Unquenched Case Research Articles

One-loop unquenched three-gluon and ghost-gluon vertices in the Curci-Ferrari model

In this article we study the unquenched three-gluon and ghost-gluon vertex in the entire momentum range from the ultraviolet to infrared regime using the Curci-Ferrari model at one loop in Landau gauge, as an extension of the results presented in by Pelaez et al. [Phys. Rev. D 88, 125003 (2013)]. Results are compared with recent lattice data for SU(3) in the unquenched case. This calculation is a pure prediction of the model because it does not require fixing any parameter once two-point functions are fitted. An analysis of the influence of dynamical quarks at the position of the zero crossing of the three-gluon vertex is presented. Due to the recent improvements in infrared lattice data for the quenched three-gluon correlation function [Phys. Lett. B 818, 136352 (2021)], we also redo the comparison between our one-loop results in this limit and the lattice results, obtaining a very good match.

Controlling the persistence of photoconductivity through additional sub-bandgap photoexcitation in individual m-axial GaN nanowires

The persistence of photoconductivity after switching off the photoexcitation is investigated in individual m-axial n-GaN nanowires as a function of temperature. At room temperature, photoconductivity is found to decay with a time scale of several hours. The capture barrier height is estimated to be ∼450 meV from the stretched exponential fitting of the decay characteristics recorded at different temperatures. This energy value is found to be much less than the surface band-bending energy of ∼770 meV, which is believed to act as the capture barrier in this system. This finding indicates the tunneling of electrons through the top part of the band-bending barrier. Interestingly, the decay rate of photoconductivity is observed to reduce significantly when the photoconductivity in these wires is quenched by an additional sub-bandgap illumination prior to the switching off the photoexcitation. A rate equation model is proposed to explain the upward band bending at the surface as well as the persistent photoconductivity effect in terms of the transfer of holes between the valence band and acceptor-type surface states of the nanowires. Photoconductivity decay profiles simulated from the model are found to match very well with the experimental data recorded at different temperatures in both quenched and unquenched cases.

Bayesian analysis of quark spectral properties from the Dyson-Schwinger equation

We report results on the quark spectral function in the Landau gauge at finite temperature determined from its Dyson-Schwinger equation. Compared to earlier quenched results this study encompasses unquenched $N_f=2+1$ fermion flavors in the medium. For the reconstruction of real-time spectra we deploy a recent Bayesian approach (BR method) and develop a new prior in order to better assess the inherent systematic uncertainties. We identify the quark quasi-particle spectrum and analyze the (non-)appearance of zero modes at or around the pseudo-critical temperature. In both, the fully unquenched system and a simpler truncation using a model for the gluon propagator we observe a characteristic three-peak structure at zero three-momentum. The temperature dependence of these structures in case of the gluon propagator model is different than observed in previous studies. For the back-coupled and unquenched case we find interesting modifications at and around the pseudo-critical transition temperature.

Phase structure and propagators at nonvanishing temperature for QCD and QCD-like theories

We investigate the universality of truncation schemes for Dyson-Schwinger equations developed for quantum chromodynamics in theories which differ from quantum chromodynamics only in the gauge group. Our specific choices are the gauge groups $SU(2)$ and $G_2$, for which lattice calculations at nonvanishing chemical potential are possible. Thus, corresponding calculations can provide benchmarks for testing calculations with functional equations. We calculate the quark and gluon propagators and determine the chiral and dual chiral condensates at vanishing density to determine the confinement/deconfinement and chiral transitions, respectively. We can reproduce the expected type of transitions in the quenched and unquenched cases. In general, all three theories react very similarly to modifications of the employed model for the quark-gluon vertex.

Finite volume for three-flavour Partially Quenched Chiral Perturbation Theory through NNLO in the meson sector

We present a calculation of the finite volume corrections to meson masses and decay constants in three flavour Partially Quenched Chiral Perturbation Theory (PQChPT) through two-loop order in the chiral expansion for the flavour-charged (or off-diagonal) pseudoscalar mesons. The analytical results are obtained for three sea quark flavours with one, two or three different masses. We reproduce the known infinite volume results and the finite volume results in the unquenched case. The calculation has been performed using the supersymmetric formulation of PQChPT as well as with a quark-flow technique. Partial analytical results can be found in the appendices. Some examples of cases relevant to lattice QCD are studied numerically. Numerical programs for all results are available as part of the CHIRON package.

Finite volume and partially quenched QCD-like effective field theories

We present a calculation of the meson masses, decay constants and quark-antiquark vacuum expectation value for the three generic QCD-like chiral symmetry breaking patterns SU(N-F) x SU(N-F) -> SU(N-F)(V), SU(N-F) -> SO(N-F) and SU(2N(F)) -> Sp(2N(F)) in the effective field theory for these cases. We extend the previous two-loop work to include effects of partial quenching and finite volume. The calculation has been performed using the quark flow technique. We reproduce the known in finite volume results in the unquenched case. The analytical results can be found in the supplementary material. Some examples of numerical results are given. The numerical programs for all cases are included in version 0.54 of the CHIRON package. The purpose of this work is the use in lattice extrapolations to zero mass for QCD-like and strongly interacting Higgs sector lattice calculations. (Less)

Highly sensitive and selective detection of dopamine using one-pot synthesized highly photoluminescent silicon nanoparticles.

A simple and highly efficient method for dopamine (DA) detection using water-soluble silicon nanoparticles (SiNPs) was reported. The SiNPs with a high quantum yield of 23.6% were synthesized by using a one-pot microwave-assisted method. The fluorescence quenching capability of a variety of molecules on the synthesized SiNPs has been tested; only DA molecules were found to be able to quench the fluorescence of these SiNPs effectively. Therefore, such a quenching effect can be used to selectively detect DA. All other molecules tested have little interference with the dopamine detection, including ascorbic acid, which commonly exists in cells and can possibly affect the dopamine detection. The ratio of the fluorescence intensity difference between the quenched and unquenched cases versus the fluorescence intensity without quenching (ΔI/I) was observed to be linearly proportional to the DA analyte concentration in the range from 0.005 to 10.0 μM, with a detection limit of 0.3 nM (S/N = 3). To the best of our knowledge, this is the lowest limit for DA detection reported so far. The mechanism of fluorescence quenching is attributed to the energy transfer from the SiNPs to the oxidized dopamine molecules through Förster resonance energy transfer. The reported method of SiNP synthesis is very simple and cheap, making the above sensitive and selective DA detection approach using SiNPs practical for many applications.

On the foundations of partially quenched chiral perturbation theory

It has been widely assumed that partially quenched chiral perturbation theory is the correct low-energy effective theory for partially quenched QCD. Here we present arguments supporting this assumption. First, we show that, for partially quenched QCD with staggered quarks, a transfer matrix can be constructed. This transfer matrix is not Hermitian, but it is bounded, and it can be used to construct correlation functions in the usual way. Combining these observations with an extension of the Vafa--Witten theorem to the partially quenched theory allows us to argue that the partially quenched theory satisfies the cluster property. By extending Leutwyler's analysis of the unquenched case to the partially quenched theory, we then conclude that the existence and properties of the transfer matrix as well as clustering are sufficient for partially quenched chiral perturbation theory to be the correct low-energy theory for partially quenched QCD.

On the Landau-gauge adjoint quark propagator

Quarks in the adjoint representation have been a subject of study for both conceptual and practical purposes. Conceptually, their differences when it comes to confining and chiral symmetry properties has long been suspected to hold important information on the relation of these two distinguished properties of QCD-like gauge theories. Practically, they have been studied as both a possibility to access finite density quark systems as well as candidate theories for technicolor in beyond-the-standard-model settings. The most elementary object describing such particles is their propagator, though it being gauge-dependent. Its properties in the minimal Landau gauge are investigated here both in the quenched and unquenched case for a range of lattice parameters using the Wilson formulation for the gauge group SU(2). It is found that the propagator shows pronounced differences to the case of fundamental quarks, especially towards the chiral limit.

Precision Determination ofr0ΛMS¯from the QCD Static Energy

We use the recently obtained theoretical expression for the complete QCD static energy at next-to-next-to-next-to leading-logarithmic accuracy to determine r₀ΛMS by comparison with available lattice data, where r₀ is the lattice scale and ΛMS is the QCD scale. We obtain r₀ΛMS=0.637(+0.032)(-0.030) [corrected] for the zero-flavor case. The procedure we describe can be directly used to obtain r₀ΛMS in the unquenched case, when unquenched lattice data for the static energy at short distances becomes available. Using the value of the strong coupling α(s) as an input, the unquenched result would provide a determination of the lattice scale r₀.

The topological susceptibility from grand canonical simulations in the interacting instanton liquid model: Chiral phase transition and axion mass

This is the last in a series of papers on the topological susceptibility in the interacting instanton liquid model (IILM). We will derive improved finite temperature interactions to study the thermodynamic limit of grand canonical Monte Carlo simulations in the quenched and unquenched case with light, physical quark masses. In particular, we will be interested in chiral symmetry breaking. The paper culminates by giving, for the first time, a well-motivated temperature-dependent axion mass. Especially, this work finally provides a computation of the axion mass in the low temperature regime, m a 2 f a 2 = 1.46 × 10 − 3 Λ 4 1 + 0.50 T / Λ 1 + ( 3.53 T / Λ ) 7.48 . It connects smoothly to the high temperature dilute gas approximation; the latter is improved by including quark threshold effects. To compare with earlier studies, we also provide the usual power-law m a 2 = α a Λ 4 f a 2 ( T / Λ ) n , where Λ = 400 MeV , n = 6.68 and α = 1.68 × 10 − 7 .

Is the chiral phase transition induced by a metal-insulator transition?

We investigate the QCD Dirac operator with gauge configurations given by a liquid of instantons in the region of temperatures about the chiral phase transition. Both the quenched and unquenched cases are examined in detail. We present evidence of a localization transition in the low lying modes of the Dirac operator around the same temperature as the chiral phase transition. Thus both level statistics and eigenvectors of the QCD Dirac operator at the chiral phase transition have similar properties than those of a disordered conductor at the metal-insulator transition. This strongly suggests the phenomenon of Anderson localization (localization by destructive quantum interference) is the leading physical mechanism in the restoration of the chiral symmetry. Finally we argue that our findings are not in principle restricted to the ILM approximation and may also be found in lattice simulations.

Chiral phase transition and Anderson localization in the instanton liquid model for QCD

We study the spectrum and eigenmodes of the QCD Dirac operator in a gauge background given by an instanton liquid model (ILM) at temperatures around the chiral phase transition. Generically we find the Dirac eigenvectors become more localized as the temperature is increased. At the chiral phase transition, both the low lying eigenmodes and the spectrum of the QCD Dirac operator undergo a transition to localization similar to the one observed in a disordered conductor. This suggests that Anderson localization is the fundamental mechanism driving the chiral phase transition. We also find an additional temperature dependent mobility edge (separating delocalized from localized eigenstates) in the bulk of the spectrum which moves toward lower eigenvalues as the temperature is increased. In both regions, the origin and the bulk, the transition to localization exhibits features of a 3D Anderson transition including multifractal eigenstates and spectral properties that are well described by critical statistics. Similar results are obtained in both the quenched and the unquenched case though the critical temperature in the unquenched case is lower. Finally we argue that our findings are not in principle restricted to the ILM approximation and may also be found in lattice simulations.

Non-perturbative renormalisation and improvement of the local vector current for quenched and unquenched Wilson fermions

By considering the local vector current between nucleon states and imposing charge conservation, we determine its renormalisation constant and quark mass improvement coefficient for Symanzik O(a) improved Wilson fermions. The computation is first performed for quenched fermions (and for completeness also with unimproved fermions) and compared against known results. The two-flavour unquenched case is then considered.

Lattice simulations of the strange quark mass and Fritzsch texture

A number of numerical simulations of lattice gauge theory have indicated a low mass of strange quark in 100 MeV range at the scale of μ=2 GeV. In the unquenched case, which is improved over the simulation in the quenched approximation by the inclusion of u and d sea quark effects, one sees a further downward trend. Here the fermion mass spectrum of the Fritzsch texture is recalculated. In a single step supersymmetric GUT with M X ∼10 16 GeV such values of the strange quark mass can be obtained for low values of tan β. Experimental numbers m pole t =173±6 GeV and 4.1< m b ( m b )<4.4 GeV are used in this study. Since the scenario is supersymmetric, gaugino loop diagrams contribute to the masses in addition to usual tree level Yukawa contributions. Upper bound of the mixing parameter V cb is taken at 0.045.

The confining string and its breaking in QCD

We point out that the world sheet swept by the confining string in the presence of dynamical quarks can belong to two different phases, depending on the number of charge species and the quark masses. When it lies in the normal phase (as opposed to the tearing one) the string breaking is invisible in the Wilson loop, while is manifest in operators composed of disjoint sources, as observed in many numerical experiments. We work out an explicit formula for the correlator of Polyakov loops at finite temperature, which is then compared with recent lattice data, both in the quenched case and in the presence of dynamical quarks. The analysis in the quenched case shows that the free bosonic string model describes accurately the data for distances larger than ∼ 0.75 fm. In the unquenched case we derive predictions of the dependence of the static potential on the temperature which are compatible with the lattice data.

Quenched supersymmetry

We study the effects of quenching in N = 1 Super-Yang-Mills theory. While supersymmetry is broken, the Lagrangian acquires a new flavour U(1|1) symmetry. The anomaly structure thus differs from the unquenched case. We derive the corresponding low-energy effective Lagrangian. As a consequence, we predict the mass splitting expected in numerical simulations for particles belonging to the lowest-lying supermultiplet. An estimate of the systematic error due to quenching follows.

Quenched supersymmetry

We study the effects of quenching in Super-Yang-Mills theory. While supersymmetry is broken, the lagrangian acquires a new flavour U(1 | 1) symmetry. The anomaly structure thus differs from the unquenched case. We derive the corresponding low-energy effective lagrangian. As a consequence, we predict the mass splitting expected in numerical simulations for particles belonging to the lowest-lying supermultiplet.

αs from the non-perturbatively renormalised lattice three-gluon vertex

We compute the running QCD coupling on the lattice by evaluating two-point and three-point off-shell gluon Green's functions in a fixed gauge and imposing non-perturbatioe renormalisation conditions on them. Our exploratory study is performed in the quenched approximation at β = 6.0 on 16 4 and 24 4 lattices. We show that, for momenta in the range 1.8–2.3 GeV, our coupling runs according to the two-loop asymptotic formula, allowing a precise determination of the corresponding Λ parameter. The role of lattice artifacts and finite-volume effects is carefully analysed and these appear to be under control in the momentum range of interest. Our renormalisation procedure corresponds to a momentum subtraction scheme in continuum field theory, and therefore lattice perturbation theory is not needed in order to match our results to the MS scheme, thus eliminating a major source of uncertainty in the determination of α MS Our method can be applied directly to the unquenched case.

Finite temperature QED 3 with light fermions

Non-compact QED 3 is simulated both in the quenched and unquenched cases. In particular, we investigate the restoration of chiral symmetry at finite temperature. We also compute the zero temperature spectrum of the theory, including (in the quenched case) the dynamical fermion mass. From these two set of data, one can obtain estimates for the ratio of the mass gap to the critical temperature, of particular interest for applications to high- T c superconductivity.