Antihydrogen Trapping Research Articles

Adaptable platform for trapped cold electrons, hydrogen and lithium anions and cations

Cold cations, electrons and anions are ubiquitous in space, participate in star formation chemistry and are relevant to studies on the origin of molecular biology homochirality. We report on a system to generate and trap these species in the laboratory. Laser ablation of a solid target (LiH) facing a sublimating Ne matrix generates cold electrons, anions, and cations. Axial energy distributions (of {{{{{{rm{e}}}}}}}^{-}, {{{{{{rm{H}}}}}}}^{pm } and {{{{{{rm{Li}}}}}}}^{pm }) peaked at 0–25 meV are obtained in a Penning trap at 90 mT and 0.5 eV barrier. Anions can be guided and neutralized with low recoil energy by near–threshold photodetachment. An immediate prospect for this {{{{{{rm{H}}}}}}}^{-} source is to load hydrogen atoms into the ALPHA antihydrogen trap at CERN toward direct spectroscopic comparison of both conjugated species beyond 13 significant figures. The production is potentially scalable and adaptable to different species including deuterium and tritium, relevant for neutrino mass and fusion research.

On the formation of antihydrogen beams using travelling optical lattices

The production of beams of antihydrogen atoms using the dipole force provided by a travelling optical lattice to accelerate a sample of the anti-atoms held in a magnetic gradient atom trap is investigated. By considering current and near-future antihydrogen trapping capabilities we find that useful fluxes of the anti-atoms can be achieved with directional properties that can be manipulated using laser parameters such as pulse duration and frequency chirp rate. Applications of the beams are briefly discussed.

Optical trapping of antihydrogen towards an atomic anti-clock

The Anti-Matter Factory at CERN is gearing up, commissioning of the Extra Low ENergy Antiprotons (ELENA) ring is ongoing and the first anti-protons are foreseen to circulate in the decelerator very soon. The unprecedented flux of low energy antiprotons delivered by ELENA will open a new era for precision tests with antimatter including laser and microwave spectroscopy and tests of its gravitational behaviour. Here we propose a scheme to load the ultra cold anti-hydrogen atoms that will be produced by the GBAR experiment in an optical lattice tuned at the magic wavelength of the 1S-2S transition in order to measure this interval at a level comparable or even better than its matter counter part. This will provide the most sensitive test of CPT symmetry parametrised in the framework of the Standard Model Extension.

Plasma and trap-based techniques for science with antimatter

Positrons (i.e., antielectrons) find use in a wide variety of applications, and antiprotons are required for the formation and study of antihydrogen. Available sources of these antiparticles are relatively weak. To optimize their use, most applications require that the antiparticles be accumulated into carefully prepared plasmas. We present an overview of the techniques that have been developed to efficiently accumulate low energy antiparticles and create, in particular, tailored antiparticle plasmas. Techniques are also described to create tailored antiparticle beams. Many of these techniques are based on methods first developed by the nonneutral plasma community using electron plasmas for increased data rate. They have enabled the creation and trapping of antihydrogen, have been critical to studies of positron and positronium interactions with matter, including advanced techniques to characterize materials and material surfaces, and have led to the creation and study of the positronium molecule. Rather than attempting to be comprehensive, we focus on techniques that have proven most useful, applications where there has been significant, recent progress, and areas that hold promise for future advances. Examples of the latter include the ever more precise comparisons of the properties of antihydrogen and hydrogen, tests of gravity using antihydrogen and positronium atoms, and efforts to create and study phases of the many-electron, many-positron system.

Simulations of Majorana spin flips in an antihydrogen trap

The properties of antihydrogen () have, thus far, been probed at magnetic fields of ∼1 T. It may be fruitful to perform some of these measurements at magnetic fields approaching 0 T. In this case, there could occur zeros in the magnitude of the B-field. The number and properties of the magnetic field zeros are investigated. For typical magnetic field geometries in traps, the zeros will occur as two groups of 5 closely spaced points instead of as a single point. Except in special cases, results from calculations show that these 10 zeros can be treated as independent sources of spin flip probability. Although the behavior of Majorana spin flip near higher order zeros should not be important in the traps, the probability for spin flip is calculated for the case of a quadratic zero. Finally, results are presented for a simple model of how magnetic field zeros would affect the trapped population of .

Machine learning for antihydrogen detection at ALPHA

The ALPHA experiment at CERN is designed to produce and trap antihydrogen to the purpose of making a precise comparison with hydrogen. The basic technique consists of driving an antihydrogen resonance which will cause the antiatom to leave the trap and annihilate. The main background to antihydrogen detection is due to cosmic rays. When an experimental cycle extends for several minutes, while the number of trapped antihydrogen remains fixed, background rejection can become challenging. Machine learning methods have been employed in ALPHA for several years, leading to a dramatic reduction of the background contamination. This allowed ALPHA to perform the first laser spectroscopy experiment on antihydrogen.

Lyman-α source for laser cooling antihydrogen.

We present a Lyman-α laser developed for cooling trapped antihydrogen. The system is based on a pulsed Ti:sapphire laser operating at 729nm that is frequency doubled using an LBO crystal and then frequency tripled in a Kr/Ar gas cell. After frequency conversion, this system produces up to 5.7μW of average power at the Lyman-α wavelength. This laser is part of the ATRAP experiment at the antiproton decelerator in CERN.

Precision measurements on trapped antihydrogen in the ALPHA experiment.

Both the 1S-2S transition and the ground state hyperfine spectrum have been observed in trapped antihydrogen. The former constitutes the first observation of resonant interaction of light with an anti-atom, and the latter is the first detailed measurement of a spectral feature in antihydrogen. Owing to the narrow intrinsic linewidth of the 1S-2S transition and use of two-photon laser excitation, the transition energy can be precisely determined in both hydrogen and antihydrogen, allowing a direct comparison as a test of fundamental symmetry. The 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. The hyperfine spectrum of antihydrogen is determined to a relative uncertainty of 4×10-4 The excited state and the hyperfine spectroscopy techniques currently both show sensitivity at the few 100 kHz level on the absolute scale. Here, the most recent work of the ALPHA collaboration on precision spectroscopy of antihydrogen is presented together with an outlook on improving the precision of measurements involving lasers and microwave radiation. Prospects of measuring the Lamb shift and determining the antiproton charge radius in trapped antihydrogen in the ALPHA apparatus are presented. Future perspectives of precision measurements of trapped antihydrogen in the ALPHA apparatus when the ELENA facility becomes available to experiments at CERN are discussed.This article is part of the Theo Murphy meeting issue 'Antiproton physics in the ELENA era'.

Aspects of 1S-2S spectroscopy of trapped antihydrogen atoms

Antihydrogen atoms are now routinely trapped in small numbers. One of the purposes of this effort is to make precision comparisons of the 1S-2S transition in hydrogen and antihydrogen as a precision test of the CPT theorem. We investigate, through calculations and simulations, various methods by which the 1S-2S transition may be probed with only a few trapped atoms. We consider the known constraints from typical experimental geometries, detection methods, sample temperatures, laser light sources etc and we identify a viable path towards a measurement of this transition at the 10−11 level in a realistic scenario. We also identify ways in which such a first measurement could be improved upon as a function of projected changes and improvements in antihydrogen synthesis and trapping. These calculations recently guided the first observation of the 1S-2S transition in trapped antihydrogen.

Antihydrogen studies in ALPHA

The ALPHA experiment studies antihydrogen as a means to investigate the symmetry of matter and antimatter. Spectroscopic studies of the anti-atom hold the promise of the most precise direct comparisons of matter and antimatter possible. ALPHA was the first to trap antihydrogen in a magnetic trap, allowing the first ever detection of atomic transitions in an anti-atom. More recently, through stochastic heating, we have also been able to put a new limit on the charge neutrality of antihydrogen. ALPHA is currently preparing to perform the first laser-spectroscopy of antihydrogen, hoping to excite the 2s state using a two-photon transition from the 1s state. We discuss the recent results as well as the key developments that led to these successes and discuss how we are preparing to perform the first laser-spectroscopy. We will also discuss plans to use our novel technique for gravitational tests on antihydrogen for a direct measurement of the sign of the gravitational force on antihydrogen.

Confinement of antiprotons in the electrostatic space charge of positrons in a model of the ALPHA antihydrogen trap

The equilibrium of a positron plasma in a model of the ALPHA apparatus is computed using a finite-difference method. The positron plasma in the model extends to axial magnetic mirrors in absence of an octupole field. Formation of a three-dimensional electrostatic potential well is found to occur self-consistently. Well depths under various conditions are evaluated. Also, the equilibrium with an antiproton plasma confined in the potential well is computed. Conditions from the two species case are used to estimate timescales associated with three-body recombination, antiproton loss, and temperature equilibration. An equilibrium where the three-body recombination timescale is the smallest of the three is identified.

A sensitive detection method for high resolution spectroscopy of trapped antihydrogen, hydrogen and other trapped species

A method for detection of the weak 1s–2s laser excitation of a few trapped antihydrogen atoms is described. It involves the typical antihydrogen trapping environment that combines a magnetic trap for the atoms as well as a Penning trap for its constituent particles. By photoionization of the excited state the photoion can be kept in a weak Penning trap and at a suitable time be ejected towards a charged particle detector such as a microchannel plate or a channel electron multiplier. Since it does not rely on annihilation, the method is also suitable for trapped hydrogen and may find application with other species when a weak transition to a metastable state is intended and only a few trapped atoms or molecules are available.

Gravitational mass of relativistic matter and antimatter

The universality of free fall, the weak equivalence principle (WEP), is a cornerstone of the general theory of relativity, the most precise theory of gravity confirmed in all experiments up to date. The WEP states the equivalence of the inertial, m, and gravitational, mg, masses and was tested in numerous occasions with normal matter at relatively low energies. However, there is no confirmation for the matter and antimatter at high energies. For the antimatter the situation is even less clear – current direct observations of trapped antihydrogen suggest the limits −65<mg/m<110 not excluding the so-called antigravity phenomenon, i.e. repulsion of the antimatter by Earth. Here we demonstrate an indirect bound 0.96<mg/m<1.04 on the gravitational mass of relativistic electrons and positrons coming from the absence of the vacuum Cherenkov radiation at the Large Electron–Positron Collider (LEP) and stability of photons at the Tevatron collider in presence of the annual variations of the solar gravitational potential. Our result clearly rules out the speculated antigravity. By considering the absolute potential of the Local Supercluster (LS), we also predict the bounds 1−4×10−7<mg/m<1+2×10−7 for an electron and positron. Finally, we comment on a possibility of performing complementary tests at the future International Linear Collider (ILC) and Compact Linear Collider (CLIC).

Antihydrogen trapping assisted by sympathetically cooled positrons

Antihydrogen, the bound state of an antiproton and a positron, is of interest for use in precision tests of nature's fundamental symmetries. Antihydrogen formed by carefully merging cold plasmas of positrons and antiprotons has recently been trapped in magnetic traps. The efficiency of trapping is strongly dependent on the temperature of the nascent antihydrogen, which, to be trapped, must have a kinetic energy less than the trap depth of . In the conditions in the ALPHA experiment, the antihydrogen temperature seems dominated by the temperature of the positron plasma used for the synthesis. Cold positrons are therefore of paramount interest in that experiment. In this paper, we propose an alternative route to make ultra-cold positrons for enhanced antihydrogen trapping. We investigate theoretically how to extend previously successful sympathetic cooling of positrons by laser-cooled positive ions to be used for antihydrogen trapping. Using simulations, we investigate the effectiveness of such cooling in conditions similar to those in ALPHA, and discuss how the formation process and the nascent antihydrogen may be influenced by the presence of positive ions. We argue that this technique is a viable alternative to methods such as evaporative and adiabatic cooling, and may overcome limitations faced by these. Ultra-cold positrons, once available, may also be of interest for a range of other applications.

ALPHA: antihydrogen and fundamental physics

Detailed comparisons of antihydrogen with hydrogen promise to be a fruitful test bed of fundamental symmetries such as the CPT theorem for quantum field theory or studies of gravitational influence on antimatter. With a string of recent successes, starting with the first trapped antihydrogen and recently resulting in the first measurement of a quantum transition in anti-hydrogen, the ALPHA collaboration is well on its way to perform such precision comparisons. We will discuss the key innovative steps that have made these results possible and in particular focus on the detailed work on positron and antiproton preparation to achieve antihydrogen cold enough to trap as well as the unique features of the ALPHA apparatus that has allowed the first quantum transitions in anti-hydrogen to be measured with only a single trapped antihydrogen atom per experiment. We will also look at how ALPHA plans to step from here towards more precise comparisons of matter and antimatter.

Development of a Lyman-α laser system for spectroscopy and laser cooling of antihydrogen

Hydrogen Lyman-α radiation (121.56 nm) is important because it allows for the excitation and detection of ground-state hydrogen atoms by a one-photon process. The trapping of antihydrogen, recently reported by the ALPHA collaboration at CERN, has revived interest in Lyman-α lasers. In order to perform high precision tests of matter-antimatter symmetry violations or gravity-antimatter interactions with antihydrogen, laser cooling using the 1s − 2p single photon transition is essential. Recent theoretical simulations predict that even with a pulsed Lyman-α source, laser cooling of antihydrogen would be possible. Here we describe the implementation of a high power vacuum-ultraviolet (VUV) laser at the Lyman-α transition of hydrogen. The VUV light was generated using a two-photon-resonant four-wave mixing process in a phase-matched mixture of krypton and argon. Two wavelengths (ωR → 202.31 and ωT → 602.56 nm) were mixed in a sum-difference scheme (ωVUV = 2ωR − ωT) with a two-photon resonance at (4s24p55p[1/2]0 ← 4s24p6(1S0)) transition in Kr. With an Ar/Kr mixture of 3.9:1 we obtained 10 ns pulses of 0.1 μJ of energy at a repetition rate of 10 Hz.

Trapped antihydrogen: A new frontier in fundamental physics

Antihydrogen, the bound state of an antiproton and a positron, has been produced at low energies at CERN since 2002. Antihydrogen is of interest for use in precision tests of nature's fundamental symmetries. The charge conjugation/parity/time reversal (CPT) theorem, a crucial part of the foundation of the standard model of elementary particles and interactions, demands that hydrogen and antihydrogen have the same spectrum. Given the current experimental precision of measurements on the hydrogen atom, subjecting antihydrogen to rigorous spectroscopic examination would constitute a compelling, model-independent test of CPT. Antihydrogen could also be used to study the gravitational behaviour of antimatter.However, until recently, experiments have produced antihydrogen that was not confined, precluding detailed study of its structure. Experimenters working to trap antihydrogen have faced the challenge of trapping and cooling relativistic antiprotons and using them to make antihydrogen cold enough to be trapped in a magnetic minimum trap with a depth of only 50 μeV. In November 2010 the ALPHA collaboration demonstrated the first trapping of antihydrogen, thus opening the door to precision measurements on anti-atoms which can soon be subjected to many of the same techniques as developed for atoms. The prospect for such measurements improved further with ALPHA's demonstration of 1000 s confinement of the anti-atoms in the summer of 2011 and the recent first detection of resonant quantum interactions in antihydrogen.

Description and first application of a new technique to measure the gravitational mass of antihydrogen.

Physicists have long wondered whether the gravitational interactions between matter and antimatter might be different from those between matter and itself. Although there are many indirect indications that no such differences exist and that the weak equivalence principle holds, there have been no direct, free-fall style, experimental tests of gravity on antimatter. Here we describe a novel direct test methodology; we search for a propensity for antihydrogen atoms to fall downward when released from the ALPHA antihydrogen trap. In the absence of systematic errors, we can reject ratios of the gravitational to inertial mass of antihydrogen >75 at a statistical significance level of 5%; worst-case systematic errors increase the minimum rejection ratio to 110. A similar search places somewhat tighter bounds on a negative gravitational mass, that is, on antigravity. This methodology, coupled with ongoing experimental improvements, should allow us to bound the ratio within the more interesting near equivalence regime.

Construction and Operational Experience With a Superconducting Octupole Used to Trap Antihydrogen

A superconducting octupole magnet has seen extensive service as part of the ALPHA experiment at CERN. ALPHA has trapped antihydrogen, a crucial step towards performing precision measurements of anti-atoms. The octupole was made at the Direct Wind facility by the Superconducting Magnet Division at Brookhaven National Laboratory. The magnet was wound with a six-around-one NbTi cable about 1 mm in diameter. It is about 300 mm long, with a radius of 25 mm and a peak field at the conductor of 4.04 T. Specific features of the magnet, including a minimal amount of material in the coil and coil ends with low multipole content, were advantageous to its use in ALPHA. The magnet was operated for six months a year for five years. During this time it underwent about 900 thermal cycles (between 4 K and 100 K). A novel operational feature is that during the course of data-taking the magnet was repeatedly shut off from its 950 A operating current. The magnet quenches during the shutoff, with a decay constant of 9 ms. Over the course of the five years, the magnet was deliberately quenched many thousands of times. It still performs well.

Trapped Antihydrogen in Its Ground State

Antihydrogen atoms (H¯) are confined in an Ioffe trap for 15-1000 s-long enough to ensure that they reach their ground state. Though reproducibility challenges remain in making large numbers of cold antiprotons (p¯) and positrons (e(+)) interact, 5±1 simultaneously confined ground-state atoms are produced and observed on average, substantially more than previously reported. Increases in the number of simultaneously trapped H¯ are critical if laser cooling of trapped H¯ is to be demonstrated and spectroscopic studies at interesting levels of precision are to be carried out.