QCD Background Research Articles

Jet substructure by accident

Abstract We propose a new search strategy for high-multiplicity hadronic final states. When new particles are produced at threshold, the distribution of their decay products is approximately isotropic. If there are many partons in the final state, it is likely that several will be clustered into the same large-radius jet. The resulting jet exhibits substructure, even though the parent states are not boosted. This “accidental” substructure is a powerful discriminant against background because it is more pronounced for high-multiplicity signals than for QCD multijets. We demonstrate how to take advantage of accidental substructure to reduce backgrounds without relying on the presence of missing energy. As an example, we present the expected limits for several R-parity violating gluino decay topologies. This approach allows for the determination of QCD backgrounds using data-driven methods, which is crucial for the feasibility of any search that targets signatures with many jets and suppressed missing energy.

Searching for dark matter at the LHC with a mono-Z

We investigate a mono-Z process as a potential dark matter search strategy at the LHC. In this channel a single Z boson recoils against missing transverse momentum, attributed to dark matter particles, $\chi$, which escape the detector. This search strategy is related, and complementary to, monojet and monophoton searches. For illustrative purposes we consider the process $q\bar{q} -> \chi\chi Z$ in a toy dark matter model, where the Z boson is emitted from either the initial state quarks, or from the internal propagator. Among the signatures of this process will be a pair of muons with high pT that reconstruct to the invariant mass of the Z, and large amounts of missing transverse energy. Being a purely electroweak signal, QCD and other Standard Model backgrounds are relatively easily removed with modest selection cuts. We compare the signal to Standard Model backgrounds and demonstrate that, even for conservative cuts, there exist regions of parameter space where the signal may be clearly visible above background in future LHC data, allowing either new discovery potential or the possibility of supplementing information about the dark sector beyond that available from other observable channels.

Using jet substructure at the LHC to search for the light Higgs bosons of theCP-violating MSSM

The CP-violating version of the Minimal Supersymmetric Standard Model (MSSM) is an example of a model where experimental data do not preclude the presence of light Higgs bosons in the range around 10 -- 110 GeV. Such light Higgs bosons, decaying almost wholly to b-bbar pairs, may be copiously produced at the LHC, but would remain inaccessible to conventional Higgs searches because of intractable QCD backgrounds. We demonstrate that a significant number of these light Higgs bosons would be boosted strongly enough for the pair of daughter $b$-jet pairs to appear as a single `fat' jet with substructure. Tagging such jets could extend the discovery potential at the LHC into the hitherto-inaccessible region for light Higgs bosons.

Next-to-leading order QCD corrections to electroweak Zjj production in the POWHEG BOX

We present an implementation of electroweak Z-boson production in association with two jets at hadron colliders in the POWHEG framework, a method that allows the interfacing of NLO-QCD calculations with parton-shower Monte Carlo programs. We focus on the leptonic decays of the weak gauge boson, and take photonic and non-resonant contributions to the matrix elements fully into account. We provide results for observables of particular importance for the suppression of QCD backgrounds to vector-boson fusion processes by means of central-jet-veto techniques. While parton-shower effects are small for most observables associated with the two hardest jets, they can be more pronounced for distributions that are employed in central-jet-veto studies.

Revisiting the Twist-3 Distribution Amplitudes of K Meson Within the QCD Background Field Approach

In the present paper, we investigate the kaon twist-3 distribution amplitudes (DAs) φKp,σ within the QCD background field approach. The SUf (3)-breaking effects are studied in detail under a systematical way, especially the sum rules for the moments of φKp,σ are obtained by keeping all the mass terms in the s-quark propagator consistently. After adding all the uncertainties in quadrature, the first two Gegenbauler moments of φKp,σ are a1K,p(1 GeV) = − 0.376+0.103−0.148, a2K,p(1 GeV) = 0.701+0.481−0.491, a1K,σ(1 GeV) = −0.160+0.051−0.074, and a2K,σ(1 GeV) = 0.369+0.163−0.149, respectively. Their normalization parameters μpK|1 GeV = 1.188+0.039−0.043 GeV and μσK|1 GeV = 1.021+0.036−0.055 GeV. A detailed discussion on the properties of φKp,σ moments shows that the higher-order s-quark mass terms can indeed provide sizable contributions. Furthermore, based on the newly obtained moments, a model for the kaon twist-3 wavefunction ΨKp,σ (x, k⊥) with a better end-point behavior is constructed, which shall be useful for perturbative QCD calculations. As a byproduct, we make a discussion on the properties of the pion twist-3 DAs.

Tracking the identities of boosted particles

We show that the tracking system in a collider detector can be used to efficiently identify boosted massive particles from their QCD backgrounds. We examine variables defined with tracking information which are sensitive to jet radiation patterns, including charged particle multiplicity and $N$-subjettiness. These variables are barely correlated with variables sensitive to the hard splitting scale in the jet, such as the filtered jet mass. Therefore these two kinds of variables should be combined to optimize the discriminating power. We illustrate the method with $W$ jet tagging. It is shown that for jet ${p}_{T}=500\text{ }\text{ }\mathrm{GeV}$, one can gain a factor of 1.6 in statistical significance by combining filtered jet mass and charged particle multiplicity, over filtered mass alone. Adding $N$-subjettiness increases the factor to 1.8.

Associated Higgs plus vector boson test of a fermiophobic Higgs boson

Production in association with an electroweak vector boson V is a distinctive mode of production for a Higgs boson H without tree-level couplings to fermions, known as a fermiophobic Higgs boson. We focus on HV associated production with H decay into a pair of photons, and V into a pair of jets, with the goal of distinguishing a fermiophobic Higgs boson from the standard model Higgs boson. Performing a simulation of the signal and pertinent QCD backgrounds, and using the same event selection cuts employed by the LHC ATLAS Collaboration, we argue that existing LHC data at 7 TeV with 4.9 fb^{-1} of integrated luminosity may distinguish a fermiophobic Higgs boson from a standard model Higgs boson near 125 GeV at about 1.9 standard deviation signal significance (1.9 sigma) per experiment. At 8 TeV we show that associated production could yield 2.8 sigma significance per experiment with 10 fb^{-1} of data.

LHC discovery potential for non-standard Higgs bosons in the 3b channel

Abstract In a variety of well motivated models, such as two Higgs Doublet Models (2HDMs) and the Minimal Supersymmetric Standard Model (MSSM), there are neutral Higgs bosons that have significantly enhanced couplings to b-quarks and tau leptons in comparison to those of the SM Higgs. These so called non-standard Higgs bosons could be copiously produced at the LHC in association with b quarks, and subsequently decay into b-quark pairs. However, this production channel suffers from large irreducible QCD backgrounds. We propose a new search strategy for non-standard neutral Higgs bosons at the 7 TeV LHC in the 3b’s final state topology. We perform a simulation of the signal and backgrounds, using state of the art tools and methods for different sets of selection cuts, and conclude that neutral Higgs bosons with couplings to b-quarks of about 0.3 or larger, and masses up to 400 GeV, could be seen with a luminosity of 30 fb−1. In the case of the MSSM we also discuss the complementarity between the 3b channel and the inclusive tau pair channel in exploring the supersymmetric parameter space.

Three-particle templates for a boosted Higgs boson

We explore the ability of three-particle templates to distinguish color neutral objects from QCD background. This method is particularly useful to identify the standard model Higgs, as well as other massive neutral particles. Simple cut-based analysis in the overlap distributions of the signal and background is shown to provide a significant rejection power. By combining with other discriminating variables, such as planar flow, and several variables that depend on the partonic template, three-particle templates are used to characterize the influence of gluon emission and color flow in collider events. The performance of the method is discussed for the case of a highly boosted Higgs in association with a leptonically-decaying W boson.

Real time algorithms in the ATLAS tau trigger system at 7 TeV center of mass energy

The ATLAS hadronic tau trigger plays an important role in many analyses. Among these analyses are searches for H0, H±, W′, and Z′ in the tau decay channel. In order to achieve the needed sensitivity in these measurement it is important to reduce the QCD background, but at the same time to keep the signal efficiency high. Furthermore it is important to understand the trigger efficiency in real data. This paper summarizes the performance of the tau trigger in data collected by the ATLAS detector in 2011.

Four jets as a probe of O(100 GeV) physics beyond standard model at hadron colliders

O(100 GeV) physics beyond the standard model could be overlooked provided that it is hidden in jets or the untouched Higgs sector. The top-quark forward-backward asymmetry measurements as well as dijet bump, which is observed by CDF in association with charged lepton plus missing energy (supposed arising from $W$ decay), may indicate the existence of a new color-octet axial vector ${Z}_{C}$ with a mass of about 145 GeV. Here, ${Z}_{C}$ decays into two jets with a certain branching ratio. In this paper, we investigated the possibility to discover ${Z}_{C}$ pair via analyzing the four jets as the final states, which are heavily polluted by huge QCD background. Our simulation showed that, however, both Tevatron and LHC have the excellent chance to discover ${Z}_{C}$ through analyzing the four-jet events with accessible integrated luminosity and good control of QCD background.

Identify charged Higgs boson inW±H∓associated production at the LHC

We investigate the possibility to discover the charged Higgs via $pp\to W^{\pm}H^{\mp}\to l+\met+b\bar{b}jj$ process at LHC, which suffers from large QCD backgrounds. We optimize the kinematic cuts to suppress the backgrounds, so that the reconstruction of the charged Higgs through hadronic decay is possible. The angular distribution of the b-jet from $H^{\pm}$ decay is investigated as a way to identify the charged scalar from vector bosons.

The FastTracker Real Time Processor and Its Impact on Muon Isolation, Tau and b-Jet Online Selections at ATLAS

As the LHC luminosity is ramped up to 3 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\,\times 10^{34}~{\hbox {cm}}^{2}~s^{1}$</tex></formula> and beyond, the high rates, multiplicities, and energies of particles seen by the detectors will pose a unique challenge. Only a tiny fraction of the produced collisions can be stored offline and immense real-time data reduction is needed. An effective trigger system must maintain high trigger efficiencies for the physics we are most interested in while suppressing the enormous QCD backgrounds. This requires massive computing power to minimize the online execution time of complex algorithms. A multi-level trigger is an effective solution to meet this challenge. The Fast Tracker (FTK) is an upgrade to the current ATLAS trigger system that will operate at full Level-1 output rates and provide high-quality tracks reconstructed over the entire inner detector by the start of processing in the Level-2 Trigger. FTK solves the combinatorial challenge inherent to tracking by exploiting the massive parallelism of associative memories that can compare inner detector hits to millions of pre-calculated patterns simultaneously. The tracking problem within matched patterns is further simplified by using pre-computed linearized fitting constants and relying on fast DSPs in modern commercial FPGAs. Overall, FTK is able to compute the helix parameters for all tracks in an event and apply quality cuts in less than 100 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\hbox {s}}$</tex> </formula> . The system design is defined and the performance presented with respect to high transverse momentum (high- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$p_{\rm T}$</tex></formula> ) Level-2 objects: b jets, tau jets, and isolated leptons. We test FTK algorithms using the full ATLAS simulation with WH events up to <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$3\times 10^{34}~{\hbox {cm}}^{2}{\hbox {s}}^{1}$</tex></formula> luminosity and compare the FTK results with the offline tracking capability. We present the architecture and the reconstruction performance for the mentioned high- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$p_{\rm T}$</tex></formula> Level-2 objects.

PQCD physics of multiparton interactions

We study production of two pairs of jets in %hard hadron--hadron collisions in view of extracting contribution of {\em double hard interactions} of three and four partons ($3\to4$, $4\to4$). Such interactions, in spite of being power suppressed at the level of the total cross section, become comparable with the standard hard collisions of two partons, $2\to4$, in the {\em back-to-back kinematics} when the transverse momentum imbalances of two pairing jets are relatively small. We express differential and total cross sections for two-dijet production in double parton collisions through the generalized two-parton distributions, $_2$GPDs \cite{BDFS1}, that contain large-distance two-parton correlations of non-perturbative origin as well as small-distance correlations due to parton evolution. We find that these large- and small-distance correlations participate in different manner in 4-jet production, and treat them in the leading logarithmic approximation of pQCD that resums collinear logarithms in all orders. A special emphasis is given to $3\to4$ double hard interaction processes that occur as an interplay between large- and short-distance parton correlations and were not taken into consideration by approaches inspired by the parton model picture. We demonstrate that the $3\to4$ mechanism, being of the same order in $\as$ as the $4\to4$ process, turns out to be {\em geometrically enhanced} compared to the latter and should contribute significantly to 4-jet production. The framework developed here takes into systematic consideration perturbative $Q^2$ evolution of $_2$GPDs. It can be used as a basis for future analysis of NLO corrections to multi-parton interactions (MPI) at LHC and Tevatron colliders, in particular for improving evaluation of QCD backgrounds to new physics searches.

A unique signal of excited bosons in dijet data from pp collisions

With this note we would like to draw attention to a possible novel signal of new physics in dijet data at the hadron colliders. Usually it is accepted that almost all exotic models predict that these two jets populate the central (pseudo)rapidity region where y 1,2 ≅ 0. Contrary, the excited bosons do not contribute into this region, but produce an excess of dijet events over the almost flat QCD background in χ = exp|y 1−y 2| away from this region. For the special choice of parameters this could lead to a dip in the centrality ratio distribution over the dijet invariant mass instead of a bump, expected for the most exotic models.

Hard QCD Results with Jets at the LHC

Hard QCD results in proton-proton collisions at sqrt(s) = 7 TeV with jets from data recorded up to the end of 2010 by the CMS and ATLAS experiments at the LHC are reported. Inclusive jet and di-jet cross section measurements as well as observables sensitive to multi-jet activity are shown and compared to simulations based on leading log parton showers as well as NLO QCD predictions. Novel approaches to identify highly boosted massive final states by exploiting the jet substructure are tested on the dominant QCD background.

SEARCHES FOR DIJET RESONANCES AT HADRON COLLIDERS

We review the experimental searches for new particles in the dijet mass spectrum conducted at the CERN [Formula: see text], the Fermilab Tevatron Collider, and the CERN Large Hadron Collider. The theory of the QCD background and new particle signals is reviewed, with emphasis on the choices made by the experiments to model the background and signal. The experimental techniques, data, and results of dijet resonance searches at hadron colliders over the last quarter century are described and compared. Model independent and model specific limits on new particles decaying to dijets are reviewed, and a detailed comparison is made of the recently published limits from the ATLAS and CMS experiments.

Forward dijets and search for new physics at the LHC

Forward dijets and possible signals of hypothetical extra-dimension graviton at the LHC are discussed. Studies for large mass dijet production at s = 7 and 14 TeV in forward rapidity region are presented with a model of extra-dimension graviton t-channel exchange in trans-Planckian scattering regime. The model is implemented into Monte Carlo event generator. QCD background for dijets with large effective masses was studied with different Monte Carlo event generators, including BFKL-motivated generators.

First Search for Multijet Resonances ins=1.96 TeVpp¯Collisions

We present the first model independent search for three-jet hadronic resonances within multijet events in $\sqrt{s} = 1.96$ TeV $p\bar{p}$ collisions at the Fermilab Tevatron using the CDF II detector. Pair production of supersymmetric gluinos and squarks with hadronic R-parity violating decays is employed as an example of a new physics benchmark for this signature. Selection criteria based on the kinematic properties of an ensemble of jet combinations within each event help to extract signal from copious QCD background. Our background estimates include all-hadronic $t\bar{t}$ decays that have a signature similar to the signal. No significant excess outside the top quark mass window is observed in data with an integrated luminosity of 3.2 fb$^{-1}$. We place 95% confidence level limits on the production cross section $\sigma(p \bar{p} \rightarrow XX^{'}) \times \rm{BR}$($\widetilde{g}\widetilde{g}$$ \rightarrow 3 {\rm jet}+3 {\rm jet})$ where $X, X' = \widetilde{g}, \widetilde{q}$, or $\widetilde{\bar{q}}$, with $\widetilde{q}$, $\widetilde{\bar{q}} \rightarrow \widetilde{g}$ + jet, as a function of gluino mass, in the range of 77 GeV/$c^2$ to 240 GeV/$c^2$.

Search for Higgs bosons in supersymmetric cascade decays and neutralino dark matter

The Minimal Supersymmetric Extension of the Standard Model (MSSM) is a well motivated theoretical framework, which contains an extended Higgs sector, including a light Higgs with Standard Model-like properties in most of the parameter space. Due to the large QCD background, searches for such a Higgs, decaying into a pair of bottom quarks, is very challenging at the LHC. It has been long realized that the situation may be ameliorated by searching for Higgs bosons in supersymmetric decay chains. Moreover, it has been recently suggested that the bobber decay channel may be observed in standard production channels by selecting boosted Higgs bosons, which may be easily identified from the QCD background. Such boosted Higgs bosons are frequent in the MSSM, since they are produced from decays of heavy colored supersymmetric particles. Previous works have emphasized the possibility of observing boosted Higgs bosons in the light higgsino region. In this work, we study the same question in the regions of parameter space consistent with a neutralino dark matter relic density, analyzing its dependence on the non-standard Higgs boson, slepton and squark masses, as well as on the condition of gaugino mass unification. In general, we conclude that, provided sleptons are heavier than the second lightest neutralinos, the presence of boosted Higgs is a common MSSM feature, implying excellent prospects for observation of the light MSSM Higgs boson in the near future.