Ab initioCalculation of thenp→dγRadiative Capture Process

Abstract

Lattice QCD calculations of two-nucleon systems are used to isolate the short-distance two-body electromagnetic contributions to the radiative capture process np → dγ, and the photo-disintegration processes γ ðÃÞ d → np. In nuclear potential models, such contributions are described by phenomenological meson-exchange currents, while in the present work, they are determined directly from the quark and gluon interactions of QCD. Calculations of neutron-proton energy levels in multiple background magnetic fields are performed at two values of the quark masses, corresponding to pion masses of m π ∼ 450 and 806 MeV, and are combined with pionless nuclear effective field theory to determine the amplitudes for these low-energy inelastic processes. At m π ∼ 806 MeV, using only lattice QCD inputs, a cross section σ 806 MeV ∼ 17 mb is found at an incident neutron speed of v ¼ 2; 200 m=s. Extrapolating the short-distance contribution to the physical pion mass and combining the result with phenomenological scattering information and one-body couplings, a cross section of σ lqcd ðnp → dγÞ ¼ 334.9ð þ5.2 −5.4 Þ mb is obtained at the same incident neutron speed, consistent with the experimental value of σ expt ðnp → dγÞ ¼ 334.2ð0.5Þ mb. The radiative capture process, np → dγ, plays a critical role in big bang nucleosynthesis (BBN) as it is the starting point for the chain of reactions that form most of the light nuclei in the cosmos. Studies of radiative capture [1][2][3], and the inverse processes of deuteron electro-and photodisintegration, γ ðÃÞ d → np [4][5][6][7], have constrained these cross sections and have also provided critical insights into the interactions between nucleons and photons. They conclusively show the importance of non-nucleonic degrees of freedom in nuclei, which arise from meson-exchange currents (MECs) in the context of nuclear potential models [8,9]. Nevertheless, in the energy range relevant for BBN, experimental investigations are challenging [10]. For the analogous weak interactions of multinucleon systems, considerably less is known from experiment but these processes are equally important. The weak two-nucleon interactions currently contribute the largest uncertainty in calculations of the rate for proton-proton fusion in the Sun [11][12][13][14][15][16][17], and in neutrino-disintegration of the deuteron [18], which is a critical process needed to disentangle solar neutrino oscillations. Given the phenomenological importance of electroweak interactions in light nuclei, direct determinations from the underlying theory of strong interaction, quantum chromodynamics (QCD), are fundamental to future theoretical progress. Such determinations are also of significant phenomenological importance for calibrating long-baseline neutrino experiments and for investigations of double beta decay in nuclei. In this Letter, we take the initial steps towards meeting this challenge and present the first lattice QCD (LQCD) calculations of the np → dγ process. The results are in good agreement with experiment and show...

Determination of the properties of vector mesons in external magnetic field by quenched SU(3) lattice QCD

“…The inclusion of dynamical quarks into a consideration will most likely not lead to a radical change in the magnetic properties of ρ meson, although it becomes an unstable particle in full QCD. For example, in 2+1 full QCD the g-factor of the ρ ± meson is equal to 2.21 (8) at the physical point [30]. Quenched lattice theory predicts the value g = 2.25(0.34) in the chiral limit [24] and g = 2.20(0.15) at the lowest pion mass [27].…”

“…8) where S pl,rt = (1/2)Tr(1 − U pl,rt ) is the plaquette (denoted by pl) or 1×2 rectangular loop term (rt), u 0 = (W 1×1 ) 1/4 = (1/2)TrU pl 1/4 is the input tadpole factor computed at zero temperature [35]. This action suppresses ultraviolet dislocations which leads to non-physical near-zero modes of the Wilson-Dirac operator and difficulties in choice of the ρ parameter (see subsection 2.1).…”

“…The magnetic polarizabilities of pions have been measured in the experiments [18][19][20], have been calculated in the chiral perturbation theory [21,22] and in the lattice gauge theory [23]. The magnetic polarizabilities of baryons have been obtained in full lattice QCD [8]. The magnetic moment of the ρ meson has been explored in [24][25][26][27][28][29][30].…”

“…Then the magnetic field was introduced in two flavor lattice QCD simulation [3] and used as the probe of QCD properties [4]. The energies of hadrons in magnetic fields have been calculated on the lattice [5][6][7][8], in theoretical models [9][10][11][12][13] and in the framework of the QCD sum rules [14,15]. The notion of the hadron polarizability was initially JHEP09(2017)142 discussed in [16,17].…”

Spectra and scattering of light lattice nuclei from effective field theory

“…Both two-nucleon elastic scattering [41] and neutron radiative capture on the proton [42] have been considered directly on the lattice. Our strategy is, instead, to analyze reactions outside the lattice box with π EFT once its LECs have been determined from binding energies at LO and, eventually, also two-nucleon scattering observables at NLO.…”