arXiv subjects

William Detmold

Publications and source records attributed to William Detmold.

133 records · Page 8Linked to original sources

arXiv2006

Generalised parton distributions of the pion in partially-quenched chiral perturbation theory

We consider the pion matrix elements of the isoscalar and isovector combinations of the vector and tensor twist-two operators that determine the moments of the various pion generalised parton distributions. Our analysis is performed using partially-quenched chiral perturbation theory. We work in the SU(2) and SU(4|2) theories and present our results at infinite volume and also at finite volume where some subtleties arise. These results are useful for extrapolations of lattice calculations of these matrix elements at small momentum transfer to the physical regime.

hep-lat↗

arXiv2005

Deep-inelastic scattering and the operator product expansion in lattice QCD

We discuss the determination of deep-inelastic hadron structure in lattice QCD. By using a fictitious heavy quark, direct calculations of the Compton scattering tensor can be performed in Euclidean space that allow the extraction of the moments of structure functions. This overcomes issues of operator mixing and renormalisation that have so far prohibited lattice computations of higher moments. This approach is especially suitable for the study of the twist-two contributions to isovector quark distributions, which is practical with current computing resources. While we focus on the isovector unpolarised distribution, our method is equally applicable to other quark distributions and to generalised parton distributions. By looking at matrix elements such as $<π^\pm| T [V^μ(x) A^ν(0)]|0>$ (where $V^μ$ and $A^ν$ are vector and axial-vector heavy-light currents) within the same formalism, moments of meson distribution amplitudes can also be extracted.

hep-lat↗

arXiv2005

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.

hep-ph↗

arXiv2005

Target mass effects in deep-inelastic scattering on the deuteron

We calculate the effects of the target mass on deep-inelastic electron scattering on targets of spin one such as the deuteron, focusing on the novel structure functions that enter. It is important to understand these effects given the recent HERMES measurements of the $b_1(x,Q^2)$ structure function of the deuteron. We also derive the spin-one analogues of the Wandzura-Wilczek relations and discuss possible calculations in lattice QCD.

hep-ph↗

arXiv2005

Twist-two matrix elements at finite and infinite volume

We present one-loop results for the forward twist-two matrix elements relevant to the unpolarised, helicity and transversity baryon structure functions, in partially-quenched (N_f=2 and N_f=2+1) heavy baryon chiral perturbation theory. The full-QCD limit can be straightforwardly obtained from these results and we also consider SU(2|2) quenched QCD. Our calculations are performed in finite volume as well as in infinite volume. We discuss features of lattice simulations and investigate finite volume effects in detail. We find that volume effects are not negligible, typically around 5--10% in current partially-quenched and full QCD calculations, and are possibly larger in quenched QCD. Extensions to the off-forward matrix elements and potential difficulties that occur there are also discussed.

hep-lat↗

arXiv2004

Nucleon Properties at Finite Volume: the Epsilon Prime Regime

We study the properties of the nucleon in highly asymmetric volumes where the spatial dimensions are small but the time dimension is large in comparison to the inverse pion mass. To facilitate power-counting at the level of Feynman diagrams, we introduce $ε^\prime$-power-counting which is a special case of Leutwyler's $δ$-power-counting. Pion zero-modes enter the $ε^\prime$-counting perturbatively, in contrast to both the $ε$- and $δ$-power-countings, since $m_q < q\bar{q}> V$ remains large. However, these modes are enhanced over those with non-zero momenta and enter at lower orders in the $ε^\prime$-expansion than they would in large volume chiral perturbation theory. We discuss an application of $ε^\prime$-counting by determining the nucleon mass, magnetic moment and axial matrix element at the first nontrivial order in the $ε^\prime$-expansion.

hep-lat↗

arXiv2004

Electroweak Matrix Elements in the Two-Nucleon Sector from Lattice QCD

We demonstrate how to make rigorous predictions for electroweak matrix elements in nuclear systems directly from QCD. More precisely, we show how to determine the short-distance contributions to low-momentum transfer electroweak matrix elements in the two-nucleon sector from lattice QCD. In potential model descriptions of multi-nucleon systems, this is equivalent to uniquely determining the meson-exchange currents, while in the context of nuclear effective field theory, this translates into determining the coefficients of local, gauge-invariant, multi-nucleon-electroweak current operators. The energies of the lowest-lying states of two nucleons on a finite volume lattice with periodic boundary conditions in the presence of a background magnetic field are sufficient to determine the local four-nucleon operators that contribute to the deuteron magnetic moment and to the threshold cross-section of n + p -> d + gamma. Similarly, the energy-levels of two nucleons immersed in a background isovector axial weak field can be used to determine the coefficient of the leading local four-nucleon operator contributing to the neutral- and charged-current break-up of the deuteron. This is required for the extraction of solar neutrino fluxes at SNO and future neutrino experiments.

hep-lat↗