880 research outputs found

### Moments and power corrections of longitudinal and transverse proton structure functions from lattice QCD

We present a simultaneous extraction of the moments of $F_2$ and $F_L$
structure functions of the proton for a range of photon virtuality, $Q^2$. This
is achieved by computing the forward Compton amplitude on the lattice utilizing
the second-order Feynman-Hellmann theorem. Our calculations are performed on
configurations with two different lattice spacings and volumes, all at the
$SU(3)$ symmetric point. We find the moments of $F_{2}$ and $F_{L}$ in good
agreement with experiment. Power corrections turn out to be significant. This
is the first time the $Q^2$ dependence of the lowest moment of $F_2$ has been
quantified.Comment: 14 pages, 11 figures, 2 tables. Version to appear in PR

### Quasi-degenerate baryon energy states, the Feynman-Hellmann theorem and transition matrix elements

The standard method for determining matrix elements in lattice QCD requires the computation of three-point correlation functions. This has the disadvantage of requiring two large time separations: one between the hadron source and operator and the other from the operator to the hadron sink. Here we consider an alternative formalism, based on the Dyson expansion leading to the Feynman- Hellmann theorem, which only requires the computation of two-point correlation functions. Both the cases of degenerate energy levels and quasi-degenerate energy levels which correspond to diagonal and transition matrix elements respectively can be considered in this formalism. As an example numerical results for the Sigma to Nucleon vector transition matrix element are presented.M. Batelaan, K. U. Can, R. Horsley, Y. Nakamura, H. Perlt, P. E. L. Rakow, G. Schierholz, H. StÃ¼ben, R. D. Young and J. M. Zanott

### Constraining beyond the Standard Model nucleon isovector charges

At the TeV scale, low-energy precision observations of neutron
characteristics provide unique probes of novel physics. Precision studies of
neutron decay observables are susceptible to beyond the Standard Model (BSM)
tensor and scalar interactions, while the neutron electric dipole moment,
$d_n$, also has high sensitivity to new BSM CP-violating interactions. To fully
utilise the potential of future experimental neutron physics programs, matrix
elements of appropriate low-energy effective operators within neutron states
must be precisely calculated. We present results from the QCDSF/UKQCD/CSSM
collaboration for the isovector charges $g_T,~g_A$ and $g_S$ using lattice QCD
methods and the Feynman-Hellmann theorem. We use a flavour symmetry breaking
method to systematically approach the physical quark mass using ensembles that
span five lattice spacings and multiple volumes. We extend this existing
flavour breaking expansion to also account for lattice spacing and finite
volume effects in order to quantify all systematic uncertainties. Our final
estimates of the isovector charges are
$g_T~=~1.009(20)_{\text{stat}}(03)_{\text{sys}},~g_A=1.246(69)_{\text{stat}}(05)_{\text{sys}}$
and $g_S~=~1.06(10)_{\text{stat}}(03)_{\text{sys}}$ renormalised, where
appropriate, at $\mu=2~\text{GeV}$ in the $\overline{\text{MS}}$ scheme.Comment: 16 pages, 11 figures, 6 table

### The strong CP problem solved by itself due to long-distance vacuum effects

The vacuum of quantum chromodynamics has an incredibly rich structure at the nonperturbative level, which is intimately connected with the topology of gauge fields, and put to a test by the strong CP problem. We investigate the long-distance properties of the theory in the presence of the topological Î¸ term. This is done on the lattice, using the gradient flow to isolate the long-distance modes in the functional integral measure and tracing it over successive length scales. The key point is that the vacuum splits into disconnected topological sectors with markedly different physical characteristics, which gives rise to a nontrivial behavior depending on Î¸. We find that the color fields produced by quarks and gluons are screened, and confinement is lost, for bare vacuum angles |Î¸|>0, thus providing a natural solution of the strong CP problem. The renormalized vacuum angle Î¸ is found to flow to zero in the infrared limit, leading to a self-consistent solution within QCD

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