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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.

Phenomenology of Dark Matter Indirect Detection

Advisor: Marco Cirelli

Format: Master 2 internship + PhD thesis project

Timing: Internship in Spring 2021, PhD starting in Fall 2021

About 85% of the matter in the Universe is in the form of an unknown substance dubbed Dark Matter (DM). While some of its general properties are known, its actual nature is still undetermined. The most popular hypothesis is that it consists of a new, yet-to-be-discovered elementary particle. One of the possible strategies to investigate it is via the so-called Indirect Detection (ID): studying the possible excesses in cosmic rays that could be produced by the annihilations (or decays) of DM particles in the galactic halo, and comparing them with the theoretical predictions from particle physics models.

Within this broad context, the proposed PhD project will proceed in different stages. The first part of the work will consist in extending and upgrading a set of numerical tools used in the DM ID studies (PPPC4DMID, based on arXiv:1012.4515). After an initial study of the basic concepts in the literature, this part will be rather technical and will require familiarizing with the Mathematica software and other numerical tools. The second part will consist in exploiting the upgraded tools and applying them to the actual physics searches, including the comparison with data from cosmic ray experiments. Different directions are here possible, concerning gamma-rays, charged cosmic rays or neutrinos. The choice will be dictated by the emergence of possible new results and by the interest in the community.

Following the typical course, a (Master 2) internship lasting a few months during Spring 2021 is strongly encouraged. In this period the student will familiarize with the basics of the physics and with some numerical tools. Subject to mutual agreement, the student can then apply for a PhD fellowship (from the local Doctoral School or from other sources) and, if successful, start the PhD in Fall 2021, for a duration of 3 years.

Candidates should send by email to marco.cirelli_at_gmail.com: i. their CV, ii. a transcript of their academic records, iii. a short description of their interests (optional, and in any case no longer than 1 page). They should also arrange for 1 or 2 short letters of recommendation to be sent to the same address, by scientists familiar with their studies and academic record.