Search

In this letter, we use LHC data from the Drell-Yan processes $pp\to\ell_i\ell_j$ (with $i\neq j$) to derive model-independent upper limits on lepton-flavor-violating meson decays. Our analysis is based on an Effective Field Theory (EFT) approach and it does not require a specific assumption regarding the basis of effective operators. We find that current LHC data (140~$\mathrm{fb}^{-1}$) already provides competitive limits on $B(B\to \pi e \tau)$ and $B(B\to \pi \mu \tau)$ with respect to the ones obtained through experimental searches at the $B$-factories. Moreover, we derive upper limits on several decays that have not been searched for experimentally yet, such as $D^0\to e\tau$ in the charm sector, and various semileptonic decays such as $B\to \rho \mu\tau$, $B_s\to K \mu\tau$ and $B_s\to\phi\mu\tau$. Lastly, we discuss the validity of the EFT description of LHC data and the impact of loop corrections in our analysis., Comment: 11 pages, 2 tables, 4 figs; typos corrected, published in EPJC

We propose an extension of the existing experimental strategy for measuring branching fractions of top quark decays, targeting specifically $t\to j_q W$, where $j_q$ is a light quark jet. The improved strategy uses orthogonal $b$- and $q$-taggers, and adds a new observable, the number of light-quark-tagged jets, to the already commonly used observable, the fraction of $b$-tagged jets in an event. Careful inclusion of the additional complementary observable significantly increases the expected statistical power of the analysis, with the possibility of excluding $|V_{tb}|=1$ at $95\%$ C.L. at the HL-LHC, and accessing directly the standard model value of $|V_{td}|^2+|V_{ts}|^2$., Comment: 19 pages, 7 figures

We consider the Drell-Yan processes $pp \to \ell \ell^{(\prime)}$ and $pp\to \ell \bar{\nu}$ at high-$p_T$ as probes of New Physics effects in semileptonic transitions. For this purpose, we describe the $2\to2$ scattering amplitudes in terms of general form-factors which we match to the SMEFT at $\mathcal{O}(1/\Lambda^{4})$ including corrections from dimension-$8$ operators and to tree-level mediators with $\mathcal{O}$(TeV) masses arising from ultraviolet models. By using the latest LHC run-II data from monolepton and dilepton production channels, we derive constraints on the SMEFT and on leptoquark models, with the most general flavor structures. Our results are compiled into the Mathematica package 'HighPT', which provides a simple way to perform high-$p_T$ collider analyses of Drell-Yan production Beyond the Standard Model., Comment: 8 pages, 3 figures and 2 tables. Contribution to the 2022 EW session of the 56th Rencontres de Moriond

We discuss a model that can accommodate the $B$-physics anomalies, based on combining two scalar leptoquarks, $R_2$ and $S_3$, of mass $\mathcal{O}(1\,\mathrm{TeV})$, and that we proposed in our previous paper. We update the analysis of its parameter space and show that a model remains viable and consistent with a number of low energy and high energy flavor physics constraints. Since the model predicts a non-zero New Physics phase, we discuss the possibility to test its contribution to the neutron electric dipole moment and to the angular distributions of the exclusive $b\to c\tau \bar \nu$ decays. We find that the model can provide a significant enhancement to $\mathcal{B}(B\to K^{(\ast)}\nu\nu)$ and provides both the upper and lower bounds to $\mathcal{B}(B\to K^{(\ast)}\mu\tau)$., Comment: 11pages, 5 figures

Monte Carlo (MC) generators are crucial for analyzing data in particle collider experiments. However, often even a small mismatch between the MC simulations and the measurements can undermine the interpretation of the results. This is particularly important in the context of LHC searches for rare physics processes within and beyond the standard model (SM). One of the ultimate rare processes in the SM currently being explored at the LHC, $pp\to t\bar tt \bar t$ with its large multi-dimensional phase-space is an ideal testing ground to explore new ways to reduce the impact of potential MC mismodelling on experimental results. We propose a novel statistical method capable of disentangling the 4-top signal from the dominant backgrounds in the same-sign dilepton channel, while simultaneously correcting for possible MC imperfections in modelling of the most relevant discriminating observables -- the jet multiplicity distributions. A Bayesian mixture of multinomials is used to model the light-jet and $b$-jet multiplicities under the assumption of their conditional independence. The signal and background distributions generated from a deliberately mistuned MC simulator are used as model priors. The posterior distributions, as well as the signal and background fractions, are then learned from the data using Bayesian inference. We demonstrate that our method can mitigate the effects of large MC mismodellings in the context of a realistic $t\bar tt\bar t$ search, leading to corrected posterior distributions that better approximate the underlying truth-level spectra., Comment: Matches accepted version at PRD. 11 pages, 8 figures. This expanded version includes a toy model and explicit tests of the model validity for Four-Tops. Code available at https://github.com/ManuelSzewc/bayes-4tops

We revisit the possibilities of accommodating the experimental indications of the lepton flavor universality violation in $b$-hadron decays in the minimal scenarios in which the Standard Model is extended by the presence of a single $\mathcal{O}(1\,\mathrm{TeV})$ leptoquark state. To do so we combine the most recent low energy flavor physics constraints, including $R_{K^{(\ast)}}^\mathrm{exp}$ and $R_{D^{(\ast)}}^\mathrm{exp}$, and combine them with the bounds on the leptoquark masses and their couplings to quarks and leptons as inferred from the direct searches at the LHC and the studies of the large $p_T$ tails of the $pp\to \ell\ell$ differential cross section. We find that none of the scalar leptoquarks of $m_\mathrm{LQ} \simeq 1\div 2$ TeV can accommodate the $B$-anomalies alone. Only the vector leptoquark, known as $U_1$, can provide a viable solution which, in the minimal setup, provides an interesting prediction, i.e. a lower bound to the lepton flavor violating $b\to s\mu^\pm\tau^\mp$ decay modes, such as $\mathcal{B}(B\to K\mu\tau) \gtrsim 0.7\times 10^{-7}$., Comment: 12 pages, 5 figures (10 plots) [v2: references added, Fig. 3 improved]

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 $W' \to (\phi \to WW) W$ signal events., Comment: 38 pages, 23 figures

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 $K\to \pi \ell^{\pm}_k \ell^{\mp}_l$, $B\to \pi \ell^{\pm}_k \ell^{\mp}_l$ and $B\to K^{(\ast)} \ell^{\pm}_k \ell^{\mp}_l$., Comment: 14 pages, 2 figures, 5 tables, published version

We analyze the prospects of probing the $CP$-odd $i \tilde \kappa \bar t \gamma^5 t h$ 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 $\tilde \kappa$. 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 $\tilde{\kappa}$. We project future sensitivity to the signal in $pp \to t(\to \ell \nu b )h(\to b\bar b) j$. 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 $\tilde \kappa \sim 0.5$ at the high-luminosity LHC and $\tilde \kappa \sim 0.1$ at high-energy LHC with $2\sigma$ confidence., Comment: Updated sensitivity plots due to better data samples

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., Comment: 8 pages, 3 figures; v2: matches published version. Additional clarifying comments added in sections I and II. Updated references

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 $\tau\bar\tau$ 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 $B\to K\mu\tau$ and $\tau\to\mu\phi$ we argue that these model can be cornered by the LHC, Belle II and LHCb in the near future., Comment: 17 pages, 7 figures, 2 tables, Proceedings for the 10th International Workshop on the CKM Unitarity Triangle

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., Comment: 31 pages, 12 figures. v2: references added, minor typos corrected. Published in Nucl. Phys. B

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 $\tau^+ \tau^-$ 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, $\tau^+ \tau^-$ 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 $\tau^+ \tau^-$ constraints as well as caveats in interpreting them within more elaborate models are also discussed., Comment: 11 pages, 6 figures