ATHENA AD-1

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            Production of cold antihydrogen atoms in large quantities

                                                  C. Regenfus

                                                        University of Zürich

                                                            On behalf of the ATHENA collaboration


                                                           Sept. 02: > 50k cold antiatoms produced

                                                       • Introduction
                                                       • The ATHENA experiment +
                                                       • New results
                                                       • Summary                              H detector
                                                       • Outlook



  Antihydrogen candidate (real data, 4-prong event)



DPG Frühjahrstagung Aachen                                                                                 1
13.03.03 C. Regenfus Uni-Zürich           ATHENA - Cold antihydrogen production
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                                            Motivation


                                      Antihydrogen:
                      The simplest antimatter counterpart to matter
                        for testing fundamental physic principles


                         • CPT symmetry (Theoretical underpinning of field theories)
                         • Gravitational acceleration (Equivalence principle)



 A very high precision can be achieved by comparing antihydrogen to hydrogen




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13.03.03 C. Regenfus Uni-Zürich     ATHENA - Cold antihydrogen production
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                  Future: high resolution laser spectroscopy

                     Atomic 1S - 2S transition by two-photon excitation
                                            (first order Doppler-free)
                                                                                  H spectroscopy
            Lyman a : D E = 10.2 eV = 2.5 x 1015 Hz = 122 nm UV
                           2 x 243 nm photons (mW)
            Lifetime of 2S state: 122 ms => precision ~10-16

                     243 nm LASER               H

                                                         M irror


                                  Need:     Cold antihydrogen ( T < mK )
                                            Capture in neutral trap               Cesar et al. (1996)
                                                                                  (Laser 3kHz, 150µK)
                                            Hydrogen reference cell


                       Gravitation: atomic fountain / interferometry

DPG Frühjahrstagung Aachen                                                                              3
13.03.03 C. Regenfus Uni-Zürich           ATHENA - Cold antihydrogen production
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                                    Present physics menu

                    Plasma studies: new kind of plasma imaging
                         • Particle losses in trap
                         • (Re)combination mechanism
                         • Production of cold antihydrogen in larger quantities


                                         Investigations

                         • Antihydrogen energy distribution (+ inner states)
                         • Laser spectroscopy on non trapped atoms
                         • Trapping H and/or creation of a H beam



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13.03.03 C. Regenfus Uni-Zürich     ATHENA - Cold antihydrogen production
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                                  The ATHENA collaboration

 Particle traps + control:
          INFN, Sez. di Genova, and Dipartimento di Fisica, Università di Genova, Italy
                             EP Division, CERN, Geneva, Switzerland
                         Department of Physics, University of Tokyo, Japan
 Precision lasers:
               Department of Physics and Astronomy, University of Aarhus, Denmark
      Instituto de Fisica, Rio de Janeiro, Centro de Educação Tecnologica do Ceara, Brazil

 Positron plasma:
                           Department of Physics, University of Wales Swansea, UK

 Detector + Analysis:
                         Physik-Institut, Zürich University, Switzerland
  INFN, Sez. di Pavia, and Dipartimento di Fisica Nucleare e Teorica, Università di Pavia, Italy
  Dipartimento di Chimica e Fisica per l'Ingegneria e per iMateriali, Università di Brescia, Italy


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13.03.03 C. Regenfus Uni-Zürich       ATHENA - Cold antihydrogen production
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                                              Experimental overview

      Scint.             Scint.               Scint.
                                                                                                          Na-22
                                              Cryostat                                                    Source
                                                                      Positron Accumulator
               3 T superconducting solenoid                                                          e+

   Antiproton
   Capture Trap                   Mixing Trap
                                                                                                    0              1m
                                  15 K , 10-11 mbar


                                        CsI crystals
                                                                           Main ATHENA features:
                                                       Antihydrogen
                                                       Detector            Open access system (no sealed vacuum)
                                                       (T = 140 K)         Powerful e+ accumulation
                                                                           Plasma diagnosis and control
 Si strip                                                                  High granularity imaging detector
detectors                                      0             10 cm

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13.03.03 C. Regenfus Uni-Zürich                ATHENA - Cold antihydrogen production
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                                      ATHENA Photo




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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                                       Penning traps
                                                                        ATHENA:
                                                     Multi-ring Penning trap (choose Vz as you like )


                                                Trapped electron at B = 3 T, E = 1 eV, U ~ 10 V
                                                • Cyclotron motion (perpendicular to B)
                                                         f = 84 GHz, r ~ 1 µm
                                                         Emission of synchrotron radiation (cooling)
                                                         t cool ~ 0.3 s
                                                • Axial motion (along B)
                                                         f ~ 7 MHz, d ~µm … cm
                                                • E x B drift (‘magnetron’) (cooling over coupling)
                                                         f ~ kHz, r ~ mm


                                                Single particle <=> Plasma
                                                Coulomb coupling parameter: Ecoul/Etherm
                                                Electrical screening distance: Debye length

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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                                  Antiproton decelerator (CERN)




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13.03.03 C. Regenfus Uni-Zürich       ATHENA - Cold antihydrogen production
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              Antiproton capture and cooling with electrons
                                                                     Degrading                            Trapping
 • Capture dynamics                                                   (t = 0)                             Potential
                                      Solenoid (3 T)                                                        5 KV
 Degrader                         Vacuum
                                           Antiproton Capture Trap                      e-
    p
                                                                                                Cold ele ctron cloud
                                                                                      (Coole d by Synchrotron Radiation,
 5 MeV                                                               Trapping                    = 0.4 s at 3 T)
                                                     50 cm
                                                                      (t = 200 ns)




 • Capture trap (50 cm)
                                                                     Cooling
                                                                      (t = 20-30 s)




                                                                                                      Univ. of Genova / IT
                                                                         10 000 p / AD shot

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13.03.03 C. Regenfus Uni-Zürich       ATHENA - Cold antihydrogen production
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                                  Positron accumulation




                                                                Accumulation rate: 106 e+/s

                                                                 150 million e+ / 5 min


                                                          After transfer: 75 x 106 in mixing trap

                                                          Positron plasma : r~2mm, l~32mm,
                                                                          n~2.5 x 108 / cm3
                                                          Lifetime: ~hours

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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                  Non destructive positron plasma diagnostics

                                                                     Complete model of plasma mode excitation
                                                                     (based on ‘Cold Fluid Theory’ * )
                                                                     PLASMA SHAPE, LENGTH, DENSITY
      drive                                         read             Plasma temperature change
                                                                                           * D. Dubin, PRL 66, 2076 (1991)
     ~ 30 MHz

                              heat




                                                                1

                                   pa d 2 f (a ) 
                  2                             2   2
            mzp         h 2           2
   kDT                                     3
              5         2         2
                                          
                                              2 2 da 2 
                                                  2         

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13.03.03 C. Regenfus Uni-Zürich               ATHENA - Cold antihydrogen production
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           Detection principle of antihydrogen annihilations

                                                           • H atom dissociates to p and e+
                                                             by contact with the trap wall or
                                                             rest gas atoms
                                                           • pN -> charged and neutral pions

                                                           • e+ e- -> 511keV photons (back to back)


                                                       Measure 1MeV on background of 2GeV


                                                                               Monte Carlo
                                                                             511 keV opening angle




        Good spatial resolution (< 1 cm ) of charged
        vertex ( at least 2 prong events)
        Time coincidence (~ 1 µs)
        High rate capability (self triggering)



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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                                        Detector development
                                  Silicon micro strip layer
                                                              • Compact design (radial thickness 3 cm)

                                                              • High granularity (8K strips, 192 crystals)

                                                              • Large solid angle (>75 %)


                                                                    Full detector installed: August 2001
                                                                 All photodiodes replaced with APDs: Spring 2002


                                                              Mechanics for 77K

         Much effort into R&D
         • Low temperature (~ 140 K)
         • High magnetic field (3 T)
         • Low power consumption
         • Light yield of pure-CsI crystals ?
         • CTE matching (Kapton, silicon, ceramics)
         • Electronic components
                                                                                  Workshop Zürich , J. Rochet

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13.03.03 C. Regenfus Uni-Zürich          ATHENA - Cold antihydrogen production
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                  Pure-CsI crystals + Avalanche Photo Diodes
  • Read out close up                                                        • Crystal APD unit




           • Crystal detector performance
   12000
                  back scatter
                                                                                                                            Pure-CsI
   10000
                                      22 Na
                                                               4000                               (511keV)
                                          T = 150 K
                                                                                      ee
                                                                                                                    ~16 times higher light yield @
    8000
                                                               3000                          a                      80K
                         511 keV
    6000
                                                                                              (511keV)
    4000                                                       2000
                            FW HM = 18%                                                                             C. Amsler, et al. :Temperature
    2000                                                                                                            dependence of pure-CsI,
                                                               1000
                                             1275 keV
                                                                                                                    scintillation light yield and decay
       0
                                                                                                                    time. NIM A 480, 494–500 (2002).
           0 60       500           1000                1500          -1.0     -0.5       0        0.5        1.0
                      Pulse height [keV]                                                cos(a)


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13.03.03 C. Regenfus Uni-Zürich                            ATHENA - Cold antihydrogen production
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                         Full GEANT Monte Carlo simulations
               E&M cascades, Hadronic Showers (GEISHA) (> 10 keV)
               Geometry from AutoCAD
               Module-by-module (in)efficiency taken into account
               Same analysis routine for MC and data

                                                                Radial vertex position

                                                                              Electrode
                                                                              (r = 1.25 cm)




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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                                    Antiproton annihilations
              Antiproton annihilation on the trap wall (real data, 3-prong event)
                                      • strip hits (inner + outer layer) => p vertex
                                       • crystals hit (matched to charged tracks)
                                  • vertex resolution, ~ 4 mm (curvature not resolved)




                                                                                         Electrode
                                                                                         position
                                                                                         (r = 1.25 cm)




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13.03.03 C. Regenfus Uni-Zürich       ATHENA - Cold antihydrogen production
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                            Plasma imaging (antiprotons only)
                    p vertex evolution in time
       70
                                                                       Antiprotons and electrons
       60                                                                    Late times
       50

       40                                                      Potential


       30                                                                    Early times

       20

       10                                                      Potential

        0
              -10        -5       0        5     10
                 Longitudinal Position [cm]

                                      Powerful plasma and loss diagnostics !

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13.03.03 C. Regenfus Uni-Zürich          ATHENA - Cold antihydrogen production
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                            Mixing trap (nested penning trap*)




             In one mixing cycle (5 min) we mix ~104 antiprotons with ~108 positrons
                                                               * G. Gabrielse et al., Phys. Lett. A129, 38 (1988)

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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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              Cooling of antiprotons by 75 million positrons




                                                           • Rapid cooling (< 20 ms)
                                                           • Decreasing energy of antiprotons
                                                           • Increasing separation of plasmas

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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                           Antiprotons in the positron plasma

                                    Energy loss by dE/dx and thermalization

       Incoming antiproton




                    e+ cloud (108/cm3)

                    T = 10K ….. 10000K
                         (by RF heating)




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13.03.03 C. Regenfus Uni-Zürich            ATHENA - Cold antihydrogen production
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                                       Antihydrogen production

    1.           Fill positron well in mixing region with 75·106 positrons;
                 allow them to cool to ambient temperature (~15 K)
    2.           Launch 104 antiprotons into mixing region
    3.           Mixing time 190 s - continuous monitoring by detector (charged trigger)
    4.           Repeat cycle every 5 minutes (data for 165 cycles)



   -125
                 antiprotons                                     For comparison:

   -100                                                          “hot” mixing = continuous RF heating
                                                                 of positron cloud
    -75
                                                                     (suppression of antihydrogen
    -50
                                                                             production)
           0       2           4    6        8   10    12
                                   Length (cm)



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13.03.03 C. Regenfus Uni-Zürich             ATHENA - Cold antihydrogen production
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          Antiproton annihilation rate (charged trigger rate)




                                                                 High initial rate ~ 100 Hz




                                                              Background trigger rate ~ 0.5 Hz


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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                                     Analysis Procedure
    Antihydrogen candidate (real data, 4-prong event)

                                                     Event reconstruction
                                                     (165 mixing cycles ~ 2 days)


                                                 • Reconstruct annihilation vertex (103 k)

                                                 • Search for ‘clean’ 511 keV-photons:
                                                            exclude crystals hit by charged particles
                                                            + its 8 nearest neighbours
                                                 • ‘511   keV’ candidate =
                                                            400… 620 keV
                                                            no hits in any adjacent crystals
                                                 • Select events with two ‘511 keV’ photons

                                                 • Reconstruction efficiency ~ 0.25 %
                                                            = “golden” events !

DPG Frühjahrstagung Aachen                                                                              24
13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                            Antihydrogen Signal (“golden” events)
                            Opening angle between two 511 keV photons
                                         (seen from charged particle vertex)
    200
    180
               Mixing with cold positrons
    160

    140                                                                             Comparison with Monte Carlo
                 131 ± 22 events
    120
                                                                         200
    100                                                                                Back-to-back 511 keV peak
                                                                         180         from antihydrogen annihilation
     80
                                                                         160
     60                      Mixing with hot positrons                                            Angular re solution from M C simulation
                                                                         140                          (normalise d to pe ak height)
     40
                                                                         120
     20
                                                                         100
      0                                                                   80
          -1         -0.5            0             0.5     1
                                  cos()                                60

                                                                          40
                            M. Amoretti et al., Nature 419, 456 (2002)
                                                                          20

    > 50,000 produced antiatoms (conservative estimate)                   0
                                                                               -1                     -0.75                          -0.5
    Background: mixing with hot positrons                                                            cos 


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13.03.03 C. Regenfus Uni-Zürich                  ATHENA - Cold antihydrogen production
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                                      Background measurements
   Opening angle between two 511 keV photons
                  (seen from charged particle vertex)

                                                                  Can antiproton annihilations on electrode
      200                                                         fake back-to-back signal?
      180        Antiprotons only
                                                                  No !
                 Cold mixing (displaced E   window)
      160
                                                                  1) Secondary e+ within 10 mm ~ 0.1 %
      140
                                                                  2) Monte Carlo - no peak
      120
                                                                  3) Measurement - no peak
      100

       80
                                                                  Histogram:
       60
                                                                  Antiproton-only data (99,610 vertices, 5,658
       40                                                         clean 2-photon events plotted).
       20                                                         Dots:
        0                                                         Antiproton + cold positrons, but analyzed
            -1         -0.5            0               0.5    1
                                                                  using an energy window displaced upward so
                                    cos()                      as not to include the 511 keV photo-peak

                 M. Amoretti et al., Nature 419, 456 (2002)


DPG Frühjahrstagung Aachen                                                                                       26
13.03.03 C. Regenfus Uni-Zürich                ATHENA - Cold antihydrogen production
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                Antihydrogen = main source of annihilations
                                                                  X-Y vertex distribution

      Time distribution of golden events and all annihilations
                                                                                   Cold




                                                                                    Hot




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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                          Physics of antihydrogen production




     ANTIHYDROGEN VERSUS BACKGROUND
     ABSOLUTE PRODUCTION RATES
     DEPENDENCE ON TEMPERATURE
     ANGULAR DISTRIBUTION



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13.03.03 C. Regenfus Uni-Zürich   ATHENA - Cold antihydrogen production
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                                            Opening angle fit

               Fit Input MC                                       Fit Result

                                  Hbar



                                                                      Data

                                                                          Fit
                     Background
                                                                     Background

                                                                        cos(q)

                                              Fit result: ~ 2/3 of the events are antihydrogen
                   cos(q)

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13.03.03 C. Regenfus Uni-Zürich          ATHENA - Cold antihydrogen production
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                                      Vertex spatial distribution fit




     Antihydrogen on trap electrode     Antihydrogen on trapped ions or rest gas   Compare to cold mix data



          Average fraction of antihydrogen
             65 ± 10 % during mixing !


                                In 2002, ATHENA produced
               =>          0.7 ± 0.3 Million antihydrogen atoms


DPG Frühjahrstagung Aachen                                                                                    30
13.03.03 C. Regenfus Uni-Zürich        ATHENA - Cold antihydrogen production
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                                  Rate of antihydrogen production



                                                                 Analysis:
                                                                 • 65 ± 10 % antihydrogen
                                                                 • ~ 50 % vertex /
                                                                 annihilation




                                                                High Initial Rate (> 100 Hz)
                                                                High S/B (~ 10:1) in first seconds




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13.03.03 C. Regenfus Uni-Zürich      ATHENA - Cold antihydrogen production
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                                     Pulsed antihydrogen production
                     Switch positron heating Off/On resp. On / Off                           We observe:
Vertex Counts




                                                                                        Annihilation rate



                                Heat On                              Heat On
                                                                                             Rise time ~ 0.4 s
                                                                                          (Positron cooling time)
                         Mixing time

                         Mixing time ->
 Vertex Z position




                                                                                         Vertex distribution
                                                                                               along z




                                  sec                                    sec

DPG Frühjahrstagung Aachen                                                                                      32
13.03.03 C. Regenfus Uni-Zürich                 ATHENA - Cold antihydrogen production
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                           Antihydrogen Production - T dependence




                                                Radiative        Three-body
                 s(T) dependence                T-0.5            T-4.5
                 Final state                    n < 10           n >> 100
                 Stability (re-ionization)      high             low
                 Expected rates                 ~ Hz             ?


DPG Frühjahrstagung Aachen                                                       33
13.03.03 C. Regenfus Uni-Zürich          ATHENA - Cold antihydrogen production
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                                                 Summary

              First production and detection of cold antihydrogen
                           - high positron accumulation rate = fast duty cycle
                           - sensitive detector = observe clear signals

              High rate production
                           - initial rate > 100 Hz, average rate ~ 10 Hz

              Antihydrogen dominates annihilation signal (~ 2/3)
              Pulsed antihydrogen production
              Temperature dependence measured
              Antihydrogen production at room temperature


DPG Frühjahrstagung Aachen                                                       34
13.03.03 C. Regenfus Uni-Zürich        ATHENA - Cold antihydrogen production
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                                                        Outlook

            Next steps - technology                                   Next steps - physics



         Study …                                                           Spectroscopy
         Formation process                                                 High precision comparison 1S-2S
                                                                           Hyperfine structure


  More …
  Increase formation rate
  More antiprotons
  Laser induced recombination


                                                                           Gravitational
  Trapping and cooling ...                                                 effects
  Anti-Hydrogen at E < 0.05 meV ?
                                                                           E ~ 0.000 1 meV
  Dense plasmas in magnetic multipole fields ?
                                                                           Atom interferometry
  Laser cooling? Collisions with ultra-cold hydrogen atoms?



DPG Frühjahrstagung Aachen                                                                                   35
13.03.03 C. Regenfus Uni-Zürich          ATHENA - Cold antihydrogen production

				
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