Nucleons in Nuclei: Interactions, Geometry, Symmetries

Transcription

1 Nucleons in Nuclei: Interactions, Geometry, Symmetries Jerzy DUDEK Department of Subatomic Research, CNRS/IN 2 P 3 and University of Strasbourg, F Strasbourg, FRANCE September 28, 2010

2 Mathematial Structure of the Effective Hamiltonian Part I Nuclear Pairing: Exact Symmetries, Exact Solutions, Pairing as a Stochastic Process

3 Mathematial Structure of the Effective Hamiltonian Mathematics of the Effective Hamiltonian The Global Structure of the N-Body Effective Hamiltonians The unknown true Hamiltonian is replaced by two effective ones

4 Mathematial Structure of the Effective Hamiltonian Mathematics of the Effective Hamiltonian The Global Structure of the N-Body Effective Hamiltonians The unknown true Hamiltonian is replaced by two effective ones Ĥ = αβ N N h αβ ĉ + αĉβ + 1 v 2 αβ;γδ ĉ + α ĉ+ β αβ=1 γδ=1 ĉδ ĉγ

5 Mathematial Structure of the Effective Hamiltonian Mathematics of the Effective Hamiltonian The Global Structure of the N-Body Effective Hamiltonians The unknown true Hamiltonian is replaced by two effective ones Ĥ = αβ N N h αβ ĉ + αĉβ + 1 v 2 αβ;γδ ĉ + α ĉ+ β αβ=1 γδ=1 ĉδ ĉγ In low-energy sub-atomic physics the theory calculations without considering the residual pairing are considered not realistic Pairing: v pairing αβ;γδ to be defined

6 Mathematial Structure of the Effective Hamiltonian Comment about Irreducible Representations Gelfand and Zetlin (1950) also obtain the matrix elements of the generators ˆN αβ within their space of U(n) irreducible representations

7 Mathematial Structure of the Effective Hamiltonian Comment about Irreducible Representations Gelfand and Zetlin (1950) also obtain the matrix elements of the generators ˆN αβ within their space of U(n) irreducible representations Thus for known physical matrices h αβ and v αβ;γδ the Hamiltonian below can be seen as a known matrix h αβ ˆN αβ v αβ;γδ ˆN αγ ˆN βδ Ĥ = αβ αβ γδ

8 Mathematial Structure of the Effective Hamiltonian Comment about Irreducible Representations Gelfand and Zetlin (1950) also obtain the matrix elements of the generators ˆN αβ within their space of U(n) irreducible representations Thus for known physical matrices h αβ and v αβ;γδ the Hamiltonian below can be seen as a known matrix h αβ ˆN αβ v αβ;γδ ˆN αγ ˆN βδ Ĥ = αβ Moreover, under the condition: P j n j = p, for n j = 0 or 1 each state can be seen as an integer corresponding to its binary representation E = nx b k 2 k k=1 αβ γδ n 1 n 2 n 3 n 4 n n n n 1=9 2=10 3=11 4=12 n n 5 n 6 n n 7 n n 8 n 5=13 6=14 7=15 8=16

9 Mathematial Structure of the Effective Hamiltonian N-Body Hamiltonians and U n -Group Generators N-Body Hamiltonians are functions of U n -group generators Ĥ = αβ h αβ ˆN αβ v αβ;γδ ˆN αγ ˆN βδ αβ γδ Two-body interactions lead to quadratic forms of ˆN αβ = c + α c β, three-body interactions to the cubic forms of ˆN αβ, etc. Hamiltonians of the N-body systems can be diagonalised within bases of the irreducible representations of unitary groups Solutions can be constructed that transform as the U n -group representations thus establishing a link H U n -formalism

10 Mathematial Structure of the Effective Hamiltonian N-Body Hamiltonians and U n -Group Generators N-Body Hamiltonians are functions of U n -group generators Ĥ = αβ h αβ ˆN αβ v αβ;γδ ˆN αγ ˆN βδ αβ γδ Two-body interactions lead to quadratic forms of ˆN αβ = c + α c β, three-body interactions to the cubic forms of ˆN αβ, etc. Hamiltonians of the N-body systems can be diagonalised within bases of the irreducible representations of unitary groups Solutions can be constructed that transform as the U n -group representations thus establishing a link H U n -formalism

11 Mathematial Structure of the Effective Hamiltonian N-Body Hamiltonians and U n -Group Generators N-Body Hamiltonians are functions of U n -group generators Ĥ = αβ h αβ ˆN αβ v αβ;γδ ˆN αγ ˆN βδ αβ γδ Two-body interactions lead to quadratic forms of ˆN αβ = c + α c β, three-body interactions to the cubic forms of ˆN αβ, etc. Hamiltonians of the N-body systems can be diagonalised within bases of the irreducible representations of unitary groups Solutions can be constructed that transform as the U n -group representations thus establishing a link H U n -formalism

12 Mathematial Structure of the Effective Hamiltonian N-Body Hamiltonians and U n -Group Generators N-Body Hamiltonians are functions of U n -group generators Ĥ = αβ h αβ ˆN αβ v αβ;γδ ˆN αγ ˆN βδ αβ γδ Two-body interactions lead to quadratic forms of ˆN αβ = c + α c β, three-body interactions to the cubic forms of ˆN αβ, etc. Hamiltonians of the N-body systems can be diagonalised within bases of the irreducible representations of unitary groups Solutions can be constructed that transform as the U n -group representations thus establishing a link H U n -formalism

13 From Many-Body - to Pairing Many-Body Problem Part II Physics of Nuclear Pairing and Nuclear Superfluidity

14 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? First Steps: Pairing on Top of the Mean Field From Many-Body - to Pairing Many-Body Problem The first step: to solve the nuclear (HF) mean-field problem Nucleons move in a deformed one-body potential representing an everage interaction among them The one-body potentials are either parametrised or calculated using Hartree-Fock method and the single nucleon levels obtained {e α : α = 1,..., n} V(3D Space) 3D Space NUCLEUS NUCLEONS

15 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Time-Independent Hamiltonians: Kramers Degeneracy We explicitly introduce the time-reversal degeneracy ˆT Ĥ ˆT 1 = Ĥ e α = eᾱ ᾱ ˆT ᾱ

16 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Time-Independent Hamiltonians: Kramers Degeneracy We explicitly introduce the time-reversal degeneracy ˆT Ĥ ˆT 1 = Ĥ e α = eᾱ ᾱ ˆT ᾱ Time-up states denoted by α α { α } Time-reversed states by { ᾱ }

17 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Pairing Hamiltonian: Its Experimental Background All the experiments show that, with no exception, all the even-even nuclei have spin zero in their ground states α > α _ > This implies the existence of the universal short range interaction that couples the time-reversed orbitals Pairing Scheme

18 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Pairing Hamiltonian: Its Experimental Background All the experiments show that, with no exception, all the even-even nuclei have spin zero in their ground states α > α _ > This implies the existence of the universal short range interaction that couples the time-reversed orbitals Pairing Scheme Implied Many-Body Hamiltonian Ĥ = α Generalized Pairing {}}{ e α (c + α c α + c+ ᾱ c ᾱ ) v 2 αᾱ;β β cα }{{} c+ ᾱ c β c β αβ G αβ ᾱ ˆT α

19 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Realistic Nucleonic Orbitals in the Mean-Field: A Few Examples of the Spatial Structure

20 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit??% Limit??% Limit??% Limit??% Density distribution ψ π ( r ) 2 Limit, for π = [2, 0, 2]1/2 orbital

21 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit 50% Limit??% Limit??% Limit??% Density distribution ψ π ( r ) 2 Limit, for π = [2, 0, 2]1/2 orbital

22 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit 50% Limit 10% Limit??% Limit??% Density distribution ψ π ( r ) 2 Limit, for π = [2, 0, 2]1/2 orbital

23 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit 50% Limit 10% Limit 3% Limit??% Density distribution ψ π ( r ) 2 Limit, for π = [2, 0, 2]1/2 orbital

24 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit 50% Limit 10% Limit 3% Limit 1% Density distribution ψ π ( r ) 2 Limit, for π = [2, 0, 2]1/2 orbital

25 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit 50% Limit 10% Limit 3% Limit 1% Limit 20% Limit??% Limit??% Limit??% Limit??% Bottom: N=3 shell b-[303]7/2, w-[312]5/2, y-[321]3/2, p-[310]1/2

26 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit 50% Limit 10% Limit 3% Limit 1% Limit 20% Limit 15% Limit??% Limit??% Limit??% Bottom: N=3 shell b-[303]7/2, w-[312]5/2, y-[321]3/2, p-[310]1/2

27 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit 50% Limit 10% Limit 3% Limit 1% Limit 20% Limit 15% Limit 12% Limit??% Limit??% Bottom: N=3 shell b-[303]7/2, w-[312]5/2, y-[321]3/2, p-[310]1/2

28 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit 50% Limit 10% Limit 3% Limit 1% Limit 20% Limit 15% Limit 12% Limit 10% Limit??% Bottom: N=3 shell b-[303]7/2, w-[312]5/2, y-[321]3/2, p-[310]1/2

29 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of Orbitals (Spherical 132 Sn) ( ψ( r ) 2 ) Limit 80% Limit 50% Limit 10% Limit 3% Limit 1% Limit 20% Limit 15% Limit 12% Limit 10% Limit 9% Bottom: N=3 shell b-[303]7/2, w-[312]5/2, y-[321]3/2, p-[310]1/2

30 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of N=3 Spherical Shell ( ψ ν ( r ) 2 ) 132 Sn: Distributions ψ ν ( r ) 2 for single proton orbitals. Top O xz, bottom O yz. Proton e ν [ν=30, 32,... 38] for spherical shell

31 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of N=3 Spherical Shell ( ψ ν ( r ) 2 ) 132 Sn: Distributions ψ ν ( r ) 2 for single proton orbitals. Top O xz, bottom O yz. Proton e ν [ν=40, 42,... 48] for spherical shell

32 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of N=3 Spherical Shell ( ψ ν ( r ) 2 ) 132 Sn: distributions ψ ν ( r ) 2 for consecutive pairs of orbitals. Top O xz, bottom O yz. Proton e ν [n=30:32,... 38:40], spherical shell

33 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Spatial Structure of N=3 Spherical Shell ( ψ ν ( r ) 2 ) 132 Sn: distributions ψ ν ( r ) 2 for consecutive pairs of orbitals. Top O xz, bottom O yz. Proton e ν [n=40:42,... 48:50], spherical shell

34 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Dichotomic Symmetries of Pairing

35 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Natural Dichotomic Symmetries: Time Reversal... There exist one-body dichotomic symmetries Ŝ 1 ˆT, ˆR x, Ŝ x,... where the subscript 1 refers to the one-body interaction Ĥ 1 = αβ α ĥ 1 β c + α c β and [Ŝ 1, ĥ 1 ] = 0

36 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Natural Dichotomic Symmetries: Time Reversal... There exist one-body dichotomic symmetries Ŝ 1 ˆT, ˆR x, Ŝ x,... where the subscript 1 refers to the one-body interaction Ĥ 1 = αβ α ĥ 1 β c + α c β and [Ŝ 1, ĥ 1 ] = 0 For Fermions Ŝ 2 1 = 1 s α = ±i

37 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Natural Dichotomic Symmetries: Time Reversal... There exist one-body dichotomic symmetries Ŝ 1 ˆT, ˆR x, Ŝ x,... where the subscript 1 refers to the one-body interaction Ĥ 1 = αβ α ĥ 1 β c + α c β and [Ŝ 1, ĥ 1 ] = 0 For Fermions Ŝ 2 1 = 1 s α = ±i This allows to introduce the basis { α, s α } (and the labelling):

38 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Natural Dichotomic Symmetries: Time Reversal... There exist one-body dichotomic symmetries Ŝ 1 ˆT, ˆR x, Ŝ x,... where the subscript 1 refers to the one-body interaction Ĥ 1 = αβ α ĥ 1 β c + α c β and [Ŝ 1, ĥ 1 ] = 0 For Fermions Ŝ 2 1 = 1 s α = ±i This allows to introduce the basis { α, s α } (and the labelling): ĥ 1 α, s α = e α, sα α, s α, Ŝ 1 α, s α = s α α, s α

39 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Exploiting the Natural Dichotomic Symmetries From Many-Body - to Pairing Many-Body Problem Therefore, there are 16 types of the two-body matrix elements, distinguished by the eigenvalues s α = ±i Ĥ = X ε α (c α+ + c α+ + c α + c X X α ) + 1 α±, β± ĥ2 γ±, δ± c 2 α± + {z } c+ β± c δ± c γ± α αβ γδ 16 families Since the residual two-body interactions are often assumed scalar, it follows that for the two-body operator Ŝ 2, the analogue of Ŝ 1 Ŝ 2 Ŝ 1 Ŝ 1 [ĥ 2, Ŝ 2 ] = 0 This implies that half of the matrix elements above simply vanish α±, β± ĥ 2 γ±, δ± δ sα s β, s γ s δ

40 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Exploiting Dichotomic Symmetries and Pairing From Many-Body - to Pairing Many-Body Problem Furthermore, because of the specific form of the nuclear pairing Hamiltonian half of the above 8 types of matrix elements are absent α+, β + ĥ2 γ, δ = 0 α, β ĥ 2 γ+, δ+ = 0 α+, β + ĥ 2 γ+, δ+ = 0 α, β ĥ 2 γ, δ = 0 α +> α > λ

41 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Exploiting Dichotomic Symmetries and Pairing From Many-Body - to Pairing Many-Body Problem Examples of the vanishing matrix elements α+, β + ĥ2 γ, δ = 0 α, β ĥ 2 γ+, δ+ = 0 α+, β + ĥ 2 γ+, δ+ = 0 α, β ĥ 2 γ, δ = 0 α +> α > λ

42 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Exploiting Dichotomic Symmetries and Pairing From Many-Body - to Pairing Many-Body Problem Examples of the vanishing matrix elements α+, β + ĥ2 γ, δ = 0 α, β ĥ 2 γ+, δ+ = 0 α+, β + ĥ 2 γ+, δ+ = 0 α, β ĥ 2 γ, δ = 0 α +> α > λ

43 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Exploiting Dichotomic Symmetries and Pairing From Many-Body - to Pairing Many-Body Problem Examples of the vanishing matrix elements α+, β + ĥ2 γ, δ = 0 α, β ĥ 2 γ+, δ+ = 0 α+, β + ĥ 2 γ+, δ+ = 0 α, β ĥ 2 γ, δ = 0 α +> α > λ

44 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Exploiting Dichotomic Symmetries and Pairing From Many-Body - to Pairing Many-Body Problem Examples of the vanishing matrix elements α+, β + ĥ2 γ, δ = 0 α, β ĥ 2 γ+, δ+ = 0 α+, β + ĥ 2 γ+, δ+ = 0 α, β ĥ 2 γ, δ = 0 α +> α > λ

45 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Final Structure of the Nuclear Pairing Hamiltonian Then the non-vanishing terms can be divided into four families Ĥ 2 = 1 2 α+β γ δ+ α+, β ĥ 2 γ, δ+ c α+c + + β c δ+ c γ α+β γ+δ α+, β ĥ2 γ+, δ c α+c + + β c δ c γ α β+ α β+ γ δ+ α, β + ĥ2 γ, δ+ c α c + + β+ c δ+ c γ γ+δ α, β + ĥ2 γ+, δ c α c β+ c δ c γ+ It turns out that the full Hamiltonian Ĥ α e α (ĉ + α ĉ α + ĉ + ᾱ ĉᾱ) + Ĥ 2 cannot connect the states that differ in terms of occupation of the + and - family states

46 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? We have just obtained the modern version of the Nuclear Pairing Hamiltonian

47 From Many-Body - to Pairing Many-Body Problem Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? We have just obtained the modern version of the Nuclear Pairing Hamiltonian In what sense are the paired-nuclei super-fluid?

48 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Collective Rotation, Moments of Inertia From Many-Body - to Pairing Many-Body Problem The first rotational transition energies are very low; for very heavy nuclei such energies e R 10 2 MeV. This energy is contributed by all the nucleons; a contribution per nucleon, is δe R e R /A 10 2 MeV/A 10 4 MeV Static Rotating

49 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Collective Rotation, Moments of Inertia From Many-Body - to Pairing Many-Body Problem These energies should be compared to the average kinetic energies of nucleons in the mean-field potential of the typical depth of V 0 60 MeV A nucleon of, say, e α 25 MeV, has the kinetic energy of the order of ˆt t α 35 MeV so that V 0 + ˆt e ν 25 MeV Rotational Spectrum Eγ ~R Potential Energy Distance r 60 MeV

50 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Collective Rotation, Moments of Inertia From Many-Body - to Pairing Many-Body Problem Consider explicitly a one-dimensional rotation about O x -axis. One may show that the perturbation is δv = ω x ĵ x Consequently the second order energy contribution is E (2) 0 = ( ω x ) 2 mi Comparison gives (m ĵ x i) 2 e (0) i e m (0) compared to E (2) 0 = 1 2 J x ωx 2 J x = 2 2 mi (m ĵ x i) 2 e (0) i e m (0) Jx rig. = [y 2 + z 2 ]ρ( r )d 3 r Jx exp. V

51 Pairing Hamiltonian from the Experimental Evidence Spherical Mean-Field: Illustrations Why Nuclear Superfluidity? Collective Rotation, Moments of Inertia From Many-Body - to Pairing Many-Body Problem Repeating the 2 nd -order perturbation calculation with pairing we obtain J pair x = 2 2 µν µ ĵ x ν 2 (u µv ν u ν v µ ) Jx rig. Jx exp. E µ + E ν By definition, within the nuclear Bardeen-Cooper-Schrieffer approach E µ = (e µ λ) 2 + 2, vµ 2 ] = 2[ 1 1 (eµ λ)/e µ and v 2 µ + uµ 2 = 1 As the pairing gap we find f µν (u µv ν u ν v µ ) 2 E µ + E ν 0 J pair x 0 When this happens we say that system approaches the super-fluid regime

52 Exact Solution of Pairing Many-Body Problem Part III A Lesson on the Exact Solutions of the Realistic Pairing Problem

53 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries Pairing, Fock-Space and Associated Notation Nuclear wave functions must be totally anti-symmetrised We formulate the problem of the motion in the Fock space We use the many-body occupation-number representation Ψ mb = (c + α 1 ) pα 1 (c + α2 ) pα 2... (c + αn ) pαn 0 > p α1, p α2,... p αn p α = 0 or 1, n j=1 p αj = p n > p Computer algorithm is constructed using bit-manipulations

54 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries Particular Symmetries of the Pairing Hamiltonian Ĥ does not couple states differing in particle-hole structure Ĥ does not couple states differing by 2 or more excited pairs Ĥ = e α c + α c α + G α,β c + β c+ β c ᾱ c α α α,β>0 <J = < configuration 1 configuration 2> = K> λ λ J Ĥ K = 0

55 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries Pairing Hamiltonian and the U(n)-Generators It follows that upon identifying ˆn αβ ĉ α + ĉ β ĝ αβ n n Ĥ = e α (ĝ α,α + ĝᾱ,ᾱ ) 1 2 G α,β ĝ β,ᾱ ĝ β,α α>0 α,β>0 Introduce linear Casimir operator Ĉ Particle No. Operator ˆN = n α ˆn αα U(n) Casimir Operator Ĉ = n α ĝαα n N + ĝ αα = ĝ α+,α+ + α α+ N α ĝ α,α ˆN ˆN 1

56 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries New Particle-Like Operators: ˆN + 1 and ˆN 1 One verifies that operators ˆN + 1 and ˆN 1 are linearly independent [Ĥ, ˆN + 1 ] = 0, [Ĥ, ˆN 1 ] = 0, [ ˆN + 1, ˆN 1 ] = 0 Introduce two linear combinations ˆN 1 ˆN ˆN 1 and ˆP 1 ˆN + 1 ˆN 1 We show straightforwardly that [Ĥ, ˆN 1 ] = 0, [Ĥ, ˆP 1 ] = 0 The Hamiltonian Ĥ is said to be ˆP 1 -symmetric

57 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries New Particle-Like Operators: ˆN + 1 and ˆN 1 Recall: Operator ˆP 1 N ˆ 1 + N ˆ 1 gives the difference between the occupation of states s α = +i and s α = i It follows that the possible eigenvalues of P 1 are P 1 = p, p 2, p 4,..., p for a system of p particles on n levels with p n/2, and P 1 = (n p), (n p 2), (n p 4),..., (n p) for a system for which n/2 p n Hamiltonian matrix splits into blocks with eigenvalues P 1 ; one shows that dim(p 1 ) = C n p+p 1 C n p P 1 2 2

58 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries Illustration of the Effect of the P 1 -Symmetry Example of Fock-space dimensions for p = 16 particles on n = 32 levels; the dimension of the full space is C16 32 = P 1 -value Dimension ± ± ± ± ± ± ± ± 16 1

59 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries New Particle-Pair-Like Operators: ˆN + 2 and ˆN 2 Our Hamiltonian does not couple states that differ in terms of the numbers of pairs; the number of broken pairs (seniority) is conserved In analogy with the previous case we define two-body operators ˆN + 2 N N c α + i c ᾱ + cᾱi i c αi and ˆN 2 (1 c α + i c ᾱ + cᾱi i c αi ) i=1 i=1 Following the same analogy we also define the linear combinations ˆN 2 = ˆN ˆN 2 and ˆP 2 = ˆN + 2 ˆN 2

60 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries New Particle-Pair-Like Operators: ˆN + 2 and ˆN 2 One can verify straightforwardly that [Ĥ, ˆN + 2 ] = 0 and [Ĥ, ˆN 2 ] = 0 and [ ˆN + 2, ˆN 2 ] = 0 It then follows immediately that [Ĥ, ˆN 2 ] = 0 and [Ĥ, ˆP 2 ] = 0 while [ ˆP 1, ˆP 2 ] = 0 The Hamiltonian Ĥ is said to be ˆP 2 -symmetric

61 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries New Particle-Pair-Like Operators: ˆN + 2 and ˆN 2 By counting numbers of pairs we obtain eigen-values of ˆP 2 -operator For p particles on n levels, and p n/2: P 2 = p n, p 2 n,..., n For p particles on n levels, and n/2 p n: P 2 = p n, p 2 n,..., 2(p n) n The dimensions of a given block characterized by the quantum numbers P 1 and P 2 are given by: dim(p 2, P 1 ) = C n p n P 2 +P 1 2 C n p n P 2 +P 1 2 p n P 2 P 1 2 C 2n p+p 2 n+p 2 2

62 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries New Particle-Pair-Like Operators: ˆN + 2 and ˆN 2 The Hamiltonian blocks for p = 16 particles on n = 32 levels; the dimension of the full space is C16 32 = Seniority P 2 Dimension P 1 -values Dimension ± ± ± ± ± ± ± ± ± ±

63 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries Yet Another Symmetry: P 12 -Symmetry Define µ i 2i 2 associated with doubly-degenerate levels ε i Define the weight factors: α i 2 µ i and ᾱ i 2 µ i +1 Define operators ˆN + 12 n (2 µ i c α + i c αi + 2 µ i +1 c ᾱ + cᾱi i ) i=1 ˆN 12 n (2 µ i + 2 µ i +1 )c α + i c ᾱ + cᾱi i c αi i=1 ˆP 12 ˆN + 12 ˆN 12

64 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries Yet Another Symmetry: P 12 -Symmetry One can show that [ ˆP 1, ˆP 2 ] = 0 and [ ˆP 1, ˆP 12 ] = 0 and [ ˆP 2, ˆP 12 ] = 0... and that for our general pairing Hamiltonian Ĥ we have [Ĥ, ˆP 1 ] = 0 and [Ĥ, ˆP 2 ] = 0 and [Ĥ, ˆP 12 ] = 0 The Hamiltonian Ĥ is said to be ˆP 12 -symmetric

65 Exact Solution of Pairing Many-Body Problem Introductory Explorations P 1 -, P 2 -, P 12 -Symmetries How Powerful This Approach Is Shows an Example: The property just observed allows for significant simplifications Example: 16 particles on 32 levels Total dimension of H Seniority P 2 Total Dimension Nb. of sub-blocs Sub-bloc dimension Details in: H. Molique and J. Dudek, Phys. Rev. C56, 1795 (1997)

66 About the Stochastic Approach Part IV Cooper-Pairs as Brownian Particles

67 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results From Quantum Mechanics to Stochastic Processes Consider a system composed of p-particles on n nucleonic levels The implied Fock space contains N = C n p many-body states { Φ K >; K = 1, 2,... N } The symbols represent N physical configurations {C K } of the type {C K } { > K ; K = 1, 2,... N } The use of the P-symmetries allows to diagonalize exactly and easily, with the help of the Lanczos method, the Hamiltonian matrices < Φ K Ĥ Φ M > of dimensions N b 10 9 to 10 (12 15)

68 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results An alternative, stochastic method is free from the disc-space limitations

69 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results An alternative, stochastic method is free from the disc-space limitations This Stochastic Method is based on fundamentally different concepts

70 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Nuclear Pairing as a Stochastic Process Starting from now on we assume that the system evolves under the influence of Hamiltonian Ĥ in terms of the single-pair transitions K K We suggest that there exist a universal probability distribution depending on the transition energy only P K K = P( E K,K ); E K,K = E K E K In other words: we assume that single-pair transition probabilities are neither dependent on the particular configuration nor on the history of the process L L

71 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Nuclear Pairing as a Stochastic Process The just formulated assumptions reduce the evolution of such a system to that of the Markov process Fock space etc. ect. / ETC* Consequently we are going to consider the underlying physical process in terms of the random walk through the Fock space

72 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Nuclear Pairing as a Stochastic Process An example of Fock space corresponding to 4 particles on 8 levels { Φ K } = { 1100, 1010, 1001, 0110, 0101, 0011 } We have the following possible transitions: To simplify the illustration we use the compact notation: 1 one pair present; 0 one pair absent

73 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Fock-State Occupation Probabilities Suppose Hamiltonian Ĥ has been diagonalised in the Fock space Ĥ Ψ K = E K Ψ K Ψ K = C K,L Φ L N b L=1 The quantum probability of finding Ψ K in one of its Fock-basis states Φ L is P q L = C K,L 2 (for a given Ψ K ) The stochastic probability of finding Ψ K in one of its Fock-basis states Φ L is P s L = N L/N total (for a given Ψ K ) where N L the number of occurrences of Φ K along the random walk and N total = the total length of the random walk

74 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Example: Exact Quantum Occupation Probabilities p = 8 particles on n = 16 levels {N b = 70 Fock Φ K states} on an equidistant model spectrum: the ground-state wave-function MODULES DES COEFFICIENTS CONFIGURATIONS

75 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic vs. Quantum Occupation Probabilities 8 particles on 16 levels (N it = iterations) MODULES OF THE COEFFICIENTS RANDOM WALK EXACT INDIVIDUAL SPECTRUM FOCK BASIS STATES

76 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic vs. Quantum Occupation Probabilities 8 particles on 16 levels (N it = iterations) - Case 2 MODULES OF THE COEFFICIENTS RANDOM WALK EXACT INDIVIDUAL SPECTRUM CONFIGURATIONS

77 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic vs. Quantum Occupation Probabilities 8 particles on 16 levels (N it = iterations) - Case 3 MODULES OF THE COEFFICIENTS RANDOM WALK EXACT INDIVIDUAL SPECTRUM CONFIGURATIONS

78 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic vs. Quantum Occupation Probabilities 8 particles on 16 levels (N it = iterations) - Case 4 MODULES OF THE COEFFICIENTS RANDOM WALK EXACT INDIVIDUAL SPECTRUM CONFIGURATIONS

79 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic vs. Quantum Occupation Probabilities 12 particles on 24 levels (N it = iterations) Fock space dimension N (24/12) = MODULES OF THE COEFFICIENTS RANDOM WALK EXACT INDIVIDUAL SPECTRUM FOCK BASIS STATES

80 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic vs. Quantum Occupation Probabilities Zooming in the previous spectrum for p = 12 and n = 24 MODULES OF THE C COEFFICIENTS RANDOM WALK EXACT FOCK BASIS STATES

81 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic vs. Quantum Occupation Probabilities 16 particles on 32 levels (N it = iterations) Fock space dimension N (32/16) = MODULES OF THE COEFFICIENTS RANDOM WALK EXACT INDIVIDUAL SPECTRUM FOCK BASIS STATES

82 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic vs. Quantum Occupation Probabilities 16 particles on 32 levels; ground-state wave-function L = KHI e+05 4e+05 6e+05 NUMBER OF ITERATIONS [ χ = 1 N 1 N K=1 ( C q 1,K C s 1,K )2 ] 1/2

83 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic Approach: Problem with Excited States? So far we have considered the ground-state wave functions

84 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic Approach: Problem with Excited States? So far we have considered the ground-state wave functions All C L,K coefficients of the ground-state wave functions (L = 1) are known to be of the same sign

85 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic Approach: Problem with Excited States? So far we have considered the ground-state wave functions All C L,K coefficients of the ground-state wave functions (L = 1) are known to be of the same sign The stochastic approach may only give the probabilities: P C 2 C so there was no problem to obtain the wave-function out of C 1,K 2

86 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Stochastic Approach: Problem with Excited States? So far we have considered the ground-state wave functions All C L,K coefficients of the ground-state wave functions (L = 1) are known to be of the same sign The stochastic approach may only give the probabilities: P C 2 C so there was no problem to obtain the wave-function out of C 1,K 2 We arrive at the problem: The wave-function of the excited states cannot be obtained in the same way...

87 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Extending the Random Walk: Excited States We consider again the full ensemble of the Fock-basis vectors { φ K ; K = 1, 2, 3,... N b }

88 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Extending the Random Walk: Excited States We consider again the full ensemble of the Fock-basis vectors { φ K ; K = 1, 2, 3,... N b } We begin the random walk starting with Φ 1 >; calculations show that in this way we obtain always the ground-state configuration

89 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Extending the Random Walk: Excited States We consider again the full ensemble of the Fock-basis vectors { φ K ; K = 1, 2, 3,... N b } We begin the random walk starting with Φ 1 >; calculations show that in this way we obtain always the ground-state configuration Next we construct the whole series of the random walk processes by beginning with Φ 2 >, Φ 3 >,...

90 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Extending the Random Walk: Excited States We consider again the full ensemble of the Fock-basis vectors { φ K ; K = 1, 2, 3,... N b } We begin the random walk starting with Φ 1 >; calculations show that in this way we obtain always the ground-state configuration Next we construct the whole series of the random walk processes by beginning with Φ 2 >, Φ 3 >, but now: how should we compare the stochastic results with the quantum case?

91 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Extending the Random Walk: Excited States We consider again the full ensemble of the Fock-basis vectors { φ K ; K = 1, 2, 3,... N b } We begin the random walk starting with Φ 1 >; calculations show that in this way we obtain always the ground-state configuration Next we construct the whole series of the random walk processes by beginning with Φ 2 >, Φ 3 >, but now: how should we compare the stochastic results with the quantum case? The random walk algorithm provides neither the signs of the C coefficients - nor the energies...

92 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Extending the Random Walk: Excited States (II) Consider a set of linearly independent vectors { Ψ L }. We will orthonormalise them, beginning with Ψ 1 > as follows: We normalise Ψ 1 >: Ψ 1 Θ 1 = 1 Ψ 1 Ψ 1 We subtract the parallel part of Ψ 2 > from Θ 1 > Ψ 2 Ψ 2 = Ψ 2 (< Θ 1 Ψ 2 >) Θ 1 We normalise this last vector: Ψ 2 Θ 2 = 1 Ψ 2 Ψ 2 We subtract the parallel part of Ψ 2 > from Θ 1 > and Θ 2 > Ψ 3 Ψ 3 Θ 1 Ψ 3 > Θ 1 Θ 2 Ψ 3 > Θ 2 >

93 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Orthonormalisation Scheme - Illustration 8 particles on 16 levels - 1 rst excited state 0.5 RANDOM WALK + ORTHONORMALISATION EXACT C COEFFICIENTS FOCK BASIS STATES... and apparently we are able to obtain the wave function of an excited state. However:

94 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Orthonormalisation Scheme - Illustration 8 particles on 16 levels - 2 nd excited state 1 RANDOM WALK + ORTHONORMALISATION EXACT C COEFFICIENTS FOCK BASIS STATES... and apparently the scheme does not seem to perform well for yet another excited state... Is it a real problem?

95 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Overlaps: Stochastic vs. Exact 1 < Ψ i Ψ 2 > exact stoch exact Ψ > i

96 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Overlaps: Stochastic vs. Exact 1 < Ψ i Ψ 2 > exact stoch exact Ψ > i

97 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Overlaps: Stochastic vs. Exact 1 < Ψ i Ψ 3 > exact stoch exact Ψ > i

98 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Overlaps: Stochastic vs. Exact 1 < Ψ Ψ > exact stoch i exact Ψ > i

99 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Overlaps: Stochastic vs. Exact 1 < Ψ i Ψ 5 > exact stoch exact Ψ > i

100 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Overlaps: Stochastic vs. Exact 1 exact stoch < Ψ i Ψ 6 > exact Ψ > i

101 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Overlaps: Stochastic vs. Exact 1 < Ψ i Ψ 7 > exact stoch exact Ψ > i

102 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Overlaps: Stochastic vs. Exact 1 < Ψ i Ψ 8 > exact stoch exact Ψ > i

103 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Overlaps: Stochastic vs. Exact 1 < Ψ i Ψ 9 > exact stoch exact Ψ > i

104 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Observations, Interpretation, Partial Conclusions We just have observed that the quantum and stochastic Fock-basis vectors are similar - but not identical More precisely: some stochastic vectors have more than 99% of overlap with one of their quantum partners some others have a strong overlap with 2 quantum partners, and several tiny overlaps with the others Observation: the stochastic basis vectors seem to be often nearly parallel to their quantum partners; sometimes they rather lie in a two-dimensional hyperplane Clearly the stochastic and quantum Fock bases are not identical; Are they equivalent i.e. differing by an orthogonal transformation?

105 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Certain Property of Eigenvectors Let us consider again a Fock basis { Φ K ; K = 1... N b } Eigenvalues and eigenvectors of Ĥ 1 in the Fock space obey: Ĥ 1 Φ N = E N Φ N with E N = e α α {Conf.} N Eigenvectors Ψ J satisfy: Ψ J = N b K=1 C JK Φ K Eigenvalues of Ĥ can be calculated knowing the {E L } energies: N b E J = CJL 2 E L + L=1 N b L,M=1 C JL C JM Φ L Ĥ2 Φ M Knowing coefficients C JL from the stochastic simulation, we orthonormalise the vectors verify whether they give eigenenergies!

106 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results The Eigenvalues of Ĥ and Stochastic Features Denoting by n the number of nucleons, we have 1 2 n G if M = L, G if M L, but Φ Φ L Ĥ(2) Φ M = M and Φ L differ by one exited pair, 0 otherwise We express unknown eigenenergies by stochastic coefficients E J = L [ C 2 JL (E L 1 2 n G ) C JL G ] C J,L+δL ; the symbol {L + δl} refers to configurations that differ from those denoted {L} by one excited pair δl

107 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results The Eigenvalues of Ĥ and Stochastic Features p=8 particles on n=16 levels - Error Fock space N = EXACT [MeV] RANDOM WALK [MeV] RELATIVE ERROR % % % % % % % % % % % % % %

108 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results The Eigenvalues of Ĥ and Stochastic Features 12 particles on 24 levels - Error Fock space N = EXACT [MeV] RANDOM WALK [MeV] RELATIVE ERROR % % % % % % % % % % % % %

109 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results The Eigenvalues of Ĥ and Stochastic Features 8 particles on 16 levels - the first 11 levels HAMILTONIAN SPECTRUM (MeV) RANDOM WALK EXACT

110 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results The Eigenvalues of Ĥ and Stochastic Features 8 particles on 16 levels - the first 33 levels HAMILTONIAN SPECTRUM (MeV) RANDOM WALK EXACT

111 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results The Eigenvalues of Ĥ and Stochastic Features 8 particles on 16 levels - All levels RANDOM WALK EXACT HAMILTONIAN SPECTRUM (MeV)

112 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results The Eigenvalues of Ĥ and Stochastic Features 12 particles on 24 levels - the first 25 levels HAMILTONIAN SPECTRUM (MeV) RANDOM WALK EXACT

113 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Question of the Universal Probability Distribution The results presented above were obtained by using, as a working hypothesis, the following form of the parametrisation of the transition probability: where P α β = K α a ( E αβ ) 2 + b E αβ + c E αβ = E α E β and where K α is a normalisation constant; a, b and c are adjustable parameters.

114 About the Stochastic Approach Summary Formulating the Concepts Testing the Method against Exact Results We discussed the problem of the nuclear pairing Hamiltonian written down in the Fock space representation (for N spaces)

115 About the Stochastic Approach Summary Formulating the Concepts Testing the Method against Exact Results We discussed the problem of the nuclear pairing Hamiltonian written down in the Fock space representation (for N spaces) We obtained the exact results using the so-called P 1, P 2 and P 12 symmetries and the Lanczos diagonalisation technique

116 About the Stochastic Approach Summary Formulating the Concepts Testing the Method against Exact Results We discussed the problem of the nuclear pairing Hamiltonian written down in the Fock space representation (for N spaces) We obtained the exact results using the so-called P 1, P 2 and P 12 symmetries and the Lanczos diagonalisation technique We have constructed the solutions to the Schrödinger equation by using the totally independent random walk (Markov chain) concepts

117 About the Stochastic Approach Summary Formulating the Concepts Testing the Method against Exact Results We discussed the problem of the nuclear pairing Hamiltonian written down in the Fock space representation (for N spaces) We obtained the exact results using the so-called P 1, P 2 and P 12 symmetries and the Lanczos diagonalisation technique We have constructed the solutions to the Schrödinger equation by using the totally independent random walk (Markov chain) concepts The eigen-energies constructed using the random walk simulations agree within a few permille level with the exact ones

118 About the Stochastic Approach Summary Formulating the Concepts Testing the Method against Exact Results We discussed the problem of the nuclear pairing Hamiltonian written down in the Fock space representation (for N spaces) We obtained the exact results using the so-called P 1, P 2 and P 12 symmetries and the Lanczos diagonalisation technique We have constructed the solutions to the Schrödinger equation by using the totally independent random walk (Markov chain) concepts The eigen-energies constructed using the random walk simulations agree within a few permille level with the exact ones Stochastic solutions are systematically higher than the exact ones

119 About the Stochastic Approach Summary Formulating the Concepts Testing the Method against Exact Results We discussed the problem of the nuclear pairing Hamiltonian written down in the Fock space representation (for N spaces) We obtained the exact results using the so-called P 1, P 2 and P 12 symmetries and the Lanczos diagonalisation technique We have constructed the solutions to the Schrödinger equation by using the totally independent random walk (Markov chain) concepts The eigen-energies constructed using the random walk simulations agree within a few permille level with the exact ones Stochastic solutions are systematically higher than the exact ones The Lanczos approach has a natural limitations related to the present-day computer memory; the stochastic simulation is extremely fast and can go in principle up to infinity

120 About the Stochastic Approach Formulating the Concepts Testing the Method against Exact Results Comments and Conclusions The Lanczos approach has a natural limitations related to the present-day computer memory; the stochastic simulation is extremely fast and can go in principle up to infinity We would like to perform more detailed tests of the structure of the universal probability distribution The fact that such a probability distribution seems to exist, acting the same way independently of the structure of the Fock-space states looks to us of extreme importance The (small) discrepancies with respect to the exact solutions can be due to the inaccuracies of the elementary probability distribution and/or to a small non-markovian corrections