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Journal of Physics Conference Series

On the temperature of antihydrogen formed in magnetic trap

Andrey A. Bobrov, Sergey Ya. Bronin, Edward A. Manykin, Boris B. Zelener, B. V. Zelener

Published Nov 1, 2016• 1 citation
Publisher

Abstract

Kinetic processes taking place after injection of antiprotons in cold positron cloud are discussed. Mixture of antiparticles is considered as low temperature non neutral weakly coupled plasma. Simple estimations of energy of antihydrogen atoms that may be formed due to three body recombination in the system are made. Dependence of atom energy on initial particles temperatures and influence of strong confining magnetic field are discussed.

DOI
10.1088/1742-6596/774/1/012160

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International Conference on Low Energy Antiproton Physics (LEAP)•Nov 23, 2017•0 citations
Kinetic processes taking place after injection of antiprotons in cold positron cloud are discussed. Mixture of antiparticles is considered as low temperature non neutral weakly coupled plasma. Simple estimations of energy of antihydrogen atoms that may be formed due to three body recombination in the system are made. Dependence of atom energy on initial particles temperatures and influence of strong confining magnetic field are discussed.
The merging of antiprotons with a positron plasma is the predominant and highest efficient method for cold antihydrogen formation used to date. We present experimental evidence that this method has serious disadvantages for producing antihydrogen cold enough to be trapped. Antihydrogen is neutral but may be trapped in a magnetic field minimum. However, the depth of such traps are of order 1 K, shallow compared to the kinetic energies in current antihydrogen experiments. Studying the spatial distribution of the antihydrogen emerging from the ATHENA positron plasma we have, by comparison with a simple model, extracted information about the temperature of the antihydrogen formed. We find that antihydrogen is formed before thermal equilibrium is attained between the antiprotons and the positrons, and thus that further positron cooling may not be sufficient for producing antihydrogen cold enough to be trapped. We discuss the implications for trapping of antihydrogen in a magnetic trap, important for ongoing work by the ALPHA collaboration.
Abstract The binding energies of antihydrogen atoms formed when antiprotons are mixed with positron plasmas having densities ranging from 1013–1015 m−3, and at temperatures of 5–30 K, have been investigated using simulations. Major changes in the distribution of binding energies are observed, with more strongly bound states evident at the higher densities, and at lower temperatures. For deeper binding, the distribution of binding energies follows a power-law which is found to be strongly dependent upon plasma properties and the strength of the applied magnetic field. The underpinning role of collisions in determining the binding energies is explored.
Over the last decades it has become routine to form beams of positrons and antiprotons and to use them to produce trapped samples of both species for a variety of purposes. Positrons can be captured efficiently, for instance using a buffer-gas system, and in such quantities to form dense, single component plasmas useful for antihydrogen formation. The latter is possible using developments of techniques for dynamically capturing and then cooling antiprotons ejected from the Antiproton Decelerator at CERN. The antiprotons can then be manipulated by cloud compression and evaporative cooling to form tailored plasmas. We will review recent advances that have allowed antihydrogen atoms to be confined for the first time in a shallow magnetic minimum neutral atom trap superimposed upon the region in which the antiparticles are held and mixed. A new mixing technique has been developed to help achieve this using autoresonant excitation of the centreofmass longitudinal motion of an antiproton cloud. This allows efficient antihydrogen formation without imparting excess energy to the antiprotons and helps enhance the probability of trapping the anti-atom. © 2012 American Institute of Physics.
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.
We propose a new mechanism that can contribute to the spontaneous recombination of antihydrogen atoms in traps containing cold positron-antiproton plasma in the presence of a constant magnetic field. The latter plays a principal role restricting the motion of the positron in the transverse plane and, finally, prolonging the time during which the positron can lose its energy due to spontaneous radiation. Classical and quantum mechanical estimates are presented under the conditions close to those experimentally realized in the existing traps.
Antihydrogen is at the focus of antimatter research.One of the main aims is to perform a direct test of the fundamental CPT symmetry by comparing the spectroscopic properties of the ground-state antihydrogen atom with those of the hydrogen atom.In current experiments synthetized antihydrogen is formed mainly via the three-body recombination process after mixing positron and antiproton clouds together in a cryogenic trap.The major three-body formation process populates mainly highly excited Rydberg states, therefore before the antihydrogen atoms reach the ground-state additional processes may take place, such as collisional (de)excitation, ionization and spontaneous or stimulated radiative transitions.We have used a classical-trajectory Monte Carlo (CTMC) code along with regular atomic codes to generate a scattering database of rate coefficients in various experimentally achievable magnetic field strengths and in low temperature positron plasma conditions.We used these rates to calculate the evolution of level population of antihydrogen during formation, scattering and flight within our experimental conditions [1].The model and the results are presented.