arXiv subjects
Publications and source records attributed to R. Frezzotti.
We study light-quark observables by means of dynamical lattice QCD simulations using two flavours of twisted mass fermions at maximal twist. We employ chiral perturbation theory to describe our data for the pion mass and decay constant. In this way, we extract precise determinations for the low-energy constants of the effective theory as well as for the light-quark mass and the chiral condensate.
We present a comprehensive investigation of light meson physics using maximally twisted mass fermions for two mass-degenerate quark flavours. By employing four values of the lattice spacing, spatial lattice extents ranging from 2.0 fm to 2.5 fm and pseudo scalar masses in the range 280 MeV to 650 MeV we control the major systematic effects of our calculation. This enables us to confront our data with chiral perturbation theory and extract low energy constants of the effective chiral Lagrangian and derived quantities, such as the light quark mass, with high precision.
We present a lattice QCD calculation of the heavy-light decay constants f_B and f_Bs performed with Nf=2 maximally twisted Wilson fermions, at four values of the lattice spacing. The decay constants have been also computed in the static limit and the results are used to interpolate the observables between the charm and the infinite-mass sectors, thus obtaining the value of the decay constants at the physical b quark mass. Our preliminary results are f_B=191(14) MeV, f_Bs=243(14) MeV, f_Bs/f_B=1.27(5). They are in good agreement with those obtained with a novel approach, recently proposed by our Collaboration (ETMC), based on the use of suitable ratios having an exactly known static limit.
We compute non-perturbatively the B-parameters of the complete basis of four-fermion operators needed to study the Kaon oscillations in the SM and in its supersymmetric extension. We perform numerical simulations with two dynamical maximally twisted sea quarks at three values of the lattice spacing on configurations generated by the ETMC. Unwanted operator mixings and O(a) discretization effects are removed by discretizing the valence quarks with a suitable Osterwalder-Seiler variant of the Twisted Mass action. Operators are renormalized non-perturbatively in the RI/MOM scheme. Our preliminary result for BK(RGI) is 0.73(3)(3).
In this paper we propose to interpret the large discretization artifacts affecting the neutral pion mass in maximally twisted lattice QCD simulations as O(a^2) effects whose magnitude is roughly proportional to the modulus square of the (continuum) matrix element of the pseudoscalar density operator between vacuum and one-pion state. The numerical size of this quantity is determined by the dynamical mechanism of spontaneous chiral symmetry breaking and turns out to be substantially larger than its natural magnitude set by the value of Lambda_QCD.
We present the results of a lattice QCD calculation of the pseudoscalar meson decay constants fpi, fK, fD and fDs, performed with Nf=2 dynamical fermions. The simulation is carried out with the tree-level improved Symanzik gauge action and with the twisted mass fermionic action at maximal twist. We have considered for the final analysis three values of the lattice spacing, a~0.10 fm, 0.09 fm and 0.07 fm, with pion masses down to mpi~270 MeV. Our results for the light meson decay constants are fK=158.1(2.4) MeV and fK/fpi=1.210(18). From the latter ratio, by using the experimental determination of Gamma(K-->mu nu_mu (gamma))/ Gamma(pi--> mu nu_mu (gamma)) and the average value of |Vud| from nuclear beta decays, we obtain |Vus|=0.2222(34), in good agreement with the determination from semileptonic Kl3 decays and the unitarity constraint. For the D and Ds meson decay constants we obtain fD=197(9) MeV, fDs=244(8) MeV and fDs/fD=1.24(3). Our result for fD is in good agreement with the CLEO experimental measurement. For fDs our determination is smaller than the PDG 2008 experimental average but in agreement with a recent improved measurement by CLEO at the 1.4 sigma level.
We present a lattice calculation of the electromagnetic form factor of the pion obtained using the tree-level Symanzik improved gauge action with two flavors of dynamical twisted Wilson quarks. The simulated pion masses range approximately from 260 to 580 MeV and the lattice box sizes are chosen in order to guarantee that M L > 4. Accurate results for the form factor are obtained using all-to-all quark propagators evaluated by a stochastic procedure. The momentum dependence of the pion form factor is investigated up to values of the squared four-momentum transfer Q**2 ~ 0.8 GeV**2 and, thanks to the use of twisted boundary conditions, down to Q**2 ~ 0.05 GeV**2. Volume and discretization effects on the form factor appear to be within the statistical errors. Our results for the pion mass, decay constant and form factor are analyzed using (continuum) Chiral Perturbation Theory at next-to-next-to-leading order. The extrapolated value of the pion charge radius is {phys} = 0.456 +/- 0.030(stat.) +/- 0.024(syst.) in nice agreement with the experimental result. The extrapolated values of the pion form factor agree very well with the experimental data up to Q**2 ~ 0.8 GeV**2 within uncertainties which become competitive with the experimental errors for Q**2 > 0.3 GeV**2. The relevant low-energy constants appearing in the chiral expansion of the pion form factor are extracted from our lattice data, which come essentially from a single lattice spacing, adding the experimental value of the pion scalar radius in the fitting procedure. Our findings are in nice agreement with the available results of ChPT analyses of pion-pion scattering data as well as with other analyses of our collaboration.
In a recent paper [hep-lat/0701012] we presented precise lattice QCD results of our European Twisted Mass Collaboration (ETMC). They were obtained by employing two mass-degenerate flavours of twisted mass fermions at maximal twist. In the present paper we give details on our simulations and the computation of physical observables. In particular, we discuss the problem of tuning to maximal twist, the techniques we have used to compute correlators and error estimates. In addition, we provide more information on the algorithm used, the autocorrelation times and scale determination, the evaluation of disconnected contributions and the description of our data by means of chiral perturbation theory formulae.
We present an update of the results for pion mass and pion decay constant as obtained by the ETM collaboration in large scale simulations with maximally twisted mass fermions and two mass degenerate flavours of light quarks. We discuss the continuum, chiral and infinite volume extrapolation of these quantities as well as the extraction of low energy constants, and investigate possible systematic uncertainties.
We present work in progress on the computation of the K-meson vector and tensor decay constants, as well as the B-parameter in Kaon oscillations. Our simulations are performed in a partially quenched setup, with two dynamical (sea) Wilson quark flavours, having a maximally twisted mass term. Valence quarks are either of the standard or the Osterwalder-Seiler maximally twisted variety. These two regularizations can be suitably combined in order to obtain a BK parameter which is both multiplicatively renormalizable and O(a) improved.
We present results on the mass of the nucleon and the Delta using two dynamical degenerate twisted mass quarks and the tree-level Symanzik improved gauge action. The evaluation is performed at four quark masses corresponding to a pion mass in the range of about 300-600 MeV on lattices of 2.1-2.7 fm. We check for cut-off effects by evaluating these baryon masses on lattices of spatial size 2.1 fm at beta=3.9 and beta=4.05 and on a lattice of 2.4 fm at beta=3.8. The values we find are compatible within our statistical errors. Lattice results are extrapolated to the physical limit using continuum chiral perturbation theory. Performing a combined fit to our lattice data at beta=3.9 and beta=4.05 we find a nucleon mass of 964\pm 28 (stat.) \pm 8 (syst.) MeV. The nucleon mass at the physical point provides an independent determination of the lattice spacing. Using heavy baryon chiral perturbation theory at O(p^3) we find a_{β=3.9}=0.0890\pm 0.0039(stat.) \pm 0.0014(syst.) fm, and a_{β=4.05}= 0.0691\pm 0.0034(stat.) \pm 0.0010(syst.) fm, in good agreement with the values determined from the pion decay constant. Isospin violating lattice artifacts in the Delta-system are found to be compatible with zero for the values of the lattice spacings used in this work. Performing a combined fit to our lattice data at beta=3.9 and beta=4.05 we find for the masses of the Delta^{++,-} and Delta^{+,0} 1316 \pm 60 (stat.) MeV and 1330 \pm 74 (stat.) MeV respectively. We confirm that in the continuum limit they are also degenerate.
We report on the scaling of basic hadronic observables in lattice QCD with N_f=2 maximally twisted Wilson dynamical quarks. We give preliminary results for some of the Gasser-Leutwyler low energy constants, the chiral condensate and the average mass of u and d quarks.
We present results for the renormalisation constants of bilinear quark operators, using the Nf=2 twisted mass Wilson action at maximal twist (which guarantees automatic O(a) improvement) and the tree-level Symanzik improved gauge action. The scale-independent renormalisation constants are computed with a new method, which makes use of both standard twisted mass and Osterwalder-Seiler fermions. Moreover, the results from an RI-MOM calculation are presented for both scale independent and scale dependent renormalisation constants.
We present the results of a lattice QCD calculation of the average up-down and strange quark masses and of the light meson pseudoscalar decay constants with Nf=2 dynamical fermions. The simulation is carried out at a single value of the lattice spacing with the twisted mass fermionic action at maximal twist, which guarantees automatic O(a)-improvement of the physical quantities. Quark masses are renormalized by implementing the non-perturbative RI-MOM renormalization procedure. Our results for the light quark masses are m_ud^{msbar}(2 GeV)= 3.85 +- 0.12 +- 0.40 MeV, m_s^{msbar}(2 GeV) = 105 +- 3 +- 9 MeV and m_s/m_ud = 27.3 +- 0.3 +- 1.2. We also obtain fK = 161.7 +- 1.2 +- 3.1 MeV and the ratio fK/fpi=1.227 +- 0.009 +- 0.024. From this ratio, by using the experimental determination of Gamma(K-> mu nu (gamma))/Gamma(pi -> mu nu (gamma)) and the average value of |Vud| from nuclear beta decays, we obtain |Vus|=0.2192(5)(45), in agreement with the determination from Kl3 decays and the unitarity constraint.
We present results of dynamical simulations with 2 flavours of degenerate Wilson twisted mass quarks at maximal twist in the range of pseudo scalar masses from 300 to 550 MeV. The simulations are performed at one value of the lattice spacing a \lesssim 0.1 fm. In order to have O(a) improvement and aiming at small residual cutoff effects, the theory is tuned to maximal twist by requiring the vanishing of the untwisted quark mass. Precise results for the pseudo scalar decay constant and the pseudo scalar mass are confronted with chiral perturbation theory predictions and the low energy constants F, \bar{l}_3 and \bar{l}_4 are evaluated with small statistical errors.
When analyzed in terms of the Symanzik expansion, lattice correlators of multi-local (gauge-invariant) operators with non-trivial continuum limit exhibit in maximally twisted lattice QCD ``infrared divergent'' cutoff effects of the type a^{2k}/(m_π^2)^{h}, 2k\geq h\geq 1 (k,h integers), which tend to become numerically large when the pion mass gets small. We prove that, if the action is O(a) improved a` la Symanzik or, alternatively, the critical mass counter-term is chosen in some ``optimal'' way, these lattice artifacts are reduced to terms that are at worst of the order a^{2}(a^2/m_π^2)^{k-1}, k\geq 1. This implies that the continuum extrapolation of lattice results is smooth at least down to values of the quark mass, m_q, satisfying the order of magnitude inequality m_q >a^2Λ^3_{\rm QCD}.
In this review we discuss the general features of maximally twisted lattice QCD. In particular, we illustrate how automatic O(a) improvement can be achieved and how it is possible to set up a lattice regularization scheme where the problem of wrong chirality mixing (be it finite or infinite) affecting the computation of the matrix elements of the CP-conserving effective weak Hamiltonian is neatly avoided, while having at the same time a positive determinant even for non-degenerate quark pairs. The question of reducing the large cutoff effects that appear when the quark mass tends to zero as a consequence of parity and iso-spin breaking in the action is also addressed. It is shown that such dangerous lattice artifacts are strongly suppressed if the clover term is added to the action or, alternatively, the critical mass is chosen so as to enforce the restoration of parity.
We present a detailed design of a (P)HMC simulation algorithm for N_f = 2 + 1 + 1 maximally twisted Wilson quark flavours. The algorithm retains even/odd and mass-shift preconditionings combined with multiple Molecular Dynamics time scales for both the light mass degenerate, u and d, quarks and the heavy mass non-degenerate, s and c, quarks. Various non-standard aspects of the algorithm are discussed, among which those connected to the use of a polynomial approximation for the inverse (square root of the squared) Dirac matrix in the s and c quark sector.