arXiv subjects

J. M. Zanotti

Publications and source records attributed to J. M. Zanotti.

At least 109 records · Page 6Linked to original sources

arXiv2010

Nucleon and $N^* (1535)$ Distribution Amplitudes

The QCDSF collaboration has investigated the distribution amplitudes and wavefunction normalization constants of the nucleon and its parity partner, the $N^* (1535)$. We report on recent progress in the calculation of these quantities on configurations with two dynamical flavors of $\mathcal{O}(a)$-improved Wilson fermions. New data at pion masses of approximately 270 MeV helps in significantly reducing errors in the extrapolation to the physical point.

hep-lat↗

arXiv2010

Renormalisation of composite operators in lattice QCD: perturbative versus nonperturbative

The perturbative and nonperturbative renormalisation of quark-antiquark operators in lattice QCD with two flavours of clover fermions is investigated within the research programme of the QCDSF collaboration. Operators with up to three derivatives are considered. The nonperturbative results based on the RI-MOM scheme are compared with estimates from one- and two-loop lattice perturbation theory.

hep-lat↗

arXiv2010

Exploring the Nucleon Structure from First Principles of QCD

Quantum Chromodynamics (QCD) is generally assumed to be the fundamental theory underlying nuclear physics. In recent years there is progress towards investigating the nucleon structure from first principles of QCD. Although this structure is best revealed in Deep Inelastic Scattering, a consistent analysis has to be performed in a fully non-perturbative scheme. The only known method for this purpose are lattice simulations. We first sketch the ideas of Monte Carlo simulations in lattice gauge theory. Then we comment in particular on the issues of chiral symmetry and operator mixing. Finally we present our results for the Bjorken variable of a single quark, and for the second Nachtmann moment of the nucleon structure functions.

hep-lat↗

arXiv2010

Pion in a Box

The residual mass of the pion in a finite spatial box at vanishing quark masses, the mass gap, is computed with two flavors of dynamical clover fermions. The result is compared with predictions of chiral perturbation theory in the delta regime.

hep-lat↗

arXiv2010

K->pi form factors with reduced model dependence

Using partially twisted boundary conditions we compute the K->pi semi-leptonic form factors in the range of momentum transfers 0 <~ q^2 <= q^2_{max}=(mK-mpi)^2 in lattice QCD with N_f=2+1 dynamical flavours. In this way we are able to determine f+(0) without any interpolation in the momentum transfer, thus eliminating one source of systematic error. This study confirms our earlier phenomenological ansatz for the strange quark mass dependence of the scalar form factor. We identify and estimate potentially significant NNLO effects in the chiral expansion that guides the extrapolation of the data to the physical point. Our main result is f+(0) = 0.9599(34)(^{+31}_{-43})(14)$, where the first error is statistical, the second error is due to the uncertainties in the chiral extrapolation of the lattice data and the last error is an estimate of potential discretisation effects.

hep-lat↗

arXiv2010

Tuning the strange quark mass in lattice simulations

QCD lattice simulations with 2+1 flavours typically start at rather large up-down and strange quark masses and extrapolate first the strange quark mass to its physical value and then the up-down quark mass. An alternative method of tuning the quark masses is discussed here in which the singlet quark mass is kept fixed, which ensures that the kaon always has mass less than the physical kaon mass. It can also take into account the different renormalisations (for singlet and non-singlet quark masses) occurring for non-chirally invariant lattice fermions and so allows a smooth extrapolation to the physical quark masses. This procedure enables a wide range of quark masses to be probed, including the case with a heavy up-down quark mass and light strange quark mass. Results show the correct order for the baryon octet and decuplet spectrum and an extrapolation to the physical pion mass gives mass values to within a few percent of their experimental values.

hep-lat↗

arXiv2009

Lattice Investigations of Nucleon Structure at Light Quark Masses

Lattice simulations of hadronic structure are now reaching a level where they are able to not only complement, but also provide guidance to current and forthcoming experimental programmes at, e.g. Jefferson Lab, COMPASS/CERN and FAIR/GSI. By considering new simulations at low quark masses and on large volumes, we review the recent progress that has been made in this exciting area by the QCDSF/UKQCD collaboration. In particular, results obtained close to the physical point for several quantities, including electromagnetic form factors and moments of ordinary parton distribution functions, show some indication of approaching their phenomenological values.

hep-lat↗

arXiv2009

Quark structure from the lattice Operator Product Expansion

We have reported elsewhere in this conference on our continuing project to determine non-perturbative Wilson coefficients on the lattice, as a step towards a completely non-perturbative determination of the nucleon structure. In this talk we discuss how these Wilson coefficients can be used to extract Nachtmann moments of structure functions, using the case of off-shell Landau-gauge quarks as a first simple example. This work is done using overlap fermions, because their improved chiral properties reduce the difficulties due to operator mixing.

hep-lat↗

arXiv2009

The consequence of excess configurational entropy on fragility: the case of a polymer/oligomer blend

By taking advantage of the molecular weight dependence of the glass transition of polymers and their ability to form perfectly miscible blends, we propose a way to modify the fragility of a system, from fragile to strong, keeping the same glass properties, i.e. vibrational density of states, mean-square displacement and local structure. Both slow and fast dynamics are investigated by calorimetry and neutron scattering in an athermal polystyrene/oligomer blend, and compared to those of a pure 17-mer polystyrene considered to be a reference, of same Tg. Whereas the blend and the pure 17-mer have the same heat capacity in the glass and in the liquid, their fragilities differ strongly. This difference in fragility is related to an extra configurational entropy created by the mixing process and acting at a scale much larger than the interchain distance, without affecting the fast dynamics and the structure of the glass.

cond-mat.soft↗

arXiv2009

Results from 2+1 flavours of SLiNC fermions

QCD results are presented for a 2+1 flavour fermion clover action (which we call the SLiNC action). A method of tuning the quark masses to their physical values is discussed. In this method the singlet quark mass is kept fixed, which solves the problem of different renormalisations (for singlet and non-singlet quark masses) occuring for non-chirally invariant lattice fermions. This procedure enables a wide range of quark masses to be probed, including the case with a heavy up-down quark mass and light strange quark mass. Preliminary results show the correct splittings for the baryon (octet and) decuplet spectrum.

hep-lat↗

arXiv2009

A Non-Perturbative Operator Product Expansion

Nucleon structure functions can be observed in Deep Inelastic Scattering experiments, but it is an outstanding challenge to confront them with fully non-perturbative QCD results. For this purpose we investigate the product of electromagnetic currents (with large photon momenta) between quark states (of low momenta). By means of an Operator Product Expansion the structure function can be decomposed into matrix elements of local operators, and Wilson coefficients. For consistency both have to be computed non-perturbatively. Here we present precision results for a set of Wilson coefficients. They are evaluated from propagators for numerous quark momenta on the lattice, where the use of chiral fermions suppresses undesired operator mixing. This over-determines the Wilson coefficients, but reliable results can be extracted by means of a Singular Value Decomposition.

hep-lat↗

arXiv2009

The spin structure of the pion

We present the first calculation of the transverse spin structure of the pion in lattice QCD. Our simulations are based on two flavors of non-perturbatively improved Wilson fermions, with pion masses as low as 400 MeV in volumes up to (2.1 fm)^3 and lattice spacings below 0.1 fm. We find a characteristic asymmetry in the spatial distribution of transversely polarized quarks. This asymmetry is very similar in magnitude to the analogous asymmetry we previously obtained for quarks in the nucleon. Our results support the hypothesis that all Boer-Mulders functions are alike.

hep-lat↗

arXiv2009

Non-perturbative improvement of stout-smeared three flavour clover fermions

We discuss a 3-flavour lattice QCD action with clover improvement in which the fermion matrix has single level stout smearing for the hopping terms together with unsmeared links for the clover term. With the (tree-level) Symanzik improved gluon action this constitutes the Stout Link Non-perturbative Clover or SLiNC action. To cancel O(a) terms the clover term coefficient has to be tuned. We present here results of a non-perturbative determination of this coefficient using the Schroedinger functional and as a by-product a determination of the critical hopping parameter. Comparisons of the results are made with lowest order perturbation theory.

hep-lat↗

arXiv2009

Precision electromagnetic structure of decuplet baryons in the chiral regime

The electromagnetic properties of the baryon decuplet are calculated in quenched QCD on a 20^3 x 40 lattice with a lattice spacing of 0.128 fm using the fat-link irrelevant clover (FLIC) fermion action with quark masses providing a pion mass as low as 300 MeV. Magnetic moments and charge radii are extracted from the electric and magnetic form factors for each individual quark sector. From these, the corresponding baryon properties are constructed. We present results for the higher order moments of the spin-3/2 baryons, including the electric quadrupole moment E2 and the magnetic octupole moment M3. The world's first determination of a non-zero M3 form factor for the Delta baryon is presented. With these results we provide a conclusive analysis which shows that decuplet baryons are deformed. We compare the decuplet baryon results from a similar lattice calculation of the octet baryons. We establish that the environment sensitivity is far less pronounced in the case of the decuplet baryons compared to that in the octet baryons. A surprising result is that the charge radii of the decuplet baryons are generally smaller than that of the octet baryons. The magnetic moment of the Delta^+ reveals a turn over in the low quark mass region, making it smaller than the proton magnetic moment. These results are consistent with the expectations of quenched chiral perturbation theory. A similar turn over is also noticed in the magnetic moment of the Sigma^*0, but not for Xi^* where only kaon loops can appear in quenched QCD. The electric quadrupole moment of the Omega^- baryon is positive when the negative charge factor is included, and is equal to 0.86 +- 0.12 x 10^-2 fm^2, indicating an oblate shape.

hep-lat↗

arXiv2009

Electroproduction of the N*(1535) resonance at large momentum transfer

We report on the first lattice calculation of light-cone distribution amplitudes of the N*(1535) resonance, which are used to calculate the transition form factors at large momentum transfers using light-cone sum rules. In the region Q^2 > 2 GeV^2, where the light-cone expansion is expected to converge, the results appear to be in good agreement with the experimental data.

hep-ph↗

arXiv2008

Kl3 and pion form factors using partially twisted boundary conditions

We compute the Kl3 and pion form factors using partially twisted boundary conditions. The twists are chosen so that the Kl3 form factors are calculated directly at zero momentum transfer (q^2=0), removing the need for a q^2 interpolation, while the pion form factor is determined at values of q^2 close to q^2=0. The simulations are performed on an ensemble of the RBC/UKQCD collaboration's gauge configurations with Domain Wall Fermions and the Iwaski gauge action with an inverse lattice spacing of 1.73(3) GeV. Simulating at a single pion mass of 330 MeV, we find the pion charge radius to be < r^2>_{330 MeV}=0.354(31) fm^2 which, using NLO SU(2) chiral perturbation theory, translates to a value of =0.418(31) fm^2 for a physical pion. For the value of the Kl3 form factor, f_{Kπ}^+(q^2), determined directly at q^2=0, we find a value of f_{Kπ}^+(0)=0.9742(41) at this particular quark mass, which agrees well with our earlier result (0.9774(35)) obtained using the standard, indirect method.

hep-lat↗

arXiv2008

Kl3 semileptonic form factor from 2+1 flavour lattice QCD

We present the first results for the Kl3 form factor from simulations with 2+1 flavours of dynamical domain wall quarks. Combining our result, namely f_+(0)=0.964(5), with the latest experimental results for Kl3 decays leads to |V_{us}|=0.2249(14), reducing the uncertaintity in this important parameter. For the O(p^6) term in the chiral expansion we obtain Δf=-0.013(5).

hep-lat↗

arXiv2008

Investigations of hadron structure on the lattice

Lattice simulations of hadronic structure are now reaching a level where they are able to not only complement, but also provide guidance to current and forthcoming experimental programmes at, e.g. Jefferson Lab, COMPASS/CERN and FAIR/GSI. In this talk I review the progress that has been made in this exciting area in the past year and discuss the advances that we can expect to see in the coming year.Topics to be covered include form factors (including transition form factors), moments of ordinary parton and generalised parton distribution functions, moments of distribution amplitudes, and magnetic and electric polarisabilities. I will also highlight the progress being made in determining disconnected contributions to hadronic properties. Of particular interest here is the size of the contribution to various nucleonic properties coming from strange quarks.

hep-lat↗