880 research outputs found

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

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    We present a simultaneous extraction of the moments of F2F_2 and FLF_L structure functions of the proton for a range of photon virtuality, Q2Q^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)SU(3) symmetric point. We find the moments of F2F_{2} and FLF_{L} in good agreement with experiment. Power corrections turn out to be significant. This is the first time the Q2Q^2 dependence of the lowest moment of F2F_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

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    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. Stüben, R. D. Young and J. M. Zanott

    Constraining beyond the Standard Model nucleon isovector charges

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    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, dnd_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 gT, gAg_T,~g_A and gSg_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 gT = 1.009(20)stat(03)sys, gA=1.246(69)stat(05)sysg_T~=~1.009(20)_{\text{stat}}(03)_{\text{sys}},~g_A=1.246(69)_{\text{stat}}(05)_{\text{sys}} and gS = 1.06(10)stat(03)sysg_S~=~1.06(10)_{\text{stat}}(03)_{\text{sys}} renormalised, where appropriate, at μ=2 GeV\mu=2~\text{GeV} in the MS\overline{\text{MS}} scheme.Comment: 16 pages, 11 figures, 6 table

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

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    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 θ 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 θ. We find that the color fields produced by quarks and gluons are screened, and confinement is lost, for bare vacuum angles |θ|>0, thus providing a natural solution of the strong CP problem. The renormalized vacuum angle θ is found to flow to zero in the infrared limit, leading to a self-consistent solution within QCD
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