The proton-proton fusion reaction, pp → de + ν, is studied in pionless effective field theory (EFT) with di-baryon fields up to next-to leading order. With the aid of the di-baryon fields, the effective range corrections are naturally resummed up to the infinite order and thus the calculation is greatly simplified. Furthermore, the low-energy constant which appears in the axial-current-di-baryon-di-baryon contact vertex is fixed through the ratio of two-and one-body matrix elements which reproduces the tritium lifetime very precisely. As a result we can perform a parameter free calculation for the process. We compare our numerical result with those from the accurate potential model and previous pionless EFT calculations, and find a good agreement within the accuracy better than 1%.
PACS
IntroductionThe proton-proton fusion process, pp → de + ν e , is a fundamental reaction for the nuclear astrophysics, especially important for the understanding of the star evolutions [1] and solar neutrinos [2,3,4]. However, the process has never been studied experimentally because the event is extremely unlikely to take place in the laboratory at the proton energies in the sun. The calculation of the transition rate and its uncertainty has naturally become a challenge to nuclear theory. The first calculation of the process was carried out by Bethe and Critchfield [5] in 1938. This estimation was improved by Salpeter [6] 2 in 1952. Later, small corrections, such as the electromagnetic radiative corrections, were considered by Bahcall and his collaborators [8,9] in the framework of effective range theory. Recently, accurate phenomenological potential models were employed to study the process [10,11]. Furthermore, in Ref.[12] the two-nucleon current operators were calculated from heavybaryon chiral perturbation theory (HBχPT) up to next-to-next-to-next-to leading order (N 3 LO), and Park et al. obtained quite an accurate estimation (∼ 0.3% uncertaity) for the process by fixing an unknown parameter, so-called low energy constant (LEC), which appears in the two-nucleon-axial-current contact interaction in terms of the tritium lifetime [13,14].The kinetic energy relevant to the pp fusion process at the core of the sun is quite low, kT c ≃ 1.18 keV, where T c is the core temperature of the sun, T c ≃ 13.7 × 10 6 K, and k is the Boltzmann constant. The proton momentum at the core, p c ≃ 2m p kT c ≃ 1.5 MeV, where m p is the proton mass, is still significantly small compared to the pion mass, m π ≃ 140 MeV. Therefore, we may regard the pion as a heavy degree of freedom for the pp fusion process. It may be convenient and suitable to employ a pionless effective field theory (EFT) [15], in which the pions are integrated out of the effective Lagrangian for the process in question. The pp fusion process in the pionless theory has been studied by Kong and Ravndal [16] up to next-to leading order (NLO) and by Butler and Chen [17] up to fifth order (N 4 LO). Thanks to the perturbative scheme in EFT, the accuracy of the N 4 LO calculation w...