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7 Identical Particles 7 Identical Particles One of the most striking facts that helps us understand the Universe is that, apart from details of their motion, all electrons are identical. This fact, though convenient, has no ex- planation within Quantum Mechanics. In Quantum Field Theory, one learns that particles are simply quantised modes of excitation of a field. We thus replace the vast number of electrons our Universe contains with a single quantum field that has hugely many excita- tions. From this perspective, the fact that all elementary particles of the same species are identical is more or less a tautology, no more surprising than the fact that one factor of x in the monomial xn is just the same as another. 7.1 Bosons and Fermions Consider a system consisting of N particles, individually described by some state in a Hilbert space a where a = 1;:::;N, and suppose αa a gives a complete specification H th j i 2 H of the state of the a particle, so αa collectively labels all the quantum numbers describing the particle's energy, orbital angular momentum, spin etc.. In the case that the particles are distinguishable { meaning they are each from different species, such as an electron, a proton or a neutron { a complete set of states of the full system is then labelled by linear combinations of states N O α1; α2; ::: ; αN = α1 α2 αN a : (7.1) j i j ij i · · · j i 2 H a=1 in accordance with the discussion of section 2.3.1. However, suppose particles 1 and 2 are indistinguishable; for example, they might both be electrons. The state (7.1) thus describes a situation where there is one electron with quantum numbers given by α1 and another whose same properties are labelled by α2. Since both electrons are identical, it can make no difference which is which. Consequently, there can be no physical difference between this state and the state in which these two particles are exchanged. This does not imply that the two states are identical, but does imply that they can only differ by a phase: iφ α2; α1; ::: = e α1; α2; ::: ; (7.2) j i j i 61 where φ is independent of the particular values of the labels αa, but may depend on the species of particle we exchange. If we exchange the pair of particles once again we find iφ 2iφ α1; α2; ::: = e α2; α1; ::: = e α1; α2; ::: (7.3) j i j i j i 61There's a (non-examinable!) exception to this in the case of two spatial dimensions, where the phase may depend on the homotopy class of the path the particles take whilst they are being exchanged. In 2 + 1 dimensions, these paths can become braided (tangled) and the multi{particle state transforms in a representation of the braid group, first studied in the maths literature by Emile Artin. This leads to the possibility of anyonic statistics, which play an important role in some condensed matter systems. 3 Fortunately, we're interested in quantum mechanics on R and in higher dimensions it turns out the paths can always be disentangled, so anyons do not arise. { 97 { using the fact that the phase is independent of the particular values of the quantum numbers 2iφ αa. Thus e = 1, so eiφ = 1 : (7.4) ± The choice of sign depends only on the species of the particle. Identical particles which are symmetric under exchange are called62 bosons, while those that are antisymmetric are said to be fermions. If all N of the particles in our system are identical, then the argument immediately extends to say that the state must be either completely symmetric or completely antisym- metric under exchange of any pair, which case being determined by whether the particles are bosons or fermions. That is, for bosons the state of the system is actually described by a vector N N O α1; α2; ::: ; αN Sym ; (7.5) j i 2 H ⊂ H whereas for fermions the state lies in the antisymmetric part N N ^ O α1; α2; ::: ; αN : (7.6) j i 2 H ⊂ H As a simple example, a pair of identical fermions with quantum numbers α1 and α2 live in the antisymmetric state 1 fermi = ( α1; α2 α2; α1 ) ; (7.7) j i p2 j i − j i whereas the system would have to be described by the symmetric combination 1 bose = ( α1; α2 + α2; α1 ) (7.8) j i p2 j i j i if instead these particles were identical bosons. A further fact that also comes from quantum field theory is that particles with integer spin are always bosons, whilst particles with (odd) half{integer spin are always fermions. Among the elementary particles, examples of bosons thus include the W and Z bosons (hence the name!) and the photon, while examples of fermions include electrons, neutrinos and quarks. You'll learn about how this connection between spin and statistics arises if you take the Part III QFT course. The above considerations apply to composite particles as well as to elementary ones. When we exchange a pair of identical composite particles, we exchange all of their con- stituents. Thus, if the composites are made up from an odd number of fermions and any number of bosons, upon exchange we'll acquire an odd number of minus signs, so the whole state must be antisymmetric. For example, a proton consists of three (valence) quarks and many gluons, so is a fermion. So too is a neutron. On the other hand, composites consist- ing of an even number of fermions and any number of bosons must be symmetric under 62They're named after Satyendra Nath Bose and Enrico Fermi, respectively, who first studied their properties. { 98 { exchange with an identical composite. For example, a hydrogen atom consists of a proton and an electron, each of which are fermions, so the composite Hydrogen atom is itself a boson. This behaviour is consistent with addition of angular momentum. When we add the angular momenta of an odd number of particles each with odd half{integer spin, the total angular momentum is also an odd half{integer, whereas when we combine the angular momenta of an even number of particles each with odd half{integer spin, the total angular momentum is an integer. Even without quantum field theory, it would be impossible for all integer spin particles to be fermions, because a composite consisting of an even number of such integer spin particles would also have integer spin, but would be a boson. 7.1.1 Pauli's Exclusion Principle The fact that the state of identical fermions must be totally antisymmetric has an im- mediate, striking consequence: no two fermions can be put in exactly the same state simultaneously. This fact is known as Pauli's exclusion principle. As an example, since we 0 0 must always have σ,σ0 (x; x ) = σ0,σ(x ; x), if both fermions happen to have the same 0 − 0 Sz eigenvalue we find σ,σ(x; x ) = σ,σ(x ; x). Hence the spatial wavefunction of the − system necessarily vanishes at x = x0 so there is zero probability that the two particles are located at the same place. More generally, the state Ψ of N identical fermions is represented in terms of the j i determinants (α ) (α ) (α ) 1 1 1 2 1 N ··· 1 2(α1) 2(α2) 2(αN ) α1; α2; : : : ; αN Ψ = . ··· . (7.9) h j i p . .. N! . N (α1) N (α2) N (αN ) ··· that in this context are known as Slater determinants. Again, α1; α2; : : : ; αn represents j i a state in which some fermion has quantum numbers α1, another has α2 and so on, where the labels αi denote a complete set of quantum numbers for single particle states, including (say) their momentum, spin-z value, etc., while 1(α1) = α1 1 is the amplitude for the h j i ‘first’ fermion to have quantum numbers α1. If αi = αj for any i = j, then two columns of 6 this determinant coincide. The amplitude : : : ; αi; : : : ; αi;::: Ψ thus vanishes identically, h j i so there is zero probability of any two fermions being found in an identical state. It's worth stressing that we've learned only that the state of a pair of identical fermions must be antisymmetric under exchange of every one of their quantum numbers. The Hilbert 2s+1 space of a single such spin-s fermion is itself a tensor product fermion = spat C , H H ⊗ including a factor that describes the fermion's spatial wavefunction (including details of 2s+1 its likely position, orbital angular momentum etc.) and a separate factor spin = C H ∼ spanned by the possible spin states s ; s 1 ;:::; s of this spin-s particle. We must fj i j − i j − ig exchange both the spin and spatial parts of the state in order to find a physically equivalent state. { 99 { 0 0 In the example of our electron pair, σ,σ0 (x; x ) is certainly equivalent to σ0,σ(x ; x), since both correspond to the electron at x having spin σ and the electron found at x0 having spin σ0. However, antisymmetry of the total state does not imply any relation between 0 0 the states σ,σ0 (x; x ) and σ0,σ(x; x ): these are distinct physical possibilities because the electron at x has different spin in the two cases. We saw in section 6.1.2 that the spin wavefunctions of the two electrons could be combined into a triplet of spin-1 states 1 1; 1 = ; 1; 0 = ( + ) ; 1; 1 = (7.10) j i j" ij" i j i p2 j" ij# i j# ij" i j − i j# ij# i which are symmetric, and a single antisymmetric state 1 0; 0 = ( ) (7.11) j i p2 j" ij# i − j# ij" i of spin-zero.
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