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An exotic atom is an otherwise normal atom in which one or more sub-atomic particles have been replaced by other particles of the same charge.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,
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,
139.8, 193.0 and 197.0 nm. These lines correspond to transitions between states of the form
. Such transitions are improbable. However, the probability is increased by a factor of
when the laser frequencies sum to within 10 GHz of an intermediate state
. 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