HOME

TheInfoList



OR:

Antiprotonic helium is a three-body atom composed of an
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 ...
and an
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
orbiting around a
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table ...
nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...
. It is thus made partly of matter, and partly of antimatter. The atom is electrically neutral, since both electrons and antiprotons each have a charge of −1, whereas helium nuclei have a charge of +2. It has the longest lifetime of any experimentally producible matter-antimatter bound state.


Production

These
exotic atom An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge.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 (muonic atoms ...
s can be produced by simply mixing antiprotons with ordinary helium gas; the antiproton spontaneously removes one of the two electrons contained in a normal helium atom in a chemical reaction, and then begins to orbit the helium nucleus in the electron's place. This will happen in the case of approximately 3% of the antiprotons introduced to the helium gas. The antiproton's orbit, which has a large
principal quantum number In quantum mechanics, the principal quantum number (symbolized ''n'') is one of four quantum numbers assigned to each electron in an atom to describe that electron's state. Its values are natural numbers (from 1) making it a discrete variable. Ap ...
and
angular momentum quantum number The azimuthal quantum number is a quantum number for an atomic orbital that determines its orbital angular momentum and describes the shape of the orbital. The azimuthal quantum number is the second of a set of quantum numbers that describe t ...
of around 38, lies far away from the surface of the helium nucleus. The antiproton can thus orbit the nucleus for tens of microseconds, before finally falling to its surface and annihilating. This contrasts with other types of exotic atoms, X^+, which typically decay within picoseconds.


Laser spectroscopy

Antiprotonic helium atoms are under study by the ASACUSA experiment at
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 ...
. In these experiments, the atoms are first produced by stopping a beam of antiprotons in helium gas. The atoms are then irradiated by powerful laser beams, which cause the antiprotons in them to resonate and jump from one atomic orbit to another. As in spectroscopy of other bound states, Doppler broadening and other effects present challenges to precision. Researchers use a variety of techniques to obtain accurate results. One way to exceed Doppler-limited precision is two-photon spectroscopy. The ASACUSA Collaboration has studied ^3He^+ and ^4He^+ atoms with the occupying a high Rydberg state with large principal and orbital quantum numbers, n \sim l + 1 \sim 38 using 2-photon spectroscopy. Counterpropagating Ti:Sapphire lasers with pulses of duration 30-100 ns excited nonlinear 2-photon transitions in the deep UV, including spectral lines of wavelengths, \lambda = 139.8, 193.0 and 197.0 nm. These lines correspond to transitions between states of the form (n, l) \rightarrow (n-2, l-2). Such transitions are improbable. However, the probability is increased by a factor of 10^5 when the laser frequencies sum to within 10 GHz of an intermediate state (n-1, l-1). States were selected pairwise such that Auger emission to ^He^2+ and rapid annihilation produced a detectable Čerenkov signal. The reduced Doppler shift resulted in narrower spectral lines accurate to between 2.3 and 5 ppb. Comparison of the results with three-body quantum electrodynamics calculations made possible a determination of the antiproton to electron mass ratio of 1836.1526736(23). In 2022 ASACUSA found unexpected narrowing of antiprotonic helium spectral lines.


Measurement of the mass ratio between the antiproton and electron

By measuring the particular
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is e ...
of the laser light needed to resonate the atom, the ASACUSA experiment determined the mass of the antiproton, which they measured at times more massive than an
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
. This is the same as the mass of a "regular" proton, within the level of certainty of the experiment. This is a confirmation of a fundamental symmetry of nature called CPT (short for charge, parity, and time reversal). This law says that all physical laws would remain unchanged under simultaneous reversal of the
charge Charge or charged may refer to: Arts, entertainment, and media Films * ''Charge, Zero Emissions/Maximum Speed'', a 2011 documentary Music * ''Charge'' (David Ford album) * ''Charge'' (Machel Montano album) * '' Charge!!'', an album by The Aqu ...
axis, parity of the space axes, and the orientation of the
time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, t ...
axis. One important prediction of this theory is that particles and their antiparticles should have exactly the same mass.


Comparison of antiproton and proton masses and charges

By comparing the above results on laser spectroscopy of antiprotonic helium with separate high-precision measurements of the antiproton's
cyclotron A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. ''Method and apparatus for the acceleration of ions'', filed: Jan ...
frequency carried out by the ATRAP and BASE collaborations at CERN, the
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
and
electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respecti ...
of the antiproton can be precisely compared with the proton values. The most recent such measurements show that the antiproton's mass (and the absolute value of the charge) is the same as the proton's to a precision of 0.5 parts in a billion.


Antiprotonic helium ions

An antiprotonic helium ion is a two-body object composed of a helium nucleus and orbiting antiproton. It has an electric charge of +1. Cold ions with lifetimes of up to 100 ns were produced by the ASACUSA experiment in 2005.


Pionic helium

In 2020 ASACUSA in collaboration with the Paul Scherrer Institut (PSI) reported the experimental verification of long lived
pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more ge ...
ic helium by spectroscopic measurements; the first time in an atom containing a lepton. Its existence had been predicted in 1964 by George Condo at University of Tennessee to explain some anomalies from bubble chamber tracks but no definite proof of its existence had ever been obtained. In the experiment negatively charged pions from a ring
cyclotron A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. ''Method and apparatus for the acceleration of ions'', filed: Jan ...
were magnetically focused into a tank filled with superfluid helium so that they would expel an electron from the atom and take its place. Later, to confirm the production, laser light was fired at various frequencies until they found a specific one at 1631 nm where the pion would resonate undergoing a quantum jump from its orbit into an inner one and eventually into the nucleus which would break down into a proton, a neutron and a deuterium. The experiment proved highly technical to perform and took 8 years, including the design and construction of the experiment.


See also

*
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 annihi ...
*
Protonium Protonium (symbol: Pn), also known as antiprotonic hydrogen, is a type of exotic atom in which a proton (symbol: p) and an antiproton (symbol: ) orbit each other. Since protonium is a bound system of a particle and its corresponding antiparticl ...


References

{{Reflist


Further reading


ASACUSA improves measurement of antiproton mass
Antimatter Atomic physics Exotic atoms Helium