Appendix
The Chiral Condensate Lattice Tight-Binding Hopping Hamiltonian Model
The Appendix provides detailed derivation of the tight-binding symmetric analytical solvable Hamiltonian model based on the geometric structure of the proposed Pmmm chiral condensate unit cell. The derivation of Eqs. (6a-e) proceeds through the following steps:
1. Identify the eight Pmmm space group Wyckoff positions: quark at Wyckoff 1a (0,0,0) + 1b (a/2,a/2,a/2); quark at Wyckoff 3c representative (a/2,0,a/2) and (0,a/2,0); at (a/2,a/2,0) and (0,0,a/2); and at (a/2,0,0) and (0,a/2,a/2).
2. Introduce the tight-binding Hamiltonian model for the chiral condensate Pmmm space group lattice.
3. Introduce the hopping selection rules (↔ and ↔ only), setting all other hopping amplitudes to zero generating the symmetric analytically solvable block-diagonal Hamiltonian: ⊕ .
4. Calculate the structure factors with the allowed hopping selection rules with nearest-neighbor approximation: B = , C = , F = , G = .
5. Introduce the decoupled 4×4 quark block and antiquark block .
6. Calculate analytically the energy bands by diagonalization of the 4×4 block symmetric Hamiltonian using symmetric/antisymmetric basis unitary transformation.
A.1 The Pmmm space group eight high-symmetry Wyckoff positions
The complete set of lattice points for each species within the conventional unit cell (including all boundary-shared points) is identified bellow. The proposed QCD chiral condensate lattice has a cubic unit cell of side with space group Pmmm (No. 47, point group D2h, order 8). This cell is assumed to arise by symmetry breaking from the parent cubic space group Pm3̄m (No. 221, point group Oh, order 48) upon condensation of the four light quark flavors into the high-symmetry Wyckoff positions.
quark (Wyckoff 1a + 1b): (0,0,0), (a,0,0), (0, a,0), (0,0, a), (0, a, a), (a, a,0), (a,0, a), (a, a, a) and (a/2, a/2, a/2)
The Wyckoff 1a site at (0,0,0) has site symmetry Oh (the full octahedral group, order 48), the highest possible site symmetry in Pm3̄m. The Wyckoff 1b site at (a/2, a/2, a/2) has the same site symmetry Oh. Both are invariant under all 48 symmetry operations of the parent cubic group, making the d quark a scalar singlet under the cubic point group.
antiquark (Wyckoff 3d, z-direction): (0, 0, a/2), (a, 0, a/2), (0, a, a/2), (a, a, a/2), (a/2 ,a/2, 0), (a/2, a/2, a)
The representative internal position is (a/2, a/2, 0) in fractional coordinates (½, ½, 0). The site symmetry of the 3d position in Pm3̄m is D4h (order 16). The three members of positions (0, ½, ½), (½, 0, ½), (½, ½, 0) in fractional coordinates, transform as the T1u polar vector representation of Oh. The antiquark is associated with the z-component of this triplet.
quark (Wyckoff 3c, xz-direction): (0, a/2,0), (a, a/2,0), (0, a/2, a), (a, a/2, a), (a/2 ,0, a/2), (a/2, a, a/2)
The representative internal position is (½, 0, ½) in fractional coordinates, which is the xz-face center. The quark is associated with the xz-component of the T1u polar vector triplet.
antiquark (Wyckoff 3d, x-direction): (a/2,0,0), (a/2,a,0), (a/2,0,a), (a/2,a,a), (0,a/2,a/2), (a,a/2,a/2).
Wyckoff assignments for the four quark species in the Pmmm unit cell
| Species |
Wyckoff position |
Representative position |
Site symmetry |
Per cell |
|
1a + 1b |
(0,0,0) and (a/2, a/2, a/2) |
Oh (order 48) |
2 |
|
3d (z-direction) |
(a/2, a/2,0) |
D4h (order 16) |
2 |
|
3c (xz-direction) |
(a/2, 0, a/2) |
D4h (order 16) |
2 |
|
3d (x-direction) |
(a/2, 0, 0) |
D4h (order 16) |
2 |
| Total |
|
|
|
8 |
The proposed unit cell contains 8 quark/antiquark positions per primitive cell: 2+ 2+2+ 2.
A.2 The Tight-Binding Hamiltonian Model
The tight-binding Hamiltonian in second-quantized form is written as [
29,
40]:
The on-site part assigns an energy to each quark flavor ∈ {, , , }, representing the effective single-particle energy of that species at its Wyckoff position:
where R labels the primitive cell index and i, j label the two inequivalent sites of each species within the cell. The index i = 1 refers to the Wyckoff 1a-type site (corner-derived) and i = 2 to the Wyckoff 1b-type site (body-center-derived) for the
quark; similarly j = 1 refers to the xz-face-center site and j = 2 to the y-edge-midpoint site for the
quark and similarly the k and l indexes represent the two positions of the
and
antiquarks.
The hopping part describes the quantum mechanical amplitude for a quark to tunnel from one Wyckoff site to a neighboring Wyckoff site:
where
is the hopping integral from species β at cell R′ to species α at cell R. The hopping integrals are determined by the overlap of the quark wavefunctions at the two sites, which depends on both the distance and the symmetry of the path between them.
A.3 The Hopping Selection Rules
We assume that the only symmetry allowed hoppings are: ↔ (quark sector hopping) and ↔ (antiquark sector hopping) where both and quarks and and antiquark numbers are conserved in these quark exchange reactions. As shown below, these selection rules decouple the tight-binding hoppings Hamiltonian model. Under these selection rules only two hopping amplitudes are non-zero:
: ↔ hopping amplitude (quark sector)
: ↔ hopping amplitude (antiquark sector)
All other hopping amplitudes , , are set to zero in the model producing a block-diagonal Hamiltonian model:
where
operates in the {
,
} quark subspace and
operate in the {
,
} antiquark subspace.
A.4 Structure Factor Calculation with Nearest-Neighbors Approximation
According to Bloch theorem [
40], the eigenstates of a periodic Hamiltonian can be written as Bloch waves:
where R is the Bravais lattice vector of primitive cell R,
is the position of site
within the lattice cell, and N is the total number of cells. In the Bloch basis the hopping Hamiltonian matrix element between species
and
is:
where ΔR = R′ − R is the lattice vector difference. In the nearest-neighbor approximation, only the shortest displacement vectors
contribute, and the hopping amplitude
is assumed equal for equivalent neighbors. The Hamiltonian matrix element then simplifies to the structure factor:
where the sum runs over all nearest-neighbor displacement vectors
connecting a Wyckoff site β to Wyckoff site α. For centrosymmetric lattices such as Pmmm, the structure factors are real, because every hop vector
is accompanied by its inversion-related symmetric partner
, and
+
=
is real. For the Pmmm chiral condensate lattice the cosine factors appear with arguments
because the nearest-neighbor vectors between Wyckoff sites are of magnitude
, half the lattice constant. cx, cy and cz are defined as:
Structure Factor B(k): d ↔ u Hopping via the xz Face-Diagonal
Site at Wyckoff 1a:
Site u1 at Wyckoff 3c: [xz face-center representative]
Both sites belong to the same primitive cell R = 0. The intra-cell displacement vector from to is:
There are four equivalent displacement vectors:
n1 = 0, n3 = 0: |δ| = a/√2
n1 = −1, n3 = 0: , |δ| = a/√2
n1 = 0, n3 = −1: , |δ| = a/√2
n1 = −1, n3 = −1: , |δ| = a/√2
The 4 displacement vectors lie in the xz plane (y = 0 component), reflecting the xz-type character of the 3c Wyckoff position occupied by u1.
The structure factor B(k) is the sum over all 4 nearest-neighbor vectors, multiplied by the hopping amplitude :
Structure Factor C(k): d ↔ u Hopping via the y Direction
Site at Wyckoff 1a: Site at Wyckoff 3c: [y-edge midpoint representative]
− =
There are two equivalent displacement vectors:
n1=0, n2=0, n3=0: , |δ| = a/2
n1=0, n2=−1, n3=0: , |δ| = a/2
Structure Factor F :1 ↔1 Hopping via the xy Face-Diagonal
Site1 is at (a/2, a/2, 0) (xy-face center). Site 1 is at (a/2, 0, 0) (x-edge midpoint). The nearest-neighbor displacement vectors connecting these two site types lie in the xy plane:
δ = (±a/2, ±a/2, 0) 4 xy face-diagonal vectors
There are four equivalent displacement vectors:
The Fourier sum similar to B(k) calculation above gives:
Note that the structure factor F involves cx and cy the xy face-diagonal in contrast to the quark sector B factor which involves cx and cz. This xy vs xz distinction is the direct geometric consequence of the antiquark species occupying a different set of Wyckoff positions.
Structure Factor G :1 ↔2 Hopping via the z Direction
Site 2 is at (0, a/2, a/2) (yz-face centre). The displacement vectors from 1 at (a/2, a/2, 0) to ū2 involve z-direction hops:
There are tow equivalent displacement vectors:
δ = (0, 0, ±a/2) 2 z-direction vectors
The antiquark G factor involves cz alone corresponding to z-direction hopping while the quark sector C factor involves cy alone. The substitution y ↔ z relative to the quark sector reflects the axis permutation in the Pmmm unit cell between the quark and antiquark Wyckoff positions.
The Inversion-Symmetry Cross-Coupling Pattern
The structure factors for are obtained by the same Fourier sum, but starting from instead of . The key result derivable by an identical calculation is that the roles of B and C are crossed for compared to position:
↔ u1: structure factor B =
↔ u2: structure factor C =
↔ u1: structure factor C =
↔ u2: structure factor B =
This inversion symmetry is mandated by the Pmmm space group: the inversion operation i maps (x, y, z) → (−x, −y, −z), which maps at (0,0,0) ↔ at (a/2,a/2,a/2) [related by inversion through the sub-cell center (a/4,a/4,a/4) shown in Figure 1a] and simultaneously maps u1 at (a/2,0,a/2) ↔ u2 at (0,a/2,0). Inversion therefore swaps both the d-site labels and the u-site labels, crossing the coupling assignments. This crossed pattern produces the distinctive off-diagonal block structure of shown in Section A.4, and the inversion symmetry generates the symmetric/antisymmetric band splitting B ± C that appears in the energy bands.
A.5 The decoupled 4×4 Quark Block and antiquark block
Assembling the structure factors into the 4×4 quark block, with basis ordering |, |〉, |1〉,|2〉:
The antiquark sector is derived analogously, but the geometric roles of the coordinate axes are permuted. The antiquark occupies the z-direction 3d position while occupies the x-direction 3d position. This axis permutation produces a different pair of structure factors. The structure factors with basis ordering |1〉, |2〉, |1〉, |2〉:
A.6 The Energy Bands Calculation
Each 4×4 block can be diagonalized analytically by exploiting the Z2 × Z2 symmetry of the block: both and are invariant under the simultaneous exchange ↔ combined with u1 ↔ u2 (or 1 ↔ 2 combined with 1 ↔ 2). This symmetry is generated by the Pmmm inversion operation which maps each Wyckoff site to its inversion-related partner as seen in Figure 1a and Figure 1b.
The unitary symmetric (+) and antisymmetric (−) transformation is:
|〉 = (|〉 + |〉)/√2
|〉 = (|〉 − |〉)/√2
|〉 = (|1〉 + | 〉)/√2
|〉 = (|1〉 − |2〉)/√2
In the symmetric and antisymmetric basis the 4×4 quark block becomes block-diagonal after the unitary transformation ‘ = U Uᵀ, separating into two independent 2×2 matrices and :
Symmetric Quark Block (|d+〉, |u+〉)
The off-diagonal element B + C = + = (.
The eigenvalues of the 2×2 symmetric matrix is:
Antisymmetric Quark Block (|d−〉, |u−〉)
The off-diagonal element is B − C = − = (.
The eigenvalues of the 2×2 anti-symmetric matrix is:
Antiquark Bands
The same procedure applied to with symmetric/antisymmetric combinations of |〉, |〉, |〉and |〉gives:
off-diagonal element F + G = + = :
off-diagonal element F − G = − = :
K-Space high-symmetry point behavior
At k-space Γ point (kx = ky = kz = 0): cx = cy = cz = 1, giving B + C = and B − C = . The symmetric and antisymmetric bands are split by , which may represent the chiral condensate lattice mass analog of the QCD pion mass .
At k-space X point (kx = π/a, ky = kz = 0): cx = 0, cy = cz = 1. Then B = 0 and C = , giving B + C = B − C = C. The symmetric and antisymmetric bands are degenerate at the X point.
At k-space R point (k = (π/a, π/a, π/a)): cx = cy = cz = 0. All structure factors vanish: B = C = F = G = 0. All eight bands collapse to the four on-site energies, complete decoupling of all sites at the Brillouin zone corner.
Adding a Lorentz contraction to the k-space Hamiltonian
Note that the structure factors anddepend on cosines functions of the momentum in the x, y and z directions,. If we assume that the chiral condensate lattice length contracts in the direction of motion of the embedded particle with a Lorentz contraction, the embedded electron performing a quantum walk in the Pmmm chiral condensate lattice velocity in the x direction for example will increase since the cosine term will increase reaching the limit value of 1 with higher momentum. A three-dimensional DTQW simulation of the embedded electron in the proposed Pmmm QCD chiral condensate lattice will be presented in a future work.
Appendix Summary
The tight-binding hopping Hamiltonian constructed for the Pmmm unit cell, constrained by the Pmmm symmetry and the hopping selection rules (↔, ↔allowed, all other hoppings forbidden), yields an 8×8 k-space Hamiltonian that is real, symmetric, and analytically solvable, and is block-diagonal in two 4×4 sectors. Each 4×4 block further decomposes into two analytically solvable 2×2 matrices via the symmetric and antisymmetric transformation of the two equivalent sites within each sector. The resulting eight closed-form energy bands are a direct consequence of the Pmmm space group geometry and the enforced hopping selection rules.
The proposed Pmmm space group chiral lattice unit cell of Figure 2a may be the geometric framework for the QCD chiral condensate. The condensation of the four quarks into specific high symmetry Wyckoff positions reduces the space group symmetry from Pm3̄m (No. 221, point group Oh, order 48) to Pmmm (No. 47, point group D2h, order 8). The geometric symmetry breaking of the Pm3̄m to Pmmm space group, may play the role of the spontaneous breaking of the chiral symmetry group SU(2)L × SU(2)R → SU(2)V that characterizes the QCD vacuum. According to Goldstone’s theorem, each continuously broken symmetry generator produces one massless Goldstone boson. The three broken rotational directions of the cube, the three independent ways of rotating between x, y, z that are no longer symmetries in Pmmm space group, correspond to three independent broken generators, producing three Goldstone bosons. The multiplicity-3 of the Wyckoff positions 3c and 3d, which transforms as the T1u irreducible representation of Oh, a polar vector representation provides the geometric origin of the SU(2) isospin triplet structure of the pion field. The QCD pion triplet is encoded in the geometry of the Pm3̄m to Pmmm symmetry reduction at the level of the proposed lattice structure of the chiral condensate lattice of the vacuum.
The proposed Pmmm chiral condensate unit cell of Figure 2a may provide the geometric framework for the QCD chiral condensate. The condensation of the four light quarks and antiquarks into specific high-symmetry Wyckoff positions reduces the space group symmetry from Pm3̄m (No. 221, point group Oh, order 48) to Pmmm (No. 47, point group D2h, order 8). This geometric symmetry reduction from Pm3̄m to Pmmm may play the role of the spontaneous breaking of the chiral symmetry group SU(2)L × SU(2)R → SU(2)V that characterizes the QCD vacuum. According to Goldstone’s theorem, each continuously broken symmetry generator produces one massless Goldstone boson. The three broken rotational directions of the cube, the three independent ways of rotating between x, y, and z that are no longer symmetries of the Pmmm space group, correspond to three independent broken generators, producing three Goldstone bosons identified as the pions π+, π−, and π0. The multiplicity-3 of the Wyckoff positions 3c and 3d, which transforms as the T1u irreducible representation of Oh, a polar vector representation, provides the geometric origin of the SU(2) isospin triplet structure of the pion field. The QCD pion triplet is therefore encoded in the geometry of the Pm3̄m → Pmmm symmetry reduction at the level of the crystal structure of the proposed chiral condensate vacuum