Energetic Jets Research Articles

Jet-photon conversion with energy loss in heavy ion collisions

The rate of high energy photons produced from energetic jets during their propagation through the QGP at RHIC and LHC is studied by taking into account the contribution of jet quenching in the medium. It is shown that the jet quenching effect reduces the rate of jet-photon conversion at large transverse momemtum by about 40% at RHIC with √s = 200 AGeV, and by about 80% at LHC with √s = 5500 AGeV.

A new analysis ofPP→bb̄ℓνjjat the LHC: Higgs andWboson associated production with two tag jets

Higgs production in association with a W boson and two energetic tag jets at the LHC is studied for MH = 120 GeV, with the Higgs decaying to b and the W to lν (l = e, μ), guaranteeing high trigger efficiency. All parton level backgrounds are analyzed, including the effect of fake b-tagging. We discuss two detection strategies: in the first, more traditional, one, two jets are required to be b-tagged while in the second, which has not been previously examined in detail, only one tag is required. After all selection cuts about 80 and 200 events are foreseen in the two cases for a standard luminosity of 100 fb−1 with a S/B ratio of 1/25 and 1/60 respectively. The corresponding statistical significancies, S/B1/2 are 1.81 and 1.82.

Structure, transport and potential vorticity of the Gulf Stream at 68°W: Revisiting older data sets with new techniques

The stream-coordinates mean structure of the Gulf Stream at 68°W is derived using new methods for both defining stream coordinates and interpreting bottom pressure and inverted echo sounder travel times collected during the extensive Synoptic Ocean Prediction experiment. These new analyses provide pictures of the vertical structure of Gulf Stream flows that are demonstrably dynamically consistent with the density field at all depths, in contrast to previous work that relies on simple vertical interpolations to fill gaps between sparse current meter measurements. This new view of the Gulf Stream suggests a slightly higher total mean transport, with the increases coming from both baroclinic and barotropic components, and slightly stronger recirculation cells, particularly on the southern side. The recirculation of the Gulf Stream appears to have a weak baroclinic component, perhaps 10% of the total. A significant advantage of the methodology is the ability to obtain sensible vertical and horizontal gradients of currents and density so that the vertical and cross-stream structures of the components of the mean potential vorticity can be clearly imaged. One new feature from this calculation is that the along-stream gradient of the cross-stream velocity, a term that is often ignored in potential vorticity analyses, is non-negligible (though small) and is asymmetric about the current axis. Both the derived structure and implied dynamics of the circulation can be significantly altered by small changes to the method of calculating daily stream coordinates, e.g., by carefully filtering out observations in rings or not. Arrays of pressure-equipped inverted echo sounders provide the opportunity (at reasonable cost) for properly defining the stream coordinates of energetic jets such as the Gulf Stream.

Nucleosynthesis in Magnetically Driven Jets from Collapsars

We have made detailed calculations of the composition of magnetically driven jets ejected from collapsars, or rapidly rotating massive stars, based on long-term magnetohydrodynamic simulations of their core collapse with various distributions of magnetic field and angular momentum before collapse. We follow the evolution of the abundances of about 4000 nuclides from the collapse phase to the ejection phase and through the jet generation phase using a large nuclear reaction network. We find that the r-process successfully operates only in energetic jets (>1051 ergs), such that U and Th are synthesized abundantly, even when the collapsar has a relatively weak magnetic field (1010 G) and a moderately rotating core before the collapse. The abundance patterns inside the jets are similar to those of the r-elements in the solar system. About 0.01-0.06 M☉ of neutron-rich, heavy nuclei are ejected from a collapsar with energetic jets. The higher energy jets have larger amounts of 56Ni, varying from 3.7 × 10−4 to 0.06 M☉. Less energetic jets, which eject small amounts of 56Ni, could induce a gamma-ray burst (GRB) without a supernova, such as GRB 060505 or GRB 060614. Considerable amounts of r-elements are likely to be ejected from GRBs with hypernovae, if both the GRB and hypernova are induced by jets that are driven near the black hole.

Transverse momentum and centrality dependence of dihadron correlations in Au+Au collisions atsNN=200GeV: Jet quenching and the response of partonic matter

Azimuthal angle (Delta phi) correlations are presented for charged hadrons from dijets for 0.4 < p(T)< 10 GeV/c in Au+Au collisions at root s(NN)=200 GeV. With increasing p(T), the away-side distribution evolves from a broad and relatively flat shape to a concave shape, then to a convex shape. Comparisons to p+p data suggest that the away-side can be divided into a partially suppressed region centered at Delta phi similar to pi and an enhanced region centered at Delta phi similar to pi +/- 1.1. The p(T) spectrum for the head region softens toward central collisions, consistent with the onset of jet quenching. The spectral slope for the shoulder region is independent of centrality and trigger p(T), which offers constraints on energy transport mechanisms and suggests that it contains the medium response to energetic jets.

Glueball-induced partonic energy loss in quark-gluon plasma

We discuss the energy loss of energetic parton jets in quark-gluon plasma above the deconfinement temperature T{sub c} by the interaction with scalar and pseudoscalar glueballs. It is shown that the loss by this mechanism is quite important and may play the important role of the observed jet quenching.

Partons and Jets at Strong Coupling from AdS/CFT

Calculations using the AdS/CFT correspondence can be used to unveil the short-distance structure of a strongly coupled plasma, as it would be seen by a `hard probe'. The results of these calculations admit a natural physical interpretation in terms of parton evolution in the plasma: via successive branchings, essentially all partons cascade down to very small values of the longitudinal momentum fraction x and to transverse momenta smaller than the saturation momentum Q_s\sim T/x. This scale Q_s controls the energy loss and the transverse momentum broadening of an energetic jet propagating through the plasma. This picture has some striking consequences, like the absence of jets in electron-proton annihilation at strong coupling, of the absence of particle production at forward and backward rapidities in hadron-hadron collisions, which look very different from the corresponding predictions of perturbative QCD and also from the known experimental situation.

Interacting jets from binary protostars

We investigate potential models that could explain why multiple proto-stellar systems predominantly show single jets. During their formation, stars most frequently produce energetic outflows and jets. However, binary jets have only been observed in a very small number of systems. We model numerically 3D binary jets for various outflow parameters. We also model the propagation of jets from a specific source, namely L1551 IRS 5, known to have two jets, using recent observations as constraints for simulations with a new MHD code. We examine their morphology and dynamics, and produce synthetic emission maps. We find that the two jets interfere up to the stage where one of them is almost destroyed or engulfed into the second one. We are able to reproduce some of the observational features of L1551 such as the bending of the secondary jet. While the effects of orbital motion are negligible over the jets dynamical timeline, their interaction has significant impact on their morphology. If the jets are not strictly parallel, as in most observed cases, we show that the magnetic field can help the collimation and refocusing of both of the two jets.

Searches for direct pair production of supersymmetric top and supersymmetric bottom quarks inpp¯collisions ats=1.96 TeV

We search for direct pair production of supersymmetric top quarks and supersymmetric bottom quarks in proton-antiproton collisions at sqrt(s)=1.96 TeV, using 295 pb^-1 of data recorded by the Collider Detector at Fermilab (CDF II) experiment. The supersymmetric top (supersymmetric bottom) quarks are selected by reconstructing their decay into a charm (bottom) quark and a neutralino, which is assumed to be the lightest supersymmetric particle. The signature of such processes is two energetic heavy-flavor jets and missing transverse energy. The number of events that pass our selection for each search process is consistent with the expected standard model background. By comparing our results to the theoretical production cross sections of the supersymmetric top and supersymmetric bottom quarks in the minimal supersymmetric standard model, we exclude, at a 95% confidence level in the frame of that model, a supersymmetric top quark mass up to 132 GeV/c^2 for a neutralino mass of 48 GeV/c^2, and a supersymmetric bottom quark mass up to 193 GeV/c^2 for a neutralino mass of 40 GeV/c^2.

Low-Frequency Variability in the Midlatitude Baroclinic Atmosphere Induced by an Oceanic Thermal Front

AbstractThis study examines the flow induced by an east–west-oriented oceanic thermal front in a highly idealized baroclinic model. Previous work showed that thermal fronts could produce energetic midlatitude jets in an equivalent-barotropic atmosphere and that barotropic instabilities of this jet had dominant periods of 25–30 and 65–75 days.The present study extends this work to a two-mode baroclinic free atmosphere. The baroclinic jet produced in this case is subject to both barotropic and baroclinic instabilities. A barotropic symmetric instability propagates westward with periods of roughly 30 days and is similar to those found in the equivalent-barotropic model. A baroclinic instability results in standing-dipole anomalies and oscillates with a period of 6–8 months. A mixed barotropic–baroclinic instability results in anomalies that propagate northward, perpendicular to the jet, with a period of 2–3 months. The later anomalies are reminiscent of the 70-day oscillation found over the North Atlantic in observed fields.The atmospheric flow has two distinct states: the flow in the high-energy state exhibits two large gyres and a strong eastward jet; its antisymmetric component is dominant. The low-energy flow is characterized by small gyres and a weak jet.The model’s dynamics depends on the layer-depth ratio. When the model is nearly equivalent-barotropic, symmetric oscillatory modes dominate. As the two layers become nearly equal, antisymmetric oscillatory modes become significant and the mean energy of the flow increases.When the oceanic thermal front’s strength T* is weak (T* ≤ 1.5°C), the flow is steady. For intermediate values of the strength (1.5°C &amp;lt; T* &amp;lt; 3°C), several oscillatory instabilities set in. As the frontal strength increases further (T* ≥ 3°C), the flow becomes more turbulent. These results all depend on the atmospheric model’s horizontal resolution being sufficiently high.

Evaluation of HYCOM in the Kuroshio Extension region using new metrics

A high resolution (0.08° at equator) HYbrid Coordinate Ocean Model (HYCOM) simulation is evaluated using observations for the period 1993–2003 for a western Pacific region containing the Kuroshio Extension (KE) (25–45°N and 135–180°E). Comparisons are made for the KE path and strength and for the upper ocean heat budget. The mean strength and path agree well with observations, except near the KE separation point, where the jet is as much as 2° of latitude too far south. However, the model path variations are considerably more energetic than observed, and this likely obscures the observed tendency for weaker meandering when the KE is strong. The model accurately reproduces seasonal variations of upper ocean heat content, but the long‐period (about 10‐yr) variations of heat content and KE strength clearly differ in the region upstream of 150°E. The long‐period variations in model SSH do not show the same relationship to wind forcing that is seen in the observations and in a low‐resolution ocean model simulation. The HYCOM upper ocean heat budget is similar to a diagnostic heat budget inferred from observations in that the dominant contribution is from lateral fluxes (advection); however, advection fluctuations (again primarily in the upstream region) are much larger in the model. This evaluation of HYCOM shows realistic mean quantities and realistic variations away from the separation region. However, an overly energetic jet upstream of 150°E can obscure the longer period variability and its contribution to the upper ocean heat budget.

New VLA observations of the SNR Puppis A: the radio properties and the correlation with the X-ray emission

Context. High-resolution, high-sensitivity multifrequency radio images of supernova remnants (SNRs) are essential in advancing the understanding of both the global SNR dynamics and particle acceleration mechanisms. Aims. In this paper we report on a new study of the SNR Puppis A based on VLA observations at 1425 MHz; the improvement represents a factor of two in angular resolution and almost ten times in sensitivity compared to the best previous image of Puppis A. This new image is used to compare with re-processed 327 MHz data and ROSAT and Chandra images to investigate morphological and spectral characteristics. Methods. The observations were carried out with the VLA in the DnC and CnB configurations in 2004. After combining with singledish data from Parkes, an angular resolution of 34 �� × 16 �� and an rms noise of 0.5 mJy beam −1 , were achieved. Archival VLA data at 327 MHz were also processed. The spectral index distribution was then determined by a direct comparison of the homogenized data at both 327 and 1425 MHz. In addition, to identify different spectral components, tomographic spectral analysis was performed. Results. The new 1425 MHz radio image reveals a highly structured border encircling a diffuse, featureless interior. In particular, the northern half of Puppis A displays a complex structure along the periphery, consisting of short arcs resembling “wave-like” features. These are oriented essentially perpendicular to the shock front on the NE side, but are tangential to the shock on the NW side. A remarkable correspondence between such “wave-like” features and spectral changes is observed. On the other hand, the brightest radio features (located to the E of the SNR and also detected in X-rays) have no counterpart in the spectral index distribution. Based on a uniform compilation of integrated flux densities between 19 and 8400 MHz, a global spectral index α = −0.52± 0.03 (S ∝ ν α )h as been determined. The new 1425 MHz image of Puppis A was compared with the ROSAT X-ray image of the entire SNR and with the resolved arcsec Chandra image of the bright E region. There is good overall agreement between the radio and high resolution X-ray images. At the arcsec level, however, the agreement of the radio and X-ray images is less striking. A noticeable similarity is found between Puppis A, with its E and W extensions, and analogous morphological features observed in the SNR W50. This suggests that Puppis A could be another case of a SNR shaped by the action of energetic jets.

Jets of energetic particles generated by magnetic reconnection at a three-dimensional magnetic null

AbstractMagnetic reconnection is a candidate mechanism for particle acceleration in a variety of astrophysical contexts. It is now widely accepted that reconnection plays a key role in solar flares, and reconstructions of coronal magnetic fields indicate that three-dimensional (3D) magnetic null points can be present during flares. We investigate particle acceleration during spine reconnection at a 3D magnetic null point, using a test particle numerical code. We observe efficient particle acceleration and find that two energetic populations are produced: a trapped population of particles that remain in the vicinity of the null, and an escaping population, which leave the configuration in two symmetric jets along field lines near the spine. While the parameters used in our simulation aim to represent solar coronal plasma conditions of relevance for acceleration in flares, the fact that the 3D spine reconnection configuration naturally results in energetic particle jets may be of importance in other astrophysical situations. We also compare the results obtained for the spine reconnection regime with those for the other possible mode of 3D reconnection, fan reconnection. We find that in the latter case energetic particle jets are not produced, though acceleration is observed.

Jets of Energetic Particles Generated by Magnetic Reconnection at a Three-dimensional Magnetic Null

We investigate particle acceleration during magnetic reconnection at a three-dimensional magnetic null point, in the spine reconnection regime, using a test particle numerical code. We observe efficient particle acceleration and find that two energetic populations are produced: a trapped population of particles that remain in the vicinity of the null and an escaping population, which leave the configuration in two symmetric jets along field lines near the spine. While the parameters used in our simulation aim to represent solar coronal plasma conditions of relevance for acceleration in flares, the fact that the reconnection configuration we studied naturally results in energetic particle jets may be of importance in other astrophysical contexts.

Ultraluminous starbursts from supermassive black hole-induced outflows

I argue that there are two modes of global star formation. Discs and smaller spheroids form stars relatively inefficiently as a consequence of supernova-triggered negative feedback via a sequence of ministarbursts (S mode), whereas massive spheroids formed rapidly with high efficiency via the impact of active galactic nucleus (AGN) jet-triggered positive feedback (J mode) that generates and enhances ultraluminous starbursts. Supermassive black hole growth by accretion is favoured in the gas-rich protospheroid environment as mergers build up the mass of the host galaxy and provide a centrally concentrated gas supply. Quasi-spherical outflows arise and provide the source of porosity as the energetic jets from the accreting central supermassive black hole (SMBH) are isotropised by the inhomogeneous interstellar medium in the protospheroid core. Super-Eddington outflows occur and help to generate both the SMBH at high redshift and the strong positive feedback on protospheroid star formation that occurs as dense interstellar clouds are overpressured and collapse. SMBH form before the bulk of spheroid stars, and the correlation between spheroid velocity dispersion and supermassive black hole mass arises as AGN-triggered outflows limit the gas reservoir for spheroid star formation. The super-Eddington phase plausibly triggers a top-heavy initial mass function (IMF) in the region of influence of the SMBH. The Compton-cooled Eddington-limited outflow phase results in a spheroid core whose phase space density scales as the inverse 5/2 power of the core mass, and whose mass scales as the 2/3 power of SMBH mass. This latter scaling suggests that SMBH growth (and hence spheroid formation) is antihierarchical.

Measurement of the cross section fortt¯production inpp¯collisions using the kinematics of lepton+jets events

We present a measurement of the top pair production cross section in $p\bar{p}$ collisions at $\sqrt{s}$=1.96 TeV. We collect a data sample with an integrated luminosity of 194$\pm$11 pb$^{-1}$ with the CDF II detector at the Fermilab Tevatron. We use an artificial neural network technique to discriminate between top pair production and background processes in a sample of 519 lepton+jets events, which have one isolated energetic charged lepton, large missing transverse energy and at least three energetic jets. We measure the top pair production cross section to be $\sigma_{t\bar{t}}= 6.6$$\pm 1.1 \pm 1.5$ pb, where the first uncertainty is statistical and the second is systematic.

A Failed Gamma‐Ray Burst with Dirty Energetic Jets Spirited Away? New Implications for the Gamma‐Ray Burst–Supernova Connection from SN 2002ap

The Type Ic supernova (SN) 2002ap is an interesting event with very broad spectral features like the famous energetic SN 1998bw associated with a gamma-ray burst (GRB) 980425. Here we examine the jet hypothesis from SN 2002ap recently proposed based on the redshifted polarized continuum found in a spectropolarimetric observation. We show that jets should be moving at about 0.23c to a direction roughly perpendicular to us, and the degree of polarization requires a jet kinetic energy of at least 5 × 1050 ergs, a similar energy scale to the GRB jets. The weak radio emission from SN 2002ap has been used to argue against the jet hypothesis, but we argue that this is not a problem because the jet is expected to be freely expanding and unshocked. However, the jet cannot be kept ionized because of adiabatic cooling without external photoionization or a heating source. We explored various ionization possibilities and found that only the radioactivity of 56Ni is a plausible source, indicating that the jet is formed and ejected from the central region of the core collapse, not from the outer envelope of the exploding star. Then we point out that, if the jet hypothesis is true, the jet will eventually sweep up enough interstellar medium and generate shocks in a few to 10 yr, producing strong radio emission that can be spatially resolved, giving us a clear test for the jet hypothesis. Discussions are also given on what the jet would imply for the GRB-SN connection, when it is confirmed. We suggest the existence of two distinct classes of GRBs from similar core-collapse events but by completely different mechanisms. Cosmologically distant GRBs having an energy scale of ~1050-1051 ergs are collimated jets generated by the central activity of core collapses, associated with 56Ni ejection along with the jets. SN 2002ap can be considered as a failed GRB of this type with large baryon contamination. On the other hand, much less energetic ones including GRB 980425 are rather isotropic, which may be produced by hydrodynamical shock acceleration at the outer envelope. We propose that the radioactive ionization for the SN 2002ap jet may give a new explanation also for the X-ray line features often observed in GRB afterglows.

Dijet angular distributions in photoproduction of charm at HERA

Dijet angular distributions of photoproduction events in which a D∗± meson is produced in association with one of two energetic jets have been measured with the ZEUS detector at HERA, using an integrated luminosity of 120 pb−1. Differential cross sections as a function of the angle between the charm-jet and the proton-beam direction in the dijet rest frame have been measured for samples enriched in direct or resolved photon events. The results are compared with predictions from leading-order parton-shower Monte Carlo models and with next-to-leading-order QCD calculations. The angular distributions show clear evidence for the existence of charm originating from the photon.

Jet‐induced Nucleosynthesis in Misaligned Microquasars

The jet axes and the orbital planes of microquasar systems are usually assumed to be approximately perpendicular, eventhough this is not currently an observational requirement. On the contrary, in one of the few systems where the relative orientations are well-constrained, V4641Sgr, the jet axis is known to lie not more than ~36 degrees from the binary plane. Such a jet, lying close to the binary plane, and traveling at a significant fraction of the speed of light may periodically impact the secondary star initiating nuclear reactions on its surface. The integrated yield of such nuclear reactions over the age of the binary system (less the radiative mass loss) will detectably alter the elemental abundances of the companion star. This scenario may explain the anomalously high Li enhancements (roughly ~20-200 times the sun's photospheric value; or, equivalently, 0.1-1 times the average solar system value) seen in the companions of some black-hole X-ray binary systems. (Such enhancements are puzzling since Li nuclei are exceedingly fragile - being easily destroyed in the interiors of stars - and Li would be expected to be depleted rather than enhanced there.) Gamma-ray line signatures of the proposed process could include the 2.22 MeV neutron capture line as well as the 0.478 MeV 7Li* de-excitation line, both of which may be discernable with the INTEGRAL satellite if produced in an optically thin region during a large outburst. For very energetic jets, a relatively narrow neutral pion gamma-decay signature at 67.5 MeV could also be measurable with the GLAST satellite. We argue that about 10-20% of all microquasar systems ought to be sufficiently misaligned as to be undergoing the proposed jet-secondary impacts.

The HRX-BL Lac sample – Evolution of BL Lac objects

The unification of X-ray and radio selected BL Lacs has been an outstanding problem in the blazar research in the past years. Recent investigations have shown that the gap between the two classes can be filled with intermediate objects and that apparently all differences can be explained by mutual shifts of the peak frequencies of the synchrotron and inverse Compton component of the emission. We study the consequences of this scheme using a new sample of X-ray selected BL Lac objects comprising 104 objects with z<0.9 and a mean redshift z=0.34. 77 BL Lacs, of which the redshift could be determined for 64 (83%) objects, form a complete sample. The new data could not confirm our earlier result, drawn from a subsample, that the negative evolution vanishes below a synchrotron peak frequency log (peak-frequency) = 16.5. The complete sample shows negative evolution at the 2 sigma level (<V_e/V_a> = 0.42 +- 0.04). We conclude that the observed properties of the HRX BL Lac sample show typical behaviour for X-ray selected BL Lacs. They support an evolutionary model, in which flat-spectrum radio quasars (FSRQ) with high energetic jets evolve towards low frequency peaked (mostly radio-selected) BL Lac objects and later on to high frequency peaked (mostly X-ray selected) BL Lacs.