Trapped Antihydrogen in Its Ground State

Abstract

Antihydrogen atoms ((\bar{H})) are confined in a magnetic quadrupole trap for 15 to 1000 s - long enough to ensure that they reach their ground state. This milestone brings us closer to the long-term goal of precise spectroscopic comparisons of (\bar{H}) and H for tests of CPT and Lorentz invariance. Realizing trapped (\bar{H}) requires characterization and control of the number, geometry, and temperature of the antiproton ((\bar{p})) and positron ((e^+)) plasmas from which (\bar{H}) is formed. An improved apparatus and implementation of plasma measurement and control techniques make available (10^7 \bar{p}) and (4 \times 10^9 e^+) for (\bar{H}) experiments - an increase of over an order of magnitude. For the first time, (\bar{p}) are observed to be centrifugally separated from the electrons that cool them, indicating a low-temperature, high-density (\bar{p}) plasma. Determination of the (\bar{p}) temperature is achieved through measurement of the (\bar{p}) evaporation rate as their confining well is reduced, with corrections given by a particle-in-cell plasma simulation. New applications of electron and adiabatic cooling allow for the lossless reduction in (\bar{p}) temperature from thousands of Kelvin to 3.5 K or colder, the lowest ever reported. The sum of the 20 trials performed in 2011 in which (\bar{p}) and (e^+) mix to form (\bar{H}) in the presence of a magnetic quadrupole trap reveals a total of (105 \pm 21) trapped (\bar{H}), or (5 \pm 1) per trial on average. This result paves the way towards the large numbers of simultaneously trapped (\bar{H}) that will be necessary for laser spectroscopy.