87 research outputs found
Higher Loop Results for the Plaquette, Using the Clover and Overlap Actions
We calculate the perturbative value of the free energy in QCD on the lattice.
This quantity is directly related to the average plaquette.
Our calculation is done to 3 loops using the clover action for fermions; the
results are presented for arbitrary values of the clover coefficient, and for a
wide range of fermion masses.
In addition, we calculate the 2 loop result for the same quantity, using the
overlap action.Comment: 3 pages, 1 figure. Presented at Lattice2004(improved
The Lattice Free Energy of QCD with Clover Fermions, up to Three-Loops
We calculate the perturbative value of the free energy in Lattice QCD, up to
three loops. Our calculation is performed using Wilson gluons and the
Sheikholeslami - Wolhert (clover) improved action for fermions.
The free energy is directly related to the average plaquette. To carry out
the calculation, we compute all relevant Feynman diagrams up to 3 loops, using
a set of automated procedures in Mathematica; numerical evaluation of the
resulting loop integrals is performed on finite lattice, with subsequent
extrapolation to infinite size.
The results are presented as a function of the fermion mass m, for any
SU(N_c) gauge group, and for an arbitrary number of fermion flavors. In order
to enable independent comparisons, we also provide the results on a per diagram
basis, for a specific mass value.Comment: 13 pages, 5 figures, 8 table
The Lattice Free Energy with Overlap Fermions: A Two-Loop Result
We calculate the 2-loop partition function of QCD on the lattice, using the
Wilson formulation for gluons and the overlap-Dirac operator for fermions.
Direct by-products of our result are the 2-loop free energy and average
plaquette. Our calculation serves also as a prototype for further higher loop
calculations in the overlap formalism. We present our results as a function of
a free parameter entering the overlap action; the dependence on the
number of colors and fermionic flavors is shown explicitly.Comment: 10 pages, 5 figures. Final version to appear in Physical Review D. A
missing overall factor was inserted in Eq. 12; it affects also Eq. 1
On the zero crossing of the three-gluon vertex
We report on new results on the infrared behaviour of the three-gluon vertex
in quenched Quantum Chormodynamics, obtained from large-volume lattice
simulations. The main focus of our study is the appearance of the
characteristic infrared feature known as 'zero crossing', the origin of which
is intimately connected with the nonperturbative masslessness of the
Faddeev-Popov ghost. The appearance of this effect is clearly visible in one of
the two kinematic configurations analyzed, and its theoretical origin is
discussed in the framework of Schwinger-Dyson equations. The effective coupling
in the momentum subtraction scheme that corresponds to the three-gluon vertex
is constructed, revealing the vanishing of the effective interaction at the
exact location of the zero crossing.Comment: 6 pages, 4 figure
Infrared regime of SU(2) with one adjoint Dirac flavor
SU(2) gauge theory with one Dirac flavour in the adjoint representation is investigated on a lattice. Initial results for the gluonic and mesonic spectrum, static potential from Wilson and Polyakov loops, and the anomalous dimension of the fermionic condensate from the Dirac mode number are presented. The results found are not consistent with conventional confining behaviour, instead tentatively pointing towards a theory lying within or very near the onset of the conformal window, with the anomalous dimension of the fermionic condensate in the range 0.9≲γ∗≲0.95. The implications of our work for building a viable theory of strongly interacting dynamics beyond the standard model are discussed
Instanton liquid properties from lattice QCD
We examined the instanton contribution to the QCD configurations generated from lattice QCD for N-F = 0, N-F = 2 + 1 and NF = 2 + 1 + 1 dynamical quark flavors from two different and complementary approaches. First via the use of Gradient flow, we computed instanton liquid properties using an algorithm to localize instantons in the gauge field con figurations and studied their evolution with flow time. Then, the analysis of the running at low momenta of gluon Green\u27s functions serves as an independent confirmation of the instanton density which can also be derived without the use of the Gradient flow
SO(2N) and SU(N) gauge theories in 2+1 dimensions
We perform an exploratory investigation of how rapidly the physics of SO(2N)
gauge theories approaches its N=oo limit. This question has recently become
topical because SO(2N) gauge theories are orbifold equivalent to SU(N) gauge
theories, but do not have a finite chemical potential sign problem. We consider
only the pure gauge theory and, because of the inconvenient location of the
lattice strong-to-weak coupling 'bulk' transition in 3+1 dimensions, we largely
confine our numerical calculations to 2+1 dimensions. We discuss analytic
expectations in both D=2+1 and D=3+1, show that the SO(6) and SU(4) spectra do
indeed appear to be the same, and show that a number of mass ratios do indeed
appear to agree in the large-N limit. In particular SO(6) and SU(3) gauge
theories are quite similar except for the values of the string tension and
coupling, both of which differences can be readily understood.Comment: 27 pages, 9 figure
The Lorentz-invariant boundary action of the confining string and its universal contribution to the inter-quark potential
We study the boundary contribution to the low energy effective action of the
open string describing the confining flux tube in gauge theories. The form of
the boundary terms is strongly constrained by the requirement of Lorentz
symmetry, which is spontaneously broken by the formation of a long confining
flux tube in the vacuum. Writing the boundary action as an expansion in the
derivatives of the Nambu-Goldstone modes describing the transverse fluctuations
of the string, we single out and put in a closed form the first few Lorentz
invariant boundary terms. We also evaluate the leading deviation from the
Nambu-Goto string produced by the boundary action on the vacuum expectation
value of the Wilson loop and we test this prediction in the 3d Ising gauge
model. Our simulation attains a level of precision which is sufficient to test
the contribution of this term.Comment: 17 pages, 5 figures, LateX 2e. V2: Final version published on JHEP.
Fixed typos in eq.s 2.2, 2.3, 3.7, 3.8, A.4. Extended explanation of the
procedures used in sec 2 to determine the possible boundary terms up to field
redefinitions and of the procedure used in sec 4 to take the continuum limit.
V3: typos corrected in eq.s (4.3) (4.5) and (4.6), acknowledgements adde
Linear broadening of the confining string in Yang-Mills theory at low temperature
The logarithmic broadening predicted by the systematic low-energy effective
field theory for the confining string has recently been verified in numerical
simulations of (2+1)-d SU(2) lattice Yang-Mills theory at zero temperature. The
same effective theory predicts linear broadening of the string at low non-zero
temperature. In this paper, we verify this prediction by comparison with very
precise Monte Carlo data. The comparison involves no additional adjustable
parameters, because the low-energy constants of the effective theory have
already been fixed at zero temperature. It yields very good agreement between
the underlying Yang-Mills theory and the effective string theory.Comment: 10 pages, 3 figures. Version published in JHEP; improved figures 1
and
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