Pion vector form factor from lattice QCD at the physical point
Résumé
We present an investigation of the electromagnetic pion form factor, Fπ(Q2), at small values of the four-momentum transfer Q2 (≲0.25 GeV2), based on the gauge configurations generated by the European Twisted Mass Collaboration with Nf=2 twisted-mass quarks at maximal twist including a clover term. Momentum is injected using nonperiodic boundary conditions and the calculations are carried out at a fixed lattice spacing (a≃0.09 fm) and with pion masses equal to its physical value, 240 MeV and 340 MeV. Our data are successfully analyzed using chiral perturbation theory at next-to-leading order in the light-quark mass. For each pion mass two different lattice volumes are used to take care of finite size effects. Our final result for the squared charge radius is ⟨r2⟩π=0.443(29) fm2, where the error includes several sources of systematic errors except the uncertainty related to discretization effects. The corresponding value of the SU(2) chiral low-energy constant ℓ¯6 is equal to ℓ¯6=16.2(1.0).
Mots clés
form factor: electromagnetic
quantum chromodynamics
lattice field theory
numerical calculations
quark: Wilson
fermion: clover
quark: mass: twist
quark: flavor: 2
perturbation theory: chiral
finite size: effect
form factor: vector
effect: discrete
higher-order: 1
low-energy constant
boundary condition
charge radius
quark: mass dependence
low-energy constant: SU(2)
pi: form factor