Quark seesaw, vectorlike fermions and diphoton excess

Journal of High Energy Physics, Feb 2016

We present a possible interpretation of the recent diphoton excess reported by the early \( \sqrt{s}=13 \) TeV LHC data in quark seesaw left-right models with vectorlike fermions proposed to solve the strong CP problem without the axion. The gauge singlet real scalar field responsible for the mass of the vectorlike fermions has the right production cross section and diphoton branching ratio to be identifiable with the reported excess at around 750 GeV diphoton invariant mass. Various ways to test this hypothesis as more data accumulates at the LHC are proposed.

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Quark seesaw, vectorlike fermions and diphoton excess

HJE Quark seesaw, vectorlike fermions and diphoton excess P.S. Bhupal Dev 0 1 2 Rabindra N. Mohapatra 0 1 2 3 Yongchao Zhang 0 1 2 4 0 Boulevard du Triomphe , CP225, 1050 Brussels , Belgium 1 College Park, MD 20742 , U.S.A 2 Saupfercheckweg 1 , D-69117 Heidelberg , Germany 3 Maryland Center for Fundamental Physics, Department of Physics, University of Maryland 4 Service de Physique Theorique, Universite Libre de Bruxelles We present a possible interpretation of the recent diphoton excess reported by the early p s = 13 TeV LHC data in quark seesaw left-right models with vectorlike fermions proposed to solve the strong CP problem without the axion. The gauge singlet real scalar eld responsible for the mass of the vectorlike fermions has the right production cross section and diphoton branching ratio to be identi able with the reported excess at around 750 GeV diphoton invariant mass. Various ways to test this hypothesis as more data accumulates at the LHC are proposed. Beyond Standard Model; Quark Masses and SM Parameters 1 Introduction Scalar sector 2 3 4 5 6 Production and decay of the singlet High-scale validity Discussions Conclusion A Exact formulae for the Z and ZZ channel 1 3 { 1 { e ! ; ; L;R 2 3 1 Introduction In the early run II data from the p s = 13 TeV Large Hadron Collider (LHC), both CMS [1] and ATLAS [2] experiments have reported an excess of events over the SM background with invariant mass around 750 GeV. The signal cross section is reported to be (6 3) fb by CMS [1] and (10 3) fb by ATLAS [2]. While this excess has a local statistical signi cance of around 2:6 (CMS) to 3.9 (ATLAS) and needs more data to rmly rule out the possibility of a statistical uctuation, it has nonetheless generated a great deal of recent activity in the theory community as a possible signal of beyond the Standard Model (SM) physics and many possible interpretations have been advanced; for a non-exhaustive list of ideas and speculations, see [3{98]. In this note we add another one in the context of a theory proposed many years ago as a solution to the strong CP problem without an axion [ 99, 100 ]. The model is based on the assumption that there exist TeV-scale vectorlike fermions which are responsible for the seesaw masses for the quarks and charged leptons [101{104] in the context of a left-right (LR) symmetric model based on the gauge group SU(3)C SU(2)L assigned to the gauge group as follows: SM fermions : QL;R = L;R = Vectorlike fermions : P 3; 1; 1; + N 3; 1; 1; E(1; 1; 1; 2): (1.1) The Higgs sector of the model consists of SU(2)L;R doublets L;R which break the left and right SU(2)'s and a real singlet S that gives mass to the vectorlike fermions. An appropriate discrete Z2 symmetry forbids the bare mass of the vectorlike fermions. Under this Z2 symmetry, the Higgs elds L and S are odd as are the right-handed (RH) chirality of the vectorlike fermions; all other elds are even. The Yukawa couplings are given in this case by the Lagrangian LY = yU QL ~LPR + yDQL LNR + yE L LER + (L $ R) +fU PLSPR + fDNLSNR + fE ELSER + H:c: : where ~L;R = i 2 L;R ( 2 being the second Pauli matrix), and yF , fF (with F = U; D; E) are the Yukawa couplings with potential beyond SM CP violations. Once both the doublets and the singlet obtain their non-vanishing vacuum expectation values (VEVs) vL; vR; vS respectively, we get the seesaw form for the 2 2 mass matrix for a single quark or lepton Both the two new VEVs are assumed to be at the (multi-)TeV scale, whereas vL ' 246:2 GeV is the electroweak scale. Clearly, the simple seesaw mass formula in eq. (1.4) is not a good approximation for the top quark, as it is expected that the matrix entries yF vR and fF vS are of similar magnitude, which implies a large \right-handed" mixing of the top quark and its partner through sin Rt should take into consideration the whole 2 p12 yT vR=fT vS. Therefore in general, one y 2 mass matrix (1.3) and diagonalize MF MF to get the mass eigenvalues of the SM quarks and their partners. As far as the avor structure and quark mixing are concerned, we can have either heavy quarks with degenerate masses (respectively for the up and down type avors) in which case the SM quark mixings are completely determined by the avor structure of the Yukawa couplings yU; D [108], or the couplings yU; D are hierarchical but diagonal (e.g. from some discrete symmetry assignments) and the matrices fU; D are of order O(1) in which case we have avor anarchic [109]. As a direct result of the Lagrangian in (1.2), the heavy vectorlike quarks decay dominantly to the SM gauge and Higgs bosons plus SM quarks, especially for the top and bottom partners. Due to the Goldstone equivalence theorem, the branching ratios for the decays to W , Z and Higgs are approximately 2 : 1 : 1. The current LHC constraints put a 95% con dence level (CL) lower limit on the top partner mass from 715{950 GeV and on the bottom partner mass from 575{813 GeV [110], dependi (...truncated)


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P. S. Bhupal Dev, Rabindra N. Mohapatra, Yongchao Zhang. Quark seesaw, vectorlike fermions and diphoton excess, Journal of High Energy Physics, 2016, pp. 186, Volume 2016, Issue 2, DOI: 10.1007/JHEP02(2016)186