arXiv subjects

Gunnar Bali

Publications and source records attributed to Gunnar Bali.

35 records · Page 2Linked to original sources

arXiv2015

Charmed baryon spectroscopy and light flavour symmetry from lattice QCD

We determine the ground state and first excited state masses of singly and doubly charmed spin 1/2 and 3/2 baryons with positive and negative parity. Configurations with $N_f=2+1$ non-perturbatively improved Wilson-clover fermions were employed, with the same quark action also being used for the valence quarks, including the charm. The spectrum is calculated for pion masses in the range $M_π\sim 259-460$ MeV at a lattice spacing $a\sim 0.075$ fm. Finite volume effects are studied comparing lattices with two different linear spatial extents ($1.8\,{\rm fm}$ and $2.4\,{\rm fm}$). The physical point is approached from the SU(3) limit keeping the flavour averaged light quark mass fixed. The baryon masses are extrapolated using expansions in the strange-light quark mass difference. Most particles fall into the expected SU(3) multiplets with well constrained extrapolations, the exceptions having a possibly more complex internal structure. Overall agreement is found with experiment for the masses and splittings of the singly charmed baryons. As part of the calculation an analysis of the lower lying charmonium, $D$ and $D_s$ spectra was performed in order to assess discretisation errors. The gross spectra are reproduced, including the $D^*_{s0}$, $D_{s1}$ and $D_1$ mesons, while at this single lattice spacing hyperfine splittings come out $10-20$ MeV too low.

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arXiv2012

Charmed hadron spectroscopy on the lattice for $N_f=2+1$ flavours

We study the spectra of charmonia, charmed mesons, singly and doubly charmed baryons using Lattice QCD, with 2+1 flavours of fermions. In the case of mesons, we include higher spin states, while for baryons, both positive and negative parity channels were investigated. By means of the variational method, we were able to extract a clean signal from the correlation functions and information about the excited states.

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arXiv2011

Potentials between pairs of static-light mesons

We give an update on our ongoing investigations of potentials between pairs of static-light mesons in Nf=2 Lattice QCD, in different spin and isospin channels. The question of attraction and repulsion is particularly interesting with respect to the X(3872) charmonium state and charged candidates such as the Z+(4430). We employ the nonperturbatively improved Sheikholeslami-Wohlert fermion and the Wilson gauge actions at two lattice spacings a approx. 0.084 fm and a approx. 0.077 fm with a pseudoscalar mass of mPS approx. 770 MeV and mPS approx. 400 MeV respectively. We use stochastic all-to-all propagator techniques, improved by a hopping parameter expansion. The analysis is based on the variational method, utilizing various source and sink interpolators.

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arXiv2010

Disconnected contributions to hadronic structure

We present an update of an on-going project to determine the disconnected contributions to hadronic structure, specifically, the scalar matrix element and the quark contribution to the spin of the nucleon.

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arXiv2010

Meson and baryon masses with low mode averaging

We describe and test a method known in the literature as low mode averaging to improve Euclidean two-point functions in lattice QCD using the low-lying eigenmodes of the Wilson-Dirac operator D. The contribution from the low modes is averaged over all positions of the quark sources while the contribution from high modes is calculated in the traditional way using one source point per lattice. We apply this method to different baryon and meson two-point functions and we compare the improvements using the eigenmodes of the non-hermitian operator D and the eigenmodes of the hermitian operator Q=gamma5 D. The convergence strongly depends on the parity of the states.

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arXiv2010

Hunting the static energy renormalon

We employ Numerical Stochastic Perturbation Theory (NSPT) together with twisted boundary conditions (TBC) to search for the leading renormalon in the perturbative expansion of the static energy. This renormalon is expected to emerge four times faster than the one for the gluon conden- sate in the plaquette. We extract the static energy from Polyakov loop calculations up to 12 loops and present preliminary results, indicating a significant step towards confirming the theoretical expectation.

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arXiv2010

Static-light meson-meson potentials

We investigate potentials between pairs of static-light mesons in Nf=2 Lattice QCD, in different spin channels. The question of attraction and repulsion is particularly interesting with respect to the X(3872) charmonium state and charged candidates such as the Z+(4430). We employ the nonperturbatively improved Sheikholeslami-Wohlert fermion and the Wilson gauge actions at a lattice spacing a approx. 0.084 fm and a pseudoscalar mass mPS approx. 760 MeV. We use stochastic all-to-all propagator techniques, improved by a hopping parameter expansion. The analysis is based on the variational method, utilizing various source and sink interpolators.

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arXiv2010

Improved Semileptonic Form Factor Calculations in Lattice QCD

We investigate the computational efficiency of two stochastic based alternatives to the Sequential Propagator Method used in Lattice QCD calculations of heavy-light semileptonic form factors. In the first method, we replace the sequential propagator, which couples the calculation of two of the three propagators required for the calculation, with a stochastic propagator so that the calculations of all three propagators are independent. This method is more flexible than the Sequential Propagator Method but introduces stochastic noise. We study the noise to determine when this method becomes competitive with the Sequential Propagator Method, and find that for any practical calculation it is competitive with or superior to the Sequential Propagator Method. We also examine a second stochastic method, the so-called ``one-end trick", concluding it is relatively inefficient in this context. The investigation is carried out on two gauge field ensembles, using the non-perturbatively improved Wilson-Sheikholeslami-Wohlert action with N_f=2 mass-degenerate sea quarks. The two ensembles have similar lattice spacings but different sea quark masses. We use the first stochastic method to extract ${\mathcal O}(a)$-improved, matched lattice results for the semileptonic form factors on the ensemble with lighter sea quarks, extracting f_+(0).

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arXiv2009

A Stochastic Method for Semileptonic Form Factor Calculations on the Lattice

We investigate an alternative to the Sequential Propagator Method used in Lattice QCD calculations of semileptonic form factors. We replace the sequential propagator with a stochastic propagator so that, in principle, all momentum and sink smearing combinations are available with only a single spin-color inversion. Practically, the stochastic noise is significant and must be reduced at the cost of more inversions. We study the behavior of the stochastic noise and compare the computational costs of this stochastic technique and the Sequential Propagator Method. We also present preliminary semileptonic form factor results using the stochastic technique on N_f=2 configurations with a non-perturbatively improved Sheikoleslami-Wohlert action generated by the QCDSF collaboration. At a fixed cost, measured in terms of the number of heavy-quark inversions, the method provides more correlators for the extraction of the form factors at various q^2's than the Sequential Propagator Method. These additional correlators reduce the total statistical errors of certain kinematic points, although the stochastic error is still comparable to the gauge error at other points.

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arXiv2009

Strangeness and charm content of the nucleon

We present results on the scalar strangeness and charm contents of the nucleon and of the disconnected contributions to the nucleon spin. These are obtained on two flavour non-perturbatively improved Sheikholeslami-Wilson configurations at a pseudoscalar mass of 290 MeV. We quote f_{T_s} = m_s /m_N< = 0.070(22) as our preliminary value for the strange quark fraction of the nucleon mass and a modified minimal subtraction scheme value Delta s = -0.015(10), with as yet unknown systematics, for the strangeness contribution to the spin.

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arXiv2009

Mixing of S-Wave Charmonia with $\mathrm{D}\overline{\mathrm{D}}$ Molecule States

Charmonium states can decay into pairs of $D$ and $\overline{D}$ mesons if their masses are above the allowed decay thresholds. In general $c\bar{c}$ states near threshold will also undergo mixing with $D\overline{D}$ molecular (or tetraquark) states, by creation and annihilation of light quark-antiquark pairs. The investigation of such effects sheds light on the higher Fock state contributions to charmonium wavefunctions and on mass shifts, relative to a scenario where such mixing effects are neglected. A variational approach is applied to a mixing matrix between operators of both sectors, of $c\bar{c}$ and of $D\overline{D}$ molecular type. The efficient calculation of several diagrams appearing in this matrix requires all-to-all propagators, which are realized by sophisticated stochastic estimator techniques. The runs are performed on $n_F=2$ $24^3\times 48$ lattice volumes with $m_π \approx 280$ MeV, using the non-perturbatively improved clover Wilson action, both for valence and for sea quarks.

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arXiv2009

Static-static-light baryonic potentials

We determine doubly heavy baryonic potentials as a function of the distance between the two static sources, coupled to a light relativistic quark, for different quantum numbers. We use the variational method to compute the ground state and the first two excitations. These can be used as an input to nonrelativistic models or to NRQCD calculations of properties of doubly heavy baryons. We compare our findings with a factorization model. We employ all-to-all propagator methods, improved by an additional hopping parameter expansion and Wuppertal smearing on QCDSF configurations with two sea quark flavors.

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arXiv2009

$η'$-$η_c$-mixing with improved stochastic estimators

Charmonia are flavour singlet mesons and thus in principle contributions from disconnected quark line diagrams might affect their masses, either directly or via mixing with other flavour singlet channels. We present a first study that takes both effects into account. We employ improved stochastic all-to-all propagator techniques (including new methods) to calculate the diagrams that appear within the mixing matrix between the $η'$ and the $η_c$. The runs are initially performed on $N_f=2$ $16^3\times 32$ configurations with the non-perturbatively improved Sheikholeslami-Wilson action, both for valence and sea quarks.

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arXiv2009

Hunting for the strangeness content of the nucleon

We present results for the strangeness contribution to the nucleon, , and to the spin of the nucleon, Delta s. By combining several variance reduction techniques for all-to-all propagators we are able to obtain gains in terms of computer time of factors of 25-30 for the disconnected loop that is needed within the calculation of Delta s, relative to the standard approach of just employing time partitioning/dilution. For , the error is dominated by the gauge noise.

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arXiv2008

Mesons at large Nc from lattice QCD

We calculate the pion and rho meson masses in quenched SU(N) gauge theories for N = 2, 3, 4 and 6. Extrapolating these results to the chiral and large-N limits, we find m_rho = (1.670 +/- 0.024) sqrt(sigma) = (741 +/- 11) MeV for the rho meson mass at a fixed lattice spacing a \approx 0.2093 sqrt(sigma) \approx 0.093 fm where we use the (arbitrary) value (444 MeV)^2 for the string tension. We estimate a continuum limit large-N value, m_rho = 1.77 +/- 0.05 sqrt(sigma). We find 1/N^2 corrections to be small and we compare our results to predictions from AdS/QCD.

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arXiv2007

Charmonium spectrum including higher spin and exotic states

We study the charmonium spectrum including higher spin and exotic states. We use the Sheikholeslami-Wilson (clover) action for $N_f=2$ sea quarks as well as for the charm valence quark. In order to access excited states we apply a variational method with a basis of highly optimized operators.

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arXiv2007

Meson masses at large N_c

We analyze the meson spectrum in SU(N_c) lattice gauge theory. We use Wilson quarks to measure the pseudoscalar and vector masses in SU(2), SU(3), SU(4) and SU(6), and extrapolate to the large-N_c limit. We find that finite-N_c corrections are small at all values of the quark mass.

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