Proton and neutron electromagnetic form factors from lattice QCD

Abstract

The electromagnetic form factors of the proton and the neutron are computed within lattice QCD using simulations with quarks masses fixed to their physical values. Both connected and disconnected contributions are computed. We analyze two new ensembles of N f = 2 and N f = 2 + 1 + 1 twisted mass clover-improved fermions and determine the proton and neutron form factors, the electric and magnetic radii, and the magnetic moments. We use several values of the sink-source time separation in the range of 1.0 fm to 1.6 fm to ensure ground state identification. Disconnected contributions are calculated to an unprecedented accuracy at the physical point. Although they constitute a small correction, they are non-negligible and contribute up to 15% for the case of the neutron electric charge radius.

Recent progress on the study of nucleon structure from lattice QCD and future perspectives

“…It is only recently that we are able to compute these diagrams, which are needed for studying sea quarks effects. This has become possible using a combination of stochastic techniques, dilution and deflation of lower modes [13]. The intrinsic spin 1 2 ∆Σ q carried by a quark of flavor q is given by…”

“…In Fig. 11 we show results on the electric and magnetic form factors for the neutron [13] using the twisted mass ensemble cB211.072.64. The disconnected contributions are included.…”

“…Figure 11: Neutron electric (left) and magnetic (right) form factors as a function of Q 2 . Filled circles show the lattice QCD results obtained using the cB211.072.64 ensemble [13] and black crosses are experimental results (see Ref. [13] for the the detailed bibliography on the experimental results).…”

“…Filled circles show the lattice QCD results obtained using the cB211.072.64 ensemble [13] and black crosses are experimental results (see Ref. [13] for the the detailed bibliography on the experimental results). The fits to lattice QCD results use the Galster-like form for the electric form factor and a dipole form for the magnetic.…”

Disconnected loop subtraction methods in lattice QCD

“…In this work we concentrate on lattice operators which respond well to unpartitioned noise [1]: scalar and vector operators. Most physical amplitudes in QCD are affected by such loop effects; some examples include nucleon electromagnetic form factors [2,3], the strangeness and charm contents of the nucleon [4,5,6], the determination of the mass of flavor singlet mesons [7], multiquarks and scattering states [8], hadronic scattering lengths and structure functions [9,10], and electron or muon hadronic g − 2 loop contributions [11,12]. To tackle such difficult lattice problems one projects out operator expectation values using unbiased noise stochastic estimates [13,14,15].…”