1,853 research outputs found
On the dynamical breaking of chiral symmetry: a new mechanism
We consider a U(1) gauge theory, minimally coupled to a massless Dirac field,
where a higher-derivative term is added to the pure gauge sector, as in the
Lee-Wick models. We find that this term can trigger chiral symmetry breaking at
low energy in the weak coupling regime. Then, the fermion field acquires a mass
that turns out to be a function of both the energy scale associated to the
higher-derivative term and the gauge coupling. The dependence of the fermion
mass on the gauge coupling is non-perturbative. Extensions to SU(N) gauge
theories and fermion-scalar interactions are also analyzed, as well as to
theories with massive gauge fields. A few implications of these results in the
framework of quark-mass generation are discussed.Comment: 15 pages 2 figures, a few comments and 4 references added. To appear
in Physical Review
Spontaneous symmetry breaking in a two-doublet lattice Higgs model
An SU(2) lattice gauge theory with two doublets of complex scalar fields is
considered. All continuous symmetries are identified and, using the
nonperturbative methods of lattice field theory, the phase diagram is mapped
out by direct numerical simulation. Two-doublet models contain phase
transitions that separate qualitatively distinct regions of the parameter
space. In some regions global symmetries are spontaneously broken. For some
special choices of the model parameters, the symmetry-breaking order parameter
is calculated. The pattern of symmetry breaking is verified further through
observation of Goldstone bosons.Comment: 24 pages, 13 figures, references added, published versio
Two flavor color superconductivity in nonlocal chiral quark models
We study the competence between chiral symmetry restoration and two flavor
color superconductivity (2SC) using a relativistic quark model with covariant
nonlocal interactions. We consider two different nonlocal regulators: a
Gaussian regulator and a Lorentzian regulator. We find that although the phase
diagrams are qualitative similar to those obtained using models with local
interactions, in our case the superconducting gaps at medium values of the
chemical potential are larger. Consequently, we obtain that in that region the
critical temperatures for the disappearance of the 2SC phase might be of the
order of 100-120 MeV. We also find that for ratios of the quark-quark and
quark-antiquark couplings somewhat above the standard value 3/4, the end point
and triple point in the phase diagram meet and a phase where both the
chiral and diquark condensates are non-negligible appears.Comment: 15 pages incl. 5 Postscript figure
Thermodynamics of lattice QCD with 3 flavours of colour-sextet quarks
We have been studying QCD with 2 flavours of colour-sextet quarks to
distinguish whether it is QCD-like or conformal. For comparison we are now
studying QCD with 3 flavours of colour-sextet quarks, which is believed to be
conformal in the chiral limit. Here we present the results of simulations of
lattice QCD with 3 colour-sextet quarks at finite temperatures on lattices of
temporal extent and 6, with masses small enough to yield access to the
chiral limit. As for the 2-flavour case, we find well-separated deconfinement
and chiral-symmetry restoration transitions, both of which move to appreciably
weaker couplings as is increased from 4 to 6. If this theory is
conformal, we would expect there to be a bulk chiral transition at a fixed
coupling. For this reason we conclude that for and 6, the chiral and
hence the deconfinement transitions are in the strong-coupling domain where the
theory is essentially quenched. The similarity between the behaviours of the 2
and 3 flavour theories suggested that the and 6 transitions for the
2-flavour theory also lie in the strong-coupling domain. The phase structure of
both theories is very similar.Comment: 17 pages Latex(Revtex), 7 postscript figure
Diquark effects in light baryon correlators from lattice QCD
We study the role of diquarks in light baryons through point to point baryon
correlators. We contrast results from quenched simulations with ones with two
flavors of dynamical overlap fermions. The scalar, pseudoscalar and axial
vector diquarks are combined with light quarks to form color singlets. The
quenched simulation shows large zero mode effects in correlators containing the
scalar and pseudoscalar diquark. The two scalar diquarks created by gamma_5 and
gamma_0gamma_5 lead to different behavior in baryon correlators, showing that
the interaction of diquarks with the third light quark matters: we do not see
an isolated diquark. In our quark mass range, the scalar diquark created by
gamma_5 seems to play a greater role than the others.Comment: 12 pages, 11 figure
Finite Density QCD: a New Approach
We introduce a new approach to analyze the phase diagram of QCD at finite
chemical potential and temperature, test it in the Gross-Neveu model at finite
baryon density, and apply it to the study of the chemical potential-temperature
phase diagram of QCD with four degenerate flavors of Kogut-Susskind type.Comment: 21 pages, 9 figures. Some comments and references adde
Thermodynamics of Two Flavor QCD to Sixth Order in Quark Chemical Potential
We present results of a simulation of 2-flavor QCD on a 4x16^3 lattice using
p4-improved staggered fermions with bare quark mass m/T=0.4. Derivatives of the
thermodynamic grand canonical partition function Z(V,T,mu_u,mu_d) with respect
to chemical potentials mu_(u,d) for different quark flavors are calculated up
to sixth order, enabling estimates of the pressure and the quark number density
as well as the chiral condensate and various susceptibilities as functions of
mu_q = (mu_u + mu_d)/2 via Taylor series expansion. Furthermore, we analyze
baryon as well as isospin fluctuations and discuss the relation between the
radius of convergence of the Taylor series and the chiral critical point in the
QCD phase diagram. We argue that bulk thermodynamic observables do not, at
present, provide direct evidence for the existence of a chiral critical point
in the QCD phase diagram. Results are compared to high temperature perturbation
theory as well as a hadron resonance gas model.Comment: 38 pages, 30 encapsulated postscript figures, typo corrected, 1
footnote adde
The QCD phase diagram: A comparison of lattice and hadron resonance gas model calculations
We compare the lattice results on QCD phase diagram for two and three flavors
with the hadron resonance gas model (HRGM) calculations. Lines of constant
energy density have been determined at different baryo-chemical
potentials . For the strangeness chemical potentials , we use two
models. In one model, we explicitly set for all temperatures and
baryo-chemical potentials. This assignment is used in lattice calculations. In
the other model, is calculated in dependence on and
according to the condition of vanishing strangeness. We also derive an
analytical expression for the dependence of on by applying
Taylor expansion of . In both cases, we compare HRGM results on
diagram with the lattice calculations. The agreement is excellent,
especially when the trigonometric function of is truncated up to the
same order as done in lattice simulations. For studying the efficiency of the
truncated Taylor expansion, we calculate the radius of convergence. For zero-
and second-order radii, the agreement with lattice is convincing. Furthermore,
we make predictions for QCD phase diagram for non-truncated expressions and
physical masses. These predictions are to be confirmed by heavy-ion experiments
and future lattice calculations with very small lattice spacing and physical
quark masses.Comment: 25 pages, 8 eps figure
EoS of finite density QCD with Wilson fermions by Multi-Parameter Reweighting and Taylor expansion
The equation of state (EoS), quark number density and susceptibility at
nonzero quark chemical potential are studied in lattice QCD simulations
with a clover-improved Wilson fermion of 2-flavors and RG-improved gauge
action. To access nonzero , we employ two methods : a multi-parameter
reweighting (MPR) in and and Taylor expansion in . The use
of a reduction formula for the Wilson fermion determinant enables to study the
reweighting factor in MPR explicitly and heigher-order coefficients in Taylor
expansion free from errors of noise method, although calculations are limited
to small lattice size. As a consequence, we can study the reliability of the
thermodynamical quantities through the consistency of the two methods, each of
which has different origin of the application limit.
The thermodynamical quantities are obtained from simulations on a lattice with an intermediate quark mass(. The MPR
and Taylor expansion are consistent for the EoS and number density up to
and for the number susceptibility up to . This
implies within a given statistics that the overlap problem for the MPR and
truncation error for the Taylor expansion method are negligible in these
regions.
In order to make MPR methods work, the fluctuation of the reweighting factor
should be small. We derive the equation of the reweighting line where the
fluctuation is small, and show that the equation of the reweighting line is
consistent with the fluctuation minimum condition.Comment: 20 pages, 11 figures. Accepted to JHEP. Discussions are added.
Figures for Taylor coefficients (Fig. 7) are modifie
Results from Lattice QCD
I present our recent results on the critical end point in the \mu_B-T phase
diagram of QCD with two flavours of light dynamical quarks and compare them
with similar results from other groups. Implications for a possible energy scan
at the RHIC are discussed. I also comment briefly on the new results of great
relevance to heavy ion collisions from finite temperature lattice QCD
simulations on speed of sound, specific heat and on the fate of J/\psi.Comment: Invited Plenary Talk given at 5th International Conference on Physics
and Astrophysics of Quark Gluon Plasma, Kolkata, India, February 8-12, 2005;
LaTeX in Journal of Physics G style; 9 pages including figure
- …