arXiv subjects

Leandro Da Rold

Publications and source records attributed to Leandro Da Rold.

At least 19 recordsLinked to original sources

arXiv2025

Di-Higgs to 4b with Bayesian inference: improving simulation estimates

Measuring di-Higgs production in the four-bottom channel is challenged by overwhelming QCD backgrounds and imperfect simulations. We develop a Bayesian mixture model that simultaneously infers signal and background fractions and their individual shapes directly in the signal region. The likelihood is a nuanced combination of a one-dimensional kinematic discriminator and per-jet flavour scores; with their correlations incorporated via kinematic bins. Monte Carlo informs weak Dirichlet priors, while the posterior adjusts to the interplay of the model, priors and observed data. Using pseudo-data simulated with standard tools and with controlled mismatches, we show that the method corrects biased priors, delivers calibrated 68-95% credible intervals for the signal count, and improves dataset-level ROC/AUC relative to simple cut-and-count baselines. This study highlights how Bayesian inference can harvest information present in the signal region and self-calibrate model parameters, providing a robust route to increased sensitivity in di-Higgs searches.

hep-ph↗

arXiv2025

Exploring enhanced non-resonant di-Higgs production at the HL-LHC with neural networks

We investigate di-Higgs production in the $b\bar{b}\gamma\gamma$ final state at the LHC, focusing on scenarios where the gluon fusion process is enhanced by new colored scalars, which could be identified as squarks or leptoquarks. We consider two benchmarks characterized by the mass of the lightest colored scalar, BM$_{\mathrm{L}}$ and BM$_{\mathrm{H}}$, corresponding to 464 GeV and 621 GeV, respectively. Using Monte Carlo simulations for both the signal and the dominant backgrounds, we perform a discovery analysis with deep neural networks, exploring various architectures and input variables. Our results show that the discrimination power is maximized by employing two dedicated classifiers, one trained against QCD backgrounds and another against backgrounds involving single-Higgs processes. Furthermore, we demonstrate that including high-level features -- such as the invariant masses $m_{\gamma\gamma}$, $m_{bb}$, and $m_{hh}$, as well as the transverse momenta and angular separations of the photon and $b$-jet pairs -- significantly improves the performance compared to using only low-level features as the invariant mass and momenta of the final particles. For the latter case, we find that architectures processing photon and $b$-jet variables separately can enhance the significance for BM$_{\mathrm{H}}$. Projecting for an integrated luminosity of 3 ab$^{-1}$, we obtain a significance of 7.3 for BM$_{\mathrm{L}}$, while it drops to 3.1 for BM$_{\mathrm{H}}$. In the particular case of BM$_{\mathrm{L}}$, discovery level significance can be reached at 1.7 ab$^{-1}$.

hep-ph↗

arXiv2025

Lepton flavor from a horizontal symmetry in a slice of AdS$_5$

We build a model of lepton flavor in a slice of AdS$_5$. We add to the 5D SM fields a set of neutrino fields, as well as a horizontal U(1) symmetry and a flavon field, all propagating in the bulk. The electroweak and U(1) symmetries are spontaneously broken by a potential localized on the infrared boundary. We show that in a flavor anarchic scenario, by suitable choice of the 5D masses and the U(1) charges, the masses of the SM leptons and the PMNS matrix can be naturally generated. The neutrino masses are Dirac like, with a normal ordered hierarchical spectrum, $\Delta m_{32}^2\approx m_3^2$, $\Delta m_{21}^2\approx m_2^2\gg m_1^2$, and a suppressed $\theta_{13}$ mixing angle. We find configurations where the charged lepton mixing angles are suppressed by powers of the Cabibbo angle ($\lambda_C$) relative to vanilla anarchic partial compositeness, consequently reducing CP and lepton flavor violation. Specifically, the Wilson coefficient for the electron electromagnetic dipole moment exhibits $\lambda_C^2$ suppression, while those governing $\mu\to e\gamma$ and $\mu-e$ vector operators are suppressed by $\lambda_C^{3/2}$ compared to the anarchic scenario without U(1) horizontal symmetry.

hep-ph↗

arXiv2024

Testing the lepton content of the proton at HERA and EIC

Although protons are baryons with an overall vanishing lepton number, they possess a non-trivial leptonic content arising from quantum fluctuations which can be described by lepton parton distribution functions (PDFs) of the proton. These PDFs have been recently computed and can be used to define lepton-induced processes at high-energy colliders. In this article, we propose a novel way to test the computation of lepton PDFs of the proton by analyzing both non-resonant di-lepton and resonant Z gauge boson production processes induced by leptons within the proton at proton-electron colliders like HERA and EIC. Despite the fact that lepton PDFs of the proton are known to be small, this work demonstrates that both processes imply a measurable yield of events at HERA and EIC, which could be used to test these PDFs.

hep-ph↗

arXiv2024

The flavor of a minimal composite $S_1$ leptoquark and the $b\to cτ\barν$ anomaly

Several experiments have measured a deviation in $B\to D^{(*)}$ semileptonic decays, that point to new physics at the TeV scale violating lepton flavor universality. A scalar leptoquark $S_1$, with a suitable structure of couplings in flavor space, is known to be able to solve this anomaly modifying $b\to cτ\barν$. In the context of composite Higgs models, we consider a theory containing $H$ and $S_1$ as Nambu-Goldstone bosons (NGBs) of a new strongly interacting sector, with ordinary resonances at a scale ${\cal O}(10)$ TeV. Assuming anarchic partial compositeness of the Standard Model (SM) fermions we calculate the potential of the NGBs that is dominated by the fermions of the third generation, we compute $R_{D^{(*)}}$ and estimate the corrections to flavor observables by the presence of $S_1$. We find that the SM spectrum and $m_{S_1}\sim$ TeV can be obtained with a NGB decay constant of order $\sim 5$ TeV. We obtain a robust correlation between the main corrections to $R_{D^{(*)}}$, $B_{K^{(*)}νν}$ and $g_τ/g_μ$, that leads to a sever bound on $R_{D^{(*)}}$, roughly $2σ$ below the experimental value. Besides the bounds on the flavor observables $g_τ^W$, BR$(τ\toμγ)$ and $Δm_{B_s}$ are saturated, with the first one requiring a coupling between resonances $g_*\lesssim 2$, whereas the second one demands $m_{S_1}\gtrsim 1.7$ TeV, up to corrections of ${\cal O}(1)$.

hep-ph↗

arXiv2024

Double Higgs production at the HL-LHC: probing a loop-enhanced model with kinematical distributions

We study di-Higgs production via gluon fusion at the high luminosity LHC in the presence of new physics, focusing on the $b\bar bγγ$ final states. Taking a minimal set of three scalar leptoquarks (LQs) with cubic and quartic interactions with the Higgs and choosing four benchmark points with a light LQ, we perform a detailed analysis of differential distributions of the di-Higgs production cross section, studying the imprints of the new physics states running in the loops. Simulating the signal and main backgrounds, we study the influence of the new physics in differential distributions such as the invariant mass of the subsystems of final particles, the transverse momentum, and angular variables, finding in particular a resonance peak associated with the light LQ. It turns out that the angular separation of the photons, which is correlated with the resonance LQ peak, is a very sensitive observable that helps in discriminating the new physics signal from the Standard Model background. We find that for two of our benchmarks discovery could be reached with 3 ab$^{-1}$, whereas exclusion limits at 95% C.L. could be claimed with 0.60-0.75 ab$^{-1}$. For the other two benchmarks that have heavier LQ masses significances of order 2$σ$ are possible for 3 ab$^{-1}$. A similar analysis could be applied to other loop-enhanced models.

hep-ph↗

arXiv2024

Probing new physics with charge asymmetries in 2 same-sign leptons plus jets final states at the LHC

We study the impact of new physics models in the charge asymmetry defined for LHC final states consisting of two same-sign leptons (2SS$l$, with $l= e, μ$) plus jets ($N_\text{jets}\geq2$), with a center-of-mass energy of $\sqrt{s}=13$ TeV, where the main SM contribution is $t\bar{t}W$ production. Concretely, we consider three different new physics sources for the charge asymmetries: a heavy neutral scalar/pseudoscalar arising from the general two Higgs doublet model, an effective theory with dimension-6 four-quark operators, and a simplified $R$-parity-violating supersymmetric model with electroweakino production (higgsino-like or wino-like). We propose measuring the charge asymmetries differentially with respect to several kinematic observables, and inclusively/exclusively with the number of $b$-tagged jets in the final state ($N_b\geq\{1, 2, 3\}$). Results are compared with the SM prediction using the $χ^2$ criteria, expressing sensitivity in terms of the normal Gaussian significance. We show that some of the proposed new physics scenarios can be probed at the LHC even for the already recorded integrated luminosity of 139 fb$^{-1}$. Finally, we also estimate the prospects for the potential LHC sensitivity to the considered new physics models in its high-luminosity phase.

hep-ph↗

arXiv2024

Improvement and generalization of ABCD method with Bayesian inference

To find New Physics or to refine our knowledge of the Standard Model at the LHC is an enterprise that involves many factors. We focus on taking advantage of available information and pour our effort in re-thinking the usual data-driven ABCD method to improve it and to generalize it using Bayesian Machine Learning tools. We propose that a dataset consisting of a signal and many backgrounds is well described through a mixture model. Signal, backgrounds and their relative fractions in the sample can be well extracted by exploiting the prior knowledge and the dependence between the different observables at the event-by-event level with Bayesian tools. We show how, in contrast to the ABCD method, one can take advantage of understanding some properties of the different backgrounds and of having more than two independent observables to measure in each event. In addition, instead of regions defined through hard cuts, the Bayesian framework uses the information of continuous distribution to obtain soft-assignments of the events which are statistically more robust. To compare both methods we use a toy problem inspired by $pp\to hh\to b\bar b b \bar b$, selecting a reduced and simplified number of processes and analysing the flavor of the four jets and the invariant mass of the jet-pairs, modeled with simplified distributions. Taking advantage of all this information, and starting from a combination of biased and agnostic priors, leads us to a very good posterior once we use the Bayesian framework to exploit the data and the mutual information of the observables at the event-by-event level. We show how, in this simplified model, the Bayesian framework outperforms the ABCD method sensitivity in obtaining the signal fraction in scenarios with $1\%$ and $0.5\%$ true signal fractions in the dataset. We also show that the method is robust against the absence of signal.

hep-ph↗

arXiv2022

A composite Froggatt-Nielsen model of flavor

A natural composite Higgs demands the presence of light resonances at the TeV scale, that in general are in conflict with bounds from flavor and CP violation. We propose a composite model with a Froggatt-Nielsen mechanism, that offers new possibilities for the origin of flavor. We analyse the interplay of partial compositeness and the horizontal U(1) symmetry in achieving the quark masses and mixing angles. We study the contributions to $ΔF=2$ 4-fermion operators, as well as to $ΔF=1$ and neutron dipole operators. We find scenarios in which the contribution to Left-Right and Right-handed operators involving the first and second generations can be suppressed, in particular for a region of parameter space it is possible to simultaneously suppress the mixed-chirality contribution to $K^0-\bar K^0$ mixing by one power of the Cabibbo angle, $λ_C$, and the dipole moments by $λ_C^2$ compared with anarchic partial compositeness, possibly making the resonances accessible at LHC. 4-fermion operators of $B_s$-meson mixing and Left-handed operators are not suppressed.

hep-ph↗

arXiv2021

Interpretation of LHC excesses in ditop and ditau channels as a 400-GeV pseudoscalar resonance

Since the discovery in 2012 of the Higgs boson at the LHC, as the last missing piece of the Standard Model of particle physics, any hint of new physics has been intensively searched for, with no confirmation to date. There are however slight deviations from the SM that are worth investigating. The CMS collaboration has reported, in a search for heavy resonances decaying in $t \bar t$ with a 13-TeV center-of-mass energy and a luminosity of 35.9 fb$^{-1}$, deviations from the SM predictions at the 3.5$σ$ level locally (1.9$σ$ after the look-elsewhere effect). In addition, in the ditau final state search performed by the ATLAS collaboration at $\sqrt{s}=13$ TeV and $\mathcal{L}=139$ fb$^{-1}$, deviations from the SM at the 2$σ$ level have been also observed. Interestingly, both slight excesses are compatible with a new pseudoscalar boson with a mass around 400 GeV that couples at least to fermions of the third generation and gluons. Starting from a purely phenomenological perspective, we inspect the possibility that a 400-GeV pseudoscalar can account for these deviations and at the same time satisfy the constraints on the rest of the channels that it gives contributions to and that are analyzed by the ATLAS and CMS experiments. After obtaining the range of effective couplings compatible with all experimental measurements, we study the gauge invariant UV completions that can give rise to this type of pseudoscalar resonance, which can be accommodated in an $SO(6)/SO(5)$ model with consistency at the 1$σ$ level and in a $SO(5)\times U(1)_{P}\times U(1)_{X}/SO(4)\times U(1)_X$ at the 2$σ$ level, while exceedingly large quartic couplings would be necessary to account for it in a general two Higgs doublet model.

hep-ph↗

arXiv2021

Enhancement of the double Higgs production via leptoquarks at the LHC

Measurements of single Higgs production and its decays are in good agreement with the Standard Model. There is still room for large modifications in double Higgs production at LHC, though these effects may be correlated with large corrections to other observables, in particular single Higgs production. In this work we address the issue of enhancing double Higgs production in the presence of scalar leptoquarks while satisfying all experimental constraints. We show at leading order that including more than one species of leptoquarks, large cubic interactions with the Higgs can lead to sizable enhancement of di-Higgs production cross section at LHC, while at the same time keeping other Higgs observables and precision measurements under control. For masses above 800 GeV these corrections are in general below 30%, whereas in a viable scenario in which one of the leptoquarks can be light, specifically in the mass range $400-600$ GeV, we show that it is possible to roughly double the SM cross section for di-Higgs production, implying that possible first hints of it may be probed at the high luminosity LHC at $\mathcal{L}\sim 2$ ab$^{-1}$.

hep-ph↗

arXiv2021

A model for the Singlet-Triplet Leptoquarks

The deviations of $B$-meson decays measured in $R_{D^{(*)}}^{τ\ell}$ and $R_{K^{(*)}}^{μe}$ can be explained by the presence of two scalar leptoquarks, a singlet $S_1$ and a triplet $S_3$, mostly coupled to the third generation. We consider a theory of resonances, as an effective description of a strongly interacting theory, that generates the leptoquarks and the Higgs as Nambu-Goldstone bosons, with the rest of the resonances at a scale of order $10-30\ {\rm TeV}$. We assume anarchic partial compositeness for the flavor of the SM fermions. Under these hypothesis we study whether it is possible to reproduce the deviations in the $B$-decays without being in conflict with flavor and electroweak bounds. We find a tension between $R_{D^{(*)}}^{τ\ell}$ and some flavor observables, dominated by flavor violating $τ$ decays and $Δm_{B_s}$, that require a tuning of order $10-25\%$. We also compute the potential of the scalars showing that leptoquarks with masses ${\cal}O(2-3)\ {\rm TeV}$ can be naturally expected in the model. We discuss briefly the phenomenology of the other resonances.

hep-ph↗

arXiv2020

Containing COVID-19 outbreaks using a Firewall

COVID-19 outbreaks have proven to be very difficult to isolate and extinguish before they spread out. An important reason behind this might be that epidemiological barriers consisting in stopping symptomatic people are likely to fail because of the contagion time before onset, mild cases and/or asymptomatics carriers. Motivated by these special COVID-19 features, we study a scheme for containing an outbreak in a city that consists in adding an extra firewall block between the outbreak and the rest of the city. We implement a coupled compartment model with stochastic noise to simulate a localized outbreak that is partially isolated and analyze its evolution with and without firewall for different plausible model parameters. We explore how further improvements could be achieved if the epidemic evolution would trigger policy changes for the flux and/or lock-down in the different blocks. Our results show that a substantial improvement is obtained by merely adding an extra block between the outbreak and the bulk of the city.

physics.soc-ph↗

arXiv2019

A vector leptoquark for the B-physics anomalies from a composite GUT

A vector leptoquark at the TeV scale, mostly coupled to the fermions of the third generation, is the preferred option to explain the hints of lepton flavor universality violation in the decays of B-mesons. It seems interesting to assume that this leptoquark belongs to the same beyond the Standard Model sector that solves the hierarchy problem, since the third generation of fermions play the leading role in the instability of the Higgs potential. We present a composite Grand Unified Theory with resonances at the TeV that contains the required vector leptoquark and develops the Higgs as a pseudo Nambu-Goldstone boson. We show that anarchic partial compositeness of the Standard Model fermions can accommodate the couplings of Left-handed currents required by the B-anomalies, predicting very small couplings to the Right-handed currents without any additional hypothesis. By making use of an effective theory description of the strong dynamics, in terms of weakly coupled resonances, we are able to compute the corrections to B-physics, as well as the one-loop potential for the pseudo Nambu-Goldstone bosons. The theory has a rich phenomenology and a candidate for dark matter.

hep-ph↗

arXiv2019

The Minimal Simple Composite Higgs Model

Most of the analysis of composite Higgs have focussed on the Minimal Composite Higgs Model, based on the coset SO(5)$\times$U(1)$_X$/SO(4)$\times$U(1)$_X$. We consider a model based on the coset of simple groups SO(7)/SO(6), with SO(4)$\times$U(1)$_X$ embedded into SO(6). This extension of the minimal model leads to a new complex pNGB that has hypercharge and is a singlet of SU(2)$_L$, with properties mostly determined by the pattern of symmetry breaking and a mass of order TeV. Composite electroweak unification also leads to new bosonic and fermion resonances with exotic charges, not present in the minimal model. The lightest of these resonances is stable, and in some cases could provide candidates for dark matter. A new rich phenomenology is expected at LHC.

hep-ph↗

arXiv2018

Composite Higgs and leptoquarks from a simple group

We propose a composite grand unified theory to study the anomalies in the semileptonic $B$ decays. We show a simple group containing the custodial and Standard Model gauge symmetries, that can deliver a set of composite pseudo Nambu-Goldstone bosons: the Higgs, a colorless SU(2)$_L$-fourplet and three leptoquarks: a triplet and two doublets. We give a description in terms of an effective theory of resonances. By assuming anarchic partial compositeness of the Standard Model fermions, we find representations for the composite fermions that allow to obtain the Higgs Yukawa couplings, as well as leptoquark interactions explaining the deviations in $R^{μe}_{K^{(*)}}$. We calculate the one-loop potential, we show that it can trigger electroweak symmetry breaking and we find a region of the parameter space that can reproduce the Standard Model spectrum. The model predicts leptoquark masses of order $0.4-1.3$ TeV, corrections to some electroweak observables, with $Zb_L\bar b_L$ saturating the current bounds, and a very reach phenomenology at LHC. We also study the possibility of explaining $R^{τ\ell}_{D^{(*)}}$.

hep-ph↗

arXiv2018

A composite pNGB leptoquark at the LHC

The measurements of $R_K^{(*)}$ and $R_{D}^{(*)}$ by BaBar, Belle and the LHCb collaborations could be showing a hint of lepton flavor universality violation that can be accommodated by the presence of suitable leptoquarks at the TeV scale. We consider an effective description, with leptoquarks arising as composite pseudo Nambu-Goldstone bosons, as well as anarchic partial compositeness of the SM fermions. Considering the $R_K^{(*)}$ anomaly within this framework, we study pair production of $S_3\sim(\bar 3,3)_{1/3}$ at the LHC. We focus on the component $S_3^{1/3}$ of the triplet, which decays predominantly into $tτ$ and $bν$, and study the bounds from existing searches at $\sqrt{s}=13$ TeV at the LHC. We find that sbottom searches in the $b\bar{b}$+MET final state best explore the region in parameter space preferred by our model and currently exclude $S_3^{1/3}$ masses up to $\sim$1 TeV. Additional searches, considering the $tτ$ and $tμ$ decay modes, are required to probe the full physical parameter space. In this paper we also recast existing studies on direct leptoquark searches in the $tτtτ$ channel and SM $t\bar{t}t\bar{t}$ searches, and obtain the regions in parameter space currently excluded. Practically the whole physical parameter space is currently excluded for masses up to $\sim$0.8 TeV, which could be extended up to $\sim$1 TeV with the full Run 3 dataset. We conclude that pair production searches for this leptoquark can benefit from considering the final state $t τb$ +MET, where the largest branching ratio is expected. We appraise that future explorations of leptoquarks explaining the B-anomalies with masses beyond the TeV should also consider single and non-resonant production in order to extend the mass reach.

hep-ph↗

arXiv2017

Measuring $|V_{td}|$ at LHC

We propose a direct measurement of the CKM element $V_{td}$ at the LHC. Taking profit of the imbalance between $d$ and $\bar d$ quark content in the proton, we show that a non-zero $V_{td}$ induces a charge asymmetry in the $tW$ associated production. The main backgrounds to this process, $t\bar t$ production, and $tW$ associated production mediated by $V_{tb}$, give charge symmetric contributions at leading order in QCD. Therefore, using specific kinematic features of the signal, we construct a charge asymmetry in the di-lepton final state which, due also to a reduction of systematic uncertainties in the asymmetry, is potentially sensitive to $V_{td}$ suppressed effects. In particular, using signal and background simulations up to detector level, we show that this new observable could improve the current direct upper bound on $|V_{td}|$ already with existing LHC data. We also project that $|V_{td}|$ values down to $\sim 10$ times the Standard Model prediction could be probed in the high luminosity phase of the LHC.

hep-ph↗