PCAC relation of the Domain-wall fermion

In this section we summarize the partially conserved axial vector current (PCAC) relations associated with chiral symmetry. We discuss how it holds on the lattice and see how it affects the correlation function. We refer to [74,75] in this section.

First, we review the PCAC relation in the continuum theory. We define isovector axial currents and pseudoscalar density operators

A^a_µ ≡ ¹

2ψγ¯ µγ5τ^aψ, (2.102)

P^a ≡ ¹

2ψγ¯ ₅τ^aψ, (2.103)

where,ψ = (u,d)^T andτ^a is the Pauli matrix. We consider an infinitesimal chiral rotation

δψ(x) = ¹

2ω^a(x)γ₅τ^aψ(x)_{, (2.104)} δψ¯(x) = ¹

2ω^a(x)ψ^¯(x)γ₅τ^a. (2.105) According to the Ward-Takahashi identity, we can relate the change of the actionS and an operatorOunder this transformation as

h0|δSO|0i= h0|δO|0i. (2.106) The expression of the change of the action is

δS=

Z

d⁴xω^a

−∂_µA^a_µ+2mP^a

, (2.107)

where we ignore the isospin breaking, namelym = m_u = m_d. Specifyingω^b(x) = ωδ_abδ(x−y)andO=P^a(y), one can derive

δO=2ωδ(x−y)ψψ^¯ (y). (2.108) Therefore, we obtain an identity

h0|∂µA^a_µ(x)P^a(y)|0i −2mh0|P^a(x)P^a(y)|0i=2δ(x−y)h0|ψψ^¯ (y)|0i. (2.109) Focusing on the low energy, we derive the PCAC relation from (2.109). We consider correlators with a time separationtby summing over the spatial coordinatex. For simplicity, we assumey = (_0,0). The first term in the l.h.s. of (2.109) yields a non- zero value only ifµ=4 since the operator∂µA_µ^a(x)is projected to zero momentum.

For larget, the contribution ofπmesons is dominant. By the definition of the pion

decay constant, the matrix element of the axial current is written as

∂th0|A^a₄(x)|π^a(p=0)i=m²_πfπe^−mπt. (2.110) Therefore, we can express an asymptotic form of the correlator

∑

x

h0|∂tA^a₄(t,x)P^a(0,0)|0i '^mπfπ

2 e^−mπthπ^a(₀)|P^a|0i. (2.111) At the same time, the correlator of the pseudoscalar can be expressed as

∑

x

h0|P^a(t,x)P^a(0,0)|0i '^e

−mπt

2mπ

h0|P^a|π^a(0)ihπ^a(0)|P^a|0i. (2.112) Now the r.h.s. of (2.109) can be ignored since we considert 0. From the terms proportional toe^−mπt, we obtain

m²_πfπ =2mh0|P^a|π^a(0)i. (2.113) This equation relate the quark mass , which explicitly breaks the chiral symmetry, and the pion mass. The equation implies that the axial current is conserved in the chiral limitm→0. Therefore, (2.113) is called the PCAC relation.

On the lattice, we may expect a relation similar to (2.113) with some modifica- tions. As we see2.7, the local currentA^a_µ(x)needs renormalization. In addition, the correction for the chiral symmetry is necessary due to the lattice regularization. The MDW fermions used in this work is defined in a finiteL_s, which slightly breaks the chiral symmetry. Thus, the quark mass is modified from the bare massm_bareas

m= m_bare+m_res, (2.114)

wherem_resis a residual quark mass parametrizing the symmetry breaking. We ex- press the PCAC relation in our setup as

m²_πfπ =Z_Ah0|∂µA^a_µ|π^a(0)i=2(m_bare+mres)h0|P^a|π^a(0)i. (2.115) In this work, we use JLQCD ensembles with the MDW fermion where the resid- ual mass m_res . 1 MeV [76]. The residual mass is estimated by the braking of the Ginsparg-Wilson relation. This indicates the good chiral property of our lattices. As we mentioned in Sec. 2.7, for the renormalization constantsZ_V = Z_AandZ_S = Z_P hold from the chiral symmetry up toO(m²_res)[77]. In the ss¯channel we focus on other chapters, the residual mass is negligible compared to the strange quark mass

ms∼100 MeV. Thus, by using MDW fermions, the computations can be performed in the same way as in the continuum theory.

Chapter 3

Two-point correlation function and QCD sum rule

3.1 Dispersion relation and spectral functions

In this section, we discuss a dispersion relation bridging the gap between (per- turbative) QCD and the nature of hadrons. The basis of that is the analyticity of correlators. We will see that a spectral function appears in the derivation of the rela- tion. The dispersion relation is one of the key concepts in QCD sum rules.

We define the hadronic vacuum polarization (HVP) function as a Fourier trans- form of the current-current correlator,

(q_µq_ν−q²g_µν)_Π(q²) =i Z

d⁴x e^iqxhJ_µ(x)J_ν(0)i, (3.1) whereJµ =qγ¯ µqis the quark vector current. The Lorentz tensorqµqν−q²gµνin the l.h.s. results from the current conservation. The HVP function is analytic except for the real axisq² > 0 where poles and a branch cut¹ may exist. Using the Cauchy’s integral theorem, the function may be rewritten by an integral,

Π(q²) = ¹ 2πi

I

dsΠ(s)

s−q². (3.2)

The integration contour encircling the pole ats =q²is shown Fig.3.1. The integrals around the positive real axis take the following form,

Z _∞+ie

0+ie dsΠ(s) s−q² +

Z _0−ie

∞−ieds Π(s) s−q² =

Z

dsΠ(s+ie)−_Π(s−ie)

s−q² , (3.3)

where e → 0⁺. The discontinuity reduces to the imaginary part by the Schwarz reflection principle,

Π(s+ie)−_Π(s−ie) =2iImΠ(s+ie). (3.4)

1The branch point corresponds to the threshold energy of hadrons.

Res Ims

q² =−Q²

FIGURE3.1: The integration contour in the complexs-plane.

If the function behaves asΠ(s) ∼ 0 at |s| ∼ ∞, it would be written in terms of a spectral functionρ(s)_:

Π(q²) =

Z _∞

0 ds ρ(s)

s−q², (3.5)

ρ(s) = ¹

πImΠ(s+ie). (3.6)

The equation (3.5) is called the dispersion relation. The integral in the dispersion relation, however, diverges since the spectral function does not vanish in the limit s → ∞. This is attributed to the ultraviolet (UV) divergence of the correlator (3.1).

We can avoid this problem by subtracting once, for instance,Π(₀)_{at a point}q² =₀ since the divergence is logarithmic. Finally, we obtain a once-subtracted dispersion relation,

Π(q²)−_Π(0) =q² Z _∞

0 ds ρ(s)

s(s−q²)^{. (3.7)}

The dispersion relation links two kinematic regions, namely,q² < 0 andq² > 0.

In the deep Euclidean regionQ² ≡ −q² 0, the perturbative expansion and OPE are applicable. In contrast, the spectral functionρ(_s) is understood as the density of hadronic states of a given energy. The relation (3.7) enables us to investigate the hadronic processes from QCD.

We demonstrate the role of the spectral function in a physical process. Let us consider electromagnetic currents with the electric charge of the quark, which ap- pears in the process of the electron-positron annihilation into hadrons. Summing these currents over flavors,

J_µ^EM= ²

3uγ¯ µu−¹

3dγ¯ µd−¹

3sγ¯ µs+· · · , (3.8)

we define the HVP functionΠ^EM(q²)andρ^EM(s)as (3.1) and (3.6), respectively. Ac- cording to the optical theorem, the functionΠ^EM(q²)in the time-like regionq² > 0 is intimately related with the total cross sectionσ(e⁺e⁻→hadrons;s),

σ(e⁺e⁻→_hadrons;s) = ^16π

2α²

s ImΠ^EM(s+ie)_{. (3.9)} Here,αis the fine structure constant and higher QED corrections are neglected. On the other hand, the total cross sectionσ(e⁺e⁻→µ⁻µ⁺;s)to the leading order in the massless limitm²_µ sis expressed as

σ(e⁺e⁻ →µ⁻µ⁺;s) = ^4πα

2

3s . (3.10)

Thus, the spectral function is proportional to the ratio of these experimental observ- ables,²

ρ^EM(s) = ¹ 12π²

σ(e⁺e⁻→hadrons;s)

σ(e⁺e⁻ →µ⁻µ⁺;s) ^{. (3.11)} The spectral function appears in the physical process and is measurable experimen- tally.³