At a Glance
◇ Construction of a theory that extends the Gauge-Higgs unified model to the Grand Unified Theory.
◇ Success in reproducing the quark*1/lepton*2 mass hierarchy *3.
Associate Professor Nobuhito Maru of the Graduate School of Science, Osaka City University and Yoshiki Yatagai, a first-year doctoral student of the same graduate school, constructed a theory that extends the 5-dimensional Gauge-Higgs unified theory, in which the Higgs field in the standard model of elementary particles is identified with a part of the higher-dimensional gauge field, to the Grand Unified Theory. In doing this they were able to reproduce not only the breaking of electroweak symmetry*4 and the mass of the Higgs boson*5, but also the quark/lepton mass hierarchy - all without unnatural fine-tuning of parameters.
The results of this research were published in "The European Physical Journal C" on October 9, 2020 (Friday).
*1 Quark: A general term for the six types of elementary particles that make up the protons (hadrons) that form the atomic nucleus.
*2 Lepton: A general term for six types of elementary particles such as electrons and electron neutrinos. Along with quarks, they make up the fundamental components of matter.
*3 Mass hierarchy: A six-digit hierarchy between the quark/lepton mass that cannot be predicted in the Standard Model.
*4 Electroweak symmetry breaking: A phenomenon which symmetry is spontaneously broken in the electroweak theory that unifies electromagnetic force and weak force.
*5 Higgs boson: A particle that acts as the origin of mass according to the Higgs mechanism used in the Standard Model.
In this research, as one of the extensions, we constructed the 5D SU (6) grand Gauge-Higgs unification Theory and solved the above problems at once. The Grand Unified Theory is an ambitious theory that unifies the three forces (electromagnetic force, weak force, and strong force) into one. In the Gauge-Higgs unification theory, the Higgs boson is identified with an extra-dimensional component of the 5-dimensional Gauge boson, and unifies the Higgs and Gauge boson. Such an ambitious theory has yet to be completed, with constructing a realistic model as an important research subject.
As a first step of this study, by carefully choosing not only quark/lepton but also extra particles that are not included in the standard model, we realized the electroweak symmetry breaking that occurs in the Standard Model, and successfully constructed a simple model that reproduces the Higgs boson mass. In addition, by introducing the kinetic term of the Gauge boson localized at the boundary of the fifth space, we also succeeded in reproducing quark/lepton mass hierarchy, including the top quark mass, which is difficult to reproduce, without making unnatural fine-tuning of parameters.
The key to success is in the grand Gauge-Higgs unification theory constructed this time, the parameters within the Higgs potential and the Yukawa coupling was restricted due to the requirement of gauge symmetry. This meant that the number of parameters that could not be determined in the Standard Model was very small. This remarkable property enhanced the theory's predictions and led to the above-mentioned results.
Japan Society for the Promotion of Science Scientific Research Fund Basic Research C JSPS KAKENHI Grant Number 17K05420
Journal: The European Physical Journal C 80 933 (2020) (IF = 4.4)
Title: Improving Fermion Mass Hierarchy in Grand Gauge-Higgs Unification with Localized Gauge Kinetic Terms
Authors: Nobuhito Maru, Yoshiki Yatagai