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abstract: 'We study the lepton sector of a four generations model based on the discrete flavor group $A_5$. The best features of the three family $A_4$ model survive, including the tribimaximal pattern of three generation neutrino mixings. At leading order the three light neutrino mass relations of $m_{\nu_1} = m_{\nu_3}$ and $m_{\nu_2} = 0$ are predicted. The splitting of the neutrino masses can be naturally obtained as a result of the breaking of $A_{5}$ down to $A_4$ and a degenerate spectrum is preferred in our model. The electron mass is zero at tree level, but calculable through quantum corrections in our $A_5$ model.'
author:
- 'Chian-Shu Chen$^{1,2}$[^1], Thomas W. Kephart$^{3}$[^2], and Tzu-Chiang Yuan$^{2}$[^3]'
title: '**An $A_5$ Model of Four Lepton Generations**'
---

Introduction
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Recently there has been considerable efforts to explore the possibility of a fourth generation of fermions in the standard model (SM). In particular these studies of fourth generation include its constraints by current electroweak precision data [@He:2001tp; @Erler-Langacker; @Chanowitz; @PQ-Sher], its interplay with Higgs physics [@Higgs4th], CP violation effects in $B$-physics [@LHCb] and accessibility of the fourth generation quarks [@Holdom] and leptons [@CRW] at the Large Hadron Collider (LHC) . For a recent review of 