Sub-Doppler two-photon laser spectroscopy of antiprotonic helium and the antiproton-toelectron

Transcription

1 Sub-Doppler two-photon laser spectroscopy of antiprotonic helium and the antiproton-toelectron mass ratio Fukuoka, August 2012 Masaki Hori Max Planck Institute of Quantum Optics A. Sótér, D. Barna, A. Dax, R. Hayano, S. Friedrich, B. Juhász, T. Pask, E. Widmann, D. Horváth, L. Venturelli, N. Zurlo

2 First a reminder: CPT symmetry: Physical laws are identical under the combined transformations of charge conjugation, parity, and time reversal. CPT theorem: any Lorentz invariant, local quantum field theory with a Hermitian Hamiltonian must have CPT symmetry - from Wikipedia Consequence: particles and antiparticles have the same mass, and additive quantum numbers of same absolute value but opposite sign. mass charge

3 PS179 (1984) Antiproton stopped in matter typically annihilates within < 1 ps, releasing 2 GeV of energy. How this happens: antiproton replaces atomic electrons, electromagnetically cascades (Auger emission, radiative deexciation), and annihilates via strong interaction with nucleus.

4 Antiprotons are long-lived in helium! Empty target Neon Helium Krypton Xenon Long-lived antiprotons I. Iwasaki et al., KEK (1991) T. Yamazaki et al., PS205 at CERN ( ).

5 4 R R p He + (39,35) p He + (37,34) 0 Antiprotonic helium: 3-body atom. Electron in 1s-ground state, p - He 2+ distance (a.u.) Antiproton in Rydberg state n=30-40, l=n+1. Long-lived because the antiproton has little overlap with nucleus Electron cannot be ionized (He has large ionization potential 25 ev). Electron cloud protects antiproton against collisions with other He.

6 Physics motivations for precisely studying this object by laser spectroscopy Determination of antiproton-to-electron mass ratio to 1.3 x Dimensionless fundamental constant of nature. Determination of electron mass in a.u. to 1.3 x 10-9 One of the data points for CODATA2010 average. When combined with cyclotron frequency of antiprotons in a Penning trap measured by TRAP collaboration, comparison of antiproton and proton mass and charge to 7 x CPT consistency test in PDG2012.

7 Relevance to this conference? Theoretical methods developed by Few-Body community was vital to the high-precision study of this atom: * Gaussian expansion method (Kamimura, Kino, Hiyama et al.) Used to calculate the non-relativistic wavefunctions of atom * Complex coordinate rotation method Evaluate atomic states with large coupling to continuum

8 Non-relativistic energy of He ground state calculated by variational methods: Frankowski, Pekeris (1966) (a.u.) Freund, Huxtable, Morgan (1984) Thakkar, Koga (1994) Drake, Yan (1994) Goldman (1998) Drake (1999) Korobov (2000) digits of improvement in 34 years. Non-relativistic bound-state 3-body wavefunction is well-understood.

9 Calculated two-photon transition frequency (n,l)=(36,34) (34,32) V.I. Korobov (2010) Non-relativistic energy MHz Relativistic correction of electron Anomalous magnetic moment of electron One transverse photon exchange Relativistic correction of heavy particles Finite charge radius of helium nucleus 4.7 One-loop self-energy correction Vacuum polarization Recoil corrections order R α 3 (m/m) 1.4 All R α 4 order corrections All R α 5 order corrections Transition frequency (2.1)(0.3) MHz Several parts in seems feasible in the near future.

10 Energy levels of antiprotonic helium atom

11 Laser spectroscopy of antiprotonic helium Resonant laser beam p He e Atomic capture Auger emission 2-body antiprotonic helium ion Annihilation and detection

12 N. Morita et al., PRL 72, 1180 (1994)

13 Experimental and theoretical transition frequencies p 4 He + Kino et al. p 3 He + Korobov et al. (40,35) (39,34) (39,35) (38,34) (38,34) (37,33) (37,35) (38,34) (36,34) (37,33) (37,34) (36,33) (36,33) (35,32) (35,33) (34,32) (34,33) (35,32) (33,32) (32,31) (34,32) (33,31) (32,31) (31,30) (32,31) (31,30) th exp exp (ppb) th exp exp (ppb) M. Hori et al., PRL 91, (2003)

14 Optical spectroscopy of H cesium clocks Laser spectroscopy improved precision by 7 orders of magnitude in 40 years optical atomic clocks T.W. Hänsch (2005)

15 Precision of past single-photon laser experiments limited by thermal Doppler broadening of the resonance lines. Circumvented this by nonlinear sub- Doppler two-photon spectroscopy: 1: Synthesize 10 million atoms 2: Irradiate atom with two counterpropagating laser beams 3: At > 1 MW-scale powers, antiproton can absorb two photons simultaneously from each laser beam, and passes through a virtual intermediate state Γ = ν ν ( ν 1 + ν ) 2 ν 1 + ν 2 kt M

16 Antiproton Decelerator at CERN Antiprotons are created in 26-GeV proton collisions with an iridium target. Deceleration from 3.5 GeV/c to 100 MeV/c. Stoachastic and electron cooling A beam of 3 million antiprotons are provided every minute to ATRAP, ALPHA, AEGIS, ASACUSA 16 CERN (2000)

17 Experimental setup

18 Radiofrequency quadrupole decelerator 2 MeV/m deceleration field! 5 MeV! 50 kev!

19 Experimental setup

20 Frequency stability of laser 40 Heterodyne signal Frequency Chirp (MHz) 20 0 New: Ti:sapphire Time (ns) 2006: cw pulse-amplified dye Number of laser pulses Instantaneous frequency measured by heterodyne spectrometer. Active chirp compensation using intracavity EOM s.

21 4.5 MHz linewidth observed in 6s-8s Cs two-photon transition. ns dye laser 20 mj (2005) Fluorescence sig. (arb.u.) 80 G. Hagel et al. (1999) 82 P. Fendel et al. (2007) Frequency (MHz) 84 Ti:S laser 100 mj (2009) 3 Chirp uncorrected Corrected Frequency (MHz) M. Hori and A. Dax, Optics Letters 34, 1273 (2009).

22 Two-photon signal intensity (arb.u.) (a) (b) First two-photon spectroscopy of antiprotonic 3He and 4He Doppler broadened 4He (c) (d) 4He 139 nm 3He 197 nm 193 nm Laser frequency offset (GHz)

23 Experiment-theory (Korobov) comparison of spin-averaged transition frequency 35

24

25 CODATA 2002 values used for calculation R(4He)=1.673(1) fm, R(3He)=1.844(45) fm R(pbar)= negligible

26 New antiproton-to-electron mass ratio (75) CODATA (23) Nature 475, 484 (2011)

27 Relative precision Antiproton charge and mass over the years LEAR AD RFQD Frequency comb Two-photon CODATA 98 proton / electron mass ratio CODATA Years elapsed

28 Conclusions 1. First sub-doppler two-photon spectroscopy of antiprotonic helium of two transitions in 4He and one transition in 3He. Results agreed with 3-body QED calculations. 2. Determined antiproton-to-electron mass ratio to 1.3 ppb. Result agrees with CODATA proton value (0.4 ppb). 3. Further improvement partially hindered by theoretical uncertainty (QED terms α radiative recoil corrections) Future prospects Cold atoms by collisional buffer cooling? Better lasers, better detectors ELENA (colder antiproton beams at 100 kev of higher luminosity)