Protonium
Encyclopedia
Protonium is an exotic atom
in which a proton
and an antiproton
orbit
each other. Protonium is an electrically neutral boson
with zero baryon number.
There are two known methods to generate protonium. One method involves violent particle collisions. The other method involves putting antiprotons and proton
s into the same magnetic cage. The latter method was first used during the experiment ATHENA
(ApparaTus for High precision Experiment on Neutral Antimatter) by Evandro Lodi Rizzini's working group at the CERN
laboratory in Geneva
in 2002, but it was not until 2006 that scientists realized protonium was also generated during the experiment.
Reactions involving a proton and an antiproton at high energies give rise to many-particle final states. In fact, such reactions are the basis of particle colliders such as the Tevatron
at Fermilab
. Indirect searches for protonium at LEAR
(Low Energy Antiproton Ring at CERN
) have used antiprotons impinging on nuclei such as helium
, with unclear results. Very low energy collisions in the range of 10 eV
to 1 keV
may lead to the formation of protonium.
Planned experiments will use traps as the source of low energy antiprotons. Such a beam would be allowed to impinge on atomic hydrogen
targets, in the field of a laser, which is meant to excite the bound proton-antiproton pairs into an excited state of protonium with some efficiency (whose computation is an open theoretical problem). Unbound particles are rejected by bending them in a magnetic field. Since the protonium is uncharged, it will not be deflected by such a field. This undeflected protonium, if formed, would be allowed to traverse a meter of high vacuum, within which it is expected to decay via annihilation of the proton and antiproton. The decay products would give unmistakable signatures of the formation of protonium.
Theoretical studies of protonium have mainly used non-relativistic quantum mechanics
. These give predictions for the binding energy
and lifetime of the states. Computed lifetimes are in the range of 0.1 to 10 microsecond
s. Unlike the hydrogen atom in which the dominant interactions are due to the Coulomb attraction
of the electron and the proton, the constituents of protonium interact predominantly through the strong interaction
. Thus multiparticle interactions involving meson
s in intermediate states may be important. Hence the production and study of protonium would be of interest also for the understanding of internucleon forces.
Exotic atom
An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge. For example, electrons may be replaced by other negatively charged particles such as muons or pions...
in which a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
and an antiproton
Antiproton
The antiproton 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....
orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
each other. Protonium is an electrically neutral boson
Boson
In particle physics, bosons are subatomic particles that obey Bose–Einstein statistics. Several bosons can occupy the same quantum state. The word boson derives from the name of Satyendra Nath Bose....
with zero baryon number.
There are two known methods to generate protonium. One method involves violent particle collisions. The other method involves putting antiprotons and proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s into the same magnetic cage. The latter method was first used during the experiment ATHENA
Athena
In Greek mythology, Athena, Athenê, or Athene , also referred to as Pallas Athena/Athene , is the goddess of wisdom, courage, inspiration, civilization, warfare, strength, strategy, the arts, crafts, justice, and skill. Minerva, Athena's Roman incarnation, embodies similar attributes. Athena is...
(ApparaTus for High precision Experiment on Neutral Antimatter) by Evandro Lodi Rizzini's working group at the CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
laboratory in Geneva
Geneva
Geneva In the national languages of Switzerland the city is known as Genf , Ginevra and Genevra is the second-most-populous city in Switzerland and is the most populous city of Romandie, the French-speaking part of Switzerland...
in 2002, but it was not until 2006 that scientists realized protonium was also generated during the experiment.
Reactions involving a proton and an antiproton at high energies give rise to many-particle final states. In fact, such reactions are the basis of particle colliders such as the Tevatron
Tevatron
The Tevatron is a circular particle accelerator in the United States, at the Fermi National Accelerator Laboratory , just east of Batavia, Illinois, and is the second highest energy particle collider in the world after the Large Hadron Collider...
at Fermilab
Fermilab
Fermi National Accelerator Laboratory , located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics...
. Indirect searches for protonium at LEAR
Lear
Lear or Leir can refer to:* Leir of Britain, a legendary king of the Britons* King Leir, an anonymous play based on the legend of Leir of Britain, published in 1605...
(Low Energy Antiproton Ring at CERN
CERN
The European Organization for Nuclear Research , known as CERN , is an international organization whose purpose is to operate the world's largest particle physics laboratory, which is situated in the northwest suburbs of Geneva on the Franco–Swiss border...
) have used antiprotons impinging on nuclei such as helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, with unclear results. Very low energy collisions in the range of 10 eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
to 1 keV
Kev
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...
may lead to the formation of protonium.
Planned experiments will use traps as the source of low energy antiprotons. Such a beam would be allowed to impinge on atomic hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
targets, in the field of a laser, which is meant to excite the bound proton-antiproton pairs into an excited state of protonium with some efficiency (whose computation is an open theoretical problem). Unbound particles are rejected by bending them in a magnetic field. Since the protonium is uncharged, it will not be deflected by such a field. This undeflected protonium, if formed, would be allowed to traverse a meter of high vacuum, within which it is expected to decay via annihilation of the proton and antiproton. The decay products would give unmistakable signatures of the formation of protonium.
Theoretical studies of protonium have mainly used non-relativistic quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
. These give predictions for the binding energy
Binding energy
Binding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
and lifetime of the states. Computed lifetimes are in the range of 0.1 to 10 microsecond
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
s. Unlike the hydrogen atom in which the dominant interactions are due to the Coulomb attraction
Coulomb's law
Coulomb's law or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. It was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism...
of the electron and the proton, the constituents of protonium interact predominantly through the strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
. Thus multiparticle interactions involving meson
Meson
In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...
s in intermediate states may be important. Hence the production and study of protonium would be of interest also for the understanding of internucleon forces.