Quantum ChromoDynami s (QCD) is a gauge theory of strong nu lear
in-tera tions between the onstituents of hadrons. The hadrons are a lass of
parti les in luding baryons (e.g. the nu leon) and mesons (e.g. the pion).
The theory is based on the prin iple of lo al gauge invarian e with a
non-Abelian SU(3) gauge group [3, 4℄. The fundamental degrees of freedom of
the theory are quarks and gluons. The Lagrangian density of QCD an be
written as:
L QCD = L quark + L gluon + L int ,
(1.1)where
L quark
is the purely fermioni (quark) part,L gluon
the purely bosoni(gluon) part and
L int
the intera tion part that ouples quarksand gluons.Let us now onsider the dierent parts that onstitute the QCD
La-grangian. The quark term is 1
:
L quark =
N f
X
f =1
ψ ¯ f (x)(iγ µ ∂ µ − m f )ψ f (x),
(1.2)where
N f
is the number of avours2,ψ f (x)
is the quark (spinor) eldor-responding to avour
f
andm f
is thef
-avour bare quark mass and the1
Throughoutthethesis,weemploytheEinsteinsummation onventionforDira indi es
(denoted byGreekletters)andSU(3)-groupgeneratorindi es(denotedbyLatinletters).
2
The Standard Model in orporates 6 avours of quarks (up, down, strange, harm,
bottom, top). However, investigating the low-energy properties of QCD with Latti e
QCDmethods, oneusuallyrestri tsoneselftothelightest2,3or4avours.
{γ µ , γ ν } = 2η µν ,
(1.3)where
η µν =
diag(1, −1, −1, −1)
isthe metri tensor.The gluon part reads:
L gluon = − 1
4 F µν a (x)F a µν (x),
(1.4)where
F µν a (x)
is the eld strength tensor, whi h is relatedto the gluon eldomponents
A a µ (x)
:F µν a (x) = ∂ µ A a ν (x) − ∂ ν A a µ (x) − gf abc A b µ (x)A c ν (x),
(1.5)where
g
isthe bare oupling onstantandf abc
are the stru ture onstantsofSU(3), satisfying the ommutation relations:
[t a , t b ] = if abc t c ,
(1.6)where
t a
are the generators of the group SU(3).The purely bosoni part of the Lagrangian is invariant with respe t to
the lo al gauge transformation. If we want the fermioni part to obey the
lo algauge symmetry aswell, we haveto introdu e a term that ouples the
fermions and bosons, i.e. des ribes the intera tion between them. This is
the basi buildingprin ipleof alllo algaugetheories. It wasrst dis overed
in the ase of the ele tromagneti intera tion, where a term that ouples
ele trons and photons is ne essary to guarantee the lo al gauge invarian e.
In the ase ofQCD, the sum
L quark + L gluon
is not invariant withrespe t tothe lo al SU(3) transformation and the way to guarantee this invarian e is
to introdu e the intera tion term
L int
that ouples the quark eldsψ
andgluon elds
A µ
:L int = g
N f
X
f =1
ψ ¯ f (x)γ µ A µ (x)ψ f (x),
(1.7)where the gluon eld
A µ
is related toits omponentsin the followingway:A µ (x) = t a A a µ (x).
(1.8)Conventionally,onewritestheterms
L quark
andL int
together,introdu ing the ovariant derivativeD µ
:D µ (x) = ∂ µ − igA µ (x).
(1.9)L QCD =
N f
X
f =1
ψ ¯ f (x)(iγ µ D µ − m f )ψ f (x) − 1
4 F µν a (x)F a µν (x).
(1.10)Letusalsodenethe ( lassi al)QCDa tion,whi his the integralof the
Lagrangian density overspa e-time:
S QCD = Z
d 4 x L QCD .
(1.11)Anelegant(and relevantfrom the point of viewof Latti e QCD) way to
quantize a lassi al theory, like the one given by the lassi al QCD a tion
(1.11), is to use the Feynman path integral formalism [5℄. The expe tation
value of any observable
O
is given by:hOi = 1 Z
Z
D ¯ ψDψDA O[ψ, ¯ ψ, A] e iS QCD [ψ, ¯ ψ,A] ,
(1.12)with the partition fun tion:
Z = Z
D ¯ ψDψDA e iS QCD [ψ, ¯ ψ,A] .
(1.13)Itisworthtoemphasizethatalleldsinthepathintegralare lassi al. Su h
path integral an not beevaluated analyti ally (ex ept for few spe ial ases
mu h simpler than QCD) and one has to swit h to approximate methods.
Formany theories, likeQuantumEle troDynami s (QED), averysu essful
method is perturbation theory. It onsists in expanding the path integral
with respe t to a small parameter (e.g. the ne stru ture onstant
α ≈ 1/137.036
inQED)anddroppingtermsbeyondsomeorder. Forexample,themost re ent al ulation of the anomalous magneti moment of the ele tron
(usually parametrized in terms of the so- alled
g
-fa tor) up to fourth-order inα
agrees with experiment up to 10 signi ant digits, making it one ofthe most pre isely veried predi tion of physi s the ele tron
g
-fa tor isg e = 2a e +2
,wherethetheoreti alvalue:a th e = 1 159 652 182.79(7.71)×10 −12
andtheexperimentalone:
a exp e = 1 159 652 180.73(0.28)×10 −12
[6℄. However,for perturbative methods to work, there has to be a small parameter with
respe t to whi h one expands the path integral. In the ase of QCD, the
oupling onstant of the olour intera tion depends on energy and one has
to onsider two regimes. For high energy or large momentum transfer, the
QCD oupling onstantissmallenoughforperturbativemethodstowork. In
this regime,theintera tionof quarksand gluons anbearbitrarilyweakand
by Gross, Politzer and Wil zek. However, in the ase of low energy orsmall
momentum transfer, this oupling onstant be omes of the order of unity
and perturbation theory is bound to fail the strong intera tions be ome
strongindeed. Quantitatively,theenergys alewhenithappens
Λ
strong≈ 250
MeV, where the value is not pre isely dened and depends on the hosen
observable. Anyway, its approximate value implies that a vast number of
relevant phenomena in QCD, su h asthe onnement of quarks and gluons
into hadrons, happen in the non-perturbative regime. Thus, one needs
non-perturbativemethods,su hasLatti eQCD,whi histheonlyknownmethod
of extra ting quantitative predi tions about the low-energy regimeof QCD.
This approa h onsists in dis retizing the QCD path integral. In this way,
one obtainsafullyregularized andwell-dened theory, whi h an bestudied
numeri ally, but also analyti ally the dis retized version of QCD enabled
manyrelevant on eptualdevelopmentsandledtoimportantinsightintothe
nature of strong intera tions.
However, the os illating exponential
e iS QCD [ψ, ¯ ψ,A]
renders the numeri alevaluation of the QCD path integral unfeasible from the pra ti al point of
view. Fortunately, integrals like (1.12) are tra table, if one swit hes from
Minkowski spa e-time with metri tensor
η µν
with signature e.g.(+ − −−)
toEu lideanspa e-timewithsignature
(++++)
. Thisisa hievedbyanalytiontinuation (Wi k rotation of the time dire tion:
t → −iτ
). In order thatthe Eu lidean formulation an be ontinued ba k to physi al (Minkowski)
spa e,the Eu lidean orrelationfun tionshavetosatisfya ertain ondition,
alled the Osterwalder-S hrader ree tion positivity [7, 8℄. This ondition
ensures that the transition probabilities between gauge-invariant states are
non-negative and the quantum me hani al Hamiltonian has only real and
positiveeigenvalues [9℄.
The QCDLagrangian density inEu lidean spa e reads [10℄:
L E QCD =
N f
X
f =1
ψ ¯ f (x)(γ µ E D µ + m f )ψ f (x) − 1
4 F µν a (x)F a µν (x)
(1.14)and the Eu lidean gammamatri es satisfy:
{γ µ , γ ν } = 2δ µν ,
(1.15)where
δ µν =
diag(1, 1, 1, 1)
isthe Eu lidean metri tensor. Theexpe tation value of any observableO
is then given by:hOi = 1 Z E
Z
D ¯ ψDψDA O[ψ, ¯ ψ, A] e −S QCD E [ψ, ¯ ψ,A] ,
(1.16)where
S QCD E = R d 4 xL E QCD
is the Eu lidean a tion and the Eu lideanparti-tion fun tion reads:
Z E = Z
D ¯ ψDψDA e −S QCD E [ψ, ¯ ψ,A] .
(1.17)The os illating exponential in (1.12) is repla ed by the well-behaved fa tor
e −S QCD E
and thus the multi-dimensional integral (1.16) an beevaluated nu-meri ally,atleastinprin iple,e.g. withMonteCarlomethods. Formally,thequantum eld theory dened by the partition fun tion (1.17) an be
inter-preted as a statisti al me hani al system and the exponential
e −S E QCD
playsthe role of aBoltzmann fa tor.
Fromnowon, wewillworkonlywiththe Eu lideanformulationof SU(3)
non-Abelian gauge theory (QCD) and hen e we drop the supers ript
E
andthe subs ript
QCD
thatremind us of it.Now,wewilldis ussafewimportantfeaturesof ontinuumQCDthatare
relevant from the point of view of further onsiderations, espe iallythe role
of hiral symmetry and spontaneous hiral symmetry breaking[10, 11,4℄.
Tobespe i , letusrestri tourselvestotwoavours ofquarks (
u
andd
quarks). The lassi alQCD Lagrangian an be rewritten as:
L = ¯uγ µ D µ u + ¯ dγ µ D µ d + ¯ um u u + ¯ dm d d − 1
4 F µν a F a µν ≡
≡ L u + L d + L m u + L m d + L gluon ,
(1.18)where
u ≡ ψ u
andd ≡ ψ d
are the orresponding spinors and we have sep-arated the mass terms in the fermioni Lagrangian. We an de ompose thequark Lagrangian further by dening left-handed and right-handed quark
spinor elds:
q R ≡ P + q, q L ≡ P − q, q = u, d,
(1.19)where:
P ± = 1 ± γ 5
2 .
(1.20)Eq. (1.19) impliesfor the onjugate spinor elds:
¯
q R = ¯ qP − , q ¯ L = ¯ qP + .
(1.21)Thus, the rst twoterms in Lagrangian (1.18)be ome:
L u + L d = ¯ u L γ µ D µ u L + ¯ u R γ µ D µ u R + ¯ d L γ µ D µ d L + ¯ d R γ µ D µ d R =
(1.22)terms we obtain:
i.e. the mass terms oupleelds of opposite hiralities.
Let us now onsider the massless terms
L u
andL d
in the Lagrangian.They are invariant with respe t to the following transformations,
respe -tively:
where
L
andR
areunitary2×2
matri es,i.e. elementsofthe(avour)groupU(2). This means that the Lagrangian
L u + L d
is invariant with respe t tothe group U(2)
L ×
U(2)R
.Let us take a loser look at the possible forms of transformations. The
masslessquark Lagrangianisinvariantunder fourSU(2)
×
U(1)ve tortrans-formations: (ve tor) Noether urrents
j i µ
asso iated with these 4 transformations and hen e 4 onserved hargesQ i = R d 3 xj i 0
the baryon number (i = 0
) andthe isospin (
i = 1, 2, 3
).Inaddition,therearetransformationsinvolving
γ 5
, alled hiralrotations:u
Togetherwithtransformations(1.25),themasslessquarkLagrangian
L u +L d
is invariant underthe symmetry groupSU(2)
R ×
SU(2)L ×
U(1)V ×
U(1)A
.However, it an be shown that the fermion integration measure in the
quantized theory is not invariant under the transformation (1.26) for
i = 0
,whi hredu es the fullsymmetry to SU(2)
R ×
SU(2)L ×
U(1)V
. This istheso- alled axial anomaly and it has important onsequen es e.g. for the meson
spe trum the hiral avour singlet symmetry an not be broken
sponta-neously and hen e there isno Goldstone boson asso iatedwith spontaneous
breaking of this symmetry. This implies that the mass of the avour
sin-glet
η ′
meson does not vanish in the limit of vanishing quark masses (asopposed to the mass of the
η
meson, whi h is one of the pseudo-Goldstone bosons),but it isrelatedto topologi alu tuationsof the QCD va uumviathe Witten-Veneziano formula[12, 13℄:
f π 2
2N f m 2 η + m 2 η ′ − 2m 2 K = χ top ,
(1.27)where
f π
is the pion de ay onstant,m x
the mass of thex
meson andχ top
the topologi al sus eptibility, whi h willbe dened later.
Letus now onsider the mass terms of the QCD Lagrangian
L m u + L m d
.They areinvariantwithrespe t tothe transformation(1.25)for
i = 0
,sothebaryon number is onserved also in the massive theory. For
i = 1, 2, 3
thetransformation(1.25)isasymmetryonlyif thequarkmassesare equal
m u = m d
. Hen e, theisospinis onserved inthemassivetheory,butonlyformass-degenerate quarks. However, the mass terms
L m u + L m d
are not invariantunder hiralrotations(1.26),whi his ausedbythefa tthattheexponential
in (1.26) is the same for the spinor
(u d) T
and the onjugate spinoru ¯ ¯ d
,
whi h is, in turn, due to the anti ommutation relation
{γ µ , γ 5 } = 0
. Thus,the symmetry ofthe quantum QCDLagrangianisbroken toSU(2)
V ×
U(1)V
in the mass-degenerate ase and to U(1)
V ×
U(1)V
ifm u 6= m d
.In the ase of arbitrary number
N f
of quark avours, the analysis iseasilygeneralized (the matri es
u i
are nowtheN f × N f
identity matrix andN f 2 − 1
generatorsofthe avour groupSU(N f
))andthefullsymmetry ofthequantized massless QCD Lagrangian is SU(
N f
)R ×
SU(N f
)L ×
U(1)V
, whi his redu ed to SU(
N f
)V ×
U(1)V
in the mass-degenerate ase and further to U(1)V × . . . ×
U(1)V
(withN f
fa tors U(1)V
) in the ase of dierent quarkmasses. Thus, in the latter ase, the only exa t symmetry is the baryon
number onservation.
However,sin etheisospinsymmetryisonlyslightlybrokenforthelightest
twoquarks, itisoftentreatedasexa t 3
,while theheavierquarksare treated
separately. Moreover, sin e the up and down quarks are so light, ompared
to the heavier quarks (
m u ≈ m d ≈
a few MeV, whereas alreadym s ≈ 100
MeV), the full symmetry of the massless Lagrangian with
N f = 2
avoursSU(2)
R ×
SU(2)L ×
U(1)V
remains an important approximate symmetry and is the basis ofN f = 2
hiral perturbation theory (χ
PT ). At low energy,the quarks and gluons are onned into hadrons and hen e one an dene
an ee tive eld theory, in whi h the fundamental degrees of freedom are
not quarksand gluons,but lighthadrons. Two-avour
χ
PT wasformulatedby Gasser and Leutwyler [14℄. The Lagrangian of this theory is onstru ted
from elds des ribing the pions (
π ±
,π 0
) in a way whi h is onsistent with3
InLatti eQCDoneusuallysimulatesthelightesttwoquarksasmass-degenerate.
ganized in terms of expansion parameters
p/Λ χ
andm π /Λ χ
, wherep
is themomentum,
m π
thepion massandΛ χ = (4πf ) 2
thetypi alhadroni s ale≈
1 GeV,with
f
the pion de ay onstantin the hirallimit. Thereare manyappli ations of
χ
PT in the analysis of the low-energy regime of QCD, e.g.pion s attering experiments. Moreover, it is also essential in the analysis of
Latti eQCDdata, sin emost of ontemporaryLatti eQCDsimulationsare
performed at unphysi al values of the pion mass 4
hen e an extrapolation
to the physi al point (physi al pion mass) is ne essary and is performed by
tting
χ
PT formulas. What is more, even thoughthe strange quarkmass ismu h larger than the mass of the up and down quarks, it is still relatively
small ompared to the typi al QCD s ale of
≈
1 GeV and the symmetrySU(3)
R ×
SU(3)L ×
U(1)V
of the masslessN f = 3
Lagrangian is also anap-proximate symmetry and forms the basis of
N f = 3
hiral perturbation the-ory,whi hisalsoof use inthe analysis oflow-energyQCDexperiments,e.g.in luding the kaons (also in kaon physi s from Latti e QCD). Three-avour
χ
PT was also introdu ed by Gasser and Leutwyler [15℄ as a generalization of the two-avour ase to in lude the strange quark. The three-avourLa-grangianin ludes,besidesthepionelds,alsootherlightpseudos alarmeson
elds (of the remaining pseudo-Goldstone bosons the kaons
K ±
,K 0
,K ¯ 0
and the
η
meson). Quantitatively, the expli it breaking of hiral symmetry by the quark masses an be expressed by the ratiosm 2 π /(4πf ) 2 ≈ 0.007
andm 2 K /(4πf ) 2 ≈ 0.09
. In this sense, the expli itbreaking by the strangequarkmass isroughlya10%ee t, whileforthe lightestquarksitisa<1% ee t.
Obviously,itisnot possibletotreat the
N f = 4
symmetryasapproximately valid,sin ethe harmquarkisalreadyheavy(m c ≈ 1.3
GeV)andthemesonsontaining itare mu hheavier than the s ale
Λ χ
.However, if hiralsymmetrywasbroken onlyexpli itly,wewouldobserve
degenerate multiplets of hadrons e.g. there should be s alar mesons with
massesverysimilartothepseudos alarones. Also,inthis aseoneshouldnot
expe t su h big dieren e between the masses of the pions and kaons. The
explanation of these phenomena an beprovided by anassumption that the
hiral symmetry of QCD is not only expli itly broken by the quark masses,
butalsospontaneouslybroken. Wespeakofspontaneoussymmetrybreaking
if asymmetrywhi hispresentatthe Lagrangianlevelisabsentinthe
phys-i al ground state 5
. If a ontinuous symmetry is broken spontaneously, then
4
Some ollaborationshavere entlystartedorarepreparingsimulationsatthephysi al
pion mass.
5
A learexampleis provided byferromagnets. EventhoughtheHamiltonian ofsu h
system is invariantwith respe t to a simultaneous ip of allspins, in an experimentall
spins are aligned, i.e. only one of two degenerate ground states must be hosen the
interpreted asthe wouldbe-Goldstonebosons of hiralsymmetrybreaking,
wheretheprex wouldbe-referstothefa tthattheyare notmassless,but
have asmallmass ( ompared tothe masses of other hadrons) that isdue to
(small) expli it breaking of hiral symmetry by the quark masses.
Also, spontaneous breaking of hiral symmetry an be observed in the
mass dieren e of parti les that are hiral partners and should have the
same mass,if hiralsymmetry wasexa t. Sin e hiralsymmetry isexpli itly
broken bythe quark masses,the experimental massvalues of hiral partners
should not be equal, but they should be lose to ea h other, be ause the
masses of the lightquarks are so small. This is not observed. Forexample,
the ve tor mesons
ρ
anda 1
have massesequalto,respe tively, 770 and 1260 MeV,whi hisamu hlargerdieren ethanone wouldexpe tfromthesmallexpli itbreakingof hiralsymmetry[16℄. Anotherexampleisthenu leonand
itsnegative-paritypartner,usuallydenotedby
N ∗
[11,17℄. Theexperimental value of the nu leon mass ism N ≈ 940
MeV, whilem N ∗ ≈ 1535
MeV.Spontaneous hiralsymmetrybreakingissignalledbyanon-zerovalueof
the hiral ondensate
h0|¯uu|0i
,where|0i
is theva uumstate. This quantityemerges in hiral perturbation theory as an important low-energy onstant
B 0
:B 0 = −f −2 h0|¯uu|0i,
(1.28)where the tree-level pion de ay onstant
f
is another low-energy onstant.A well-known relationthat involves the hiral ondensate is the Gell-Mann,
Oakes, Renner (GMOR) relation[18℄:
f 2 m 2 π = −(m u + m d )h0|¯uu|0i,
(1.29)whi h an be derived in
χP T
. As su h, it is desirable to assess the valueof the hiral ondensate from experiment thus the value of
B 0
would beknown. It has been argued that the best estimate an be obtained from
the low-energypion-pions attering [19, 20℄. However, the al ulation of the
ondensatefromempiri aldatarequiressomemodelassumptions,i.e. one in
fa t has to assumethat spontaneous hiral symmetry breaking takes pla e.
Therefore,animportant he k would beto al ulatethe ondensate
non-perturbativelyfromrstprin iples,withoutanyadditionalassumptions. One
su hway isprovidedby Latti eQCD.Indeed, Latti eQCDsimulations
on-rmthatitisnon-zeroatzerotemperature(areviewofresultsonthistopi is
provided e.g. in. [21℄). However, thereexists atemperaturewherethe hiral
ondensate vanishes,thus signalling hiral symmetry restoration. Moreover,
spin-ip symmetryisspontaneouslybroken.
ment temperature, i.e. the temperature at whi h the quark-gluon plasma
formsand quarksand gluons areno longer onned intohadrons. Up tothe
presentday,thisissuehasnotbeenresolved ompletely,butitisastronghint
that Latti eQCD al ulations point to the fa t that both temperatures are
equal, up tostatisti alerror. This strongly suggests that spontaneous hiral
symmetry breaking isrelated to onnement and onrms that
understand-ing hiral symmetry and spontaneous hiral symmetry breaking is essential
to fully omprehend QCD. However, mu h more pre ise results are needed
to unambiguously resolve this question. In Latti e QCD investigations of
these phenomena it is therefore essential to take hiral symmetry properly
into a ount,i.e. fermionswith good hiralpropertieshave tobeused. This
is one of the motivations for employing overlap fermions, whi h will be the
main subje t of this thesis.