulting in large shifts [1,2,5] in the transition frequencies. We here report on new laser spectroscopy measurements at ultralow target densities of 10 17 cm ÿ3 , some 10 4 times lower than those previously used. These experiments were made possible by a radio frequency quadrupole decelerator (RFQD, Fig. 1) [6], which decelerated the 5.3-MeV antiprotons used previously [2] to energies E 10-120 keV. The collisional shifts thus became negligible (jj 1 MHz) compared to the natural widths of the transitions, so that the observed frequencies were effectively in vacuo. We also extended our high-precision studies to the isotope p 3 He , and to transitions which could not be observed at high densities, thus increasing the number of measured frequencies with small natural widths (ÿ < 50 MHz) from 4 [2] to 13.In these experiments, laser pulses induced antiproton transitions from metastable pHe states ( Fig. 2) with large principal (n 38) and angular momentum (' n) quantum numbers, to states with short Auger lifetimes [1]. The two-body pHe 2 ions formed after Auger emission suffered collisional Stark effects, which caused the rapid annihilation of the antiproton in the helium nucleus. The resulting spike in the rate of annihilations signaled the transition frequency. In previous experiments made at higher densities, collisions produced large frequency shifts (jj 0:5-5:0 GHz) which were of the order of 10 ÿ6 -10 ÿ5 of the measured transition frequencies . The frequencies 0...

Systematic study of the decay rates of antiprotonic helium states

“…Detailed studies have been done for its metastable-state region, where the principal quantum number n and angular momentum l are both around 38, and where lifetimes against annihilation can be as long as several microseconds. Recently stringent CPT-violation limits on the antiproton mass and charge were determined [4] at CERN Antiproton Decelerator using a laser spectroscopic method to measure some transition energies of the pHe + atom to a precision of 50-200 ppb. The measured transition energies were compared with two independent variational three-body calculations with QED corrections by Korobov [5][6][7] and by Kino [8,9].…”

Development of precise nanosecond and picosecond dye lasers for spectroscopy of antiprotonic atoms

“…Some ∼ 20 transition frequencies of the p 4 He + and p 3 He + isotopes [2,3], and the populations [11,12] and Auger rates [13] of the associated states were systematically measured using the laser.…”

“…However, fluctuations in their M. Hori Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Strasse 1, 85748 Garching, Germany M. Hori (B) · A. Dax Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan e-mail: Masaki.Hori@cern.ch frequency and linewidth, and the difficulty in calibrating their wavelengths generally limit the fractional precision attainable with these lasers to around 1 part in 10 6 . Over the years we refined these nanosecond lasers, thereby measuring the pHe + frequencies with successively higher precisions, from 2 parts in 10 7 [2] to 1 part in 10 8 [3,4]. By comparing the results with three-body QED calculations [7][8][9], the antiproton-to-electron mass ratio was recently derived [4].…”

“…The ASACUSA collaboration of CERN has recently carried out several laser [1][2][3][4] and microwave [5] spectroscopy experiments on antiprotonic helium atoms ( pHe + ≡ p + He 2+ + e − ) and ions [6]. In these experiments, laser transitions of wavelengths λ = 264-726 nm were induced between Rydberg states of the antiproton, with principal and angular momentum quantum numbers of n ∼ − 1 ∼ 40.…”

Precise laser spectroscopy of antiprotonic helium at CERN’s Antiproton Decelerator

“…The AD provided pulsed beams of length 200-300 ns containing ∼ 2 × 10 7 antiprotons of energy 5.3 MeV at a repetition rate of 0.01 Hz. These antiprotons were decelerated to ∼ 100 keV using a radiofrequency quadruple decelerator [3,13], before being allowed to stop in a cryogenic helium target (Fig. 2).…”