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Proposals for Master or PhD thesis at LPTHE

Applications of supergravity and string theory to High-energy physics

Advisor: Ignatios Antoniadis

High-energy physics enters a new era with the Large Hadron Collider (LHC) at CERNsearching for new particles and forces at energies ten times bigger than previously, creating conditions similar to those of the early Universe, just after the Big Bang. At the same time, several new observations arise from experiments in astrophysics and cosmology, which complete our understanding of the same physics, while the direct and indirect search of dark matter continues. There is a real opportunity to extract the underlying microscopic theory, beyond the current standard models of particle physics and cosmology. The subject of this thesis is to explore several theoretical ideas and proposals, based on supersymmetry, string theory, extra dimensions and holography, in order to make progress in this direction. Indeed, supersymmetry is space-time symmetry motivated by theoretical arguments and experimental indications. On the other hand, string theory renders compatible two fundamental theories: quantum mechanics and general relativity, by replacing the notion of point particles by extended objects. Two important consequences are the existence of extra dimensions and the possible brane-world description of our Universe.

Several research directions are possible. Central questions are: (i) the origin of the different scales associated with a theory describing cosmology, gravitation and particle physics; (ii) the role of supersymmetry at a fundamental level and its possible non- linear realisation at low energies. Possible research objectives are: (1) Particle physics and cosmology of (approximate) de Sitter vacua in supergravity, towards the description of both inflation and present dark energy. (2) Non-linear supersymmetry and cosmology. (3) In the context of string theory, compactification methods, mechanisms of supersymmetry breaking, the structure of the effective supergravity and its properties, the computation of non-perturbative effects, the propagation of strings in curved spacetime, and the study of gravitational phenomena in regions of strong curvature where the effective field theory approach breaks down.

The expected duration of the thesis is three years. During the first months, the candidate should rather study the literature in this highly competitive subject, 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.

Pushing the precision frontier of LHC physics: Automating QCD resummation for new physics

Advisor: Benjamin Fuks, Co-advisor: Hua-Sheng Shao

The Standard Model of particle physics is an extremely successful theory, with (almost) all observations performed so far perfectly matching the predictions. However, the present experimental status also reveals several of the conceptual issues and practical limitations of the Standard Model, like the absence of a candidate for dark matter or the hierarchy problem. The Standard Model is therefore acknowledged as an effective theory that should originate from a more fundamental one yet to be discovered, and new phenomena are expected at energies well below the Planck scale. Whilst there are currently numerous intriguing features that provide hints that new physics could be accessible at energies that are already probed, none of these are yet significant enough to exclude the standard paradigm. However, as more data is being collected, we have reason to be optimistic towards the close future.

One interesting avenue in the searches for new physics relies on indirect probes in which precision theoretical predictions are confronted to accurate experimental measurements. The state-of-the-art for new physics predictions for the LHC consists in Monte Carlo simulations in which matrix elements including the next-to-leading order corrections driven by quantum chromodynamics (QCD) are matched with parton showers. While the accuracy of parton shower is quite limited so far, QCD resummation procedure provides a systematic way to improve it. In more details, fixed-order QCD perturbative computations are only justified when the perturbative series is controlled by a small expansion parameter. Close to the production threshold or when QCD emission are soft and/or collinear, large logarithmic terms need to be resummed. Whilst such a resummation can be handled through parton showers, its accuracy is limited so that analytical computations are in order to provide a better description at the hard-scattering level.

This thesis fits within this context and aims to automate perturbative QCD resummation calculations for various observables relevant for the LHC, both in the framework of the Standard Model and for new physics. The planned research will extend two existing programs developed at the LPTHE and that are widely used in the high-energy physics community, namely MADGRAPH5_aMC@NLO [1] and RESUMMINO [2]. Whilst we plan to start studying standard candles (total rates, invariant mass and transverse-momentum distributions of one or more final state particles) in the Standard Model, phenomenological applications to new physics will be conducted later on. We indeed plan to focus both on well-motivated ultraviolet-complete models (such as models featuring a strong dynamics at a high-energy scale or supersymmetric models) and effective field theories extending the Standard Model by higher-dimensional operators.

The technical part of the project consists in understanding how next-to-leading order calculations in QCD work and in devising an efficient method to extract the components allowing for performing all-order QCD resummation. Whilst a first training on the Standard Model and supersymmetry for which analytical results exist will be achieved, generalisation to any process from any theory will be foreseen in a second step. The deliverable consists in the building of a plugin linking MADGRAPH5_aMC@NLO and RESUMMINO, so that any physicist could compute precision predictions for any model. Moreover, tables of total and differential cross sections relevant for LHC and future collider physics will be provided, as this is known as relevant inputs for the high-energy physics community.

Within this project, the candidate is expected to develop a deep knowledge of new physics models and phenomenology, as well as acquire strong computing skills in QCD. Achieving the predefined goals will allow the candidate to obtain a strong expertise both in the development and in the usage of various tools widely used in our community, which is a valuable expertise for most research groups in the world.

[1] J. Alwall, R. Frederix, S. Frixione, V. Hirschi, F. Maltoni, O. Mattelaer, H.S. Shao, T. Stelzer, P. Torrielli and M. Zaro, The automated computation of tree-level and next-to-leading order differential cross sections, and their matching to parton shower simulations, JHEP 1407 (2014) 079.

[2] B. Fuks, M. Klasen, D.R. Lamprea and M. Rothering, Precision predictions for electroweak superpartner production at hadron colliders with Resummino, Eur. Phys. J. C 73 (2013) 2480.