Antiprotonic helium
Encyclopedia
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 antiprotons have a charge of -1, whereas helium nuclei have a charge of +2. The antiprotonic helium atom is called an atomcule, as it has a structure like an atom, but has two nuclei like a molecule.
and angular momentum quantum number 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 long lifetime is seen only in the case of antiprotons mixed in helium; in all other substances, antiprotons annihilate a million times faster, in less than a picosecond.
. 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.
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
. 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 appear the same if you simultaneously reversed the charge
, parity
, and the flow of time
(or equivalently, the direction of motion) in the universe. One of the important predictions of this theory is that particles and their antiparticles should have exactly the same mass.
frequency carried out by the ATRAP collaboration at CERN, the mass
and electric charge
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 0.7 parts in a billion.
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....
and an electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
orbiting around a 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...
nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
. It is thus made partly of matter, and partly of antimatter. The atom is electrically neutral, since both electrons and antiprotons have a charge of -1, whereas helium nuclei have a charge of +2. The antiprotonic helium atom is called an atomcule, as it has a structure like an atom, but has two nuclei like a molecule.
Production
These exotic atoms 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 numberPrincipal quantum number
In atomic physics, the principal quantum symbolized as n is the firstof a set of quantum numbers of an atomic orbital. The principal quantum number can only have positive integer values...
and angular momentum quantum number 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
Annihilation
Annihilation is defined as "total destruction" or "complete obliteration" of an object; having its root in the Latin nihil . A literal translation is "to make into nothing"....
. This long lifetime is seen only in the case of antiprotons mixed in helium; in all other substances, antiprotons annihilate a million times faster, in less than a picosecond.
Laser spectroscopy
Antiprotonic helium atoms are under study by the ASACUSA experiment at CERNCERN
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...
. 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.
Measurement of the mass ratio between the antiproton and electron
By measuring the particular frequencyFrequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
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
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
. 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
CPT symmetry
CPT symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity, and time simultaneously.-History:...
(short for Charge, Parity, and Time reversal). This law says that all physical laws would appear the same if you simultaneously reversed the charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
, parity
Parity (physics)
In physics, a parity transformation is the flip in the sign of one spatial coordinate. In three dimensions, it is also commonly described by the simultaneous flip in the sign of all three spatial coordinates:...
, and the flow of time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
(or equivalently, the direction of motion) in the universe. One of the important predictions 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 cyclotronCyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...
frequency carried out by the ATRAP collaboration at CERN, the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
and electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
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 0.7 parts in a billion.