Physics PhD Thesis Defense by Seçil Benli








Title: The Masses and Mixings of Neutral Fermions in a Grand Unified E6 Model


Speaker: Seçil Benli


Time: August 14, 2017, 15:30


Place: SCI 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul


Thesis Committee Members:


Prof.Dr.Tekin Dereli (Advisor, Koç University)

Prof.Dr.Cihan Saçlıoglu (Sabancı University)

Prof.Dr.Özgür Müstecaplıoğlu (Koç University)

Prof.Dr.Erkcan Özcan (Boğaziçi University)

Assoc.Prof.Dr.Kaan Güven (Koç University)




The modern theory of elementary particle physics is based on the Standard Model and the symmetry groups in question are Lie groups as far as continuous symmetries are considered. The strong, weak and electromagnetic interactions among the elementary particles are associated with the Lie algebras of SU(3)C, SU(2) and U(1) in the Standard Model. There are many reasons not to believe that the Standard Model is a final theory. New physics on which we will concentrate in this thesis is based on the idea of unification where three types of known gauge interactions are unified at the same very high energy scale by linking the three of the four forces in nature by combining strong and electroweak forces, called the Grand Unified Theory (GUT). We here ground on the exceptional E6 group for Grand Unification to give the masses and mixing of neutral leptons. We start by giving some algebraic foundations of exceptional groups including a discussion of the division algebras of quaternions and octonions. Then, we focus on E6 model where the fundamental representation {27} includes the fermionic content of the model. A brief review of the assignment of elementary fermions and bosons to irreducible multiplets in E6 models is followed by a discussion of different, hierarchical symmetry breaking chains from E6 down to SU(3)CxU(1)EM. We first discuss the conventional symmetry breaking chain E6 ®SO(10) ®SU(5) ®U(2)EWxSU(3)C ®U(1)QEDxSU(3)C that involves the intermediate Georgi-Glashow SU(5) symmetry. Next we concentrate on the symmetry breaking chain E6 ®SO(10) ®SU(2)LxSU(2)RxSU(4)C ®U(2)EWxSU(3)C ®U(1)EMxSU(3)C with an intermediate Pati-Salam symmetry for which (B-L) is conserved. In particular, the mass/mixing matrix of electrically neutral fermions (i.e.neutrinos) that would be derived from Yukawa couplings is constructed. The pattern of neutrino masses and some bounds on mixing parameters are discussed.