arXiv subjects

Miguel Faria

Publications and source records attributed to Miguel Faria.

6 recordsLinked to original sources

arXiv2026

Hawking emission of massive vector fields by Kerr black holes

We compute, for the first time, the Hawking emission spectrum of massive vector (Proca) fields by spinning Kerr black holes, determining the associated greybody factors and the resulting mass and spin loss functions. We show, in particular, that the scalar (longitudinal) polarization of the Proca field has a spectrum approaching that of a free scalar field in the massless limit (in which it becomes a pure gauge mode), although we find substantial differences for finite mass. The contribution of the two vector (transverse) polarization modes coincides, as expected, with the one obtained by Page for the Maxwell field in the massless limit. The black hole's evaporation rate is dominated by the scalar mode for slowly spinning black holes and by the two vector modes as the black hole approaches extremality. As for other fields, we find that Proca Hawking emission is Boltzmann-suppressed for Hawking temperatures $T_H\lesssim |\mu-\Omega_H|$, where $\mu$ is the field mass and $\Omega_H$ is the angular velocity of the black hole's horizon. This implies that highly spinning black holes can efficiently emit massive vector fields at temperatures parametrically below the field's mass. Finally, we also find that superradiant emission is more pronounced for massive vector fields, with a maximum amplification factor of $\simeq 7\%$ (compared to $\simeq 4\%$ for massless photons).

gr-qc↗

arXiv2026

MATH-PT: A Math Reasoning Benchmark for European and Brazilian Portuguese

The use of large language models (LLMs) for complex mathematical reasoning is an emergent area of research, with fast progress in methods, models, and benchmark datasets. However, most mathematical reasoning evaluations exhibit a significant linguistic bias, with the vast majority of benchmark datasets being exclusively in English or (at best) translated from English. We address this limitation by introducing {\sc Math-PT}, a novel dataset comprising 1,729 mathematical problems written in European and Brazilian Portuguese. {\sc Math-PT} is curated from a variety of high-quality native sources, including mathematical Olympiads, competitions, and exams from Portugal and Brazil. We present a comprehensive benchmark of current state-of-the-art LLMs on {\sc Math-PT}, revealing that frontier reasoning models achieve strong performance in multiple choice questions compared to open weight models, but that their performance decreases for questions with figures or open-ended questions. To facilitate future research, we release the benchmark dataset and model outputs.

cs.CL↗

arXiv2026

AMALIA Technical Report: A Fully Open Source Large Language Model for European Portuguese

Despite rapid progress in open large language models (LLMs), European Portuguese (pt-PT) remains underrepresented in both training data and native evaluation, with machine-translated benchmarks likely missing the variant's linguistic and cultural nuances. We introduce AMALIA, a fully open LLM that prioritizes pt-PT by using more high-quality pt-PT data during both the mid- and post-training stages. To evaluate pt-PT more faithfully, we release a suite of pt-PT benchmarks that includes translated standard tasks and four new datasets targeting pt-PT generation, linguistic competence, and pt-PT/pt-BR bias. Experiments show that AMALIA matches strong baselines on translated benchmarks while substantially improving performance on pt-PT-specific evaluations, supporting the case for targeted training and native benchmarking for European Portuguese.

cs.CL↗

arXiv2025

"Teammates, Am I Clear?": Analysing Legible Behaviours in Teams

In this paper we investigate the notion of legibility in sequential decision-making in the context of teams and teamwork. There have been works that extend the notion of legibility to sequential decision making, for deterministic and for stochastic scenarios. However, these works focus on one agent interacting with one human, foregoing the benefits of having legible decision making in teams of agents or in team configurations with humans. In this work we propose an extension of legible decision-making to multi-agent settings that improves the performance of agents working in collaboration. We showcase the performance of legible decision making in team scenarios using our proposed extension in multi-agent benchmark scenarios. We show that a team with a legible agent is able to outperform a team composed solely of agents with standard optimal behaviour.

cs.AI↗

arXiv2025

Stupendously Large Primordial Black Holes from the QCD axion

The inflationary diffusion of (pseudo-)scalar fields with discrete symmetries can seed the formation of a gas of closed domain walls after inflation, when the distance between degenerate minima in field space is not too far from the inflationary Hubble scale. Primordial black holes (PBHs) can then be formed once sufficiently heavy domain walls re-enter the Hubble sphere. In this scenario, inflation determines a distinctive PBH mass distribution that is rather flat and can thus lead to a sizable total abundance of PBHs, while avoiding some of the downsides of PBH formation from critical collapse. We show that generic QCD axion models, with decay constant close to the inflationary Hubble scale, can yield up to $1\%$ of the dark matter (DM) today in the form of PBHs, while being compatible with isocurvature constraints from Cosmic Microwave Background observations. This occurs for values of axion decay constants around $f_a\simeq 10^{8}~\text{GeV}$, that is the region targeted by axion helioscopes and partially constrained by astrophysical observations. The resulting PBHs have \textit{stupendously} large masses, above $10^{11}M_\odot$, and their existence can be probed by Large Scale Structure observations. Larger PBH abundances can be generated by axion-like particles. Alternatively, in scenarios where isocurvature constraints can be relaxed, we find that the totality of the DM can be produced by the QCD axion misalignment mechanism, accompanied by a ${\cal O}(10^{-3})$ DM fraction in PBHs of masses $(10^5-10^6)~M_\odot$. These can act as seeds for the formation of massive black holes at large redshifts, as suggested by recent JWST observations.

astro-ph.CO↗

arXiv2022

"Guess what I'm doing": Extending legibility to sequential decision tasks

In this paper we investigate the notion of legibility in sequential decision tasks under uncertainty. Previous works that extend legibility to scenarios beyond robot motion either focus on deterministic settings or are computationally too expensive. Our proposed approach, dubbed PoL-MDP, is able to handle uncertainty while remaining computationally tractable. We establish the advantages of our approach against state-of-the-art approaches in several simulated scenarios of different complexity. We also showcase the use of our legible policies as demonstrations for an inverse reinforcement learning agent, establishing their superiority against the commonly used demonstrations based on the optimal policy. Finally, we assess the legibility of our computed policies through a user study where people are asked to infer the goal of a mobile robot following a legible policy by observing its actions.

cs.RO↗