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arXiv:0708.3717 · Physical Review Letters

Evidence For The Production Of Slow Antiprotonic Hydrogen In Vacuum

Nicola Zurlo, M. Amoretti, C. Amsler, G. Bonomi, C. Carraro, Claudio Lenz Cesar, M. Charlton, M. Doser, A. Fontana, R. Funakoshi, P. Genova, R. Hayano, L. V. Jørgensen, A. Kellerbauer, Vittorio Lagomarsino, Rolf Landua, Evandro Lodi Rizzini, M. M. Macri, N. Madsen, Giulio Manuzio, Daniel Mitchard, P. Montagna, Lawrence Gene C. Posada, H.S. Pruys, C. Regenfus, Alberto Rotondi, G. Testera, Dirk Peter van der Werf, A. Variola, L. Venturelli, Y. Yamazaki

Published Oct 9, 2006• 41 citations
arXivHTMLPDF

Abstract

We present evidence showing how antiprotonic hydrogen, the quasistable antiproton (p)-proton bound system, has been synthesized following the interaction of antiprotons with the molecular ion H2+ in a nested Penning trap environment. From a careful analysis of the spatial distributions of antiproton annihilation events, evidence is presented for antiprotonic hydrogen production with sub-eV kinetic energies in states around n=70, and with low angular momenta. The slow antiprotonic hydrogen may be studied using laser spectroscopic techniques.

DOI
10.1103/physrevlett.97.153401

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We present evidence showing how antiprotonic hydrogen, the quasistable antiproton (p)-proton bound system, has been synthesized following the interaction of antiprotons with the molecular ion H2+ in a nested Penning trap environment. From a careful analysis of the spatial distributions of antiproton annihilation events, evidence is presented for antiprotonic hydrogen production with sub-eV kinetic energies in states around n=70, and with low angular momenta. The slow antiprotonic hydrogen may be studied using laser spectroscopic techniques.
Antihydrogen is formed when antiprotons are mixed with cold positrons in a nested Penning trap. We present experimental evidence, obtained using our antihydrogen annihilation detector, that the spatial distribution of the emerging antihydrogen atoms is independent of the positron temperature and axially enhanced. This indicates that antihydrogen is formed before the antiprotons are in thermal equilibrium with the positron plasma. This result has important implications for the trapping and spectroscopy of antihydrogen.
This work reports on experiments in which antihydrogen atoms have been produced in cryogenic Penning traps from antiproton and positron plasmas by two different methods and on experiments that have been carried out subsequently in order to investigate the antihydrogen atoms. By the first method antihydrogen atoms have been formed during the process of positron cooling of antiprotons in so called nested Penning traps and detected via a field ionization method. A linear dependence of the number of detected antihydrogen atoms on the number of positrons has been found. A measurement of the state distribution has revealed that the antihydrogen atoms are formed in highly excited states. This suggests along with the high production rate that the antihydrogen atoms are formed by three-body recombination processes and subsequent collisional deexcitations. However current theory cannot yet account for the measured state distribution. Typical radii of the detected antihydrogen atoms lie in the range between 0.4 µm and 0.15 µm. The deepest bound antihydrogen atoms have radii below 0.1 µm. Antihydrogen atoms with that size have chaotic positron orbits so that for the first time antihydrogen atoms have been detected that cannot be described by the GCA-model. The kinetic energy of the weakest bound antihydrogen atoms has been measured to about 200 meV, which corresponds to an antihydrogen velocity of approximately 6200 m/s. A simple model suggests that these atoms are formed from only one deexcitation collision and methods that might lead to a decrease of the antihydrogen velocity are presented. By the second method antihydrogen atoms have been synthesized in charge-exchange processes. Lasers are used to produce a Rydberg cesium beam within the cryogenic Penning trap that collides with trapped positrons so that Rydberg positronium atoms are formed via charge-exchange reactions. Due to their charge neutrality the Rydberg positronium atoms are free to leave the positron trapping region. The Rydberg positronium atoms that collide with nearby stored antiprotons form antihydrogen atoms in charge-exchange reactions. So far, 14 +/- 4 antihydrogen atoms have been detected background-free via a field-ionization method. The antihydrogen atoms produced via the two-step charge-exchange mechanism are expected to have a temperature of 4.2 K, the temperature of the antiprotons from which they are formed. A method is proposed by which the antihydrogen temperature can be determined with an accuracy of better than 1 K from a measurement of the time delay between antihydrogen annihilation events and the laser pulse that initiates the antihydrogen production via the production of Rydberg cesium atoms. First experiments have been carried out during the last days of the 2004 beam time, but the number of detected antihydrogen annihilations has been too low for a determination of the antihydrogen temperature. Trapped antiprotons have been directly exposed to laser light delivered by a Titanium:Sapphire laser in order to investigate if the laser light causes any loss on the trapped antiprotons. Experiments have shown that no extra loss occurs for laser powers of less than 590 mW. This is an important result against the background of the future plan to confine antihydrogen atoms in a combined Penning-Ioffe trap and then to carry out laser spectroscopy on these atoms, since it reveals that laser light does not cause an increase of the pressure in the trapping region to the extend that annihilations with the background gas become noticeable. The ATRAP Collaboration plans to precisely investigate antihydrogen atoms. The ultimate goal is to test the CPT-theorem by a high precision measurement of the 1S-2S transition of antihydrogen and a comparison with the precisely known value of the corresponding transition in hydrogen. This thesis presents the achievement of the first step towards this challenging goal: the production of cold antihydrogen itself.
Slow antihydrogen is now produced by two different production methods. In Method I, large numbers of H atoms are produced during positron‐cooling of antiprotons within a nested Penning trap (Fig. 1). In a just‐demonstrated Method II, lasers control the production of antihydrogen atoms via charge exchange collisions. Field ionization detection makes it possible to probe the internal structure of the antihydrogen atoms being produced — most recently revealing atoms that are too tightly bound to be well described by the guiding center atom approximation. The speed of antihydrogen atoms has recently been measured for the first time. After the requested overview, the recent developments are surveyed.
Lasers are used for the first time to control the production of antihydrogen (H ). Sequential, resonant charge exchange collisions are involved in a method that is very different than the only other method used so far-producing slow H during positron cooling of antiprotons in a nested Penning trap. Two attractive features are that the laser frequencies determine the H binding energy, and that the production of extremely cold H should be possible in principle-likely close to what is needed for confinement in a trap, as needed for precise laser spectroscopy.
Precision comparison experiments on bound states of matter and antimatter rely on the production of corresponding systems at low temperatures and in sufficient numbers. In this paper we propose a scheme for the pulsed production of highly excited protonium (Pn) in a Penning-Malmberg trap at low kinetic energies of tens of meV. The scheme relies on the resonant-charge-exchange reaction ${\mathrm{H}}^{*}+\overline{p}\ensuremath{\rightarrow}{\mathrm{Pn}}^{*}+{e}^{\ensuremath{-}}$ where Rydberg excited hydrogen and antiprotons (p‾\overline{p}p​) interact to form Pn∗{\mathrm{Pn}}^{*}Pn∗. The reagent H(n=30,l=2n=30,l=2n=30,l=2) is created from laser photodetached and excited hydrogen anions (${\mathrm{H}}^{\ensuremath{-}}$), which are initially trapped and mixed in a plasma together with electrons and antiprotons at low kinetic energies. We discuss a three-step pulsed laser excitation using rate equations. A semiclassical Monte Carlo approach leads to a formation rate of 105{10}^{5}105 Pn per 20 s when assuming a production temperature of 100 K. The formed Pn are internally excited in states with average principal quantum number $\ensuremath{\langle}n\ensuremath{\rangle}\ensuremath{\approx}1200$ having lifetimes that can reach seconds. The proposed scheme is therefore particularly interesting for experiments aiming at the study of cold antimatter and purely baryonic systems for precision experiments (charge neutrality, gravity, spectroscopy), as performed at the antiproton decelerator facility at CERN.
Complete five-body dynamical calculations of antiproton (p‾)(\overline{p})(p​) capture by the hydrogen molecule (H2)({\mathrm{H}}_{2})(H2​) have been carried out using a generalization of the Kirschbaum-Wilets method (belonging to a class of quasiclassical methods sometimes called fermion molecular dynamics). The differences between capture by H2{\mathrm{H}}_{2}H2​ and the H atom are found to be dramatic. The effects due to the two-center structure, rotational motions, and vibrational motions are distinguished. Of particular importance, the vibrational degree of freedom enables the molecule to capture antiprotons having lab energies above 100 eV, whereas atomic capture cuts off sharply above the ionization threshold of 27 eV (in the lab system). Antiproton capture by the atom is calculated by the same method as well as by the classical-trajectory Monte Carlo method, which is applicable only to the atom. The initial quantum numbers (assigned quasiclassically) of the p‾p\overline{p}pp​p formed are found to be shifted to significantly smaller values for the molecular target; the nnn distribution is also narrower for the molecular target as compared with the atomic target.