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New Journal of Physics

Diagnosing antihydrogen energy distributions using spatio-temporal annihilation patterns

Roya Ahemeh, R Akbari, L O de Araujo Azevedo, C. J. Baker, William Alan Bertsche, N. M. Bhatt, G. Bonomi, A. Capra, I Carli, Claudio Lenz Cesar, M. Charlton, April Cridland Mathad, A. Del Vincio, D. Duque Quiceno, S Eriksson, A Evans, J. Fajans, Tim Friesen, M C Fujiwara, L. M. Golino, M. B. Gomes Gonçalves, Jeffrey Scott Hangst, M. E. Hayden, Danielle Louise Hodgkinson, Christopher Aled Isaac, A. J. Uribe Jimenez, Stephen A. Jones, S. Jonsell, N. Madsen, V. R. Marshall, J. T. K. McKenna, Takamasa Momose, J. Nauta, Art Olin, A. N. Oliveira, A Powell, C. Ø. Rasmussen, Thomas G. Robertson-Brown, F. Robicheaux, R. L. Sacramento, E. Sarid, Joos Schoonwater, D. M. Silveira, J Singh, G Smith, Chukman So, S Stracka, J. Suh, A. G. Swadling, T. D. Tharp, K. A. Thompson, R. I. Thompson, E. Thorpe-Woods, M. Urioni, Dirk Peter van der Werf, Pooja Woosaree, Jonathan Syrkin Wurtele

Published Aug 1, 2026• 0 citations
Publisher

Abstract

Abstract Precision measurements of the fundamental properties of trapped antihydrogen in the (ALPHA) experiment at CERN require an accurate determination of the antihdyrogen energy distribution. We introduce a new diagnostic technique, termed Spatio-Temporal Annihilation Mapping (STAM), which measures this distribution by exploiting the dynamics of antihydrogen in adiabatically time-varying magnetic fields. By analyzing the spatial and temporal patterns of annihilations recorded during adiabatic release from magnetic confinement and comparing them with detailed simulations, we infer the underlying energy distributions. STAM enables the simultaneous measurement of axial and transverse energy components, achieving a relative precision of approximately 10% over a mean energy range of ∼ 10–100 mK in the ALPHA trap. The technique is applied to both uncooled and laser-cooled antihydrogen populations, with results benchmarked against established laser-based diagnostics. While laser-based methods provide high-resolution measurements of the transverse energy and limited sensitivity to the axial component, an accurate determination of the axial energy remains challenging. In addition to complementing existing techniques, STAM is expected to access energy regimes that are currently inaccessible to laser-based diagnostics due to intrinsic limitations of those methods. These capabilities make STAM broadly applicable to future high-precision spectroscopic and gravitational studies with antihydrogen. Furthermore, STAM provides a novel probe of off-axis magnetic field structures, addressing a critical gap in current magnetometry techniques within the ALPHA experiment.

DOI
10.1088/1367-2630/ae9c66

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Abstract Precision measurements of the fundamental properties of trapped antihydrogen in the (ALPHA) experiment at CERN require an accurate determination of the antihdyrogen energy distribution. We introduce a new diagnostic technique, termed Spatio-Temporal Annihilation Mapping (STAM), which measures this distribution by exploiting the dynamics of antihydrogen in adiabatically time-varying magnetic fields. By analyzing the spatial and temporal patterns of annihilations recorded during adiabatic release from magnetic confinement and comparing them with detailed simulations, we infer the underlying energy distributions. STAM enables the simultaneous measurement of axial and transverse energy components, achieving a relative precision of approximately 10% over a mean energy range of ∼ 10–100 mK in the ALPHA trap. The technique is applied to both uncooled and laser-cooled antihydrogen populations, with results benchmarked against established laser-based diagnostics. While laser-based methods provide high-resolution measurements of the transverse energy and limited sensitivity to the axial component, an accurate determination of the axial energy remains challenging. In addition to complementing existing techniques, STAM is expected to access energy regimes that are currently inaccessible to laser-based diagnostics due to intrinsic limitations of those methods. These capabilities make STAM broadly applicable to future high-precision spectroscopic and gravitational studies with antihydrogen. Furthermore, STAM provides a novel probe of off-axis magnetic field structures, addressing a critical gap in current magnetometry techniques within the ALPHA experiment.
Antihydrogen, the bound state of a positron and an antiproton, is the simplest pure anti-atomic system and an excellent candidate to test the symmetry between matter and antimatter. This thesis focuses on the magnetic confinement of antihydrogen and the first ever resonant interaction with trapped antihydrogen, as performed by the ALPHA collaboration. The ALPHA apparatus and the techniques that have been developed to form, trap, probe, and detect antihydrogen atoms will be described in detail. The first successful demonstration of trapped antihydrogen will then be described. In the initial demonstrations, 38 trapped antihydrogen atoms were detected after being confined for at least 172 ms. Since then, over 400 antihydrogen atoms have been trapped and confinement times of 1000 s (over 15 minutes) have been demonstrated. Spectroscopy of these trapped antihydrogen atoms is the next major step forward. As an initial proof-of-principle demonstration, ALPHA induced and observed reso- nant positron spin flip (PSR) transitions between the ground states of antihydrogen. Because of the strong magnetic field dependence of these transition frequencies, the success of this experiment relied heavily on the ability to measure the magnetic field seen by the antihydrogen atoms. A novel method to measure the magnetic field in situ by detecting the cyclotron resonance of a trapped electron plasma is presented. This method allowed ALPHA to measure the magnetic field strength at the minimum of the magnetic antihydrogen trap to within 1.4 parts in 103. Hardware improvements and further study should allow this resolution to be improved by several orders of magnitude. The cyclotron resonance measurements can also be applied as a roughdiagnostic of a microwave field within the ALPHA apparatus. This allowed for important diagnostics of the microwave field used to excite the PSR transitions. Finally, the experimental results demonstrating resonant PSR transitions in antihydrogen are presented. This experiment is the first ever spectroscopic measurement of antihydrogen and an important step towards future precision spectroscopy.
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Recently, antihydrogen atoms were trapped at CERN in a magnetic minimum (minimum-B) trap formed by superconducting octupole and mirror magnet coils. The trapped antiatoms were detected by rapidly turning off these magnets, thereby eliminating the magnetic minimum and releasing any antiatoms contained in the trap. Once released, these antiatoms quickly hit the trap wall, whereupon the positrons and antiprotons in the antiatoms annihilate. The antiproton annihilations produce easily detected signals; we used these signals to prove that we trapped antihydrogen. However, our technique could be confounded by mirror-trapped antiprotons, which would produce seemingly identical annihilation signals upon hitting the trap wall. In this paper, we discuss possible sources of mirror-trapped antiprotons and show that antihydrogen and antiprotons can be readily distinguished, often with the aid of applied electric fields, by analyzing the annihilation locations and times. We further discuss the general properties of antiproton and antihydrogen trajectories in this magnetic geometry, and reconstruct the antihydrogen energy distribution from the measured annihilation time history.
Antihydrogen, the simplest pure-antimatter atomic system, holds the promise of direct tests of matter-antimatter equivalence and CPT invariance, two of the outstanding unanswered questions in modern physics. Antihydrogen is now routinely produced in charged-particle traps through the combination of plasmas of antiprotons and positrons, but the atoms escape and are destroyed in a minuscule fraction of a second. The focus of this work is the production of a sample of cold antihydrogen atoms in a magnetic atom trap. This poses an extreme challenge, because the state-of-the-art atom traps are only approximately 0.5 K deep for ground-state antihydrogen atoms, much shallower than the energies of particles stored in the plasmas. This thesis will outline the main parts of the ALPHA experiment, with an overview of the important physical processes at work. Antihydrogen production techniques will be described, and an analysis of the spatial annihilation distribution to give indications of the temperature and binding energy distribution of the atoms will be presented. Finally, we describe the techniques needed to demonstrate confinement of antihydrogen atoms, apply them to a data taking run and present the results, making a definitive identification of trapped antihydrogen atoms.
Magnetically trapped antihydrogen atoms can be cooled by expanding the volume of the trap in which they are confined. We report a proof-of-principle experiment in which antiatoms are deliberately released from expanded and static traps. Antiatoms escape at an average trap depth of 0.08±0.01K (statistical errors only) from the expanded trap while they escape at average depths of 0.22±0.01 and 0.17±0.01K from two different static traps. (We employ temperature-equivalent energy units.) Detailed simulations qualitatively agree with the escape times measured in the experiment and show a decrease of 38% (statistical error<0.2%) in the mean energy of the population after the trap expansion without significantly increasing antiatom loss compared to typical static confinement protocols. This change is bracketed by the predictions of one-dimensional and three-dimensional semianalytic adiabatic expansion models. These experimental, simulational, and model results are consistent with obtaining an adiabatically cooled population of antihydrogen atoms that partially exchanged energy between axial and transverse degrees of freedom during the trap expansion. This result is important for future antihydrogen gravitational experiments which rely on adiabatic cooling, and it will enable antihydrogen cooling beyond the fundamental limits of laser cooling.
Antihydrogen, the simplest pure-antimatter atomic system, holds the promise of direct tests of matter-antimatter equivalence and CPT invariance, two of the outstanding unanswered questions in modern physics. Antihydrogen is now routinely produced in charged-particle traps through the combination of plasmas of antiprotons and positrons, but the atoms escape and are destroyed in a minuscule fraction of a second. The focus of this work is the production of a sample of cold antihydrogen atoms in a magnetic atom trap. This poses an extreme challenge, because the state-of-the-art atom traps are only approximately 0.5 K deep for ground-state antihydrogen atoms, much shallower than the energies of particles stored in the plasmas. This thesis will outline the main parts of the ALPHA experiment, with an overview of the important physical processes at work. Antihydrogen production techniques will be described, and an analysis of the spatial annihilation distribution to give indications of the temperature and binding energy distribution of the atoms will be presented. Finally, we describe the techniques needed to demonstrate confinement of antihydrogen atoms, apply them to a data taking run and present the results, making a definitive identification of trapped antihydrogen atoms.