Antihydrogen

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Antihydrogen consists of an antiproton and a positron 3D image of Antihydrogen.jpg
Antihydrogen consists of an antiproton and a positron
Simplified model of an antihydrogen atom in ground state Antihydrogen.gif
Simplified model of an antihydrogen atom in ground state

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.

Contents

Experimental history

Accelerators first detected hot antihydrogen in the 1990s. ATHENA studied cold
H
 in 2002. It was first trapped by the Antihydrogen Laser Physics Apparatus (ALPHA) team at CERN [2] [3] in 2010, who then measured the structure and other important properties. [4] ALPHA, AEgIS, and GBAR plan to further cool and study
H
 atoms.

1s–2s transition measurement

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:

fdd = 2466061103064(2) kHz
fcc = 2466061707104(2) kHz

A single-photon transition between s states is prohibited by quantum selection rules, so to elevate ground state positrons to the 2s level, the confinement space was illuminated by a laser tuned to half the calculated transition frequencies, stimulating allowed two photon absorption.

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

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:

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 significant difference between the off-resonance and no laser runs, but a 58% drop in the number of events detected after the resonance runs. They were also able to count annihilation events during the runs and found a higher level during the resonance runs, again with no significant difference between the off-resonance and no laser runs. The results were in good agreement with predictions based on normal hydrogen and can be "interpreted as a test of CPT symmetry at a precision of 200 ppt." [6]

Characteristics

The CPT theorem of particle physics predicts antihydrogen atoms have many of the characteristics regular hydrogen has; i.e. the same mass, magnetic moment, and atomic state transition frequencies (see atomic spectroscopy ). [7] For example, excited antihydrogen atoms are expected to glow the same color as regular hydrogen. Antihydrogen atoms should be attracted to other matter or antimatter gravitationally with a force of the same magnitude that ordinary hydrogen atoms experience. [2] This would not be true if antimatter has negative gravitational mass, which is considered highly unlikely, though not yet empirically disproven (see gravitational interaction of antimatter ). [8] Recent theoretical framework for negative mass and repulsive gravity (antigravity) between matter and antimatter has been developed, and the theory is compatible with CPT theorem. [9]

When antihydrogen comes into contact with ordinary matter, its constituents quickly annihilate. The positron annihilates with an electron to produce gamma rays. The antiproton, on the other hand, is made up of antiquarks that combine with quarks in either neutrons or protons, resulting in high-energy pions, that quickly decay into muons, neutrinos, positrons, and electrons. If antihydrogen atoms were suspended in a perfect vacuum, they should survive indefinitely.

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 ATRAP and ATHENA collaborations at CERN, brought together positrons and antiprotons in Penning traps, resulting in synthesis at a typical rate of 100 antihydrogen atoms per second. Antihydrogen was first produced by ATHENA in 2002, [20] and then by ATRAP [21] and by 2004, millions of antihydrogen atoms were made. The atoms synthesized had a relatively high temperature (a few thousand kelvins), and would hit the walls of the experimental apparatus as a consequence and annihilate. Most precision tests require long observation times.

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 antideuterium (
D
), antitritium (
T
), and antihelium (
He
) are much more difficult to produce. Antideuterium, [26] [27] antihelium-3 (3
He
) [28] [29] and antihelium-4 (4
He
) nuclei [30] have been produced with such high velocities that synthesis of their corresponding atoms poses several technical hurdles.

See also

Related Research Articles

<span class="mw-page-title-main">Antimatter</span> Material composed of antiparticles of the corresponding particles of ordinary matter

In modern physics, antimatter is defined as matter composed of the antiparticles of the corresponding particles in "ordinary" matter, and can be thought of as matter with reversed charge, parity, and time, known as CPT reversal. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioactive decay, but only a tiny fraction of these have successfully been bound together in experiments to form antiatoms. Minuscule numbers of antiparticles can be generated at particle accelerators; however, total artificial production has been only a few nanograms. No macroscopic amount of antimatter has ever been assembled due to the extreme cost and difficulty of production and handling. Nonetheless, antimatter is an essential component of widely available applications related to beta decay, such as positron emission tomography, radiation therapy, and industrial imaging.

<span class="mw-page-title-main">Positron</span> Anti-particle to the electron

The positron or antielectron is the particle with an electric charge of +1e, a spin of 1/2, and the same mass as an electron. It is the antiparticle of the electron. When a positron collides with an electron, annihilation occurs. If this collision occurs at low energies, it results in the production of two or more photons.

<span class="mw-page-title-main">Antimatter rocket</span> Rockets using antimatter as their power source

An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several designs that attempt to accomplish this goal. The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket.

<span class="mw-page-title-main">Antiproton</span> Subatomic particle

The antiproton,
p
, is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy.

ATHENA, also known as the AD-1 experiment, was an antimatter research project at the Antiproton Decelerator at CERN, Geneva. In August 2002, it was the first experiment to produce 50,000 low-energy antihydrogen atoms, as reported in Nature. In 2005, ATHENA was disbanded and many of the former members of the research team worked on the subsequent ALPHA experiment and AEgIS experiment.

The Antihydrogen Trap (ATRAP) collaboration at the Antiproton Decelerator facility at CERN, Geneva, is responsible for the AD-2 experiment. It is a continuation of the TRAP collaboration, which started taking data for the TRAP experiment in 1985. The TRAP experiment pioneered cold antiprotons, cold positrons, and first made the ingredients of cold antihydrogen to interact. Later ATRAP members pioneered accurate hydrogen spectroscopy and observed the first hot antihydrogen atoms.

<span class="mw-page-title-main">Gravitational interaction of antimatter</span> Theory of gravity on antimatter

The gravitational interaction of antimatter with matter or antimatter has been observed by physicists. As was the consensus among physicists previously, it was experimentally confirmed that gravity attracts both matter and antimatter at the same rate within experimental error.

<span class="mw-page-title-main">PS210 experiment</span> Scientific experiment

The PS210 experiment was the first experiment that led to the observation of antihydrogen atoms produced at the Low Energy Antiproton Ring (LEAR) at CERN in 1995. The antihydrogen atoms were produced in flight and moved at nearly the speed of light. They made unique electrical signals in detectors that destroyed them almost immediately after they formed by matter–antimatter annihilation.

Gerald Gabrielse is an American physicist. He is the Board of Trustees Professor of Physics and director of the Center for Fundamental Physics at Northwestern University, and Emeritus George Vasmer Leverett Professor of Physics at Harvard University. He is primarily known for his experiments trapping and investigating antimatter, measuring the electron g-factor, and measuring the electron electric dipole moment. He has been described as "a leader in super-precise measurements of fundamental particles and the study of anti-matter."

<span class="mw-page-title-main">Antiproton Decelerator</span> Particle storage ring at CERN, Switzerland

The Antiproton Decelerator (AD) is a storage ring at the CERN laboratory near Geneva. It was built from the Antiproton Collector (AC) to be a successor to the Low Energy Antiproton Ring (LEAR) and started operation in the year 2000. Antiprotons are created by impinging a proton beam from the Proton Synchrotron on a metal target. The AD decelerates the resultant antiprotons to an energy of 5.3 MeV, which are then ejected to one of several connected experiments.

Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA), AD-3, is an experiment at the Antiproton Decelerator (AD) at CERN. The experiment was proposed in 1997, started collecting data in 2002 by using the antiprotons beams from the AD, and will continue in future under the AD and ELENA decelerator facility.

High-precision experiments could reveal small previously unseen differences between the behavior of matter and antimatter. This prospect is appealing to physicists because it may show that nature is not Lorentz symmetric.

<span class="mw-page-title-main">ALPHA experiment</span> Antimatter gravitation experiment

The Antihydrogen Laser Physics Apparatus (ALPHA), also known as AD-5, is an experiment at CERN's Antiproton Decelerator, designed to trap antihydrogen in a magnetic trap in order to study its atomic spectra. The ultimate goal of the experiment is to test CPT symmetry through comparing the respective spectra of hydrogen and antihydrogen. Scientists taking part in ALPHA include former members of the ATHENA experiment (AD-1), the first to produce cold antihydrogen in 2002.

AEgIS, AD-6, is an experiment at the Antiproton Decelerator facility at CERN. Its primary goal is to measure directly the effect of Earth's gravitational field on antihydrogen atoms with significant precision. Indirect bounds that assume the validity of, for example, the universality of free fall, the Weak Equivalence Principle or CPT symmetry also in the case of antimatter constrain an anomalous gravitational behavior to a level where only precision measurements can provide answers. Vice versa, antimatter experiments with sufficient precision are essential to validate these fundamental assumptions. AEgIS was originally proposed in 2007. Construction of the main apparatus was completed in 2012. Since 2014, two laser systems with tunable wavelengths and synchronized to the nanosecond for specific atomic excitation have been successfully commissioned.

<span class="mw-page-title-main">GBAR experiment</span> Experiment at the Antiproton Decelerator

GBAR, AD-7 experiment, is a multinational collaboration at the Antiproton Decelerator of CERN.

The rotating wall technique is a method used to compress a single-component plasma confined in an electromagnetic trap. It is one of many scientific and technological applications that rely on storing charged particles in vacuum. This technique has found extensive use in improving the quality of these traps and in tailoring of both positron and antiproton plasmas for a variety of end uses.

<span class="mw-page-title-main">John H. Malmberg</span> American physicist

John Holmes Malmberg was an American plasma physicist and a professor at the University of California, San Diego. He was known for making the first experimental measurements of Landau damping of plasma waves in 1964, as well as for his research on non-neutral plasmas and the development of the Penning–Malmberg trap.

<span class="mw-page-title-main">Penning–Malmberg trap</span> Electromagnetic device used to confine particles of a single sign of charge

The Penning–Malmberg trap, named after Frans Penning and John Malmberg, is an electromagnetic device used to confine large numbers of charged particles of a single sign of charge. Much interest in Penning–Malmberg (PM) traps arises from the fact that if the density of particles is large and the temperature is low, the gas will become a single-component plasma. While confinement of electrically neutral plasmas is generally difficult, single-species plasmas can be confined for long times in PM traps. They are the method of choice to study a variety of plasma phenomena. They are also widely used to confine antiparticles such as positrons and antiprotons for use in studies of the properties of antimatter and interactions of antiparticles with matter.

<span class="mw-page-title-main">Jeffrey Hangst</span> Experimental particle physicist

Jeffrey Scott Hangst is an experimental particle physicist at Aarhus University, Denmark, and founder and spokesperson of the ALPHA collaboration at the Antiproton Decelerator (AD) at CERN, Geneva. He was also one of the founding members and the Physics Coordinator of the ATHENA collaboration at the AD facility.

<span class="mw-page-title-main">Stefan Ulmer (physicist)</span> Particle physicist

Stefan Ulmer is a particle physicist, professor of Physics at Heinrich Heine University Düsseldorf and chief scientist at the Ulmer Fundamental Symmetries Laboratory, RIKEN, Tokyo. He is the founder and the spokesperson of the BASE experiment (AD-8) at the Antiproton Decelerator facility at CERN, Geneva. Stefan Ulmer is well known for his contributions to improving Penning trap techniques and precision measurements on antimatter. He is the first person to observe spin transitions with a single trapped proton as well as single spin transitions with a single trapped antiproton, a significant achievement towards a precision measurement of the antiproton magnetic moment.

References

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