arXiv subjects

Darius A. Faroughy

Publications and source records attributed to Darius A. Faroughy.

31 records · Page 2Linked to original sources

arXiv2020

Uncovering hidden patterns in collider events with Bayesian probabilistic models

Individual events at high-energy colliders like the LHC can be represented by a sequence of measurements, or 'point patterns' in an observable space. Starting from this data representation, we build a simple Bayesian probabilistic model for event measurements useful for unsupervised event classification in beyond the standard model (BSM) studies. In order to arrive to this model we assume that the event measurements are exchangeable (and apply De Finetti's representation theorem), the data is discrete, and measurements are generated from multiple 'latent' distributions, called 'themes'. The resulting probabilistic model for collider events is a mixed-membership model known as Latent Dirichlet Allocation (LDA), a model extensively used in natural language processing applications. By training on point patterns in the primary Lund plane, we demonstrate that a two-theme LDA model can learn to distinguish in unlabelled dijet events the hidden new physics patterns produced by a BSM signature from a much larger QCD background. Based on 1904.04200 and 2005.12319.

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arXiv2020

Lepton Flavor Violation and Dilepton Tails at the LHC

Starting from a general effective Lagrangian for lepton flavor violation (LFV) in quark-lepton transitions, we derive constraints on the effective coefficients from the high-mass tails of the dilepton processes $pp \to \ell_k \ell_l$ (with $k\neq l$). The current (projected) limits derived in this paper from LHC data with $36~\mathrm{fb}^{-1}$ ($3~\mathrm{ab}^{-1}$) can be applied to generic new physics scenarios, including the ones with scalar, vector and tensor effective operators. For purely left-handed operators, we explicitly compare these LHC constraints with the ones derived from flavor-physics observables, illustrating the complementarity of these different probes. While flavor physics is typically more constraining for quark-flavor violating operators, we find that LHC provides the most stringent limits on several flavor-conserving ones. Furthermore, we show that dilepton tails offer the best probes for charm-quark transitions at current luminosities and that they provide competitive limits for tauonic $b\to d$ transitions at the high-luminosity LHC phase. As a by-product, we also provide general numerical expressions for several low-energy LFV processes, such as the semi-leptonic decays $K\to π\ell^{\pm}_k \ell^{\mp}_l$, $B\to π\ell^{\pm}_k \ell^{\mp}_l$ and $B\to K^{(\ast)} \ell^{\pm}_k \ell^{\mp}_l$.

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arXiv2020

Probing the $CP$ nature of the top quark Yukawa at hadron colliders

We analyze the prospects of probing the $CP$-odd $i \tilde κ\bar t γ^5 t h$ interaction at the LHC and its projected upgrades, the high-luminosity and high-energy LHC, directly using associated on-shell Higgs boson and top quark or top quark pair production. To this end we first construct a $CP$-odd observable based on top quark polarization in $W b \to t h$ scattering with optimal linear sensitivity to $\tilde κ$. For the corresponding hadronic process $pp \to t h j$ we present a method of extracting the phase-space dependent weight function that allows to retain close to optimal sensitivity to $\tildeκ$. We project future sensitivity to the signal in $pp \to t(\to \ell νb )h(\to b\bar b) j$. Based on these insights we propose novel $CP$-odd observables for top quark pair production in association with the Higgs, $pp \to t \bar t h$, with semileptonically decaying tops and $h\to \bar b b$, that rely solely on measuring the momenta of leptons and $b$-jets from the decaying tops without having to distinguish the charge of the $b$-jets. Among the many possibilities we single out an observable that can potentially probe $\tilde κ\sim 0.5$ at the high-luminosity LHC and $\tilde κ\sim 0.1$ at high-energy LHC with $2σ$ confidence.

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arXiv2020

Learning the latent structure of collider events

We describe a technique to learn the underlying structure of collider events directly from the data, without having a particular theoretical model in mind. It allows to infer aspects of the theoretical model that may have given rise to this structure, and can be used to cluster or classify the events for analysis purposes. The unsupervised machine-learning technique is based on the probabilistic (Bayesian) generative model of Latent Dirichlet Allocation. We pair the model with an approximate inference algorithm called Variational Inference, which we then use to extract the latent probability distributions describing the learned underlying structure of collider events. We provide a detailed systematic study of the technique using two example scenarios to learn the latent structure of di-jet event samples made up of QCD background events and either $t\bar{t}$ or hypothetical $W' \to (ϕ\to WW) W$ signal events.

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arXiv2020

Flavour symmetries in the SMEFT

We analyse how $U(3)^5$ and $U(2)^5$ flavour symmetries act on the Standard Model Effective Field Theory, providing an organising principle to classify the large number of dimension-six operators involving fermion fields. A detailed counting of such operators, at different order in the breaking terms of both these symmetries, is presented. A brief discussion about possible deviations from these two reference cases, and a simple example of the usefulness of this classification scheme for high-$p_T$ analyses at the LHC, are also presented.

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arXiv2019

Uncovering latent jet substructure

We apply techniques from Bayesian generative statistical modeling to uncover hidden features in jet substructure observables that discriminate between different a priori unknown underlying short distance physical processes in multi-jet events. In particular, we use a mixed membership model known as Latent Dirichlet Allocation to build a data-driven unsupervised top-quark tagger and $t\bar t$ event classifier. We compare our proposal to existing traditional and machine learning approaches to top jet tagging. Finally, employing a toy vector-scalar boson model as a benchmark, we demonstrate the potential for discovering New Physics signatures in multi-jet events in a model independent and unsupervised way.

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arXiv2018

Direct searches motivated by recent $B$-anomalies

We discuss the physics case for direct searches at the LHC motivated by the $B$-physics anomalies. After correlating semi-tauonic $B$ decays to di-tau production at the LHC, and discussing the possible models solving the $B$-anomalies, we show how current LHC data in $τ\barτ$ tails exclude most beyond the SM scenarios except for a handful of leptoquark (LQ) models. We analyze in detail the impact of LHC searches for some of these LQ solutions using current data. In particular, we focus on the well known $U_1\sim({\bf3},{\bf1})_{2/3}$ vector LQ solution as well as the GUT-inspired scalar LQ solutions, $R_2\sim({\bf3},{\bf2})_{7/6}$ and $S_3\sim({\bf\bar3},{\bf3})_{1/3}$. By exploiting the complementarity between high-$p_T$ searches in di-tau tails and the lepton flavor violating decays $B\to Kμτ$ and $τ\toμϕ$ we argue that these model can be cornered by the LHC, Belle II and LHCb in the near future.

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arXiv2018

Closing the window on single leptoquark solutions to the $B$-physics anomalies

We examine various scenarios in which the Standard Model is extended by a light leptoquark state to solve for one or both $B$-physics anomalies, viz. $R_{D^{(\ast)}}^\mathrm{exp}> R_{D^{(\ast)}}^\mathrm{SM}$ or/and $R_{K^{(\ast)}}^\mathrm{exp}< R_{K^{(\ast)}}^\mathrm{SM}$. To do so we combine the constraints arising both from the low-energy observables and from direct searches at the LHC. We find that none of the scalar leptoquarks of mass $m_\mathrm{LQ} \simeq 1$ TeV can alone accommodate the above mentioned anomalies. The only single leptoquark scenario which can provide a viable solution for $m_\mathrm{LQ} \simeq 1÷2$ TeV is a vector leptoquark, known as $U_1$, which we re-examine in its minimal form (letting only left-handed couplings to have non-zero values). We find that the limits deduced from direct searches are complementary to the low-energy physics constraints. In particular, we find a rather stable lower bound on the lepton flavor violating $b\to s\ell_1^\pm\ell_2^\mp$ modes, such as $\mathcal{B}(B\to Kμτ)$. Improving the experimental upper bound on $\mathcal{B}(B\to Kμτ)$ by two orders of magnitude could compromise the viability of the minimal $U_1$ model as well.

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arXiv2018

Scalar leptoquarks from GUT to accommodate the $B$-physics anomalies

We address the $B$-physics anomalies within a two scalar leptoquark model. The low-energy flavor structure of our set-up originates from two $SU(5)$ operators that relate Yukawa couplings of the two leptoquarks. The proposed scenario has a UV completion, can accommodate all measured lepton flavor universality ratios in $B$-meson decays, is consistent with related flavor observables, and is compatible with direct searches at the LHC. We provide prospects for future discoveries of the two light leptoquarks at the LHC and predict several yet-to-be-measured flavor observables.

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arXiv2018

Anomalies in Bottom from new physics in Top

We analyze the possibility to accommodate current $b \to s \ell^+ \ell^-$ anomalies with TeV-scale mediators that couple to right-handed top quarks and muons, contributing to $b \to s \ell^+ \ell^-\,$ at the one-loop level. We use the Standard Model Effective Field Theory (SMEFT) framework but also look at specific scenarios by taking into account all possible irreducible representations of the Lorentz and Standard Model gauge group for the mediators. From a global fit of $b \to s \ell^+ \ell^-$ data and LEP-I observables we find that the Wilson coefficients of two SMEFT operators: $\mathcal{O}_{\ell u}= (\bar \ell_{ L μ} γ^α \ell_{ L μ}) (\bar t_R γ_α t_R )$ and $\mathcal{O}_{e u}= (\bar μ_{R} γ^α μ_{R}) (\bar t_R γ_α t_R )$ need to satisfy $\mathcal{C}_{eu} \sim \mathcal{C}_{\ell u}$. New physics enters then in $b \to s \ell^+ \ell^-$ mainly through the operator $\mathcal{O}_9 = ( \bar s γ_μ P_{L} b )( \bar \ell γ^μ \ell )$ of the Weak Effective Theory. After discussing all possible mediators, we concentrate on two scenarios: A vector boson in the irreducible representation $Z_μ^{\prime} \sim (1,1,0)$ of the Standard Model gauge group with vectorial coupling to muons, and a combination of two leptoquarks: the scalar $R_2 \sim (3,2,7/6)$ and the vector $\widetilde U_{1 μ} \sim (3,1,5/3)$. We derive LHC constraints by recasting di-muon resonance, $pp \to t \bar t t \bar t$ and SUSY searches. Additionally, we analyze the prospects for discovering these mediators during the high-luminosity phase of the LHC.

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arXiv2017

The role of the $S_3$ GUT leptoquark in flavor universality and collider searches

We investigate the ability of the $S_3$ scalar leptoquark to address the recent hints of lepton universality violation in $B$ meson decays. The $S_3$ leptoquark with quantum numbers $(\overline{\mathbf{3}},\mathbf{3},1/3)$ naturally emerges in the context of an $SU(5)$ GUT model without any conflict with the stringent limits from observed nucleon stability. Scalar leptoquark $S_3$ with left-handed couplings to 2nd and 3rd generations of charged leptons and down-type quarks seems well-suited to address both $R_{K^{(*)}}$ and $R_{D^{(*)}}$. We quantify this suitability with numerical fits to a plethora of relevant flavor observables. The proposed $SU(5)$ model calls for a second leptoquark state, i.e., $\tilde{R}_2$ with quantum numbers $(\mathbf{3},\mathbf{2},1/6)$, if one is to generate gauge coupling unification and neutrino mass. We accordingly include it in our study to investigate $\tilde{R}_2$'s ability to offset adverse effects of $S_3$ and thus improve a quality of numerical fits. A global fit of the leptoquark Yukawa couplings shows that large couplings of light $S_3$ to $τ$ leptons are preferred. We furthermore identify $B \to K^{(*)} \barνν$ as the most sensitive channel to probe the preferred region of parameter space. Large couplings of $S_3$ to $τ$ leptons are finally confronted with the experimental searches for $τ$ final states at the Large Hadron Collider. These searches comprise a study of decay products of the leptoquark pair production, as well as, and more importantly, an analysis of the high-mass $ττ$ final states.

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arXiv2017

Four Tops for LHC

We design a search strategy for the Standard Model $t\bar t t \bar t$ production at the LHC in the same-sign dilepton and trilepton channels. We study different signal features and, given the small expected number of signal events, we scrutinize in detail all reducible and irreducible backgrounds. Our analysis shows that by imposing a set of basic jet and lepton selection criteria, the SM $pp \to t\bar t t \bar t$ process could be evidenced in the near future, within Run-II, when combining both multi-lepton search channels. We argue that this search strategy should also be used as a guideline to test New Physics coupling predominantly to top-quarks. In particular, we show that a non-resonant New Physics enhancement in the four-top final state would be detectable through this search strategy. We study two {\it top-philic} simplified models of this kind, a neutral scalar boson and a $Z^\prime$, and present current and future exclusion limits on their mass and couplings.

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arXiv2016

Confronting lepton flavor universality violation in B decays with high-$p_T$ tau lepton searches at LHC

We confront the indications of lepton flavor universality (LFU) violation observed in semi-tauonic $B$ meson decays with new physics (NP) searches using high $p_T$ tau leptons at the LHC. Using effective field theory arguments we correlate possible non-standard contributions to semi-tauonic charged currents with the $τ^+ τ^-$ signature at high energy hadron colliders. Several representative standard model extensions put forward to explain the anomaly are examined in detail: (i) weak triplet of color-neutral vector resonances, (ii) second Higgs doublet and (iii) scalar or (iv) vector leptoquark. We find that, in general, $τ^+ τ^-$ searches pose a serious challenge to NP explanations of the LFU anomaly. Recasting existing 8 TeV and 13 TeV LHC analyses, stringent limits are set on all considered simplified models. Future projections of the $τ^+ τ^-$ constraints as well as caveats in interpreting them within more elaborate models are also discussed.

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