Supersymmetric Nonlinear Sigma Models

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server OU-HET 348 TIT/HET-448 hep-th/0006025 June 2000 Supersymmetric Nonlinear Sigma Models a b Kiyoshi Higashijima ∗ and Muneto Nitta † aDepartment of Physics, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan bDepartment of Physics, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan Abstract Supersymmetric nonlinear sigma models are formulated as gauge theories. Auxiliary chiral superfields are introduced to impose supersymmetric constraints of F-type. Target manifolds defined by F-type constraints are al- ways non-compact. In order to obtain nonlinear sigma models on compact manifolds, we have to introduce gauge symmetry to eliminate the degrees of freedom in non-compact directions. All supersymmetric nonlinear sigma models defined on the hermitian symmetric spaces are successfully formulated as gauge theories. 1Talk given at the International Symposium on \Quantum Chromodynamics and Color Con- finement" (Confinement 2000) , 7-10 March 2000, RCNP, Osaka, Japan. ∗e-mail: [email protected]. †e-mail: [email protected] 1 Introduction Two dimensional (2D) nonlinear sigma models and four dimensional non-abelian gauge theories have several similarities. Both of them enjoy the property of the asymptotic freedom. They are both massless in the perturbation theory, whereas they acquire the mass gap or the string tension in the non-perturbative treatment. Although it is difficult to solve QCD in analytical way, 2D nonlinear sigma models can be solved by the large N expansion and helps us to understand various nonperturbative phenomena in four dimensional gauge theories. In the nonperturbative treatment of nonlinear σ models, auxiliary field method play an important role. As an example, let us consider the supersymmetric 2D nonlinear σ model with O(N) symmetry. The bosonic field A~(x) takes the value N 1 √N on the real N 1 dimensional sphere S − with a radius . The corresponding − g N fermionic field ψ~(x) is a Majorana (real) spinor which has the four-fermi type interaction with O(N) symmetry and is called the Gross-Neveu model. The supersymmetric nonlinear sigma model with O(N) symmetry is defined simply as a combination of these two models 1 ¯ g2 ¯ (x)= (∂A~)2 + ψi∂~ /ψ~ + (ψ~ψ~)2 L 2{ } 8N where A~2 = N and A~ ψ~ =0. g2 · This model enjoys three kinds of symmetry: O(N) symmetry, discrete chiral symmetry ψ γ ψ and the supersymmetry which mixes bosonic and fermionic → 5 fields. We can find the solution in the large N limit where N with g fixed. →∞ In order to study the phase structure, it is crucial to introduce auxiliary superfields defined in the superspace with the bosonic coordinate xµ and the two component fermionic coordinate θ 1 Φ (x, θ)=A (x)+θψ¯ (x)+ θθF¯ (x). 0 0 0 2 0 A (x) ψ~¯ ψ~ describes the scalar bound state of two fermions as the auxiliary field 0 ∼ · of the Gross-Neveu model. ψ (x) A~ ψ~ describes the fermionic bound state of the 0 ∼ · ~2 N fermion and the boson. F0(x) is the Lagrange multiplier for the constraint: A = g2 1 Table 1: Phase Structure of the O(N) model symmetry order parameter perturbative nonperturbative O(N) A~ h i × chiral symmetry A h 0i × supersymmetry F h 0i of the bosonic nonlinear σ model. These auxiliary fields also play the role of order parameters of various symmetry. With the auxiliary field, the nonlinear lagrangian reduces to a very simple form 1 ¯ ¯ ¯ N (x)= (∂A~)2+ψi∂~ /ψ~+F~ 2 + 2A A~ F~ A ψ~ψ~ + F A~2 ψ¯ ψ~A~ ψψ~ A~ F . 0 0 0 0 0 2 0 L 2{ } · − − − −g In the large N field theory, we take into account the dominant vacuum fluctuations of O(N) vector fields A,~ ψ~ and F~ which can be integrated out easily since the lagrangian is simply of quadratic form in these variables. As is shown in the table 1, O(N) symmetry recovers and A~(x) acquire a mass proportional to A .Chiral h 0i symmetry breaks down and fermions have the same mass with bosons. In this talk, we generalize the auxiliary field formulation to nonlinear σ models with = 2 supersymmetry in two dimensions, which is equivalent to =1 N N supersymmetry in four dimensions. 2 Nonlinear σ Models in Four Dimensions When a global symmetry group G breaks down to its subgroup H by a vacuum expectation value v = φ , there appear massless Nambu-Goldstone (NG) bosons h i corresponding to broken generators in G/H. At low energies, interactions among these NG bosons are described by nonlinear σ models [1]. In supersymmetric theories, target manifolds of nonlinear σ models must be Kähler manifolds [2, 11]. A 2 manifold whose metric is given by a Kähler potential K(A,¯ A) ∂2K(A,¯ A) g = , (1) mn¯ ∂A¯m¯ ∂An ia called the Kähler manifold. Since NG bosons must be scalar components of complex chiral superfields 1 φ(x, θ)=A(x)+θψ(x)+ θ2F (x), (2) 2 there are two possibilities. If the coset G/H itself is a Kähler manifold, both Re A(x) and Im A(x) must be NG-bosons. The effective lagrangian in this case is uniquely determined by the metric of the coset manifold G/H [3, 4, 5, 6]. On the other hand, if the coset G/H is a submanifold of a Kähler manifold, there is at least one chiral superfield whose real or imaginary part is not a NG-boson. This additional massless boson is called the quasi-Nambu-Goldstone(QNG) boson [7, 8]. In this case, the Kähler metric in the direction of QNG boson is not determined by the metric of its subspace G/H, and the effective lagrangian is not unique. In this article, we will confine ourselves to the case of Kähler G/H. The lagrangian of the nonlinear σ model on a Kähler manifold is m¯ µ n i ¯m¯ µ n n µ ¯ m¯ (x)=gmn∂µA¯ ∂ A + gmn ψ σ (Dµψ) + ψ σ¯ (Dµψ) L ¯ 2 ¯ 1 k l ¯m¯ ¯n¯ + 4 Rkml¯ n¯(ψ ψ )(ψ ψ ), (3) where n n n l m n nk¯ (Dµψ) =(δm∂µ +Γlm∂µA )ψ , Γlm = g ∂lgkm¯ . (4) Once we know the Kähler potential K(A,¯ A), we can calculate the metric by (1), the connection by (4) and the lagrangian by (3). This lagrangian is suitable for perturbative calculations. For the nonperturbative study, however, the auxiliary field formulation is hoped for. 3 Auxiliary Field Formulation N 1 Let us start from a known example, the CP − model. Chiral superfield φi(x, θ)(i = 1, 2, ,N) belongs to a fundamental representation of G = SU(N)whichisthe ··· 3 N 1 isometry of CP − . We introduce U(1) gauge symmetry iΛ(x,θ) φ(x, θ) φ0(x, θ)=e φ(x, θ)(5) −→ to require that φ(x, θ)andφ0(x, θ) are physically indistinguishable. With a complex chiral superfield, eiΛ(x,θ) is an arbitrary complex number. U(1) gauge symmetry is C thus complexified to U(1) . The identification φ φ0 defines the complex projective ∼ N 1 space CP − . In order to impose local U(1) gauge symmetry, we have to introduce V V iΛ+iΛ a U(1) gauge field V (x, θ, θ¯) with the transformation property e e e− ∗ . −→ Then the lagrangian with a local U(1) gauge symmetry is given by 2 2 V = d θd θ¯(e φ~∗ φ~ cV )(6) L Z · − where the last term V is called the Fayet-Iliopoulos D-term. In this model, real scalar superfield V (x, θ, θ¯) is the auxiliary superfield. The Kähler potential K(φ, φ∗) is obtained by eliminating V by using the equation of motion for V 2 2 2 2 = d θd θK¯ (φ, φ∗)=c d θd θ¯log (φ~∗ φ~). (7) L Z Z · N 1 This Kähler potential reduces to the standard Fubini-Study metric of CP − N 1 − K(φ, φ∗)=c log (1 + φi∗φi)(8) i X=1 by a choice of gauge fixing φN (x, θ)=1. (9) N 1 The lagrangian of the CP − model is obtained by substituting the Kähler potential (8) to eqns. (1), (4) and (3). The global symmetry G = SU(N), the isometry of N 1 the target space CP − , is linearly realized on our φi fields and our lagrangian (6) with auxiliary field V is manifestly invariant under G. The gauge fixing condition (9) is not invariant under G = SU(N) and we have to perform an appropriate gauge transformation simultaneously to compensate the change of φN caused by the SU(N) transformation. Therefore the global symmetry G = SU(N) is nonlinearly realized in the gauge fixed theory. In this sense, our lagrangian (6) use the linear realization of G in contrast to the nonlinear lagrangian in terms of the Kähler potential which use the nonlinear realization of G. 4 Similarly, we introduce M (M<N)copiesφj (j =1, 2, ,M) of fundamental ··· representations of SU(N) and impose the local gauge symmetry U(M)toidentify ΦU Φ(U U(M)) where Φ = (φ ,φ , ,φM )isanN M matrix of chiral ∼ ∈ 1 2 ··· × superfields. Then we obtain the nonlinear σ model on the Grassmann manifold SU(N)/SU(N M) U(M). The lagrangian with an auxiliary U(M) gauge field − × V reads 2 2 V = d θd θ¯ tr Φ†Φe c trV (10) L − Z N 1 As in the CP − model, the auxiliary field V can be eliminated by the use of its equation motion.

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