PROPOSITION DE STAGE 2011
Master 2 (stage de pré-thèse)
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STAGE DE THESE
Groupe du laboratoire : Théorie
Responsable du stage : Emi KOU
Adresse et lieu du stage : LAL – Bât 208
 : 01 64 46 85 99
e-mail : kou @ lal.in2p3.fr
Titre du stage : Extension of the Standard Model with a fourth generation
Précisions sur le sujet proposé :
The current target of particle physics is the discovery of a signal beyond the Standard Model
(SM). As the LHC experiment has finally started its operation, experimental data will soon be
available to confront the theoretical ideas beyond SM proposed during the last three decades.
In this thesis, we propose to focus on the fourth generation extension of SM (SM4). Indeed,
although there are overwhelming experimental evidences for at least three generations of
fermions, the theory allows the existence of more generations. We will investigate the
theoretical and phenomenological aspects of SM4.
Direct searches, i.e. producing the new heavier fermions at high energy colliders, would of
course be the cleanest signal, but they are not the only possible path to discovery. Indeed,
loops with virtual fourth-generation heavier fermions have a tremendous potential because
various observables would then get corrections sizeable enough compared to SM predictions
that they could be measured. A non-exhaustive list of fourth-generation-sensitive observables
comes as the flavour changing neutral current, the Higgs mass, and more generally electroweak precision data, then, the measurements of the violation of the discrete symmetry called
CP (Charge-Parity). Generating more CP violation than in SM is especially interesting since it
may be related to the matter-antimatter asymmetry of the universe.
The SM4 is one of the simplest extensions of the SM. However, the theoretical consequences
of having another generation of heavier fermion are very diverse. A particularly exiting
prospect is that for such heavy particles a Yukawa-type interaction could become so strong
that it creates a bound-state of those particles, just like the strong interaction creating the
hadrons. This so-called « composite state » would explicitly break the electro-weak symmetry
and it would therefore provide an alternative mechanism to the spontaneous symmetry
breaking mediated by the Higgs boson. It means that this bound-state could be related to the
origin of particle masses instead or concurrently with the Higgs boson.
During the thesis, the student will investigate direct and indirect searches of SM4 as well as
new theoretical ideas emerging from the possible existence of fourth generation fermions,
such as the one highlighted in the previous paragraphs. The student will work in the theory
group of LAL, which is currently composed of 1 CNRS researcher, 1 postdoc and 1 PhD
student. The members of the group are encouraged to work in a close contact with the
experimental groups in LAL as well as the theorists in LPT.
Précisions sur le stage :
When extending the SM from three generations to four, the model receives additional free
parameters, namely the heavy fermion masses and the 4x4 Cabibbo-Kobayashi-Maskawa
(CKM) matrix elements, in which our knowledge of the electro-weak interaction in SM4 is
engraved. In order to make theoretical predictions for the signals of SM4, we first need to
understand the precise constraints on these parameters coming from past experiments. During
the “stage”, the student will work on the fit of these parameters using the flavour experiments
as well as the electro-weak precision data in order to obtain these constraints.