Relativistic Quantum Mechanics

Transcription

1 Relativistic Quantum Mechanics Wayne Polyzou The University of Iowa Relativistic Quantum Mechanics p.1/42

2 Collaborators F. Coester (ANL), B. Keister (NSF), W. H. Klink (Iowa), G. L. Payne (Iowa) P. L. Chung, H, C, Jean, Y. Huang, F. Bulut B. Kessler, V. Wessels, E. Sengbusch Relativistic Quantum Mechanics p.2/42

3 Motivation Scattering experiments at the Thomas Jefferson National Laboratory use high-current electron beams to deduce the structure of baryons, mesons, and light nuclei. c p x c.197(gev fm) x =.2fm c p 1GeV m p c 2 Relativistic quantum mechanics is needed to interpret these experiments! Relativistic Quantum Mechanics p.3/42

4 Example Assume a model quark-antiquark Hamiltonian: M = 2 m 2 + p 2 + αr β r + γ s 1 s 2 e λr2 Choose model parameters to fit experimental data. Use the resulting model to make predictions. Relativistic Quantum Mechanics p.4/42

5 Pion form factor 1 Pion Form Factor m=.23 cl=.1 cc=-.4 css=1.67 F(Q) Q 2 Thu Mar 25 23:28: Relativistic Quantum Mechanics p.5/42

6 Pion form factor 0.5 Pion Form Factor m=.23 cl=.1 cc=-.4 css= F(Q)Q Q 2 Thu Mar 25 23:36: Relativistic Quantum Mechanics p.6/42

7 Pion form factor This example makes a prediction, based on a relativistic model, for proposed measurements. The nominal RMS radius of the pion in this model is 3.88 fm, which is slightly more than half of the charge radius. This large difference is a relativistic effect. Relativistic Quantum Mechanics p.7/42

8 Quantum Theory States are represented by unit vectors, ψ, in a complex vector space φ ψ = ψ φ The predictions of a quantum theory are the probabilities P ψφ = ψ φ 2 Relativistic Quantum Mechanics p.8/42

9 Relativity Inertial coordinate systems are coordinate systems where free particles move with constant velocity. The probabilities measured in equivalent experiments done in different inertial coordinate systems are identical. Relativistic Quantum Mechanics p.9/42

10 Relativity Because solutions of the Schrödinger equation are not observable in quantum theory, relativity does not require that the Schrödinger equation have the same form in every inertial coordinate system. This is an important difference between the classical and quantum formulation of relativistic invariance. Relativistic Quantum Mechanics p.10/42

11 Relativistic Invariance Galilean Relativity: X and X inertial x ij = x ij t ij = t ij Special Relativity: X and X inertial x ij 2 c 2 t ij 2 = x ij 2 c 2 t ij 2 Relativistic Quantum Mechanics p.11/42

12 Relativity The Michelson-Morley experiment verified that special relativity gives the observed relation between inertial coordinate systems. The most general transformation relating two inertial coordinate systems is a Poincaré transformation: x µ x µ = Λ µ νx ν + a µ g µν = Λ µ αλ ν βg αβ Relativistic Quantum Mechanics p.12/42

13 Relativity in Quantum Theory X X φ, ψ φ, ψ φ ψ 2 = φ ψ 2 Must hold for all ψ, φ and all inertial coordinate systems X and X Relativistic Quantum Mechanics p.13/42

14 Wigner s Theorem φ ψ 2 = φ ψ 2 φ = U φ ψ = U ψ ψ φ = ψ φ or φ = A φ ψ = A ψ ψ φ = ψ φ Relativistic Quantum Mechanics p.14/42

15 Wigner s Theorem For rotations, translations, and rotationless Lorentz transformations: U = U 1/2 U 1/2 A = A 1/2 A 1/2 The correspondence between states must be unitary! ψ = U ψ Relativistic Quantum Mechanics p.15/42

16 Wigner s Theorem X }{{} (Λ,a) X }{{} (Λ,a ) X } {{ } (Λ,a ) U(Λ, a )U(Λ, a) = U(Λ Λ, Λ a + a ) U(Λ, a) is a unitary representation of the Poincaré group Relativistic Quantum Mechanics p.16/42

17 Elements of Relativistic Quantum Theory Complex vector space, V (Hilbert space) Unitary representation of the Poincaré group U(Λ, a) : V V U(Λ, a) contains the dynamics (time evolution). Relativistic Quantum Mechanics p.17/42

18 Dynamics Space Time Diagram t x Relativistic Quantum Mechanics p.18/42

19 Dynamics Λ = γ 0 0 γβ γβ 0 0 γ a = (0, 0, 0, z) b = (0, 0, 0, γz) t := (γβz, 0) (Λ 1, b)(λ, a) = (I, (t, 0)) Relativistic Quantum Mechanics p.19/42

20 Dynamics U(Λ, a) = e i λ i G i G i = G i G i {H, P, J, K} [G i, G j ] = ig ijk G k [P i, K j ] = iδ ij H Relativistic Quantum Mechanics p.20/42

21 Dynamics Galilean Invariant Dynamics {H, P, J, K, M} [P i, K j ] = iδ ij M Poincaré Invariant Dynamics {H, P, J, K} [P i, K j ] = iδ ij (H 0 + V ) Puts non-linear constraints on {H, P, J, K} for consistent initial value problem! Relativistic Quantum Mechanics p.21/42

22 One-Particle Relativistic Quantum Mechanics Problem Find the single particle Hilbert space V Find U(Λ, a) on V Relativistic Quantum Mechanics p.22/42

23 One-Particle Relativistic Quantum Mechanics Ψ Φ = Ψ Φ U(Λ, a) {H, P, J, K} Relativistic Quantum Mechanics p.23/42

24 One-Particle Relativistic Quantum Mechanics Construct a maximal set of commuting Hermitian operators out of {H, P, J, K}: Use the commutations relations of {H, P, J, K} to determine the eigenvalues of these operators. Let V be the space of square integrable functions of the eigenvalues. Use the transformation properties of the observables to determine U(Λ, a). Relativistic Quantum Mechanics p.24/42

25 Commuting Observables where M := {M, j 2, P, j z } H 2 P P j = J { 1 H, K} P + ( P (H J P K)) P MH(M + H) Relativistic Quantum Mechanics p.25/42

26 Commuting Observables The mass and spin of the particle fix the eigenvalue of m and j 2 The commutation relations imply [j x, j y ] = ij z which means j must be integer of half integer and j z { j, j + 1, j 1, j} Relativistic Quantum Mechanics p.26/42

27 One-Particle Hilbert Space Lorentz transformations can generate any momentum < P i < Ψ( p, µ) = m, j; p, µ Ψ j Ψ Φ := d 3 pψ ( p, µ)φ( p, µ) µ= j Ψ Ψ < Relativistic Quantum Mechanics p.27/42

28 Basis vectors Start with eigenstate with P = 0 and j z = j: (m, j) 0, j := 0, j Define the lowering operator j = j x ij y 0, µ 1 = 1 (j + µ)(j µ + 1) j 0, µ This defines m, j; 0, µ := 0, µ Relativistic Quantum Mechanics p.28/42

29 Basis vectors The commutation relations [j x, j y ] = ij z and definitions imply for any rotation R U(R, 0) 0, µ = j µ = j 0, µ D j µ µ (R) Relativistic Quantum Mechanics p.29/42

30 Basis vectors Let Λ( p ) : (m, 0) = ( p 2 + m 2, p) : p, µ := U(Λ( p), 0) 0, µ (1 + p 2 m 2) 1/4 the factor (1 + p 2 m ) 1/4 is determined by the 2 requirement that U(Λ, 0) be unitary. U(I, a) 0, µ := e ia0m 0, µ Relativistic Quantum Mechanics p.30/42

31 One-Body Problem Any Poincare transformation can be decomposed into a product of the previously defined transformations U(Λ, a) = U(Λ(p ), 0)U(I, Λ 1 (p )a) U(Λ 1 (p )ΛΛ(p), 0)U(Λ }{{} 1 (p), 0) rotation p = Λp Relativistic Quantum Mechanics p.31/42

32 The two-body problem The one-body solution is well known. It provides clues on how to solve the two-body problem Relativistic Quantum Mechanics p.32/42

33 The Currie Jordan Sudarshan Theorem {X i, P j } = δ ij {X i, J j } = ɛ ijk X k {X i, K j } = X j {X i, H} world-line condition Poisson brackets of {H, P, J, K} satisfy Lie algebra of Poincaré group. Relativistic Quantum Mechanics p.33/42

34 The Currie-Jordan-Sudarshan Theorem Can only be satisfied for free particles! The CJS theorem suggests that it might be difficult to formulate relativistic models for systems of interacting particles. The out in relativistic quantum mechanics is that there is no position operator! Relativistic Quantum Mechanics p.34/42

35 Position Let 0, 0 be a vector corresponding the a particle at the origin at time t = 0 p 0; 0 = p = 0 U (Λ( p), 0)U(Λ( p), 0) 0; 0 = N ( p 2 + m 2 ) 1/4 p x; t = 0 = p U(I, x ) 0; 0 Relativistic Quantum Mechanics p.35/42

36 Position ic 2 0 x = p x = 2 (2π)2 D + (0, x ) e i p x (2π) 3/2 ( p 2 + m 2 ) 1/4 d 3 p (2π) 3 ( p 2 + m 2 ) 1/2e i p x = ( ) mc x e mc x 0 Relativistic Quantum Mechanics p.36/42

37 The Two-Body Problem The one-body solution has clues about how to solve the two-body problem Find M, j 2, P, j z for the non-interacting two-body system. Construct basis of simultaneous eigenstates of these operators Relativistic Quantum Mechanics p.37/42

38 The Two-Body Problem Add interactions to M that commute with and do not depend on P and j z Construct simultaneous eigenstates of M + V, j 2, P, j z Construct U(Λ, a) following the one body construction Relativistic Quantum Mechanics p.38/42

39 The Two-Body Problem Construct p 1, µ 1 ; p 1, µ 1 m 12, j 12 ; p 12, µ 12 ; l 12, s 12 m 12, j 12 ; p 12, µ 12 ; l 12, s 12 The coefficients above differ from the corresponding non-relativistic coefficients. Relativistic Quantum Mechanics p.39/42

40 Two-Body Problem m 12, j 12 ; p 12, µ 12 ; l 12, s 12 V m 12, j 12; p 12, µ 12; l 12, s 12 = δ( p 12 p 12)δ j12 j 12 δ µ 12 µ m 12 12, l 12, s 12 V j m 12, l 12, s 12 M, j 12 ; p 12, µ 12 Relativistic Quantum Mechanics p.40/42

41 Two-Body Problem M, j 12 ; 0 12, j 12 (j 12 ) M, j 12 ; 0 12, µ 12 (U(Λ(p)) M, j 12 ; p 12, µ 12 Relativistic Quantum Mechanics p.41/42

42 The Two-Body Problem U I (Λ, a) M, j 12 ; p 12, µ 12 This construction leads to a U(Λ, a) for the interacting system on the two body Hilbert space. The dynamical problem is to diagonalize M = m 12 + V in the basis of free particle eigenstates. Relativistic Quantum Mechanics p.42/42

43 Beyond-Two The construction has been generalized to treat any number of particle and with limited particle production. The requirement of cluster properties adds new dynamical non-linear constraints for more than two particles. The have been treated. U(Λ, a) implies that spinor, tensor, and four vector operators are interaction dependent Relativistic Quantum Mechanics p.43/42

44 Conclusions Relativistic quantum mechanics of particles is a useful and practical tool for modeling quantum systems where relativity is important. It has been successfully applies to model spectral, electromagnetic, and scaling properties of baryons, mesons, and the two and three nucleon systems. Open question on how to treat problems with particle production are being explored Relativistic Quantum Mechanics p.44/42