Antiproton Decelerator

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Antiproton Decelerator
Type Storage ring
purpose decelerate antiprotons
Installation 1999
scope 188 m
Initial impulse 3.5 GeV / c
Final pulse 100 MeV / c
Cooling methods

The Antiproton Decelerator (abbreviation AD ; German: antiprotons -Entschleuniger) is a storage ring at CERN in Geneva . The aim of the AD is to slow down the antiprotons generated by the proton synchrotron and to make them available for the various antimatter experiments.


At CERN, antiprotons have been generated with the proton beam of the proton synchrotron since the late 1970s and captured for experiments with antimatter in the "Antiproton Accumulator" (AA), "Antiproton Collector" (AC) and "Low Energy Antiproton Ring" ( LEAR ) facilities , collected and slowed down. In 1995 the PS210 experiment at the LEAR storage ring at CERN showed that it is in principle possible to produce anti-hydrogen . However, only 9 anti-hydrogen atoms with a kinetic energy of about 1.2  GeV could be produced. With anti-hydrogen, this corresponds to a speed of 0.9  c , or a temperature of 1.4 · 10 13  Kelvin. Because of this high temperature, one speaks of "hot" anti-hydrogen. Since one can also use antiatoms theories such as B. can check the CPT theorem and various predictions about antigravity , it is of particular interest to carry out experiments on antihydrogen. In order to carry out high-precision experiments, one needs much larger amounts and anti-hydrogen atoms that are several orders of magnitude colder. This could not be achieved with the PS210 structure. In 1996 the facilities were shut down in favor of the LHC .

Because of the continued great interest in cooled antiprotons, it was decided to build the antiproton decelerator based on the components of the AC . The renovation plans were approved in February 1999. In 1999 the Antiproton Decelerator was functional and is able to deliver 2 · 10 7 antiprotons with a kinetic energy of 5.3 MeV. After the completion of the AD, various antimatter experiments were set up inside the storage ring. Many deal with the production of cold anti-hydrogen (e.g. ATHENA, ATRAP), others use the antiprotons for other purposes such as e.g. B. ASACUSA, which takes measurements on exotic atoms .


With the ELENA decelerator ring, a further improvement of the beam from the Antiproton Decelerator has been achieved since 2019. A synchrotron ring, the main component of ELENA, should ultimately slow down the antiprotons from 5.3 MeV to 0.1 MeV, and an integrated electron cooler should also cool them. As of 2019, however, work is still ongoing to connect all experiments of the antiproton decelerator complex to ELENA.


Antiproton generation

Cross-section for the formation of a proton-antiproton pair as a function of the primary proton energy.

Since antiprotons do not occur naturally on Earth, they have to be created artificially. This is usually done by pairing . A charged particle (e.g. a proton p) with high kinetic energy is shot at a target . If the beam particle hits an atomic nucleus, it interacts with a proton in the nucleus and a particle-antiparticle pair is generated. Under certain circumstances, a proton-antiproton pair is formed.

The antiproton generated in this way is separated from the protons and the other generated particle-antiparticle pairs by a mass spectrometer , so that only antiprotons are left in the beam tube. Because of the conservation of four momentum , the minimum kinetic energy is


This corresponds to an impulse of 6.5 GeV / c. Since the protons are bound in the core of the target material, the actual energy is somewhat lower and depends on the material used. Copper , iridium and beryllium are common here .

Since the formation of antiprotons with proton pulses higher than 6.5 GeV / c is much more likely (see cross-section graph), protons with a pulse of 26 GeV / c are used, which corresponds to a kinetic energy of about 25 GeV. A particle accelerator is required in order to be able to provide this very high energy . In the case of the AD, this is the proton synchrotron , which is also used as a pre-accelerator for the LHC .


The antiproton production and storage complex at a glance. The red beam tube in the left area of ​​the image is used to inject protons for calibration purposes.

The AD is a storage ring with a circumference of 188 m. It essentially consists of the parts of the Antiproton Collector , a storage ring that was previously used to collect antiprotons and was also used in the PS210 experiment. However, many parts were heavily modified. All power converters have been better stabilized and the vacuum has been improved by a factor of 20 (with AD a few 10 −8 ) pascals compared to its predecessor . Acceleration cavities are used to slow down the antiprotons, but they are operated “upside down” so that the particles are slower after passing through the cavity. In order to reduce the emittance of the particle beam, the AD has the ability to use the two standard cooling methods, stochastic cooling and electron cooling . After the deceleration procedure, the antiprotons are passed on to the experiments with a kicker . A kicker is an electromagnet that can be switched on quickly and thus changes the path of the particles. A kind of switch for charged particles can be realized by clever control.

The AD can be filled with protons for calibration purposes. Since protons have the opposite charge of antiprotons, the deflection magnets deflect them in the opposite direction. However, so that they can still be saved, they can be shot in the opposite direction using a second jet pipe (the red loop in the picture).

In order to use the space of the hall optimally, the experiments are set up inside the AD-Ring.


In order to calibrate the AD and to synchronize the components, it is put into operation with protons. The advantage that protons have over antiprotons is the fact that they are available in much larger quantities, since they can be shot directly from the proton synchrotron into the AD and need not be produced in an intermediate step via pair creation. Typically 3 · 10 10 protons are available, while in operation it is only 5 · 10 7 . In this way, the signals from the measuring devices become stronger and you get a better signal-to-noise ratio .


The individual braking and cooling processes in chronological order.

A deceleration cycle begins with the antiprotons from the target being shot into the AD with an impulse of 3.5 GeV / c. Since the emittance is still very high, it is reduced with the help of the stochastic cooling method (see cooling graph). After the emittance has been sufficiently reduced, the actual braking process begins. In a few seconds, the antiprotons are brought to a pulse of 2 GeV / c with the help of the cavities. At the same time, however, the emittance is increased again, which is why stochastic cooling must be used again. It should be noted that cooling measures only serve to reduce the emittance and are not responsible for the fact that the particle package becomes slower overall. If the antiprotons were to be brought directly to the desired pulse of 100 MeV / c, too many antiprotons would be lost in the beam pipe due to the increasing emittance. After the second cooling, it can be slowed down again and the emittance reduced by means of electron cooling. This is repeated one more time to achieve the desired pulse of 100 MeV / c. After this braking process, about 2 · 10 7 slow antiprotons are available. If you compare this with the 10 13 protons that hit the target, you need an average of 5 · 10 5 protons to generate a slow antiproton. The cooled antiprotons are guided to the experiments with the help of a kicker. After the slow antiprotons have been passed on to the experiments, the AD can be refilled with fast antiprotons and the whole process begins again.


After the completion of the AD, various antimatter experiments were set up. A selection of these is described below.


Since only nine very hot anti-hydrogen atoms could be produced in the PS210 experiment, the ATHENA collaboration wanted to show that it is possible to produce larger amounts of cold anti-hydrogen. In order to achieve this, an apparatus was produced that can be divided into three sections: First, the antiproton trap, in which the antiprotons from AD are caught and further cooled, second, the positron generation, collection and cooling area, and finally the Mixing area in which the two components of the anti-hydrogen are brought together and can recombine .

Antiproton trap

Procedure for antiproton accumulation

The antiprotons from the AD ring have a pulse of 100 MeV / c, which corresponds to a temperature of 6.2 · 10 10  Kelvin. It is therefore necessary to cool them down further. According to the Bethe formula , charged particles lose kinetic energy when they penetrate a solid, which is why a 130 µm thick aluminum foil is placed in the way of the antiproton packet. Since protons are present in the nuclei of the aluminum atoms , one might think that the antiprotons annihilate immediately upon contact , but the annihilation rate is strongly dependent on the interaction time, which is very short. Therefore only a very small percentage of the antiprotons are lost through annihilation. Then the still high-energy antiprotons get into the prepared collection trap. The collecting trap is a cylindrical Penning trap . In contrast to the classic Penning trap, the electric quadrupole field is not achieved by hyperbolic electrodes , but by segmented cylinder electrodes in which each ring has a different potential . This makes it possible to form a potential well in which charged particles can be trapped (see picture on the right). Since the anti protons must come only in the potential well, it is opened in the first 200 ns after the incidence of the anti-proton packet on the aluminum foil on one side, while on the other hand, a voltage of 5 k V is applied. Antiprotons that have less than 5 keV kinetic energy after passing through the aluminum foil cannot overcome the potential mountain and are reflected. However, these are only less than 0.1% of all antiprotons, so that of the original 2 · 10 7 only about 10 4 remain. In order for the reflected antiprotons to remain in the trap, a voltage of 5 kV must also be applied to the other side of the trap after about 0.5 µs. The antiprotons now swing back and forth between the two potential walls of the trap. To brake the 5 keV protons anti few meV, one has before the arrival of the Bunchs cold (about 15 K and 1.3 MeV) electron precharged in the case. Since electrons and antiprotons are negatively charged, it is not a problem to catch them in the same trap. If the antiprotons now fly through the cold electrons, they give their temperature to the colder electrons and thus lose energy. The heated electrons in turn give off their energy through synchrotron radiation in the magnetic field of the trap. Antiprotons, which are approx. 1,800 times heavier than positrons, also emit synchrotron radiation, but the radiation power depends very much on the mass of the particles and increases rapidly with falling mass. After a few seconds, the antiprotons have completely given their thermal energy to the electrons, which in turn have reduced the temperature through synchrotron radiation. Finally, the trapped particles are in thermal equilibrium with the surrounding cooled superconducting magnets at around 15 K and are now ready to be transferred into the mixed trap.

Positron generation and accumulation

To produce the positrons for the anti-hydrogen, one could proceed in exactly the same way as with the production of the antiprotons, but here nature provides a simpler way. The radioactive isotope 22 Na decays with a probability of 90% through β + decay into 22 Ne , a positron, an electron neutrino and a high-energy photon .

The resulting fast positron is now also caught in a cylindrical Penning trap and cooled down. On the one hand there is nitrogen gas in the trap at a very low pressure. If the positrons move through the gas, they excite it. This happens inelastically, so that the positrons lose kinetic energy and are slowed down. Again, antimatter (positrons ) come into contact with normal matter ( shell electrons of nitrogen) and begin to annihilate. However, the pressure of the nitrogen gas is very low and the positron flux of 5 · 10 6 positrons per second is so high that losses are negligible. The other method used is the rotating wall technique , in which a rotating electric field is superimposed on the trap potential, which in the magnetic field of the coil leads to a compression of the positron cloud. The time it takes for AD to slow down the antiprotons is used to accumulate the positrons in the Penning trap. So in the end there are over 3 · 10 8 positrons in the trap.

The mixed trap

How to mix antiprotons and positrons

Now you have created the two components of an anti-hydrogen atom and you have to bring them into the same area of ​​space so that they can recombine. As with the other two traps, a cylindrical Penning trap, which consists of many individual ring electrodes, is used to realize the complex potential. First, the positrons are transported into the mixed trap. This is done by setting the potential on one side of the positron trap to zero; the positrons flow out of the positron trap due to their low intrinsic speed, like gas from a gas cylinder. The potential of the mixed trap at this point is  similar to that of the antiproton trap at t = 200 ns. As soon as the positrons have flown into the empty mixed trap, the potential on the other side of the mixed trap is increased and the positrons are trapped in the mixed trap. During this process you lose about 50% of the positrons. The positron cloud is then compressed axially so that it does not fill the entire volume of the mixed trap. Now you want to add the antiprotons, but you run into the problem that the two particles are charged differently (antiproton negative, positron positive), which means that they cannot be stored together in a normal Penning trap. Clearly speaking, one can say that a potential well for positrons represents a potential mountain for antiprotons. To solve this problem, the potential that can be seen in the picture under 1) is applied to the trap. The positrons as well as the antiprotons are each trapped in their potential well, which is "open" in a different direction depending on their charge. In order to get the antiprotons into the mixed area, change the potential so that it takes on the dashed curve in Figure 2). This allows the antiprotons to "slide" into the mixed trap. After the antiprotons have been transferred to the larger potential well, the old potential is applied to the trap again (Fig. 3). The resulting trap is called a nested Penning trap because it combines two Penning traps. Although the picture gives the impression that the two types of particles are separate from each other, one must remember that they can be in the same trap volume and recombine with one another. They are only held in the correct position by the potential.

When a positron and an antiproton have come together, electrically neutral anti-hydrogen is created. This neutral anti-hydrogen is no longer held by the trap potential and the magnetic field, and so the anti-atom can move freely inside the trap until it hits the ring electrodes of the Penning trap. There the two particles annihilate with their respective material partner from the electrode material. Characteristic annihilation radiation is emitted. This radiation can be detected with a detector and thus it can be counted how many anti-hydrogen atoms were produced.

In 2002 ATHENA was able to produce a total of 5 · 10 5 cold anti-hydrogen atoms in this way . The kinetic energy was 0.2 eV, which corresponds to a temperature of about 2000 ° C. This is not "cold" in the sense of a few millikelvin , but if you compare the temperature with the 1.4 · 10 13  Kelvin for PS210, the expression is justified. However, no high-precision experiments were carried out on ATHENA, only the production of large amounts of cold anti-hydrogen was demonstrated. In the meantime, the project has been discontinued in favor of the follow-up experiments AEGIS and ACE.


ATRAP was created at the same time as ATHENA at AD. At ATRAP, too, the goal was to produce cold anti-hydrogen. The two experiments are very similar, except for the type of positron accumulation, which is described below for ATRAP.

Positron generation and accumulation

The positron production and storage area.

There are currently two effective ways of slowing down the fast positrons by inelastic processes. The ATRAP collaboration chose a different path than ATHENA. The fast positrons emitted by 22 Na (as with ATHENA) were first decelerated by a 10 µm thick titanium foil and then hit a 2 µm thick tungsten crystal . Inside the crystal there is then the possibility that a positively charged positron and a negatively charged electron can combine to form a positronium atom. During this process, the positrons lose a large part of their energy, so that it is no longer necessary here, as with ATHENA, to slow them down further with nitrogen gas. If the positronium atom now reaches the Penning trap at the end of the apparatus, it is ionized there and the positron is trapped in the trap.

Since the positron accumulation was not particularly efficient in this way, the ATRAP experiment has now switched to the method used at ATHENA.

Current development

In contrast to ATHENA, ATRAP has not yet been discontinued and has been continuously improved and expanded. ATRAP now has a Penning-Ioffe trap that can store the electrically neutral anti-hydrogen with the help of magnetic quadrupole fields. This is possible because the magnetic moment of anti-hydrogen is non-zero.


The ASACUSA experiment has specialized in producing exotic atoms in the form of antiprotonic helium , i.e. a helium atom in which one shell electron has been replaced by an antiproton. If one examines these atoms with spectroscopic methods, one can test different aspects of the CPT theorem . Among other things, this predicts that the masses of the proton and the antiproton are identical. The formula

links the wavelength of the emitted light to be measured with the atomic number , the Rydberg constant , the main quantum numbers involved in the transition and the nuclear mass and the mass of the antiproton . This formula is only a first approximation which relativistic and QED effects such as B. neglecting the Lamb shift . Nevertheless, it illustrates the idea behind the measurement quite well.

Except for the wavelength and the antiproton mass , all observables are known. The antiproton mass can therefore be determined very precisely by measuring the wavelength with great precision and compared with the mass of the proton. If the values deviate from one another within the measurement error , then the CPT theorem is refuted.

ASACUSA measured several radiation transitions with high precision, but could not detect any deviations in the masses. So the CPT theorem still holds.

ACE (AD-4)

The potential benefits of using antiprotons in radiation therapy for malignant tumors is being explored by the ACE collaboration. Due to the released annihilation energy, the dose is roughly doubled compared to protons in the Bragg peak with the same dose in the input channel. This could protect the healthy tissue in the vicinity of the tumor. In addition, the detection of high-energy pions promises opportunities for online dose verification.


ALPHA is engaged in the production, capture and measurement of anti-hydrogen molecules. To do this, positrons and antiprotons are first stored in a Penning trap , and then brought together in a magnetic octupole trap ( Ioffe trap ). The anti-hydrogens are detected indirectly by the annihilation particles in a silicon vertex detector , photons in the case of positrons and pions for the antiproton.

In 2010, ALPHA was the first to catch anti-hydrogen. In 2011 it was possible to store 309 anti-hydrogen atoms for over 1000 seconds (over a quarter of an hour). The first measurement of a transition in anti-hydrogen was published in 2012 by the same group. In 2016, the 1S – 2S transition of antihydrogen was accurately measured for the first time. As predicted by the CPT theorem, the spectral lines of the 1S – 2S transition of hydrogen and anti-hydrogen agree to an accuracy of 2 · 10 −10 .


As already mentioned above, there are various quantum theoretical descriptions of gravitation, which do not exclude the possibility that antimatter in the earth's gravitational field could experience a different acceleration of gravity than normal matter. To check this, the AEGIS collaboration was founded. At the moment the experiment is still in the planning and preparation phase, but the basic structure has already been determined.

Anti-hydrogen was chosen as the test specimen. The reason for this lies in the electrical neutrality and the relatively simple production of anti-hydrogen. Other experiments that used charged antiparticles as test specimens (e.g. antiprotons) failed because of the electrical and magnetic forces acting on them due to weak fields that are omnipresent or generated by traps. This is understandable if one compares the electrical Coulomb force F C with the gravitational force F G of two electrons.

In this case, gravity is 4.2 · 10 42 times weaker than the electrical force.

Measuring principle

An overview of the complete measurement process from AEGIS.

First, positrons with kinetic energies of 100 eV to a few keV are shot at a target made of a nanoporous , non-conductive solid. Nanoporous here means that the pore size is in the range from 0.3 to 30 nm. The incident positron is decelerated very quickly in the material and, under certain circumstances, can enter into a bond state with a shell electron from the insulator; this is how positronium is created . Since the dielectric constant in the pores is smaller than in the solid body and thus the binding energy of the positronium increases, it preferably collects in these free spaces. There the positronium repeatedly hits the wall and loses more and more kinetic energy until it is as large as the thermal energy of the target material. By cooling down the insulator, very cold and therefore very slow positronium can be accumulated. Once the positronium has thermalized , it can diffuse out of the insulator . During this entire process, a large proportion of the positrons are lost through annihilation. However, by appropriately dimensioning the positron flux, a sufficiently large number of thermal positronium can be provided. If the positronium is now brought together with the antiprotons that were previously accumulated and cooled in a Penning trap, anti-hydrogen is formed. However, this reaction has a very low probability, since in positronium in the ground state the positron is very strongly bound to the electron. In order to reduce the binding energy, the positronium can be excited to high main quantum numbers in the range of n  = 30 ... 40 with the help of lasers . Figuratively speaking, the two particles move away from each other and feel the mutual attraction less. In the case of highly excited states (one also speaks of Rydberg states ), the probability of anti-hydrogen formation increases approximately to the fourth power of the principal quantum number n . So the formation equation looks like this:

The star means that the atom is in a Rydberg state.

Antihydrogen is electrically neutral and can leave the trap in any direction, including in the direction of the Stark acceleration electrodes (see picture). Since an anti-hydrogen jet is required for the measurement, the slow anti-hydrogen must be accelerated in one direction. However, due to the electrical neutrality, this cannot be achieved with a homogeneous electrical field. Anti-hydrogen, however, has an electrical dipole moment and can therefore be accelerated in an electrical gradient field . This fact is comparable to the everyday experience that a jet of water (which is electrically neutral) can be deflected with a charged comb. The water is accelerated towards the ridge in the inhomogeneous electric field of the ridge. As this technique is related to the Stark effect in antihydrogen, it is also called Stark acceleration. The speed v that is to be achieved will be approx. 400 m / s. In order to measure the gravitational acceleration g , the beam is allowed to fly a certain distance L. In the time T  =  L / v the anti-hydrogen atoms “fall” in the gravitational field of the earth. So the antiatoms perform a horizontal throw . During the fall, the beam is deflected by the distance δ x from the horizontal. Since the velocity v is very small, classical Newtonian mechanics can be applied and obtained

By measuring the displacement δ x one can determine the gravitational acceleration g for antimatter. In the AEGIS experiment, this is done with a spatially resolving moiré detector . The first target for the measurement accuracy was a measurement deviation of 1%.

Related projects

With the Fermilab Antiproton Accumulator, the USA also has an antiproton storage ring. In 1997, with the E862 experiment, 66 anti-hydrogen atoms were produced in a similar way to the PS210 experiment.

With the FAIR accelerator center, a similar facility will also be available in Germany from around 2020. To this end, the existing accelerator facility at GSI will be greatly expanded. Although this facility will be in Germany, it is designed as an international project, similar to CERN.


  • Wolfgang Demtröder: Laser spectroscopy, basics and techniques. Springer, 2007, ISBN 978-3-540-33792-8
  • Ingolf V. Hertel, Claus-Peter Schulz: Atoms, Molecules and Optical Physics 1. Springer, 2008, ISBN 978-3-540-30613-9
  • Frank Hinterberger: Physics of Particle Accelerators and Ion Optics. Springer, 2008, ISBN 978-3-540-75281-3
  • Seminar lecture on antimatter: Antimatter (English; PDF; 3.4 MB)

Web links

Individual evidence

  1. CERN: The Antimatter Factory - What is the AD? ( English ) Archived from the original on January 17, 2010. Accessed June 16, 2016th
  2. Website of the PS210 experiment
  3. The first experimental evidence of (hot) anti-hydrogen: G. Baur et al. "Production of Antihydrogen", Phys. Lett. B 368 (1996) p. 251
  4. Possible deviation from the "normal" gravitational force: Goldman et al. "Experimental Evidence for Quantum Gravity?" Phys. Let. B 171 (1986) p. 217
  5. a b Overview of the AD: S. Maury "THE ANTIPROTON DECELERATOR (AD)" (pdf; 433 kB)
  6. W. Oelert (January 23, 2015): The ELENA project at CERN. arxiv : 1501.05728 [abs]
  7. ^ Announcement on the commissioning of ELENA: Anaïs Schaeffer "Exceptionally slow antiprotons" (March 7, 2019)
  8. Overview article on the subject of antiprotons: Antimatter Decelerator - Story
  9. Experimental area inside the AD: M. Giovannozzi et al. "Experimental Area of ​​the CERN Antiproton Decelerator" (pdf; 1.2 MB)
  10. Welcome to the ATHENA Experiment
  11. Review article on ATHENA: Amoretti et al. "The ATHENA antihydrogen apparatus" NIM A 518 (2004) 679
  12. website of the ATRAP experiment
  13. G. Gabrielse et al. "Background-Free Observation of Cold Anti-Hydrogen with Field-Ionization Analysis of Its States", Phys. Rev. Lett. 89 (2002) p. 213401
  14. G. Gabrielse et al. Antihydrogen Production within a Penning-Ioffe Trap , Phys. Rev. Lett. 100 (2008) p. 113001
  15. website of the ASACUSA experiment
  16. Review article on the ASACUSA experiment: T. Azuma et al. ATOMIC SPECTROSCOPY AND COLLISIONS USING SLOWANTIPROTONS (pdf; 1.7 MB)
  17. M. Hori et al. Sub-ppm laser spectroscopy of antiprotonic helium and a CPT-violation limit on the antiprotonic charge and mass , Phys. Rev. Lett. 87 (2001) 093401 (pdf; 201 kB)
  18. Homepage of the ACE collaboration, ad4 homepage ( Memento of the original from December 21, 2010 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /
  19. ALPHA experiment
  20. Andresen, G. et al .: Trapped antihydrogen . In: Nature . 468, No. 7321, 2010. doi : 10.1038 / nature09610 .
  21. ALPHA Collaboration: Confinement of antihydrogen for 1000 seconds . In: Cornell University . 2011. arxiv : 1104.4982 .
  22. ALPHA Collaboration: Resonant quantum transitions in trapped antihydrogen atoms . In: Nature . 2012.
  23. M. Ahmadi, BXR Alves, CJ Baker, W. Bertsche, E. Butler, A. Capra, C. Carruth, CL Cesar, M. Charlton, S. Cohen, R. Collister, S. Eriksson, A. Evans, N. Evetts, J. Fajans, T. Friesen, MC Fujiwara, DR Gill, A. Gutierrez, JS Hangst, WN Hardy, ME Hayden, CA Isaac, A. Ishida, MA Johnson, SA Jones, S. Jonsell, L. Kurchaninov, N. Madsen, M. Mathers, D. Maxwell, JTK McKenna, S. Menary, JM Michan, T. Momose, JJ Munich, P. Nolan, K. Olchanski, A. Olin, P. Pusa, C. Ø . Rasmussen, F. Robicheaux, RL Sacramento, M. Sameed, E. Sarid, DM Silveira, S. Stracka, G. Stutter, C. So, TD Tharp, JE Thompson, RI Thompson, DP van der Werf, JS Wurtele: Observation of the 1S – 2S transition in trapped antihydrogen . In: Nature . Accelerated Article Preview Published, December 19, 2016, ISSN  1476-4687 , doi : 10.1038 / nature21040 .
  24. A. Kellerbauer et al. Proposed antimatter gravity measurement with an antihydrogen beam NIM B 266 (2008) 351 (pdf; 237 kB)
  25. ^ Website of the E862 experiment: Antihydrogen at Fermilab - Observation of Antihydrogen Atoms. (No longer available online.) FNAL , 2000, archived from the original on May 10, 2014 ; accessed on June 16, 2016 .

Coordinates: 46 ° 14 '2.2 "  N , 6 ° 2' 47"  E ; CH1903:  492,603  /  121234