qcd thermal sum rules: general aspects and applications
TRANSCRIPT
QCD Thermal Sum Rules: General
Aspects and Applications
M. Loewe
Pontificia Universidad Católica de Chile
Morelia, Mexico, April 2011
This talk is based on several articles written along
many years in collaboration with C. A. Dominguez
(University of Cape Town, South Africa) and
different coworkers.
I acknowledge support form : FONDECYT 1095217 and
Proyecto Anillos ACT119 (CHILE) and NRF (South Africa)
• The plan is the following:
• 1) A general introduction to QCD Sum Rules
• 2) The extension to a finite temperature scenario
• 3) Comments on the light-light and heavy-light quarks systems.
• 4) Surviving of Charmonium states
The Idea of QCD Sum Rules is to establish a connection between fundamental aspects of QCD and the hadronic world.
It is based on:
1) The analytic structure of correlators (Cauchy Theorem) which implies global duality
2) The validity of the OPE (also in the presence
of non perturbative effects)
It involves both perturbative as non perturbative
contributions.
The usual resumation implies a
shift in the pole of the propagator
Let us consider a two point correlator
Confinement effects are parametrized through quark and
gluon condensates
The red area denotes
the quark condensate
The red area denotes
the gluon condensate
(infrared dynamics)
There are many other
Condensates…..
Here we have the
four-quark condensate
Dimension 6
In principle there are several condensates of increasing
order. In this way we will get an expansion of the form
For example (light quark sector) for the
Axialvector-Axialvector
correlator we have the current Aμ
The Fourier-transform of the correlator produces the tensor
such that
where
Notice that the operators are ordered by increasing
dimensionality and the coefficients fall down by powers
of Q2 (i.e. non-perturbative contributions). In most
applications higher condensates may be ignored
The correlator can also be expressed in the hadronic sector.
Bound states correspond to poles on the real axis and
resonances lie on the second Riemann sheet. (Complex
s-plane)
In principle we have also cuts associated to non resonant
multiple particle production.
By integrating along the contour, Cauchy’s theorem
produces immediately a set of FESR.
On the cut we have the hadronic resonances.
On the circle a description in terms of QCD
degrees of freedom is assumed to be valid.
• hadron
Since the Hadronic spectral function has the form
We immediatelly find
And this provides us with the following set of Finite
Energy Sum Rules
Where the dimensionless perturbative PQCD
moments M N are given by
If PQCD reduces only to the quark loop we
have (axial-axial correlator)
Thermal Extension of the QCD Sum Rules
• There are important differences:
• 1) The vacuum is populated (a thermal vacuum)
• 2) A new analytic structure in the complex
s-plane appears, due to scattering. This effect
turns out to be very important
Let us consider a typical current correlator in a
thermal vacuum (a populated vacuum)
We can write the standard spectral
representation
Normal analytic structure of the spectral function
for current correlators in the s plane (at zero
temperature):
New effect in the pressence of a populated vacuum:
Annihilation + Scattering contributions to the spectral
function
Annihilation term (survives when T → 0)
New Effect:The current may scatter off particles in
the populated vacuum. Notice that this term
vanishes when T→0 (Bochkarev and Schaposnikov, Nucl.
Phys. B268 (1986) 220)
New cut associated to a scattering process with
quarks (antiquarks) in the populated vacuum
• Let us consider a toy model: The non
linear O(N) σ model
Such that f is the coupling
constant
In the Large N limit the theory is asymptotically free
There are condensates
All other condensates factorize.
Where the α field is given by
S
Supose we are interested in the Green function
associated to the propagation of the α field. At the one
Loop order
Expanding
• Using the scalar current
the OPE provides exactly the same
answer.
For the finite temperature corrections we can do
the calculation exactly
with
• Our results for the light-light and heavy light quark systems provides us with the following picture
1) The hadronic mass remains almost constant as function of T up to T → Tc .
2) The hadronic widths grow with T, having a divergent behavior when T → Tc.
3) In the case of effective coupling constants like gρππ ,
gπNN, the axial vector coupling of the nucleon gA, as well as form factors like Fπ, it turns out that the couplings
vanish and the mean square associated radius diverge
when T→Tc
In all cases, the continuum threshold S0(T) moves
to the left, going to the threshold where the
resonances start to appear. In fact Tc is defined
as the temperature where this occurs.
As an example, and without going into details let
us consider now the three point πNN correlator.
Here we have to deal with the three point correlator
The nucleon (η) and pion (J 5) interpolating
currents are defined as
We have the following definition
Relevant
diagrams
Thermal behavior of the coupling constant
I will present our last results in the
Charmonium sector
1)The vector J/Ψ channelC. A. Dominguez, M. Loewe, J. C. Rojas and Y. Zhang
Phys. Rev D 81 (2010) 014007
2) The pseudo- and scalar sectors (ηc, χ)C. A. Dominguez, M. Loewe, J. C. Rojas and Y. Zhang
Phys. Rev D 83 (2011) 034033
a)Our goal is to discuss the thermal behavior of the
hadronic parameters of Charmonium (vector channel
J/Ψ, pseudoscalar ηc and scalar ) from the
perspective of
Thermal Hilbert Moment QCD Sum Rules
c) Total width
d) Continuum Threshold
Thermal behavior of J/Ψ is an important
signal in the Physics of heavy Ion collisions.
a) Mass
b) Coupling: Leptonic Decay
Going into the details:
Charmonium (vector channel) seems to have a curious
bahaviour at finite temperature: Surviving beyond the
critical temperatureA Finite Temperature QCD Sum Rule Discussion
The basic object:
Satisfies a once substracted dispersion relation
The Thermal Average is defined as
The vector current is
The imaginary part involves two pieces:
a) Time like region (q2 ≥ 4mQ2)
In the rest frame
b) Space-like region (q2 ≤ 0). Im ΠS(q2,T) Vanishes for T=0:
In the rest frame of the thermal bath reduces to
The only non-perturbative relevant object is the Gluon Condensate
Going into the hadron representation of our correlator (cero width)
The leptonic decay constant is
In the finite width case
To complete the hadronic parametrization we need the hadronic scattering
term due to the current scattering off heavy-light pseudoscalar D-mesons
This term is 2-3 orders of magnitude smaller than it’s QCD counterpart
To eliminate the substraction we use Hilbert moments:
The Sum Rules
Cauchy Theorem implies quark-hadron duality.
The Hilbert Moments become Finite Energy Sum Rules (FESR)
where
and
The non perturbative term (dimension 4)
where and
The thermal dependence (low T) of the gluon condensate
can be approximate as
Hypergeometric
function
A subtle point
A gluonic twist-two term in the OPE has been considered
by Klingl et al and computed on the lattice. This implies
a modification of the non perturbative QCD moments
For small N the corrections are of the order 2-6% independent
of Q2
Lattice QCD is the right tool to go beyond the low
temperature regime (Boyd and Miller)
T* ≈ 150 MeV T*C ≈ 250
MeVFirst we determine s0 and Q2 at T=0 using as imput: MV = 3.097 GeV,
fV= 196 MeV and ΓV = 93.2 KeV, mQ= 1.3 GeV
In the zero width approximation we find
Given the extreme small total width of the J/Ψ this relation also holds
at finite width
This relation depends on s0 and Q2. Together with
the duality relation, N=1 we get
Reproduces the experimantal
values of mass coupling and
width within 1%.
Valid up to Q2~20 GeV2
as N up to ~ 10The different choices do not
produce a qualitative change at finite T
other than 10% of Tc (Tc = 140 MeV)
Let us go to our results……
The mass, in the zero
width follows from
The width can be
determined from
Finally, the leptonic decay
constant
can be determined from e.g.
The rise of the coupling beyond Tc is the result of a subtle
balance between different contributions:
If T increases, s0(T) decreases while the QCD scattering
moment increases ≈ annhilation moment at T ~ 160 MeV.
The non perturbative moment stays approx. constant and
negative mostly cancelling the annhilation moment.
The hadronic moment dividing the QCD moment is a
decreasing function of T implying an increase in the
coupling.
This behavior (coupling and width) is stable if Q2 ~ 10-20
GeV2 and N ~ 1-10
• The pseudoscalar and scalar channels
(ηc and χ)
with
We follow almost the same procedure.
The main difference comes from the absence of
scattering terms in both channels.
Our results:
• S0(T)/S0(0) as function of T/Tc for Tc= 180 MeV
Curve (a) is for ηc and curve (b) for χc.
Evolution of the ratio of the ηc mass M(t)/M(0)
for Tc= 180 MeV (Similar results for χc)
The ratio of the ηc width Γ(T)/Γ(0) for Tc= 180MeV
The ratio of the χc widths as function of T/Tc for
Tc= 180 MeV
The ratio of the leptonic decay constants f as function
of T/Tc. Curve (a) for ηc and curve (b) for χc
Other Approaches to this Problem
• 1) The Lattice community has found strong evidence
supporting the surviving of 1S charmonium sates beyond
the critical temperature. (Umeda, Nomura, Matsufuru: Eur.
Phys.J.C 39S1 (2005) 9; S. Datta, F. Karsch, P. Petreczky,
Wetzorke: Phys. Rev. D 69 (2004) (094507
With the kernel
There has been many attempts to understand
the problem form the perspective of Potential
Models
• A, Mócsy and P. Petreczky: PRL 99 (2007) 211602;
PRD 73 (2006) 074007. Several screened potentials are
used
• However, this approach fails to reproduce the results
from the lattice!! Probably this is related to the analytic
structure. The central cut here is absent.
Conclusions:
At low T the J/ψ behaves as other light-light and heavy-light
quark resonances: s0 and fV decrease and the width
increases with increasing T. However…..
As T approaches Tc the coupling increases and the
width decreases.
This scenario is new: the scattering
QCD contribution to the spectral function as well
as the peculiar Gluon condensate thermal behavior
are responsible for this.
A strong indication for surviving of
Charmonium beyond Tc
•Thank You