Physics-mathematics connection
There is a natural connection between particle physics and representation theory, as first noted in the 1930s by Eugene Wigner. It links the properties of elementary particles to the structure of Lie groups and Lie algebras. According to this connection, the different quantum states of an elementary particle give rise to an irreducible representation of the Poincaré group. Moreover, the properties of the various particles, including their spectra, can be related to representations of Lie algebras, corresponding to "approximate symmetries" of the universe.
General picture.
Symmetries of a quantum system.
In quantum mechanics, any particular one-particle state is represented as a vector in a Hilbert space formula_1. To help understand what types of particles can exist, it is important to classify the possibilities for formula_1 allowed by symmetries, and their properties. Let formula_1 be a Hilbert space describing a particular quantum system and let formula_4 be a group of symmetries of the quantum system. In a relativistic quantum system, for example, formula_4 might be the Poincaré group, while for the hydrogen atom, formula_4 might be the rotation group SO(3). The particle state is more precisely characterized by the associated projective Hilbert space formula_7, also called ray space, since two vectors that differ by a nonzero scalar factor correspond to the same physical quantum state represented by a "ray" in Hilbert space, which is an equivalence class in formula_1 and, under the natural projection map formula_9, an element of formula_7.
By definition of a symmetry of a quantum system, there is a group action on formula_7. For each formula_12, there is a corresponding transformation formula_13 of formula_7. More specifically, if formula_15 is some symmetry of the system (say, rotation about the x-axis by 12°), then the corresponding transformation formula_13 of formula_7 is a map on ray space. For example, when rotating a "stationary" (zero momentum) spin-5 particle about its center, formula_15 is a rotation in 3D space (an element of formula_19), while formula_13 is an operator whose domain and range are each the space of possible quantum states of this particle, in this example the projective space formula_7 associated with an 11-dimensional complex Hilbert space formula_1.
Each map formula_13 preserves, by definition of symmetry, the ray product on formula_7 induced by the inner product on formula_1; according to Wigner's theorem, this transformation of formula_7 comes from a unitary or anti-unitary transformation formula_27 of formula_1. Note, however, that the formula_27 associated to a given formula_13 is not unique, but only unique "up to a phase factor". The composition of the operators formula_27 should, therefore, reflect the composition law in formula_4, but only up to a phase factor:
formula_33,
where formula_34 will depend on formula_15 and formula_36. Thus, the map sending formula_15 to formula_27 is a "projective unitary representation" of formula_4, or possibly a mixture of unitary and anti-unitary, if formula_4 is disconnected. In practice, anti-unitary operators are always associated with time-reversal symmetry.
Ordinary versus projective representations.
It is important physically that in general formula_41 does not have to be an ordinary representation of formula_4; it may not be possible to choose the phase factors in the definition of formula_27 to eliminate the phase factors in their composition law. An electron, for example, is a spin-one-half particle; its Hilbert space consists of wave functions on formula_44 with values in a two-dimensional spinor space. The action of formula_19 on the spinor space is only projective: It does not come from an ordinary representation of formula_19. There is, however, an associated ordinary representation of the universal cover formula_47 of formula_19 on spinor space.
For many interesting classes of groups formula_4, tells us that every projective unitary representation of formula_4 comes from an ordinary representation of the universal cover formula_51 of formula_4. Actually, if formula_1 is finite dimensional, then regardless of the group formula_4, every projective unitary representation of formula_4 comes from an ordinary unitary representation of formula_51. If formula_1 is infinite dimensional, then to obtain the desired conclusion, some algebraic assumptions must be made on formula_4 (see below). In this setting the result is a theorem of Bargmann. Fortunately, in the crucial case of the Poincaré group, Bargmann's theorem applies. (See Wigner's classification of the representations of the universal cover of the Poincaré group.)
The requirement referred to above is that the Lie algebra formula_59 does not admit a nontrivial one-dimensional central extension. This is the case if and only if the second cohomology group of formula_59 is trivial. In this case, it may still be true that the group admits a central extension by a "discrete" group. But extensions of formula_4 by discrete groups are covers of formula_4. For instance, the universal cover formula_63 is related to formula_4 through the quotient formula_65 with the central subgroup formula_66 being the center of formula_63 itself, isomorphic to the fundamental group of the covered group.
Thus, in favorable cases, the quantum system will carry a unitary representation of the universal cover formula_51 of the symmetry group formula_4. This is desirable because formula_1 is much easier to work with than the non-vector space formula_7. If the representations of formula_51 can be classified, much more information about the possibilities and properties of formula_1 are available.
The Heisenberg case.
An example in which Bargmann's theorem does not apply comes from a quantum particle moving in formula_74. The group of translational symmetries of the associated phase space, formula_75, is the commutative group formula_75. In the usual quantum mechanical picture, the formula_75 symmetry is not implemented by a unitary representation of formula_75. After all, in the quantum setting, translations in position space and translations in momentum space do not commute. This failure to commute reflects the failure of the position and momentum operators—which are the infinitesimal generators of translations in momentum space and position space, respectively—to commute. Nevertheless, translations in position space and translations in momentum space "do" commute up to a phase factor. Thus, we have a well-defined projective representation of formula_75, but it does not come from an ordinary representation of formula_75, even though formula_75 is simply connected.
In this case, to obtain an ordinary representation, one has to pass to the Heisenberg group, which is a nontrivial one-dimensional central extension of formula_75.
Poincaré group.
The group of translations and Lorentz transformations form the Poincaré group, and this group should be a symmetry of a relativistic quantum system (neglecting general relativity effects, or in other words, in flat spacetime). Representations of the Poincaré group are in many cases characterized by a nonnegative mass and a half-integer spin (see Wigner's classification); this can be thought of as the reason that particles have quantized spin. (Note that there are in fact other possible representations, such as tachyons, infraparticles, etc., which in some cases do not have quantized spin or fixed mass.)
Other symmetries.
While the spacetime symmetries in the Poincaré group are particularly easy to visualize and believe, there are also other types of symmetries, called internal symmetries. One example is color SU(3), an exact symmetry corresponding to the continuous interchange of the three quark colors.
Lie algebras versus Lie groups.
Many (but not all) symmetries or approximate symmetries form Lie groups. Rather than study the representation theory of these Lie groups, it is often preferable to study the closely related representation theory of the corresponding Lie algebras, which are usually simpler to compute.
Now, representations of the Lie algebra correspond to representations of the universal cover of the original group. In the finite-dimensional case—and the infinite-dimensional case, provided that applies—irreducible projective representations of the original group correspond to ordinary unitary representations of the universal cover. In those cases, computing at the Lie algebra level is appropriate. This is the case, notably, for studying the irreducible projective representations of the rotation group SO(3). These are in one-to-one correspondence with the ordinary representations of the universal cover SU(2) of SO(3). The representations of the SU(2) are then in one-to-one correspondence with the representations of its Lie algebra su(2), which is isomorphic to the Lie algebra so(3) of SO(3).
Thus, to summarize, the irreducible projective representations of SO(3) are in one-to-one correspondence with the irreducible ordinary representations of its Lie algebra so(3). The two-dimensional "spin 1/2" representation of the Lie algebra so(3), for example, does not correspond to an ordinary (single-valued) representation of the group SO(3). (This fact is the origin of statements to the effect that "if you rotate the wave function of an electron by 360 degrees, you get the negative of the original wave function.") Nevertheless, the spin 1/2 representation does give rise to a well-defined "projective" representation of SO(3), which is all that is required physically.
Approximate symmetries.
Although the above symmetries are believed to be exact, other symmetries are only approximate.
Hypothetical example.
As an example of what an approximate symmetry means, suppose an experimentalist lived inside an infinite ferromagnet, with magnetization in some particular direction. The experimentalist in this situation would find not one but two distinct types of electrons: one with spin along the direction of the magnetization, with a slightly lower energy (and consequently, a lower mass), and one with spin anti-aligned, with a higher mass. Our usual SO(3) rotational symmetry, which ordinarily connects the spin-up electron with the spin-down electron, has in this hypothetical case become only an "approximate" symmetry, relating "different types of particles" to each other.
General definition.
In general, an approximate symmetry arises when there are very strong interactions that obey that symmetry, along with weaker interactions that do not. In the electron example above, the two "types" of electrons behave identically under the strong and weak forces, but differently under the electromagnetic force.
Example: isospin symmetry.
An example from the real world is isospin symmetry, an SU(2) group corresponding to the similarity between up quarks and down quarks. This is an approximate symmetry: while up and down quarks are identical in how they interact under the strong force, they have different masses and different electroweak interactions. Mathematically, there is an abstract two-dimensional vector space
formula_83
and the laws of physics are "approximately" invariant under applying a determinant-1 unitary transformation to this space:
formula_84
For example, formula_85 would turn all up quarks in the universe into down quarks and vice versa. Some examples help clarify the possible effects of these transformations:
In general, particles form isospin multiplets, which correspond to irreducible representations of the Lie algebra SU(2). Particles in an isospin multiplet have very similar but not identical masses, because the up and down quarks are very similar but not identical.
Example: flavour symmetry.
Isospin symmetry can be generalized to flavour symmetry, an SU(3) group corresponding to the similarity between up quarks, down quarks, and strange quarks. This is, again, an approximate symmetry, violated by quark mass differences and electroweak interactions—in fact, it is a poorer approximation than isospin, because of the strange quark's noticeably higher mass.
Nevertheless, particles can indeed be neatly divided into groups that form irreducible representations of the Lie algebra SU(3), as first noted by Murray Gell-Mann and independently by Yuval Ne'eman.
Notes.
 ;templatestyles src="Reflist/styles.css" / ;