The Antiproton Decelerator (AD) is a

storage ring A storage ring is a type of circular particle accelerator in which a continuous or pulsed particle beam may be kept circulating typically for many hours. Storage of a particular particle depends upon the mass, momentum and usually the charge of t ...

at the

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in a northwestern suburb of Gene ...

laboratory near

Geneva Geneva ( ; french: Genève ) frp, Genèva ; german: link=no, Genf ; it, Ginevra ; rm, Genevra is the List of cities in Switzerland, second-most populous city in Switzerland (after Zürich) and the most populous city of Romandy, the French-speaki ...

. It was built from the

Antiproton Collector The Antiproton Collector (AC) was part of the antiparticle factory at CERN designed to decelerate and store antimatter, to study the properties of antimatter and to create atoms of antihydrogen. It was built in 1986 around the existing Antiprot ...

(AC) to be a successor to the

Low Energy Antiproton Ring The Low Energy Anti-Proton Ring (LEAR) was a particle accelerator at CERN which operated from 1982 until 1996. The ring was designed to decelerate and store antiprotons, to study the properties of antimatter and to create atoms of antihydrogen. ...

(LEAR) and started operation in the year 2000.

Antiproton The antiproton, , (pronounced ''p-bar'') 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. The exis ...

s are created by impinging a proton beam from the

Proton Synchrotron The Proton Synchrotron (PS, sometimes also referred to as CPS) is a particle accelerator at CERN. It is CERN's first synchrotron, beginning its operation in 1959. For a brief period the PS was the world's highest energy particle accelerator. It ...

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. The major goals of experiments at AD are to spectroscopically observe the antihydrogen and to study the effects of

gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...

on antimatter. Though each experiment at AD has varied aims ranging from testing antimatter for cancer therapy to

CPT symmetry Charge, parity, and time reversal symmetry is a fundamental symmetry of physical laws under the simultaneous transformations of charge conjugation (C), parity transformation (P), and time reversal (T). CPT is the only combination of C, P, and T ...

and antigravity research.

History

From 1982 to 1996, CERN operated the Low Energy Antiproton Ring (LEAR), through which several experiments with slow-moving antiprotons were carried out. During the end stages of LEAR, the physics community involved in those

antimatter In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter. Antimatter occurs in natural processes like cosmic ray collisions and some types of radioac ...

experiments wanted to continue their studies with the slow antiprotons. The motivation to build the AD grew out of the Antihydrogen Workshop held in

Munich Munich ( ; german: München ; bar, Minga ) is the capital and most populous city of the States of Germany, German state of Bavaria. With a population of 1,558,395 inhabitants as of 31 July 2020, it is the List of cities in Germany by popu ...

in 1992. This idea was carried forward quickly and AD's feasibility study was completed by 1995. In 1996, the CERN Council asked the

(PS) division to look into the possibility of generating slow antiproton beams. The PS division prepared a design study in 1996 with the solution to use the

antiproton collector The Antiproton Collector (AC) was part of the antiparticle factory at CERN designed to decelerate and store antimatter, to study the properties of antimatter and to create atoms of antihydrogen. It was built in 1986 around the existing Antiprot ...

(AC), and transform it into a single Antiproton Decelerator Machine. The AD was approved in February 1997. AC modification, AD installation, and commissioning process were carried out in the next three years. By the end of 1999, the AC ring was modified into a decelerator and cooling system- forming the Antiproton Decelerator.

Decelerator

AD's oval-shaped perimeter has four straight sections where the deceleration and cooling systems are placed. There are several

dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system ...

and

quadrupole A quadrupole or quadrapole is one of a sequence of configurations of things like electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure refl ...

magnets in these sections to avoid beam dispersion. Antiprotons are cooled and decelerated in a single 100-second cycle in the AD synchrotron.

Production of antiprotons

AD requires about

\mathrm

protons of momentum 26 GeV/c to produce

\mathrm

antiprotons per minute. The high-energy protons coming from the proton synchrotron are made to collide with a thin, highly dense rod of

iridium Iridium is a chemical element with the symbol Ir and atomic number 77. A very hard, brittle, silvery-white transition metal of the platinum group, it is considered the second-densest naturally occurring metal (after osmium) with a density of ...

metal of 3-mm diameter and 55 cm in length. The iridium rod embedded in

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...

and enclosed by a sealed water-cooled

titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

case remains intact. But the collisions create a lot of energetic particles, including the antiprotons. A magnetic bi-conical aluminum horn-type lens collects the antiprotons emerging from the target. This collector takes in the antiprotons, and they are separated from other particles using deflection through electromagnetic forces. Experimental area at CERNs Antiproton Decelerator (AD) Hall

Experimental area at CERNs Antiproton Decelerator (AD) Hall

Deceleration, accumulation and cooling down

The

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the upp ...

(RF) systems carry out the tasks of deceleration and bunching of cooled antiprotons at 3.5 GeV/c. There are numerous magnets inside, which focus and bend the randomly moving antiproton into a

collimated A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction p ...

beam. Simultaneously the

electric field An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...

s decelerate them. Stochastic cooling and

electron cooling Electron cooling is a method to shrink the emittance (size, divergence, and energy spread) of a charged particle beam without removing particles from the beam. Since the number of particles remains unchanged and the space coordinates and their der ...

stages designed inside the AD decrease the energy of beams as well as limit the antiproton beam from any significant distortions. Stochastic cooling is applied for antiprotons at 3.5 GeV/c and then at 2 GeV/c, followed by electron cooling at 0.3 GeV/c and at 0.1 GeV/c. The final output beam has a momentum of 0.1 GeV/C (

kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...

equal to 5.3 MeV). These antiprotons move with the speed of about one-tenth that of light. But the experiments need much lower energy beams (3 to 5 KeV). So the antiprotons are again decelerated to ~5 KeV, using the degrader foils. This step accounts for the loss of 99.9% of antiprotons. The collected antiprotons are then temporarily stored in the

Penning trap A Penning trap is a device for the storage of charged particles using a homogeneous axial magnetic field and an inhomogeneous quadrupole electric field. This kind of trap is particularly well suited to precision measurements of properties of i ...

s; before being fed into the several AD experiments. The Penning traps can also form antihydrogen by combining antiprotons with the

positron The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. It has an electric charge of +1 '' e'', a spin of 1/2 (the same as the electron), and the same mass as an electron. When a positron collides ...

ELENA

ELENA (Extra Low ENergy Antiproton) is a 30 m hexagonal storage ring situated inside the AD complex. It is designed to further decelerate the antiproton beam to an energy of 0.1 MeV for more precise measurements. The first beam circulated ELENA on 18 November 2016. GBAR was the first experiment to use a beam from ELENA, with the rest of the AD experiments to follow suit after LS2 when beam transfer lines from ELENA will have been laid to all the experiments using the facility.

AD experiments

ATHENA

ATHENA, AD-1 experiment, was an

research project that took place at the Antiproton Decelerator. In August 2002, it was the first experiment to produce 50,000 low-energy antihydrogen atoms, as reported in ''

Nature Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...

''. In 2005, ATHENA was disbanded and many of the former members worked on the subsequent

ALPHA experiment The Antihydrogen Laser Physics Apparatus (ALPHA), also known as AD-5, is an experiment at the Antiproton Decelerator at CERN, designed to trap neutral antihydrogen in a magnetic trap, and conduct experiments on them. The ultimate goal of thi ...

ATRAP

The Antihydrogen Trap (ATRAP) collaboration, responsible for the AD-2 experiment, is a continuation of the

TRAP A trap is a mechanical device used to capture or restrain an animal for purposes such as hunting, pest control, or ecological research. Trap or TRAP may also refer to: Art and entertainment Films and television * ''Trap'' (2015 film), Fil ...

collaboration, which started taking data for the PS196 experiment in 1985. The TRAP experiment (PS196) pioneered cold

antiproton The antiproton, , (pronounced ''p-bar'') 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. The exis ...

s, cold

s, and first made the ingredients of cold antihydrogen to interact. Later ATRAP members pioneered accurate

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...

and observed the first hot antihydrogen atoms.

ASACUSA

Atomic Spectroscopy and Collisions Using Slow Antiprotons (ASACUSA), AD-3, is an experiment testing for CPT-symmetry by

laser spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...

antiprotonic helium Antiprotonic helium is a three-body atom composed of an antiproton and an electron orbiting around a helium nucleus. It is thus made partly of matter, and partly of antimatter. The atom is electrically neutral, since both electrons and antiprot ...

and

microwave spectroscopy Microwave spectroscopy is the spectroscopy method that employs microwaves, i.e. electromagnetic radiation at GHz frequencies, for the study of matter. History The ammonia molecule NH3 is shaped like a pyramid 0.38 Å in height, with an equilatera ...

of the hyperfine structure of antihydrogen. It compares matter and antimatter using antihydrogen and antiprotonic helium and looks into matter-antimatter collisions. It also measures atomic and nuclear cross-sections of antiprotons on various targets at extremely low energies.

ACE

The Antiproton Cell Experiment (ACE), AD-4, started in 2003. It aims to assess fully the effectiveness and suitability of antiprotons for

cancer therapy Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal bl ...

. The results showed that antiprotons required to break down the

tumor cells A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

were four times less than the number of protons required. The effect on healthy tissues due to antiprotons was significantly less. Although the experiment ended in 2013, further research and validation still continue, owing to the long procedures of bringing in novel medical treatments.

ALPHA

The Antihydrogen Laser Physics Apparatus (ALPHA), the AD-5 experiment, is designed to trap neutral antihydrogen in a magnetic trap, and conduct experiments on them. The ultimate goal of this endeavour is to test

through comparison of the

atomic spectra Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...

and antihydrogen (see

hydrogen spectral series The emission spectrum of atomic hydrogen has been divided into a number of spectral series, with wavelengths given by the Rydberg formula. These observed spectral lines are due to the electron making transitions between two energy levels in an ...

). The ALPHA collaboration consists of some former members of the ATHENA collaboration (the first group to produce cold antihydrogen, in 2002), as well as a number of new members.

AEgIS

AEgIS, Antimatter Experiment: gravity, Interferometry, Spectroscopy, AD-6, is an experiment at the Antiproton Decelerator. AEgIS would attempt to determine if

affects

in the same way it affects normal

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic partic ...

by testing its effect on an antihydrogen beam. The first phase of the experiment created antihydrogen using the charge exchange reaction between antiprotons from the Antiproton Decelerator (AD) and

positronium Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom, specifically an onium. Unlike hydrogen, the system has no protons. The system is unstable: the two particles annih ...

, producing a pulse of antihydrogen atoms. These atoms are sent through a series of

diffraction grating In optics, a diffraction grating is an optical component with a periodic structure that diffracts light into several beams travelling in different directions (i.e., different diffraction angles). The emerging coloration is a form of structura ...

s, ultimately hitting a surface and thus annihilating. The points where the antihydrogen annihilates are measured with a precise detector. Areas behind the gratings are shadowed, while those behind the slits are not. The annihilation points reproduce a periodic pattern of light and shadowed areas. Using this pattern, it can be measured how many atoms of different velocities are vertically displaced due to gravity during n their horizontal flight. Therefore, the Earth's gravitational force on antihydrogen can be determined.

GBAR

GBAR (Gravitational Behaviour of Anti hydrogen at Rest), AD-7 experiment, is a multinational collaboration at the Antiproton Decelerator of CERN. The GBAR project aims to measure the free-fall acceleration of ultra-cold neutral anti-hydrogen

atoms Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, an ...

in the terrestrial

gravitational field In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenome ...

. By measuring the free fall acceleration of anti-hydrogen and comparing it with acceleration of normal hydrogen, GBAR is testing the

equivalence principle In the theory of general relativity, the equivalence principle is the equivalence of gravitational and inertial mass, and Albert Einstein's observation that the gravitational "force" as experienced locally while standing on a massive body (suc ...

proposed by

Albert Einstein Albert Einstein ( ; ; 14 March 1879 – 18 April 1955) was a German-born theoretical physicist, widely acknowledged to be one of the greatest and most influential physicists of all time. Einstein is best known for developing the theory ...

. The equivalence principle says that the gravitational force on a particle is independent of its internal structure and composition.

BASE

BASE (Baryon Antibaryon Symmetry Experiment), AD-8, is a multinational collaboration at the Antiproton Decelerator of CERN. The goal of the Japanese/German BASE collaboration are high-precision investigations of the fundamental properties of the antiproton, namely the

charge-to-mass ratio The mass-to-charge ratio (''m''/''Q'') is a physical quantity relating the ''mass'' (quantity of matter) and the ''electric charge'' of a given particle, expressed in units of kilograms per coulomb (kg/C). It is most widely used in the electrod ...

and the

magnetic moment In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...

. The single antiprotons are stored in an advanced

system, which has a double-trap system at its core, for high precision frequency measurements and for single particle spin flip

. By measuring the spin flip rate as a function of the frequency of an externally applied magnetic-drive, a resonance curve is obtained. Together with a measurement of the cyclotron frequency, the magnetic moment is extracted.

PUMA

The PUMA (antiProton Unstable Matter Annihilation experiment), AD-9, aims to look into the quantum interactions and

annihilation In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total energy ...

processes between the antiprotons and the exotic slow-moving nuclei. PUMA's experimental goals require about one billion trapped antiprotons made by AD and ELENA to be transported to the ISOLDE-nuclear physics facility at CERN, which will supply the exotic nuclei. Antimatter has never been transported out of the AD facility before. Designing and building a trap for this transportation is the most challenging aspect for the PUMA collaboration.

References

INSPIRE-HEP INSPIRE-HEP is an open access digital library for the field of high energy physics (HEP). It is the successor of the Stanford Physics Information Retrieval System (SPIRES) database, the main literature database for high energy physics since the 1970 ...