The latest progress in calculating electroweak gauge boson production in association with QCD jets at hadron colliders is summarized. Particular emphasis is given to the recently completed QCD one-loop calculations of W+3jets and Wb final states. Furthermore recent developments in improving Monte Carlo event generators by means of combining tree-level matrix elements with parton showers is reviewed.
Top plus jets production at hadron collider allows us to study the couplings of the top quark. In the Standard Model, two single top processes contribute to the top-jets final state. Beyond the Standard Model, additional direct top production can occur. All three processes probe top gauge couplings including flavor mixing. The structure of accompanying QCD jets allows us to separate the direct top signal from the QCD backgrounds as well as to disentangle the three top plus jets production mechanisms orthogonally to the usual bottom tags.
A brief summary of the current status of the next-to-leading order QCD corrections to top quark pair production and the associated production of top anti-top with jet(s) in different configurations, i.e. with one jet, two jets and another top anti-top pair, is presented.
There exist several recent studies of the top-quark CEDM in the context of searching for CP violating signals in top-quark pair production at the LHC. Most of these studies constrain the CEDM either from deviations in the top-pair cross section from its standard model value, or from T-odd asymmetries in the dimuon channel. Motivated by ATLAS and CMS interest, we extend the study of T-odd asymmetries to the lepton plus jets channel. At the parton level, using MadGraph, we identify the most promising signals and their statistical sensitivity. We find that the signals with larger sensitivity to t…
We present results for the differential distributions of jets from non-leptonic decays of polarized top quarks within the Standard Model, including QCD radiative corrections. Our work extends existing results which are only available for semileptonic top quark decays at the parton level. For $t(\uparrow)$ -> b-jet + 2 light jets we compute in particular the QCD-corrected top-spin analysing power of the b-quark jet and the least energetic light jet. The dependence of the results on the choice of the jet recombination scheme is found to be small. In addition we compute the spin analysing power o…
The degenerate stop NLSP and neutralino LSP scenario is well motivated but hard to detect at the collider experiments. We propose a novel signature for detection this scenario at the Large Hadron Collider (LHC) and demonstrate its feasibility.
We present calculations of Higgs boson production via gluon-gluon fusion in association with one or two additional jets at next-to-leading order in QCD. The calculation of H+jet is exact in the treatment of the top-quark mass, whereas for the H+2 jets calculation the two-loop virtual amplitudes are approximated via a reweighting with leading-order mass effects, while keeping all top-quark mass effects in the real radiation contributions. For H+jet production, this study extends a previous calculation, revealing an error in the previous results. For total and differential cross sections, we pre…
Theoretical issues are discussed for the measurement of the top-mass using jets, including perturbative and non-perturbative effects that relate experimental observables to the Lagrangian mass, and appropriate choices for mass schemes. Full account for these issues is given for e+e--> t-tbar using a factorization theorem for event shapes for massive quarks. Implications for the Tevatron top-mass measurement are discussed. A mass-scheme, the "MSR-mass", is introduced which allows for a precise description of observables sensitive to scales R << m, but at the same time does not introduce perturb…
We compute the differential and total rate of the semileptonic decay of polarized top-quarks $t\to \ell ν_\ell + b{\rm jet} + {\rm jet}$ at next-to-leading order (NLO) in the QCD coupling with an off-shell intermediate $W$ boson. We present several normalized distributions, in particular those that reflect the $t$-spin analyzing powers of the lepton, the b-jet and the $W^+$ boson at LO and NLO QCD.
The scalar partner of the top quark is relatively light in many models of supersymmetry breaking. We study the production of top squarks (stops) at the Tevatron collider and their subsequent decay through baryon-number violating couplings such that the final state contains no leptons. Performing a detector-level analysis, we demonstrate that, even in the absence of leptons or missing energy, stop masses upto 210 \gev/c^2 can be accessible at the Tevatron.
We propose a novel model-independent method to search for W' bosons at the Large Hadron Collider by looking at dijets where one jet is identified as a boosted-top jet and another jet is tagged as a boosted-bottom jet. Performing a detector simulated signal and background study, we demonstrate that the reach in effective coupling g' is improved over existing analysis methods by up to factor of 2 for W' masses above 1.5 TeV, and the reach in mass is extended by 600 GeV to 2.6 TeV at an 8 TeV Large Hadron Collider. We introduce a method for distinguishing high-energy b jets from light-quark jets,…
In this talk we present a calculation of Higgs production via gluon fusion in association with two jets, including the full top-quark mass dependence, and compare it to the large top-mass limit. We find that the large top-mass limit is a good approximation as long as the Higgs mass is smaller than the top quark pair mass, and the jet transverse energies are smaller than the top mass. In addition, we compare Higgs production via gluon fusion and via weak-boson fusion, and consider final-state distributions, like the rapidity interval between the jets and the jet-jet azimuthal decorrelation, whi…
We present theoretical predictions for the production of top-quark pairs with up to three jets at the next-to leading order in perturbative QCD. The relevant calculations are performed with Sherpa and OpenLoops. To address the issue of scale choices and related uncertainties in the presence of multiple scales, we compare results obtained with the standard scale HT/2 at fixed order and the MINLO procedure. Analyzing various cross sections and distributions for tt+0,1,2,3 jets at the 13 TeV LHC we find a remarkable overall agreement between fixed-order and MINLO results. The differences are typi…
We review various aspects of jet physics in the context of hadron colliders. We start by discussing the definitions and properties of jets and recent development in this area. We then consider the question of factorization for processes with jets, in particular for cases in which jets are produced in special configurations, like for example in the region of forward rapidities. We review numerous perturbative methods for calculating predictions for jet processes, including the fixed-order calculations as well as various matching and merging techniques. We also discuss the questions related to n…
At the LHC associated top quark and Higgs boson production with a Higgs decay to bottom quarks has long been a heavily disputed search channel. Recently, it has been found to not be viable. We show how it can be observed by tagging massive Higgs and top jets. For this purpose we construct boosted top and Higgs taggers for Standard Model processes in a complex QCD environment.
Unveiling the Secrets of New Physics Through Top Quark Tagging
Institute of Physics, Bhubaneswar（布巴内斯瓦尔物理研究所）；Homi Bhabha National Institute（霍米·巴巴国立研究所）；Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所）；Kaiping Neutrino Research Center（开平中微子研究中心）
本文综述了顶夸克标记在LHC上用于Beyond Standard Model搜索的现状及应用，探讨了基于机器学习的分类方法在区分顶夸克喷注与QCD喷注中的最新进展。
Many new physics models predict resonances with masses in the TeV range which decay into a pair of top quarks. With its large cross section, t\bar t production at the Large Hadron Collider (LHC) offers an excellent opportunity to search for such particles. The identification of very energetic top quarks is crucial in such an analysis. We consider in detail the t\bar t\to\ell^\pmνb\bar bq\bar q' (\ell=e, μ) final state for high p_T top quarks. In this phase space region, two or more of the final state quarks can merge into a single jet due to the large Lorentz boost of the parent top quark. As …
We study the sensitivity of top pair production with six-fermion decay at the LHC to the presence and nature of an underlying Z' boson, accounting for full tree-level Standard Model ttbar interference, with all intermediate particles allowed off-shell. We concentrate on the lepton-plus-jets final state and simulate experimental conditions, including kinematic requirements and top quark pair reconstruction in the presence of missing transverse energy and combinatorial ambiguity in jet-top assignment. We focus on the differential mass spectra of the cross section and asymmetry observables, espec…
A method of top tagging is introduced. Using the anti-kt algorithm to define jets, events with nj = 2 fat jets of cone size R = 1.5 are decomposed into R = 0.6 sub-jets and retained if nj (R= 0.6) \geq 4 . One pair of sub-jets reconstructs the W-mass and another jet is tagged as a b-jet, as necessary for hadronic or semileptonic events of tt origin. This 'hollow cone' method distinguishes the tt events from the light parton QCD dijet events. Our simulations are made for the LHC at 7 TeV.
A method is introduced for distinguishing top jets (boosted, hadronically decaying top quarks) from light quark and gluon jets using jet substructure. The procedure involves parsing the jet cluster to resolve its subjets, and then imposing kinematic constraints. With this method, light quark or gluon jets with pT ~ 1 TeV can be rejected with an efficiency of around 99% while retaining up to 40% of top jets. This reduces the dijet background to heavy t-tbar resonances by a factor of ~10,000, thereby allowing resonance searches in t-tbar to be extended into the all-hadronic channel. In addition,…
In this talk the application of the recently introduced methods to merge NLO calculations of successive jet multiplicities to the production of top pairs in association with jets will be discussed, in particular a fresh look is taken at the top quark forward-backward asymmetries. Emphasis will be put on the achieved theoretical accuracy and the associated perturbative and non-perturbative error estimates.
The sensitivity of top quark pair production at the Large Hadron Collider to the presence and nature of an underlying Z' boson is studied, accounting for six-fermion decay with full tree-level Standard Model ttbar interference and all intermediate particles allowed off-shell. Focus is placed on the lepton-plus-jets final state, emulating experimental conditions, including kinematic requirements and top quark pair reconstruction in the presence of missing transverse energy and combinatorial ambiguity in jet-top assignment. Considering a resonance with mass of 4 TeV, and assuming 300 inverse fb …
Real emission corrections to gg -> H, which lead to H+2 jet events, are calculated at order alpha_s^4. Contributions include top-quark triangles, boxes and pentagon diagrams and are evaluated analytically for arbitrary top mass m_t. This new source of H+2 jet events is compared to the weak-boson fusion cross section for a range of Higgs boson masses. The heavy top-mass approximation appears to work well for intermediate Higgs-boson masses, provided that the transverse momenta of the final-state partons are smaller than the top-quark mass.
We present differential cross sections for the production of top-quark pairs in conjunction with up to two jets, computed at next-to leading order in perturbative QCD and consistently merged with a parton shower in the Sherpa+OpenLoops framework. Top quark decays including spin correlation effects are taken into account at leading order accuracy. The calculation yields a unified description of top-pair plus multi-jet production, and detailed results are presented for various key observables at the Large Hadron Collider. A large improvement with respect to the multi-jet merging approach at lead…
Recently, an anomaly of W+jets events at large invariant masses has been reported by CDF. Many interpretations as physics beyond the Standard Model are being offered. We show how such an invariant mass peak can arise from a slight shift in the relative normalization of the top and WW backgrounds.
We perform perturbative QCD factorization of infrared radiations associated with an energetic $b$ quark from a polarized top quark decay, taking the semi-leptonic channel as an example. The resultant formula is expressed as a convolution of an infrared-finite heavy-quark kernel with a $b$-quark jet function. Evaluating the heavy-quark kernel up to leading order in the coupling constant and adopting the jet function from QCD resummation, we predict the dependence of the spin analyzing power for a polarized top quark on the invariant mass of the $b$-quark jet. It is observed that the spin analyz…
We investigate the possibility that the W+4 jets background is a source of the anomalously large top-antitop (tt) charge asymmetry observed at the Tevatron. We simulate the tt reconstruction of the signal and background events at the matrix-element level and find that the reconstructed tt candidates from the W +4 jets background could give large forward-backward asymmetry. We suggest serious re-evaluation of the W+4 jets background for the tt candidate events by studying the distributions of the reconstructed tt systems in their rapidity difference, the transverse momentum of the top quark, an…
We propose a method to identify jets consisting of all the visible remnants of boosted top particles when these decay semileptonically to electrons. Within these jets, the electron shower overlaps with the shower initiated by the $b$ quark, which makes the identification of the electron hard. Even if an electron inside a jet is identified, it is difficult to pinpoint whether the electron rich jet is indeed due to top quark decay or not, since the invisible neutrino carries away a nontrivial part of the energy-momentum of the original top quark. Broadly speaking, the method proposed here has th…
We report the results of a next-to-leading order simulation of top quark pair production in association with two jets. With our inclusive cuts, we show that the corrections with respect to leading order are negative and small, reaching 11%. The error obtained by scale variation is of the same order. Additionally, we reproduce the result of a previous study of top quark pair production in association with a single jet.
A new jet observable, dipolarity, is introduced that can distinguish whether a pair of subjets arises from a color singlet source. This observable is incorporated into the HEPTopTagger and is shown to improve discrimination between top jets and QCD jets for moderate to high pT.
I present an analysis of fully differential single-top-quark production plus jets at next-to-leading order. I describe the effects of jet definitions, top-quark mass, and higher orders on the shapes and normalizations of the kinematic distributions, and quantify all theoretical uncertainties. I explain how to interpret next-to-leading-order jet calculations, and compare them to showering event generators. Using the program ZTOP, I show that HERWIG and PYTHIA significantly underestimate both s-channel and t-channel single-top-quark production, and propose a scheme to match the relevant samples …
In this article we calculate the next-to-leading order (NLO) QCD corrections for single on-shell top-quark production in association with two jets at proton-proton colliders. The tW channel is assumed to be measured independently. The QCD corrections to the inclusive cross section are about 28 (22)\% for top (anti-top) quark production at the 13 TeV LHC. Theoretical errors are dominated by scale uncertainties, which are found to be around 5\% at NLO. Results for various kinematical distributions are also provided using a well-motivated dynamical scale. The QCD corrections are found to have a n…
A method of top tagging is introduced. Using the anti-kt algorithm to define jets, events with nj = 2 fat jets of cone size R = 1.5 are decomposed into R = 0.6 sub-jets and retained if nj (R= 0.6) >= 4 . One pair of sub-jets reconstructs the W-mass and another jet is tagged as a b-jet, as necessary for hadronic or semileptonic events of tt origin. This 'hollow cone' method distinguishes the tt events from the light parton QCD dijet events.
A report on the recent next-to-leading order QCD calculations to ttbb and ttjj at the CERN Large Hardon Collider is given. The elements of the calculation are briefly summarized and results for integrated and differential cross sections are presented.
The production of top-quark pairs that subsequently decay hadronically and leptonically (lepton+jets channel) is one of the key processes for the study of top-quark properties at the LHC. In this article, NLO QCD corrections of order $\mathcal{O}{\left(α_{\rm s}^3α^{4}\right)}$ to the hadronic process ${\rm p}{\rm p}\to μ^-\barν_μ{\rm b}\bar{\rm b} {\rm j} {\rm j}$ are presented. The computation includes off-shell as well as non-resonant contributions, and experimental event selections are used in order to provide realistic predictions. The results are provided in the form of cross sections an…
This paper introduces and applies a method for propagating changes in jet energy corrections into a D0 top quark mass measurement. It will serve as a source for the PRL presenting it's final results, and will be later published in a journal, or as a part of the author's PhD thesis. Motivation for the present measurement arises from the recalibration of the D0 flavor-dependent jet energy corrections. The recalibration was driven by the observation that the corrections for D0 Run IIb differ notably from those of Run IIa. The recalibration analysis was able to show that the Run IIb corrections sh…
We analyze the energy spectra of $single$ b-jets and B-hadrons resulting from the production and decay of top quarks within the SM at the LHC at the NLO QCD. For both hadrons and jets, we calculate the correlation of the peak of the spectrum with the top quark mass, considering the "energy-peak" as an observable to determine the top quark mass. Such a method is motivated by our previous work where we argued that this approach can have reduced sensitivity to the details of the production mechanism of the top quark, whether it is higher-order QCD effects or new physics contributions. As part of …
We develop a new method for tagging jets produced by hadronically decaying top quarks. The method is an application of shower deconstruction, a maximum information approach that was previously applied to identifying jets produced by Higgs bosons that decay to bottom quarks. We tag an observed jet as a top jet based on a cut on a calculated variable that is an approximation to the ratio of the likelihood that a top jet would have the structure of the observed jet to the likelihood that a non-top QCD jet would have this structure. We find that the shower deconstruction based tagger can perform b…
An azimuthal angle correlation between the two hardest jets is studied in the ttbar production process at the 14 TeV LHC. The event samples are generated by merging the tree level matrix elements for the ttbar plus up to 2 or 3 partons with parton showers. The generated event samples show a strong correlation in the azimuthal angle difference between the two hardest jets, as predicted in the analysis based on the tree level matrix elements for the ttbar+2 partons. The effects of merging the matrix elements for the ttbar+3 partons on the correlation are studied in detail. It is found that they …
Fermionic top-partners arise in models such as Composite Higgs and Little Higgs. They modify Higgs properties, in particular how the Higgs couples to top quarks. Alas, there is a low-energy cancellation acting in the coupling of the Higgs boson to gluons and photons. As a result of this cancellation, no information about the spectrum and couplings of the top-partners can be obtained in gluon fusion to Higgs, just the overall new physics scale. In this paper we show that this is not the case when hard radiation is taken into account. Indeed, differential distributions in Higgs plus jets are sen…
We study the reconstruction of high p_T hadronically-decaying top quarks at the LHC. The main challenge in identifying energetic top quarks is that the decay products become increasingly collimated. This reduces the efficacy of conventional methods that exploit the topology of the top decay chain. We focus on the cases where the decay products of the top quark are reconstructed as a single jet, a "top-jet". The most basic "top-tag" method based on jet mass measurement is considered in detail. To analyze the top-tagging method, theoretical and experimental aspects of the QCD jet background are …
We study an interesting region of phase space at the LHC for pair-produced stops decaying into hadronic top quarks and light neutralinos. After imposing a sizeable cut on the missing transverse energy, which is the key variable for reducing backgrounds, we have found that the two hadronic tops are likely to merge into a single fat jet. We develop a jet-substructure-based strategy to tag the two merged top-jets and utilize the MT2 variable to further reduce the backgrounds. We obtain about a 50% increase to the ratio of the signal over background and a mild increase on the signal discovery sign…
We consider top-quarks produced at large energy in e+e- collisions, and address the question of what top-mass can be measured from reconstruction. The production process is characterized by well separated scales: the center-of-mass energy, Q, the top mass, m, the top decay width, Gamma_t, and also LambdaQCD; scales which can be disentangled with effective theory methods. In particular we show how the mass measurement depends on the way in which soft radiation is treated, and that this can shift the mass peak by an amount of order Q LambdaQCD/m. We sum large logs for Q >> m >> Gamma_t > LambdaQ…
Hadronic decays of boosted resonances, e.g., top quark jets, at hadronic super colliders are frequent predictions in TeV-scale extensions of the Standard Model of Particle Physics. In such scenarios, accurate modeling of QCD radiation is necessary for trustworthy predictions. We present the automation of fully differential, next-to-leading-order (NLO) in QCD corrections with parton shower (PS) matching for an effective Left-Right Symmetric Model (LRSM) that features $W_R^\pm, Z_R$ gauge bosons and heavy Majorana neutrinos $N$. Publicly available universal model files require remarkably fewer u…
The top quark is the heaviest known elementary particle of the Standard Model (SM) of particle physics and, therefore, it is expected to have large couplings to hypothetical new physics in many models beyond the SM (BSM). Various studies have predicted the presence of multi-lepton anomalies at the LHC. One of those anomalies is the excess production of two same-sign leptons and three isolated leptons in association with $b$-jets. These are reasonably well described by a 2HDM+$S$ model, where $S$ is a singlet scalar. Both the ATLAS and CMS experiments have reported sustained excesses in these f…
We report on the calculation of the next-to-leading order QCD corrections to the production of ttbar pairs in association with two hard jets at the Fermilab TeVatron and CERN Large Hadron Collider. Results for the integrated and differential cross sections are given. The corrections with respect to leading order are negative and moderate. A study of the scale dependence of our NLO predictions indicates that the residual theoretical uncertainty, due to higher order corrections, is 21% for the TeVatron and 15% for the LHC. In case of the TeVatron, the forward-backward asymmetry of the top quark …
We propose a new method for measuring the top-quark width based on the on-/off-shell ratio of $b$-charge asymmetry in $pp\to Wbj$ production at the LHC. The charge asymmetry removes virtually all backgrounds and related uncertainties, while remaining systematic and theoretical uncertainties can be taken under control by the ratio of cross sections. Limited only by statistical error, in an optimistic scenario, we find that our approach leads to good precision at high integrated luminosity, at a few hundred MeV assuming 300-3000 fb$^{-1}$ at the LHC. The approach directly probes the total width,…
Angular correlation of jets produced in association with a massive scalar, vector or tensor boson is crucial in the determination of their spin and CP properties. We study jet angular correlations in events with a high mass bottom quark pair or a top quark pair and two jets at the LHC, whose cross-section is dominated by the virtual gluon fusion sub-processes when appropriate kinematic selection cuts (vector-boson fusion cuts) are applied. We evaluate helicity amplitudes for sub-processes initiated by qq, qg and gg collisions in the limit where the intermediate gluons are collinear to the init…
In the top quark pair production in association with the Higgs boson decaying to a b quark pair t-tbar H (b-bbar), the final state has an irreducible nonresonant background from the production of a top quark pair in association with a b quark pair t-tbar b-bbar. Therefore, understanding of the t-tbar b-bbar process precisely in particular differential cross-section as functions of the properties of the additional b jets not from the top quark decay is essential for improving the sensitivity of a search for the t-tbar H b-bbar process. The two additional b jets can be identified by using variou…
Multiplicity distributions in QCD jets and jet topics
Key Laboratory of Nuclear Physics and Ion-beam Application (MOE), Institute of Modern Physics, Fudan University（复旦大学现代物理研究所核物理与离子束应用教育部重点实验室）；Shanghai Research Center for Theoretical Nuclear Physics, NSFC and Fudan University（国家自然科学基金委与复旦大学理论物理上海研究中心）；Instituto Galego de Física de Altas Enerxías IGFAE, Universidade de Santiago de Compostela（圣地亚哥德孔波斯特拉大学加利西亚高能物理研究所）；Axencia Galega de Innovación (GAIN), Xunta de Galicia（加利西亚自治区创新署）
