Antihydrogen

Antimatter
Antiparticles Positron Antiproton Antineutron Antihydrogen Antihelium Onia Antiprotonic hydrogen Antiprotonic helium Muonium True muonium Pionium Positronium Quarkonium
Concepts and phenomena Annihilation Baryogenesis Baryon asymmetry Comet CP violation Gravitational interaction Positron emission
Devices Cloud chamber Particle accelerator Antiproton decelerator Relativistic Heavy Ion Collider Penning trap
Uses Positron emission tomography Fuel Weapon
People and bodies Carl David Anderson Paul Dirac Andrei Sakharov CERN
v t e

Antihydrogen (
H
) is the antimatter counterpart of hydrogen. Whereas the common hydrogen atom is composed of an electron and proton, the antihydrogen atom is made up of a positron and antiproton. Scientists hope that studying antihydrogen may shed light on the question of why there is more matter than antimatter in the observable universe, known as the baryon asymmetry problem.^[1] Antihydrogen is produced artificially in particle accelerators.

Experimental history

Accelerators first detected hot antihydrogen in the 1990s.

In 2016, the ALPHA experiment measured the atomic electron transition between the two lowest energy levels of antihydrogen, 1s–2s. The results, which are identical to that of hydrogen within the experimental resolution, support the idea of matter–antimatter symmetry and CPT symmetry.^[5]

In the presence of a magnetic field the 1s–2s transition splits into two hyperfine transitions with slightly different frequencies. The team calculated the transition frequencies for normal hydrogen under the magnetic field in the confinement volume as:

f_dd = 2466061103064(2) kHz

f_cc = 2466061707104(2) kHz

A single-photon transition between s states is prohibited by quantum

Antihydrogen atoms excited to the 2s state can then evolve in one of several ways:

• They can emit two photons and return directly to the ground state as they were
• They can absorb another photon, which ionizes the atom
• They can emit a single photon and return to the ground state via the 2p state—in this case the positron spin can flip or remain the same.

Both the ionization and spin-flip outcomes cause the atom to escape confinement. The team calculated that, assuming antihydrogen behaves like normal hydrogen, roughly half the antihydrogen atoms would be lost during the resonant frequency exposure, as compared to the no-laser case. With the laser source tuned 200 kHz below half the transition frequencies, the calculated loss was essentially the same as for the no-laser case.

The ALPHA team made batches of antihydrogen, held them for 600 seconds and then tapered down the confinement field over 1.5 seconds while counting how many antihydrogen atoms were annihilated. They did this under three different experimental conditions:

• Resonance: exposing the confined antihydrogen atoms to a laser source tuned to exactly half the transition frequency for 300 seconds for each of the two transitions,
• Off-resonance: exposing the confined antihydrogen atoms to a laser source tuned 200 kilohertz below the two resonance frequencies for 300 seconds each,
• No-laser: confining the antihydrogen atoms without any laser illumination.

The two controls, off-resonance and no-laser, were needed to ensure that the laser illumination itself was not causing annihilations, perhaps by liberating normal atoms from the confinement vessel surface that could then combine with the antihydrogen.

The team conducted 11 runs of the three cases and found no

As an anti-element, it is expected to have exactly the same properties as hydrogen.[10] For example, antihydrogen would be a gas under standard conditions and combine with antioxygen to form antiwater,
H
₂
O
.

Production

The first antihydrogen was produced in 1995 by a team led by Walter Oelert at CERN^[11] using a method first proposed by Charles Munger Jr, Stanley Brodsky and Ivan Schmidt Andrade.^[12]

In the LEAR, antiprotons from an accelerator were shot at xenon clusters,^[13] producing electron-positron pairs. Antiprotons can capture positrons with probability about 10⁻¹⁹, so this method is not suited for substantial production, as calculated.^[14][15][16] Fermilab measured a somewhat different cross section,^[17] in agreement with predictions of quantum electrodynamics.^[18] Both resulted in highly energetic, or hot, anti-atoms, unsuitable for detailed study.

Subsequently, CERN built the Antiproton Decelerator (AD) to support efforts towards low-energy antihydrogen, for tests of fundamental symmetries. The AD supplies several CERN groups. CERN expects their facilities will be capable of producing 10 million antiprotons per minute.^[19]

Low-energy antihydrogen

Experiments by the

ALPHA, a successor of the ATHENA collaboration, was formed to stably trap antihydrogen.^[19] While electrically neutral, its spin magnetic moments interact with an inhomogeneous magnetic field; some atoms will be attracted to a magnetic minimum, created by a combination of mirror and multipole fields.^[22]

In November 2010, the ALPHA collaboration announced that they had trapped 38 antihydrogen atoms for a sixth of a second,^[23] the first confinement of neutral antimatter. In June 2011, they trapped 309 antihydrogen atoms, up to 3 simultaneously, for up to 1,000 seconds.^[24] They then studied its hyperfine structure, gravity effects, and charge. ALPHA will continue measurements along with experiments ATRAP, AEgIS and GBAR.

In 2018, AEgIS has produced a novel pulsed source of antihydrogen atoms with a production time spread of merely 250 nanoseconds. ^[25] The pulsed source is generated by the charge exchange reaction between Rydberg positronium atoms -- produced via the injection of a pulsed positron beam into a nanochanneled Si target, and excited by laser pulses -- and antiprotons, trapped, cooled and manipulated in electromagnetic traps. The pulsed production enables the control of the antihydrogen temperature, the formation of an antihydrogen beam, and in the next phase a precision measurement on the gravitational behaviour using an atomic interferometer, the so-called Moiré deflectormeter.

Larger antimatter atoms

Larger antimatter atoms such as

• Gravitational interaction of antimatter

References