In this se tion, we show how one an extra t hadron properties from
Lat-ti e QCD simulations, on entratingon the quantities of interestfor further
onsiderations.
Letus onsider a generalzero-momentumtwo-point orrelation fun tion
of the form
C(t) ≡ h0|O i (t) ¯ O j (0)|0i
, whereO i (t)
is some interpolating op-erator orresponding to the state with quantum numbers of the hadron wewant to analyze. Sin e:
O i (t) = e Ht O i e −Ht ,
(1.82)insertinga ompleteset of energy eigenstatesinto
C(t)
yields(wetakei = j
where
E n
is the energy of the staten
(1/2E n
is a normalization fa tor for energy eigenstates).One an immediatelysee that in the limitof large Eu lidean time
t
, theabove expression isdominated by the lowest energy state
|1i
:C(t) −−−→ t→∞ |h0|O i |1i| 2
2E 1 e −E 1 t ,
(1.84)where
E 1
isthe energy of this state, i.e. the mass of the lightest parti le. Inthisway,one anextra tthismassbytting(insomeinterval
t ∈ [t min , t max ]
)the orrelation fun tion with an exponential fun tion
A exp(−m 1 t)
, whereA
,m 1
are tting parameters, whi h provide estimates for the parti le massE 1 = m 1
and thematrix element|h0|O i |1i| 2 = 2Am 1
. Tond the tinterval[t min , t max ]
,one usually omputes the so- alledee tive mass:m
e(t) ≡ log
C(t) C(t + 1)
(1.85)
and plotsittond theplateauregion,i.e. the regionwherethe ontribution
of the ex ited states is negligible and the ee tive mass is stable, up to
statisti al u tuations.
Sin e a latti e omputation is usually performed with a nite latti e
ex-tent
T
in the temporal dire tion11 with e.g. periodi boundary onditionsin time, the large-time form of the orrelation fun tion is modied in the
followingway:
In su h ase,the ee tive massattime
t
an beextra ted bysolvingnumer-i ally the equation
C(t)/C(t + 1) = cosh E 1 t − T 2 / cosh E 1 t + 1 − T 2
.
11
However, it is sometimespossible to onsider latti es with innite time extent. An
examplewillbegivenin thenext hapter.
Wenow on entrateonmeson orrelatorsinthe aseof
N f = 2
degeneratequarks. The generalform of aninterpolating operatorfor mesons is:
O i (~x, t) = ¯ ψ(~x, t)Γ i ψ(~x, t),
(1.87)where
Γ
denotes any Dira matrix (an identity matrix, a gamma matrix ora ombinationof gammamatri es).
Expli itly introdu ing Dira (
µ
,ν
,ρ
,σ
) and olour (a
,b
) indi es, theorrelation fun tion an be writtenas:
C(t) = X
~ x
h0| ¯ ψ µ a (~x, t)Γ i µν ψ ν a (~x, t) ¯ ψ ρ b (~0, 0)Γ j ρσ ψ σ b (~0, 0)|0i,
(1.88)where the sum over
~x
Fourier-transforms the orrelation fun tion to zero momentum. Contra tingfermion elds pairwise intofermion propagators:h0|ψ µ a (~x, t) ¯ ψ ν b (0, 0)|0i = S µν ab (~x, t;~0, 0),
(1.89)a ording to Wi k's theorem, one nds that there are two possible
on-tra tions (
ψ ν a (~x, t) ↔ ¯ ψ µ a (~x, t)
,ψ σ b (~0, 0) ↔ ¯ ψ b ρ (~0, 0)
andψ a ν (~x, t) ↔ ¯ ψ ρ b (~0, 0)
,ψ b σ (~0, 0) ↔ ¯ ψ a µ (~x, t)
),whi h lead to:C(t) = X
~ x
Tr
(S(~x, t; ~x, t) Γ i )
Tr(S(~0, 0;~0, 0) Γ j ) +
− X
~ x
Tr
(S(~x, t;~0, 0) Γ i S(~0, 0; ~x, t) Γ j ),
(1.90)where the tra e is overspin and olour.
The rst term in the above expression an be represented by a
dis on-ne ted diagramand ontributesonlytoavour singletmesons. Lateron, we
willbeinterested onlyinavournon-singletmesons,i.e. ones thatare
repre-sentedby onne teddiagrams, orrespondingtothese ondtermintheabove
expression. Hen e, we now drop the rst term and use the
γ 5
-hermiti ity property of the propagator:S(~0, 0; ~x, t) = γ 5 S † (~x, t;~0, 0)γ 5
to rewrite:C(t) = − X
~ x
Tr
(S(~x, t;~0, 0) Γ i γ 5 S † (~x, t;~0, 0) γ 5 Γ j ).
(1.91)Inthisway,toevaluatethis orrelatoritisenoughto omputethepropagator
from a given sour e (lo atedat the origin (
~0, 0
) in the above formula) to allpossiblesinks(alllatti esites(
~x, t
)). Su hpropagatoris alledapoint-to-all propagator. This an be done by solving the following matrix equation:Dψ ˆ µa = η µa
(1.92)Table 1.1: Meson interpolatingoperators.
J P C
lassi ationdenotes parti le spinJ
, parityP
and harge onjugationC
[63℄.hannel
J P C Γ
pseudos alar
0 −+ γ 5
,γ 0 γ 5
s alar
0 ++
1,γ 0
ve tor
1 −− γ i
,γ 0 γ i
axialve tor
1 ++ γ i γ 5
tensor
1 +− γ i γ j
12 times for ea h spin- olour ombination
µa
, with a point sour eη µa
, i.e.a ve tor
(0 . . . 010 . . . 0) T
, where the only non-zero number is pla ed in oneof the rst 12 entries, orrespondingto12 spin- olour omponentsatlatti e
site
(0, 0, 0, 0)
.The solutionof this equation:
ψ µa = ˆ D −1 η µa
(1.93)is the point-to-all quarkpropagator, denoted by
S(~x, t;~0, 0)
ineq. (1.91), inwhi hthe spin- olour indi esare suppressed.
Obviously, the Dira equation (1.92) does not have to be solved with a
point sour e lo ated at the origin. Other hoi es of the sour e an be e.g.
pointsour es with randomlo ationof the sour eor sto hasti sour es. The
latter areof spe ial relevan e fromthepointofviewof thiswork and willbe
dis ussed later.
Table1.1summarizesthemost ommonlyusedmesoninterpolating
oper-ators. The names of dierent hannels ome from the transformation
prop-erties of parti les with respe t to spin and parity. Here we have assumed
that the
Γ
matrix at the sour e (denoted byΓ j
in eq. (1.91)) and at thesink (
Γ i
) are thesame. However, itis alsopossibleto onstru t mesonswithΓ i 6= Γ j
, e.g.Γ i = γ 5
,Γ j = γ 0 γ 5
, whi h belongs to the pseudos alar hannel and hen eit analsobeusedtoextra t themass ofthepseudos alarmeson.From the point of view of further onsiderations, the most important
meson hannel will be the pseudos alar one. The PP orrelation fun tion
(
Γ i = γ 5 ≡ P
,Γ j = γ 5 ≡ P
) is the simplest orrelation fun tion that an be onstru ted. Puttingits gammamatrix stru ture ineq. (1.91),one obtains:C P P (t) = − X
~ x
Tr
(S(~x, t;~0, 0) S † (~x, t;~0, 0)).
(1.94)son (pion)
m π
from the de ay of the PP orrelator and also the pion de ayonstant
f π
from the matrix element|h0|P |πi|
:f π = 2m
m 2 π |h0|P |πi|,
(1.95)where
m
is the bare quark mass.Anequivalent denition of the pion de ay onstant reads:
f π = Z A
m π |h0|A 0 |πi|,
(1.96)where
Z A
isthe renormalization onstant ofthe axial urrent and|h0|A 0 |πi|
the matrix element of this urrent.
Forthe ase of overlapfermions, the
O(a)
-improved interpolating opera-tors formesons are onstru ted inthe following way [64℄:O i ov (~x, t) = ¯ ψ(~x, t)Γ i 1 − a ˆ D ov (0) 2
!
ψ(~x, t) = 1
1 − am 2 ψ(~x, t)Γ ¯ i ψ(~x, t),
(1.97)
where the last equality holds for orrelation fun tions at non-zero physi al
distan e.
We also give here the expressions for baryon interpolating operators
for the proton
p
(uud), the neutronn
(udd) and the deltas∆ ++
(uuu),∆ +
(uud),
∆ 0
(udd),∆ −
(ddd) [65, 66℄, i.e. the o tet and the de uplet baryonsthat ontain only lightquarks (up and down, no strangequarks).
J p = ǫ abc u T a Cγ 5 d b u c ,
(1.98)J n = ǫ abc d T a Cγ 5 u b d c ,
(1.99)J ∆ µ ++ = ǫ abc u T a Cγ µ u b u c ,
(1.100)J ∆ µ + = 1
√ 3 ǫ abc 2 u T a Cγ µ d b u c + u T a Cγ µ u b d c ,
(1.101)J ∆ µ 0 = 1
√ 3 ǫ abc 2 d T a Cγ µ u b d c + d T a Cγ µ d b u c ,
(1.102)J ∆ µ − = ǫ abc d T a Cγ µ d b d c ,
(1.103)where
C = γ 4 γ 2
isthe harge onjugation matrix.The two-point orrelation fun tionfor baryon
B
reads:C B (t) = 1
2 Tr(1 ± γ 4 ) X
~ x
hJ B (~x, t) ¯ J B (~0, 0)i,
(1.104)where
(1 ± γ 4 )/2
isthe parityproje tor. Forexample,the physi alprotonisdes ribed by the orrelation fun tion
C p (t)
with proje tiontopositiveparityand the negativeparityproje tion orresponds tothebaryon
N ∗
, mentionedearlier inthe ontext of spontaneous hiral symmetry breaking.
We will be interested in light baryon masses, whi h are evaluated in an
analogouswayasinthe aseofmesons,i.e. fromtheexponentialfall-oofthe
orresponding orrelation fun tion. The ee tive masses are thus extra ted
numeri ally fromthe ratiosof the orrelationfun tionsof the form(1.86) at
two subsequent timesli es.
Wenishbyshortlydis ussingthedegenera iesbetweenthelightbaryons
in the ase of fermions that preserve isospin symmetry (e.g. overlap) and
violate it (e.g. twisted mass). In the overlap ase, the proton
p
and neutronn
are degenerate,aswellasalldeltabaryons. Forthe twisted mass ase, the degenera y isredu ed, but stillholds betweenp
n
,∆ ++
∆ −
and∆ +
∆ 0
,whi h isdue to
γ 5
-hermiti ity. Therefore, we willalways refer tothe proton and neutron asthenu leonN
, butwe willdistinguish between∆ ++
and∆ +
in the twisted mass ase.
Tree-level s aling test
In this hapter we willshow the results of tree-level s aling tests of overlap,
twistedmassand Creutzfermionsand thusexpli itlydemonstratethe
O(a)
-improvementintheobservables[67,55,56℄. Wewill onsiderthreequantities
the pseudos alar meson mass and de ay onstant and the pseudos alar
orrelationfun tionataxedphysi aldistan e. Wewillalsoanalyzethe ase
when the pseudos alar orrelation fun tion is onstru ted with propagators
orresponding to two dierent fermion dis retizations.
2.1 Fermion propagators
Thetree-leveltestofdierentkindsoflatti efermions onsistsinanalyti ally
evaluating the momentum-spa e fermion propagator and then using it to
onstru t the relevant orrelation fun tion, from whi h the observables of
interest an beextra ted.
2.1.1 Overlap fermions
The startingpointforthe evaluationofthe tree-leveloverlap fermion
propa-gatoristhe freemasslessoverlapDira operatorinmomentumspa e 1
,whi h
was given by Lüs her [29℄:
a ˆ D
ov(p) = 1 −
1 − iaγ µ ˚ p µ − a 2 2 p ˆ 2
1 + a 4 2
X
µ<ν
ˆ
p 2 µ p ˆ 2 ν −1/2
.
(2.1)The massive operator is,a ording to (1.73):
a ˆ D ov (p, m) =
1 − am 2
a ˆ D ov (p) + am,
(2.2)1
An expli itderivationof thisoperatorisgivenin AppendixB.
where
m
is the bare overlap quark mass.The expression for the quarkpropagator in momentum spa e
S ov (p)
anbefound by omputing the inverse of the above Dira operator
a ˆ D ov (p, m)
:S ov (p) = −i(1 − ma 2 )F (p) −1/2 ˚ p µ γ µ + M(p)
1(1 − ma 2 ) 2 F (p) −1 P
µ ˚ p 2 µ + M(p) 2
(2.3)where 1 is the identity matrix in Dira spa e and we have introdu ed the
fun tions:
The propagator has a matrix stru ture in Dira spa e and for later
onve-nien e we write ithere in terms of its omponents:
S ov (p) = S µ ov (p)γ µ + S 0 ov (p)
1.
(2.6)2.1.2 Wilson twisted mass fermions
The twisted mass fermion propagator an be found as an inverse of the
followingDira operator inmomentum spa e:
D ˆ
tm(p) = i˚ p µ γ µ
1f + ar
2 p ˆ 2 µ
11f + m
11f + iµγ 5 τ 3 ,
(2.7)where the relevant notation has been introdu ed in Se tion 1.3.3 and we
show here expli itly the matrix stru ture in avour spa e. The rst three
terms have a trivial stru ture in avour spa e (1
f
is the identity matrix inthis spa e), but the twisted mass term
iµγ 5 τ 3
breaks the isospin symmetrybetween up and down quarks and hen e it modies the expression for the
tree-levelWilsonpropagatorinmomentumspa e(1.51)inthefollowingway:
S ˆ
tm(p) = −i˚ p µ γ µ
1f + ( ar 2 P
µ p ˆ 2 µ + m)
11f − iµγ 5 τ 3
P
µ ˚ p 2 µ + ( ar 2 P
µ p ˆ 2 µ + m) 2 + µ 2 .
(2.8)The propagator has a matrix stru ture in Dira and avour spa e and we
again write it here in terms of its omponents, expli itly distinguishing
be-tween up and down quark propagators:
S tm,u (p) = S µ tm (p)γ µ + S 5 tm (p)γ 5 + S 0 tm (p)
1,
(2.9)S tm,d (p) = S µ tm (p)γ µ − S 5 tm (p)γ 5 + S 0 tm (p)
1,
(2.10)where the propagators of the two avours dier only in the sign of the
γ 5
-matrix oe ient.
Toobtainautomati
O(a)
-improvement(maximaltwist), inthe free the-ory it is enough to set the bare untwisted quark massm
to0. Su h variantof twisted mass fermions is usually referred to as maximally twisted mass
(MTM) fermions.
2.1.3 Creutz fermions
It an beshown [55℄ that the momentum spa e tree-level Dira operatorfor
Creutz fermions an be writtenas:
D Creutz (p) = i X
All notationused inthis subse tion is explainedinAppendix B. This yields
the following formof the fermion propagator:
S Creutz (p) = −i P
The tree-level Dira operator for the variant suggested by Bori i is:
D Borici (p) = i X
where we haveagain introdu ed anauxiliaryfun tion
G µ (p)
.The matrix stru ture of both Creutz and Bori i fermions is of the same
form asin the ase of overlap (2.6).