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.
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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)