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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 Outlook. H detector.
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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 • Outlook H detector Antihydrogen candidate (real data, 4-prong event) ATHENA - Cold antihydrogen production
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 ATHENA - Cold antihydrogen production
Future: high resolution laser spectroscopy Atomic 1S - 2S transition by two-photon excitation (first order Doppler-free) 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 H spectroscopy Need: Cold antihydrogen ( T < mK ) Capture in neutral trap Hydrogen reference cell Cesar et al. (1996) (Laser 3kHz, 150µK) Gravitation: atomic fountain / interferometry ATHENA - Cold antihydrogen production
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 ATHENA - Cold antihydrogen production
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 ATHENA - Cold antihydrogen production
Experimental overview Scint. Scint. Scint. 15 K , 10-11 mbar Main ATHENA features: Open access system (no sealed vacuum) Powerful e+ accumulation Plasma diagnosis and control High granularity imaging detector ATHENA - Cold antihydrogen production
ATHENA Photo ATHENA - Cold antihydrogen production
Penning traps ATHENA: Multi-ring Penning trap (choose Vzas 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 ATHENA - Cold antihydrogen production
Antiproton decelerator (CERN) ATHENA - Cold antihydrogen production
Antiproton capture and cooling with electrons • Capture dynamics • Capture trap (50 cm) 10 000 p / AD shot ATHENA - Cold antihydrogen production
Positron accumulation Accumulation rate: 106 e+/s 150 million e+ / 5 min After transfer: 75 x 106 in mixing trap Positronplasma : r~2mm, l~32mm, n~2.5 x 108 / cm3 Lifetime: ~hours ATHENA - Cold antihydrogen production
Non destructive positron plasma diagnostics Complete model of plasma mode excitation (based on ‘Cold Fluid Theory’ * ) PLASMA SHAPE, LENGTH, DENSITY Plasma temperature change drive read * D. Dubin, PRL 66, 2076 (1991) ~ 30 MHz heat ATHENA - Cold antihydrogen production
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) ATHENA - Cold antihydrogen production
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 ATHENA - Cold antihydrogen production
Pure-CsI crystals + Avalanche Photo Diodes • Read out close up • Crystal APD unit • Crystal detector performance Pure-CsI ~16 times higher light yield @ 80K C. Amsler, et al. :Temperature dependence of pure-CsI, scintillation light yield and decay time. NIM A 480, 494–500 (2002). ATHENA - Cold antihydrogen production
Full GEANT Monte Carlo simulations Electrode (r = 1.25 cm) 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 ATHENA - Cold antihydrogen production
Antiproton annihilations Electrode position (r = 1.25 cm) • 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) ATHENA - Cold antihydrogen production
Plasma imaging (antiprotons only) p vertex evolution in time Powerful plasma and loss diagnostics ! ATHENA - Cold antihydrogen production
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) ATHENA - Cold antihydrogen production
Cooling of antiprotons by 75 million positrons • Rapid cooling (< 20 ms) • Decreasing energy of antiprotons • Increasing separation of plasmas ATHENA - Cold antihydrogen production
Antiprotons in the positron plasma Energy loss by dE/dx and thermalization Incoming antiproton e+ cloud (108/cm3) T = 10K ….. 10000K (by RF heating) ATHENA - Cold antihydrogen production
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) For comparison: “hot” mixing = continuous RF heating of positron cloud (suppression of antihydrogen production) ATHENA - Cold antihydrogen production
Antiproton annihilation rate (charged trigger rate) High initial rate ~ 100 Hz Background trigger rate ~ 0.5 Hz ATHENA - Cold antihydrogen production
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 ! ATHENA - Cold antihydrogen production
Antihydrogen Signal (“golden” events) Opening angle between two 511 keV photons (seen from charged particle vertex) Comparison with Monte Carlo M. Amoretti et al., Nature419, 456 (2002) > 50,000 produced antiatoms (conservative estimate) Background: mixing with hot positrons ATHENA - Cold antihydrogen production
Background measurements Opening angle between two 511 keV photons (seen from charged particle vertex) Can antiproton annihilations on electrode fake back-to-back signal? No ! 1) Secondary e+ within 10 mm ~ 0.1 % 2) Monte Carlo - no peak 3) Measurement - no peak Histogram: Antiproton-only data (99,610 vertices, 5,658 clean 2-photon events plotted). Dots: Antiproton + cold positrons, but analyzed using an energy window displaced upward so as not to include the 511 keV photo-peak M. Amoretti et al., Nature419, 456 (2002) ATHENA - Cold antihydrogen production
Antihydrogen = main source of annihilations Cold Hot X-Y vertex distribution Time distribution of golden events and all annihilations ATHENA - Cold antihydrogen production
Physics of antihydrogen production ANTIHYDROGEN VERSUS BACKGROUND ABSOLUTE PRODUCTION RATES DEPENDENCE ON TEMPERATURE ANGULAR DISTRIBUTION PRELIMINARY ATHENA - Cold antihydrogen production
Opening angle fit Fit Input MC Hbar Data Fit Background Background cos(qgg) cos(qgg) Fit Result PRELIMINARY Fit result: ~ 2/3 of the events are antihydrogen ATHENA - Cold antihydrogen production
Vertex spatial distribution fit PRELIMINARY 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 => ATHENA - Cold antihydrogen production
Rate of antihydrogen production Analysis: • 65 ± 10 % antihydrogen • ~ 50 % vertex / annihilation PRELIMINARY High Initial Rate (> 100 Hz) High S/B (~ 10:1) in first seconds ATHENA - Cold antihydrogen production
Pulsed antihydrogen production Vertex Counts Heat On Mixing time -> Vertex Z position sec sec Switch positron heating Off/On resp. On / Off We observe: Annihilation rate Heat On Rise time ~ 0.4 s (Positron cooling time) Mixing time PRELIMINARY Vertex distribution along z ATHENA - Cold antihydrogen production
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 ? ATHENA - Cold antihydrogen production
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 ATHENA - Cold antihydrogen production
Outlook Next steps - physics Next steps - technology Study … Formation process Spectroscopy High precision comparison 1S-2S Hyperfine structure More … Increase formation rate More antiprotons Laser induced recombination Gravitational effects E ~ 0.000 1 meV Atom interferometry Trapping and cooling ... Anti-Hydrogen at E < 0.05 meV ? Dense plasmas in magnetic multipole fields ? Laser cooling? Collisions with ultra-cold hydrogen atoms? ATHENA - Cold antihydrogen production