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## Proposals for Master or PhD thesis at LPTHE

### Challenges in supersymmetric theories of fields and strings

*Advisor: Ignatios Antoniadis*

The last decade has been very important for our understanding of the Universe. The discovery of the Higgs boson at the Large Hadron Collider at CERN has completed the searches for the building blocks of the Standard Model (SM) of particle physics. On the other hand, recent cosmological observations have confirmed the SM of cosmology. Measurements have allowed establishing the content of our Universe: besides the visible matter, there is a large amount of dark matter and dark energy. Moreover, the recent discovery of gravitational waves has opened a whole new way of perceiving the Universe. Despite this success of our present theories, the Standard Models of particle physics and cosmology and General Relativity, several fundamental questions emerged and have become of central importance for research in fundamental physics. Those relevant to the proposed thesis are:

The LHC has not shown yet any signal of new physics beyond the SM. A well motivated extension is based on supersymmetry that relates bosons and fermions, providing a dark matter candidate and explaining the stability of the mass hierarchy problem. What is the role of supersymmetry in Nature and what is its breaking scale?

What is the origin of inflation and what is the inflation field: a fundamental scalar, or composite, or an effective degree of freedom? Are there associated symmetries protecting its mass? What determines the initial conditions for inflation?

What is the nature of dark energy? Is it a cosmological constant or the effect of a dynamical field? What makes it so small?

What is the origin of the very different scales appearing in particle physics, gravitation and cosmology? Is the physics associated to these scales independent or are there connections due to the same underlying theory?

String theory has been proposed as the candidate to describe gravity at high energies where quantum effects begin to become important. It ought to shed light on the dynamics of the early Universe. Two important ingredients of string theory, necessary for its theoretical consistency, are extra dimensions and supersymmetry. In fact, string (or M-) theory appears in a few forms living in ten (or eleven) dimensions. Unfortunately, at the present stage of knowledge, going from ten or eleven to four dimensions leads to an extremely large number of vacua. The presence of this vast landscape of ground states for string theory weakens its predictive power. Still, the question whether there exist any de Sitter vacua amongst the plethora of possibilities is a long-standing issue and a subject of an ongoing debate today.

The expected duration of the thesis is three years. During the first months, the candidate should rather study the literature in these highly competitive subjects, in order to improve her/his level and to be able to choose the precise research direction depending on personal interests.

A Master internship is possible, prior to the start of the PhD thesis.

### Internship/PhD in theoretical high energy physics

*Advisor: Karim Benakli*

We are looking for a Master-2 student with a background in high energy theoretical physics interested in one of the following three topics, to be discussed to agree on a choice with the student:

1) Supersymmetry breaking models: supersymmetry, a symmetry with several virtues for theories beyond the Standard Model, is not observed experimentally, so it must be broken if it plays a role in nature.

2) The weak gravity conjecture: this conjecture states that for any abelian gauge theory, there exists a state for which the gravitational force (Newton) is weaker than the gauge interaction (Coulomb).

3) The study of the equations of motion describing the propagation of particles in non-trivial media. In particular, for particles with spin > 1, the description of the propagation of longitudinal helicity modes presents interesting properties and requires a particular analysis.

A Ph.D. in theoretical high-energy physics is also possible on similar topics, subject to the usual condition of successful application to the Doctoral School.

Contact: Karim Benakli by e-mail at kbenakli_AT_lpthe.jussieu.fr