Physics at CERN’s Antiproton Decelerator

The Antiproton Decelerator (AD) facility of CERN began operation in 1999 to serve experiments for studies of CPTCPT invariance by precision laser and microwave spectroscopy of antihydrogen (H¯) and antiprotonic helium (p¯He+) atoms. The first 12 years of AD operation saw cold H¯ synthesized by overlapping clouds of positrons (e+e+) and antiprotons (p¯) confined in magnetic Penning traps. Cold H¯ was also produced in collisions between Rydberg positronium (PsPs) atoms and p¯. Ground-state H¯ was later trapped for up to ∼1000 s in a magnetic bottle trap, and microwave transitions excited between its hyperfine levels. In the p¯He+ atom, deep ultraviolet transitions were measured to a fractional precision of (2.3–5)×10−9 by sub-Doppler two-photon laser spectroscopy. From this the antiproton-to-electron mass ratio was determined as Mp¯/me=1836.1526736(23), which agrees with the pp value known to a similar precision. Microwave spectroscopy of p¯He+ yielded a measurement of the p¯ magnetic moment with a precision of 0.3%. More recently, the magnetic moment of a single p¯ confined in a Penning trap was measured with a higher precision, as μp¯=−2.792845(12)μnucl in nuclear magnetons. Other results reviewed here include the first measurements of the energy loss (−dE/dx−dE/dx) of 1–100 keV p¯ traversing conductor and insulator targets; the cross sections of low-energy (<10<10  keV) p¯ ionizing atomic and molecular gas targets; and the cross sections of 5 MeV p¯ annihilating on various target foils via nuclear collisions. The biological effectiveness of p¯ beams destroying cancer cells was measured as a possible method for radiological therapy. New experiments under preparation attempt to measure the gravitational acceleration of H¯ or synthesize H¯+. Several other future experiments will also be briefly described.