question: How does a neutrally charged particle have an antiparticle?
comment: So proton and neutrons are composed of even smaller particles called quarks that have a charge. A proton is composed of two "Up" quarks and one "down" quark. An up quark has a charge of 2/3 and a down quark a charge of -1/3. 2/3+2/3-1/3 =1 so a proton has a charge of 1 & #x200B; A neutron is composed of two down quarks and one up quark: 2/3-1/3-1/3 = 0 no charge. & #x200B; Antiparticles are actually composed of anti-quarks. The quarks themselves are what have the opposite charges. & #x200B; So an antineutron is composed of two anti-down quarks and one anti-up. anti-down quarks have charge of +1/3 and anti-up quarks have a charge of -2/3 So you still end up with zero net charges. & #x200B; Edit: corrected charges for anti-quarks
passages: Although particles and their antiparticles have opposite charges, electrically neutral particles need not be identical to their antiparticles. The neutron, for example, is made out of quarks, the antineutron from antiquarks, and they are distinguishable from one another because neutrons and antineutrons annihilate each other upon contact. However, other neutral particles are their own antiparticles, such as photons, Z bosons,  mesons, and hypothetical gravitons and some hypothetical WIMPs.
  
  Section::::History.
  
  Section::::History.:Experiment.
   Antiprotons were routinely produced at Fermilab for collider physics operations in the Tevatron, where they were collided with protons. The use of antiprotons allows for a higher average energy of collisions between quarks and antiquarks than would be possible in proton-proton collisions. This is because the valence quarks in the proton, and the valence antiquarks in the antiproton, tend to carry the largest fraction of the proton or antiproton's momentum.
   BULLET::::- opposite electric charges q and -q.
  
  Section::::Quantum field theory.
  
  One may try to quantize an electron field without mixing the annihilation and creation operators by writing
  
  where we use the symbol "k" to denote the quantum numbers "p" and σ of the previous section and the sign of the energy, "E(k)", and "a" denotes the corresponding annihilation operators. Of course, since we are dealing with fermions, we have to have the operators satisfy canonical anti-commutation relations. However, if one now writes down the Hamiltonian
   where n denotes the charge conjugate state, that is, the antiparticle. This behaviour under CPT symmetry is the same as the statement that the particle and its antiparticle lie in the same irreducible representation of the Poincaré group. Properties of antiparticles can be related to those of particles through this. If T is a good symmetry of the dynamics, then
  
  where the proportionality sign indicates that there might be a phase on the right hand side. In other words, particle and antiparticle must have
  
  BULLET::::- the same mass m
  
  BULLET::::- the same spin state J
   Section::::Occurrence in nature.
  
  Antiprotons have been detected in cosmic rays for over 25 years, first by balloon-borne experiments and more recently by satellite-based detectors. The standard picture for their presence in cosmic rays is that they are produced in collisions of cosmic ray protons with nuclei in the interstellar medium, via the reaction, where A represents a nucleus:
  
  The secondary antiprotons () then propagate through the galaxy, confined by the galactic magnetic fields. Their energy spectrum is modified by collisions with other atoms in the interstellar medium, and antiprotons can also be lost by "leaking out" of the galaxy.
   The magnitude of properties of the antiproton are predicted by CPT symmetry to be exactly related to those of the proton. In particular, CPT symmetry predicts the mass and lifetime of the antiproton to be the same as those of the proton, and the electric charge and magnetic moment of the antiproton to be opposite in sign and equal in magnitude to those of the proton. CPT symmetry is a basic consequence of quantum field theory and no violations of it have ever been detected.
  
  Section::::Occurrence in nature.:List of recent cosmic ray detection experiments.
   These particle–antiparticle pairs carry various kinds of charges, such as color charge if they are subject to QCD such as quarks or gluons, or the more familiar electromagnetic charge if they are electrically charged leptons or quarks, the most familiar charged lepton being the electron and since it is the lightest in mass, the most numerous due to the energy-time uncertainty principle as mentioned above; e.g., virtual electron–positron pairs. Such charged pairs act as an electric dipole. In the presence of an electric field, e.g., the electromagnetic field around an electron, these particle–antiparticle pairs reposition themselves, thus partially counteracting the field (a partial screening effect, a dielectric effect). The field therefore will be weaker than would be expected if the vacuum were completely empty. This reorientation of the short-lived particle-antiparticle pairs is referred to as "vacuum polarization".
   Section::::Properties.
  
  Unparticles would have properties in common with neutrinos, which have almost zero mass and are therefore nearly scale invariant. Neutrinos barely interact with matter – most of the time physicists can infer their presence only by calculating the "missing" energy and momentum after an interaction. By looking at the same interaction many times, a probability distribution is built up that tells more specifically how many and what sort of neutrinos are involved. They couple very weakly to ordinary matter at low energies, and the effect of the coupling increases as the energy increases.
   Section::::Experimental status.:Direct CP violation.
  
  The kind of CP violation discovered in 1964 was linked to the fact that neutral kaons can transform into their antiparticles (in which each quark is replaced with the other's antiquark) and vice versa, but such transformation does not occur with exactly the same probability in both directions; this is called "indirect" CP violation.
   BULLET::::- LEAR collaboration at CERN:
  
  BULLET::::- Antihydrogen Penning trap of Gabrielse et al.:
  
  BULLET::::- BASE experiment at CERN:
  
  BULLET::::- APEX collaboration at Fermilab: for → + anything
  
  BULLET::::- APEX collaboration at Fermilab: for → +
   The neutral kaons are initially produced in the annihilation channels 
  
  BULLET::::- p → π
  
  BULLET::::- p → π
   Another hypothesis concerns "neutrinoless double-beta decay", which, if it exists, would violate lepton number conservation and imply a minuscule splitting (or difference) between the physical masses of what are conventionally called a "neutrino" and its corresponding "antineutrino" having the opposite sign in its lepton number. Searches for this mechanism are underway but have not yet found strong evidence for it. If they were to, then what are now called antineutrinos could not be true antiparticles. The resulting "six" distinct neutrinos would have no distinct antiparticle partner. Cosmic ray neutrino experiments detect neutrinos from space to study both the nature of neutrinos and the cosmic sources producing them.
   then one sees immediately that the expectation value of "H" need not be positive. This is because "E(k)" can have any sign whatsoever, and the combination of creation and annihilation operators has expectation value 1 or 0.
  
  So one has to introduce the charge conjugate "antiparticle" field, with its own creation and annihilation operators satisfying the relations
  
  where "k" has the same "p", and opposite σ and sign of the energy. Then one can rewrite the field in the form
  
  where the first sum is over positive energy states and the second over those of negative energy. The energy becomes
   When a proton encounters its antiparticle (and more generally, if any species of baryon encounters the corresponding antibaryon), the reaction is not as simple as electron-positron annihilation. Unlike an electron, a proton is a composite particle consisting of three "valence quarks" and an indeterminate number of "sea quarks" bound by gluons. Thus, when a proton encounters an antiproton, one of its quarks, usually a constituent valence quark, may annihilate with an antiquark (which more rarely could be a sea quark) to produce a gluon, after which the gluon together with the remaining quarks, antiquarks, and gluons will undergo a complex process of rearrangement (called hadronization or fragmentation) into a number of mesons, (mostly pions and kaons), which will share the total energy and momentum. The newly created mesons are unstable, and unless they encounter and interact with some other material, they will decay in a series of reactions that ultimately produce only gamma rays, electrons, positrons, and neutrinos. This type of reaction will occur between any baryon (particle consisting of three quarks) and any antibaryon consisting of three antiquarks, one of which corresponds to a quark in the baryon. (This reaction is unlikely if at least one among the baryon and anti-baryon is exotic enough that they share no constituent quark flavors.) Antiprotons can and do annihilate with neutrons, and likewise antineutrons can annihilate with protons, as discussed below.
   The name "baryon", introduced by Abraham Pais, comes from the Greek word for "heavy" (βαρύς, "barýs"), because, at the time of their naming, most known elementary particles had lower masses than the baryons. Each baryon has a corresponding antiparticle (antibaryon) where their corresponding antiquarks replace quarks. For example, a proton is made of two up quarks and one down quark; and its corresponding antiparticle, the antiproton, is made of two up antiquarks and one down antiquark.
   BULLET::::- PhaD - 547 and 13
  
  BULLET::::- PhaE - 161 and 3
  
  BULLET::::- PhaF - 92 and 3
  
  BULLET::::- PhaG - 120 and 3
  
  All are predicted to be integral membrane proteins.
  
  Corresponding values for the "S. aureus" Mnh system are: 
  
  BULLET::::- MnhA - 801 and 18
  
  BULLET::::- MnhB - 142 and 4
  
  BULLET::::- MnhC - 113 and 3
  
  BULLET::::- MnhD - 498 and 13
  
  BULLET::::- MnhE - 159 and 4
  
  BULLET::::- MnhF - 97 and 3
  
  BULLET::::- MnhG - 118 and 3
   Because of their infraparticle properties, charged particles do not have a sharp delta function density of states like an ordinary particle, but instead the density of states rises like an inverse power at the mass of the particle. This collection of states which are very close in mass to m consist of the particle together with low-energy excitation of the electromagnetic field.
  
  Section::::Noether's theorem for gauge transformations.
   Section::::Properties.
  
  Quantum states of a particle and an antiparticle can be interchanged by applying the charge conjugation (C), parity (P), and time reversal (T) operators. If formula_1 denotes the quantum state of a particle (n) with momentum p, spin J whose component in the z-direction is σ, then one has
   The antiproton-driven Inertial confinement fusion (ICF) Rocket concept uses pellets for the D-T reaction. The pellet consists of a hemisphere of fissionable material such as U with a hole through which a pulse of antiprotons and positrons is injected. It is surrounded by a hemisphere of fusion fuel, for example deuterium-tritium, or lithium deuteride. Antiproton annihilation occurs at the surface of the hemisphere, which ionizes the fuel. These ions heat the core of the pellet to fusion temperatures.
   The antiprotons were produced by accelerating protons onto a target. The resulting antiprotons emitted by the target material had a large divergence, which called for special devices to focus them. Instead of quadrupole magnets, which are conventionally used to focus particle beams, rods of solid lithium with an applied high gradient magnetic field were implemented.
   Within Dirac's theory, the problem of infinite charge of the universe remains. Some bosons also have antiparticles, but since bosons do not obey the Pauli exclusion principle (only fermions do), hole theory does not work for them. A unified interpretation of antiparticles is now available in quantum field theory, which solves both these problems by describing antimatter as negative energy states of the same underlying matter field i.e. particles moving backwards in time.
  
  Section::::Particle–antiparticle annihilation.
   Such is the case for charged leptons, like the electron; but within the standard model, the right-handed neutrino does not exist, so even with a Yukawa coupling neutrinos remain massless. In other words, there are no mass terms for neutrinos under the Standard Model: the model only contains a left-handed neutrino and its antiparticle, a right-handed antineutrino, for each generation, produced in weak eigenstates during weak interactions. (See neutrino masses in the Standard Model for a detailed explanation.)
   where "E" is an infinite negative constant. The vacuum state is defined as the state with no particle or antiparticle, "i.e.", formula_11 and formula_12. Then the energy of the vacuum is exactly "E". Since all energies are measured relative to the vacuum, H is positive definite. Analysis of the properties of "a" and "b" shows that one is the annihilation operator for particles and the other for antiparticles. This is the case of a fermion.
   Transport of the positively charged proton is typically electrogenic, i.e. it generates an electrical field across the membrane also called the membrane potential. Proton transport becomes electrogenic if not neutralized electrically by transport of either a corresponding negative charge in the same direction or a corresponding positive charge in the opposite direction. An example of a proton pump that is not electrogenic, is the proton/potassium pump of the gastric mucosa which catalyzes a balanced exchange of protons and potassium ions.
   This approach is due to Vladimir Fock, Wendell Furry and Robert Oppenheimer. If one quantizes a real scalar field, then one finds that there is only one kind of annihilation operator; therefore, real scalar fields describe neutral bosons. Since complex scalar fields admit two different kinds of annihilation operators, which are related by conjugation, such fields describe charged bosons.
  
  Section::::Quantum field theory.:Feynman–Stueckelberg interpretation.
  