Background-Free Observation of Cold Antihydrogen with Field-Ionization Analysis of Its States

A background-free observation of cold antihydrogen atoms is made using field ionization followed by antiproton storage, a detection method that provides the first experimental information about antihydrogen atomic states. More antihydrogen atoms can be field ionized in an hour than all the antimatter atoms that have been previously reported, and the production rate per incident high energy antiproton is higher than ever observed. The high rate and the high Rydberg states suggest that the antihydrogen is formed via three-body recombination.

Figure 1

Overview of the trap and detectors. Antiprotons are loaded from below (left), into the trap electrodes below the rotatable electrode. Positrons are simultaneously loaded from above (right) into the electrodes above the rotatable electrode. $\overline{\mathrm{H}}$ formation is observed within the lower region detailed in the next figure.

Figure 2

(a) Electrodes for the nested Penning trap. Inside is a representation of the magnitude of the electric field that strips $\overline{\mathrm{H}}$ atoms. (b) Potential on axis for positron cooling of antiprotons (solid line) during which $\overline{\mathrm{H}}$ formation takes place, with the (dashed line) modification used to launch $\overline{p}$ into the well. (c) Antiprotons from $\overline{\mathrm{H}}$ ionization are released from the ionization well during a 20 ms time window. (d) No $\overline{p}$ are counted when no $e^{+}$ are in the nested Penning trap.

Figure 3

(a) Antiproton average energy decreases exponentially in time until the antiprotons and positrons have the lowest relative velocity. Cooling then continues but at a 10 times slower rate. (b)–(e) Energy spectra of the $\overline{p}$ as a function of the positron cooling time. (For this example, 5000 $\overline{p}$ are used, along with $200000$ $e^{+}$ in a 15 V well.)

Figure 4

(a) The number of field-ionized $\overline{\mathrm{H}}$ atoms increases with the number of $e^{+}$ in the nested Penning trap of Fig. 2, and then levels off. (b) This number decreases when the ionization well is moved away from the nested Penning trap.