arXiv subjects

J. A. Aguilar-Saavedra

Publications and source records attributed to J. A. Aguilar-Saavedra.

At least 37 records · Page 2Linked to original sources

arXiv2023

Confronting the 95 GeV excesses within the UN2HDM

We consider the small excesses around 95 GeV found in several searches for a new scalar in $γγ$, $ττ$ and $b \bar b$ final states. Instead of trying to accommodate them all, as is usually done in the literature, in the context of a given Standard Model~(SM) extension, we investigate whether it would be possible that one or two of these excesses correspond to an actual new scalar, while the remaining ones are merely statistical fluctuations. To this end, we use as benchmark model the UN2HDM, a SM extension with one scalar doublet, one scalar singlet, and an extra $\text{U}(1)'$ symmetry, which has been previously studied in the context of multiboson cascade decays. We show that most of the possibilities where the excesses in one or two of these channels disappear in the future can be accommodated by type-I or type-III UN2HDMs.

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arXiv2023

Laboratory-frame tests of quantum entanglement in $H \to WW$

Quantum entanglement between the two $W$ bosons resulting from the decay of a Higgs boson may be investigated in the dilepton channel $H \to WW \to \ell ν\ell ν$ using laboratory-frame observables that only involve the charged leptons $\ell=e,μ$. The dilepton invariant mass distribution, already measured by the ATLAS and CMS Collaborations at the LHC, can be used to observe the quantum entanglement of the $WW$ pair with a statistical sensitivity of $7σ$ with Run 2 data, and of $6σ$ when including theoretical systematics. As a by-product, the relation between $W$ rest frame (four-dimensional) angular distributions, $H \to WW$ decay amplitudes, and spin correlation coefficients, is written down.

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arXiv2022

Multiboson signals in the UN2HDM

We address multiboson production from a heavy $Z'$ resonance in the context of the UN2HDM, a standard model extension with an additional $\text{U}(1)'$ symmetry and an enlarged scalar sector with an extra doublet and a singlet. After taking into account theoretical and experimental constraints on the model, it turns out this type of signals - mostly uncovered by current searches - could be sizeable. We focus on three benchmark scenarios, each of them predicting up to 4000 multiboson events with the LHC Run 2 collected luminosity. Anomaly-detection methods could uncover those signals, if present in data.

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arXiv2022

Crafting polarisations for top, $W$ and $Z$

We put forward a method to tune the polarisation state of decaying heavy particles (top quarks and $W/Z$ bosons) in a pre-existing Monte Carlo sample. With this technique, dubbed as `custom angle replacement', the decay angular distributions are modified in such a way that the desired polarisation state is reproduced, while all the production kinematics are unchanged. A non-trivial test of this approach is presented for the top quark semileptonic decay $t \to W b \to \ell νb$, with $\ell = e,μ$, in which the decay distribution is four-dimensional and involves three different Lorentz frames. The proposed method is ideally suited to obtain event samples with polarised heavy particles, as required by experimental measurements of polarisation and spin correlations.

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arXiv2022

Testing entanglement and Bell inequalities in $H \to ZZ$

We discuss quantum entanglement and violation of Bell inequalities in the $H\rightarrow ZZ$ decay, in particular when the two $Z-$bosons decay into light leptons. Although such process implies an important suppression of the statistics, this is traded by clean signals from a "quasi maximally-entangled" system, which makes it very promising to check these crucial phenomena at high energy. In this paper we devise a novel framework to extract from $H \to ZZ$ data all significant information related to this goal, in particular spin correlation observables. In this context we derive sufficient and necessary conditions for entanglement in terms of only two parameters. Likewise, we obtain a sufficient and improved condition for the violation of Bell-type inequalities. The numerical analysis shows that with a luminosity of $L = 300 \text{fb}^{-1}$ entanglement can be probed at $> 3σ$ level. For $L = 3 \text{ab}^{-1}$ (HL-LHC) entanglement can be probed beyond the $5σ$ level, while the sensitivity to a violation of the Bell inequalities is at the $4.5σ$ level

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arXiv2022

Understanding template methods for top polarisation measurements

Top polarisation measurements at the LHC are often performed using template methods. I discuss the role of quantum interference between polarisation states - which is mostly overlooked in the literature. Furthermore, I argue which is the correct definition of the `templates' of definite polarisations, in order to match the experimental measurements with meaningful theoretical predictions.

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arXiv2022

Improved tests of entanglement and Bell inequalities with LHC tops

We discuss quantum entanglement in top pair production at the LHC. Near the $t \bar t$ threshold, entanglement observables are enhanced by suppressing the contribution of $q \bar q$ subprocesses, which is achieved by a simple cut on the velocity of the $t \bar t$ system in the laboratory frame. Furthermore, we design new observables that directly measure the relevant combinations of $t\bar t$ spin correlation coefficients involved in the measurement of entanglement and Bell inequalities. As a result, the statistical sensitivity is enhanced, up to a factor of 7 for Bell inequalities near threshold.

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arXiv2022

Semi-dark Higgs decays: sweeping the Higgs neutrino floor

We study exotic Higgs decays $h \to Z X$, with $X$ an invisible beyond the Standard Model (SM) particle, resulting in a semi-dark final state. Such exotic Higgs decays may occur in theories of axion-like-particles (ALPs), dark photons or pseudoscalar mediators between the SM and dark matter. The SM process $h\to Zν\barν$ represents an irreducible "neutrino floor" background to these new physics searches, providing also a target experimental sensitivity for them. We analyze $h \to Z + \text{invisible}$ searches at the LHC and a future ILC, showing that these exotic Higgs decays can yield sensitivity to unexplored regions of parameter space for ALPs and dark matter models.

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arXiv2022

WIMP Dark Matter in the U$μν$SSM

The U$μν$SSM is a $U(1)'$ extension of the $μν$SSM supersymmetric model, where baryon-number-violating operators as well as explicit mass terms are forbidden, and the potential domain wall problem is avoided. The gauge anomaly-cancellation conditions impose the presence of exotic quark superfields in the spectrum of U$μν$SSM models, and allow the presence of several singlet superfields under the standard model gauge group, in addition to the right-handed neutrino superfields. The gauge structure implies an additional discrete $Z_2$ symmetry in the superpotential, ensuring the stability of a singlet which behaves as WIMP dark matter without invoking $R$-parity. We analyze this novel possibility in detail, using the fermionic component of the singlet as the dark matter candidate. In particular, we compute its amount of relic density via $Z'$, Higgs-right sneutrino and dark matter mediated annihilations, and its potential signals in dark matter direct detection experiments. The constraints on the parameter space due to $Z'$ direct searches at the LHC are imposed in the analysis, as well as those from the hadronization inside the detector of the exotic quarks. Large regions of the parameter space turn out to be in the reach of the upcoming Darwin experiment.

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arXiv2022

A template method to measure the $t \bar t$ polarisation

We develop a template method for the measurement of the polarisation of $t \bar t$ pairs produced in hadron collisions. The method would allow to extract the individual fractions of $t_L \bar t_L$, $t_R \bar t_R$, $t_L \bar t_R$ and $t_R \bar t_L$ pairs with a fit to data, where $L,R$ refer to the polarisation along any axis. These polarisation fractions have not been independently measured at present. Secondarily, the method also provides the net polarisation of $t$ and $\bar t$, as well as their spin correlation for arbitrary axes.

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arXiv2022

Anomaly detection from mass unspecific jet tagging

We introduce a novel anomaly search method based on (i) jet tagging to select interesting events, which are less likely to be produced by background processes; (ii) comparison of the untagged and tagged samples to single out features (such as bumps produced by the decay of new particles) in the latter. We demonstrate the usefulness of this method by applying it to a final state with two massive boosted jets: for the new physics benchmarks considered, the signal significance increases an order of magnitude, up to a factor of 40. We compare to other anomaly detection methods in the literature and discuss possible generalisations.

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arXiv2022

Taming modeling uncertainties with Mass Unspecific Supervised Tagging

We address the modeling dependence of jet taggers built using the method of Mass Unspecific Supervised Tagging, by using two different parton showering and hadronisation schemes. We find that the modeling dependence of the results - estimated by using different schemes in the design of the taggers and applying them to the same type of data - is rather small, even if the jet substructure varies significantly between the two schemes. These results add great value to the use of generic supervised taggers for new physics searches.

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arXiv2021

Exotic diboson $Z'$ decays in the U$μν$SSM

Searches for new leptophobic resonances at high energy colliders usually target their decay modes into pairs of light quarks, top quarks, or standard model bosons. Additional decay modes may also be present, producing signatures to which current searches are not sensitive. We investigate the performance of generic searches that look for resonances decaying into two large-radius jets. As benchmark for our analysis we use a supersymmetric $\text{U}(1)'$ extension of the Standard Model, the so-called U$μν$SSM, where all the SM decay modes of the $Z'$ boson take place, plus additional (cascade) decays into new scalars. The generic searches use a generic multi-pronged jet tagger and take advantage of the presence of $b$ quarks in the large-radius jets, and are sensitive to all these $Z'$ decay modes (except into light quarks) at once. For couplings that are well below current experimental constraints, these generic searches are sensitive at the $3σ-4σ$ level with Run 2 LHC data.

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arXiv2021

Pulling the Higgs and Top needles from the jet stack with Feature Extended Supervised Tagging

Jet tagging has become an essential tool for new physics searches at the high-energy frontier. For jets that contain energetic charged leptons we introduce Feature Extended Supervised Tagging (FEST) which, in addition to jet substructure, considers the features of the charged lepton within the jet. With this method we build dedicated taggers to discriminate among boosted $H \to \ell νq \bar q$, $t \to \ell νb$, and QCD jets (with $\ell$ an electron or muon). The taggers have an impressive performance, allowing for overall light jet rejection factors of $10^4-10^5$, for top quark / Higgs boson efficiencies of $0.5$. The taggers are also excellent in the discrimination of Higgs bosons from top quarks and vice versa, for example rejecting top quarks by factors of $100-300$ for Higgs boson efficiencies of $0.5$. We demonstrate the potential of these taggers to improve the sensitivity to new physics by using as example a search for a new $Z'$ boson decaying into $Z H$, in the fully-hadronic final state.

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arXiv2021

More light on Higgs flavor at the LHC: Higgs couplings to light quarks through $h + γ$ production

Higgs production in association with a photon at hadron colliders is a rare process, not yet observed at the LHC. We show that this process is sensitive to significant deviations of Higgs couplings to first and second generation SM quarks (particularly the up-type) from their SM values, and use a multivariate neural network analysis to derive the prospects of the High Luminosity LHC to probe deviations in the up and charm Higgs Yukawa couplings through $h + γ$ production.

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arXiv2021

$U(1)'$ extensions of the $μν$SSM

In the $μν$SSM, the presence of $R$-parity violating couplings involving right-handed (RH) neutrinos solves simultaneously the $μ$- and $ν$-problems. We explore extensions of the $μν$SSM adding a $U(1)'$ gauge group, which provides the RH neutrinos with a non-vanishing charge. In these models, dubbed U$μν$SSM, the anomaly cancellation conditions impose the presence of exotic quarks in the spectrum that are vector-like under the standard model (SM) gauge group: either three pairs $SU(2)$ quark singlets, or a pair of quark singlets together with a pair of quark doublets. Several extra singlets under the SM group can also be present, with the $U(1)'$ charges making distinctions among them, and therefore allowing different types of couplings. Some of these singlets dynamically generate Majorana masses for the RH neutrinos, and others can be candidates for dark matter. The useful characteristics of models with $U(1)'$s are also present in U$μν$SSM models: baryon-number-violating operators as well as explicit Majorana masses and $μ$ terms are forbidden, and the domain wall problem is avoided. The phenomenology of U$μν$SSM models is very rich. We analyze the experimental constraints on their parameter space, specially on the mass and mixing of the new $Z'$ boson. In addition to the exotic quarks, which can hadronize inside the detector or decay producing SM particles, the U$μν$SSM models can also have new signals such as decays of the $Z'$ to sparticle pairs like right sneutrinos, charginos or neutralinos. Besides, $Z'$ and Higgs mediated annihilations and interactions with the visible sector of WIMP dark matter particles, can also be present.

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arXiv2021

Mass Unspecific Supervised Tagging (MUST) for boosted jets

Jet identification tools are crucial for new physics searches at the LHC and at future colliders. We introduce the concept of Mass Unspecific Supervised Tagging (MUST) which relies on considering both jet mass and transverse momentum varying over wide ranges as input variables - together with jet substructure observables - of a multivariate tool. This approach not only provides a single efficient tagger for arbitrary ranges of jet mass and transverse momentum, but also an optimal solution for the mass correlation problem inherent to current taggers. By training neural networks, we build MUST-inspired generic and multi-pronged jet taggers which, when tested with various new physics signals, clearly outperform the variables commonly used by experiments to discriminate signal from background. These taggers are also efficient to spot signals for which they have not been trained. Taggers can also be built to determine, with a high degree of confidence, the prongness of a jet, which would be of utmost importance in case a new physics signal is discovered.

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arXiv2020

Jet tagging made easy

We develop taggers for multi-pronged jets that are simple functions of jet substructure (so-called `subjettiness') variables. These taggers can be approximately decorrelated from the jet mass in a quite simple way. Specifically, we use a Logistic Regression Design (LoRD) which, even being one of the simplest machine learning classifiers, shows a performance which surpasses that of simple variables used by the ATLAS and CMS Collaborations and is not far from more complex models based on neural networks. Contrary to the latter, our method allows for an easy implementation of tagging tasks by providing a simple and interpretable analytical formula with already optimised parameters.

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