Mike Charlton Physics, Swansea Antimatter at Low Energies Mike Charlton Physics, Swansea Antimatter at Low Energies - Fudan Summer School - July 16

Antimatter at Low Energies - Fudan Summer School - July 16 Overview of Lectures III. Antihydrogen   - Motivation - Basic formation processes - Formation and detection of antihydrogen - Antihydrogen trapping - Physics with antihydrogen - Prospects IV. Applications of Antiparticles - Use of positrons in condensed matter - Use of positrons in medicine - The positron as a sensitive probe in atomic physics - Positrons in space - The positive muon Antimatter at Low Energies - Fudan Summer School - July 16

Antimatter at Low Energies - Fudan Summer School - July 16 Overview of Lecture Antihydrogen   - Motivation - Basic formation processes - Formation and detection of antihydrogen - Antihydrogen trapping - Physics with antihydrogen - Prospects Antimatter at Low Energies - Fudan Summer School - July 16

Antimatter at Low Energies - Fudan Summer School - July 16 Motivation Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms Spontaneous Radiative Recombination (SRR) Stimulated Radiative Recombination Three-Body Recombination (TBR) The Positronium Reaction We will take a brief look at SRR, TBR and the Positronium Reaction Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms Spontaneous Radiative Recombination (SRR) ─ Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms The radiative process, cross section quoted by Holzscheiter, Charlton and Nieto, Phys. Rep. 402 (2004) 1 Antihydrogen principal quantum number nH = 1 binding energy Kinetic energy of e+ in antiproton rest frame The formulae apply for Ee << E0 and clearly favour low nH. Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms We can now go on to estimate a “total” radiative cross section and attempt to construct a rate for the reaction. Positron and antiproton spatial density profiles This is the phase space overlap of the positron and antiproton clouds Positron velocity distribution This is the recombination rate coefficient Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms To proceed we need to make assumptions about the densities, the overlap of the clouds and the velocity distribution …. We usually take the easy route and assume complete and constant overlap with constant density clouds with Maxwellian temperature distributions. N.B. This isn’t necessarily the situation that pertains to the experiments, but it is a start … here we find, Antiproton number Positron density Averaged rate coefficient Under these conditions the rate of events, per antiproton, can be found from (see e.g. Stevefelt et al, Phys Rev A 12 (1975) 1246) per second with Te in K and ne in cm-3 Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms Three-Body Recombination (TBR) + + e Two unbound positrons with an antiproton. The positrons undergo an elastic collision; Energy change ΔE ~ E ~ kTe The length scale for this reaction is governed by the length scale of the interactions. This is the Thomson length, RT, found by equating thermal and Coulomb energies as – Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms This is not the end of the story for the TBR since an extra collisional-radiative term needs to be added to those for SRR and TBR. This is empirically determined and is due to radiative decay of some of the high-lying states formed in TBR. The full rate looks like This rate is for full overlap, in equilibrium and with no applied fields … so there are many further things to consider … Also the antihydrogen formed by the TBR is weakly bound and can be field ionized in plasma/trap fields, or in collisions. Thus, there may be a reaction cycle of the form …this can be an important antiproton transport mechanism … Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms Basic comparator of the SRR and TBR Radiative Three-body Rate T dependence T-0.6 T-4.5 Final state n < 10 n >> 10 Stability (re-ionization) high low Expected rates ~10s Hz fast ??? Lets have a look at an example Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms CAUTION!! 3-body and radiative labels reversed!! Around 100 K Thanks to Paul Bowe, Aarhus, for this plot and numerous discussions Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms (Simulations) Binding energies Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms (Simulations) Time evolution of antihydrogen formation positions 1015m-3 5x1013m-3 At the higher ne the repeated formation and destruction of antihydrogen results in radial transport of the antiprotons to the plasma edge Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms First suggested some time ago: Humberston et al, J. Phys. B 20 (1987) L25 Importance of excited states pointed out: Charlton, Phys. Letts A 143 (1990) 143 The scaled (principal quantum number to the 4th power) cross sections for antihydrogen formation and Ps destruction. The velocity scale is in terms of the quasi-classical orbit speed of the positron in the positronium. Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Formation Mechanisms Double Rydberg charge exchange method; Hessels et al, Phys. Rev. A 57 (1998) 1668 Implemented by ATRAP: Storry et al., PRL, 93 (2004) 263401 Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection Sympathetic compression of an antiproton cloud by electrons Typically use a fixed frequency rotating wall technique at 10 MHz G. Andresen et al., PRL, 101 (2008) 203401 Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection Evaporative cooling of antiproton and positrons to lower temperatures prior to mixing 9 K 23 K 19 K 325 K 57 K 1040 K Typically (9 ± 4) K is lowest achievable at the lowest well available at which (6 ± 1) % of the initial antiprotons remain Andresen et al., PRL (2010) 105 013003 Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection Nested Penning trap arrangement; Gabrielse et al, Phys. Letts A129 (1988) 38 Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection Autoresonant injection of antiprotons in to the positron plasma to achieve robust mixing with minimum added kinetic energy Chirped driven harmonic oscillator Andresen et al. PRL 106 025002 (2011) Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection a) Antiproton annihilation b) Cosmic ray Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection Potential energy surface of an electron/positron in a combined Coulomb and static electric field in the z-direction: from Lankhuijzen and Noordam, Adv. At. Mol. Opt. Phys. 38 (1997) 121 Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Production and Detection ATRAP Field ionized antiprotons None present without e+ Gabrielse et al., PRL 89 (2002) 213401 Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Trapping Exploit the magnetic moment of the atom μ is the Bohr magneton, μB, for the ground state Breit-Rabi diagram for the ground state of hydrogen Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Trapping Classic Ioffe-Pritchard Geometry quadrupole winding Mirror coils Based on Berkeley/Swansea results: standard quadrupole arrangement was rejected by ALPHA as the magnetic field gradient across charged plasmas is too great; see Fajans et al., Phys. Rev. Lett. 95 155001 (2005) Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Trapping In general for a multipole of order m (i.e. a quadrupole has order 2, and octupole order 4) The radial dependence goes as follows with Bw the field at the Penning trap wall, at a radius, Rw Quadrupole B Octupole Radius in trap Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Trapping N.B. Well depth ~ 0.7 K/T Technical details of the trap in Bertsche et al, NIM A 566 (2006) 746 Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Trapping 30,000 pbars at 200K 2M positrons at 40 K (evaporatively cooled) Auto-resonant injection and mix for 1 sec. Clear the charge particles Turn off the neutral trap (1/e time ~ 9 ms) Search for pbar annihilations from Hbar (bias fields to eject any charged particles still trapped) Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Trapping Published in Nature 468 (2010) 673 Initial publication – 38 events Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Trapping 309 events – Nature Physics 7 (2011) 558 Antimatter at Low Energies - Fudan Summer School - July 16

Antihydrogen Trapping 309 events – Nature Physics 7 (2011) 558 Antimatter at Low Energies - Fudan Summer School - July 16

Physics with Antihydrogen Antimatter at Low Energies - Fudan Summer School - July 16

Physics with Antihydrogen On resonance – 15 MHz scan width for 15 s each – 6 repeats Antimatter at Low Energies - Fudan Summer School - July 16

Physics with Antihydrogen On resonance – 15 MHz scan width for 15 s each – 6 repeats Off resonance – B shift Antimatter at Low Energies - Fudan Summer School - July 16

Physics with Antihydrogen On resonance – 15 MHz scan width for 15 s each – 6 repeats Off resonance – B shift On resonance – frequency shift Antimatter at Low Energies - Fudan Summer School - July 16

Physics with Antihydrogen Extra annihilations on resonance – the microwaves force the antihydrogen into the untrapped states First observation of a resonant quantum transitions in an anti-atom: Nature 483 (2012) 439 Antimatter at Low Energies - Fudan Summer School - July 16

Physics with Antihydrogen Analysis of the up/down annihilation positions versus time (red dots, data: green dots, simulations) during the magnet shutdown F = Mg/M, ratio of grav. to inertial mass C. Amole et al., Nature Comm. 4 (2013) 1785 Antimatter at Low Energies - Fudan Summer School - July 16

Physics with Antihydrogen ALPHA’s reverse cumulative average analysis Data Red: y-direction Green: x-direction Simulations Dash: “antigravity” at given |F| Line: gravity at given |F| Grey bands: 90% confidence limits on simulations C. Amole et al., Nature Comm. 4 (2013) 1785 Antimatter at Low Energies - Fudan Summer School - July 16

Physics with Antihydrogen Antimatter at Low Energies - Fudan Summer School - July 16

Antimatter at Low Energies - Fudan Summer School - July 16 Prospects Antimatter at Low Energies - Fudan Summer School - July 16

Antimatter at Low Energies - Fudan Summer School - July 16 Prospects System for 1S-2S laser spectroscopy Antimatter at Low Energies - Fudan Summer School - July 16

Antimatter at Low Energies - Fudan Summer School - July 16 Prospects A very bright and busy future awaits … CERN has started work on ELENA an extra ring to decelerate antiprotons to about 100 keV – this will increase our capture efficiency for low energy antiprotons by a factor of around 100. Antimatter at Low Energies - Fudan Summer School - July 16