The rule that the interchange of opposite members of a multiplet of subatomic particles in a system or process does not affect any property of the system or the outcome of the process.
A multiplet is a group of subatomic particles that are (nearly) identical except that each member of the multiplet carries a different amount of electrical charge. The simplest example is the two-member multiplet consisting of the proton and neutron. Both are called nucleons, and both are the building blocks of the atomic nucleus. They have the same spin (½ ℏ, where ℏ is Planck's constant divided by 2π) and nearly the same mass, but the proton carries one unit of positive electrical charge while the neutron has no charge. It is the proton's charge that attracts and binds negatively charged electrons to make atoms. Many other multiplets have been found in the particles created in accelerators. These include the pion with three charge states (π+, π0, π−) and the delta with four (Δ++, Δ+, Δ0, Δ−). See also: Baryon; Meson; Neutron; Nucleon; Proton
The mathematical framework that best describes the behavior of particles within a multiplet mirrors the framework for spin, with the value of the isospin being (N − 1)/2, where the number of multiplet members is N. If a process or property is the same for all members of the multiplet, that property obeys isospin (I-spin) symmetry. Charge symmetry is a special case in which the property is the same when opposite members of the multiplet are swapped. See also: I-spin; Spin (quantum mechanics)
For the nucleon, charge symmetry requires that swapping neutrons and protons in a nucleus or a subatomic reaction has no physical effect. Thus, charge symmetry would predict that the triton (nucleus made of one proton and two neutrons) would have the same spin and mass as the helium-3 (3He) nucleus (two protons and one neutron). This rule has proven useful in the classification of subatomic particles and the states of the atomic nucleus. In addition, if there is a reaction among subatomic particles that can proceed through different members of a multiplet, the relative rates of these reactions are related by the mathematical rules that govern isospin. See also: Elementary particle; Nuclear structure; Symmetry laws (physics)
Charge symmetry breaking
The predictions of the charge symmetry rule are not exactly obeyed. Neutrons have more mass than protons by 0.14%. Such small deviations are called charge symmetry breaking. They arise from two causes:
1. The change in electric force effects that accompanies the swap of a multiplet member with one electrical charge for another with a different charge. While the strong force between nucleons binds together the nucleus and dominates nuclear properties, the electrical force of repulsion between similar charges makes the force between two protons more repulsive than the force between two neutrons.
2. The heavier mass of the down quark compared to the up quark. Thus, the neutron (composed of two down quarks and one up quark) is heavier than the proton (two up quarks and one down quark).
By measuring the small deviations in nuclear properties and reactions that are a result of charge symmetry breaking, scientists hope to untangle these two causes and measure the size of the quark mass difference effect.
Consequences of charge symmetry breaking
The small mass difference between the neutron and the proton makes it possible for the neutron to decay into a proton, an electron, and an antineutrino while the proton is stable. Left by itself, the neutron will decay with a half-life of about 10 min. The neutron can survive for a long time only if it is bound into a stable nucleus or is a part of a massive object such as a neutron star. In these situations, there is not enough available energy to create the decay products. See also: Neutron star
The stability of the proton compared to the neutron led to the abundance of hydrogen (the atom made of a single proton and electron) after the universe was formed in the big bang. This makes possible the process of fusion that powers the Sun and stars. It leads to the formation of water and the many chemicals that are essential for life on Earth. See also: Big bang theory
Effects that measure charge symmetry breaking
With the discovery of charmed particles in 1975, it was confirmed that protons and neutrons, as well as many other subatomic particles, were composed of a new kind of particles called quarks. Because quarks are permanently confined inside the nucleons and other particles, their masses cannot be directly measured. For this information, we must rely on experiments that are sensitive to some effect caused by the quark mass in combination with theoretical models that relate the mass to the effect. Measurements of charge symmetry breaking can provide information on the mass difference between the up and down quarks. See also: Charm; Quarks
The mass difference between the neutron and the proton (1.2933317 ± 0.0000005 MeV/c2, where c is the speed of light) is one such piece of information that is known to great precision. Another piece of information is the scattering length, a measure of the strength of the force between two nucleons when they collide at low speeds. After the effects of the electric force are subtracted using theory, the proton-proton length (−17.3 ± 0.4 femtometer, where the negative value indicates attraction) still differs from the neutron-neutron length (−18.8 ± 0.3 fm), an indication of charge symmetry breaking. The neutron-proton length of −23.77 ± 0.09 fm differs by an even larger amount due to the electric effects of charge independence breaking. If all three lengths had been the same, this property would have obeyed isospin symmetry. See also: Scattering experiments (nuclei)
At its longest ranges, the strong force that binds nucleons can be described by the exchange between nucleons of short-lived subatomic particles called mesons. Charge symmetry breaking may cause mesons that are otherwise similar but belong to different multiplets with different isospin values to mix. This allows, for example, a ρ meson (isospin = 1) to transform in flight into an ω meson (isospin = 0). Other mixings may occur (pion into η or η′ meson) but have not yet been precisely measured.
The scattering of neutrons and protons from each other at speeds generated in accelerators depends on the orientation of their spin axes during the collision. A small difference of 0.6% in the scattering probability appears in the comparison of neutron versus proton spin effects, a difference attributed to charge symmetry breaking.
Effective field theories
The model connecting such experimental results with the underlying quark properties is chiral effective field theory. First developed by Steven Weinberg, this model tracks nucleons and pions within a framework that obeys the rules of quark interactions. This model points to experiments in which the pion scatters from nucleons as the best place to obtain information on the difference between the up and down quark masses. This has led to two observations of charge symmetry breaking in experiments that produce electrically neutral pions, both of which reported results in 2003. See also: Quantum field theory
Neutron-proton fusion with pion production
An experiment conducted by Allena Opper, Elie Korkmaz, and their colleagues measured the rates at which the fusion of a neutron and proton gave rise to a deuteron (the neutron-proton nuclear bound state) and a pion emerging at different angles. A difference in the rates at forward and backward angles, as shown in the illustration, is a signature of charge symmetry breaking. See also: Deuteron
The illustration shows three views of the reaction. In illus.a, the neutron and proton enter from the left and right, producing a deuteron and pion that go out back-to-back; the scattering angle is θ. Charge symmetry requires the same reaction rate if the neutron and proton are swapped (illus.b). When this picture is turned around (illus.c) to place the proton and neutron as they were in illus.a, then θ is on the left instead of the right. So charge symmetry requires that just as many deuterons should go to the left as to the right at the same angle θ. Using a magnetic spectrometer located at the TRIUMF cyclotron laboratory in Vancouver, British Columbia, the experimental team was able to measure a small “fore-aft” asymmetry in this reaction rate equal to 0.17 ± 0.10%, the amount of charge symmetry breaking in this process. See also: Particle accelerator
Deuteron-deuteron fusion with pion production
In this experiment, conducted by Edward Stephenson, Andrew Bacher, and their colleagues at the Indiana University Cyclotron Facility, an electron-cooled storage ring was used to bring two deuterons together to make a helium-4 (4He) nucleus and a pion. In this case, swapping neutrons and protons in either the deuteron or 4He nucleus changes nothing as these nuclei are self-conjugate (members of multiplets with only a single member and isospin equal to 0). But the pion is part of a multiplet having three members (π+, π0, and π−) and an isospin value of 1. So this reaction is forbidden to occur to the extent that the amount of isospin before and after the reaction must remain the same. Using a very sensitive magnetic channel for 4He and lead-glass Cerenkov detectors to see the light from the decay of the pion, the team was able to observe this reaction for the first time. The rate is very small; only 1 out of 1010 collisions between two deuterons resulted in the production of 4He and a pion. See also: Cerenkov radiation
Interpretation of results
The interpretation of these results is underway. In each case, all indirect mechanisms for breaking charge symmetry must be included along with the quark mass difference and electromagnetic effects. These include meson mixing, as described above. In addition, there may be states in either the deuteron or the 4He nucleus that do not maintain a single value of isospin and would allow these symmetry-breaking effects to appear. The importance of such mechanisms is not well known, and will have to be determined self-consistently among all of the results of charge symmetry breaking. It is likely that such a determination will emerge and yield a better insight into the properties of the quarks inside the proton and neutron.