Precision experiments with stored ions and antimatterMax Planck Institute for Nuclear PhysicsUniversity of HeidelbergEuropean Research Council
Ultracold Ions and Antimatter Research
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Antimatter gravity experiment

Antimatter has been one of the most fascinating fields of research ever since the prediction of its existance by Paul Dirac in 1931 and the discovery of the anti-electron by Carl Anderson in 1932. Dirac's postulate has been fully incorporated into the Standard Model of Particle Physics, which predicts that each of the fundamental particles has an equivalent antimatter partner.
Presumably, equal amounts of matter and antimatter were formed in the Big Bang, but ordinary matter is clearly prevalent in the observable Universe today. This imbalance could be explained by a slight difference in one of the fundamental properties of particle-antiparticle pairs (such as charge or mass), a violation of CPT symmetry for which there isn't yet any experimental evidence. Elementary antimatter particles naturally occur in radioactive decays and in cosmic radiation and some of them, such as the positron and the antiproton, have been studied extensively and even compared to their matter equivalents. On the other hand, laser spectroscopy holds the prospect of allowing even more precise comparisons between matter and antimatter systems. Furthermore, a neutral anti-atom could be used to test the effect of gravity on antimatter for the first time, because it is immune to stray electromagnetic fields that have hampered such studies with charged antimatter particles in the past.

The ATHENA experiment at CERN
The ATHENA experiment at CERN was the first to produce large amounts of cold antihydrogen in 2002

In 2002, our ATHENA experiment at CERN's Antiproton Decelerator (AD) was the first to produce copious amounts of cold antihydrogen, the simplest atomic antimatter system. The antiprotons supplied by the AD were trapped and cooled, and brought into overlap with positrons from a radioactive sodium source in a cylindrical Penning trap.
The produced anti-atoms, no longer confined in the charged-particle trap, drifted radially outward and annihilated on the electrodes. ATHENA's sophisticated detector allowed the temporally and spacially resolved reconstruction of these annihilation events. During the data taking periods in 2003 and 2004, the experimental parameters were optimized in order to maximize the antihydrogen production rate, and the temperature and internal quantum states of the anti-atoms were determined. Data taking with ATHENA ended in 2004, and its scientists devised more advanced antimatter experiments.

Antihydrogen annihilation
Antihydrogen annihilation event recorded with the ATHENA detector

Together with other ATHENA collaborators, along with new groups from other institutes, our group devised and set up a successor experiment with the aim of measuring the effect of gravity on antimatter. The goal of the AEGIS experiment (Antimatter Experiment: Gravity, Interferometry, Spectroscopy) is to create a cold, horizontal beam of antihydrogen and to study its free fall in the Earth's gravitational field with a matter wave interferometer or a moiré deflectometer (its classical counterpart). Monte Carlo simulations have shown that a determination of the gravitational acceleration on antimatter of the order of 1% will be feasible. Construction of AEGIS began in early 2010. The MPIK group, together with our colleagues from the University of Genoa, is responsible for the design, construction and operation of the AEGIS ion traps. AEGIS will use a novel technique for antihydrogen production, combining positronium (the bound system of a positron and an electron) with antiprotons.

Overview sketch of
	the AEGIS experiment
The AEGIS experiment will study the gravitational interaction of antimatter

In 2014, we performed a proof-of-principle measurement of the force acting on a beam of antiprotons using a deflectometer. The force (due to electromagnetic fringe fields) was measured to be a few hundred attonewtons, while the deflection of 10 μm was exactly the one expected in a full-scale antihydrogen experiment. More recently, we used annihilation spectroscopy to determine the wavelength of the n = 1 → 3 transition in positronium. This is the first time this fundamental property has ever been measured.

Starting in 2019, The new ELENA storage ring, which is currently being installed, will supply antiprotons to the AD experiments at a 50 times lower energy than before, paving the way for yet more precise measurements with antimatter than are currently possible.

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