Energetic Jets Research Articles

A Turnover in the Radio Light Curve of GW170817

Abstract We present 2–9 GHz radio observations of GW170817 covering the period 125–200 days post-merger, taken with the Australia Telescope Compact Array (ATCA) and the Karl G. Jansky Very Large Array (VLA). Our observations demonstrate that the radio afterglow peaked at 149 ± 2 days post-merger and is now declining in flux density. We see no evidence for evolution in the radio-only spectral index, which remains consistent with optically thin synchrotron emission connecting the radio, optical, and X-ray regimes. The peak implies a total energy in the synchrotron-emitting component of a few × 1050 erg. The temporal decay rate is most consistent with mildly or non-relativistic material and we do not see evidence for a very energetic off-axis jet, but we cannot distinguish between a lower-energy jet and more isotropic emission.

Merger and reconnection of Weibel separated relativistic electron beam

The relativistic electron beam (REB) propagation in a plasma is fraught with beam plasma instabilities. The prominent amongst them is the collisionless Weibel destabilization which spatially separates the forward propagating REB and the return shielding currents. This results in the formation of REB current filaments which are typically of the size of electron skin depth during the linear stage of the instability. It has been observed that in the nonlinear stage, the size of filaments increases as they merge with each other. With the help of 2-D particle-in-cell simulations in the plane perpendicular to the REB propagation, it is shown that these mergers occur in two distinct nonlinear phases. In the first phase, the total magnetic energy increases. Subsequently, however, during the second phase, one observes a reduction in magnetic energy. It is shown that the transition from one nonlinear regime to another occurs when the typical current associated with individual filaments hits the Alfvén threshold. In the second nonlinear regime, therefore, the filaments can no longer permit any increase in current. Magnetic reconnection events then dissipate the excess current (and its associated magnetic energy) that would result from a merger process leading to the generation of energetic electron jets in the perpendicular plane. At later times when there are only few filaments left, the individual reconnection events can be clearly identified. It is observed that in between such events, the magnetic energy remains constant and shows a sudden drop as and when two filaments merge. The electron jets released in these reconnection events are thus responsible for the transverse heating which has been mentioned in some previous studies [Honda et al., Phys. Plasmas 7, 1302 (2000)].

Search for dark matter and other new phenomena in events with an energetic jet and large missing transverse momentum using the ATLAS detector

Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses proton-proton collision data corresponding to an integrated luminosity of 36.1 fb−1 at a centre-of-mass energy of 13 TeV collected in 2015 and 2016 with the ATLAS detector at the Large Hadron Collider. Events are required to have at least one jet with a transverse momentum above 250 GeV and no leptons (e or μ). Several signal regions are considered with increasing requirements on the missing transverse momentum above 250 GeV. Good agreement is observed between the number of events in data and Standard Model predictions. The results are translated into exclusion limits in models with pair-produced weakly interacting dark-matter candidates, large extra spatial dimensions, and supersymmetric particles in several compressed scenarios.

Study of performance of the ATLAS transition radiation tracker in run 1 of the LHC: Tracking characteristics

The ATLAS Transition Radiation Tracker (TRT) contains more than 350000 large straw tubes and it is the outermost of the three subsystems of the ATLAS Inner Detector (ID). The TRT contributes substantially to the ATLAS ID resolution for the tracks of high-energy particles, providing excellent particle identification capabilities and electron-pion separation. Basic performance parameters of the TRT related to its tracking function are described in this paper. The data used in this study were collected during the first period of the Large Hadron Collider (LHC) operation in 2012 with a proton collision energy of 8 TeV. The tracking performance of the TRT has been studied in the case of operating with a Xe-based gas mixture and as a function of the straw occupancy. Special attention was paid to investigation of tracking parameters inside hadronic jets. The experimental data and simulation are in reasonable agreement, even within the dense cores of the most energetic jets.

The ATLAS Run-2 Trigger Menu for Higher Luminosities: Design, Performance and Operational Aspects

The ATLAS experiment aims at recording about 1 kHz of physics collisions, starting with an LHC design bunch crossing rate of 40 MHz. To reduce the massive background rate while maintaining a high selection effciency for rare physics events (such as beyond the Standard Model physics), a two-level trigger system is used. Events are selected based on physics signatures such as the presence of energetic leptons, photons, jets or large missing energy. The trigger system exploits geometrical information on candidate objects, as well as multi-variate methods to carry out the necessary physics filtering. In total, the ATLAS online selection consists of thousands of different individual triggers. A trigger menu is a compilation of these triggers which specifies the physics algorithms to be used during data taking and the bandwidth a given trigger is allocated to. Trigger menus reflect not only the physics goals of the collaboration for a given run, but also take into consideration the instantaneous luminosity of the LHC and limitations from the ATLAS detector readout and online processing farm. For the 2017 run, the ATLAS trigger has been enhanced to be able to handle higher instantaneous luminosities (up to 2:0x1034 cm-2s-1) and to ensure the selection robustness against higher average multiple interactions per bunch crossing. In these proceedings, we describe the design criteria for the trigger menus used for Run 2 at the LHC. We discuss several aspects of the process, from the validation of the algorithms, the fine-tuning of the prescales, and the monitoring tools that ensure the smooth operation of the trigger during data taking. We also report on the physics performance of a few trigger algorithms.

Reconfinement and loss of stability in jets from active galactic nuclei

Jets powered by active galactic nuclei appear impressively stable compared with their terrestrial and laboratory counterparts-they can be traced from their origin to distances exceeding their injection radius by up to a billion times. However, some less energetic jets get disrupted and lose their coherence on the scale of their host galaxy. Quite remarkably, on the same scale, these jets are expected to become confined by the thermal pressure of the intra-galactic gas. Motivated by these observations, we have started a systematic study of active galactic nuclei jets undergoing reconfinement via computer simulations. Here, we show that in the case of unmagnetized relativistic jets, the reconfinement is accompanied by the development of an instability and transition to a turbulent state. During their initial growth, the perturbations have a highly organized streamwise-oriented structure, indicating that it is not the Kelvin-Helmholtz instability, the instability which has been the main focus of the jet stability studies so far. Instead, it is closely related to the centrifugal instability. This instability is likely to be behind the division of active galactic nuclei jets into two morphological types in the Fanaroff-Riley classification.

Jet evolution in a dense medium: event-by-event fluctuations and multi-particle correlations

We study the gluon distribution produced via successive medium-induced branchings by an energetic jet propagating through a weakly-coupled quark-gluon plasma. We show that under suitable approximations, the jet evolution is a Markovian stochastic process, which is exactly solvable. For this process, we construct exact analytic solutions for all the n-point correlation functions describing the gluon distribution in the space of energy [M. A. Escobedo, E. Iancu, Event-by-event fluctuations in the medium-induced jet evolution, JHEP 05 (2016) 008. arXiv:arXiv:1601.03629, doi:http://dx.doi.org/10.1007/JHEP05(2016)008, M. A. Escobedo, E. Iancu, Multi-particle correlations and KNO scaling in the medium-induced jet evolution, JHEP 12 (2016) 104. arXiv:arXiv:1609.06104, doi:http://dx.doi.org/10.1007/JHEP12(2016)104]. Using these results, we study the event-by-event distribution of the energy lost by the jet at large angles and of the multiplicities of the soft particles which carry this energy. We find that the event-by-event fluctuations are huge: the standard deviation in the energy loss is parametrically as large as its mean value [M. A. Escobedo, E. Iancu, Event-by-event fluctuations in the medium-induced jet evolution, JHEP 05 (2016) 008. arXiv:arXiv:1601.03629, doi:http://dx.doi.org/10.1007/JHEP05(2016)008]. This has important consequences for the phenomenology of di-jet asymmetry in Pb+Pb collisions at the LHC: it implies that the fluctuations in the branching process can contribute to the measured asymmetry on an equal footing with the geometry of the di-jet event (i.e. as the difference between the in-medium path lengths of the two jets). We compute the higher moments of the multiplicity distribution and identify a remarkable regularity known as Koba-Nielsen-Olesen (KNO) scaling [M. A. Escobedo, E. Iancu, Multi-particle correlations and KNO scaling in the medium-induced jet evolution, JHEP 12 (2016) 104. arXiv:arXiv:1609.06104, doi:http://dx.doi.org/10.1007/JHEP12(2016)104]]. These predictions could be tested via event-by-event measurements of the di-jet asymmetry.

Search for supersymmetry in final states with two same-sign or three leptons and jets using 36 fb\u22121 of sqrt{s}=13 TeV pp collision data with the ATLAS detector

A search for strongly produced supersymmetric particles using signatures involving multiple energetic jets and either two isolated same-sign leptons (e or μ), or at least three isolated leptons, is presented. The analysis relies on the identification of b-jets and high missing transverse momentum to achieve good sensitivity. A data sample of proton-proton collisions at sqrt{s}=13 TeV recorded with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to a total integrated luminosity of 36.1 fb−1, is used for the search. No significant excess over the Standard Model prediction is observed. The results are interpreted in several simplified supersymmetric models featuring R-parity conservation or R-parity violation, extending the exclusion limits from previous searches. In models considering gluino pair production, gluino masses are excluded up to 1.87 TeV at 95% confidence level. When bottom squarks are pair-produced and decay to a chargino and a top quark, models with bottom squark masses below 700 GeV and light neutralinos are excluded at 95% confidence level. In addition, model-independent limits are set on a possible contribution of new phenomena to the signal region yields.

Extracting jet quenching parameters from large pT hadron suppression at RHIC/LHC

The large pT hadron suppression in high-energy heavy-ion collisions at RHIC/LHC is studied in a NLO pQCD parton model in which the fragmentation function is modified due to jet quenching. The energy loss of the energetic parton jets is incorporated in the modified fragmentation functions which utilize two important parameters to characterize the jet transport properties, the jet transport parameter qˆ0 and the mean free path λ0, both at the initial time τ0 in the center of the fireball created in central AA collisions. We performed a phenomenological study of simultaneous χ2/d.o.f fits to RHIC as well as LHC data to constrain the two parameters for energetic quarks, qˆ0≈1.1±0.2 GeV2/fm and λ0≈0.4±0.03 fm in central Au+ Au collisions at sNN=200 GeV, while qˆ0≈1.7±0.3 GeV2/fm, and λ0≈0.5±0.05 fm in central Pb + Pb collisions at sNN=2.76 TeV. Numerical results for the best fit give a multiple scattering picture for the hard jets with a moderate averaged number of gluon emissions. Based on the best constraints for λ0 and τ0, the estimated value for the mean-squared transverse momentum broadening is moderate which implies that the hard jets go through the medium with small reflection.

Event-by-event picture for the medium-induced jet evolution

We discuss the evolution of an energetic jet which propagates through a dense quark-gluon plasma and radiates gluons due to its interactions with the medium. Within perturbative QCD, this evolution can be described as a stochastic branching process, that we have managed to solve exactly. We present exact, analytic, results for the gluon spectrum (the average gluon distribution) and for the higher n-point functions, which describe correlations and fluctuations. Using these results, we construct the event-by-event picture of the gluon distribution produced via medium-induced gluon branching. In contrast to what happens in a usual QCD cascade in vacuum, the medium-induced branchings are quasi-democratic, with offspring gluons carrying sizable fractions of the energy of their parent parton. We find large fluctuations in the energy loss and in the multiplicity of soft gluons. The multiplicity distribution is predicted to exhibit KNO (Koba-Nielsen-Olesen) scaling. These predictions can be tested in Pb+Pb collisions at the LHC, via event-by-event measurements of the di-jet asymmetry.Based on [1, 2].

Productions of η, ρ0 and ϕ at large transverse momentum in Heavy ion Collisions

The suppression of the productions of the η meson in relativistic heavy-ion collisions and its ratio of η/π0 are computed theoretically in the framework of the perturbative QCD(pQCD) to confront the experimental data which matches well. We explore how the hadron production ratios as η/π0 would further disclose the informations of the production suppressions due to the energy loss of the energetic jet that propagating though the QGP medium. Also, we present our further studies on vector mesons such as ρ0 and ϕ within the same framework. The theoretical predictions based on pQCD are thus firstly given which give a decent description on the experimental measurements. It paved the way to the uniformly understanding of the strong suppression of single hadron productions at large transverse momentum which is a convincing evidence of the jet quenching effect.

Formation of periodic magnetic field structures in overdense plasmas

AbstractThe role played by the angle of incidence of a short pulse laser in the generation of magnetic field via Weibel instability from overdense plasmas is investigated with the help of three-dimensional particle-in-cell simulations. The simulations have been done for different cases by varying the angle of incidence. When the laser pulse is incident normally (θ = 0°), it gets self-focused at a very short distance and the axial intensity rises up to several times the fundamental laser intensity. Strong current filamentation is observed, which causes the generation of high magnetic fields across the current filament. When the laser is incident obliquely at θ = 30° and 60°, periodic density ripple-like structures are formed on the plasma surface which is due to emission of energetic electron jets caused by vacuum heating. The periodic structures carry forward and return currents, which results in the formation of periodic magnetic field structures having strong magnetic fields. At θ = 60°, the inter spacing distance coincides with the incident laser wavelength. The magnetic energy is found to be highest in case of normal incidence, which is due to strong current filamentation. The filamentation becomes weaker as the angle of incidence is increased.

Search for dark matter produced with an energetic jet or a hadronically decaying W or Z boson at sqrt{s}=13 TeV

A search for dark matter particles is performed using events with large missing transverse momentum, at least one energetic jet, and no leptons, in proton-proton collisions at sqrt{s}=13 TeV collected with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 12.9 fb−1. The search includes events with jets from the hadronic decays of a W or Z boson. The data are found to be in agreement with the predicted background contributions from standard model processes. The results are presented in terms of simplified models in which dark matter particles are produced through interactions involving a vector, axial-vector, scalar, or pseudoscalar mediator. Vector and axial-vector mediator particles with masses up to 1.95 TeV, and scalar and pseudoscalar mediator particles with masses up to 100 and 430 GeV respectively, are excluded at 95% confidence level. The results are also interpreted in terms of the invisible decays of the Higgs boson, yielding an observed (expected) 95% confidence level upper limit of 0.44 (0.56) on the corresponding branching fraction. The results of this search provide the strongest constraints on the dark matter pair production cross section through vector and axial-vector mediators at a particle collider. When compared to the direct detection experiments, the limits obtained from this search provide stronger constraints for dark matter masses less than 5, 9, and 550 GeV, assuming vector, scalar, and axial-vector mediators, respectively. The search yields stronger constraints for dark matter masses less than 200 GeV, assuming a pseudoscalar mediator, when compared to the indirect detection results from Fermi-LAT.

Factorization for substructures of boosted Higgs jets

We present a perturbative QCD factorization formula for substructures of an energetic Higgs jet, taking the energy profile resulting from the H→bb¯ decay as an example. The formula is written as a convolution of a hard Higgs decay kernel with two b-quark jet functions and a soft function that links the colors of the two b quarks. We derive an analytical expression to approximate the energy profile within a boosted Higgs jet, which significantly differs from those of ordinary QCD jets. This formalism also extends to boosted W and Z bosons in their hadronic decay modes, allowing an easy and efficient discrimination of fat jets produced from different processes.

Measurement of inclusive jet spectra in pp, p–Pb, and Pb–Pb collisions with the ALICE detector

Highly energetic jets are sensitive probes of the kinematic properties and the topology of high energy hadron collisions. Jets are collimated sprays of charged and neutral particles, which are produced in fragmentation of hard scattered partons from an early stage of the collision. In ALICE, jets have been measured in pp, p–Pb, and Pb–Pb collisions at several collision energies. While analyses of Pb–Pb events unveil properties of the hot and dense medium formed in heavy-ion collisions, pp and p–Pb collisions can shed light on hadronization and cold nuclear matter effects in jet production. Additionally, pp and p–Pb collisions serve as a baseline for disentangling hot and cold nuclear matter effects. A possible modification of the initial state is tested in p–Pb analyses. For the extraction of a jet signal, the exact evaluation of the background from the underlying event is an especially important ingredient. Due to the different nature of underlying events, each collision system requires a different analysis technique for removing the effect of the background on the jet sample. The focus of this publication is on the ALICE measurements of nuclear modification factors connecting p–Pb and Pb–Pb events to pp collisions. Furthermore, the radial jet structure is explored by comparing jet spectra reconstructed with different resolution parameters.

Interplay between collective expansion and Mach cone

By using a hybrid dynamical model which describes space-time evolution ofthe bulk medium, (mini-)jet propagation and interactions between medium and (mini-)jets, we study hydrodynamic responses to (mini-)jet propagation in high energy nuclear collisions. When an energetic jet traverses the bulk matter, it loses its energy into the matter and forms a Mach-cone like structure. On the other hand, the bulk matter expands radially due to pressure gradient. As a result, there happens an interplay between radial expansion and the Mach cone. We discuss possible phenomena and observables related with this in asymmetric gamma-jet events. We also discuss phenomena in which many mini-jets propagate the bulk matter at once in an event and calculate higher harmonics of azimuthal angle distribution.

Design and test performance of the ATLAS Feature Extractor trigger boards for the Phase-1 Upgrade

In Run 3, the ATLAS Level-1 Calorimeter Trigger will be augmented by an Electron Feature Extractor (eFEX), to identify isolated e/γ and τ particles, and a Jet Feature Extractor (jFEX), to identify energetic jets and calculate various local energy sums. Each module accommodates more than 450 differential signals that can operate at up to 12.8 Gb/s, some of which are routed over 30 cm between FPGAs. Here we present the module designs, the processes that have been adopted to meet the challenges associated with multi-Gb/s PCB design, and the results of tests that characterize the performance of these modules.

Galaxy Formation from the Primordial Black Holes

Supermassive black hole (SMBH) of size MBH = 106-10 M ⊙ is common in the Universe and it defines the center of the galaxy. A galaxy and the SMBH are generally thought to have co-evolved. However, the SMBH cannot evolve so fast as commonly observed even at redshift z > 6. Therefore, we explore a natural hypothesis that the SMBH has been already formed mature at z ⪆ 10 before stars and galaxies. The SMBH forms energetic jets and out-flows which trigger massive star formation in the ambient gas. They eventually construct globular clusters and classical bulge as well as the body of elliptical galaxies. We propose simple models which implement these processes. We point out that the globular clusters and classical bulges have a common origin but are in different phases. The same is true for the elliptical and spiral galaxies. Physics behind these phase division is the runaway star formation process with strong feedback to SMBH. This is similar to the forest-fire model that displays self-organized criticality.

Event-by-event picture for the medium-induced jet evolution

We discuss the evolution of an energetic jet which propagates through a dense quark-gluon plasma and radiates gluons due to its interactions with the medium. Within perturbative QCD, this evolution can be described as a stochastic branching process, that we have managed to solve exactly. We present exact, analytic, results for the gluon spectrum (the average gluon distribution) and for the higher n-point functions, which describe correlations and fluctuations. Using these results, we construct the event-by-event picture of the gluon distribution produced via medium-induced gluon branching. In contrast to what happens in a usual QCD cascade in vacuum, the medium-induced branchings are quasi-democratic, with offspring gluons carrying sizable fractions of the energy of their parent parton. We find large fluctuations in the energy loss and in the multiplicity of soft gluons. The multiplicity distribution is predicted to exhibit KNO (Koba-Nielsen-Olesen) scaling. These predictions can be tested in Pb+Pb collisions at the LHC, via event-by-event measurements of the di-jet asymmetry. Based on JHEP 1605 (2016) 008 and arXiv:1609.06104

Search for dark matter in proton-proton collisions at 8 TeV with missing transverse momentum and vector boson tagged jets

A search is presented for an excess of events with large missing transverse momentum in association with at least one highly energetic jet, in a data sample of proton-proton collisions at a centre-of-mass energy of 8 TeV. The data correspond to an integrated luminosity of 19.7 inverse femtobarns collected by the CMS experiment at the LHC. The results are interpreted using a set of simplified models for the production of dark matter via a scalar, pseudoscalar, vector, or axial vector mediator. Additional sensitivity is achieved by tagging events consistent with the jets originating from a hadronically decaying vector boson. This search uses jet substructure techniques to identify hadronically decaying vector bosons in both Lorentz-boosted and resolved scenarios. This analysis yields improvements of 80% in terms of excluded signal cross sections with respect to the previous CMS analysis using the same data set. No significant excess with respect to the standard model expectation is observed and limits are placed on the parameter space of the simplified models. Mediator masses between 80 and 400 GeV in the scalar and pseudoscalar models, and up to 1.5 TeV in the vector and axial vector models, are excluded.