arXiv subjects

Rui Santos

Publications and source records attributed to Rui Santos.

At least 37 records · Page 2Linked to original sources

arXiv2024

How Books Tell a History of Statistics in Portugal: Works of Foreigners, Estrangeirados, and Others

Foreigners and "estrangeirados", an expression meaning "people going to a foreign country ["estrangeiro"] getting there further education", had a leading role in the development of Mathematical Statistics in Portugal. In what concerns Statistics, "estrangeirados" in the nineteenth century were mainly liberal intellectuals exiled for political reasons. From 1930 onwards, the research funding authority sent university professors abroad, and hired foreign researchers to stay in Portuguese institutions, and some of them were instrumental in the importation of new concepts and methods of inferential statistics. After 1970, there was a huge program of sending young researchers abroad for doctoral studies. At the same time, many new universities and polytechnic institutes have been created in Portugal. After that, aside from foreigners who choose to have a research career in those institutions and the "estrangeirados" who had returned and created programs of doctoral studies, others, who hadn't the opportunity of studying abroad, began to play a decisive role in the development of Statistics in Portugal. The publication of handbooks on Probability and Statistics, thesis and core papers in Portuguese scientific journals, and also of works for the layman, reveals how Statistics progressed from descriptive to a mathematical discipline used for inference in all fields of knowledge, from natural sciences to methodology of scientific research.

stat.OT↗

arXiv2024

Tables with Critical Values for the Meta-Analysis of Genuine and Fake $\boldsymbol{p}$-Values

The classical theory for the meta-analysis of $p$-values is based on the assumption that if the overall null hypothesis is true, then all $p$-values used in a chosen combined test statistic are genuine, i.e., are observations from independent and identically distributed standard uniform random variables. However, the pressure felt by most researchers to publish, which is worsen by publication bias, can originate fake $p$-values to be reported, usually Beta(1,2) distributed. In general, the existence of fake $p$-values in a sample of $p$-values to be combined is unknown, and if, for some reason, there is information that they do exist, their number will most likely be unknown as well. Moreover, even if fake $p$-values are accounted for, the cumulative distribution function of classical combined test statistics does not have a closed-form expression that facilitates its practical usage. To overcome this problem, tables with estimated critical values are supplied for the commonly used combined tests for the meta-analysis of $p$-values when a few of them are fake ones, i.e., Beta(1,2) distributed.

stat.CO↗

arXiv2024

Block Expanded DINORET: Adapting Natural Domain Foundation Models for Retinal Imaging Without Catastrophic Forgetting

Integrating deep learning into medical imaging is poised to greatly advance diagnostic methods but it faces challenges with generalizability. Foundation models, based on self-supervised learning, address these issues and improve data efficiency. Natural domain foundation models show promise for medical imaging, but systematic research evaluating domain adaptation, especially using self-supervised learning and parameter-efficient fine-tuning, remains underexplored. Additionally, little research addresses the issue of catastrophic forgetting during fine-tuning of foundation models. We adapted the DINOv2 vision transformer for retinal imaging classification tasks using self-supervised learning and generated two novel foundation models termed DINORET and BE DINORET. Publicly available color fundus photographs were employed for model development and subsequent fine-tuning for diabetic retinopathy staging and glaucoma detection. We introduced block expansion as a novel domain adaptation strategy and assessed the models for catastrophic forgetting. Models were benchmarked to RETFound, a state-of-the-art foundation model in ophthalmology. DINORET and BE DINORET demonstrated competitive performance on retinal imaging tasks, with the block expanded model achieving the highest scores on most datasets. Block expansion successfully mitigated catastrophic forgetting. Our few-shot learning studies indicated that DINORET and BE DINORET outperform RETFound in terms of data-efficiency. This study highlights the potential of adapting natural domain vision models to retinal imaging using self-supervised learning and block expansion. BE DINORET offers robust performance without sacrificing previously acquired capabilities. Our findings suggest that these methods could enable healthcare institutions to develop tailored vision models for their patient populations, enhancing global healthcare inclusivity.

cs.CV↗

arXiv2024

Impact of New Experimental Data on the C2HDM: the Strong Interdependence between LHC Higgs Data and the Electron EDM

The complex two-Higgs doublet model (C2HDM) is one of the simplest extensions of the Standard Model with a source of CP-violation in the scalar sector. It has a $\mathbb{Z}_2$ symmetry, softly broken by a complex coefficient. There are four ways to implement this symmetry in the fermion sector, leading to models known as Type-I, Type-II, Lepton-Specific and Flipped. In the latter three models, there is a priori the surprising possibility that the 125 GeV Higgs boson couples mostly as a scalar to top quarks, while it couples mostly as a pseudoscalar to bottom quarks. This ``maximal'' scenario was still possible with the data available in 2017. Since then, there have been more data on the 125 GeV Higgs boson, direct searches for CP-violation in angular correlations of $τ$-leptons produced in Higgs boson decays, new results on the electron electric dipole moment, new constraints from LHC searches for additional Higgs bosons and new results on $b\to s γ$ transitions. Highlighting the crucial importance of the physics results of LHC's Run 2, we combine all these experiments and show that the ``maximal'' scenario is now excluded in all models. Still, one can have a pseudoscalar component in $h τ\barτ$ couplings in the Lepton-Specific case as large as $87\%$ of the scalar component for all mass orderings of the neutral scalar bosons.

hep-ph↗

arXiv2024

Freeze-in as a Complementary Process to Freeze-Out

There are many extensions of the Standard Model with a dark matter (DM) candidate obtained via the freeze-out mechanism. It can happen that after all experimental and theoretical constraints are taken into account, all parameter points have a relic density below the experimentally measured value. This means that the models solve only partially the DM problem, and at least one more candidate is needed. In this work we show that it is possible to further extend the model with a DM candidate obtained via the freeze-in mechanism to be in agreement with the relic density experimental measurement. Once the relic density problem is solved with this addition, new questions are raised. This new model with at least two DM candidates could have a freeze-out undetectable DM particle both in direct and indirect detection. This could happen if the freeze-out DM particle would have a very low density. Hence, a collider DM hint via excess in the missing energy with no correspondence in direct and indirect detection experiments, could signal the existence of a Feebly Interacting Massive Particle (FIMP). Conversely, if a DM particle is found and a particular model can explain all observables except the correct relic density, an extension with an extra FIMP would solve the problem. The freeze-in DM candidate, due to the small portal couplings, will not change the remaining phenomenology.

hep-ph↗

arXiv2024

Simulation-based inference in the search for CP violation in leptonic WH production

Sources of CP violation beyond the Standard Model (BSM) are required to explain the baryonic asymmetry of the Universe. In this work, we study BSM CP-violating components in the HWW interaction in WH production, parametrized by an effective dimension-6 CP-odd operator. We explore a machine learning simulation-based inference method that estimates a detector-level optimal observable - SALLY - comparing it with energy-dependent and angular observables, exploring different binnings for their distributions. We show that in regions of phase space where the interference between SM and the effective operator dominates, a SALLY observable leads to optimal limits. In regions where effects of the quadratic term of the effective operator start becoming dominant, such an observable still leads to optimal limits. This work aims to test current multivariate techniques and inform analysis strategies for LHC Run 3 and beyond.

hep-ph↗

arXiv2023

Precision Muon-Related Observables as a Tool to Constrain New Physics Models

We propose a set of precision muon-related observables that serve as a tool to constrain new physics models. Using LEP's precision measurements on the $Z$-boson pole, we derive bounds on the new physics quantum contributions to the decay $Z \to μ^+ μ^-$. We show that the new precision observables have a real impact on two specific models that solve the $g-2$ anomaly and provide a sound dark matter candidate.

hep-ph↗

arXiv2023

Dark Coloured Scalars Impact on Single and Di-Higgs Production at the LHC

The search for Dark Matter (DM) at colliders is primarily pursued via the detection of missing energy in particular final states. These searches are based on the production and decay processes where final states include DM particles and at least one Standard Model (SM) particle. DM will then reveal itself as missing energy. An alternative form to get a hint of a dark sector is via loop contribution to SM processes. In this case, it is not even relevant if the new particles have their origin in the dark sector of the model. In this work we discuss the impact of an arbitrary number of coloured scalars in single Higgs and double Higgs production at the Large Hadron Collider (LHC), and we show their complementarity. We determine the range of variation of the corrections relative to the SM for an arbitrary number of coloured scalars $n$, and discuss in more detail the cases $n=1$ and $n=2$.

hep-ph↗

arXiv2023

Search for an invisible scalar in $t \bar{t}$ final states at the LHC

We use the current $t\bar t$ experimental analysis to look for Dark Matter (DM) particles hidden in the final state. We present a phenomenological study where we successfully perform the reconstruction of a $t\bar{t}$ system in the presence of a scalar mediator $Y_0$, that couples to both Standard Model (SM) and to DM particles. We use a \texttt{MadGraph5\_aMC@NLO} simplified DM model, where signal samples of $pp \rightarrow t\bar{t}Y_0$ are generated at the Large Hadron Collider (LHC) with both Charge-Parity (CP) -even and CP-odd couplings of $Y_0$ to the top quarks. Different mass scales for the $Y_0$ mediator are considered, from the low mass region ($\sim$ 0~GeV) to masses close to the Higgs boson mass (125~GeV). The dileptonic final states of the $t\bar{t}$ system were used in our analysis. The reconstruction of the $t\bar{t}$ system is done with a kinematic fit, without reconstructing the mediator. All relevant SM backgrounds for the dileptonic $t\bar{t}$ search at the LHC are considered. Furthermore, CP angular observables were used to probe the CP-nature of the coupling between the mediator and top-quarks, which allowed to set confidence level (CL) limits for those Yukawa couplings as a function of the mediator mass.

hep-ph↗

arXiv2023

Electroweak Corrections to Higgs Boson Decays in a Complex Singlet Extension of the SM and their Phenomenological Impact

The complex singlet extension CxSM of the Standard Model (SM) is a simple extension of the SM with two visible Higgs bosons in the spectrum and a Dark Matter (DM) candidate. In this paper we complete the computation of the next-to-leading (NLO) electroweak (EW) corrections to on-shell and non-loop-induced Higgs decays. Our calculations are implemented in the code EWsHDECAY which also includes the relevant QCD corrections. Performing an extensive parameter scan in the model and including all relevant theoretical and experimental single- and di-Higgs as well as DM constraints, we obtain a viable parameter sample. We find that current DM constraints are able to test the model in DM mass regions where collider searches are not sensitive. The relative EW corrections turn out to be large for scenarios with relatively large couplings, threshold effects or small leading-order (LO) widths. Otherwise, they are of typical EW size and can amount up to about 20-25%. The theory uncertainty derived from the change of the renormalization scheme dependence then is of a few per cent. While the NLO corrections applied in the constraints due to single- and di-Higgs searches impact the validity of specific parameter points, the overall shape of the allowed parameter region is not yet sensitive to the EW corrections. This picture will change with further increased experimental precision in the future and necessitates precise predictions on the theory side as presented in this paper.

hep-ph↗

arXiv2023

Exploring the viability of pseudo Nambu-Goldstone boson as ultralight dark matter in a mass range relevant for strong gravity applications

We study a simple extension of the Standard Model featuring a dark sector with an ultralight pseudo Nambu-Goldstone boson as dark matter candidate. We focus on the mass range $\mathcal{O}(10^{-20} - 10^{-10})$ eV, relevant for strong gravity applications, and explore its production and evolution in the early Universe. The model is formulated in such a way that dark matter does not couple directly to photons or other Standard Model particles avoiding some of the most stringent cosmological bounds related to axion-like particles. In this work, two different scenarios are considered depending on whether dark matter is produced in a pre-inflationary or post-inflationary regime. We also discuss the effect from emergent topological defects such as cosmic strings and domain walls, and estimate the spectrum of stochastic gravitational waves produced by their decay, enabling to test the model at current and future gravitational-wave experiments.

hep-ph↗

arXiv2023

Direct detection of pseudo-Nambu-Goldstone dark matter in a two Higgs doublet plus singlet extension of the SM

We calculate the leading radiative corrections to the dark-matter-nucleon scattering in the pseudo-Nambu-Goldstone dark matter model augmented with a second Higgs doublet (S2HDM). In this model, the cross sections for the scattering of the dark-matter on nuclei vanishes at tree-level in the limit of zero momentum-transfer due to a U(1) symmetry. However, this symmetry is softly broken in order to give a mass to the dark-matter particle. As a consequence, non-vanishing scattering cross sections arise at the loop level. We find that the current cross-section limits from dark-matter direct-detection experiments can hardly constrain the parameter space of the S2HDM. However, the loop-corrected predictions for the scattering cross sections can be well within the reach of future direct-detection experiments. As a consequence, future phenomenological analyses of the S2HDM should take into account cross-section predictions beyond tree-level and the experimental constraints from dark-matter direct-detection experiments.

hep-ph↗

arXiv2023

Deep Learning Searches for Vector-Like Leptons at the LHC and Electron/Muon Colliders

The discovery potential of both singlet and doublet vector-like leptons (VLLs) at the Large Hadron Collider (LHC) as well as at the not-so-far future muon and electron machines is explored. The focus is on a single production channel for LHC direct searches while double production signatures are proposed for the leptonic colliders. A Deep Learning algorithm to determine the discovery (or exclusion) statistical significance at the LHC is employed. While doublet VLLs can be probed up to masses of 1 TeV, their singlet counterparts have very low cross sections and can hardly be tested beyond a few hundreds of GeV at the LHC. This motivates a physics-case analysis in the context of leptonic colliders where one obtains larger cross sections in VLL double production channels, allowing to probe higher mass regimes otherwise inaccessible even to the LHC high-luminosity upgrade.

hep-ph↗

arXiv2022

P-even, CP-violating Signals in Scalar-Mediated Processes

Most studies of Higgs sector CP violation focus on the detection of CP-violating neutral Higgs-fermion Yukawa couplings, which yield P-odd, CP-violating phenomena. There is some literature on purely bosonic signatures of Higgs sector CP violation, where the simultaneous observation of three processes (suitably chosen) constitutes a signal of P-even CP violation. However, in the examples previously analyzed, some of the processes are strongly suppressed in the approximate Higgs alignment limit (corresponding to the existence of a Standard Model like Higgs boson as suggested by LHC data), in which case the proposed CP-violating signals are difficult to observe in practice. In this paper, we extend the existing literature by examining processes that do not vanish in the Higgs alignment limit and whose simultaneous observation would provide unambiguous evidence for scalar-mediated P-even CP violation. We assess the discovery potential of such signals at various future multi-TeV lepton (and $γγ$) colliders. The potential for detecting loop-induced P-even, CP-violating phenomena is also considered.

hep-ph↗

arXiv2022

Benchmarking Di-Higgs Production in Various Extended Higgs Sector Models

We present a comprehensive study on Higgs pair production in various archetypical extended Higgs sectors such as the real and the complex 2-Higgs-Doublet Model, the 2-Higgs-Doublet Model augmented by a real singlet field and the Next-to-Minimal Supersymmetric extension of the Standard Model. We take into account all relevant theoretical and experimental constraints, in particular the experimental limits on non-resonant and resonant Higgs pair production. We present the allowed cross sections for Standard Model (SM)-like Higgs pair production and the ranges of the SM-like Yukawa and trilinear Higgs self-coupling that are still compatible with the applied constraints. Furthermore, we give results for the pair production of a SM-like with a non-SM-like Higgs boson and for the production of a pair of non-SM-like Higgs bosons. We find that di-Higgs production in the models under investigation can exceed the SM rate substantially, not only in the non-resonance region but also due to resonant enhancement. We give several benchmarks with interesting features such as large cross sections, the possibility to test CP violation, Higgs-to-Higgs cascade decays or di-Higgs production beating single Higgs production. In all of our benchmark points, the next-to-leading order QCD corrections are included in the large top-mass limit. For these points, we found that, depending on the model and the Higgs pair final state, the corrections increase the leading order cross section by a factor of 1.79 to 2.24. We also discuss the relation between the description of Higgs pair production in an effective field theory approach and in the specific models investigated here.

hep-ph↗

arXiv2022

CP-violation, Asymmetries and Interferences in $t \bar{t} ϕ$

We use the associated production of top-quark pairs with a generic scalar boson at the LHC to explore the sensitivity of a large set of observables to the sign of the CP mixing angle, present in the coupling between the scalar boson and the top quarks. The mass of the scalar boson is set to $m_ϕ=125$ GeV and its coupling to top-quarks is varied such that $α=$ 0$^\circ$, 22.5$^\circ$, 45.0$^\circ$, 67.5$^\circ$, 90.0$^\circ$, 135.0$^\circ$ and 180.0$^\circ$. Dileptonic final states of the $t\bar{t}ϕ$ system are used, with the scalar boson decaying as $ϕ\rightarrow b\bar{b}$. A new method to reconstruct the scalar mass, originally designed for the low mass regime is used, improving the resolution of the Higgs mass by roughly a factor of two. A full phenomenological analysis is performed using Standard Model background and signal events generated with MadGraph5aMC@NLO and reconstructed using a kinematical fit. The most sensitive CP-observables are selected to compute Confidence Level (CL) limits as a function of the sign of the top quark Yukawa couplings to the scalar boson. We also explore the sensitivity to interference terms using differential distributions and angular asymmetries. Given the significant difference between the pure scalar and pure pseudo-scalar production cross section values, it is unlikely the $t\bar{t}ϕ$ channel alone will be sensitive to the sign of the CP-mixing angle or interference terms, even at the end of the LHC. Using the $b_2^{t\bar{t}ϕ}$ and $b_4^{t\bar{t}ϕ}$ variables, exclusion limits at 95\% CL for the CP-even and CP-odd components of the top quark Yukawa couplings are expected to be set to $\tildeκ \in$ [-0.698,+0.698] and $|κ| \in$ [0.878,1.04], respectively, at the end of the High Luminosity phase of the LHC (HL-LHC) by using the dileptonic decay channel alone.

hep-ph↗

arXiv2022

Gravitational waves and the Higgs mass

One of the simplest models that allows for an electroweak (EW) strong first order phase transition (SFOPT) while having a Higgs with the observed mass of 125 GeV is the singlet extension of the Standard Model (SM). We discuss two different phases of a complex singlet extension of the SM. If the singlet has a zero vacuum expectation value (VEV) the situation is exactly the same as in the SM with no SFOPT. However, when the singlet acquires a VEV, a SFOPT is possible, giving rise to detectable gravitational waves (GWs) by LISA. The model also provides a dark matter candidate. We have studied the impact of the SM particle masses on the strength of the GWs spectrum. It turns out that the variation of the Higgs boson and top quark masses within one standard deviation of the experimentally measured value has a noticeable impact on GWs detectability prospects.

hep-ph↗

arXiv2022

Impact of electroweak group representation in models for $B$ and $g-2$ anomalies from Dark Loops

We discuss two models which are part of a class providing a common explanation for lepton flavor universality violation in $b \to s l^+ l^- $ decays, the dark matter (DM) problem and the muon $(g-2)$ anomaly. The $B$ meson decays and the muon $(g-2)$ anomalies are explained by additional one-loop diagrams with DM candidates. The models have one extra fermion field and two extra scalar fields relative to the Standard Model (SM). The $SU(3)$ quantum numbers are fixed by the interaction with the SM fermions in a new Yukawa Lagrangian that connects the dark and the visible sectors. We compare two models, one where the fermion is a singlet and the scalars are doublets under $SU(2)_L$ and another one where the fermion is a doublet and the scalars are singlets under $SU(2)_L$. We conclude that both models can explain all new physics phenomena simultaneously, while satisfying all other flavor and DM constraints. However, there are crucial differences between how the DM constraints affect the two models leading to a noticeable difference in the allowed DM mass range.

hep-ph↗