arXiv subjects

Bruce Mellado

Publications and source records attributed to Bruce Mellado.

At least 37 records · Page 2Linked to original sources

arXiv2023

Searches for Additional Scalars at Future $e^{+} e^{-}$ Colliders

We present growing excesses consistent with a 95 GeV scalar. We provide a comprehensive analysis of the Two Higgs Doublet Model and an additional singlet (2HDM+S) at future $e^{+} e^{-}$ collider. In particular, we provide a precise mass reconstruction measurement for the scalar, $m_{S}$, using the recoil mass method through $e^{+} e^{-} \to Z S$ where $Z \to μ_{+} μ_{-}$ and $S \to b \bar{b}$ at $\sqrt{s} = 250$~GeV and $\sqrt{s} = 200$~GeV. Furthermore, we employ Deep Neural Network to analyze the properties and behaviour of the scalar particle with a mass most importantly to provide enhanced resolution for the separation between beyond the Standard Model (SM) signal and SM background in the region 95 - 96 GeV in the $S \to b \bar{b}$ for $μ_{+} μ_{-}$ channel. A 95 GeV scalar can be observed with $5σ$ significance at $15(10)$ fb$^{-1}$ integrated luminosity for $\sqrt{s} = 250(200)$~GeV. This strengthens the discovery of the potential of the future $e^{+} e^{-}$ collider.

hep-ph↗

arXiv2023

Constraining 2HDM+S model through W-boson mass measurements

Following a discussion on $W$-boson mass observed at the CDF and ATLAS, we explore the parameter space allowed in the 2HDM+$S$ model. Further, the model parameter space is constrained through vector-like leptons via muon $g-2$ measurements. We show our results for additional scalar mass fixed to $m_S \approx 95$ and $150$~GeV keeping the standard Higgs-boson mass at 125~GeV in all four types of 2HDM+$S$ model. The chosen mass of the singlet scalar is motivated by the excesses seen at the CMS and ATLAS data in proton-proton collisions at the Large Hadron Collider.

hep-ph↗

arXiv2023

Accumulating Evidence for the Associated Production of a New Higgs Boson at the Large Hadron Collider

In the last decades, the Standard Model (SM) of particle physics has been extensively tested and confirmed, with the announced discovery of the Higgs boson in 2012 being the last missing puzzle piece. Even though since then the search for new particles and interactions has been further intensified, the experiments ATLAS and CMS at the Large Hadron Collider (LHC) at CERN did not find evidence for the direct production of a new state. However, in recent years deviations between LHC data and SM predictions in multiple observables involving two or more leptons (electrons or muons) have emerged, the so-called ``multi-lepton anomalies'', pointing towards the existence of a beyond the SM Higgs boson $S$. While from these measurements its mass cannot be exactly determined, it is estimated to lay in the range between $130\,$GeV and $160\,$GeV. Motivated by this observation, we perform a search for signatures of $S$, by using existing CMS and ATLAS analyses. Combining channels involving the associate productions of SM gauge bosons ($γγ$ and $Zγ$), we find that a simplified model with a new scalar with $m_S= 151.5\,$GeV is preferred over the SM hypothesis by 4.3$σ$ (3.9$σ$) locally (globally). On the face of it, this provides a good indication for the existence of a new scalar resonance $S$ decaying into photons, in association with missing energy and allows for a connection to the long-standing problem of Dark Matter. Furthermore, because $S$ is always produced together with other particles, we postulate the existence of a second new (heavier) Higgs boson $H$ that decays into $S$ and propose novel searches to discover this particle, which can be performed by ATLAS and CMS.

hep-ph↗

arXiv2023

Differential $eμbb $ Cross-Sections and New Higgses at the Electroweak Scale

ATLAS found that none of their Standard Model simulations can describe the measured differential lepton distributions in their $t \bar{t}$ analysis reasonably well. Therefore, we study the possibility that this measurement has a new physics contamination. We consider a benchmark model motivated by the indications for di-photon resonances: A heavy scalar decays into two lighter Higgs bosons with masses of 152\,GeV and 95\,GeV, with subsequent decay to $WW$ and $bb$, respectively. In this setup, the description of data is improved by at least $5.6 σ$.

hep-ph↗

arXiv2023

$SU(2)_L$ triplet scalar as the origin of the 95 GeV excess?

We explore the possibility that an $SU(2)_L$ triplet scalar with hypercharge $Y=0$ is the origin of the $95\,$GeV diphoton excess. For a small mixing angle with the Standard Model Higgs, its neutral component has naturally a sizable branching ratio to $γγ$ such that its Drell-Yan production via $pp\to W^*\to H H^\pm$ is sufficient to obtain the desired signal strength, where $H^\pm$ is the charged Higgs component of the triplet. The predictions of this setup are: 1) The $γγ$ signal has a $p_T$ spectrum different from gluon fusion but similar to associated production. 2) Photons are produced in association with tau leptons and jets, but generally do not fall into the vector-boson fusion category. 3) The existence of a charged Higgs with $m_{H^\pm}\approx\!(95\pm5)\,$GeV leading to $σ(pp\to ττνν)\approx0.4\,$pb, which is of the same level as the current limit and can be discovered with Run 3 data. 4) A positive definite shift in the $W$ mass as suggested by the current global electroweak fit.

hep-ph↗

arXiv2023

COVID-19 South African Vaccine Hesitancy Models Show Boost in Performance Upon Fine-Tuning on M-pox Tweets

Very large numbers of M-pox cases have, since the start of May 2022, been reported in non-endemic countries leading many to fear that the M-pox Outbreak would rapidly transition into another pandemic, while the COVID-19 pandemic ravages on. Given the similarities of M-pox with COVID-19, we chose to test the performance of COVID-19 models trained on South African twitter data on a hand-labelled M-pox dataset before and after fine-tuning. More than 20k M-pox-related tweets from South Africa were hand-labelled as being either positive, negative or neutral. After fine-tuning these COVID-19 models on the M-pox dataset, the F1-scores increased by more than 8% falling just short of 70%, but still outperforming state-of-the-art models and well-known classification algorithms. An LDA-based topic modelling procedure was used to compare the miss-classified M-pox tweets of the original COVID-19 RoBERTa model with its fine-tuned version, and from this analysis, we were able to draw conclusions on how to build more sophisticated models.

cs.CL↗

arXiv2023

Searching for Low-Mass Resonances Decaying into $W$ Bosons

In this article, we recast and combine the CMS and ATLAS analyses of the Standard Model Higgs boson decaying to a pair of $W$ bosons in order to search for low-mass resonances in this channel. We provide limits on the corresponding cross section assuming direct production via gluon fusion. For the whole range of masses we consider (90$\,$GeV to 200$\,$GeV), the observed limit on the cross section turns out to be weaker than the expected one. Furthermore, at $\approx95\,$GeV the limit is weakest and a new scalar decaying into a pair of $W$ bosons (which subsequently decay leptonically) with a cross section $\approx0.5\,$pb is preferred over the Standard Model hypothesis by $\gtrsim 2.5\,σ$. In light of the excesses in the $γγ$, $τ^+τ^-$ and $b\bar b$ channels at similar masses, this strengthens the case for such a new Higgs boson. Furthermore, this analysis also gives room for the scalar candidate at 151$\,$GeV decaying into $W$ bosons.

hep-ph↗

arXiv2023

Detecting the Presence of COVID-19 Vaccination Hesitancy from South African Twitter Data Using Machine Learning

Very few social media studies have been done on South African user-generated content during the COVID-19 pandemic and even fewer using hand-labelling over automated methods. Vaccination is a major tool in the fight against the pandemic, but vaccine hesitancy jeopardizes any public health effort. In this study, sentiment analysis on South African tweets related to vaccine hesitancy was performed, with the aim of training AI-mediated classification models and assessing their reliability in categorizing UGC. A dataset of 30000 tweets from South Africa were extracted and hand-labelled into one of three sentiment classes: positive, negative, neutral. The machine learning models used were LSTM, bi-LSTM, SVM, BERT-base-cased and the RoBERTa-base models, whereby their hyperparameters were carefully chosen and tuned using the WandB platform. We used two different approaches when we pre-processed our data for comparison: one was semantics-based, while the other was corpus-based. The pre-processing of the tweets in our dataset was performed using both methods, respectively. All models were found to have low F1-scores within a range of 45$\%$-55$\%$, except for BERT and RoBERTa which both achieved significantly better measures with overall F1-scores of 60$\%$ and 61$\%$, respectively. Topic modelling using an LDA was performed on the miss-classified tweets of the RoBERTa model to gain insight on how to further improve model accuracy.

cs.CY↗

arXiv2023

Growing Excesses of New Scalars at the Electroweak Scale

We combine searches for scalar resonances at the electroweak scale performed by the Large Hadron Collider experiments ATLAS and CMS where persisted excesses have been observed in recent years. Using both the side-bands of Standard Model Higgs analyses as well as dedicated beyond the Standard Model analyses, we find significant hints for new scalars at $\approx 95\,$GeV ($S^\prime$) and $\approx152\,$GeV ($S$). The presence of a $95\,$GeV scalar is preferred over the Standard Model hypothesis by $3.8σ$, while interpreting the $152\,$GeV excesses in a simplified model with resonant pair production of $S$ via a new heavier scalar $H(270)$, a global significance of $\approx5σ$ is obtained. While the production mechanism of the $S^\prime$ cannot yet be determined, data strongly favours the associated production of $S$, i.e. via the decay of a heavier boson $H$ ($pp\to H\to SS^*$). A possible alternative or complementary decay chain is $H\rightarrow SS^{\prime}$, where $S\to WW^*$ ($S^{\prime}$) would be the source of the leptons ($b$-quarks) necessary to explain the multi-lepton anomalies found in Large Hadron Collider data.

hep-ph↗

arXiv2023

Consistency and Interpretation of the LHC (Di-)Di-Jet Excesses

ATLAS observed a limit for {the cross section of di-jets resonances, which is weaker than expected for a} mass slightly below $\approx$1\TeV. In addition, CMS reported hints for the (non-resonant) pair production of di-jet resonances $X$ via a particle $Y$ at a very similar mass range with a local (global) significance of 3.6\,$σ$ (2.5\,$σ$) at $m_X\approx950\,$GeV. In this article we show that using the preferred range for $m_X$ from the ATLAS analysis, one can reinterpret the CMS analysis of di-di-jets in terms of a resonant search with $Y\to XX$, with a significantly reduced look-elsewhere effect, finding an excess for $m_Y\!\approx\!3.6$\TeV with a significance of $4.0\,σ$ ($3.2\,σ$) locally (globally). We present two possible UV completions capable of explaining the (di-)di-jet excesses, one containing two scalar di-quarks, the other one involving heavy gluons based on an $SU(3)_1\!\times\! SU(3)_2\!\times\! SU(3)_3$ gauge symmetry, spontaneously broken to $SU(3)$ color. In the latter case, non-perturbative couplings are required, pointing towards a composite or extra-dimensional framework. In fact, using 5D-AdS space-time, one obtains the correct mass ratio for $m_X/m_Y$, assuming the $X$ is the lowest lying resonance, and predicts a third (di-)di-jet resonance with a mass around $\approx2.2$\TeV.

hep-ph↗

arXiv2023

Feasibility of the observation of a heavy scalar through the fully hadronic final state at the LHeC

The proposed future Large Hadron Electron Collider provides sufficient center of mass energies, $\sqrt{s}$, to probe heavy particles decaying into $W^\pm(Z)-$boson of mass $>2m_W$ $(2m_Z)$. In this work we present a study to produce one such heavy $C P$ even scalar $H$ of mass $2m_h < m_H < 2 m_t$ through charged-current production mode where $H \to W^+W^-$, where hadronic decay of $W^\pm-$boson is considered to reconstruct $m_H$. Due to the presence of missing energy and forward jet in this channel, it is challenging to reconstruct $m_H$ with above final state and thus we employed three different reconstruction methods and discuss the significance of each one. For this analysis we consider a benchmark value of $m_H = 270$\,GeV and $\sqrt{s} \approx 1.3$\,TeV with an assumed luminosity of 1\,ab$^{-1}$.

hep-ph↗

arXiv2023

Connecting multi-lepton anomalies at the LHC and in Astrophysics with MeerKAT/SKA

Multi-lepton anomalies at the Large Hadron Collider (LHC) are reasonably well described by a two Higgs doublet model with an additional singlet scalar. Here, we demonstrate that using this model with parameters set by the LHC, we are also able to describe the excesses in gamma-ray flux from the galactic centre and the cosmic-ray spectra from AMS-02. This is achieved through Dark Matter (DM) annihilation via the singlet scalar. Of great interest is the flux of synchrotron emissions which results from annihilation of DM in Milky-Way satellites. We make predictions for MeerKAT/SKA observations of the nearby dwarf galaxy Reticulum II and we demonstrate the power of this instrument as a new frontier in indirect dark matter searches. Since the dark matter sector of the aforementioned two Higgs doublet model is unconstrained by current LHC data, we also demonstrate a synergy between particle and astrophysical searches in order to motivate further exploration of this promising model.

astro-ph.HE↗

arXiv2022

Twitter-Based Gender Recognition Using Transformers

Social media contains useful information about people and the society that could help advance research in many different areas (e.g. by applying opinion mining, emotion/sentiment analysis, and statistical analysis) such as business and finance, health, socio-economic inequality and gender vulnerability. User demographics provide rich information that could help study the subject further. However, user demographics such as gender are considered private and are not freely available. In this study, we propose a model based on transformers to predict the user's gender from their images and tweets. We fine-tune a model based on Vision Transformers (ViT) to stratify female and male images. Next, we fine-tune another model based on Bidirectional Encoders Representations from Transformers (BERT) to recognize the user's gender by their tweets. This is highly beneficial, because not all users provide an image that indicates their gender. The gender of such users could be detected form their tweets. The combination model improves the accuracy of image and text classification models by 6.98% and 4.43%, respectively. This shows that the image and text classification models are capable of complementing each other by providing additional information to one another. We apply our method to the PAN-2018 dataset, and obtain an accuracy of 85.52%.

cs.CL↗

arXiv2021

Machine learning approach for the search of resonances with topological features at the Large Hadron Collider

The observation of resonances is unequivocal evidence of new physics beyond the Standard Model at the Large Hadron Collider (LHC). So far, inclusive and model dependent searches have not provided evidence of new resonances, indicating that these could be driven by subtle topologies. Here, we use machine learning techniques based on weak supervision to perform searches. Weak supervision based on mixed samples can be used to search for resonances with little or no prior knowledge on the production mechanism. Also, it offers the advantage that sidebands or control regions can be used to effectively model backgrounds with minimal reliance on simulations. However, weak supervision alone is found to be highly inefficient in identifying corners of the multi-dimensional space of interest. Instead, we propose an approach to search for new resonances that involves a classification procedure that is signature and topology based. A combination of weak supervision with Deep Neural Network algorithms are applied following this classification. The performance of this strategy is evaluated on the production of SM Higgs boson decaying to a pair of photons inclusively and in exclusive regions of phase space tailored for specific production modes at the LHC. After verifying the ability of the methodology to extract different SM Higgs boson signal mechanisms, a search for new phenomena in high-mass final states is setup for the LHC.

hep-ex↗

arXiv2021

An investigation of over-training within semi-supervised machine learning models in the search for heavy resonances at the LHC

In particle physics, semi-supervised machine learning is an attractive option to reduce model dependencies searches beyond the Standard Model. When utilizing semi-supervised techniques in training machine learning models in the search for bosons at the Large Hadron Collider, the over-training of the model must be investigated. Internal fluctuations of the phase space and bias in training can cause semi-supervised models to label false signals within the phase space due to over-fitting. The issue of false signal generation in semi-supervised models has not been fully analyzed and therefore utilizing a toy Monte Carlo model, the probability of such situations occurring must be quantified. This investigation of $Zγ$ resonances is performed using a pure background Monte Carlo sample. Through unique pure background samples extracted to mimic ATLAS data in a background-plus-signal region, multiple runs enable the probability of these fake signals occurring due to over-training to be thoroughly investigated.

hep-ex↗

arXiv2021

Understanding two same-sign and three leptons with $b$-jets in four top quark events at the LHC

The top quark is the heaviest known elementary particle of the Standard Model (SM) of particle physics and, therefore, it is expected to have large couplings to hypothetical new physics in many models beyond the SM (BSM). Various studies have predicted the presence of multi-lepton anomalies at the LHC. One of those anomalies is the excess production of two same-sign leptons and three isolated leptons in association with $b$-jets. These are reasonably well described by a 2HDM+$S$ model, where $S$ is a singlet scalar. Both the ATLAS and CMS experiments have reported sustained excesses in these final states. This includes corners of the phase-space where production of top quark pairs in association with a $W$ boson contributes to. Here, we investigate the production of two same-sign and three leptons from the production of four top quark final states. Our focus is on understanding the differences between the SM and BSM production mechanisms of four top quarks from $t\overline{t} A$ ($A \rightarrow t\overline{t}$) using Machine Leaning techniques with twelve discriminating kinematic variables.

hep-ph↗

arXiv2021

Unveiling Hidden Physics at the LHC

The field of particle physics is at the crossroads. The discovery of a Higgs-like boson completed the Standard Model (SM), but the lacking observation of convincing resonances Beyond the SM (BSM) offers no guidance for the future of particle physics. On the other hand, the motivation for New Physics has not diminished and is, in fact, reinforced by several striking anomalous results in many experiments. Here we summarise the status of the most significant anomalies, including the most recent results for the flavour anomalies, the multi-lepton anomalies at the LHC, the Higgs-like excess at around 96 GeV, and anomalies in neutrino physics, astrophysics, cosmology, and cosmic rays. While the LHC promises up to 4/ab of integrated luminosity and far-reaching physics programmes to unveil BSM physics, we consider the possibility that the latter could be tested with present data, but that systemic shortcomings of the experiments and their search strategies may preclude their discovery for several reasons, including: final states consisting in soft particles only, associated production processes, QCD-like final states, close-by SM resonances, and SUSY scenarios where no missing energy is produced. New search strategies could help to unveil the hidden BSM signatures, devised by making use of the CERN open data as a new testing ground. We discuss the CERN open data with its policies, challenges, and potential usefulness for the community. We showcase the example of the CMS collaboration, which is the only collaboration regularly releasing some of its data. We find it important to stress that individuals using public data for their own research does not imply competition with experimental efforts, but rather provides unique opportunities to give guidance for further BSM searches by the collaborations. Wide access to open data is paramount to fully exploit the LHCs potential.

hep-ph↗

arXiv2021

The anatomy of the multilepton anomalies at the LHC and a candidate for a singlet scalar

In this presentation an account of the multi-lepton (electrons and muons) anomalies at the LHC is given. These include the excess production of opposite sign leptons with and without b-quarks, including a corner of the phase-space with a full hadronic jet veto; same sign leptons with and without b-quarks; three leptons with and without b-quarks, including also the presence of a $Z$. Excesses emerge in corners of the phase space where a range of SM processes dominate, indicating that the potential mismodeling of a particular SM process is unlikely to explain them. A procedure is implemented that avoids parameter tuning or scanning the phase-space in order to nullify potential look-else-where effects or selection biases. The internal consistency of these anomalies and their interpretation in the framework of a simplified model are presented. Motivated by the multi-lepton anomalies, a search for narrow resonances with $S\rightarrowγγ, Zγ$ in association with light jets, $b$-jets or missing transverse energy is performed. The maximum local (global) significance is achieved for $m_S=151.5$\,GeV with 5.1$σ$ (4.8$σ$).

hep-ph↗