Strong isospin violation and chiral logarithms in the baryon spectrum

• DOI: 10.48550/arxiv.1612.07733
• Publication Date: 2016-01-01
• Authors: Brantley, David A.; Joo, Balint; Mastropas, Ekater...; Mereghetti, Emanuele; Monge-Camacho, Henry; Tiburzi, Brian C.; Walker-Loud, Andre
• Keywords: Nuclear Theory (nucl-th); High Energy Physics - Phenomenology; High Energy Physics - Lattice; High Energy Physics - Phenomenology (hep-ph); Nuclear Theory; 0103 physical sciences; High Energy Physics - Lattice (hep-lat); FOS: Physical sciences; 01 natural sciences

We present a precise lattice QCD calculation of the contribution to the neutron-proton mass splitting arising from strong isospin breaking, $m_n-m_p|_{QCD}=2.32\pm0.17$ MeV. We also determine $m_{��^-} - m_{��^0}|_{QCD} = 5.44\pm0.31$ MeV. The calculation is performed at three values of the pion mass, with several values of the quark mass splitting and multiple lattice volumes, but only a single lattice spacing and an estimate of discretization errors. The calculations are performed on the anisotropic clover-Wilson ensembles generated by the Hadron Spectrum Collaboration. The omega-baryon mass is used to set the scale $a_t^{-1}=6111\pm127$ MeV, while the kaon masses are used to determine the value of the light-quark mass spitting. The nucleon mass splitting is then determined as a function of the pion mass. We observe, for the first time, conclusive evidence for non-analytic light quark mass dependence in lattice QCD calculations of the baryon spectrum. When left as a free parameter, the fits prefer a nucleon axial coupling of $g_A=1.24(56)$. To highlight the presence of this chiral logarithm in the nucleon mass splitting, we also compute the isospin splitting in the cascade-baryon system which is less sensitive to chiral dynamics. Finally, we update the best lattice QCD determination of the CP-odd pion-nucleon coupling that would arise from a non-zero QCD theta-term, $\bar{g}_0 / (\sqrt{2}f_��) = (14.7\pm1.8\pm1.4) \cdot 10^{-3} \bar��$. The original lattice QCD correlation functions, analysis results and extrapolated quantities are packaged in HDF5 files made publicly available including a simple Python script to access the numerical results, construct effective mass plots along with our analysis results, and perform the extrapolations of various quantities determined in this work.<br/>16 pages including figures and tables. The original lattice QCD correlation functions and our analysis results can be found at this address http://ntc0.lbl.gov/~walkloud/mn_mns_mp/ stored in HDF5 format. Additionally, there are a number of Python files which can be used to access the original results and our analysis, see the README.txt to get started<br/>