Hadronic Jets Research Articles

Model-independent search for the presence of new physics in events including H → γγ with sqrt{s} = 13 TeV pp data recorded by the ATLAS detector at the LHC

A model-independent search for new physics leading to final states containing a Higgs boson, with a mass of 125.09 GeV, decaying to a pair of photons is performed with 139 fb−1 of sqrt{s} = 13 TeV pp collision data recorded by the ATLAS detector at the Large Hadron Collider at CERN. This search examines 22 final states categorized by the objects that are produced in association with the Higgs boson. These objects include isolated electrons or muons, hadronically decaying τ-leptons, additional photons, missing transverse momentum, and hadronic jets, as well as jets that are tagged as containing a b-hadron. No significant excesses above Standard Model expectations are observed and limits on the production cross section at 95% confidence level are set. Detector efficiencies are reported for all 22 signal regions, which can be used to convert detector-level cross-section limits reported in this paper to particle-level cross-section constraints.

Medium Response and Jet–Hadron Correlations in Relativistic Heavy-Ion Collisions

The study of high-energy heavy-ion collisions at the Relativistic Heavy Ion Collider and the Large Hadron Collider has evolved from a qualitative understanding to the precise extraction of the properties of the quantum chromodynamics medium at extremely high temperatures. Jet quenching has offered unique insights into the transport properties of the quark–gluon plasma (QGP) created in these energetic collisions. Apart from medium modification of jets, jet-induced medium excitation constitutes another crucial aspect of jet–QGP interaction and is indispensable in understanding the soft components of jets. We review recent theoretical and phenomenological developments regarding medium response to jet energy loss, including an overview of both weakly and strongly coupled approaches for describing the thermalization and propagation of energy deposition from jets, effects of medium response on jet observables, and exploration of its unique signatures in jet–hadron correlations. Jet-induced medium excitation is shown to be an essential component in probing the in-medium dynamics of jets and a critical step toward precise extraction of the QGP properties.

Uncovering tau leptons-enriched semi-visible jets at the LHC

This Letter proposes a new signature for confining dark sectors at the Large Hadron Collider. Under the assumption of a QCD-like hidden sector, hadronic jets containing stable dark bound states could manifest in proton-proton collisions. We present a simplified model with a Z' boson yielding the production of jets made up of dark bound states and subsequently leading to the decays of those that are unstable to tau leptons and Standard Model quarks. The resulting signature is characterised by non-isolated tau lepton pairs inside semi-visible jets. We estimate the constraints on our model from existing CMS and ATLAS analyses. We propose a set of variables that leverage the leptonic content of the jet and exploit them in a supervised jet tagger to enhance the signal-to-background separation. Furthermore, we discuss the performance and limitations of current triggers for accessing sub-TeV Z' masses, as well as possible strategies that can be adopted by experiments to access such low mass regions. We estimate that with the currently available triggers, a high mass search can claim a 5 sigma discovery (exclusion) of the Z' boson with a mass up to 4.5 TeV (5.5 TeV) with the full Run 2 data of the LHC when the fraction of unstable dark hadrons decaying to tau lepton pairs is around 50%, and with a coupling of the Z' to right-handed up-type quarks of 0.25. Furthermore, we show that, with new trigger strategies for Run 3, it may be possible to access Z' masses down to 700 GeV, for which the event topology is still composed of two resolved semi-visible jets.

Measurements of Zγ+jets differential cross sections in pp collisions at sqrt{s} = 13 TeV with the ATLAS detector

Differential cross-section measurements of Zγ production in association with hadronic jets are presented, using the full 139 fb−1 dataset of sqrt{s} = 13 TeV proton–proton collisions collected by the ATLAS detector during Run 2 of the LHC. Distributions are measured using events in which the Z boson decays leptonically and the photon is usually radiated from an initial-state quark. Measurements are made in both one and two observables, including those sensitive to the hard scattering in the event and others which probe additional soft and collinear radiation. Different Standard Model predictions, from both parton-shower Monte Carlo simulation and fixed-order QCD calculations, are compared with the measurements. In general, good agreement is observed between data and predictions from MATRIX and MiNNLOPS, as well as next-to-leading-order predictions from MadGraph5_aMC@NLO and Sherpa.

Flavor Identification of Reconstructed Hadronic Jets.

Identifying the flavor of reconstructed hadronic jets is critical for precision phenomenology and the search for new physics at collider experiments, as it allows one to pinpoint specific scattering processes and reject backgrounds. Jet measurements at the LHC are almost universally performed using the anti-k_{T} algorithm; however, no approach exists to define the jet flavor for this algorithm that is infrared and collinear safe. We propose a new approach, a flavor-dressing algorithm, that is infrared and collinear safe in perturbation theory and can be combined with any definition of a jet. We test the algorithm in an e^{+}e^{-} environment and consider the pp→Z+b-jet process as a practical application at hadron colliders.

Identification of b jets using QCD-inspired observables

We study the issue of separating hadronic jets that contain bottom quarks ($b$ jets) from jets featuring light partons only. We develop a novel approach to $b$ tagging that exploits the application of QCD-inspired jet substructure observables such as one-dimensional jet angularities and the two-dimensional primary Lund plane. We demonstrate that these observables can be used as inputs to modern machine-learning algorithms to efficiently separate $b$ jets from light ones. In order to test our tagging procedure, we consider simulated events where a $Z$ boson is produced in association with jets and show that using jet angularities as an input for a deep neural network, as well as using images obtained from the primary Lund jet plane as input to a convolutional neural network, one can achieve tagging accuracy comparable with the accuracy of conventional track-based taggers. We argue that the complementary usage of the track-based taggers together with the ones based upon QCD-inspired observables could improve $b$-tagging accuracy.

Exploring the role of composition and mass loading on the properties of hadronic jets

ABSTRACTAstrophysical jets are relativistic outflows that remain collimated for remarkably many orders of magnitude. Despite decades of research, the origin of cosmic rays (CRs) remains unclear, but jets launched by both supermassive black holes in the centre of galaxies and stellar-mass black holes harboured in X-ray binaries (BHXBs) are among the candidate sources for CR acceleration. When CRs accelerate in astrophysical jets, they initiate particle cascades that form γ-rays and neutrinos. In the so-called hadronic scenario, the population of accelerated CRs requires a significant amount of energy to properly explain the spectral constraints, similarly to a purely leptonic scenario. The amount of energy required often exceeds the Eddington limit or even the total energy available within the jets. The exact energy source for the accelerated protons is unclear, but due to energy conservation along the jets, it is believed to come from the jet itself via transfer of energy from the magnetic fields or kinetic energy from the outflow. To address this hadronic energy issue and to self-consistently evolve the energy flux along the flows, we explore a novel treatment for including hadronic content, in which instabilities along the jet/wind border play a critical role. We discuss the impact of the different jet compositions on the jet dynamics for a pair dominated and an electron-proton jet and, consequently, the emitted spectrum, accounting for both leptonic and hadronic processes. Finally, we discuss the implications of this mass-loading scenario to address the proton energy issue.

Jet fragmentation via azimuthal angular correlations of heavy flavors in pp collisions at s=7 TeV

Measurements in heavy-flavor azimuthal angular correlation provide insight into the production, propagation, and hadronization of heavy-flavor jets in ultrarelativistic hadronic and heavy-ion collisions. These measurements across different particle species help to isolate the possible modification in particle production and fragmentation due to different mass and quark contents. Jet correlation studies give direct access to the initial parton dynamics produced in these collisions. This article studies the azimuthal angular correlations of heavy-flavor hadrons (charm and beauty mesons and charm baryons) in $pp$ collisions at $\sqrt{s}=7$ TeV using pythia8. We study the production of heavy-flavor jets with different parton-level processes, including multiparton interactions and different color reconnection prescriptions. The heavy-flavor hadrons correlations are calculated in the different triggers and associated ${p}_{\mathrm{T}}$ intervals to characterize the impact of hard and soft scattering. The yields and the widths associated with the near-side (NS) and away-side (AS) correlation peaks are calculated and studied as a function of associated ${p}_{\mathrm{T}}$ for different trigger ${p}_{\mathrm{T}}$ ranges.

Analyzing N-Point Energy Correlators inside Jets with CMS Open Data.

Jets of hadrons produced at high-energy colliders provide experimental access to the dynamics of asymptotically free quarks and gluons and their confinement into hadrons. In this Letter, we show that the high energies of the Large Hadron Collider (LHC), together with the exceptional resolution of its detectors, allow multipoint correlation functions of energy flow operators to be directly measured within jets for the first time. Using Open Data from the CMS experiment, we show that reformulating jet substructure in terms of these correlators provides new ways of probing the dynamics of QCD jets, which enables direct imaging of the confining transition to free hadrons as well as precision measurements of the scaling properties and interactions of quarks and gluons. This opens a new era in our understanding of jet substructure and illustrates the immense unexploited potential of high-quality LHC data sets for elucidating the dynamics of QCD.

Particle flow reconstruction for the CMS Phase-II Level-1 Trigger

The upgrade of the CMS detector for the high-luminosity LHC will include track-finding for the first time in the Level-1 trigger, enabling Particle Flow reconstruction of every event in addition to comprehensive pileup mitigation. The Correlator trigger will reconstruct isolated leptons and photons, hadronic jets, and energy sums, assisted in many cases by machine learning to benefit from the complete particle-level event record. We present the logic of these algorithms, possible implementations using large FPGAs and their demonstration in prototype hardware, in addition to the expected physics performance.

Calorimetry with Extremely Fine Spatial Segmentation

Particle Flow Algorithms (PFAs) attempt to measure each particle in a hadronic jet individually, using the detector subsystem that provides the best energy/momentum resolution. Calorimeters that can exploit the power of PFAs emphasize spatial granularity over single particle energy resolution. In this context, the CALICE Collaboration developed the Digital Hadron Calorimeter (DHCAL). The DHCAL uses Resistive Plate Chambers (RPCs) as active media and is read out with 1 × 1 cm2 pads and digital (1-bit) resolution. In order to obtain a unique dataset of electromagnetic and hadronic interactions with unprecedented spatial resolution, the DHCAL went through a broad test beam program. In addition to conventional calorimetry, the DHCAL offers detailed measurements of event shapes, rigorous tests of simulation models and various analytical tools to improve calorimetric performance. Here we report on the results from the analysis of DHCAL data and comparisons with the Monte Carlo simulations.

Crilin: A CRystal calorImeter with Longitudinal InformatioN for a future Muon Collider

The measurement of physics processes at new energy frontier experiments requires excellent spatial, time, and energy resolutions to resolve the structure of collimated high-energy jets. In a future Muon Collider, the beam-induced backgrounds (BIB) represent the main challenge in the design of the detectors and of the event reconstruction algorithms. The technology and the design of the calorimeters should be chosen to reduce the effect of the BIB, while keeping good physics performance. Several requirements can be inferred: i) high granularity to reduce the overlap of BIB particles in the same calorimeter cell; ii) excellent timing (of the order of 100 ps) to reduce the out-of-time component of the BIB; iii) longitudinal segmentation to distinguish the signal showers from the fake showers produced by the BIB; iv) good energy resolution (less than 10%/√E) to obtain good physics performance, as has been already demonstrated for conceptual particle flow calorimeters. Our proposal consists of a semi-homogeneous electromagnetic calorimeter based on lead fluoride crystals (PbF2) read out by surface-mount UV-extended Silicon Photomultipliers (SiPMs): the Crilin calorimeter. In this paper, the performance of the Crilin calorimeter in the Muon Collider framework for hadron jets reconstruction has been analyzed. We report the characterisation of individual components together with the development of a small-scale prototype, consisting of 2 layers of 3 × 3 crystals each.

Hadron pair production in semi-inclusive electron positron annihilation process at twist-4

Fragmentation functions are important quantities in describing the hadronization process in high energy reactions. They can induce various azimuthal modulations which can be measured to reveal correlations between transverse momenta and polarizations. Without introducing initial state uncertainties, electron positron annihilation process is known as an ideal process to investigate the fragmentation functions. In this paper, therefore, we calculate the hadron pair production in the semi-inclusive electron positron annihilation process e^+ + e^- rightarrow h_1+h_2 + {bar{q}} +X at twist-4 to study dihadron fragmentation functions. Here {bar{q}} denotes an antiquark that corresponds to a jet of hadrons in experiments. Together with single hadron fragmentation functions, dihadron fragmentation functions can provide additional ways to extract nucleon parton distribution functions from the semi-inclusive deeply inelastic scattering experiments with two detected final state hadrons. We calculate the differential cross section of the hadron pair production semi-inclusive electron positron annihilation process at twist-4 level. The calculation is carried out by using the collinear expansion method. We also calculate azimuthal asymmetries in terms of dihadron fragmentation functions. Contributions from four-quark correlator are also taken into account. Both the electromagnetic and weak interactions are considered in this paper.

Search for events with a pair of displaced vertices from long-lived neutral particles decaying into hadronic jets in the ATLAS muon spectrometer in pp collisions at s=13 TeV

A search for events with two displaced vertices from long-lived particles (LLP) pairs using data collected by the ATLAS detector at the LHC is presented. This analysis uses 139~fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=13$ TeV recorded in 2015-2018. The search employs techniques for reconstructing vertices of LLPs decaying to jets in the muon spectrometer displaced between 3 m and 14 m with respect to the primary interaction vertex. The observed numbers of events are consistent with the expected background and limits for several benchmark signals are determined. For the Higgs boson with a mass of 125 GeV, the paper reports the first exclusion limits for branching fractions into neutral long-lived particles below 0.1%, while branching fractions above 10% are excluded at 95% confidence level for LLP proper lifetimes ranging from 4 cm to 72.4 m. In addition, the paper present the first results for the decay of LLPs into into $t\bar{t}$ in the ATLAS muon spectrometer.

Quantum Machine Learning for b-jet charge identification

Machine Learning algorithms have played an important role in hadronic jet classification problems. The large variety of models applied to Large Hadron Collider data has demonstrated that there is still room for improvement. In this context Quantum Machine Learning is a new and almost unexplored methodology, where the intrinsic properties of quantum computation could be used to exploit particles correlations for improving the jet classification performance. In this paper, we present a brand new approach to identify if a jet contains a hadron formed by a b or overline{b} quark at the moment of production, based on a Variational Quantum Classifier applied to simulated data of the LHCb experiment. Quantum models are trained and evaluated using LHCb simulation. The jet identification performance is compared with a Deep Neural Network model to assess which method gives the better performance.

Search for neutral long-lived particles in pp collisions at sqrt{s} = 13 TeV that decay into displaced hadronic jets in the ATLAS calorimeter

A search for decays of pair-produced neutral long-lived particles (LLPs) is presented using 139 fb−1 of proton-proton collision data collected by the ATLAS detector at the LHC in 2015–2018 at a centre-of-mass energy of 13 TeV. Dedicated techniques were developed for the reconstruction of displaced jets produced by LLPs decaying hadronically in the ATLAS hadronic calorimeter. Two search regions are defined for different LLP kinematic regimes. The observed numbers of events are consistent with the expected background, and limits for several benchmark signals are determined. For a SM Higgs boson with a mass of 125 GeV, branching ratios above 10% are excluded at 95% confidence level for values of c times LLP mean proper lifetime in the range between 20 mm and 10 m depending on the model. Upper limits are also set on the cross-section times branching ratio for scalars with a mass of 60 GeV and for masses between 200 GeV and 1 TeV.

Two-loop non-planar hexa-box integrals with one massive leg

Based on the Simplified Differential Equations approach, we present results for the two-loop non-planar hexa-box families of master integrals. We introduce a new approach to obtain the boundary terms and establish a one-dimensional integral representation of the master integrals in terms of Generalised Polylogarithms, when the alphabet contains non-factorisable square roots. The results are relevant to the study of NNLO QCD corrections for W, Z and Higgs-boson production in association with two hadronic jets.

Production of strange hadrons in jets and underlying events in pp and p–Pb collisions with ALICE

The production of strange (, Λ) and multi-strange (Ξ± and Ω±) hadrons in jets and underlying events in pp and p–Pb collisions is studied with ALICE at the LHC. Transverse momentum (p T) differential density distribution of particles produced in a jet is compared to that of inclusive particle production and that in underlying events. The particle yield ratios of and as a function of p T are also investigated in jets and underlying events. The production of the multi-strange hadrons, Ξ± and Ω±, and the corresponding ratio in jets and underlying events are measured for the first time. The p T-differential density distribution of hadrons associated with hard scattering decreases slower than that for inclusive production. The baryon-to-meson and baryon-to-baryon ratios measured in jets exhibit clear differences from values obtained from the inclusive spectra in the intermediate p T range. The p T distribution of strange hadrons produced in jets is also studied in pp collisions at and and in p–Pb collisions at . The p T spectra of particle produced in jets are independent of collision systems and collision energies. No apparent collision energy and collision system dependence is observed for the particle yield ratios of and associated with energetic jets. These new results are compared to PYTHIA 8 event generator. The ratio are generally reproduced by the model, but large discrepancies between data and PYTHIA simulations are observed for .

Characterization of mixed Bi4(Ge[formula omitted]Si[formula omitted])3O12 for crystal calorimetry at future colliders

Bismuth germanate (BGO) is a well known high density scintillating material widely used in many applications such as high energy physics and medical imaging. Bismuth silicate (BSO) features properties similar to BGO in terms of stopping power and Cherenkov photon yield with a lower scintillation light output but faster decay time, thus being more attractive for applications in high-rate environments. Mixed crystals such as Bi4(GexSi1−x)3O12 (BGSO, with x varying from 0 to 1) make it possible to optimize decay time and light yield based on the detector needs.A characterization campaign of the optical and scintillation properties of two sets of BGSO mixed crystals with Ge fraction varying from 0 to 100% was performed. A coincidence time resolution (CTR) at 511 keV of 208 ±2 ps FWHM was measured for a 2 × 2 × 3 mm3 pixel with 40% Ge, while the optimum value obtained for the effective decay time is 49.9 ±1.8 ns for a 6 × 6 × 0.7 mm3 plate-shaped sample with 30% Ge. Furthermore the smallest slow decay time component achieved is 101 ±2 ns and is obtained for the plate-shaped sample with 30% Ge, while the largest is 236 ±5 ns for a pure BGO sample with the same geometry.In addition we demonstrated the possibility to efficiently separate the Cherenkov and scintillation light produced in a pure BSO sample. Such a technique could be exploited in a crystal-based dual-readout calorimeter to improve the energy resolution for hadronic showers and jets.

The time substructure of jets and boosted object tagging

We propose the study of the time substructure of jets, motivated by the fact that the next generation of detectors at particle colliders will resolve the time scale over which jet constituents arrive. This effect is directly related to the fragmentation and hadronization process, which transforms partons into massive hadrons with a distribution of velocities. We review the basic predictions for the velocity distribution of jet hadrons, and suggest an application for this information in the context of boosted object tagging. By noting that the velocity distribution is determined by the properties of the color string which ends on the parton that initiates the jet, we observe that jets originating from boosted color singlets, such as Standard Model electroweak bosons, will exhibit velocity distributions that are boosted relative to QCD jets of similar jet energy. We find that by performing a simple cut on the corresponding distribution of charged hadron arrival times at the detector, we can discriminate against QCD jets that would otherwise give a false positive under a traditional spatial substructure-based boosted object tagger.