704 research outputs found

    Moments of Isovector Quark Distributions in Lattice QCD

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    We investigate the connection of lattice calculations of moments of isovector parton distributions to the physical regime through extrapolations in the quark mass. We consider the one pion loop renormalisation of the nucleon matrix elements of the corresponding operators and thereby develop formulae with which to extrapolate the moments of the unpolarised, helicity and transversity distributions. These formulae are consistent with chiral perturbation theory in the chiral limit and incorporate the correct heavy quark limits. In the polarised cases, the inclusion of intermediate states involving the Δ\Delta isobar is found to be very important. The results of our extrapolations are in general agreement with the phenomenological values of these moments where they are known, and for the first time we perform an extrapolation of the low moments of the isovector transversity distribution which is consistent with chiral symmetry.Comment: 3 pages, 3 figures. Talk given by W. Detmold. at QNP200

    QCD inequalities for hadron interactions

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    We derive generalisations of the Weingarten--Witten QCD mass inequalities for particular multi-hadron systems. For systems of any number of identical pseudo-scalar mesons of maximal isospin, these inequalities prove that interactions between the constituent mesons must be repulsive and that no bound states can form in these channels. Similar constraints in less symmetric systems are also extracted. These results are compatible with experimental results (where known) and recent lattice QCD calculations, and also lead to a more stringent bound on the nucleon mass than previously derived, mN3/2mπm_N \ge 3/2 m_\pi.Comment: 5 pages, 2 figure

    Universality of the EMC Effect

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    Using effective field theory, we investigate nuclear modification of nucleon parton distributions (for example, the EMC effect). We show that the universality of the shape distortion in nuclear parton distributions (the factorisation of the Bjorken x and atomic number (A) dependence) is model independent and emerges naturally in effective field theory. We then extend our analysis to study the analogous nuclear modifications in isospin and spin dependent parton distributions and generalised parton distributions.Comment: 8 pages, 2 figure

    Lattice QCD study of mixed systems of pions and kaons

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    The different ground state energies of N-pion and M-kaon systems for N+M <=12 are studied in lattice QCD. These energies are then used to extract the various two- and three- body interactions that occur in these systems. Particular attention is paid to additional thermal states present in the spectrum because of the finite temporal extent. These calculations are performed using one ensemble of 2+1 flavor anisotropic lattices with a spatial lattice spacing a_s ~ 0.125 fm, an anisotropy factor {\xi}=a_s/a_t=3.5, and a spatial volume L^3 ~ (2.5 fm)^3. The quark masses used correspond to pion and kaon masses of m_{\pi} ~ 383 MeV and m_K ~ 537 MeV, respectively. The isospin and strangeness chemical potentials of these systems are found to be in the region where chiral perturbation theory and hadronic models predict a phase transition between a pion condensed phase and a kaon condensed phase

    ΛbΛ+\Lambda_b \to \Lambda \ell^+ \ell^- form factors, differential branching fraction, and angular observables from lattice QCD with relativistic bb quarks

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    Using (2+1)(2+1)-flavor lattice QCD, we compute the 10 form factors describing the ΛbΛ\Lambda_b \to \Lambda matrix elements of the bsb \to s vector, axial vector, and tensor currents. The calculation is based on gauge field ensembles generated by the RBC and UKQCD Collaborations with a domain-wall action for the uu, dd, and ss quarks and the Iwasaki gauge action. The bb quark is implemented using an anisotropic clover action, tuned nonperturbatively to the physical point, and the currents are renormalized with a mostly nonperturbative method. We perform simultaneous chiral, continuum, and kinematic extrapolations of the form factors through modified zz expansions. Using our form factor results, we obtain precise predictions for the ΛbΛ(p+π)μ+μ\Lambda_b \to \Lambda(\to p^+ \pi^-) \mu^+ \mu^- differential branching fraction and angular observables in the Standard Model.Comment: 33 pages, 9 figures, form factor parameters included as ancillary file

    Composite Vector Particles in External Electromagnetic Fields

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    Lattice quantum chromodynamics (QCD) studies of electromagnetic properties of hadrons and light nuclei, such as magnetic moments and polarizabilities, have proven successful with the use of background field methods. With an implementation of nonuniform background electromagnetic fields, properties such as charge radii and higher electromagnetic multipole moments (for states of higher spin) can be additionally obtained. This can be achieved by matching lattice QCD calculations to a corresponding low-energy effective theory that describes the static and quasi-static response of hadrons and nuclei to weak external fields. With particular interest in the case of vector mesons and spin-1 nuclei such as the deuteron, we present an effective field theory of spin-1 particles coupled to external electromagnetic fields. To constrain the charge radius and the electric quadrupole moment of the composite spin-1 field, the single-particle Green's functions in a linearly varying electric field in space are obtained within the effective theory, providing explicit expressions that can be used to match directly onto lattice QCD correlation functions. The viability of an extraction of the charge radius and the electric quadrupole moment of the deuteron from the upcoming lattice QCD calculations of this nucleus is discussed.Comment: 38 page

    Dark Nuclei II: Nuclear Spectroscopy in Two-Colour QCD

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    We consider two-colour QCD with two flavours of quarks as a possible theory of composite dark matter and use lattice field theory methods to investigate nuclear spectroscopy in the spin J=0J=0 and J=1J=1 multi-baryon sectors. We find compelling evidence that J=1J=1 systems with baryon number B=2,3B=2,3 (and their mixed meson-baryon counterparts) are bound states - the analogues of nuclei in this theory. In addition, we estimate the σ\sigma-terms of the J=0J=0 and J=1J=1 single baryon states which are important for the coupling of the theory to scalar currents that may mediate interactions with the visible sector.Comment: 40 pages, 26 figure

    Baryon properties in meson mediums from lattice QCD

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    We present results for the ground-state mass shifts of octet baryons due to the presence of a medium of pions or kaons from a lattice QCD calculation performed at a single value of the quark mass, corresponding to a pion mass of mπm_\pi ~ 390 MeV, and a spatial volume V ~ (4fm)^3. We use a canonical approach in which correlators are formed using a single baryon propagator and a fixed number of meson propagators, up to n=9. From the ground-state energies we calculate two- and three-body interaction parameters. We also extract combinations of low-energy constants by comparing our results to tree level chiral perturbation theory at non-zero isospin/kaon chemical potential.Comment: 7 pages, 3 figures, Proceedings of the 31st International Symposium on Lattice Field Theory (Lattice 2013), July 29 - August 3, 2013, Mainz, German
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