Measurements with Positronium

1. Measurements with Positronium P. Crivelli One year fellowship from the SNF for prospective researchers at the host institute: CEA, Saclay (France) A.Badertschera, A.S.Belovb, W. Fetschera, U.Gendottia, S.N.Gninenkob, A.Rubbiaa , D.Sillouc a ETH, Zürich, Switzerland b Inst. Nucl. Research, INR Moscow, Russia c LAPP, France http://neutrino.ethz.ch/Positron

2. Outline • Introduction • Summary of the ETHZ activity 2002-2006 with Positronium • Outlook

3. Positronium • A particularly simple particle-antiparticle system determined by QED: purely leptonic. • Bounds and self-annihilates through the same interaction • Two ground states Orthopositronium (o-Ps) triplet spin state 3S1 Parapositronium (p-Ps) Singlet spin state 1S0 t ≈142ns t≈125ps Compared to p-Ps the long lifetime of o-Ps has an enhancement factor of 103 on the sensitivity to an admixture of new physics.

4. 2002: Exotic three body decay search Badertscher et al., Phys. Lett. B542, (2002)29-34 From our search for an exotic three body decay of o-Ps 4.4x10-5 at 90% CL Theory (2000) We excluded this decay channel as a possible source of the o-Ps lifetime discrepancy

5. 2003: Workshop on Positronium at ETHZ Proceedings, Int. J. Mod. Phys. A19 (2004) No23 The main outcomes • o-Ps is an ideal probe for new physics, i.e. to search for hidden-sectors: extra-dimension, dark matter of mirror particle type or milli-charged particles predicted in GUT • Precision of the lifetime measurement 200 times worse than the recent theoretical calculations -> need of a new experiment with a higher precision to check the QED corrections Positronium = interesting for research

6. 2003: Orthopositronium and new physics Gninenko, Krasnikov, Rubbia, Phys.Rev.D67:075012,2003 • New physics could be signaled by an o-Ps -> invisible decay with an • experimentally interesting branching ratio of the order of 10-8 . • The models that predict such a decay are: • Extra-dimensions • Milli-charged particles • Dark matter of a mirror particle type e e Am m 2 e e g A’m e’ e’

7. The search for o-Ps->invisible decays P. Crivelli, PhD Thesis, No. 16117, ETHZ, Switzerland (2006) The o-Ps invisible decay would appear as an event compatible with zero-energy deposition in a hermetic g-detector surrounding the o-Ps formation region. MPs =1.022MeV • Design criteria: • Hermetic calorimeter: Escaping probability for annihilation photons <109 • Region around the target with less dead material as possible • High fraction of produced o-Ps-> high statistics and background from gammas suppression • Efficient positron tagging system to provide a clean trigger • Veto of charged particle in the crystal used to identify the 1.27 MeV from the 22Na source emitted with the positron and used as a requirement for the trigger.

8. The calorimeter the o-Ps->invisible search P. Crivelli, PhD Thesis, No. 16117, ETHZ, Switzerland (2006) NIM logic for trigger LED pulser Light tight Box HV controller o-Ps formation region VME crate The o-Ps production Target g The 4p BGO calorimeter surrounding the o-Ps formation region (100 BGO crystals kindly lend us from PSI) The positron source The scintillating fiber to tag the positron

9. Results of the o-Ps -> invisible search A. Badertscher et al., hep-ex/ 0609059 Data taking period: 5 months 1.39x1010 triggers Since no event is observed in the signal region, this result provides an upper limit on the o-Ps -> invisible This limit is 7 times more stringent than what was previously reported by the Tokyo group. Simulations and an extrapolation of the data show that performing some improvements in the trigger rate with this experimental setup one could gain a factor 5 in the sensitivity.

10. The slow positron Beam 1 Mbq 22Na source of positron (prepared at PSI) &Tungsten moderator chamber Calorimeter e+ flux Positronium formation region Magnetic coils for positron transportation (quasi-uniform longitudinal field of 70 Gauss) Beam pipe (10-8-10-9 mBar)

11. The goal of the slow positron beam • Fundamental Physics: • A new lifetime measurement of o-Ps with an improved precision to check the QED corrections (the present result are 200 times worse than the recent theoretical calculations). • A search for dark matter of mirror particle type. • Positronium spectroscopy measurements. • Efficient anti-hydrogen production using the reaction: Ps*+pbar->Hbar+e- with the goal of measuring gravity fall for antimatter • Applied Physics: • Characterization of nano-porous materials using the positron annihilation spectroscopy technique (PALS) • Therefore, the final beam construction had to compromise several • design criteria.

12. The design of the slow positron beam • Design criteria: • Definition of the time t0 for the positronium formation in the vacuum cavity using two different solutions: • A Bunched short pulse of positrons • B Precise tagging using secondary electron detection • Monoenergetic beam with varying implantation energy range from 100 eV to 10 keV • Beam intensity of 103 positrons per second at the target region • In case of bunched beam: • 1. pulse duration at the target DT <3 ns for an initial 2.pulse duration of 300 - 400 ns • 3.Repetition rate 0.3-1.0 MHz • 4.High peak/noise ratio, (single) Gaussian shape of the pulse, • Beam spot size at the target position of the order of a few millimeters

13. Scheme of the bunching system Potential seen by the e+

14. Results in e+ bunching compression Alberola et al., Nucl. Instr. Method A 560 (2006) 224-232 ~ 2.3 ns (FWHM) for an Initial pulse of 300ns ~1.4 ns (FWHM) for an initial pulse of 120ns Compression factor 100! (5 times better than reported previously by two groups in Japan) MCP used to measure the time spectra Repetition period ~ 1 ms

15. Slow Positrons ~200 eV Acceleration Electrode 1-20 kV Sample Deflection Plates MCP (START SIGNAL) Secondary electrons Annihilation photons (STOP SIGNAL) The secondary electron detection • The secondary electrons emitted when the positron hit the surface of the target • are used to tag the positrons. Two-fold use: • Tagging system for o-Ps studies in vacuum (definition of positronium formation • Time) • Spectrometer for PALS (Positron Annihilation Lifetime Spectrometry)

16. Results of the e+ time distribution with Al target Pure aluminum target ~700 ps (FWHM) • Time distribution of positrons at the target tagged through the secondary electron detection and the annihilation photons.

17. Summary of the Positronium Activity 2002: -Search for a three body exotic decay of ortho-Positronium (oPs) -Design of a slow positron beam 2003: -Workshop at ETHZ on Positronium Physics -Design of the experiment o-Ps->invisible using an aerogel target -Slow positron DC beam working 2004: -Engineering run for o-Ps-> invisible -Design and construction of the bunching system 2005: -Slow positron bunched beam working -Improvements of the detector for o-Ps-> invisible 2006: -Results of the o-Ps->invisible search -Tagging system

18. List of publications • A. Badertscheret al., Phys. Lett. B542, (2002) 29-34 • P. Crivelli, Can.J.Phys.80:1281-1285 (2002) • S. Gninenko, N. Krasnikov, A. Rubbia, Phys.Rev.D67:075012 (2003) • M. Felcini, S. Gninenko, A. Nyffeler, A. Rubbia : Proceedings, Int. J. Mod. Phys. A19 (2004) No.23 • P. Crivelli, PhD Thesis, No. 16117, ETHZ, Switzerland (2006) • N. Alberola et al., Nucl. Instr. Method A 560 (2006) 224-232 • A. Badertscher et al., hep-ex/ 0609059 • + 2 Diploma and 3 Semester Works

19. 2006 Increase of the beam intensity • A 40 times more intense source have been produced by irradiating a pure Al foil with the proton beam here at PSI . The installation is planned for mid-October. • The moderator efficiency: annealing process and moderator choice have been optimized. The best results were obtained using a pattern of 12 tungsten meshes (improvement of a factor 1.5) The expected number of positrons is: 1000e+/s

20. 2006-2007: Positronium formation Study U. Gendotti, PhD started this year The nearest goal: controlled production of o-Ps to be used for future experiments, new lifetime measurement, invisible decay search of o-Ps in vacuum, positronium spectroscopy measurement, very efficient anti-hydrogen formation. • Study the production of o-Ps in thin SiO2 films with open interconnected porosity (developed as low k-dielectrica for the microelectronics industry) , i.e • the velocity distribution as a function of the target temperature using a time of flight technique (TOF) and the fraction of positronium formed in the target using the positron lifetime annihilation spectroscopy (PALS). Movable slit PMT BGO Lead Magnetic coils e+ beam Positronium formation region & emission in vacuum (~30%) TOF length

21. Design of the TOF and PALS experiment • The designed detector will serve for both, TOF and PALS spectroscopy of the thin SiO2 films is finished. Moving slit • The detector is under construction, the first measurements are • expected for the beginning of next year.

22. Acknowledgements This research is supported by the SNF with a grant for beginner researchers of 1 year. I would like to thanks the PSI for the essential help in preparing the source for both the invisible decay search and the slow positron beam and for lending us the BGO crystals. I’m in debt with Prof. A. Rubbia, Prof. S. Gninenko, Dr. A. Badertscher, Prof. W. Fetscher, Dr. A. Belov, U. Cug Gendotti, Dr. D. Sillou, Leo Knecht and Prof. R. Eichler for being the co-referee of my thesis. And thanks you very much to all of you for your attention…