Featured Researches

High Energy Physics Lattice

Apparent convergence of Padé approximants for the crossover line in finite density QCD

We propose a novel Bayesian method to analytically continue observables to real baryochemical potentialμBin finite density QCD. Taylor coefficients atμB=0and data at imaginary chemical potentialμIBare treated on equal footing. We consider two different constructions for the Padé approximants, the classical multipoint Padé approximation and a mixed approximation that is a slight generalization of a recent idea in Padé approximation theory. Approximants with spurious poles are excluded from the analysis. As an application, we perform a joint analysis of the available continuum extrapolated lattice data for both pseudocritical temperatureTcatμIBfrom the Wuppertal-Budapest Collaboration and Taylor coefficientsκ2andκ4from the HotQCD Collaboration. An apparent convergence of[p/p]and[p/p+1]sequences of rational functions is observed with increasingp.We present our extrapolation up toμB≈600MeV.

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High Energy Physics Lattice

Asymptotic low-temperature critical behavior of two-dimensional multiflavor lattice SO(Nc) gauge theories

We address the interplay between global and local gauge nonabelian symmetries in lattice gauge theories with multicomponent scalar fields. We consider two-dimensional lattice scalar nonabelian gauge theories with a local SO(Nc) (Nc >= 3) and a global O(Nf) invariance, obtained by partially gauging a maximally O(Nf x Nc)-symmetric multicomponent scalar model. Correspondingly, the scalar fields belong to the coset S(Nf Nc-1)/SO(Nc), where S(N) is the N-dimensional sphere. In agreement with the Mermin-Wagner theorem, these lattice SO(Nc) gauge models with Nf >= 3 do not have finite-temperature transitions related to the breaking of the global nonabelian O(Nf) symmetry. However, in the zero-temperature limit they show a critical behavior characterized by a correlation length that increases exponentially with the inverse temperature, similarly to nonlinear O(N) sigma models. Their universal features are investigated by numerical finite-size scaling methods. The results show that the asymptotic low-temperature behavior belongs to the universality class of the two-dimensional RP(Nf-1) model.

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High Energy Physics Lattice

Automatic differentiation for error analysis

We present ADerrors.jl, a software for linear error propagation and analysis of Monte Carlo data. Although the focus is in data analysis in Lattice QCD, where estimates of the observables have to be computed from Monte Carlo samples, the software also deals with variables with uncertainties, either correlated or uncorrelated. Thanks to automatic differentiation techniques linear error propagation is performed exactly, even in iterative algorithms (i.e. errors in parameters of non-linear fits). In this contribution we present an overview of the capabilities of the software, including access to uncertainties in fit parameters and dealing with correlated data. The software, written in julia, is available for download and use in this https URL

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High Energy Physics Lattice

Avoiding the sign-problem in lattice field theory

In lattice field theory, the interactions of elementary particles can be computed via high-dimensional integrals. Markov-chain Monte Carlo (MCMC) methods based on importance sampling are normally efficient to solve most of these integrals. But these methods give large errors for oscillatory integrands, exhibiting the so-called sign-problem. We developed new quadrature rules using the symmetry of the considered systems to avoid the sign-problem in physical one-dimensional models for the resulting high-dimensional integrals. This article gives a short introduction to integrals used in lattice QCD where the interactions of gluon and quark elementary particles are investigated, explains the alternative integration methods we developed and shows results of applying them to models with one physical dimension. The new quadrature rules avoid the sign-problem and can therefore be used to perform simulations at until now not reachable regions in parameter space, where the MCMC errors are too big for affordable sample sizes. However, it is still a challenge to develop these techniques further for applications with physical higher-dimensional systems.

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High Energy Physics Lattice

Axial U(1) symmetry and mesonic correlators at high temperature inNf=2lattice QCD

We investigate the high-temperature phase of QCD using lattice QCD simulations withNf=2dynamical Möbius domain-wall fermions. On generated configurations, we study the axialU(1)symmetry, overlap-Dirac spectra, screening masses from mesonic correlators, and topological susceptibility. We find that some of the observables are quite sensitive to lattice artifacts due to a small violation of the chiral symmetry. For those observables, we reweight the Möbius domain-wall fermion determinant by that of the overlap fermion. We also check the volume dependence of observables. Our data near the chiral limit indicates a strong suppression of the axialU(1)anomaly at temperatures≥220 MeV.

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High Energy Physics Lattice

AxialUA(1)Anomaly: a New Mechanism to Generate Massless Bosons

Prior to the establishment ofQCDas the correct theory describing hadronic physics, it was realized that the essential ingredients of the hadronic world at low energies are chiral symmetry and its spontaneous breaking. Spontaneous symmetry breaking is a non-perturbative phenomenon and thanks to massiveQCDsimulations on the lattice we have at present a good understanding on the vacuum realization of the non-abelian chiral symmetry as a function of the physical temperature. As far as theUA(1)anomaly is concerned, and especially in the high temperature phase, the current situation is however far from satisfactory. The first part of this article is devoted to review the present status of lattice calculations, in the high temperature phase ofQCD, of quantities directly related to theUA(1)axial anomaly. In the second part I will analyze some interesting physical implications of theUA(1)anomaly, recently suggested, in systems where the non-abelian axial symmetry is fulfilled in the vacuum. More precisely I will argue that, if theUA(1)symmetry remains effectively broken, the topological properties of the theory can be the basis of a mechanism, other than Goldstone's theorem, to generate a rich spectrum of massless bosons at the chiral limit.

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High Energy Physics Lattice

Baryons in the Gross-Neveu model in 1+1 dimensions at finite number of flavors

In a recent work we studied the phase structure of the Gross-Neveu (GN) model in1+1dimensions at finite number of fermion flavorsNf=2,8,16, finite temperature and finite chemical potential using lattice field theory. Most importantly, we found an inhomogeneous phase at low temperature and large chemical potential, quite similar to the analytically solvableNf→∞limit. In the present work we continue our lattice field theory investigation of the finite-NfGN model by studying the formation of baryons, their spatial distribution and their relation to the chiral condensate. As a preparatory step we also discuss a linear coupling of lattice fermions to the chemical potential.

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High Energy Physics Lattice

Bc→Bs(d)form factors from lattice QCD

We present results of the first lattice QCD calculations ofBc→BsandBc→Bdweak matrix elements. Form factors across the entire physicalq2range are then extracted and extrapolated to the physical-continuum limit before combining with CKM matrix elements to predict the semileptonic decay ratesΓ(B+c→B0sℓ¯νℓ)=26.2(1.2)×109s−1andΓ(B+c→B0ℓ¯νℓ)=1.65(10)×109s−1. The lattice QCD uncertainty is comparable to the CKM uncertainty here. Results are derived from correlation functions computed on MILC Collaboration gauge configurations with a range of lattice spacings including 2+1+1 flavours of dynamical sea quarks in the Highly Improved Staggered Quark (HISQ) formalism. HISQ is also used for the propagators of the valence light, strange, and charm quarks. Two different formalisms are employed for the bottom quark: non-relativistic QCD (NRQCD) and heavy-HISQ. Checking agreement between these two approaches is an important test of our strategies for heavy quarks on the lattice. From chained fits of NRQCD and heavy-HISQ data, we obtain the differential decay ratesdΓ/dq2as well as integrated values for comparison to future experimental results.

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High Energy Physics Lattice

Bc→J/ψForm Factors for the fullq2range from Lattice QCD

We present the first lattice QCD determination of theBc→J/ψvector and axial-vector form factors. These will enable experimental information on the rate forBcsemileptonic decays toJ/ψto be converted into a value forVcb. Our calculation covers the full physicalq2range of the decay and uses non-perturbatively renormalised lattice currents. We use the Highly Improved Staggered Quark (HISQ) action for all valence quarks on the second generation MILC ensembles of gluon field configurations includingu,d,sandcHISQ sea quarks. Our HISQ heavy quarks have masses ranging upwards from that ofc; we are able to reach that of thebon our finest lattices. This enables us to map out the dependence on heavy quark mass and determine results in the continuum limit at theb. We use our form factors to construct the differential rates forB−c→J/ψμ−ν¯μand obtain a total rate with7%uncertainty:Γ(B−c→J/ψμ−ν¯μ)/|ηEWVcb|2=1.73(12)×1013 s−1. Including values forVcb,ηEWandτBcyields a branching fraction for this decay mode of 0.0150(11)(10)(3) ~with uncertainties from lattice QCD,ηEWVcbandτBcrespectively.

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High Energy Physics Lattice

Beauty mesons inNf=2+1+1+1lattice QCD with exact chiral symmetry

We present the first study ofNf=2+1+1+1lattice QCD with domain-wall quarks. The(b,c,s)quarks are physical, while the(u,d)quarks are heavier than their physical masses, with the pion mass∼700MeV. The gauge ensemble is generated by hybrid Monte Carlo simulation with the Wilson gauge action for the gluons, and the optimal domain-wall fermion action for the quarks. Using point-to-point quark propagators, we measure the time-correlation functions of quark-antiquark meson interpolators with quark contentsb¯b,b¯c,b¯s, andc¯c, and obtain the masses of the low-lying mesons. They are in good agreement with the experimental values, plus some predictions which have not been observed in experiments. Moreover, we also determine the masses of(b,c,s)quarks.

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