Top quarks produced in multi-TeV processes will have large Lorentz boosts, and their decay products will be highly collimated. In semileptonic decay modes, this often leads to the merging of the b-jet and the hard lepton according to standard event reconstructions, which can complicate new physics searches. Here we explore ways of efficiently recovering this signal in the muon channel at the LHC. We perform a particle-level study of events with muons produced inside of boosted tops, as well as in generic QCD jets and from W-strahlung off of hard quarks. We characterize the discriminating power of cuts previously explored in the literature, as well two new ones. We find a particularly powerful isolation variable which can potentially reject light QCD jets with hard embedded muons at the 10^3 level while retaining 80~90% of the tops. This can also be fruitfully combined with other cuts for O(1) greater discrimination. For W-strahlung, a simple pT-scaled maximum ΔR cut performs comparably to a highly idealized top-mass reconstruction, rejecting an O(1) fraction of the background with percent-scale loss of signal. Using these results, we suggest a set of well-motivated baseline cuts for any physics analysis involving semileptonic top quarks at TeV-scale momenta, using neither b-tagging nor missing energy as discriminators. We demonstrate the utility of our cuts in searching for resonances in the top-antitop invariant mass spectrum. For example, our results suggest that 100 fb^{-1} of data from a 14 TeV LHC could be used to discover a warped KK gluon up to 4.5 TeV or higher.
Top partner searches are one of the key aspects of new physics analyses at the LHC. We correct an earlier statement that supersymmetric top searches based on decays to semileptonic tops are not promising. Reconstructing the direction of the boosted leptonic top quark and correlating it with the measured missing transverse energy vector allows us to reduce the top pair background to an easily manageable level. In addition, reconstructing the full momentum of the leptonic top quark determines the stop mass based on an M_{T2} endpoint.
We address the potential of measurements with boosted single-top final states at the high-luminosity LHC (HL-LHC) and possible future hadron colliders: the high-energy LHC (HE-LHC), and the future circular collider (FCC). As new physics examples to assess the potential, we consider the search for $tbW$ anomalous couplings and for a weakly-coupled $W'$ boson. The FCC would improve by a factor of two the sensitivity to anomalous couplings of the HL-LHC. For $W'$ bosons, the FCC is sensitive to $W'$ couplings $2-5$ times smaller than the HL-LHC in the mass range 2-4 TeV, and to masses up to 30 Te…
We discuss production of charmed mesons as well as electrons/muons from semileptonic decays of charm and bottom mesons in proton-proton collisions at the LHC. The cross section for inclusive production of $c \bar c$ and $b \bar b$ pairs is calculated in the framework of the $k_{\perp}$-factorization approach. Here, the KMR and Jung CCFM unintegrated gluon distribution functions are used. Theoretical uncertainties of the model related to the choice of renormalization and factorization scales as well as due to the quark mass are also discussed. The hadronization of charm and bottom quarks is inc…
Study of polarization of heavy particles is an important branch of research in today's collider studies. The massive $W$ boson has two types of polarization states, which usually are studied via the angular distribution of its decay products. We have studied polarization of hadronic and boosted $W$ boson using jet substructure technique at 14 TeV LHC. Two different methods, viz. N-subjettiness and Soft Drop, were used to find the subjets, which are approximately considered to be the two hadronic decay products of $W$, inside boosted $W$ jets. These subjets were then used to find the distributi…
Longitudinal vector boson scattering provides an important probe of electroweak symmetry breaking, bringing sensitivity to physics beyond the Standard Model as well as constraining properties of the Higgs boson. It is a difficult process to study due to the small production cross section and challenging separation of the different polarization states. We study the sensitivity to longitudinal $WV$ vector boson scattering at the high-luminosity Large Hadron Collider in semileptonic final states. While these are characterized by larger background contributions compared to fully leptonic final sta…
It is widely believed that fully hadronic final states are not competitive in searches for new physics at the Large Hadron Collider due to the overwhelming QCD backgrounds. In this letter, we present a particle-level study of the topology arising when a TeV-scale resonance decays to two Higgs bosons and these subsequently decay to $b\bar{b}$, leading to two back-to-back boosted dijet systems. We show that selecting events with this topology dramatically reduces all backgrounds, thus enabling very competitive searches for new physics in a variety of models. For a resonance with mass 1 TeV and w…
Searching for the Higgs boson is of first priority in LHC experiments. Once a Higgs candidate is found, we then need to know whether it is the standard model (SM) Higgs boson or a Higgs boson in new physics beyond the SM. So far we do not know what model of new physics will actually reflect the property of the nature. A model-independent way of measuring the anomalous couplings of the Higgs boson will provide a no-lose study, i.e., if we find nonvanishing anomalous couplings (deviation from the SM couplings), we can conclude that it is a new physics effect. In a previous paper \cite{QKLZ09}, w…
At the LHC, a generic supersymmetric cascade can be a source of top quark. Specifically third generation squarks and gluino are the major sources of top quark which could also be boosted. In this article, we have shown that jet substructure algorithm can be very useful in identifying such boosted top quarks in the cascade. We take inclusive three jets plus zero lepton plus missing energy final state and try to reconstruct at least one hadronically decaying top quark by using top tagging techniques which has good prospect at the LHC.
We study the process $pp \to Z(\ell^+ \ell^-)h(b\bar b)$ in the Standard Model Effective Field Theory (SMEFT) at high energies using subjet techniques to reconstruct the Higgs boson. We show that at high energies this process probes four directions in the dimension 6 EFT space, namely the operators that contribute to the four contact interactions, $hZ_μ\bar{f}γ^μf$, where $f=u_L, u_R,d_L$ and $d_R$. These four directions are, however, already constrained by the $Z$-pole and diboson measurements at LEP. We show that by utilising the energy growth of this process in the SMEFT and the accuracy th…
We make a full tree level study of the signatures of anomalous gauge couplings of the Higgs boson at the CERN LHC via the semileptonic decay mode in WW scatterings. Both signals and backgrounds are studied at the hadron level for the Higgs mass in the range 115 GeV to 200 GeV. We carefully impose suitable kinematical cuts for suppressing the backgrounds. To the same sensitivity as in the pure leptonic mode, our result shows that the semileptonic mode can reduce the required integrated luminosity by a factor of 3. If the anomalous couplings in nature are actually larger than the sensitivity bou…
Naturalness considerations, together with the non-observation of superpartners of the Standard Model particles at the Large Hadron Collider (LHC) so far, favor supersymmetric (SUSY) models in which third generation squarks are significantly lighter than those of the first two generations. In such models, gluino pair-production is typically the dominant SUSY production process at the LHC, and it often leads to final states with multiple top quarks. Some of these top quarks may be relativistic in the lab frame, in which case their hadronic decays may produce "top jets". We propose that the recen…
Studies of the boosted sector in top-quark physics have known a fast-growing development with the arrival of high-energy data at LHC. This short review summarizes the current status of the boosted top-tagging techniques in ATLAS and CMS and presents an overview of some of the most noticeable developments.
We investigate existing and future hadron-collider constraints on the top dipole chromomagnetic and chromoelectric moments, two quantities that are expected to be modified in the presence of new physics. We focus first on recent measurements of the inclusive top pair production cross section at the Tevatron and at the Large Hadron Collider. We then analyse the role of top-antitop events produced at very large invariant masses, in the context of the forthcoming 13-14 TeV runs of the LHC, and at a future 100 TeV proton-proton collider. In this latter case, the selection of semileptonic decays to…
Studies of highly-boosted top quarks produced inclusively in pp collisions at 14 TeV are discussed. The hadronic decays of boosted top quarks was studied in a data-driven approach by analysing shapes of jet-mass distributions. Using Monte Carlo models after a fast detector simulation, it is shown that inclusive production of boosted top quarks can be observed if it has a cross section at least twice larger than the prediction from the approximate next-to-next-to-leading-order (aNNLO) calculation for the ttbar process. The ttbar process with the nominal aNNLO strength can be measured using the …
We investigate the discovery potential of singly produced top-philic resonances at the high luminosity (HL) LHC in the four-top final state. Our analysis spans over the fully-hadronic, semi-leptonic, and same-sign dilepton channels where we present concrete search strategies adequate to a boosted kinematic regime and high jet-multiplicity environments. We utilize the Template Overlap Method (TOM) with newly developed template observables for tagging boosted top quarks, a large-radius jet variable $M_J$ and customized b-tagging tactics for background discrimination. Our results show that the sa…
Instead of the usual dijet decay, the coloron may mainly decay into its own "Higgs bosons", which subsequently decay into many jets. This is a general feature of the renormalizable coloron model, where the corresponding "Higgs bosons" are a color-octet $Θ$ and a color-singlet $ϕ_I$. In this paper, we perform a detailed collider study for the signature of $pp \rightarrow G' \rightarrow (Θ\rightarrow gg) (ϕ_I \rightarrow gg q\bar{q})$ with the coloron $G'$ as a six-jet resonance. For a light $ϕ_I$ below around 0.5 TeV, it may be boosted and behave as a four-prong fat jet. We also develop a jet-s…
In this paper we compute, within in the context of a relativistic quark model, the Isgur-Wise functions for exclusive semileptonic $\bar B \to X_c$ decays, where $X_c$ is any charmed mesons with total spin $J=0,1,2$ or one of their first excited states. The relevant matrix elements are computed by a direct numerical integration, in coordinate space, of the convolution of the wave function of the $B$ meson at rest and the wave function of the $X_c$ meson, boosted according with its recoil factor. Our results are compared with other predictions found in the existing literature.
We study the top quark pair events production in $pp$ collisions in the $\ell$+jets channel at the LHC energy of $\sqrt{s} = 14$ TeV for Standard Model as well as new physics processes. We explore the usage of semi-boosted topologies where the top quark decays into a boosted hadronic $W$-jet and an isolated $b$-jet and study their performance in the $t\bar{t}$ events kinematic reconstruction. An efficiency increase is observed and the correlation of selected kinematic variables between the detector and particle level is studied. Quality of the reconstructed mass line shape of a $Z'$ model with…
We investigate the sensitivity of top pair production to the properties of different Beyond the Standard Model theories embedding a new neutral boson. We include six-fermion decay, and account for the full tree-level Standard Model ttbar interference, with all intermediate particles allowed off-shell. We focus on those observables best suited to the lepton-plus-jets final state at the LHC, and simulate the resulting experimental conditions, including kinematic requirements and top quark pair reconstruction in the presence of missing transverse energy and combinatorial ambiguity in quark-top as…
A method is proposed for distinguishing highly boosted hadronically decaying W's (W-jets) from QCD-jets using jet substructure. Previous methods, such as the filtering/mass-drop method, can give a factor of ~2 improvement in S/sqrt(B) for jet pT > 200 GeV. In contrast, a multivariate approach including new discriminants such as R-cores, which characterize the shape of the W-jet, subjet planar flow, and grooming-sensitivities is shown to provide a much larger factor of ~5 improvement in S/sqrt(B). For longitudinally polarized W's, such as those coming from many new physics models, the discrimin…
NNLL$^\prime$ resummation of azimuthal decorrelation for boosted top quark pair production at the LHC
Fudan University（复旦大学）；Shanghai Research Center for Theoretical Nuclear Physics, NSFC and Fudan University（上海理论核物理研究中心，国家自然科学基金委员会与复旦大学）；Center for High Energy Physics, Peking University（北京大学高能物理中心）
Search strategies for the third-generation leptoquarks (LQs) are distinct from other LQ searches, especially when they decay to a top quark and a $τ$ lepton. We investigate the cases of all TeV-scale scalar and vector LQs that decay to either a top-tau pair (charge-$1/3$ and $5/3$ LQs) or a top-neutrino pair (charge-$2/3$ LQs). One can then use the boosted top (which can be tagged efficiently using jet-substructure techniques) and high-$p_{\rm T}$ $τ$ leptons to search for these LQs. We consider two search channels with either one or two taus along with at least one hadronically decaying boost…
Inspired by the newly observed $D^{(*)}(3000)$, and $D_{sJ}(3040)$ states, in this work we study the production of $D^{(*)}(3000)$, $D_{sJ}(3040)$ and their partners through the semileptonic decays of $B_{(s)}$ mesons, where the light-front Quark model is applied to the whole calculation. Our numerical results indicate that the $B_{(s)}$ semileptonic decays into the $2P$ states of the charmed or charmed-strange meson family have considerable branching ratios, which shows that these semileptonic decays can be accessible at future experiments, especially LHCb and the forthcoming Belle II.
After the recent discovery of a 125 GeV Higgs-like particle at the Large Hadron Collider (LHC), it is crucial to examine its role in unitarizing high energy W_LW_L scattering, which may reveal its possible deviation from a Standard Model Higgs. We perform an updated study on WW scattering in the semileptonic channel at the LHC, improved by the recently developed W jet tagging method. The resultant statistical significance of a Strongly-Interacting Light Higgs (SILH) model is about 20% larger than that based on the conventionally "gold-plated" dileptonic channel, while 200% more signal events a…
Lepton Flavour Universality tests with semileptonic $Λ_b\toΛ_c^*$ decays are important to corroborate the present anomalies in the similar ratios $R_{D^{(*)}}$, and can provide complementary constraints on possible origins of these anomalies beyond the Standard Model. In this paper we provide - for the first time - all the necessary theoretical ingredients to perform and interpret measurements of $R_{Λ_c^*}$ at the LHCb experiment. For this, we revisit the heavy-quark expansion of the relevant hadronic matrix elements, and provide their expressions to order $α_s$ and $1/m$ accuracy. Moreover, …
A new algorithm for the identification of boosted, hadronically decaying, heavy particles at the LHC is presented. The algorithm is based on the known procedure of jet clustering with variable distance parameter $R$ and adapts the jet size to its transverse momentum $p_T$. Subjets are found using a mass jump condition. The resulting algorithm - called Heavy Object Tagger with Variable $R$ (HOTVR) - features little algorithmic complexity and combines jet clustering, subjet finding and rejection of soft clusters in one sequence. While the HOTVR algorithm can be used for the identification of any…
At the LHC, a TeV-scale leptoquark (LQ) that decays dominantly to a top quark ($t$) and a light charged lepton ($\ell=e,μ$) would form a resonance system of \emph{boosted-$t$ $+$ high-$p_{\rm T}$-$\ell$}. We consider all possible vector LQ models within the Buchmüller-Rückl-Wyler classifications with the desired decay. We propose simple phenomenological Lagrangians that are suitable for bottom-up/experimental studies and, at the same time, can cover the relevant parameter spaces of these models. In this simplified framework, we study the pair and single production channels of vector LQs at the…
This report of the BOOST2012 workshop presents the results of four working groups that studied key aspects of jet substructure. We discuss the potential of the description of jet substructure in first-principle QCD calculations and study the accuracy of state-of-the-art Monte Carlo tools. Experimental limitations of the ability to resolve substructure are evaluated, with a focus on the impact of additional proton proton collisions on jet substructure performance in future LHC operating scenarios. A final section summarizes the lessons learnt during the deployment of substructure analyses in se…
We study high-pt jets from QCD and from highly-boosted massive particles such as tops, W, Z and Higgs, and argue that infrared-safe observables can help reduce QCD backgrounds. Jets from QCD are characterized by different patterns of energy flow compared to the products of highly-boosted heavy particle decays, and we employ a variety of jet shapes, observables restricted to energy flow within a jet, to explore this difference. Results from Monte Carlo generators and arguments based on perturbation theory support the discriminating power of the shapes we refer to as planar flow and angularities…
We briefly review common tools and methods to identify boosted, hadronically decaying top quarks at the LHC experiments. This includes generic jet substructure variables, specific top identification algorithms, and recent developments in deep learning techniques.
In supersymmetric extensions of the Standard Model, the superpartners of the top quark (stops) play the crucial role in addressing the naturalness problem. For direct pair-production of stops with each stop decaying into a top quark plus the lightest neutralino, the standard stop searches have difficulty finding the stop for a compressed spectrum where the mass difference between the stop and the lightest neutralino is close to the top quark mass, because the events look too similar to the large $t\bar{t}$ background. With an additional hard ISR jet, the two neutralinos from the stop decays ar…
The University of British Columbia（不列颠哥伦比亚大学）；McGill University（麦吉尔大学）；Stanford University（斯坦福大学）
Multivariate techniques based on engineered features have found wide adoption in the identification of jets resulting from hadronic top decays at the Large Hadron Collider (LHC). Recent Deep Learning developments in this area include the treatment of the calorimeter activation as an image or supplying a list of jet constituent momenta to a fully connected network. This latter approach lends itself well to the use of Recurrent Neural Networks. In this work the applicability of architectures incorporating Long Short-Term Memory (LSTM) networks is explored. Several network architectures, methods …
We analyse the semileptonic decay of a polarised top-quark with a large velocity based on the perturbative QCD factorisation framework. Thanks to the factorisation and the spin decomposition, the production part and the decay part can be factorised and the spin dependence is introduced in the decay part. The decay part is converted to the top-jet function which describes the distribution of jet observables and the spin is translated to the helicity of the boosted top. Using this top-jet function, the energy profile of b-jet is investigated and it is turned out that the sub-jet energy for the h…
Triple gauge boson production at the LHC can be used to test the robustness of the Standard Model and provide useful information for VBF di-boson scattering measurement. Especially, any derivations from SM prediction will indicate possible new physics. In this paper we present a detailed Monte Carlo study on measuring WWW production in pure leptonic and semileptonic decays, and probing anomalous quartic gauge WWWW couplings at the CERN LHC and future hadron collider, with parton shower and detector simulation effects taken into account. Apart from cut-based method, multivariate boosted decisio…
Measuring top polarization is extremely important in understanding any new physics that couples to top quark. Here, we take up the task of extracting polarization of top quark coming from TeV-scale stop decay at the upgraded high luminosity and high energy LHC. We focus on the scenario where the two tops from the stop decay lead to semi-leptonic final state. We take into account all relevant backgrounds and put kinematical cuts to suppress them. We arrive at a signal region where the desired signal can be observed with reasonable significance. Next, we examine, in such signal regions, how well…
In this paper, we study the extended Standard Model (SM) with an extra Higgs doublet and right-handed neutrinos. If the symmetry to distinguish the two Higgs doublets is not assigned, flavor changing neutral currents (FCNCs) involving the scalars are predicted even at the tree level. We investigate the constraints on the FCNCs at the one-loop level, and especially study the semileptonic $B$ meson decays, e.g. $B \to D^{(*)} τν$ and $B \to K^{(*)} ll$ processes, where the SM predictions are more than $2 σ$ away from the experimental results. We also consider the flavor-violating couplings invol…
An overview of the most recent measurements on top quark charge asymmetry in top quark pair production is presented. The results are obtained using data collected with ATLAS and CMS detectors in proton-proton collisions at centre-of-mass energies of 8 TeV. In these studies, either dileptonic or semileptonic top pair decays are analyzed. All measurements are comparable with the standard model prediction and no sign of new physics is observed.
We present a resummation-improved prediction for $VH$ + 0 jets production at the Large Hadron Collider. We focus on highly-boosted final states in the presence of jet veto to suppress the $t{\bar t}$ background. In this case, conventional fixed-order calculations are plagued by the existence of large Sudakov logarithms $α_s^n \log^m (p_T^{veto}/Q)$ for $Q\sim m_V + m_H$ which lead to unreliable predictions as well as large theoretical uncertainties, and thus limit the accuracy when comparing experimental measurements to the Standard Model. In this work, we show that the resummation of Sudakov …
Collider signals of dark photons are an exciting probe for new gauge forces and are characterized by events with boosted lepton jets. Existing techniques are efficient in searching for muonic lepton jets but due to substantial backgrounds have difficulty constraining lepton jets containing only electrons. This is unfortunate since upcoming intensity frontier experiments are sensitive to dark photon masses which only allow electron decays. Analyzing a recently proposed model of kinetic mixing, with new scalar particles decaying into dark photons, we find that existing techniques for electron je…
A method is proposed to directly probe the Higgs boson compositeness using the unique characteristics of a boosted Higgs boson produced in association with a weak gauge boson ($W^{\pm},Z$). The discovery potential for the upcoming LHC running is presented, showing that compositeness scales up to 3 TeV can be probed at the LHC with an integrated luminosity of $\mathcal{L}=3000$ fb$^{-1}$ collected at $\sqrt{s}=13$ TeV.
We analyze the observational potential of single $t'$ production in both the $t^\prime \rightarrow bW$ and $t^\prime \rightarrow th$ decay channels at 8 TeV LHC using an integrated luminosity of 25 $\text{fb}^{-1}$. Our analysis is based on a simplified model with minimal coset $SO(5)/SO(4)$ in which the $t'$ is a singlet of the unbroken SO(4). The single $t'$ production, as a consequence of electroweak symmetry breaking, is less kinematically suppressed, associated with a light forward jet and has boosted decay products at the 8 TeV LHC. Therefore it provides the most promising channel in sea…
The conventional way to search for long-lived CHArged Massive Particles (CHAMPs) is to identify slow (small $β$) tracks using delayed time of flight and high ionization energy loss. But at the 7-14 TeV center of mass energy of the LHC, a CHAMP may be highly boosted (high $β$) and therefore look more like a minimum ionizing particle, while for high momentum muons (more than $\sim$500 GeV/c) the radiative effect dominates energy deposition. This suggests a new strategy to search for CHAMPs at the LHC. Using energy deposition from different detector components, we construct a boosted decision tre…
The LHC search strategies for leptoquarks that couple dominantly to a top quark are different than for the ones that couple mostly to the light quarks. We consider charge $1/3$ ($ϕ_1$) and $5/3$ ($ϕ_5$) scalar leptoquarks that can decay to a top quark and a charged lepton ($t\ell$) giving rise to a resonance system of a boosted top and a high-$p_{\rm T}$ lepton. We introduce simple phenomenological models suitable for bottom-up studies and explicitly map them to all possible scalar leptoquark models within the Buchmüller-Rückl-Wyler classifications that can have the desired decays. We study pa…
The Semileptonic B Frontier: LHC, B-factories, Lattice QCD & the Hunt for Precision
INFN, Sezione di Bari（意大利国家核物理研究所巴里分部）；Laboratori Nazionali di Frascati（弗拉斯卡蒂国家实验室）；PRISMA ++ Cluster of Excellence & Mainz Institute for Theoretical Physics, Johannes Gutenberg University（约翰内斯·古腾堡大学）；ICCUB, Universitat de Barcelona（巴塞罗那大学）；Department of Physics and Astronomy, University of Manchester（曼彻斯特大学）；Aix Marseille Univ, CNRS/IN2P3, CPPM（艾克斯-马赛大学）；Dipartimento di Fisica e Astronomia Galileo Galilei & INFN, Università di Padova（帕多瓦大学）；Dipartimento di Fisica & INFN, Università di Torino（都灵大学）；INFN, Sezione di Roma Tre（意大利国家核物理研究所罗马第三大学分部）
The CP nature of the Higgs coupling to top quarks is addressed in this paper, in single charged lepton final states of $t\bar{t}h$ events produced in proton-proton collisions at the LHC. Pure scalar ($h=H$) and pseudo-scalar ($h=A$) Higgs boson signal events, generated with MadGraph5_aMC@NLO, are fully reconstructed using a kinematic fit. Angular distributions of the decay products, as well as CP-sensitive asymmetries, are exploited to separate and gain sensitivity to possible pseudo-scalar components of the Higgs boson and reduce the contribution from the dominant irreducible background $t\ba…
Top polarization is an important probe of new physics that couples to the top sector, and which may be discovered at the 14 TeV LHC. Taking the example of the MSSM, we argue that top polarization measurements can put a constraint on the soft supersymmetry breaking parameter A_t. In light of the recent discovery of a Higgs-like boson of mass ~125 GeV, a large A_t is a prediction of many supersymmetric models. To this end, we develop a *detector level* analysis methodology for extracting polarization information from hadronic tops using boosted jet substructure. We show that with 100 fb^(-1) of …
This is the summary report of the Top Quark Kinematics working group prepared for Snowmass 2013. We survey the current state of theoretical predictions for top pair differential distributions, in both boosted and un-boosted regimes, and present an overview of uncertainties and prospects for top spin correlations. We study the prospects for measuring the inclusive SM top pair production asymmetry AFC at LHC 14 as a function of systematic error, and show that some improvement over current systematic uncertainties, as customarily handled, is required for observing a SM-size asymmetry. Cuts on top…
We demonstrate the performance of a very efficient tagger applies on hadronically decaying top quark pairs as signal based on deep neural network algorithms and compares with the QCD multi-jet background events. A significant enhancement of performance in boosted top quark events is observed with our limited computing resources. We also compare modern machine learning approaches and perform a multivariate analysis of boosted top-pair as well as single top quark production through weak interaction at $\sqrt{s}=$14 TeV proton-proton Collider. The most relevant known background processes are inco…
