Boosted Jets Research Articles

Anomaly detection from mass unspecific jet tagging

We introduce a novel anomaly search method based on (i) jet tagging to select interesting events, which are less likely to be produced by background processes; (ii) comparison of the untagged and tagged samples to single out features (such as bumps produced by the decay of new particles) in the latter. We demonstrate the usefulness of this method by applying it to a final state with two massive boosted jets: for the new physics benchmarks considered, the signal significance increases an order of magnitude, up to a factor of 40. We compare to other anomaly detection methods in the literature and discuss possible generalisations.

Search for resonances decaying to a pair of Higgs bosons in the b overline{mathrm{b}} q overline{mathrm{q}} \u2019\u2113\u03bd final state in proton-proton collisions at sqrt{s} = 13 TeV

A search for new massive particles decaying into a pair of Higgs bosons in proton-proton collisions at a center-of-mass energy of 13 TeV is presented. Data were collected with the CMS detector at the LHC, corresponding to an integrated luminosity of 35.9 fb−1. The search is performed for resonances with a mass between 0.8 and 3.5 TeV using events in which one Higgs boson decays into a bottom quark pair and the other decays into two W bosons that subsequently decay into a lepton, a neutrino, and a quark pair. The Higgs boson decays are reconstructed with techniques that identify final state quarks as substructure within boosted jets. The data are consistent with standard model expectations. Exclusion limits are placed on the product of the cross section and branching fraction for generic spin-0 and spin-2 massive resonances. The results are interpreted in the context of radion and bulk graviton production in models with a warped extra spatial dimension. These are the best results to date from searches for an HH resonance decaying to this final state, and they are comparable to the results from searches in other channels for resonances with masses below 1.5 TeV.

Search for pair production of Higgs bosons in the boverline{b}boverline{b} final state using proton-proton collisions at sqrt{s}=13 TeV with the ATLAS detector

A search for Higgs boson pair production in the boverline{b}boverline{b} final state is carried out with up to 36.1 fb−1 of LHC proton-proton collision data collected at sqrt{s}=13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260–3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to boverline{b}boverline{b} are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.

Computing N -subjettiness for boosted jets

Jet substructure tools have proven useful in a number of high-energy particle-physics studies. A particular case is the discrimination, or tagging, between a boosted jet originated from an electroweak boson (signal), and a standard QCD parton (background). A common way to achieve this is to cut on a measure of the radiation inside the jet, i.e. a jet shape. Over the last few years, analytic calculations of jet substructure have allowed for a deeper understanding of these tools and for the development of more efficient ones. However, analytic calculations are often limited to the region where the jet shape is small. In this paper we introduce a new approach in perturbative QCD to compute jet shapes for a generic boosted jets, waiving the above limitation. We focus on an example common in the substructure literature: the jet mass distribution after a cut on the N -subjettiness τ21 ratio, extending previous works to the region relevant for phenomenology. We compare our analytic predictions to Monte Carlo simulations for both plain and SoftDrop-groomed jets. We use our results to construct analytically a decorrelated tagger.

Search for pair production of vector-like T and B quarks in single-lepton final states using boosted jet substructure in proton-proton collisions at sqrt{s}=13 TeV

A search for pair production of massive vector-like T and B quarks in proton-proton collisions at sqrt{s}=13 TeV is presented. The data set was collected in 2015 by the CMS experiment at the LHC and corresponds to an integrated luminosity of up to 2.6 fb−1. The T and B quarks are assumed to decay through three possible channels into a heavy boson (either a W, Z or Higgs boson) and a third generation quark. This search is performed in final states with one charged lepton and several jets, exploiting techniques to identify W or Higgs bosons decaying hadronically with large transverse momenta. No excess over the predicted standard model background is observed. Upper limits at 95% confidence level on the T quark pair production cross section are set that exclude T quark masses below 860 GeV in the singlet, and below 830 GeV in the doublet branching fraction scenario. For other branching fraction combinations with ℬ(T → tH) + ℬ(T → bW) ≥ 0.4, lower limits on the T quark range from 790 to 940 GeV. Limits are also set on pair production of singlet vector-like B quarks, which can be excluded up to a mass of 730 GeV. The techniques showcased here for understanding highly-boosted final states are important as the sensitivity to new particles is extended to higher masses.

Searching for charged Higgs bosons with boosted top and boosted bottom jets

At moderate values of tanβ , a supersymmetric charged Higgs boson H ± is difficult to find due its small cross section and large backgrounds. Using realistic boosted tagging rates, we present preliminary predictions for the reach for TeV-scale charged Higgs bosons at 14 TeV and 100 TeV colliders in top-Higgs associated production. We conclude that moderate values of tanβ will be possible to probe at a 100 TeV collider.

Jet shapes for boosted jet two-prong decays from first-principles

Several boosted jet techniques use jet shape variables to discriminate the multi-pronged signal from Quantum Chromodynamics backgrounds. In this paper, we provide a first-principles study of an important class of jet shapes all of which put a constraint on the subjet mass: the mass-drop parameter ($\mu^2$), the $N$-subjettiness ratio ($\tau_{21}^{(\beta=2)}$) and energy correlation functions ($C_2^{(\beta=2)}$ or $D_2^{(\beta=2)}$). We provide analytic results both for QCD background jets as well as for signal processes. We further study the situation where cuts on these variables are applied recursively with Cambridge-Aachen de-clustering of the original jet. We also explore the effect of the choice of axis for $N$-subjettiness and jet de-clustering. Our results bring substantial new insight into the nature, gain and relative performance of each of these methods, which we expect will influence their future application for boosted object searches.

Resolving boosted jets with XCone

We show how the recently proposed XCone jet algorithm smoothly interpolates between resolved and boosted kinematics. When using standard jet algorithms to reconstruct the decays of hadronic resonances like top quarks and Higgs bosons, one typically needs separate analysis strategies to handle the resolved regime of well-separated jets and the boosted regime of fat jets with substructure. XCone, by contrast, is an exclusive cone jet algorithm that always returns a fixed number of jets, so jet regions remain resolved even when (sub)jets are overlapping in the boosted regime. In this paper, we perform three LHC case studies---dijet resonances, Higgs decays to bottom quarks, and all-hadronic top pairs---that demonstrate the physics applications of XCone over a wide kinematic range.

Top Partner Discovery in the T → tZ channel at the LHC

In this paper we study the discovery potential of the LHC run II for heavy vector-like top quarks in the decay channel to a top and a $Z$ boson. Despite the usually smaller branching ratio compared to charged-current decays, this channel is rather clean and allows for a complete mass reconstruction of the heavy top. The latter is achieved in the leptonic decay channel of the $Z$ boson and in the fully hadronic top channel using boosted jet and jet substructure techniques. To be as model-independent as possible, a simplified model approach with only two free parameters has been applied. The results are presented in terms of parameter space regions for $3\sigma$ evidence or $5\sigma$ discovery for such new states in that channel.

TemplateTagger v1.0.0: A template matching tool for jet substructure

TemplateTagger is a C++ package for jet substructure analysis with Template Overlap Method. The code operates with arbitrary models within fixed-order perturbation theory and arbitrary kinematics. Specialized template generation classes allow the user to implement any model for a decay of a boosted heavy object. In addition to Template Overlap, the code provides ability to calculate other template shape and energy flow observables. We describe in detail the structure of the package, as well as provide examples of its usage. Program summaryProgram title: Template Tagger v.1.0.0Catalogue identifier: AERZ_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AERZ_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License, version 3No. of lines in distributed program, including test data, etc.: 112534No. of bytes in distributed program, including test data, etc.: 1812748Distribution format: tar.gzProgramming language: C++.Computer: PC/Mac.Operating system: Linux/Unix/Mac OS X.RAM: 50 MBClassification: 11.External routines: The code uses the PseudoJet library of the FastJet package (http://fastjet.fr/)Nature of problem:Traditional jet reconstruction techniques are inadequate to properly characterize boosted decays of heavy objects. High luminosity and pileup further complicate the matter as extra soft radiation is deposited within the large jet cones, thus both shifting and broadening the kinematic distributions boosted jets.Solution method:Template Overlap Method attempts to characterize the sub-structure of boosted jets by matching them to the underlying parton level decay configuration. Each boosted jet is analyzed with a large number of possible kinematically allowed structures called templates. The score of the best matching template is used to estimate the likelihood that the event is a heavy boosted jet of interest. The method is robust against pileup as the small template sub-cone size and the insensitivity of Template Overlap to energy depositions outside the template sub-cone limit the ability of pileup to affect the analysis.Restrictions:Depends on the problem one wishes to analyze.Running time:1–100 events per second, depending on the number of templates required for the analysis.

Top polarization and stop mixing from boosted jet substructure

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 data, left and right 600 GeV stops can be distinguished to 4σ, and 800 GeV stops can be distinguished to 3σ.

Top Polarization from Boosted Jet Substructure

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 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 data, left and right 600 GeV stops can be distinguished to 4,5σ, and 800 GeV stops can be distinguished to 3σ.

Implications of the measurement of ultramassive boosted jets at CDF

The CDF Collaboration recently reported an upper limit on boosted top pair production and noted a significant excess above the estimated background of events with two ultramassive boosted jets. We discuss the interpretation of the measurement and its fundamental implications. In case new physics is involved, the most naive contribution is from a new particle produced with a cross section that is a few times higher than that of the top quark and a sizable hadronic branching ratio. We quantify the resulting tension of a possible larger top pair cross section with the absence of excess found in events with one massive boosted jet and missing energy. The measured planar flow distribution shows deviation from CDF's Pythia QCD prediction at high planarity, while we find a somewhat smaller deviation when comparing with other Monte Carlo tools. As a simple toy model, we analyze the case of a light gluino with R-parity violation and show that it can be made consistent with the data.

Implications of the CDF [formula omitted] forward–backward asymmetry for boosted top physics

New physics at a high scale Λ can affect top-related observables at O(1/Λ2) via the interference of effective four quark operators with the SM amplitude. The (u¯γμγ5Tau)(t¯γμγ5Tat) operator modifies the large Mtt¯ forward–backward asymmetry, and can account for the recent CDF measurement. The (u¯γμTau)(t¯γμTat) operator modifies the differential cross section, but cannot enhance the cross section of ultra-massive boosted jets by more than 60%. The hint for a larger enhancement from a recent CDF measurement may not persist future experimental improvements, or may be a QCD effect that is not accounted for by leading order and matched Monte Carlo tools or naive factorization. If it comes from new physics, it may stem from new light states or an O(1/Λ4) new physics effect.

JET RESULTS, JET RECONSTRUCTION, AND UNDERLYING EVENT STUDIES AT CDF

We present recent results of jet studies from the CDF Collaboration. The inclusive jet cross section measurements using different clustering algorithms, dijet production, and vector boson plus jet production measurements, are discussed. We also present the first study of highly boosted and massive jets. The properties of these jets, including the mass of the jet and several other observables, such as angularity and planar flow, are discussed.