Physics with Trapped Antihydrogen

For more than a decade antihydrogen atoms have been formed by mixing antiprotons and positrons held in arrangements of charged particle (Penning) traps [1,2]. More recently, magnetic minimum neutral atom traps have been superimposed upon the anti-atom production region, promoting the trapping of a small quantity of the antihydrogen yield [3-5]. We will review these advances, and describe some of the first physics experiments performed on anrtihydrogen including the observation of the two-photon 1S-2S transition [6], invesigation of the charge neutrailty of the anti-atom [7,8] and studies of the ground state hyperfine splitting [9]. We will discuss the physics motivations for undertaking these experiments and describe some near-future initiatives. 1. M. Amoretti \textit{et al}. (ATHENA Collaboration), Nature \textbf{419} (2002) 456 2. G. Gabrielse \textit{et al.} (ATRAP Collaboration), Phys. Rev. Lett. \textbf{89} (2002) 213401 3. G.B. Andresen \textit{et al}. (ALPHA Collaboration), Nature \textbf{468} (2010) 673 4. G.B. Andresen \textit{et al}. (ALPHA Collaboration), Nature Phys. \textbf{7} (2011) 558 5. G. Gabrielse \textit{et al.} (ATRAP Collaboration), Phys. Rev. Lett. \textbf{108} (2012) 113002 6. M. Ahmadi \textit{et al}. (ALPHA Collaboration), Nature \textbf{541} (2017) 506 7. C. Amole \textit{et al}. (ALPHA Collaboration), Nature Commun. \textbf{5} (2014) 3955 8. M. Ahmadi \textit{et al}. (ALPHA Collaboration), Nature \textbf{529} (2016) 373 9. C. Amole \textit{et al}. (ALPHA Collaboration) Nature \textbf{483} (2012) 439