MSc thesis proposals

MEFT PROJECT 1: Scale invariance, inflation 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. Interestingly enough, the inclusion of gravity in a scale-invariant setting may have far-reaching consequences. On the one hand, the breaking of dilatation symmetry translates into the appearance of a pseudo-Goldstone boson or dilaton which, due to its small mass, could potentially contribute to the early and late time acceleration of the Universe. On the other hand, the small value of the Higgs mass at the Planck scale could be a natural consequence of asymptotic safety, as already suggested by several renormalization group studies.

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 coupling constant. 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 highly recommended. Familiarity with the Standard Model phenomenology and beyond (e.g “Modelo Standard e Nova Física“) could be also helpful but not strictly necessary.

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

MEFT PROJECT 2: On the fate of primordial dark matter halos in hierarchical structures and galaxies

According to current observations, roughly the 30% of the mass of the Universe is in the form of a cold dark matter (DM) component whose fundamental nature remains unknown. Whatever its origin, this component is expected to be gravitationally unstable, allowing for the formation of virialized structures ranging in size from superclusters of galaxies to very small clumps.

The formation of bound objects in the standard cosmological scenario is restricted to small redshifts. This result is based on i) gravity being the dominant attractive force for the clumping of matter and ii) the assumption of a nearly scale-invariant spectrum of primordial density perturbations. These two ingredients entail the absence of significant structure formation prior to matter-radiation equality. Note, however, that none of these conditions must be necessarily fulfilled in alternative cosmologies. The DM particles could be subject, for instance, to long-range attractive forces stronger than gravity, opening the possibility of forming bound structures even during radiation-domination. If still present nowadays, these primordial dark matter halos could dominate the dark matter content of the Universe while passing current microlensing and CMB bounds (c.f. 1906.05300).

The formation process of DM halos is statistically described by the so-called Press-Schechter formalism, with smaller structures forming earlier and subsequently agregating to produce a complex landscape of halos-within-halos. This well-posed setting does not account, however, for the tidal disruption effects associated with the collective gravitational field of other clumps in the hierarchial clustering. This mutual destruction mechanism is specially effective at the early stages of structure formation. Later stages are additionally influenced by the stars in the Galactic disc.

The MSc candidate will estimate the surviving probability of primordial dark matter halos produced by fifth forces. The calculations will seek to determine the density distribution of DM clumps as a function of their mass, radius, distance to the Galactic center and orbit inclination with respect to the disk plane. Potential experimental and observational consequences in scenarios involving DM annihilation will be also considered.

Supervisors: Javier Rubio and Ilidio Lopes

Further reading

[1] 1906.05300 [astro-ph.CO]
[2] 0712.3499 [astro-ph]
[3] 0511494 [astro-ph]
[4] 0612733 [astro-ph]

Requirements

A basic knowledge of General Relativity and Cosmology (e.g. Relatividade e Cosmologia”) is highly recommended. Familiarity with Astrophysics (e.g “Astrofísica“) could be also helpful but not strictly necessary.

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