Biblioteca Digital

We address the problem of Gribov copies in lattice QCD. The gluon propagator is computed, in the Landau gauge, using 302 ([beta]=5.8) 124 SU(3) configurations gauge fixed to different copies. The results of the simulation shows that: (i) the effect of Gribov copies is small (less than 10%); (ii) Gribov copies change essentially the lowest momenta components (q<2.6 GeV); (iii) within the statistical accuracy of our simulation, the effect of Gribov copies is resolved if statistical errors are multiplied by a factor of two or three. Moreover, when modelling the gluon propagator, different sets of Gribov copies produce different sets of parameters not, necessarily, compatible within one standard deviation. Finally, our data supports a gluon propagator which, for large momenta, behaves like a massive gluon propagator with a mass of 1.1 GeV.; http://www.sciencedirect.com/science/article/B6TVC-4CDJ80P-2/1/756ee6b978f0c2ae3dc804289ab89a32

An algorithm for gauge fixing to the Landau gauge in the fundamental modular region in lattice QCD is described. The method, a combination of an evolutionary algorithm with a steepest descent method, is able to solve the problem of the nonperturbative gauge fixing. The performance of the combined algorithm is investigated on 84, [beta]=5.7, and 164, [beta]=6.0, lattice SU(3) gauge configurations.; http://www.sciencedirect.com/science/article/B6TJ5-4BMTMN0-3/1/17fcaa643a6848d4ea1fe488f7188dc9

A comprehensive study of the positive and negative parity, isospin = ½, spin = ½ energy eigenstates of the nucleon is presented from lattice QCD. The existence of the Roper resonance, N[superscript]½⁺ (1440 MeV) P11, of the nucleon is experimentally well determined as signified by a four star rating in the Review of Particle Physics. The Roper state is puzzling as this state has a significantly lower mass than the adjacent N[superscript]½⁻ ground state. In a constituent quark model the Roper state sits at almost 100 MeV higher than the N[superscript]½⁻ (1535 MeV) state. In this thesis our principal investigation is centred in finding the Roper resonance using the “variational method”. The analysis is presented in quenched and dynamical QCD with two degenerate flavours of sea quarks. We extract the elusive Roper resonance using the variational approach for the first time, by using smeared-smeared correlators in constructing the correlation matrices. The Roper state has a tendency to approach the physical state at light quark masses and a significant curvature is observed as the chiral region is approached. We also observe a physical level ordering between the Roper and the negative parity ground states of the nucleon...

We study the s-wave I=2 \pi \Sigma and I=1 KN interactions from 2+1 flavor full lattice QCD simulation for relatively heavy quark mass corresponding to m_{\pi}=700 MeV. The s-wave meson-baryon potentials are obtained from the Nambu-Bethe-Salpeter amplitudes. Potentials in both channels reveal short range repulsions, which suggest the importance of the Pauli blocking effect. The I=1 KN scattering phase shifts are calculated and compared with the existing experimental data.; Comment: Prepared for The XXIX International Symposium on Lattice Field Theory (Lattice 2011), July 10-16, 2011, Squaw Valley, Lake Tahoe, California, USA

After a brief review of discovery of the H-dibaryon in lattice QCD, effect of the flavor SU(3) symmetry breaking on the H-dibaryon is studied by basing on the baryon-baryon (BB) interactions extracted from QCD on the lattice. The Schrodinger equation for Lambda Lambda - N Xi -Sigma Sigma coupled-channel is solved with the physical baryon masses and the potentials obtained from QCD at the flavor SU(3) limit. A resonant H-dibaryon is found between Lambda Lambda and N Xi thresholds in this treatment.; Comment: Two references are included in this 2nd version. 5 pages, 4 figures, 2 tables, Proceedings of the 30th International Symposium on Lattice Field Theory, June 24-29, 2012, Cairns, Australia

We explore three-nucleon forces (3NF) from lattice QCD simulations. Utilizing the Nambu-Bethe-Salpeter (NBS) wave function, two-nucleon forces (2NF) and 3NF are determined on the same footing. Quantum numbers of the three-nucleon (3N) system are chosen to be (I, J^P)=(1/2,1/2^+) (the triton channel). The enormous computational cost is reduced by employing the simplest geometrical configuration, where 3N are aligned linearly with an equal spacing. We perform lattice QCD simulations using Nf=2 dynamical clover fermion configurations generated by CP-PACS Collaboration, at the lattice spacing of a = 0.156 fm on a 16^3 x 32 lattice with a large quark mass corresponding to m(\pi) = 1.13 GeV. Repulsive 3NF is found at short distance.; Comment: Talk given at 29th International Symposium on Lattice Field Theory (Lattice 2011), Squaw Valley, Lake Tahoe, California, USA, 11-16 Jul 2011, 7 pages, 4 figures

We study the three nucleon force in Nf=2 dynamical clover fermion lattice QCD, utilizing the Nambu-Bethe-Salpeter wave function of the three nucleon system. Since parity-odd two nucleon potentials are not available in lattice QCD at this moment, we develop a new formulation to extract the genuine three nucleon force which requires only the information of parity-even two nucleon potentials. In order to handle the extremely expensive calculation cost, we consider a specific three-dimensional coordinate configuration for the three nucleons. We find that the linear setup is advantageous, where nucleons are aligned linearly with equal spacings. The lattice calculation is performed with 16^3 x 32 configurations at \beta=1.95, m_\pi=1.13 GeV generated by CP-PACS Collaboration, and the result of the three nucleon force in triton channel is presented.; Comment: Talk given at International conference on the structure of baryons (BARYONS'10), Osaka, Japan, 7-11 Dec 2010, 4 pages, 2 figures

We study the $B \to Kl^+l^-$ semileptonic decay process in three-flavor lattice QCD. We analyze several ensembles generated by the MILC collaboration at different lattice spacings and sea-quark masses. We use the asqtad improved staggered action for the light quarks and the clover action with the Fermilab interpretation for the heavy $b$ quark. We present preliminary results for the vector current induced form factors for a range of kaon energies. Our analysis includes chiral and continuum extrapolations based on SU(2) staggered \chi PT.; Comment: 7 pages, 4 figures, presented at The XXIX International Symposium on Lattice Field Theory - Lattice 2011, July 10-16, 2011, Squaw Valley, Lake Tahoe, California

We investigate baryon-baryon interactions with strangeness $S=-2$ and isospin I=0 system from Lattice QCD. In order to solve this system, we prepare three types of baryon-baryon operators ($\Lambda-\Lambda$, $N-\Xi$ and $\Sigma-\Sigma$) for the sink and construct three source operators diagonalizing the $3\times3$ correlation matrix. Combining of the prepared sink operators with the diagonalized source operators, we obtain nine effective Nambu-Bethe-Salpeter (NBS) wave functions. The $3\times3$ potential matrix is calculated by solving the coupled-channel Schr\"odinger equation. The flavor SU(3) breaking effects of the potential matrix are also discussed by comparing with the results of the SU(3) limit calculation. Our numerical results are obtained from three sets of 2+1 flavor QCD gauge configurations provided by the CP-PACS/JLQCD Collaborations.; Comment: 7 pages, 3 figures, Talk given at the XXVIII International Symposium on Lattice Field Theory (Lattice 2010), Italy

The charmed-strange meson masses are calculated on a quenched lattice QCD. The charm and strange quark propagators are calculated on the same lattice with the overlap fermion. $16^3\times 72$ lattice with Wilson gauge action at $\beta=0.6345$ are used. The charm and strange quark masses are determined by fitting the $J/\psi$ and $\phi$ masses respectively. The charmed strange meson spectrum for the scalar, axial, pseudoscalar and vector channels are calculated. They agree with experiments. In particular, we find the scalar meson mass to be 2248(78)MeV which is in agreement with that of D_{s0}^*(2317).; Comment: 7 pages, 6 figures, The XXVI International Symposium on Lattice Field Theory, July 14-19, 2008

We report the recent progress on the determination of three-nucleon forces (3NF) in lattice QCD. We utilize the Nambu-Bethe-Salpeter (NBS) wave function to define the potential in quantum field theory, and extract two-nucleon forces (2NF) and 3NF on equal footing. The enormous computational cost for calculating multi-baryon correlators on the lattice is drastically reduced by developing a novel contraction algorithm (the unified contraction algorithm). Quantum numbers of the three-nucleon (3N) system are chosen to be (I, J^P)=(1/2,1/2^+) (the triton channel), and we extract 3NF in which three nucleons are aligned linearly with an equal spacing. Lattice QCD simulations are performed using N_f=2 dynamical clover fermion configurations at the lattice spacing of a = 0.156 fm on a 16^3 x 32 lattice with a large quark mass corresponding to m(\pi)= 1.13 GeV. Repulsive 3NF is found at short distance.; Comment: Plenary talk given at the 20th International IUPAP Conference on Few-Body Problems in Physics (FB20), Fukuoka, Japan, 20-25 Aug 2012, 9 pages, 2 figures

We present our latest study of Lambda-Nucleon (LN) interaction by using lattice QCD, following up on our report at LATTICE 2008. We have calculated not only the scattering lengths but also the central and tensor potentials, which are obtained from the Bethe-Salpeter (BS) amplitude measured in lattice QCD. For these calculations, we employ two different types of gauge configurations: (i) 2+1 flavor full QCD configurations generated by the PACS-CS collaboration at $\beta=1.9$ ($a=0.0907(13)$ fm) on a $32^3\times 64$ lattice, whose spatial volume is (2.90 fm)$^3$, with the quark masses corresponding to $(m_\pi,m_K)\approx (301,592)$, $(414,637)$, $(570,724)$ and $(699,787)$ (in units of MeV). (ii) Quenched QCD configurations at $\beta=5.7$ ($a=0.1416(9)$ fm) on a $32^3\times48$ lattice, whose spatial volume is (4.5 fm)$^3$, with the quark masses corresponding to $(m_\pi,m_K)\approx (512,606)$, $(464, 586)$ and $(407,565)$. The following qualitative features are found: The LN potential has a relatively strong (weak) repulsive core in the $^1S_0$ ($^3S_1$) channel at short distance, while the potential has slight attractive region at medium distance. The tensor potential is found to be weaker than the $NN$ case. These results hold in both full and quenched QCD. The energy of the ground state on the finite lattice volume is calculated: In both spin channels...

Nucleon matters are studied based on QCD. We extract nucleon-nucleon interaction from lattice QCD simulations in a recently developed approach, and then derive the equations of state of the symmetric nuclear matter and the pure neutron matter, at zero temperature, in the Brueckner-Hartree-Fock framework. We find that QCD reproduce known features of the symmetric nuclear matter, such as the self-binding and saturation, at some values of quark mass. We find also that the pure neutron matter become more stiff at large density as quark mass decrease. We apply these equations of state to neutron star and study its mass and radius.; Comment: 7 pages, 5 figures, presented at the 31st International Symposium on Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013, Mainz, Germany

We present the first unquenched lattice-QCD calculation of the hadronic form factors for the exclusive decay $\overline{B} \rightarrow D \ell \overline{\nu}$ at nonzero recoil. We carry out numerical simulations on fourteen ensembles of gauge-field configurations generated with 2+1 flavors of asqtad-improved staggered sea quarks. The ensembles encompass a wide range of lattice spacings (approximately 0.045 to 0.12 fm) and ratios of light (up and down) to strange sea-quark masses ranging from 0.05 to 0.4. For the $b$ and $c$ valence quarks we use improved Wilson fermions with the Fermilab interpretation, while for the light valence quarks we use asqtad-improved staggered fermions. We extrapolate our results to the physical point using rooted staggered heavy-light meson chiral perturbation theory. We then parameterize the form factors and extend them to the full kinematic range using model-independent functions based on analyticity and unitarity. We present our final results for $f_+(q^2)$ and $f_0(q^2)$, including statistical and systematic errors, as coefficients of a series in the variable $z$ and the covariance matrix between these coefficients. We then fit the lattice form-factor data jointly with the experimentally measured differential decay rate from BaBar to determine the CKM matrix element...

We present a lattice-QCD calculation of the $B\to\pi\ell\nu$ semileptonic form factors and a new determination of the CKM matrix element $|V_{ub}|$. We use the MILC asqtad 2+1-flavor lattice configurations at four lattice spacings and light-quark masses down to 1/20 of the physical strange-quark mass. We extrapolate the lattice form factors to the continuum using staggered chiral perturbation theory in the hard-pion and SU(2) limits. We employ a model-independent $z$ parameterization to extrapolate our lattice form factors from large-recoil momentum to the full kinematic range. We introduce a new functional method to propagate information from the chiral-continuum extrapolation to the $z$ expansion. We present our results together with a complete systematic error budget, including a covariance matrix to enable the combination of our form factors with other lattice-QCD and experimental results. To obtain $|V_{ub}|$, we simultaneously fit the experimental data for the $B\to\pi\ell\nu$ differential decay rate obtained by the BaBar and Belle collaborations together with our lattice form-factor results. We find $|V_{ub}|=(3.72\pm 0.16)\times 10^{-3}$ where the error is from the combined fit to lattice plus experiments and includes all sources of uncertainty. Our form-factor results bring the QCD error on $|V_{ub}|$ to the same level as the experimental error. We also provide results for the $B\to\pi\ell\nu$ vector and scalar form factors obtained from the combined lattice and experiment fit...

Three-nucleon forces (3NF) are investigated from two-flavor lattice QCD simulations. We utilize the Nambu-Bethe-Salpeter (NBS) wave function to determine two-nucleon forces (2NF) and 3NF in the same framework. As a first exploratory study, we extract 3NF in which three nucleons are aligned linearly with an equal spacing. This is the simplest geometrical configuration which reduces the huge computational cost of calculating the NBS wave function. Quantum numbers of the three-nucleon system are chosen to be (I, J^P)=(1/2,1/2^+) (the triton channel). Lattice QCD simulations are performed using N_f=2 dynamical clover fermion configurations at the lattice spacing of a = 0.156 fm on a 16^3 x 32 lattice with a large quark mass corresponding to m_\pi= 1.13 GeV. We find repulsive 3NF at short distance in the triton channel. Several sources of systematic errors are also discussed.; Comment: 20 pages, 7 figures. Accepted for publication in Prog. Theor. Phys

We investigate three-nucleon forces (3NF) from lattice QCD simulations, utilizing the Nambu-Bethe-Salpeter (NBS) wave function to determine two-nucleon forces (2NF) and 3NF on the same footing. Quantum numbers of the three-nucleon (3N) system are chosen to be (I, J^P)=(1/2, 1/2^+) (the triton channel). We consider the simplest geometrical configuration where 3N are aligned linearly with an equal spacing, to reduce the enormous computational cost. Lattice QCD simulations are performed using Nf=2 dynamical clover fermion configurations at the lattice spacing of a = 0.156 fm on a 16^3 x 32 lattice with a large quark mass corresponding to m(\pi) = 1.13 GeV. We find repulsive 3NF at short distance.; Comment: Talk given at the 19th Particles and Nuclei International Conference (PANIC11), Boston, USA, 24-29 Jul 2011, 3 pages, 2 figures

4 pages, 1 figure.-- PACS nrs.: 12.38.Gc, 13.25.Es, 11.30.Rd.-- ISI Article Identifier: 000244420700019.-- ArXiv pre-print available at: http://arxiv.org/abs/hep-ph/0607220; We compute the leading-order low-energy constants of the ΔS=1 effective weak Hamiltonian in the quenched approximation of QCD with up, down, strange, and charm quarks degenerate and light. They are extracted by comparing the predictions of finite-volume chiral perturbation theory with lattice QCD computations of suitable correlation functions carried out with quark masses ranging from a few MeV up to half of the physical strange mass. We observe a ΔI = 1/2 enhancement in this corner of the parameter space of the theory. Although matching with the experimental result is not observed for the Δ = 1/2 amplitude, our computation suggests large QCD contributions to the physical ΔI = 1/2 rule in the GIM limit, and represents the first step to quantify the role of the charm-quark mass in K -> π π amplitudes. The use of fermions with an exact chiral symmetry is an essential ingredient in our computation.; Our calculations were performed on PC clusters at CILEA, DESY Hamburg, and the Universities of Rome "La Sapienza" and Valencia-IFIC, as well as on the IBM MareNostrum at the Barcelona Supercomputing Center...

We investigate two-point correlation functions of left-handed currents computed in quenched lattice QCD with the Neuberger-Dirac operator. We consider two lattice spacings a ~ 0.09, 0.12 fm and two different lattice extents L ~ 1.5, 2.0 fm; quark masses span both the p- and the epsilon-regimes. We compare the results with the predictions of quenched chiral perturbation theory, with the purpose of testing to what extent the effective theory reproduces quenched QCD at low energy. In the p-regime we test volume and quark mass dependence of the pseudoscalar decay constant and mass; in the epsilon-regime, we investigate volume and topology dependence of the correlators. While the leading order behaviour predicted by the effective theory is very well reproduced by the lattice data in the range of parameters that we explored, our numerical data are not precise enough to test next-to-leading order effects.; S. N. is supported by Marie Curie Fellowship MEIF-CT-2006-025673. S. N. and P. H. aknowledge partial support by Spanish Ministry for Education and Science under grant FPA2004- 00996 and Generalitat Valenciana under grant GVACOMP2007-156. C. P. acknowledges support from the Ramón y Cajal Programme, as well as by the research grant FPA2006-05807 and the Consolider-Ingenio 2010 Programme CPAN (CSD2007-00042) of the Spanish Ministry for Education and Science. L. G....

Predictions of heavy quark parameters are an integral component of precision tests of the Standard Model of particle physics. Experimental measurements of electroweak processes involving heavy hadrons provide stringent tests of Cabibbo-Kobayashi-Maskawa (CKM) matrix unitarity and serve as a probe of new physics. Hadronic matrix elements parameterise the strong dynamics of these interactions and these matrix elements must be calculated nonperturbatively. Lattice quantum chromodynamics (QCD) provides the framework for nonperturbative calculations of QCD processes. Current lattices are too coarse to directly simulate b quarks. Therefore an effective theory, nonrelativistic QCD (NRQCD), is used to discretise the heavy quarks. High precision simulations are required so systematic uncertainties are removed by improving the NRQCD action. Precise simulations also require improved sea quark actions, such as the highly-improved staggered quark (HISQ) action. The renormalisation parameters of these actions cannot be feasibly determined by hand and thus automated procedures have been developed. In this dissertation I apply automated lattice pertubartion theory to a number of heavy quark calculations. I first review the fundamentals of lattice QCD and the construction of lattice NRQCD. I then motivate and discuss lattice perturbation theory in detail...