Physics at CERN's Antiproton Decelerator

The Antiproton Decelerator of CERN began operation in 1999 to serve experiments for studies of CPT invariance by precision laser and microwave spectroscopy of antihydrogen ($\bar{\rm H}$) and antiprotonic helium ($\bar{p}{\rm He}^+$). The first 12 years of operation saw cold $\bar{\rm H}$ synthesized by overlapping clouds of positrons ($e^+$) and antiprotons ($\bar{p}$) confined in magnetic Penning traps. Cold $\bar{\rm H}$ was also produced in collisions between Rydberg positronium atoms and $\bar{p}$. Ground-state $\bar{\rm H}$ was later trapped for up to $\sim 1000$ s in a magnetic bottle trap, and microwave transitions excited between its hyperfine levels. In the $\bar{p}{\rm He}^+$ atom, UV transitions were measured to a precision of (2.3-5) $\times$ $10^{-9}$ by sub-Doppler two-photon laser spectroscopy. From this the antiproton-to-electron mass ratio was determined as $M_{\bar{p}}/m_e=$1836.1526736(23), which agrees with the p value. Microwave spectroscopy of $\bar{p}{\rm He}^+$ yielded a measurement of the $\bar{p}$ magnetic moment with a precision of 0.3%. More recently the magnetic moment of a single $\bar{p}$ confined in a Penning trap was measured with a higher precision, as $\mu_{\bar{p}}=-2.792845(12)$$\mu_{\rm nucl}$ in nuclear magnetons. Other measurements include the energy loss of 1-100 keV $\bar{p}$ traversing conductor and insulator targets; the cross sections of <10 keV $\bar{p}$ ionizing gas targets; and the cross sections of 5-MeV $\bar{p}$ annihilating on target foils via nuclear collisions. The biological effectiveness of $\bar{p}$ beams destroying cancer cells was measured as a possible method for radiological therapy. New experiments under preparation attempt to measure the gravitational acceleration of $\bar{\rm H}$ or synthesize $\bar{\rm H}^+$.