arXiv subjects

Stefano Capitani

Publications and source records attributed to Stefano Capitani.

At least 19 recordsLinked to original sources

arXiv2024

Taste-splittings of staggered, Karsten-Wilczek and Borici-Creutz fermions under gradient flow in 2D

Karsten-Wilczek and Borici-Creutz fermions show a near-degeneracy of the $2$ species involved, similar to the $2^{d/2}$ species of staggered fermions. Hence in $d=2$ dimensions all three formulations happen to be minimally doubled (two species). This near-degeneracy shows up both in the eigenvalue spectrum of the respective Dirac operator and in spectroscopic quantities (e.g. the pion mass), but in the former case it is easier to quantify. We use the quenched Schwinger model to determine the low-lying eigenvalues of these fermion operators at a fixed gradient flow time $\tau$ (either in lattice units or in physical units, hence keeping either $\tau/a^2$ or $e^2 \tau$ fixed at all $\beta$).

hep-lat↗

arXiv2019

Dynamical Generation of Elementary Fermion Mass: First Lattice Evidence

Using lattice simulations we demonstrate from first principles the existence of a non-perturbative mechanism for elementary particle mass generation in models with gauge fields, fermions and scalars, if an exact invariance forbids power divergent fermion masses and fermionic chiral symmetries broken at UV scale are maximally restored. We show that in the Nambu-Goldstone phase a fermion mass term, unrelated to the Yukawa operator, is dynamically generated. In models with electro-weak interactions weak boson masses are also generated opening new scenarios for beyond the Standard Model physics.

hep-th↗

arXiv2019

Precision computation of hybrid static potentials in SU(3) lattice gauge theory

We perform a precision computation of hybrid static potentials with quantum numbers $Λ_η^ε= Σ_g^-,Σ_u^+,Σ_u^-,Π_g,Π_u,Δ_g,Δ_u$ using SU(3) lattice gauge theory. The resulting potentials are used to estimate masses of heavy $\bar{c} c$ and $\bar{b} b$ hybrid mesons in the Born-Oppenheimer approximation. Part of the lattice gauge theory computation, which we discuss in detail, is an extensive optimization of hybrid static potential creation operators. The resulting optimized operators are expected to be essential for future projects concerning the computation of 3-point functions as e.g. needed to study spin corrections, decays or the gluon distribution of heavy hybrid mesons.

hep-lat↗

arXiv2019

Iso-vector axial form factors of the nucleon in two-flavour lattice QCD

We present a lattice calculation of the nucleon iso-vector axial and induced pseudoscalar form factors on the CLS ensembles using $N_{\rm f}=2$ dynamical flavours of non-perturbatively $\mathcal{O}(a)$-improved Wilson fermions and an $\mathcal{O}(a)$-improved axial current together with the pseudoscalar density. Excited-state effects in the extraction of the form factors are treated using a variety of methods, with a detailed discussion of their respective merits. The chiral and continuum extrapolation of the results is performed both using formulae inspired by Heavy Baryon Chiral Perturbation Theory (HBChPT) and a global approach to the form factors based on a chiral effective theory (EFT) including axial vector mesons. Our results indicate that careful treatment of excited-state effects is important in order to obtain reliable results for the axial form factors of the nucleon, and that the main remaining error stems from the systematic uncertainties of the chiral extrapolation. As final results, we quote $g_{\rm A} = 1.278 \pm 0.068\genfrac{}{}{0pt}{1}{+0.000}{-0.087}$, $\langle r_{\rm A}^2\rangle = 0.360 \pm 0.036\genfrac{}{}{0pt}{1}{+0.080}{-0.088}~\mathrm{fm}^2$, and $g_{\rm P} = 7.7 \pm 1.8 \genfrac{}{}{0pt}{1}{+0.8}{-2.0}$ for the axial charge, axial charge radius and induced pseudoscalar charge, respectively, where the first error is statistical and the second is systematic.

hep-lat↗

arXiv2018

Non-perturbative generation of elementary fermion masses: a numerical study

In this talk we present a numerical lattice study of an SU(3) gauge model where an SU(2) doublet of non-Abelian strongly interacting fermions is coupled to a complex scalar field doublet via a Yukawa and a Wilson-like term. The model enjoys an exact symmetry, acting on all fields, which prevents UV power divergent fermion mass corrections, despite the presence of these two chiral breaking operators in the Lagrangian. In the phase where the scalar potential is non-degenerate and fermions are massless, the bare Yukawa coupling can be set at a critical value at which chiral fermion transformations become symmetries of the theory. Numerical simulations in the Nambu-Goldstone phase of the critical theory, for which the renormalized Yukawa coupling by construction vanishes, give evidence for non-perturbative generation of a UV finite fermion mass term in the effective action.

hep-lat↗

arXiv2018

Computation of hybrid static potentials from optimized trial states in SU(3) lattice gauge theory

We compute hybrid static potentials in SU(3) lattice gauge theory using a method to automatically generate a large set of suitable creation operators from elementary building blocks. This method allows to find sets of creation operators, which generate trial states with large ground state overlaps for all investigated angular momentum and parity sectors. We present numerical results for hybrid static potentials with quantum numbers $Σ^-_g, Σ^+_u, Σ^-_u, Π_g, Π_u, Δ_g, Δ_u$.

hep-lat↗

arXiv2017

Testing a non-perturbative mechanism for elementary fermion mass generation: numerical results

Based on a recent proposal according to which elementary particle masses could be generated by a non-perturbative dynamical phenomenon, alternative to the Higgs mechanism, we carry out lattice simulations of a model where a non-abelian strongly interacting fermion doublet is also coupled to a doublet of complex scalar fields via a Yukawa and an "irrelevant" Wilson-like term. In this pioneering study we use naive fermions and work in the quenched approximation. We present preliminary numerical results both in the Wigner and in the Nambu-Goldstone phase, focusing on the observables relevant to check the occurrence of the conjectured dynamical fermion mass generation effect in the continuum limit of the critical theory in its spontaneously broken phase.

hep-lat↗

arXiv2017

Testing a non-perturbative mechanism for elementary fermion mass generation: lattice setup

In this contribution we lay down a lattice setup that allows for the non-perturbative study of a field theoretical model where a SU(2) fermion doublet, subjected to non-Abelian gauge interactions, is also coupled to a complex scalar field doublet via a Yukawa and an "irrelevant" Wilson-like term. Using naive fermions in quenched approximation and based on the renormalized Ward identities induced by purely fermionic chiral transformations, lattice observables are discussed that enable: a) in the Wigner phase, the determinations of the critical Yukawa coupling value where the purely fermionic chiral transformation become a symmetry up to lattice artifacts; b) in the Nambu-Goldstone phase of the resulting critical theory, a stringent test of the actual generation of a fermion mass term of non-perturbative origin. A soft twisted fermion mass term is introduced to circumvent the problem of exceptional configurations, and observables are then calculated in the limit of vanishing twisted mass.

hep-lat↗

arXiv2017

Computation of hybrid static potentials in SU(3) lattice gauge theory

We compute hybrid static potentials in SU(3) lattice gauge theory. We present a method to generate a large set of suitable creation operators with defined quantum numbers from elementary building blocks. We show preliminary results for several channels and discuss, which structures of the gluonic flux tube seem to be realized by the ground states in these channels.

hep-lat↗

arXiv2016

Check of a new non-perturbative mechanism for elementary fermion mass generation

We consider a field theoretical model where a SU(2) fermion doublet, subjected to non-Abelian gauge interactions, is also coupled to a complex scalar field doublet via a Yukawa and an irrelevant Wilson-like term. Despite the presence of these two chiral breaking operators in the Lagrangian, an exact symmetry acting on fermions and scalars prevents perturbative mass corrections. In the phase where fermions are massless (Wigner phase) the Yukawa coupling can be tuned to a critical value at which chiral transformations acting on fermions only become a symmetry of the theory (up to cutoff effects). In the Nambu-Goldstone phase of the critical theory a fermion mass term of dynamical origin is expected to arise in the Ward identities of the purely fermionic chiral transformations. Such a non-perturbative mechanism of dynamical mass generation can provide a "natural" (à la 't Hooft) alternative to the Higgs mechanism adopted in the Standard Model. Here we lay down the theoretical framework necessary to demonstrate the existence of this mechanism by means of lattice simulations.

hep-lat↗

arXiv2013

Numerical studies of Minimally Doubled Fermions

We have performed the first numerical study of minimally doubled fermions of the Karsten-Wilczek class in the quenched approximation. This requires fixing the counterterms, which arise due to hypercubic symmetry breaking induced by the Karsten-Wilczek term. Non-perturbative renormalisation criteria are formulated after a detailed study of the parameter dependence of mesonic observables. Minimisation of the mass anisotropy of the pseudoscalar ground state fixes non-perturbative renormalisation conditions for the counterterm coefficients. These anisotropies are mapped out by probing different euclidean components of the transfer matrix through calculations of the pseudoscalar ground state mass in different directions. The chiral behaviour of the pseudoscalar ground state is studied with the tuned Karsten-Wilczek action for multiple lattice spacings. Light pseudoscalar masses ($ M_{PS} \lesssim 250\,MeV $) were achieved in the quenched approximation without encountering exceptional configurations. The presence of quenched chiral logarithms is studied under the tentative assumption of Goldstone Boson-like behaviour.

hep-lat↗

arXiv2013

New chiral lattice actions of the Borici-Creutz type

We generalize the Borici-Creutz action in such a way that the position of the second zero and the direction which breaks the hypercubic symmetry can be arbitrarily chosen, and the action has still the correct continuum limit. Minimal doubling is guaranteed if the distance between the two zeros does not become too large. Special values of this distance could turn out to be particularly convenient for efficient numerical simulations of minimally doubled fermions.

hep-lat↗

arXiv2013

New actions for minimally doubled fermions and their counterterms

Minimally doubled fermions provide a cheap and convenient way of simulating quarks which preserve chiral symmetry. It has been established that two actions of this kind (known as Borici-Creutz and Karsten-Wilczek) require the tuning of three counterterms in order to be properly renormalized. Here we construct some more general minimally doubled actions and investigate the properties of their counterterms.

hep-lat↗

arXiv2013

Reducing the number of counterterms with new minimally doubled actions

We study a class of nearest-neighbor minimally doubled actions which depend on 2 continuous parameters. We calculate the contributions of the 3 possible counterterms in perturbation theory, and we find that for each counterterm there are curves in the parameter space on which its coefficient vanishes. One can thus construct renormalized actions that contain only 2 counterterms instead of the 3 of the standard Karsten-Wilczek or Borici-Creutz actions. Our investigations suggest the usefulness of analogous nonperturbative searches for values of the parameters for which the number of counterterms can be reduced. They can also be an inspiration to undertake a search for ultralocal minimally doubled actions with even better counterterm-reducing properties, including the optimal case in which all counterterms can be removed. Simulations of the latter actions will be much cheaper than the cases in which one needs to add counterterms to the bare actions, like the already conveniently inexpensive standard Karsten-Wilczek fermions. Finally, we also introduce minimally doubled fermions with next-to-nearest-neighbor interactions, which depend on 4 continuous parameters, as a further possibility in the search for renormalized actions with no counterterms.

hep-lat↗

arXiv2012

Excited state systematics in extracting nucleon electromagnetic form factors

We present updated preliminary results for the nucleon electromagnetic form factors for non-perturbatively $\mathcal{O}(a)$ improved Wilson fermions in $N_f=2$ QCD measured on the CLS ensembles. The use of the summed operator insertion method allows us to suppress the influence of excited states in our measurements. A study of the effect that excited state contaminations have on the $Q^2$ dependence of the extracted nucleon form factors may then be made through comparisons of the summation method to standard plateau fits, as well as to excited state fits.

hep-lat↗

arXiv2011

Scale setting via the Ω baryon mass

We present the first results of an ongoing effort to determine the lattice scale on the N_f=2 CLS lattice ensembles via the mass of the Ω baryon. Results from different methods are compared, and various sources of systematic uncertainty are discussed.

hep-lat↗

arXiv2010

Systematic errors in extracting nucleon properties from lattice QCD

Form factors of the nucleon have been extracted from experiment with high precision. However, lattice calculations have failed so far to reproduce the observed dependence of form factors on the momentum transfer. We have embarked on a program to thoroughly investigate systematic effects in lattice calculation of the required three-point correlation functions. Here we focus on the possible contamination from higher excited states and present a method which is designed to suppress them. Its effectiveness is tested for several baryonic matrix elements, different lattice sizes and pion masses.

hep-lat↗

arXiv2010

Minimally doubled fermions and their renormalization

Minimally doubled fermions have been proposed as a strictly local discretization of the QCD quark action, which also preserves chiral symmetry at finite cut-off. We study the renormalization and mixing properties of two particular realizations of minimally doubled fermions in lattice perturbation theory at one loop, and we construct conserved axial currents which have a simple form involving only nearest-neighbours sites. We also introduce a notation which allows a unified description of the renormalization properties of both actions.

hep-lat↗