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Antiproton physics in the ELENA Era

Scientific meeting








Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ


Theo Murphy international scientific meeting organised by Professor Niels Madsen, Professor Michael Charlton, Professor Jeffrey S. Hangst and Professor Walter Oelert.

Antiproton physics

The CERN Antiproton Decelerator supplies antiprotons to a lively scientific programme. The imminent addition of the ELENA machine will massively increase both the availability and the number of low energy antiprotons. The meeting gathers experts from all across antiproton science to discuss their vision of the new opportunities presented by this step change and how they will profit from and exploit them.

Recorded audio of the presentations will be available on this page after the meeting has taken place. Meeting papers will be published in a future issue of Philosophical Transactions of the Royal Society A.

Attending this event

This is a residential conference, which allows for increased discussion and networking

  • Free to attend
  • Advanced registration essential (please request an invite)
  • Catering and accommodation available to purchase during registration

Enquiries: contact the Scientific Programmes team


Event organisers

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Schedule of talks

04 September

Session 1 09:00-12:30

Theoretical aspects

6 talks Show detail Hide detail


Professor Michael Charlton, Swansea University, UK

09:15-09:45 Gravity and antimatter: theoretical aspects

Dr Diego Blas, CERN, Switzerland


Dr Blas will review the physical case of studying the gravitational properties of antimatter from a theoretical perspective. His plan is to first use an effective description to understand where one can get information about how antimatter gravitates from different phenomena. Second, he will describe which difficulties are faced in the construction of theories consistent these bounds

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09:45-10:00 Discussion

10:30-11:00 Antiatomic physics

Dr Svante Jonsell, Stockholm University, Sweden


Professor Jonsell will give an overview over what we know about antiproton and antihydrogen scattering on ordinary atoms and molecules. For antiproton scattering, most emphasis will be on inelastic processes, e.g. ionisation and antiproton capture. At the highest energies covered (~100 keV), ionisation is well described by the Born approximation. At lower energies (~10 keV), theoretical descriptions are usually based on classical trajectories in some form. Good agreement with experimental data from the ASACUSA collaboration has been obtained down to 2 keV. Below this limit very little is known from experiments, and we therefore compare classical trajectory calculations, to more advanced quantum mechanical or semi-classical calculations, where available.

For collisions involving antihydrogen Professor Jonsell will cover both inelastic processes, which may destroy antiatoms, and elastic collisions, which may eject them from a magnetic trap. At ultracold energies several quantum calculations have been performed for H and He targets. At higher energies, semi-classical approaches have been used.

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11:00-11:15 Discussion

11:15-11:45 Prospects of ELENA for testing Lorentz and CPT symmetry with antiprotons

Dr Arnaldo Vargas, Loyola University New Orleans, USA


The notion that small deviations from Lorentz symmetry might arise from quantum gravity candidate theories has motivated a world-wide systematic search for Lorentz violation.  The Standard-Model Extension (SME) is an effective field theory that facilitates the systematic search for Lorentz violation. The SME contains the standard model of particle physics, general relativity, and all the field operators that break Lorentz and CPT symmetry. Because of the deep connection between CPT symmetry and Lorenz symmetry the SME can also facilitate the systematic test of CPT symmetry.  Examples of test models derived from the SME can be found in the literature for antimatter experiments such as antihydrogen spectroscopy experiments, antiproton Penning-trap experiments, and antimatter gravity tests. The SME coefficients quantify any deviation from Lorentz and CPT symmetry, and for that reason the sensitivity of the experiments to the coefficients indicate the value of the experiments as probes of Lorentz and CPT symmetry.  The addition of ELENA to the Antiproton Decelerator at CERN will allow experiments to increase the sensitivity to coefficients for CPT and Lorentz violation, improving the limits at which these symmetries have been tested or perhaps discovering violations of them in nature.

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11:45-12:30 Discussion

Session 2 13:30-17:00

Trapped antihydrogen

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Professor Niels Madsen, Swansea University

13:30-14:00 Precision measurements of trapped antihydrogen in the ALPHA experiment at CERN

Dr Stefan Eriksson, Swansea University, UK


Precision measurements of trapped antihydrogen in the ALPHA experiment at CERN Antihydrogen, the antimatter equivalent of hydrogen, offers a unique way to test matter-antimatter symmetry. In particular, the CPT (charge, parity and time) theorem requires that hydrogen and antihydrogen have the same spectrum. Antihydrogen can reproducibly be synthesised and trapped in the laboratory for extended periods of time, offering an opportunity to study the properties of antimatter in detail. New techniques to study antihydrogen have emerged; the ALPHA collaboration at CERN can now interrogate the ground state energy structure with resonant microwaves, determine the gravitational mass to inertial mass ratio and measure charge neutrality. Very recently, the collaboration has observed the 1S-2S transition in trapped antihydrogen; the first observation of resonant interaction of light with an anti-atom. Due to the narrow intrinsic linewidth of the transition and use of two-photon laser excitation, the transition energy can be precisely determined in both hydrogen and antihydrogen, allowing a direct comparison. Our result is consistent with CPT invariance at a relative precision of around 2×10-10. This constitutes the most precise measurement of a property of antihydrogen. Here, I present the most recent work of the collaboration on antihydrogen in the ALPHA-2 apparatus and an outlook on improving the precision of measurements involving lasers and microwave radiation.

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14:00-14:15 Discussion

14:15-14:45 ATRAP

Professor Gerald Gabrielse, Harvard University, USA

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14:45-15:00 Discussion

15:30-16:00 History and prospects for antihydrogen gravitation measurements

Dr. William Bertsche, University of Manchester, UK


The ALPHA experiment at CERN is focused on performing precision measurements on low energy antihydrogen in an effort to rigorously test symmetries between matter and antimatter underlying many theories in contemporary physics. With the recent observation of the 1S-2S transition in a population of trapped antihydrogen atoms, ALPHA has entered the era on performing high precision atomic physics measurements on this trapped atom. ALPHA-g is a new experiment designed to perform direct free-fall measurements of antihydrogen in Earth’s gravitational field. The goal is to improve on the proof-of-principle experiment conducted in, with the ultimate goal of achieving a 1% measurement on antimatter gravitational acceleration g. This talk will present background on the proposed experiment as well as the current status of the project.

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16:00-16:15 Discussion

16:15-17:00 Poster session

05 September

Session 3 09:00-12:15

Antiproton physics and beams

6 talks Show detail Hide detail


Professor Jeffrey S. Hangst, Aarhus University

09:00-09:30 Challenging the Standard Model by the precise comparisons of the fundamental properties of protons and antiprotons.

Dr Stefan Ulmer, RIKEN, Japan


The Standard Model (SM) of particle physics is known to be incomplete. This inspires various searches for physics beyond, among them are tests of charge, parity, time (CPT) invariance that compare the fundamental properties of matter/antimatter conjugates at low energy and with high precision.

The Japanese/German BASE collaboration at the antiproton decelerator of CERN targets high-precision comparisons of the fundamental properties of antiprotons and protons, namely, charge-to-mass ratios and magnetic moments. To perform these tests we have developed an advanced Penning trap spectrometer which enabled the most precise measurement of the proton magnetic moment with a fractional precision of 3.3 parts in a billion, the most precise comparison of the proton-to-antiproton charge-to-mass ratio, with a fractional precision of 69 parts in a trillion, as well as the most precise measurement of the magnetic moment of the antiproton (0.8 p.p.m). Recent improvements in the stability of the apparatus enabled us to observe single antiproton spin transitions, based on this achievement a 100-fold improved measurement of the antiproton magnetic moment will become possible. This talk will summarise our most recent results and give an overview on the perspectives of BASE in the ELENA era.

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09:30-09:45 Discussion

10:15-10:45 GBAR: leading the dance with ELENA's antiprotons

Dr Pauline Comini, ETH Zürich, Switzerland


In order to observe the free fall of antihydrogen atoms, hence measuring the gravitational acceleration of antimatter on Earth, the GBAR experiment aims at producing these antiatoms with velocity of a few m.s-1. This will be achieved by implementing the sympathetic cooling of antihydrogen positive ions, an original idea proposed by J. Walz & T. Hänsch.

In its race toward ultracold antihydrogen atoms, GBAR has found a precious and necessary partner in the new antiproton decelerator ring, ELENA. This talk begins with a review of the different stages allowing GBAR to transform the 100 keV ELENA antiprotons into neV antihydrogen atoms, with particular attention to the ELENA parameters that influence most the design of the experiment.

As ELENA’s first user, GBAR is currently getting ready to receive the first 100 keV antiprotons. The present status of the installation in the AD hall and the planning of the coming years are then detailed. The recording of GBAR’s first antihydrogen free falls will provide a 1% precision on the measurement of g ̅; in order to further improve this precision to 0.01%, a future upgrade of the experiment is already being studied.

Finally, it is worth noting that each antiproton bunch delivered by ELENA to GBAR also bears the potentiality of a few hundreds of antihydrogen atoms, while a large part of the antiprotons remains untouched. Ideas proposed to exploit these beams are also sketched.

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10:45-11:00 Discussion

11:00-11:30 The ELENA facility

Dr Christian Carli, CERN, Switzerland


The CERN Antiproton Decelerator AD provides antiproton beams with a kinetic energy of 5.3 MeV to an active users community. This extraction energy is the lowest one possible under good conditions with the given circumference of the AD.

The Extra Low Energy Antiproton ring (ELENA) is a small synchrotron with a circumference a factor 6 smaller than the AD to further decelerate antiprotons from the AD from 5.3 MeV to 100 keV. Controlled deceleration in a synchrotron equipped with an electron cooler to reduce emittances in all three planes will allow the existing AD experiments to increase substantially their antiproton capture efficiencies and render new experiments possible.

ELENA ring commissioning is taking place at present and first beams to a new experiment installed in a new experimental area are foreseen this year. The transfer lines from ELENA to existing experiments in the old experimental area will be installed during CERN long Shutdown 2 (LS2) in 2019 and 2020.  The status of the project and ring commissioning will be reported.

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11:30-12:15 Discussion

Session 4 13:15-16:30

Beams of antihydrogen

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Professor Walter Oelert, Johannes Gutenberg University, Germany

13:15-13:45 Determination of the antiproton-to-electron mass ratio by laser spectroscopy of antiprotonic helium atoms

Dr Masaki Hori, Max Planck Institute of Quantum Optics, Germany


The (anti)proton-to-electron mass ratio is a fundamental dimensionless constant of nature that serves as a basis of our international system of units. The Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA) collaboration at CERN is carrying out precise laser spectroscopy experiments of antiprotonic helium. This is a three-body atom composed of a normal helium nucleus, an electron, and an antiproton occupying a highly excited Rydberg state. Various techniques such as sub-Doppler two-photon laser spectroscopy and buffer gas cooling of the atoms to cryogenic temperature T=1.5-1.7 K are employed to measure their transition frequencies to a precision of 2.5 parts in a billion. By comparing the results with three-body quantum electrodynamics calculations, the antiproton-to-electron mass ratio was determined as 1836.1526734(15). This agreed with the known proton-to-electron mass ratio at a precision of 0.8 parts per billion. This constitutes a consistency test of CPT symmetry.

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13:45-14:00 Discussion

14:00-14:30 AEgIS at ELENA: outlook for physics with a pulsed cold antihydrogen beam

Dr Michael Doser, CERN


The efficient production of cold antihydrogen atoms in particle traps at CERN's Antiproton Decelerator opened up the possibility to perform direct measurements of Earth's gravitational acceleration on purely antimatter bodies.

This is the goal of the AEgIS collaboration: measure the value of g using a pulsed source of cold antihydrogen and a moiré deflectometer/Talbot-Lau interferometer.

The milestones achieved so far by AEgIS, on the way of developing a pulsed cold antihydrogen source using resonant charge-exchange between antiprotons and cold Rydberg positronium, are presented. Cold positronium was first formed at room temperature in a dedicated setup. The spectroscopy of its 1-3 and 3-15 transitions was carried out, demonstrating the feasibility of AEgIS' proof-of-concept in-flight laser excitation. Positronium was subsequently formed from the target at 10K temperature inside the 1T magnetic field of the experiment.

In parallel, antiprotons were captured from the A.D. using aluminum degraders with 1.5% efficiency and cooled with electrons. These mixed e-/p+ plasma were radially compressed to sub-mm radii applying a rotating-wall drive  and progressively reducing the number of cooling electrons. Antiprotons were finally transferred to the antihydrogen production trap with 70% efficiency using an in-flight launch and recapture procedure. Two further critical steps that are germane mainly to charge-exchange formation of antihydrogen - cooling of antiprotons and formation of a beam of antihydrogen - are being addressed in parallel.

The coming of ELENA will allow, in the very near future, to increase the number of available antiprotons by up to a factor 50, overcoming the capture efficiency limitation of material degraders. This would be reflected  in an increase of produced antihydrogen atoms of nominally the same factor, leading to a significative reduction of measurement times. 

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14:30-14:45 Discussion

15:00-15:30 The ASACUSA antihydrogen and hydrogen program: results and prospects

Chloé Malbrunot, CERN, Switzerland


The goal of the ASACUSA CUSP collaboration at the Antiproton Decelerator of CERN is to measure the ground-state hyperfine splitting of antihydrogen using an atomic spectroscopy beamline. A milestone was achieved in 2012 through the successful detection of 80 antihydrogen atoms 2.7 meters away from their production region. This was the first observation of “cold” antihydrogen in a magnetic field free region and opened the way toward precision spectroscopy of antimatter atoms in a beam. In parallel to the progress on the antihydrogen production, the spectroscopy beamline intended to be used for antihydrogen spectroscopy was tested with a source of hydrogen. This led to a measurement at a relative precision of 10􀀀9 which constitues the most precise measurement of the hydrogen hyperfine splitting in a beam. This measurement also enabled to forecast the necessary conditions to achieve a measurement at the ppm level with antihydrogen. Unlike for hydrogen, the antihydrogen experiment is complicated by the difficulty of synthesizing enough cold antiatoms in ground state. A first measurement of the hyperfine splitting of antihydrogen with a precision 3 orders of magnitude worse than what was achieved with the same spectroscopy setup with hydrogen would however provide one of the most stringent CPT test on an absolute energy scale. My talk will present the latest developments and results with an emphasis on the spectroscopy apparatus. The coming years challenges within the ELENA era will also be discussed.

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15:30-15:45 Discussion

15:45-16:30 Overview, closing remarks and perspective on future

Professor Klaus Jungmann, University of Groningen, Netherlands

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Antiproton physics in the ELENA Era Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ