29 research outputs found
How to get from imaginary to real chemical potential
Using the exactly solvable Gross-Neveu model as theoretical laboratory, we
analyse in detail the relationship between a relativistic quantum field theory
at real and imaginary chemical potential. We find that one can retrieve the
full information about the phase diagram of the theory from an imaginary
chemical potential calculation. The prerequisite is to evaluate and
analytically continue the effective potential for the chiral order parameter,
rather than thermodynamic observables or phase boundaries. In the case of an
inhomogeneous phase, one needs to compute the full effective action, a
functional of the space-dependent order parameter, at imaginary chemical
potential.Comment: revtex, 9 pages, 10 figures; v2: add more references, modify
concluding sectio
The phase diagram of QCD with four degenerate quarks
We revisit the determination of the pseudo-critical line of QCD with four
degenerate quarks at non-zero temperature and baryon density by the method of
analytic continuation. We determine the pseudo-critical couplings at imaginary
chemical potentials by high-statistics Monte Carlo simulations and reveal
deviations from the simple quadratic dependence on the chemical potential
visible in earlier works on the same subject. Finally, we discuss the
implications of our findings for the shape of the pseudo-critical line at real
chemical potential, comparing different possible extrapolations.Comment: 8 pages, 8 figures, 2 table
Critical point of QCD from lattice simulations in the canonical ensemble
A canonical ensemble algorithm is employed to study the phase diagram of QCD using lattice simulations. We lock in the desired quark number sector
using an exact Fourier transform of the fermion determinant. We scan the phase
space below and look for an S-shape structure in the chemical potential,
which signals the coexistence phase of a first order phase transition in finite
volume. Applying Maxwell construction, we determine the boundaries of the
coexistence phase at three temperatures and extrapolate them to locate the
critical point. Using an improved gauge action and improved Wilson fermions on
lattices with a spatial extent of 1.8 \fm and quark masses close to that of
the strange, we find the critical point at and baryon
chemical potential .Comment: 5 pages, 7 figures, references added, published versio
The critical line from imaginary to real baryonic chemical potentials in two-color QCD
The method of analytic continuation from imaginary to real chemical
potentials is one of the few available techniques to study QCD at finite
temperature and baryon density. One of its most appealing applications is the
determination of the critical line for small : we perform a direct test of
the validity of the method in this case by studying two-color QCD, where the
sign problem is absent. The (pseudo)critical line is found to be analytic
around , but a very large precision would be needed at imaginary
to correctly predict the location of the critical line at real .Comment: Replaced with the version accepted for publication as a Rapid
Communication in Physical Review D
Confining strings in representations with common -ality
We study the spectrum of confining strings in SU(3) pure gauge theory, by
means of lattice Monte Carlo simulations, using torelon operators in different
representations of the gauge group. Our results provide direct evidence that
the string spectrum is according to predictions based on -ality. Torelon
correlations in the rank-2 symmetric channel appear to be well reproduced by a
two-exponential picture, in which the lowest state is given by the fundamental
string , the heavier string state is such that the ratio
is approximately given by the Casimir ratio , and the torelon has a much smaller overlap with the
lighter fundamental string state.Comment: 7 pages, 2 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
Degenerate distributions in complex Langevin dynamics: one-dimensional QCD at finite chemical potential
We demonstrate analytically that complex Langevin dynamics can solve the sign
problem in one-dimensional QCD in the thermodynamic limit. In particular, it is
shown that the contributions from the complex and highly oscillating spectral
density of the Dirac operator to the chiral condensate are taken into account
correctly. We find an infinite number of classical fixed points of the Langevin
flow in the thermodynamic limit. The correct solution originates from a
continuum of degenerate distributions in the complexified space.Comment: 20 pages, several eps figures, minor comments added, to appear in
JHE
Finite-size and Particle-number Effects in an Ultracold Fermi Gas at Unitarity
We investigate an ultracold Fermi gas at unitarity confined in a periodic box
using renormalization group (RG) techniques. Within this approach we
can quantitatively assess the long range bosonic order parameter fluctuations
which dominate finite-size effects. We determine the finite-size and
particle-number dependence of universal quantities, such as the Bertsch
parameter and the fermion gap. Moreover, we analyze how these universal
observables respond to the variation of an external pairing source. Our results
indicate that the Bertsch parameter saturates rather quickly to its value in
the thermodynamic limit as a function of increasing box size. On the other
hand, we observe that the fermion gap shows a significantly stronger dependence
on the box size, in particular for small values of the pairing source. Our
results may contribute to a better understanding of finite-size and
particle-number effects present in Monte-Carlo simulations of ultracold Fermi
gases.Comment: 13 pages, 7 figure
Deconfining Phase Transition as a Matrix Model of Renormalized Polyakov Loops
We discuss how to extract renormalized from bare Polyakov loops in SU(N)
lattice gauge theories at nonzero temperature in four spacetime dimensions.
Single loops in an irreducible representation are multiplicatively renormalized
without mixing, through a renormalization constant which depends upon both
representation and temperature. The values of renormalized loops in the four
lowest representations of SU(3) were measured numerically on small, coarse
lattices. We find that in magnitude, condensates for the sextet and octet loops
are approximately the square of the triplet loop. This agrees with a large
expansion, where factorization implies that the expectation values of loops in
adjoint and higher representations are just powers of fundamental and
anti-fundamental loops. For three colors, numerically the corrections to the
large relations are greatest for the sextet loop, ; these
represent corrections of for N=3. The values of the renormalized
triplet loop can be described by an SU(3) matrix model, with an effective
action dominated by the triplet loop. In several ways, the deconfining phase
transition for N=3 appears to be like that in the matrix model of
Gross and Witten.Comment: 24 pages, 7 figures; v2, 27 pages, 12 figures, extended discussion
for clarity, results unchange
EurA1c: the European HbA1c Trial to Investigate the Performance of HbA1c Assays in 2166 Laboratories across 17 Countries and 24 Manufacturers by Use of the IFCC Model for Quality Targets
Background: A major objective of the IFCC Committee on Education and Use of Biomarkers in Diabetes is to generate awareness and improvement of HbA1c assays through evaluation of the performance by countries and manufacturers. Methods: Fresh whole blood and lyophilized hemolysate specimens manufactured from the same pool were used by 17 external quality assessment organizers to evaluate analytical performance of 2166 laboratories. Results were evaluated per country, per manufacturer, and per manufacturer and country combined according to criteria of the IFCC model for quality targets. Results: At the country level with fresh whole blood specimens, 6 countries met the IFCC criterion, 2 did not, and 2 were borderline. With lyophilized hemolysates, 5 countries met the criterion, 2 did not, and 3 were borderline. At the manufacturer level using fresh whole blood specimens, 13 manufacturers met the criterion, 8 did not, and 3 were borderline. Using lyophilized hemolysates, 7 manufacturers met the criterion, 6 did not, and 3 were borderline. In both country and manufacturer groups, the major contribution to total error derived from between-laboratory variation. There were no substantial differences in performance between groups using fresh whole blood or lyophilized hemolysate samples. Conclusions: The state of the art is that 1 of 20 laboratories does not meet the IFCC criterion, but there are substantial differences between country and between manufacturer groups. Efforts to further improve quality should focus on reducing between-laboratory variation. With some limitations, fresh whole blood and well-defined lyophilized specimens are suitable for purpose