The Physics Society of Iran Iranian Journal of Physics Research 1682-6957 4 1 2020 02 02 Excited states energy corrections of hydrogen atom at the order of & alpha4 using NRQED as an effective field theory Excited states energy corrections of hydrogen atom at the order of & alpha4 using NRQED as an effective field theory 33 40 1488 FA S. M. Zebarjad M. Bigdeli Journal Article 2020 02 02   In this paper, we calculate Hydrogen hyperfine splitting (HFS) at the order a4 by using NRQED as an effective field theory. We also show the leading-order contribution of nuclear size to the energy correction of Hydrogen atom .    In this paper, we calculate Hydrogen hyperfine splitting (HFS) at the order a4 by using NRQED as an effective field theory. We also show the leading-order contribution of nuclear size to the energy correction of Hydrogen atom .  https://ijpr.iut.ac.ir/article_1488_8a146f1a3da4700cbf03cdc55e2daae6.pdf