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Chapter 6: Nucelon-Nucleon Interactions and Deuteron

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Chapter 6: Nucelon-Nucleon Interactions and Deuteron Chapter 6 Nucleon-Nucleon Interaction, Deuteron Protons and neutrons are the lowest-energy bound states of quarks and gluons. When we put two or more of these particles together, they interact, scatter and sometimes form bound states due to the strong interactions. If one is interested in the low-energy region where the nucleons hardly get excited internally, we can treat the nucleons as inert, structureless elementary particles, and we can understand many of the properties of the multi-nucleon systems by the nucleon-nucleon interactions. If the nucleons are non-relativistic, the interaction can be described by a potential. Since the fundamental theory governing the nucleon-nucleon interactions is QCD, the interactions shall be calculable from the physics of quarks and gluons. Nonetheless, the problem is quite complicated and only limited progress has been made from the first principles so far. Therefore, the approach we are going take is a phenomenological one: One first tries to extract the nucleon- nucleon interaction from the nucleon-nucleon scattering data or few nucleon properties, and then one tries to use these interactions to make predictions for the nuclear many-body system. In this and the following few chapters, we are going to discuss some of results of this approach. 6.1 Yukawa Interaction and One-Pion Exchange at Long Distance If we consider the nucleons as elementary particles, their strong interactions are known have a short range from α particle scattering experiments of Rutherford. In fact, the range of the interaction is 15 roughly the size of the atomic nuclei, namely on the order of a few fermis (fm) (1 fm = 10− m). The first theory of the nucleon force was put forward by H. Yukawa, who suggested that the interaction between two nucleons is effected by the exchange of a particle, just like the interaction between the electric charges by the exchange of a photon. However, because the nucleon interactions appear to be short-ranged, the particle must have a finite mass. In fact, one can correlate the range and mass roughly by the quantum uncertainty principle r 1/m , (6.1) ∼ therefore, the mass of the quanta exchanged is about 1/fm which is about 200 MeV. The particle was discovered almost 20 years later and was identified as π meson (140 MeV). The most significant aspect of the Yukawa theory is generalizing the relation between particles and forces—-the existence 99 100 CHAPTER 6. NUCLEON-NUCLEON INTERACTION, DEUTERON of strong interactions implies the existence of a new particle! This was considered a novel and radical idea at that time. The modern theory of interactions through particle exchanges is made possible by the de- velopment of quantum field theory. However, at low-energy, one can assume the interactions is instantaneous and therefore the concept of interaction potential becomes useful. To illustrate the derivation of a potential through particle exchange, consider the interaction of a scalar particle with the nucleons. Introducing a quantum field φ to represent the particle, the Yukawa interaction has the form, = gφψψ¯ . (6.2) L The scattering S-matrix between two nucleon states with momentum p1 and p2 is 4 4 i S = (2π) δ (p + p p′ p′ )igU(p′ )U(p ) igU(p′ )U(p ) (6.3) 1 2 − 1 − 2 1 1 (p p )2 m2 + iǫ 2 2 1 − 1′ − An interacting potential can be derived with the following approximation: Since particles are non- relativistic, their energy is dominated by the rest mass, and therefore 2 2 (p p′ ) = (~p ~p′) + (1/M) , (6.4) 1 − 1 − − O where M is the nucleon mass. The propagator is now energy-independent, and the interac- tion is instantaneous and can be represented by a potential. The on-shell Dirac spinor is U = E + M(χ, ~σ ~p/(E + M)χ) which to the leading order can be approximated by U = √2M(χ, 0). p · p Therefore the S-matrix can be approximated by p 2 4 4 2 g S i(2π) δ (p + p p′ p′ )(2M) χ†′ χ χ†′ χ − (6.5) ∼− 1 2 − 1 − 2 2 2 1 1 ~q2 + m2 where we introduce the center-of-mass and relative variables, R~ = (~r + ~r )/2, ~r = ~r ~r ; 1 2 1 − 2 P~ = ~p + ~p , ~p = (~p ~p )/2; (6.6) 1 2 1 − 2 and assume P~ = 0. Therefore, ~q = ~p ~p is the relative momentum. − ′ The relationship between the interaction potential V and the S-matrix is S = exp( i dtV ) i dtV = i(2π)δ(0)V (6.7) − ∼− − Z Z Therefore, we find 2 3 g ~p′ ~p′ V ~p ~p = (2π) δ(~p + ~p ~p ~p )χ†′ χ χ†′ χ − (6.8) h 1 2| | 1 2i 2 1 − 2 − 1 2 2 1 1 ~q2 + m2 where the states are normalized according to ~p ~p = χ χ(2π)3δ(~p ~p ). The δ-function in the h | i † − ′ above matrix element indicates that the interaction is translational-invariant, and V is a function of the relative coordinate ~r. The above matrix element is then 2 ~q ~r 3 g e− · V (r)d ~r = − . (6.9) ~q2 + m2 Z 6.1. YUKAWA INTERACTION AND ONE-PION EXCHANGE AT LONG DISTANCE 101 Making an inverse Fourier transformation, one gets g2 e mr V (r)= − , (6.10) −4π r which is an attractive interaction, independent of the spin of the nucleon. When m = 0, the interaction is Coulomb-like and is long range. For a finite m, the interaction is negligible beyond the distance r 1/m. ∼ Pi-meson is the lightest meson because of the spontaneous chiral symmetry breaking. It is responsible to the longest range nucleon-nucleon force. One of the important new features here is isospin symmetry. Pi-meson has iso-spin 1, and can be represented by a vector ~π = (π1,π2,π3) in isospin space. Combinations of π1,2 generate positive and negative charged pions π±, and π3 corresponds to a neutral pion π0. Using ψ to represent the nucleon field, a spinor in the isospin space ψ = (p,n), a isospin symmetric Yukawa interaction is = g ψiγ ~τψ φ~ (6.11) LY πNN 5 · where ~π is the isovector pion field. We can again consider the nucleon-nucleon interaction through one-pion exchange. Because the pion is a pseudo-scalar particle, parity conservation requires that it be emitted or absorbed in the p-wave. Therefore there is a factor of ~p associated with the interaction vertex in the momentum space. This factor is multiplied by the spin operator sigma~ forms a pseudo-scalar. Going through the same derivation as the scalar case, we find the following potential, 2 mπ r g ~ ~ e− V (~r)= 2 (~τ1 ~τ2)(~σ1 )(~σ2 ) . (6.12) 4MN · · ∇ · ∇ r where the isospin factor ~τ ~τ depends on the isospin states of the two nucleons. 1 · 2 For a system of two interacting nucleons the total isospin operator is given by T~ = ~t1 + ~t2 . (6.13) If we ignore the EM interaction and the mass difference of up and down quarks, the interaction Hamiltonian conserves isospin and so commutes with all components of isospin [H,ˆ T~] = 0 . (6.14) Then since Hˆ is invariant under rotations in isospin space, it can only depend on the isospin through T~ 2, where 2 2 T~ = (~t1 + ~t2) (6.15) = ~t2 + ~t2 + 2~t ~t (6.16) 1 2 1 · 2 3 3 1 = + + ~τ ~τ (6.17) 4 4 2 1 · 2 So Hˆ or potential can be a function of the quantity ~τ ~τ . 1 · 2 The states of good total Tˆ2 are also eigenstates of Hˆ . Then we expect the following properties of a system of 2 nucleons: 102 CHAPTER 6. NUCLEON-NUCLEON INTERACTION, DEUTERON The eigenstates with T = 1 will be degenerate (just as the S = 1 states for a system of 2 spin • 1 2 objects with a rotationally symmetric Hamiltonian), i.e., the states 1 pp nn (pn + np) (6.18) √2 all have identical interactions and will be degenerate in energy. The T = 0 combination 1 (pn np) can be different (and generally is), i.e, its energy is • √2 − different from the 3 T = 1 states and the interaction differs in this channel. It is therefore advantageous to form states of good isospin to describe a system of 2 nucleons. Note the combination ~τ ~τ is given by 1 · 2 3 3, T = 0 ~τ ~τ = 4T~ T~ = 2 T (T + 1) = − . (6.19) 1 2 1 2 2 1, T = 1 · · − Going back to the one-pion exchange potential, it can be shown that e mπ r 1 3 3 e mπ r (~σ ~ )(~σ ~ ) − = m2 ~σ ~σ + Sˆ 1+ + − . (6.20) 1 2 m r 3 π 1 2 12 m r m2 r2 m r · ∇ · ∇ π · π π π where S12 is called tensor operator, defined as ~ 2 (S ~r) 2 S12 = 2 3 · S~ (6.21) " r2 − # where S~ is the total spin. Putting together all the factors, one finds that the nucleon-nucleon potential from one pion exchange as, mπ r 2 e− V = V0(~τ1 ~τ2) ~σ1 ~σ2 + S12 1+ ~3mπr + ~3(mπr) (6.22) · · mπr h i 2 2 2 where V0 = g mπ/12M . This potential matches the phenomenological forms extracted from experimental data at large N-N separations ( 2 3 fm). ∼ − At smaller distance, there is also exchanges from scalar meson (isospin 0) of about 500 MeV. The interaction is attractive as we seen above, corresponding to a medium range attraction. Finally there are also exchanges from vector mesons, ω (isospin-0) meson and ρ meson (spin-1).
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