96 research outputs found
Nuclear Parity Violation from Lattice QCD
The electroweak interaction at the level of quarks and gluons are well
understood from precision measurements in high energy collider experiments.
Relating these fundamental parameters to Hadronic Parity Violation in nuclei
however remains an outstanding theoretical challenge. One of the most
interesting observables in this respect is the parity violating hadronic
neutral current: it is hard to measure in collider experiments and is thus the
least constrained observable of the Standard Model. Precision measurements of
parity violating transitions in nuclei can help to improve these constraints.
In these systems however, the weak interaction is masked by effects of the
seven orders of magnitude stronger non-perturbative strong interaction.
Therefore, in order to relate experimental measurements of the parity violating
pion-nucleon couplings to the fundamental Lagrangian of the SM, these
non-perturbative effects have to be well understood. In this paper, we are
going to present a Lattice QCD approach for computing the parity
violating matrix element in proton proton scattering. This process does not
involve disconnected diagrams in the isospin symmetric limit and is thus a
perfect testbed for studying the feasibility of the more involved calculation
of the parity violating pion-nucleon coupling.Comment: PoS Lattice 201
Two-nucleon scattering in multiple partial waves
We determine scattering phase shifts for S,P,D, and F partial wave channels
in two-nucleon systems using lattice QCD methods. We use a generalization of
Luscher's finite volume method to determine infinite volume phase shifts from a
set of finite volume ground- and excited-state energy levels on two volumes,
V=(3.4 fm)^3 and V=(4.5 fm)^3. The calculations are performed in the
SU(3)-flavor limit, corresponding to a pion mass of approximately 800 MeV. From
the energy dependence of the phase shifts we are able to extract scattering
parameters corresponding to an effective range expansion.Comment: 7 pages, 11 figures. Proceedings of the 33rd International Symposium
on Lattice Field Theory, July 14-18, 2015, Kobe, Japa
Calm Multi-Baryon Operators
Outstanding problems in nuclear physics require input and guidance from
lattice QCD calculations of few baryons systems. However, these calculations
suffer from an exponentially bad signal-to-noise problem which has prevented a
controlled extrapolation to the physical point. The variational method has been
applied very successfully to two-meson systems, allowing for the extraction of
the two-meson states very early in Euclidean time through the use of improved
single hadron operators. The sheer numerical cost of using the same techniques
in two-baryon systems has been prohibitive. We present an alternate strategy
which offers some of the same advantages as the variational method while being
significantly less numerically expensive. We first use the Matrix Prony method
to form an optimal linear combination of single baryon interpolating fields
generated from the same source and different sink interpolators. Very early in
Euclidean time this linear combination is numerically free of excited state
contamination, so we coin it a calm baryon. This calm baryon operator is then
used in the construction of the two-baryon correlation functions.
To test this method, we perform calculations on the WM/JLab iso-clover gauge
configurations at the SU(3) flavor symmetric point with m{\pi} 800 MeV
--- the same configurations we have previously used for the calculation of
two-nucleon correlation functions. We observe the calm baryon removes the
excited state contamination from the two-nucleon correlation function to as
early a time as the single-nucleon is improved, provided non-local (displaced
nucleon) sources are used. For the local two-nucleon correlation function
(where both nucleons are created from the same space-time location) there is
still improvement, but there is significant excited state contamination in the
region the single calm baryon displays no excited state contamination.Comment: 8 pages, 3 figures, proceedings for LATTICE 201
On the Feynman-Hellmann theorem in quantum field theory and the calculation of matrix elements
The Feynman-Hellmann theorem can be derived from the long Euclidean-time limit of correlation functions determined with functional derivatives of the partition function. Using this insight, we fully develop an improved method for computing matrix elements of external currents utilizing only two-point correlation functions. Our method applies to matrix elements of any external bilinear current, including nonzero momentum transfer, flavor-changing, and two or more current insertion matrix elements. The ability to identify and control all the systematic uncertainties in the analysis of the correlation functions stems from the unique time dependence of the ground-state matrix elements and the fact that all excited states and contact terms are Euclidean-time dependent. We demonstrate the utility of our method with a calculation of the nucleon axial charge using gradient-flowed domain-wall valence quarks on the Nf = 2 + 1 + 1 MILC highly improved staggered quark ensemble with lattice spacing and pion mass of approximately 0.15 fm and 310 MeV respectively. We show full control over excited-state systematics with the new method and obtain a value of g(A) = 1.213(26) with a quark-mass-dependent renormalization coefficient
Scaling study for 2 HEX smeared fermions: hadron and quark masses
The goal of this study is to investigate the scaling behaviour of our 2 HEX
action. For this purpose, we compute the spectrum and compare the
results to our 6 EXP action. We find a large scaling window up to along with small scaling corrections at the 2%-level and
full compatibility with our previous study. As a second important observable to
be tested for scaling, we chose the non-perturbatively renormalized quenched
strange quark mass. Here we find a fairly flat scaling with a broad scaling
range up to and perfect agreement with the
literature.Comment: PoS for the XXVIII International Symposium on Lattice Field Theory,
Lattice2010, 7 pages, 4 figure
- …