MSc thesis proposals

MEFT PROJECT 1: Scale invariance and the strong CP problem

The synergy of the Standard Model of particle physics and General Relativity has led to a consistent framework confirmed by numerous experiments and observations. In spite of their undeniable success, these theories cannot be considered as complete theories of Nature. On the one hand, they fail to explain basic observational facts such as the existence of neutrino masses, the presence of a sizable dark matter component or the matter-antimatter asymmetry of the Universe. On the other hand, they are unable to provide a satisfactory solution to several fine-tuning issues such as the strong CP problem in QCD or the so-called hierarchy problem, which leads to the instability of the Higgs mass under radiative corrections and to the infamous cosmological constant problem.

The discovery of a relatively light Higgs boson in the LHC together with the absence of new physics beyond the Standard Model has rejuvenated scale symmetry as an interesting framework to address the aforementioned Standard Model problems. The MSc candidate will complement this ambitious program by considering an axion solution to the strong CP problem within a scale invariant extension of the Standard Model non-minimally coupled to gravity. The main idea behind this proposal will be to identify the axion field in the most commonly accepted Peccei-Quinn scenario with the dilaton field needed for implementing scale invariance in the Standard Model, such that both dilatation and chiral symmetries are spontaneously broken before inflation. The target scenarios will be based on the principles of minimality and quantum stability, seeking to provide a self-consistent evolution of the Universe while avoiding the stringent cosmological bounds on the axion. Distinguishing features for experiments and observations will be also considered.

Supervisor: Javier Rubio

Further reading

[1] 1104.1392 [hep-th]
[2] 1811.05984 [astro-ph.CO]
[3] 1705.10836 [hep-ph]
[4] 1608.05414 [hep-ph]

Requirements

A basic knowledge of Quantum Field Theory (at the level of “Teoria de Campo” or “Física de Partículas“) and General Relativity (e.g. “Relatividade e Cosmologia”) is recommended.

MEFT PROJECT 2: Inflation and dark energy from the renormalisation group flow

A dynamical scalar field with a sufficiently flat potential and at most tiny couplings to ordinary matter is often advocated as a promising alternative to the cosmological constant. This idea, usually named quintessence, can be partly viewed as a late-time implementation of the successful inflationary paradigm. Since inflation and dark energy share many essential properties, it is natural to seek for a unification of these two mechanisms using a single scalar field. This appealing possibility dubbed quintessential inflation is usually formulated in the so-called Einstein frame, where the gravitational part of the action takes the usual Einstein-Hilbert form.

Although physics should be independent of the particular representation a given theory is formulated, some frames can be more convenient than others for identifying construction principles such as symmetries or distinguishing key elements affecting the observable predictions from purely accessory ones. In particular, the presence of a fixed gravitational scale in quintessential inflation scenarios obscures the impact of scale symmetry on the inflationary and dark energy observables while making dimensional arguments and estimates more complicated. In many cases, this gives rise to wrong or inaccurate statements on the role of quantum fluctuations in the early Universe or the “natural size” of physical quantities. 

To overcome this extended misunderstanding, we will reformulate the main quintessential inflation scenarios in a scaling frame encoding the effective emergence of dilatation symmetry in the early and late Universe. This will allow us to tackle the controversial trans-Planckian problem in a more robust way. Having recast the most popular quintessential inflation scenarios in this representation, we will perform a preliminary study of their potential embedding in a complete theory of gravity within the asymptotic safety paradigm.

Supervisor: Javier Rubio

Further reading

[1] 1705.00552 [gr-qc]
[2] 11605.01858 [hep-th]

Requirements

A basic knowledge of General Relativity and Cosmology (e.g. “Relatividade e Cosmologia”) is recommended.

Please do not hesitate to contact me if you need further details!