Decays of Higgs boson-like particles into multileptons is a well-motivated process for investigating physics beyond the Standard Model (SM). A unique feature of this final state is the precision with which the SM is known. As a result, simulations are used directly to estimate the background. Current searches consider specific models and typically focus on those with a single free parameter to simplify the analysis and interpretation. In this paper, we explore recent proposals for signal model agnostic searches using machine learning in the multilepton final state. These tools can be used to simultaneously search for many models, some of which have no dedicated search at the Large Hadron Collider. We find that the machine learning methods offer broad coverage across parameter space beyond where current searches are sensitive, with a necessary loss of performance compared to dedicated searches by only about one order of magnitude.
The signatures of dark matter at the LHC commonly involve, in simplified scenarios, the production of a single particle plus large missing energy, from the undetected dark matter. However, in $Z'$-portal scenarios anomaly cancellation requires the presence of extra dark leptons in the dark sector. We investigate the signatures of the minimal scenarios of this kind, which involve cascade decays of the extra $Z'$ boson into the dark leptons, identifying a four-lepton signal as the most promising one. We estimate the sensitivity to this signal at the LHC, the high-luminosity LHC upgrade, a possib…
As sequel to a recent paper we examine the phenomenology of the full electroweakino sector of the pMSSM without invoking the adhoc but often employed assumption that the heavier ones are decoupled. We identify several generic models which illustrate the importance of the heavier electroweakinos and constrain them with the LHC $3l$ + ${E\!\!\!\!/_T}$ data. The constraints are usually stronger than that for decoupled heavier electroweakinos indicating that the LHC data is already sensitive to their presence. We also take into account the constraints from the observed dark matter relic density of…
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 tha…
We study in detail the collider signatures of an $SU(2)_R$ fermionic quintuplet in the framework of left-right symmetric model in the context of the 13 TeV LHC. Apart from giving a viable dark matter candidate ($χ^0$), this model provides unique collider imprints in the form of same-sign multileptons through the decays of multi-charged components of the quintuplet. In particular, we consider the scenario where the quintuplet carries $(B - L) = 4$ charge, allowing for the presence of high charge-multiplicity particles with relatively larger mass differences among them compared to $(B - L)$ = 0 …
Leptonic signatures of Dark Matter (DM) are one of the cleanest ways to discover such a secluded form of matter at high energy colliders. We explore the full parameter space relevant to multi-lepton (2- and 3-lepton) signatures at the Large Hadron Collider (LHC) from representative minimal consistent models with scalar and fermion DM. In our analysis, we suggest a new parametrisation of the model parameter spaces in terms of the DM mass and mass differences between DM and its multiplet partners. This parametrisation allows us to explore properties of DM models in their whole parameter space. T…
During the last years ATLAS and CMS have reported a number of slight to mild discrepancies in signatures of multileptons plus $b$-jets in analyses such as $t\bar t H$, $t\bar t W^\pm$, $t\bar t Z$ and $t\bar t t\bar t$. Among them, a recent ATLAS result on $t\bar t H$ production has also reported an excess in the charge asymmetry in the same-sign dilepton channel with two or more $b$-tagged jets. Motivated by these tantalizing discrepancies, we study a phenomenological New Physics model consisting of a $Z'$ boson that couples to up-type quarks via right-handed currents: $t_Rγ^μ\bar t_R$, $t_Rγ…
In light of recent discrepancies between the modeling of $t\bar{t} W^\pm$ signatures and measurements reported by the Large Hadron Collider (LHC) experimental collaborations, we investigate in detail theoretical uncertainties for multi-lepton signatures. We compare results from the state-of-the-art full off-shell calculation and its Narrow Width Approximation to results obtained from the on-shell $t\bar{t} W^\pm$ calculation, with approximate spin-correlations in top-quark and $W$ decays, matched to parton showers. In the former case double-, single-, and non-resonant contributions together wi…
The aim of this paper is to showcase the novel multilepton ($nl + E\!\!\!\!/_T$, n = 3 - 5) signals, hitherto unexplored at the LHC, arising from the heavier electroweakinos, in several generic pMSSMs at the upcoming LHC experiments. We first briefly review our old constraints on the full electroweakino sector of these models, containing both lighter and heavier sparticles, using the ATLAS trilepton data from the LHC Run I. Next we derive new stronger constraints on this sector for the first time using the ATLAS Run II data. We identify some benchmark points and explore the prospect of observi…
We revisit the scalar singlet dark matter (DM) scenario with a pair of dark lepton partners which form a vectorlike Dirac fermionic doublet. The extra doublet couples with the Standard Model (SM) leptonic doublet and the scalar singlet via a non-SM-like Yukawa structure. As a result, (i) since the extra fermionic states interact with other dark sector particles as well as the SM via gauge and Yukawa interactions, it gives rise to new DM annihilation processes including pair annihilation as well as coannihilation channels, and (ii) such a Yukawa structure opens up new production channels for le…
A new paradigm for data-driven, model-agnostic new physics searches at colliders is emerging, and aims to leverage recent breakthroughs in anomaly detection and machine learning. In order to develop and benchmark new anomaly detection methods within this framework, it is essential to have standard datasets. To this end, we have created the LHC Olympics 2020, a community challenge accompanied by a set of simulated collider events. Participants in these Olympics have developed their methods using an R&D dataset and then tested them on black boxes: datasets with an unknown anomaly (or not). This …
Graph theory inspired anomaly detection at the LHC
University College London（伦敦大学学院）；City St. George’s, University of London（圣乔治城市大学，伦敦大学）；Stony Brook University（石溪大学）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）
测量了在7 TeV LHC上产生的截面。因此，第一次有可能测量顶夸克的Z耦合。用额外维模型来解释臭名昭著的LEP1异常，即Z与b夸克的耦合，人们期望在8 TeV的LHC数据中可能已经变得重要的速率上有一个大到2倍的过量。其他计划进行审查，也预测耦合偏差。效应的大小和具有正确手征性的顶夸克的急剧增加应该证实我们对潜在机制的解释。在LHC观测到的互补信号也简要回顾。
We investigate how to improve new physics detection strategies exploiting variational autoencoders and normalizing flows for anomaly detection at the Large Hadron Collider. As a working example, we consider the DarkMachines challenge dataset. We show how different design choices (e.g., event representations, anomaly score definitions, network architectures) affect the result on specific benchmark new physics models. Once a baseline is established, we discuss how to improve the anomaly detection accuracy by exploiting normalizing flow layers in the latent space of the variational autoencoder.
Heavy neutrinos $(N)$ remain one of most promising explanations for the origin of neutrinos' tiny masses and large mixing angles. In light of broad advances in understanding and modeling of hadron collisions at large momentum transfer, we revisit the long-standard search strategy for heavy $N$ decaying to multiple charged leptons $(\ell)$, $pp \to N\ell X \to 3\ell νX$. For electroweak and TeV-scale $N$, we propose a qualitatively new collider analysis premised on a dynamic jet veto and discriminating, on an event-by-event basis, according to the relative amount of hadronic and leptonic activi…
The OPERA collaboration has reported the observation of superluminal muon neutrinos, whose speed $v_ν$ exceeds that of light $c$, with $(v_ν- c)/c \simeq 2.5 \times 10^{-5}$. In a recent work, Cohen and Glashow (CG) have refuted this claim by noting that such neutrinos will lose energy, by pair-emission of particles, at unacceptable rates. Following the CG arguments, we point out that pair-emissions consistent with the OPERA anomaly can lead to detectable signals from decays of highly boosted top quarks at the LHC, allowing an independent test of the superluminal neutrino hypothesis.
We revisit the current LHC constraints on the electroweak-ino sector parameters in the minimal supersymmetric standard model (MSSM) that are relevant to explaining the $(g-2)_μ$ anomaly via the dominant chargino and muon sneutrino loop. Since the LHC bounds on electroweak-inos become weaker if they decay via an intermediate stau or a tau sneutrino instead of the first two generation sleptons, we perform a detailed analysis of the scenario with a bino as the lightest supersymmetric particle (LSP) and a light stau as the next-to-lightest one (NLSP). Even in this scenario, the chargino sector par…
We consider a neutrinophilic $U(1)$ extension of the standard model (SM) which couples only to SM isosinglet neutral fermions, charged under the new group. The neutral fermions couple to the SM matter fields through Yukawa interactions. The neutrinos in the model get their masses from a standard inverse-seesaw mechanism while an added scalar sector is responsible for the breaking of the gauged $U(1)$ leading to a light neutral gauge boson ($Z'$), which has minimal interaction with the SM sector. We study the phenomenology of having such a light $Z'$ in the context of neutrinophilic interaction…
There are discrepancies between the experimental results and the Standard Model predictions, in the lepton flavor universality of the semileptonic $B$ decays: $B \to D^{(*)} \ell ν$. As the new physics interpretations, new charged vector and charged scalar fields, that dominantly couple to the second and third generations, have been widely discussed. In this paper, we study the signals of the new particles at the LHC, and test the interpretations via the direct search for the new resonances. In particular, we see that the $τν$ resonance search at the LHC has already covered most of the paramet…
We propose a resolution of the discrepancy between the proton yield predicted by the statistical hadronization approach and data on hadron production in ultra-relativistic nuclear collisions at the LHC. Applying the S-matrix formulation of statistical mechanics to include pion-nucleon interactions, we reexamine their contribution to the proton yield, taking resonance widths and the presence of nonresonant correlations into account. The effect of multi-pion-nucleon interactions is estimated using lattice QCD results on the baryon-charge susceptibility. We show that a consistent implementation o…
There is a growing need for machine learning-based anomaly detection strategies to broaden the search for Beyond-the-Standard-Model (BSM) physics at the Large Hadron Collider (LHC) and elsewhere. The first step of any anomaly detection approach is to specify observables and then use them to decide on a set of anomalous events. One common choice is to select events that have low probability density. It is a well-known fact that probability densities are not invariant under coordinate transformations, so the sensitivity can depend on the initial choice of coordinates. The broader machine learnin…
The discrepancy between the measured value and the Standard Model prediction for the muon anomalous magnetic moment is one of the important issues in the particle physics. In this paper, we consider a two Higgs doublet model (2HDM) where the extra Higgs doublet couples to muon and tau in lepton flavor violating (LFV) way and the one-loop correction involving the scalars largely contributes to the muon anomalous magnetic moment. The couplings should be sizable to explain the discrepancy, so that the extra Higgs bosons would dominantly decay into $μτ$ LFV modes, which makes the model testable at…
This paper discusses model-agnostic searches for new physics at the Large Hadron Collider (LHC) using anomaly-detection techniques for the identification of event signatures that deviate from the Standard Model (SM). We investigate anomaly detection in the context of machine-learning approaches using autoencoders, and illustrate expected shapes of invariant masses in the outlier region using Monte Carlo simulations. Challenges and conceptual limitations of this approach are discussed.
