Physics at CERN’s Antiproton Decelerator

The Antiproton Decelerator (AD) facility of CERN began operation in 1999 to serve experiments for studies of C P T 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 + ) and antiprotons ( p ¯ ) confined in magnetic Penning traps. Cold H ¯ was also produced in collisions between Rydberg positronium ( P s ) 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 M p ¯ / m e = 1836.1526736 ( 23 ) , which agrees with the p 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 μ p ¯ = − 2.792845 ( 12 ) μ nucl in nuclear magnetons. Other results reviewed here include the first measurements of the energy loss ( − d E / d x ) of 1–100 keV p ¯ traversing conductor and insulator targets; the cross sections of low-energy ( < 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.