Proton-Proton Fusion and Tritium β Decay from Lattice Quantum Chromodynamics

Area of Science:

• Nuclear Physics
• Quantum Chromodynamics
• Particle Physics

Background:

• Nuclear matrix elements are crucial for understanding fusion reactions and beta decay.
• Previous calculations were limited by computational complexity and theoretical approximations.

Purpose of the Study:

• To calculate nuclear matrix elements for pp→de^{+}ν fusion and tritium β decay using lattice quantum chromodynamics (LQCD).
• To validate LQCD as a method for studying weak transition amplitudes in few-nucleon systems.

Main Methods:

• Utilized a novel implementation of the background field method within LQCD.
• Calculations were performed at the SU(3) flavor-symmetric point, corresponding to a pion mass of ~806 MeV.

Main Results:

• The Gamow-Teller matrix element for tritium β decay was calculated to be 0.979(03)(10), consistent with experimental data.
• The calculated pp→de^{+}ν fusion cross section at physical quark masses agrees with accepted values.
• The leading two-nucleon axial counterterm of pionless effective field theory was determined to be L_{1,A}=3.9(0.2)(1.0)(0.4)(0.9) fm³.

Conclusions:

• LQCD can directly study weak transition amplitudes in few-nucleon systems from fundamental quark and gluon degrees of freedom.
• This approach opens new avenues for investigating key nuclear physics quantities.
• The findings bridge the gap between fundamental theory and observable nuclear phenomena.