Study on positronium Bose-Einstein condensation

Transcription

1 Study on positronium Bose-Einstein condensation Akira Ishida a,*, K. Shu a, T. Murayoshi a, X. Fan a, T. Namba a,s. Asai a, K. Yoshioka b, M. Kuwata-Gonokami a, N. Oshima c, B. E. O Rourke c, R. Suzuki c a Dept. of Physics, Graduate School of Science, and International Center for Elementary Particle Physics (ICEPP), The University of Tokyo, b Photon Science Center (PSC), Graduate School of Engineering, The University of Tokyo, c National Institute of Advanced Industrial Science and Technology (AIST) 2017/06/09 The 3 rd China-Japan Joint Workshop on Positron Science (JWPS2017) Hefei, CHINA 1

2 Contents Introduction Motivation for Ps-BEC matter-antimatter asymmetry gamma-ray laser Our new idea to realize Ps-BEC pulsed dense positron beam + SiO 2 cavity thermalization + laser cooling Ps themalization measurement in cryogenic environment Laser development for Ps cooling 2017/06/09 2

3 Positronium (Ps) Positron Electron Bound state of an electron (e - ) + and a positron (e + ) Lightest and Exotic Atom Sensitive probe for fundamental physics Exotic atom with antiparticle Suitable for exploring the mystery of antimatter Pure leptonic system Experiments and theoretical calculations can be compared in high precision (ppm level) 3

4 Our works Hyperfine splitting (E o-ps -E p-ps ) ppm precision! First direct transition o-ps p-ps CP violation in lepton sector Search for asymmetry of annihilation gammas directions o-ps lifetime Phys. Lett. B 671(2009)219 Prog. Theor. Exp. Phys. 2015, 011C01 Phys. Rev. Lett. 104, (2010) C CP = ± (stat.) ± (sys.) 2017/06/09 4

5 Next target : Positronium Bose-Einstein condensation Bose-Einstein condensation (BEC) Almost all of atoms in a cloud occupy a single quantum state Atoms must be dense and cold T>T c T~T c T<<T c 270 mm 200 mm Important feature BEC is Atom laser Quiet and coherent: Microscopic quantum effect in macroscopic such as matter-wave interference Breakthrough to study microscopic world Before release trapped by double-well After release expand to overlap Spatial image of dense rubidium-87 around T c (critical temperature) of BEC 2017/06/09 Nobel prize 2001 Figure 1: Observation of BEC in rubidium by the JILA group. The upper left sequence of pictures shows the shadow created by absorption in the expanding atomic cloud released from the trap. Below, the same data are shown in another representation, where the distribution of the atoms in the cloud is depicted. In the first frame to the left, we see the situation just before the condensation sets in, 50 mm Science 275, 637 (1997) 60 mm 230 mm 5

6 BEC of Positronium - Antimatter - First BEC with antimatter. Hot topic in particle physics and cosmology Antimatter should not be the same as matter to explain why matters are left in the universe. matter antimatter Many experiments are searching for matter-antimatter asymmetry Ex) Antihydrogen experiments at CERN BEC with antimatter can be a good tool to search on this hot topic by using coherency 2017/06/09 6

7 Applications of Ps-BEC 1. Measure anti-matter gravity by atom-interferometer g Intensity of Ps blinks by changing path length Slow?? Deceleration by gravity shift phase of Ps in different paths Path length 20 cm to see gravity effects with weak-equivalent principle Phys. stat. sol. 4, 3419 (2007) kev gamma-ray laser 203 GHz Phys. Rev. A 92, (2015) o-ps BEC to p-ps by 203 GHz RF p-ps BEC collectively decays into coherent 511 kev gamma-rays High-resolution imaging with x10 shorter wavelength than current X-rays Macroscopic entanglement 2017/06/09 7

8 The challenge: Dense and cold Ps in a short time Conditions to realize Ps-BEC High density Low temperature For Ps, 14 K at cm -3 Critical temperature (T C ) is very high due to Ps light mass Ps annihilation life time is only 142 ns (o-ps) Necessary techniques 1. Instance (around 10 ns) creation of dense Ps 2. Fast cooling of Ps to 10 K in around 100 ns 87 Rb 1 H Ps Goal Current BEC phase over the lines 2017/06/09 8

9 Method to realize Ps-BEC New method: K. Shu et al. J. Phys. B 49, (2016) 1. Create dense positrons and convert into dense Ps at once ~10 8 positrons in nanoseconds bunch kev energy Positron Accumulator e + Internal void = trap cavity ~150 nm 150 nm 150 nm e + Ps 13,500 Ps created 4 x cm -3 Magnified view Inject into a silica (SiO 2 ) material with ~100 nm beam waist by focusing Silica as Ps converter ~50% prob positron accumulation was achieved elsewhere. We are studying new focusing system to achieve 100nm beam waist. 2017/06/09 9

10 Positron focusing Basic Idea State-of-the-art: a few mm waist 100 nm waist for BEC Principle of Positron focusing: Focusing lens Continue this process for several stages bunched e + beam Re-moderator Pararell Monochromatic Energy N. Oshima et al. J. Appl. Phys. 103, (2008). Problems to be solved : Space charge (beam), Discharge, charging up, heating up (target) Basic study is ongoing. Measurement of beam-density dependence on target using bunched positron beam is important! 2017/06/09 10

11 1 st Ps cooling: thermalization process 1st step By collisions with cold silica cavity wall = Thermalization process Cold Silica < 10 K Ps No measurement of Ps thermalization process in cryogenic environment We are measuring it for the first time. 2017/06/09 11

12 Observation of Ps thermalization process in cryogenic environment Pick-off 2γ annihilation A positron in Ps and an electron in silica by collisions 511 kev mono energy e - e + e - o-ps Velocity v Pick-off annihilation rate l 2 n sv n: Density of electrons in silica particle s: Cross section of Pick-off annihilation By measuring l 2 vs Ps life, temperature evolution of Ps can be measured 2017/06/09 Silica aerogel particle e + e - o-ps 3γ self annihilation Both a positron and an electron are in Ps 0 ~ 511 kev continuous energy spectrum 12

13 Experimental Setup 22 Na radioisotope e + The thin plastic scintillator Cold stage Ps Vac. LaBr 3 (Ce) Scintillator Fast decay constant : 20 ns Good energy resolution : 4% at 511 kev (FWHM) Silica aerogel : porous material made by silica Density: 0.11 g/cm 3 Mean free path 60 nm /06/09

14 GM 4K cryocooler Photos Heater: Tunable in 20 ~ 300 K Silica aerogel holder Silica aerogel Springs to ensure thermal coupling Scintillators and PMTs to detect Ps formation and decay. Obtain timing and Energy information 2017/06/09 14

15 Annihilation g-rays energy spectra Less Pick-off by further cooling of Ps in lower temperature Decay from self 3g annihilation l 3 is constant (7.04 mm -1 ) 300 K 210 K 130 K 20 K Decays from temperaturedependent Pick-off 2g annihilation l ns from Ps formation 2017/06/09 15

16 Deducing Pick-off annihilation rate using MC simulation Use difference between energy spectra of Pickoff 2γ/Self 3γ Pick-off 2g : 511 kev peak Self 3g : Continuous Define energy regions to enhance each annihilation event Detection efficiencies and contamination fractions are estimated by Geant4 MC simulation. 3γ self region Data Geant4 self 2γ Pick-off region Geant4 Pick-off Geant4 Total Recorded energy spectrum (Ps life ns) Accidental events are subtracted by energy spectrum in ns /06/09

17 Result of the measurement 2017/06/09 Lines: fit curves Preliminary 300 K 210 K 130 K 20 K Thermalization curves of Ps in various silica temperature Thermalization into cryogenic temperature was clearly observed Temperature evolutions of Ps are well fitted by the elasticscattering model de = 2 v (E 3 k dt LM 2 BT), v = 2E m Ps, λ 2 t = C L v Important parameter M : Effective mass of silica for elastic collision with Ps Measured M = 170±10 a.m.u Smaller (thermalize faster) than other experiments in high T or with gases 17

18 2 nd Ps cooling: Laser cooling 2nd step Irradiate 243 nm UV laser to cool Ps down to 10 K Ps Silica is transparent in UV 243 nm UV laser 2017/06/09 18

19 Ps laser cooling 2nd step Preliminary Irradiate 243 nm UV laser to cool Ps down to 10 K Without Laser Ps With Laser Silica is transparent in UV T c By 243 detailed nm UV MC laser simulation 2017/06/09 19

20 Ps laser cooling 2nd step Preliminary Irradiate 243 nm UV laser to cool Ps down to 10 K Below T C Ps-BEC possible Ps With Laser Silica is transparent in UV T c By 243 detailed nm UV MC laser simulation 2017/06/09 20

21 Principle of Laser Cooling Laser cooling: Cool atoms by absorptions of photons momenta E Ps Internal states 2p (t=3.2ns) 1s 5.1eV = 243nm UV Ps At resonant 243nm Laser wavelength l To let Ps absorb photon, use 1s - 2p transition Incident laser wavelength is detuned slightly longer than resonance 2017/06/09 21

22 Ps Laser Cooling Cycle E P Ps s Ps Excited P Ps s 1s 2p E P Ps s Ps de-excited P Ps s 2p 3.2 ns 1s Ps v Ps Random direction Doppler effect( v) Repeat this cycle Resonant 1. Only counter-propagating photons are absorbed by Doppler effect 2. Decelerate by photon s momentum l 3. Spontaneously de-excite in 3.2 ns with random direction photon (no effect on Ps temperature) 2017/06/09 22

23 Requirements for Cooling Laser First laser cooling of Ps (anti-matter systems) For Ps, several special features are necessary 1. Long time duration pulse Cooling of Ps takes around 300 ns (~ Ps life) Laser intensity (arb.) e + injection & Ps conversion Cool Ps while 300 ns Pulse duration Time (ns) 2017/06/09 23

24 Requirements for Cooling Laser 2. Wide linewidth Doppler effect is large due to Ps light mass, so laser linewidth must cover wide Doppler width At 300 K Resonant Cooling laser should cover this wide 80 pm linewidth Wide Also Wide 2017/06/09 24

25 Requirements for Cooling Laser 3. Fast shift of wavelength Resonant wavelength shifts as Ps atoms get cold Fast shift (40 pm in 300 ns) of pulse laser has never been achieved Resonant At 50 K Largely shift by cooling At 300 K Laser wavelength should be also shifted according to cooling 40 pm in 300 ns 2017/06/09 25

26 How special is laser? Ps cooling laser Common laser Time duration 300 ns CW or Pulse with 10ns or 100 fs Linewidth 80 pm < 2 pm or > 10 nm Wavelength shift 40 pm in 300 ns No example in my knowledge Even though laser optics are deeply developed, many features which Ps requires are special because laser cooling of Ps is a new challenge New design has been considered by combining sophisticated thestate-of-the-arts optics technologies 2017/06/09 26

27 Special home-made laser system Schematic diagram of the system 10 mj Ti:Sapphire Q-switched Pump Laser EOM2 Sideband Generator EOM1 Wavelength Shifter 10 mw SHG 729 nm 365 nm THG 40 mj 243 nm 2017/06/09 27

28 Laser development is going well Ti:Sapphire 740 nm Schematic diagram of the system 729 nm CW 100 ns 10 mj Ti:Sapphire Q-switched 729 Pump nm Laser Pulse 100 ns 5% M Nd:YAG LBO crystal EOM Powerlite Blue 365 nm EOM2 Sideband Generator 729 nm pulse SHG Nd:YAG pulse 365nm 200 mj EOM1 Wavelength Shifter 729 nm 365 nm 5% 10 mj 729 nm Nd:YAG 729 nm 729 nm Output Laser Diode opnext HL7301MG (InGaAsP) THG Optical cavity 10 mw 40 mj 243 nm 2017/06/ cm Gratings Ti:Sapphir e 729 nm (red) light To controller ECDL-Box

29 Summary Ps-BEC is a good candidate for the first BEC with antimatter, which has a rich potentials on both fundamental and application physics A new method has been proposed using dense positrons and cooling by the thermalization process and laser cooling. Ps Thermaliztion process in cryogenic environment has been measured for the first time. The result indicates that it is efficient enough to realize BEC with laser cooling Cooling laser for Ps requires very special optics, so new system is currently under development. Prototype long pulse mode is confirmed to be possible. Developments on creating dense, focused positrons is also under study in parallel We will do Ps laser cooling firstly and then go to BEC! 2017/06/09 29