Physics with Cold Polarised Positronium

1. Physics with Cold Polarised Positronium David B. Cassidy & Allen P. Mills Department of Physics and Astronomy, University of California, Riverside, USA cassidy@physics.ucr.edu Jefferson Lab March 2009

2. Positronium Although electrons and positrons mutually annihilate each other, they can co-exist for a short time in a bound state called positronium (chemical symbol Ps) Ps can exist in different configurations depending on the relative spin states of the positron and the electron. These are known as para-positronium (p-Ps), with total spin s = 0 and ortho positronium (o-Ps) with s = 1.

3. How long does positronium live? The Ps lifetime against annihilation depends on the number of photons the atom decays into. For s=1 this must be an odd number and for s = 0 it must be even The Ps decay rates are The ground state Ps vacuum lifetimes (inverse of the decay rate) are τp-Ps = 125 ps τo-Ps = 142 ns

4. Energy levels of hydrogen and positronium Gross energy levels are half that of H. The large Ps magnetic moment makes the hyperfine splitting much larger than is the case for H R. Ley, Appl. Surf. Sci. 194 301(2002)

5. How do we make Ps atoms? Porous silica (and similar materials) It’s actually quite easy to make Ps atoms: positrons directed onto a silica film will pick up an electron and make Ps in the bulk which may then diffuse to the surface or internal voids. e+ Ps SEQ In some samples Ps may also become trapped in a surface state where they can interact with other Ps atoms and form Ps2 molecules. a-SiO2 Ps2

6. We can also make Ps on a metal surface: Single crystal metal (Al(111)) Incident Positron pulse ~ 2 keV Positron does not diffuse to surface and annihilates in the bulk Positron trapped in a surface state t ~0.5 ns e+ emission Direct/thermal Ps emission

7. What can happen when Ps atoms interact with each other? • Nothing o-Ps + o-Ps → o-Ps + o-Ps • Spin exchange quenching (SEQ) o-Ps(m=1) + o-Ps(m = -1)→ 2 p-Ps + 2Eh • Positronium molecule (Ps2) formation X+ o-Ps(m = 1) + o-Ps(m = -1)→ X+ Ps2 + Eb

8. A decoupled Surko trap Nitrogen buffer gas e+ lifetime ~ 2 seconds efficiency ~ 20%, 4 Hz duty cycle > 50,000 e+/pulse SF6 cooling gas (~ 1 x 10-8 Torr) e+ lifetime > 500 seconds efficiency ~ 80% Maximum 108 e+ Typical pulse, 5 x 107 e+ Pulse out ~ 15 ns FWHM 50 mCi 22Na source Neon moderator > 106 e+/sec RSI 77, 073106 (2006)

9. The positron beam density can be controlled via the “rotating wall” electric field in the accumulator n2D ~ 3×1010 cm-2 y (mm) n2D ~ 8×109 cm-2 x (mm)

10. 2 kV buncher produces sub-ns pulses

11. Lead fluoride and lead tungstate detectors Al(111) target Prompt peak and later times recorded with different scope gain NIM A 580 1338 (2007)

12. We can analyze each waveform separately, and/or we can automatically determine the parameter fd which is a measure of the amount of o-Ps created fd fd

13. Single atom positronium decay in voids • Single shot lifetime spectra measure the amount and the decay rate of o-Ps. • In porous materials the decay rate, g, depends on the pore size (pick-off). • Ps decays exponentially→ gA gA< gB gB

14. How do lifetime spectra change if Ps atoms interact with each other? • If SEQ occurs, o-Ps atoms are converted to p-Ps atoms and decay rapidly (g ~ 8 ns-1) • If Ps2 molecules are formed these will also decay rapidly (g ~ 4 ns-1) • Present resolution cannot tell the difference. • Decay becomes non-linear: decay due to Ps-Ps interactions Ordinary (pick-off) decay parameter b describes strength of SEQ and/or Ps2 formation

15. If there is a Ps surface state then heating the sample will thermally desorb Ps, leading to an increase in the Ps fraction Indicates no Ps surface state Ps2 formation suppressed Sample has a Ps surface statePs2 formation allowed We were lucky with our samples: they were each suitable for one process and unsuitable for the other

16. Ps2

17. Ps-Ps interactions indicated by density dependent changes in lifetime spectra: the “quenching” effect Form of Q(T) indicates that the Ps-Ps interactions occur via two surface state atoms Nature 449 196 (2007)

18. New system recently installed: higher Ps density

19. The (near) future…… Production of a Bose-Einstein condensate of positronium This requires spin aligned Ps at high densities

20. BEC transition temp

21. How can we observe the formation of a Ps BEC? Doppler broadening of thermal Ps ensemble will be much narrower for the condensed atoms. Can be observed by laser absorption The lifetime difference between thermal and condensed Ps will be negligible.

22. Ps BEC Atom Laser

23. Concluding remarks • Ps2 formation observed, 60 years after it was predicted… • Observation via timing data and sample properties is compelling, but indirect • Excited state of Ps2 allows for a definitive spectroscopic measurement • New area of multi-positronium physics now possible. Beyond Ps2, Ps BEC formation should also be achievable using similar techniques, but polarised beam needed. Thank you for your attention