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F. Farchioni

Publications and source records attributed to F. Farchioni.

At least 37 records · Page 2Linked to original sources
arXiv2005

Twisted mass fermions: neutral pion masses from disconnected contributions

Twisted mass fermions allow light quarks to be explored but with the consequence that there are mass splittings, such as between the neutral and charged pion. Using a direct calculation of the connected neutral pion correlator and stochastic methods to evaluate the disconnected correlations, we determine the neutral pion mass. We explore the dependence on lattice spacing and quark mass in quenched QCD. For dynamical QCD, we determine the sign of the splitting which is linked, via chiral PT, to the nature of the phase transition at small quark mass.

hep-lat↗
arXiv2005

Lattice Spacing Dependence of the First Order Phase Transition for Dynamical Twisted Mass Fermions

Lattice QCD with Wilson fermions generically shows the phenomenon of a first order phase transition. We study the phase structure of lattice QCD using Wilson twisted mass fermions and the Wilson plaquette gauge action are used in a range of beta values where such a first order phase transition is observed. In particular, we investigate the dependence of the first order phase transition on the value of the lattice spacing. Using only data in one phase and neglecting possible problems arising from the phase transition we are able to perform a first scaling test for physical quantities using this action.

hep-lat↗
arXiv2005

The phase structure of lattice QCD with two flavours of Wilson quarks and renormalization group improved gluons

The effect of changing the lattice action for the gluon field on the recently observed [1] first order phase transition near zero quark mass is investigated by replacing the Wilson plaquette action by the DBW2 action. The lattice action for quarks is unchanged: it is in both cases the original Wilson action. It turns out that Wilson fermions with the DBW2 gauge action have a phase structure where the minimal pion mass and the jump of the average plaquette are decreased, when compared to Wilson fermions with Wilson plaquette action at similar values of the lattice spacing. Taking the DBW2 gauge action is advantageous also from the point of view of the computational costs of numerical simulations.

hep-lat↗
arXiv2005

Twisted mass quarks and the phase structure of lattice QCD

The phase structure of zero temperature twisted mass lattice QCD is investigated. We find strong metastabilities in the plaquette observable when the untwisted quark mass assumes positive or negative values. We provide interpretations of this phenomenon in terms of chiral symmetry breaking and the effective potential model of Sharpe and Singleton.

hep-lat↗
arXiv2004

Unquenched simulations with N_f=2 light quark flavours

The quark mass dependence of meson masses and decay constants in N_f=2 QCD is studied at a fixed gauge coupling with different dynamical quark masses. Partially Quenched Wilson Chiral Perturbation Theory (PQWchiPT) is applied to obtain the extrapolation to zero quark mass. It is shown, that in our analysis NNLO-terms play a more important role than O(a)-terms, which can be neglected. Also we compare our results with recent CP-PACS results.

hep-lat↗
arXiv2004

The Volume Source Technique for flavor singlets: a second look

We reconsider the Volume Source Technique (VST) for the determination of flavor singlet quantities on the lattice. We point out a difficulty arising in the case of fermions in real representations of the gauge group and propose an improved version of the method (IVST) based on random gauge transformations of the background configuration. We compare the performance of IVST with the method based on stochastic estimators (SET). We consider the case of the N=1 Supersymmetric Yang-Mills Theory (SYM), where just one fermionic flavor is present, the gluino in the adjoint representation, and only flavor singlet states are possible. The work is part of an inclusive analysis of the spectrum of the lightest particles of the theory, based on the simulation of the model on a $16^3\cdot32$ lattice with dynamical gluinos in the Wilson scheme.

hep-lat↗
arXiv2004

Quark mass dependence of pseudoscalar masses and decay constants on a lattice

Our previous calculations of the sea- and valence-quark mass dependence of the pseudoscalar meson masses and decay constants is repeated on a 16^3x32 lattice which allows for a better determination of the quantities in question. The conclusions are similar as before on the 16^4 lattice [1]. The two light dynamical quark flavours we simulate have masses in the range 1/4 m_s < m_{u,d} < 2/3 m_s. The sea quark mass dependence of f_pi and m_pi^2/m_q is well described by the next-to-leading order (NLO) Chiral Perturbation Theory (ChPT) formulas and clearly shows the presence of chiral logarithms. The valence quark mass dependence requires the presence of NNLO contributions in Partially Quenched ChPT (PQChPT) -- in addition to the NLO terms. The O(a) lattice artifacts in these quantities turn out to be small.

hep-lat↗
arXiv2003

Quark mass dependence of pseudoscalar masses and decay constants

The dependence of pseudo-scalar masses and decay constants on the sea and valence quark masses is investigated in the pseudo-Goldstone boson sector of QCD with two light quark flavours. The sea quark masses are at present in the range ${1/3}m_s \leq m_{ud} \leq {2/3}m_s$ whereas the valence quark masses satisfy ${1/2}m_{sea} \leq m_{val} \leq 2m_{sea}$. The values of the Gasser-Leutwyler low energy constants $L_4$, $L_5$, $L_6$ and $L_8$ are estimated. The computation is done with the Wilson-quark lattice action at gauge coupling $β=5.1$ on $16^4$ lattices. ${\cal O}(a)$ effects are taken into account by applying chiral perturbation theory for the Wilson lattice action as proposed by Rupak and Shoresh.

hep-lat↗
arXiv2003

Quark mass dependence of masses and decay constants of the pseudo-Goldstone bosons in QCD

The dependence of the pseudoscalar meson masses and decay constants on sea and valence quark masses is compared to next-to-leading order (NLO) Chiral Perturbation Theory (ChPT). The numerical simulations with two light dynamical quark flavors are performed with the Wilson-quark lattice action at gauge coupling beta=5.1 and hopping parameters kappa=0.176, 0.1765, 0.177 on a 16^4 lattice. O(a) lattice artifacts are taken into account by applying chiral perturbation theory for the Wilson lattice action. The values of the relevant combinations of Gasser-Leutwyler constants L_4, L_5, L_6 and L_8 are estimated.

hep-lat↗
arXiv2003

Partially quenched chiral perturbation theory and numerical simulations

The dependence of the pseudoscalar meson mass and decay constant is compared to one-loop Partially Quenched Chiral Perturbation Theory (PQChPT) in a numerical simulation with two light dynamical quarks. The characteristic behaviour with chiral logarithms is observed. The values of the fitted PQChPT-parameters are in a range close to the expectation in continuum in spite of the fact that the lattice spacing is still large, namely a=0.28 fm.

hep-lat↗
arXiv2002

Lattice QCD with light dynamical quarks

We report on the simulation of QCD with light dynamical quarks using the two-step multi-boson (TSMB) algorithm. In an exploratory study with two flavours of quarks at lattice spacing about 0.27 fm and with quark mass down to one sixth of the strange quark mass eigenvalue spectra and autocorrelations have been studied. Here we present results on the volume dependence as well as tests of possible algorithmic improvements.

hep-lat↗
arXiv2002

On the price of light quarks

The computational cost of numerical simulations of QCD with light dynamical Wilson-quarks is estimated. The qualitative behaviour of the pion mass and coupling at small quark masses is discussed.

hep-lat↗
arXiv2002

Numerical simulation tests with light dynamical quarks

Two degenerate flavours of quarks are simulated with small masses down to about one fifth of the strange quark mass by using the two-step multi-boson (TSMB) algorithm. The lattice size is 8^3 x 16 with lattice spacing about 0.27fm which is not far from the N_t=4 thermodynamical cross-over line. Autocorrelations of different physical quantities are estimated as a function of the quark mass. The eigenvalue spectra of the Wilson-Dirac operator are investigated.

hep-lat↗
arXiv2002

Unquenched domain wall quarks with TSMB

The numerical simulation of domain wall quarks with the two-step multi-boson (TSMB) algorithm is investigated. Tests are performed on a 8^3*4 lattice with N_f=2 quark flavours.

hep-lat↗
arXiv2002

The supersymmetric Ward identities on the lattice

Supersymmetric (SUSY) Ward identities are considered for the N=1 SU(2) SUSY Yang Mills theory discretized on the lattice with Wilson fermions (gluinos). They are used in order to compute non-perturbatively a subtracted gluino mass and the mixing coefficient of the SUSY current. The computations were performed at gauge coupling $β$=2.3 and hopping parameter $κ$=0.1925, 0.194, 0.1955 using the two-step multi-bosonic dynamical-fermion algorithm. Our results are consistent with a scenario where the Ward identities are satisfied up to O(a) effects. The vanishing of the gluino mass occurs at a value of the hopping parameter which is not fully consistent with the estimate based on the chiral phase transition. This suggests that, although SUSY restoration appears to occur close to the continuum limit of the lattice theory, the results are still affected by significant systematic effects.

hep-lat↗