Antihydrogen accumulation for fundamental symmetry tests

Abstract

Antihydrogen, a positron bound to an antiproton, is the simplest anti-atom. Its structure and properties are expected to mirror those of the hydrogen atom. Prospects for precision comparisons of the two, as tests of fundamental symmetries, are driving a vibrant programme of research. In this regard, a limiting factor in most experiments is the availability of large numbers of cold ground state antihydrogen atoms. Here, we describe how an improved synthesis process results in a maximum rate of 10.5 ± 0.6 atoms trapped and detected per cycle, corresponding to more than an order of magnitude improvement over previous work. Additionally, we demonstrate how detailed control of electron, positron and antiproton plasmas enables repeated formation and trapping of antihydrogen atoms, with the simultaneous retention of atoms produced in previous cycles. We report a record of 54 detected annihilation events from a single release of the trapped anti-atoms accumulated from five consecutive cycles.Antihydrogen studies are important in testing the fundamental principles of physics but producing antihydrogen in large amounts is challenging. Here the authors demonstrate an efficient and high-precision method for trapping and stacking antihydrogen by using controlled plasma.

Fig. 1

The ALPHA-2 central apparatus. a ALPHA-2 geometry, drawn to scale except for the radial extent of the annihilation detector. The inner diameter of the Penning–Malmberg electrodes is 44.35 mm in the central region of the atom trap and 29.6 mm at either end. Antiprotons enter from the left in this view, while positrons and electrons are loaded from the right. b Magnetic field strength on axis with the atom trap energised (the external solenoid responsible for producing a uniform 1 T field is not shown). The solid curve (red) shows the flattened atom trap field used in ref. . The dashed curve (blue), shows the on-axis field during stacking; the left and right solenoids a, b increase the field from 1 to 3 T for enhanced capture, cyclotron cooling and rotating wall efficiency of, as appropriate, positrons, electrons and antiprotons

Fig. 2

Antihydrogen synthesis sequence. Dashed and solid curves represent electrostatic potentials before and after each step in the process. Filled regions indicate self-potentials and physical extents of antiproton and positron plasmas. a Potential before evaporative cooling. Positron well depth 3.31 V. b Evaporative cooling, during which energetic positrons escape to the right (duration 600 ms). Final positron well depth 0.91 V. c Potential realignment in preparation for mixing (duration 10 ms). Final positron well depth 0.91 V. d Potential merge mixing (duration 1 s). Positrons escape to the left during mixing, resulting in further evaporative cooling. Final positron well depth 0.27 V. Remaining positrons are ejected to the right for a temperature measurement; remaining antiprotons are ejected to the left

Fig. 3

Antihydrogen stacking. The number of antihydrogen atoms detected when the magnetic minimum trap is ramped down after one or more consecutive mixing cycles. Each mixing cycle in a sequence is separated by ~4 min. The error bars are statistical and the number of replicates is indicated above each data point. The dashed line is a linear fit to the data giving an average trapping rate of 10.5 ± 0.6 detected antihydrogen atoms per mixing cycle