Implementation of the type III seesaw model in FeynRules/MadGraph and prospects for discovery with early LHC data

The European Physical Journal C, Feb 2012

We discuss the implementation of the “minimal” type III seesaw model, i.e. with one fermionic triplet, in FeynRules/MadGraph. This is the first step in order to realize a real study of LHC data recorded in the LHC detectors. With this goal in mind, we comment on the possibility of discovering this kind of new physics at the LHC running at 7 TeV with a luminosity of few fb−1.

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Implementation of the type III seesaw model in FeynRules/MadGraph and prospects for discovery with early LHC data

0 Istituto Nazionale di Fisica Nucleare , Sezione di Padova, via Marzolo 8, 35131 Padova, Italy 1 Institut de Fsica d'Altes Energies, Universitat Autnoma de Barcelona , 08193 Bellaterra, Spain We discuss the implementation of the minimal type III seesaw model, i.e. with one fermionic triplet, in FeynRules/MadGraph. This is the first step in order to realize a real study of LHC data recorded in the LHC detectors. With this goal in mind, we comment on the possibility of discovering this kind of new physics at the LHC running at 7 TeV with a luminosity of few fb1. - In a period in which LHC is running and ready to discover new physics, it is of crucial importance to have the possibility of simulating the signals that a particular kind of new physics could give in the two main detectors, ATLAS and CMS. In this paper we describe the implementation in FeynRules/MadGraph [13] of a simple extension of the standard model (SM), the minimal type III seesaw. This is a first necessary step before performing the analysis of real data, which is the ultimate goal of our work and which will be discussed in a future publication. As it is well known, oscillation experiments have proved that neutrinos oscillate and therefore are massive. However, from the theoretical point of view, the origin of this mass is still unknown. An appealing possibility, also accounting for the smallness of this mass, is the seesaw mechanism: new heavy states having a Yukawa interaction with the lepton and the Higgs doublets generate a small Majorana mass for the neutrinos, generically suppressed, with respect to charged fermion masses, by a factor v/M , where v is the Higgs vev and M the mass of the heavy particle. Depending on the nature of the heavy state, seesaw models are called type I a e-mail: b e-mail: [47], type II [812] or type III [13], corresponding to heavy fermionic singlet, scalar triplet or fermionic triplet, respectively. If one requires O(1) Yukawa couplings, M should be of the order of the grand unification scale in order to account for neutrino masses smaller than the eV. However, in principle the scale can be as low as hundreds of GeV, in which case either the Yukawas are smaller or an alternative method, such as for instance an inverse seesaw [14, 15] should be at work. In this case the heavy field responsible for neutrino masses could be discovered at the LHC. As regards collider physics, the seesaws of type II and III are more exciting, since they can be produced via gauge interactions: at difference with singlets, whose production is drastically suppressed if the Yukawa couplings are small, triplets can be produced and observed at the LHC if their mass is sufficiently small, independently of the size of the Yukawa couplings or mixing angles. In the present paper we focus on the type III seesaw, i.e. the one mediated by fermionic triplets. To simplify the implementation of the model in FeynRules, we consider a simple extension of the SM obtained by adding a single triplet. Indeed we can safely assume that, unless in case of extreme degeneracy, the lightest triplet will be the one most copiously produced and the one which will be eventually firstly discovered. In the literature few papers [1620] discussing the possibility of discovering the type III seesaw at the LHC (at 14 TeV) are present. However so far no code is publicly available to perform calculations and simulations in this model. With this paper and the publication of the implemented model at the URL http://feynrules.phys.ucl.ac.be/wiki/TypeIIISeeSaw we are going to fill this gap. Moreover we briefly discuss the physics case for LHC running at 7 TeV, suggesting that with few fb1 of luminosity a discovery is already possible. This paper is organized as follows. In Sect. 2 the model with the complete Lagrangian and all the couplings is reviewed, both in the general and in the simplified case. In Sect. 3 the implementation of the model in FeynRules and the checks performed for its validation are discussed. In Sect. 4 the physics case at 7 TeV is discussed and in Sect. 5 we conclude. 2 The model The model considered here is the one presented in Ref. [21]. It consists in the addition to the standard model of SU(2) triplets of fermions with zero hypercharge, . In this model at least two such triplets are necessary in order to have two non-vanishing neutrino masses. The beyond the standard model interactions are described by the following Lagrangian (with implicit flavour summation): 1 L = Tr[ iD/ ] 2 Tr[ M c + cM ] 2Y L L 2Y , with L (, l)T , (+, 0)T (+, (v + H + i)/ 2)T , = i2, c C T and with, for each fermionic triplet, Without loss of generality, we can assume that we start from the basis where M is real and diagonal, as well as the charged lepton Yukawa coupling, not explicitly written above. In order to consider the mixing of the triplets with the charged leptons, it is convenient to express the four degrees of freedom of each charged tr (...truncated)


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Carla Biggio, Florian Bonnet. Implementation of the type III seesaw model in FeynRules/MadGraph and prospects for discovery with early LHC data, The European Physical Journal C, 2012, pp. 1899, Volume 72, Issue 2, DOI: 10.1140/epjc/s10052-012-1899-z