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Two-Photon Laser Spectroscopy of Antiprotonic Helium and the Antiproton-To-Electron Mass Ratio

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Two-Photon Laser Spectroscopy of Antiprotonic Helium and the Antiproton-To-Electron Mass Ratio LETTER doi:10.1038/nature10260 Two-photon laser spectroscopy of antiprotonic helium and the antiproton-to-electron mass ratio Masaki Hori1,2, Anna So´te´r1, Daniel Barna2,3, Andreas Dax2, Ryugo Hayano2, Susanne Friedreich4, Bertalan Juha´sz4, Thomas Pask4, Eberhard Widmann4, Dezso˝Horva´th3,5, Luca Venturelli6 & Nicola Zurlo6 1 Physical laws are believed to be invariant under the combined trans- the thermal motion of pHe at temperature T stronglypffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi broadens the 1 2 formations of charge, parity and time reversal (CPT symmetry ). measured widths of the laser resonances, by *n 8kBT log (2)=Mc , This implies that an antimatter particle has exactly the same mass where n denotes the transition frequency, kB the Boltzmann constant, and absolute value of charge as its particle counterpart. Metastable M the atom’s mass and c the speed of light. 1 antiprotonic helium (pHe ) is a three-body atom2 consisting of a One way to reach precisions beyond this Doppler limit is provided normal helium nucleus, an electron in its ground state and an anti- by two-photon spectroscopy. For example, the 1s–2s transition fre- proton (p) occupying a Rydberg state with high principal and angu- quency in atomic hydrogen (H) has been measured to a precision of lar momentum quantum numbers, respectively n and l,suchthat 10214 using two counter-propagating laser beams, each with a fre- n < l 1 1 < 38. These atoms are amenable to precision laser spectro- quency of half the 1s–2s energy interval. This arrangement cancels scopy, the results of which can in principle be used to determine the the Doppler broadening to first order6. A similar experiment has been antiproton-to-electron mass ratio and to constrain the equality proposed for antihydrogen (H) atoms7,8, a sample of which was between the antiproton and proton charges and masses. Here we recently confined in a magnetic trap8. However, it is normally difficult report two-photon spectroscopy of antiprotonic helium, in which to apply this to pHe1 because of the small probabilities involved in the 3 1 4 1 p He and p He isotopes are irradiated by two counter-propagating nonlinear transitions of the massive antiproton. Indeed, calculations9 laser beams. This excites nonlinear, two-photon transitions of the show that gigawatt-scale laser powers would be needed to excite them antiproton of the type (n, l) R (n 2 2, l 2 2) at deep-ultraviolet wave- against annihilation within the atom’s lifetime. lengths (l 5 139.8, 193.0 and 197.0 nm), which partly cancel the Nevertheless, we have been able to induce two-photon antiproton Doppler broadening of the laser resonance caused by the thermal transitions of the type (n, l) R (n 2 2, l 2 2) (Fig. 1a), by using the fact motion of the atoms. The resulting narrow spectral lines allowed us that this small transition probability can be enhanced by factor .105 if to measure three transition frequencies with fractional precisions of the counter-propagating beams have frequencies, n1 and n2, such that 9 2.3–5 parts in 10 . By comparing the results with three-body the virtual intermediate state of the two-photon transition lies within 10 quantum electrodynamics calculations, we derived an antiproton- Dnd < 10 GHz of a real state ,(n 2 1, l 2 1). At resonance between the to-electron mass ratio of 1,836.1526736(23), where the parenthetical atom and the laser beams, the antiprotons are directly transferred error represents one standard deviation. This agrees with the between the parent and daughter states via the nonlinear transition, proton-to-electron value known to a similar precision. leaving the population in (n 2 1, l 2 1) unaffected. The first-order 1 Antiprotonic atoms (denoted pX ) can be readily synthesized in a Doppler width is then reduced by a factor of jn1 2 n2j/(n1 1 n2). given element X by replacing the atomic electrons with a negatively In these experiments, transitions were selected for laser excitation charged antiproton2. The substitution takes place spontaneously when between pairs of states with microsecond- and nanosecond-scale life- antiprotons are brought to rest in the substance concerned. However, times against Auger emission of the electron. At the two-photon res- these exotic atoms are usually destroyed within picoseconds by elec- onance, Auger decay left a pHe21 ion behind. No longer protected by tromagnetic cascade mechanisms that result in the annihilation of the electron in the way described above, the ion was rapidly destroyed the antiprotons with the nucleus of X. The pHe1 atom alone has in Stark collisions with other helium atoms. Charged pions emerged microsecond-scale lifetimes even in dense helium targets. The from the resulting antiproton annihilations, passed through an acrylic extreme longevity is due to the fact that the antiprotons trapped in sheet and produced Cherenkov radiation, which was detected by the n < l 1 1 < 38 Rydberg states have almost no overlap with the photomultipliers. The two-photon resonance condition between the nucleus, and furthermore cannot easily de-excite by Auger emission counter-propagating lasers beams and the atom was thus revealed as a of the electron owing to its large binding energy (I < 25 eV) and the sharp spike in the annihilation rate (Fig. 1b). large multiplicity (Dl) of the necessary transition. The electron protects Even under these conditions of enhanced transition probability, the antiproton during collisions with other helium atoms, making megawatt-scale laser pulses of high spectral purity and low phase noise pHe1 amenable to laser spectroscopy. are needed to excite these two-photon transitions and avoid rapid The energy levels of pHe1 have been calculated by three-body dephasing of the transition amplitude10. For this (Fig. 1c), we developed quantum electrodynamics (QED) calculations (ref. 3 and V. I. two sets of Ti:sapphire laserswith pulse lengths of 30–100 nsand among Korobov, personal communication) to precisions of 1 3 1029. The the smallest linewidths reported so far11 (,6 MHz). They were based on calculated values now include nuclear size effects, and relativistic continuous-wave lasers of wavelength 728–940 nm whose frequencies and radiative recoil corrections up to order ma6, where m and a were measured to a precision of ,1 3 10210 using a femtosecond respectively denote the electron mass and the fine structure constant. optical comb12 locked to a Global-Positioning-System-disciplined The fractional measurement precision of single-photon laser spectro- quartz oscillator. This continuous-wave seed beam was pulse-amplified scopy experiments4,5 of pHe1, however, has always been limited to to the necessary 1-MW peak power using a Ti:sapphire oscillator and 1027–10–8 by the Doppler broadening effect. As in normal atoms, amplifier. Spurious modulations in the pulsed laser frequency, or 1Max-Planck-Institut fu¨ r Quantenoptik, Hans-Kopfermann-Strasse 1, D85748 Garching, Germany. 2Department of Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. 3KFKI Research Institute for Particle and Nuclear Physics, H-1525 Budapest, Hungary. 4Stefan Meyer Institut fu¨ r Subatomare Physik, Boltzmanngasse 3, Vienna 1090, Austria. 5Institute of Nuclear Research, H-4001 Debrecen, Hungary. 6Dipartimento di Chimica e Fisica per l’Ingegneria e per i Materiali, Universita` di Brescia & Istituto Nazionale di Fisica Nucleare, Gruppo Collegato di Brescia, I-25133 Brescia, Italy. 484|NATURE|VOL475|28JULY2011 ©2011 Macmillan Publishers Limited. All rights reserved LETTER RESEARCH b c 0.0 p –0.1 Identical laser Achromatic system beam transport Laser 1 –0.2 RF quadrupole decelerator Cherenkov Average PMT signal (V) Average 2.0 2.5 3.0 counter Laser 2 Elapsed time (μs) Heterodyne spectrometer ency a (n, l) = (36, 34) Frequcomb SHG/THG crystal ULE cavity ν1 laser Ti:S amplifier Virtual state e– 2+ ser Δ He CW seedla νd Pulsed laser ν2 laser (35, 33) p illator (34, 32) Ti:S osc Figure 1 | Energy levels, Cherenkov detector signals and experimental c,Thep4He1 atoms were synthesized by decelerating a beam of antiprotons layout for two-photon spectroscopy of pHe1.a, Two counter-propagating using a radio-frequency quadrupole, and allowing them to stop in a cryogenic laser beams induced the two-photon transition (n, l) 5 (36, 34) R (34, 32) in helium target. Two Ti:sapphire pulsed lasers whose optical frequencies were p4He1 via a virtual intermediate state of the antiproton tuned close to the real stabilized to a femtosecond frequency comb were used to carry out the state (35, 33). b, Cherenkov detectors revealed the annihilation of p4He1 spectroscopy. CW, continuous wave; RF, radio frequency; SHG, second- following the nonlinear two-photon resonance induced at t 5 2.4 ms (blue). harmonic generation; THG, third-harmonic generation; ULE, ultralow When one of the lasers was detuned from resonance frequency by 2500 MHz, expansion. the two-photon signal abruptly disappeared (red). PMT, photomultiplier tube. ‘chirp’, induced during this amplification are an important source of measured linewidth (,200 MHz) represents the highest spectral reso- systematic error5,13–15 and were measured using a heterodyne spectro- lution achieved so far for an antiprotonic atom, and is more than an meter11. The precision of this laser system was verified11 to be order of magnitude smaller than the Doppler- and power-broadened ,1.4 3 1029 by measuring some two-photon transition frequencies profile of the corresponding single-photon resonance (36, 34) R (35, in rubidium and caesium at respective wavelengths of 778 and 822 nm. 33) (Fig. 2a) measured under the same target and laser power condi- It was essential to use helium targets of low enough density for the tions.This allows us todetermine the atomic transition frequency witha relaxations caused by collisions between pHe1 and other helium atoms correspondingly higher precision.
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