The ALPHA collaboration at CERN has succeeded in cooling down antihydrogen atoms – the simplest form of atomic antimatter – using laser light. The technique, known as laser cooling, was first demonstrated 40 years ago on normal matter and is a mainstay of many research fields. Its first application to antihydrogen by ALPHA, described in a paper published in Nature, opens the door to considerably more precise measurements of the internal structure of antihydrogen and of how it behaves under the influence of gravity.

Comparing such measurements with those of the well-studied hydrogen atom could reveal differences between matter and antimatter atoms. Such differences, if present, could shed light on why the universe is made up of matter only, an imbalance known as matter–antimatter asymmetry.

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The LHCb collaboration at CERN has seen, for the first time, the matter–antimatter asymmetry known as CP violation in a particle dubbed the D0 meson. The finding, presented at the annual Rencontres de Moriond conference in 2019 and in a dedicated CERN seminar, is sure to make it into the textbooks of particle physics.

CP violation is an essential feature of our universe, necessary to induce the processes that, following the Big Bang, established the abundance of matter over antimatter that we observe in the present-day universe.

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The kick-off meeting for ELENA, the Extra Low Energy Antiproton Ring, starts today at CERN, bringing together scientists from Canada, Denmark, France, Germany, Japan, Sweden, the UK and the USA. 

ELENA will consist of a small new decelerator ring that will be installed in same building that houses CERN’s existing Antiproton Decelerator (AD). It will slow antiprotons down to under a fiftieth of the current AD energy, bringing an improvement of a factor of 10-100 in antiproton trapping efficiency. At the AD, antiprotons have to be slowed down by passing them through a series of foils, a process that results in the loss of some 99.9% of the antiprotons extracted from the AD before they reach the experiments.

In Physical Review Letters, the Antihydrogen TRAP (ATRAP) experiment at CERN's Antiproton Decelerator (AD) reports a new measurement of the antiproton magnetic moment made with an unprecedented uncertainty of 4.4 parts per million (ppm). This result is 680 times more precise than previous measurements. The unusual increase in precision is due to the experiment’s ability to trap individual protons and antiprotons, and to use a huge magnetic gradient to gain sensitivity to the tiny magnetic moment. ATRAP’s new result is partly an attempt to understand the matter-antimatter imbalance of the universe, one of the great mysteries of modern physics.

Using a device called a Penning trap, a sort of electromagnetic cage, the antiproton is suspended at the centre of an iron ring electrode sandwiched between copper electrodes. Thermal contact with liquid helium keeps the electrodes at 4.2 K, providing a nearly perfect vacuum that eliminates the stray matter atoms that could otherwise annihilate the antiproton. Static and oscillating voltages applied to the electrodes allow the antiproton to be manipulated and its properties to be measured.

The ATRAP team found that the magnetic moments of the antiproton and proton are "exactly opposite": equal in strength but opposite in direction with respect to the particle spins, consistent with the prediction of the Standard Model and its CPT theorem to 5 parts per million. However, the potential for much greater measurement precision puts ATRAP in position to eventually test the Standard Model prediction much more stringently.

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The LHCb collaboration at CERN publishes a paper in Physical Review Letters on the first observation of matter-antimatter asymmetry in the decays of the particle known as the B⁰_s. It has a significance of more than 5 sigma, and is only the fourth subatomic particle known to exhibit such behaviour. 

By studying subtle differences in the behaviour of particle and antiparticles, experiments at the LHC are seeking to cast light on this dominance of matter over antimatter.

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The ALPHA collaboration at CERN¹ has published a paper in Nature Communications describing the first direct analysis of how antimatter is affected by gravity.

ALPHA was the first experiment to trap atoms of antihydrogen — neutral antimatter atoms held in place with a strong magnetic field for up to 1000 seconds. The original goal of the experiment was not to study gravity, but the researchers realised that the data they had already collected might be sensitive to gravitational effects.

Current theoretical arguments predict that hydrogen and antihydrogen atoms have the same mass and should interact with gravity in the same way. If an atom is released, it should experience a downward force whether it’s made of matter or antimatter. The ALPHA scientists have retroactively analysed how their antihydrogen atoms moved when released; this has allowed them to put a limit on anomalous gravitational effects.

In a paper published in the journal Nature Communications today, the ALPHA experiment at CERN's Antiproton Decelerator (AD) reports a measurement of the electric charge of antihydrogen atoms, finding it to be compatible with zero to eight decimal places. Although this result comes as no surprise, since hydrogen atoms are electrically neutral, it is the first time that the charge of an antiatom has been measured to high precision.

Antiparticles should be identical to matter particles except for the sign of their electric charge. So while the hydrogen atom is made up of a proton with charge +1 and an electron with charge -1, the antihydrogen atom consists of a charge -1 antiproton and a charge +1 positron. We know, however, that matter and antimatter are not exact opposites – nature seems to have a one-part in 10 billion preference for matter over antimatter, so it is important to measure the properties of antimatter to great precision: the principal goal of CERN’s AD experiments. ALPHA achieves this by using a complex system of particle traps that allow antihydrogen atoms to be produced and stored for long enough periods to study in detail. Understanding matter antimatter asymmetry is one of the greatest challenges in physics today. Any detectable difference between matter and antimatter could help solve the mystery and open a window to new physics.

To measure the charge of antihydrogen, the ALPHA experiment studied the trajectories of antihydrogen atoms released from the trap in the presence of an electric field. If the antihydrogen atoms had an electric charge, the field would deflect them, whereas neutral atoms would be undeflected. The result, based on 386 recorded events, gives a value of the antihydrogen electric charge as (-1.3±1.1±0.4) × 10^-8, the plus or minus numbers representing statistical and systematic uncertainties on the measurement.

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Geneva, 12 August 2015. In a paper published today in Nature, the Baryon Antibaryon Symmetry Experiment (BASE1) at CERN2's Antiproton Decelerator (AD), reports the most precise comparison of the charge-to-mass ratio of the proton to that of its antimatter equivalent, the antiproton. 

The charge-to-mass ratio — an important property of particles — can be measured by observing the oscillation of a particle in a magnetic field. The new result shows no difference between the proton and the antiproton, with a four-fold improvement in the energy resolution compared with previous measurements.

To perform the experiment, the BASE collaboration used a Penning-trap system comparable to that developed by the TRAP collaboration in the late 1990s at CERN. However, the method used is faster than in previous experiments. This has allowed BASE to carry out about 13 000 measurements over a 35-day campaign, in which they compare a single antiproton to a negatively-charged hydrogen ion (H-). Consisting of a hydrogen atom with a single proton in its nucleus, together with an additional electron, the H- acts as a proxy for the proton.

“We found that the charge-to-mass ratio is identical to within 69 parts per thousand billion, supporting a fundamental symmetry between matter and antimatter,” said BASE spokesperson Stefan Ulmer.

While matter and antimatter particles can differ, for example, in the way they decay (a difference often referred to as violation of CP symmetry), other fundamental properties, such as the absolute value of their electric charges and masses, are predicted to be exactly equal. Any difference – however small — between the charge-to-mass ratio of protons and antiprotons would break a fundamental law known as CPT symmetry. This symmetry reflects well-established properties of space and time and of quantum mechanics, so such a difference would constitute a dramatic challenge not only to the Standard Model, but also to the basic theoretical framework of particle physics.

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The ASACUSA experiment at CERN reports in Science a new precision measurement of the mass of the antiproton relative to that of the electron. Such measurements provide a unique tool for comparing with high precision the mass of an antimatter particle with its matter counterpart. 

This result is based on spectroscopic measurements with about 2 billion antiprotonic helium atoms cooled to extremely cold temperatures of 1.5 to 1.7 degrees above absolute zero. In antiprotonic helium atoms an antiproton takes the place of one of the electrons that would normally be orbiting the nucleus. 

The measurement of the antiproton’s mass is done by spectroscopy, by shining a laser beam onto the antiprotonic helium. ASACUSA has now managed to cool down the antiprotonic helium atoms to temperatures close to absolute zero by suspending them in a very cold helium buffer-gas. In this way, the microscopic motion of the atoms is reduced, enhancing the precision of the frequency measurement. The measurement of the transition frequency has been improved by a factor of 1.4 to 10 compared with previous experiments. Experiments were conducted from 2010 to 2014, with about 2 billion atoms, corresponding to roughly 17 femtograms of antiprotonic helium. 

According to standard theories, protons and antiprotons are expected to have exactly the same mass. The observation of even a minute breaking of CPT would call for a review of our assumptions about the nature and properties of space-time. 

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The ALPHA collaboration reports in Nature the first ever measurement on the optical spectrum of an antimatter atom. 

When the electrons move from one orbit to another they absorb or emit light at specific wavelengths, forming the atom's spectrum. ALPHA result is the first observation of a spectral line in an antihydrogen atom, allowing the light spectrum of matter and antimatter to be compared for the first time. 

Within experimental limits, the result shows no difference compared to the equivalent spectral line in hydrogen. This is consistent with the Standard Model of particle physics, the theory that best describes particles and the forces at work between them, which predicts that hydrogen and antihydrogen should have identical spectroscopic characteristics. Measuring the antihydrogen spectrum with high-precision offers an extraordinary new tool to test whether matter behaves differently from antimatter and thus to further test the robustness of the Standard Model. 

The measurement was done by observing the so-called 1S-2S transition. The 2S state in atomic hydrogen is long-lived, leading to a narrow natural line width, so it is particularly suitable for precision measurement. 

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