704 research outputs found

### Moments of Isovector Quark Distributions in Lattice QCD

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 $\Delta$
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

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, $m_N \ge 3/2
m_\pi$.Comment: 5 pages, 2 figure

### Universality of the EMC Effect

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

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

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

Using $(2+1)$-flavor lattice QCD, we compute the 10 form factors describing
the $\Lambda_b \to \Lambda$ matrix elements of the $b \to s$ 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
$u$, $d$, and $s$ quarks and the Iwasaki gauge action. The $b$ 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 $z$ expansions. Using our form factor
results, we obtain precise predictions for the $\Lambda_b \to \Lambda(\to p^+
\pi^-) \mu^+ \mu^-$ 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

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

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 $J=0$ and $J=1$ multi-baryon sectors. We find
compelling evidence that $J=1$ systems with baryon number $B=2,3$ (and their
mixed meson-baryon counterparts) are bound states - the analogues of nuclei in
this theory. In addition, we estimate the $\sigma$-terms of the $J=0$ and $J=1$
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

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
$m_\pi$ ~ 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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