arXiv subjects

Bartosz Kostrzewa

Publications and source records attributed to Bartosz Kostrzewa.

At least 37 records · Page 2Linked to original sources

arXiv2022

Pion-pole contribution to HLbL from twisted mass lattice QCD at the physical point

We report on our computation of the pion transition form factor ${\cal F}_{P\rightarrow γ^*γ^*}$ from twisted mass lattice QCD in order to determine the numerically dominant light pseudoscalar pole contribution in the hadronic light-by-light scattering contribution to the anomalous magnetic moment of the muon $a_μ=(g-2)_μ$. The pion transition form factor is computed directly at the physical point. We present first results for our estimate of the pion-pole contribution with kinematic setup for the pion at rest.

hep-lat↗

arXiv2022

Twisted mass gauge ensembles at physical values of the light, strange and charm quark masses

Lattice QCD simulations directly at physical masses of dynamical light, strange and charm quarks are highly desirable especially to remove systematic errors due to chiral extrapolations. However such simulations are still challenging. We discuss the adaption of efficient algorithms, like multi-grid methods or higher order integrators, within the molecular dynamic steps of the Hybrid Monte Carlo algorithm, that are enabling simulations of a new set of gauge ensembles by the Extended Twisted Mass collaboration (ETMC). We present the status of the on-going ETMC simulation effort that aims to enabling studies of finite size and discretization effects. We work within the twisted mass discretization which is free of odd-discretization effects at maximal twist and present our tuning procedure.

hep-lat↗

arXiv2021

Lattice QCD results for the topological up-quark mass contribution: too small to rescue the $m_u=0$ solution to the strong CP problem

A vanishing Yukawa coupling of the up quark could in principle solve the strong CP problem. To render this solution consistent with current algebra results, the up quark must receive an alternative mass contribution that conserves CP symmetry. Such a contribution could be provided by QCD through non-perturbative topological effects, including instantons. In this talk, we present the first direct lattice computation of this topological mass contribution, using gauge configurations generated by the Extended Twisted Mass collaboration. We use the Iwasaki gauge action, Wilson twisted mass fermions at maximal twist, and dynamical up, down, strange and charm quarks. Our result for the topological mass contribution is an order of magnitude too small to account for the phenomenologically required up-quark mass. This rules out the "massless" up-quark solution to the strong CP problem, in accordance with previous results relying on $χ$PT fits to lattice data. The talk is based on [Alexandrou et al., PRL 125, 232001 (2020)], where more details can be found.

hep-lat↗

arXiv2021

Gradient flow scale-setting with $N_f=2+1+1$ Wilson-clover twisted-mass fermions

We present a determination of the gradient flow scales $w_0$, $\sqrt{t_0}$ and $t_0/w_0$ in isosymmetric QCD, making use of the gauge ensembles produced by the Extended Twisted Mass Collaboration (ETMC) with $N_f=2+1+1$ flavours of Wilson-clover twisted-mass quarks including configurations close to the physical point for all dynamical flavours. The simulations are carried out at three values of the lattice spacing and the scale is set through the PDG value of the pion decay constant, yielding $w_0=0.17383(63)$ fm, $\sqrt{t_0}=0.14436(61)$ fm and $t_0/w_0=0.11969(62)$ fm. Finally, fixing the kaon mass to its isosymmetric value, we determine the ratio of the kaon and pion leptonic decay constants to be $f_K/f_π=1.1995(44)$.

hep-lat↗

arXiv2021

Quark and gluon momentum fractions in the pion from $N_f=2+1+1$ lattice QCD

We perform the first full decomposition of the pion momentum into its gluon and quark contributions. We employ an ensemble generated by the Extended Twisted Mass Collaboration with $N_f=2 + 1 +1$ Wilson twisted mass clover fermions at maximal twist tuned to reproduce the physical pion mass. We present our results in the $\overline{\mathrm{MS}}$ scheme at $2\gev$. We find $\avgx_{u+d}=0.601(28)$, $\avgx_s=0.059(13)$, $\avgx_c=0.019(05)$, and $\avgx_g=0.52(11)$ for the separate contributions, respectively, whose sum saturates the momentum sum rule.

hep-lat↗

arXiv2021

Scattering of two and three physical pions at maximal isospin from lattice QCD

We present the first direct $N_f=2$ lattice QCD computation of two- and three-$π^+$ scattering quantities that includes an ensemble at the physical point. We study the quark mass dependence of the two-pion phase shift, and the three-particle interaction parameters. We also compare to phenomenology and chiral perturbation theory (ChPT). In the two-particle sector, we observe good agreement to the phenomenological fits in $s$- and $d$-wave, and obtain $M_πa_0 = -0.0481(86)$ at the physical point from a direct computation. In the three-particle sector, we observe reasonable agreement at threshold to the leading order chiral expansion, i.e.\@ a mildly attractive three-particle contact term. In contrast, we observe that the energy-dependent part of the three-particle quasilocal scattering quantity is not well described by leading order ChPT.

hep-lat↗

arXiv2021

Ruling Out the Massless Up-Quark Solution to the Strong CP Problem by Computing the Topological Mass Contribution with Lattice QCD

The infamous strong CP problem in particle physics can in principle be solved by a massless up quark. In particular, it was hypothesized that topological effects could substantially contribute to the observed nonzero up-quark mass without reintroducing CP violation. Alternatively to previous work using fits to chiral perturbation theory, in this Letter, we bound the strength of the topological mass contribution with direct lattice QCD simulations, by computing the dependence of the pion mass on the dynamical strange-quark mass. We find that the size of the topological mass contribution is inconsistent with the massless up-quark solution to the strong CP problem.

hep-lat↗

arXiv2021

Meson-meson scattering lengths at maximum isospin from lattice QCD

We summarize our lattice QCD determinations of the pion-pion, pion-kaon and kaon-kaon s-wave scattering lengths at maximal isospin with a particular focus on the extrapolation to the physical point and the usage of next-to-leading order chiral perturbation theory to do so. We employ data at three values of the lattice spacing and pion masses ranging from around 230 MeV to around 450 MeV, applying Luescher's finite volume method to compute the scattering lengths. We find that leading order chiral perturbation theory is surprisingly close to our data even in the kaon-kaon case for our entire range of pion masses.

hep-lat↗

arXiv2020

Hadron-Hadron Interactions from $N_f=2+1+1$ Lattice QCD: The $ρ$-resonance

We present an investigation of the Rho-meson from Nf=2+1+1 flavour lattice QCD. The calculation is performed based on gauge configuration ensembles produced by the ETM collaboration with three lattice spacing values and pion masses ranging from 230 MeV to 500 MeV. Applying the Lüscher method phase shift curves are determined for all ensembles separately. Assuming a Breit-Wigner form, the Rho-meson mass and width are determined by a fit to these phase shift curves. Mass and width combined are then extrapolated to the chiral limit, while lattice artefacts are not detectable within our statistical uncertainties. For the Rho-meson mass extrapolated to the physical point we find good agreement with experiment. The corresponding decay width differs by about two standard deviations from the experimental value.

hep-lat↗

arXiv2020

The $ρ$-resonance with physical pion mass from $N_f=2$ lattice QCD

We present the first-ever lattice computation of pi pi-scattering in the I=1 channel with Nf=2 dynamical quark flavours obtained including an ensemble with physical value of the pion mass. Employing a global fit to data at three values of the pion mass, we determine the universal parameters of the rho-resonance. We carefully investigate systematic uncertainties by determining energy eigenvalues using different methods and by comparing inverse amplitude method and Breit-Wigner type parametrizations. Overall, we find mass 786(20) MeV and width 180(6) MeV, including statistical and systematic uncertainties. In stark disagreement with the previous Nf=2 extrapolations from higher than physical pion mass results, our mass value is in good agreement with experiment, while the width is slightly too high.

hep-lat↗

arXiv2020

On the generalised eigenvalue method and its relation to Prony and generalised pencil of function methods

We discuss the relation of three methods to determine energy levels in lattice QCD simulations: the generalised eigenvalue, the Prony and the generalised pencil of function methods. All three can be understood as special cases of a generalised eigenvalue problem. We show analytically that the leading corrections to an energy $E_l$ in all three methods due to unresolved states decay asymptotically exponentially like $\exp(-(E_{n}-E_l)t)$. Using synthetic data we show that these corrections behave as expected also in practice. We propose a novel combination of the generalised eigenvalue and the Prony method, denoted as GEVM/PGEVM, which helps to increase the energy gap $E_{n}-E_l$. We illustrate its usage and performance using lattice QCD examples.

hep-lat↗

arXiv2020

Quark masses and decay constants in $N_f=2+1+1$ isoQCD with Wilson clover twisted mass fermions

We present a preliminary study of the pion, kaon and D-meson masses and decay constants in isosymmetric QCD, as well as a preliminary result for the light-quark renormalized mass. The analysis is based on the gauge ensembles produced by ETMC with $N_f=2+1+1$ flavours of Wilson-clover twisted mass quarks, spanning a range of lattice spacings from $\sim0.10$ to $0.07$ fm and include configurations at the physical pion point on lattices with linear size up to $L~\sim~5.6$~fm

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

Topological susceptibility and $η'$ meson mass from $N_f=2$ lattice QCD at the physical point

In this paper we explore the computation of topological susceptibility and $η'$ meson mass in $N_f=2$ flavor QCD using lattice techniques with physical value of the pion mass as well as larger pion mass values. We observe that the physical point can be reached without a significant increase in the statistical noise. The mass of the $η'$ meson can be obtained from both fermionic two point functions and topological charge density correlation functions, giving compatible results. With the pion mass dependence of the $η'$ mass being flat we arrive at $M_{η'}= 772(18)\ \mathrm{MeV}$ without an explicit continuum limit. For the topological susceptibility we observe a linear dependence on $M_π^2$, however, with an additional constant stemming from lattice artifacts.

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

Hadron-Hadron Interactions from $N_f=2+1+1$ Lattice QCD: $I=3/2$ $πK$ Scattering Length

In this paper we report on results for the s-wave scattering length of the $π$-$K$ system in the $I=3/2$ channel from $N_f=2+1+1$ Lattice QCD. The calculation is based on gauge configurations generated by the European Twisted Mass Collaboration with pion masses ranging from about $230$ to $450\,\text{MeV}$ at three values of the lattice spacing. Our main result reads $M_π\,a_0^{3/2,\text{phys}} = -0.059(2)$. Using chiral perturbation theory we are also able to estimate $M_π\,a_0^{1/2,\text{phys}} = 0.163(3)$. The error includes statistical and systematic uncertainties, and for the latter in particular errors from the chiral and continuum extrapolations.

hep-lat↗

arXiv2018

Simulating twisted mass fermions at physical light, strange and charm quark masses

We present the QCD simulation of the first gauge ensemble of two degenerate light quarks, a strange and a charm quark with all quark masses tuned to their physical values within the twisted mass fermion formulation. Results for the pseudoscalar masses and decay constants confirm that the produced ensemble is indeed at the physical parameters of the theory. This conclusion is corroborated by a complementary analysis in the baryon sector. We examine cutoff and isospin breaking effects and demonstrate that they are suppressed through the presence of a clover term in the action.

hep-lat↗

arXiv2017

Simulation of an ensemble of $N_f=2+1+1$ twisted mass clover-improved fermions at physical quark masses

We present a general strategy aimed at generating $N_f=2+1+1$ configurations with quarks at their physical mass using maximally twisted mass fermions to ensure automatic $O(a)$ improvement, in the presence of a clover term tuned to reduce the charged to neutral pion mass difference. The target system, for the moment, is a lattice of size $64^3 \times 128$ with a lattice spacing $a\sim 0.08$ fm. We show preliminary results on the pion and kaon mass and decay constants.

hep-lat↗