Jet Cross Section Research Articles

Unsteady Predictions of Mixing Enhancement with Steady and Pulsed Control Jets

Unsteady Reynolds-averaged Navier–Stokes predictions are reported for a single round jet at high Reynolds number and high subsonic Mach number (, ) excited by steady and pulsed control jets. Comparison has been made with experimental validation data to assess the ability of -based unsteady Reynolds-averaged Navier–Stokes modeling for predicting control-jet-driven flow control of near-field jet mixing and potential core-length reduction. The well-known overprediction of the clean (unexcited) core length with this level of turbulence closure remains, but taking this into account, the relative effect of control jets on core-length reduction was predicted remarkably well. For example, steady control jets and pulsed control jets in symmetric/antisymmetric modes indicated a core-length reduction (relative to the unexcited case) of in predictions, compared to in measurements. Comparison of radial profiles showed that unsteady Reynolds-averaged Navier–Stokes modeling was also able to predict the three-dimensional near-field behavior induced by control jets throughout the jet cross section. The vortex structures produced in the pulsed-control-jet predictions were compared. Symmetric and antisymmetric modes produced different vortex structures; these caused different levels of enhanced mixing in the two azimuthal modes and explained the better performance of antisymmetric pulsing.

Measurement of differential production cross-sections for a Z boson in association with b-jets in 7 TeV proton-proton collisions with the ATLAS detector

Measurements of differential production cross-sections of a $Z$ boson in association with $b$-jets in $pp$ collisions at $\sqrt{s}=7$ TeV are reported. The data analysed correspond to an integrated luminosity of 4.6 fb$^{-1}$ recorded with the ATLAS detector at the Large Hadron Collider. Particle-level cross-sections are determined for events with a $Z$ boson decaying into an electron or muon pair, and containing $b$-jets. For events with at least one $b$-jet, the cross-section is presented as a function of the $Z$ boson transverse momentum and rapidity, together with the inclusive $b$-jet cross-section as a function of $b$-jet transverse momentum, rapidity and angular separations between the $b$-jet and the $Z$ boson. For events with at least two $b$-jets, the cross-section is determined as a function of the invariant mass and angular separation of the two highest transverse momentum $b$-jets, and as a function of the $Z$ boson transverse momentum and rapidity. Results are compared to leading-order and next-to-leading-order perturbative QCD calculations.

Toward multi-differential cross sections: measuring two angularities on a single jet

The analytic study of differential cross sections in QCD has typically focused on individual observables, such as mass or thrust, to great success. Here, we present a first study of double differential jet cross sections considering two recoil-free angularities measured on a single jet. By analyzing the phase space defined by the two angularities and using methods from soft-collinear effective theory, we prove that the double differential cross section factorizes at the boundaries of the phase space. We also show that the cross section in the bulk of the phase space cannot be factorized using only soft and collinear modes, excluding the possibility of a global factorization theorem in soft-collinear effective theory. Nevertheless, we are able to define a simple interpolation procedure that smoothly connects the factorization theorem at one boundary to the other. We present an explicit example of this at next-to-leading logarithmic accuracy and show that the interpolation is unique up to $\alpha_s^4$ order in the exponent of the cross section, under reasonable assumptions. This is evidence that the interpolation is sufficiently robust to account for all logarithms in the bulk of phase space to the accuracy of the boundary factorization theorem. We compare our analytic calculation of the double differential cross section to Monte Carlo simulation and find qualitative agreement. Because our arguments rely on general structures of the phase space, we expect that much of our analysis would be relevant for the study of phenomenologically well-motivated observables, such as $N$-subjettiness, energy correlation functions, and planar flow.

EERAD3: Event shapes and jet rates in electron–positron annihilation at order [formula omitted

The program EERAD3 computes the parton-level QCD contributions to event shapes and jet rates in electron–positron annihilation to order αs3. For three-jet production and related observables, this corresponds to next-to-next-to-leading order corrections, and allows for precision QCD studies. We describe the program and its usage in detail. Program summaryProgram title: EERAD3Catalogue identifier: AEUC_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEUC_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License, version 3No. of lines in distributed program, including test data, etc.: 119253No. of bytes in distributed program, including test data, etc.: 1368922Distribution format: tar.gzProgramming language: FORTRAN77 (with some FORTRAN90 features).Computer: All.Operating system: All.Has the code been vectorised or parallelised?: Program code allows the storage and adaptation of Monte Carlo information for producing statistically independent results on many cores. A tool for the combination of these is provided with the code.RAM: 6 MBytesClassification: 11.2, 11.5.Nature of problem:Computation of second-order QCD corrections to event shapes and jet cross sections in electron–positron annihilation.Solution method:Generation of events at parton-level, supplemented by a subtraction method to combine infrared singular contributions among different partonic channels.Running time:The high-precision/high-resolution distributions in [1] required about 700 CPU-days on an Intel Q6800 2.9 GHz. Reasonable precision can be obtained with run times of about 100 hours per coefficient for distributions, or within 5 hours if only single moments are computed.

What have we learned about the quark–gluon plasma with the ATLAS detector at the LHC?

Recent results from lead–lead and proton–lead collisions, measured by the ATLAS experiment at the LHC, are presented. In lead–lead collisions, electroweak bosons are found to be produced proportionally to the number of binary nucleon–nucleon collisions, and to have rapidity distributions compatible with perturbative QCD calculations, suggesting no need for large nuclear PDF effects. Conversely, the large suppression of inclusive jets, the elliptic flow of hadrons at high p T and the direct measurements of jet v 2 support the need for a path-length-dependent energy loss in the hot, dense medium. Proton–lead measurements provide new insights into particle production in small, longitudinally asymmetric systems, but require further insights into the fluctuating nature of proton–proton collisions. The modification factors for charged hadrons show a non-trivial dependence on centrality and rapidity, with a “Cronin” peak appearing only in the most central events, and in the lead-going direction. Finally the measurements of inclusive jets in proton–lead show a striking scaling in the R CP suppression variable that is only a function of the jet momentum, while the suppression factor relative to PYTHIA jet cross sections shows an enhanced yield in peripheral events, and a suppressed yield in central events.

Slot jet impingement heat transfer in the presence of jet-axis switching

Jet-axis switching is an established phenomenon whereby a non-circular, three-dimensional free jet undergoes a major change in cross-sectional shape with increasing downstream distance from the jet origin. This phenomenon has been demonstrated here to also occur for impinging jets. The focus of the present work is to investigate the heat transfer and fluid flow characteristics of rectangular slot jets which experience jet-axis switching. The jets in question have initial cross-sectional aspect ratios of 5:1 and 10:1. The jet cross sections, although highly skewed at first, evolve through near circularity and subsequently become skewed in the direction perpendicular to that of the initial skewness. In addition to the two initial aspect ratios, parametric variations were made of the Reynolds number, the distance of the impingement plate from the jet origin, and the contraction experienced by the flow passing through the aperture of the jet-forming orifice (i.e., the blockage ratio). The investigation was implemented by means of numerical simulation from which local and average Nusselt numbers were determined as functions of the foregoing parameters. Higher Reynolds numbers, greater downstream distances of the impingement plate, and greater blockages served to enhance the Nusselt number values. It remains to be seen whether there is an initial jet aspect ratio that is large enough to preclude axis switching and thereby allow two-dimensional modeling.

The effect of LHC jet data on MSTW PDFs.

We consider the effect on LHC jet cross sections on partons distribution functions (PDFs), in particular the MSTW2008 set of PDFs. We first compare the published inclusive jet data to the predictions using MSTW2008, finding a very good description. We also use the parton distribution reweighting procedure to estimate the impact of these new data on the PDFs, finding that the combined ATLAS 2.76 and 7 TeV data, and CMS 7 TeV data have some significant impact. We then also investigate the impact of ATLAS, CMS and DØ dijet data using the same techniques. In this case we investigate the effect of using different scale choices for the NLO cross section calculation. We find that the dijet data is generally not completely compatible with the corresponding inclusive jet data, often tending to pull PDFs, particularly the gluon distribution, away from the default values. However, the effect depends on the dijet dataset used as well as the scale choice. We also note that conclusions may be affected by limiting the pull on the data luminosity chosen by the best fit, which is sometimes a number of standard deviations. Finally we include the inclusive jet data in a new PDF fit explicitly. This enables us to check the consistency of the exact result with that obtained from the reweighting procedure. There is generally good, but not full quantitative agreement. Hence, the conclusion remains that MSTW2008 PDFs already fit the published jet data well, but the central values and uncertainties are altered and improved, respectively, to a significant, but not dramatic extent by inclusion of these data.

The high-pressure liquid jet incremental forming for the aluminum sheet

High-pressure liquid jet incremental forming, a new die-less forming process, utilizes liquid as a flexible tool to deform the sheet metal. In this article, an oil was used as the forming liquid. Theoretical analysis and numerical simulation were conducted to obtain the influence of jet nozzle geometry on the jetting pressure and velocity, and then, the best conic angle of the nozzle was proven to be 13°. Through analyzing the simulation results, it is found that the preferable jetting distance is between 15 and 30 mm and the dynamic pressure distribution approximately obeys a Gaussian distribution on the cross section of the oil jet. Aluminum sheets with a thickness of 0.3 mm were used in the local bulging experiment and simulation to investigate the process parameters of high-pressure oil jet forming. High-pressure liquid jet incremental forming is one of several high-speed forming methods. To obtain the precise simulation results, the dynamic effect of jetting pressure was studied by using Abaqus/Explicit. The results show that using loading time 0.02 s in the simulation is suitable for the simulation of the dynamic effect caused by the high-pressure oil jet. By comparing the deformation of the sheet under different oil pressures, it can be found that the deformation of the aluminum sheet becomes severe as the oil pressure increases, and the proper oil pressure is 15 MPa for the 0.3-mm-thick LF21 aluminum sheet. To investigate the influence of jetting path on the surface quality of the sheet metal parts, the forming processes of the 0.3-mm-thick LF21 aluminum sheets, with different path intervals, were simulated. The simulation results show that a better forming quality of the workpiece can be obtained when the jetting path interval is 4 mm.

Computation of multi-leg amplitudes with NJET

In these proceedings we report our progress in the development of the publicly available C++ library NJet for accurate calculations of high-multiplicity one-loop amplitudes. As a phenomenological application we present the first complete next-to-leading order (NLO) calculation of five jet cross section at hadron colliders.

Gamma-ray flare of PKS 1222+216 in 2010: effect of jet dynamics at the recollimation zone

The $\gamma$-ray flare of PKS 1222+216, observed in June 2010, is interpreted as an outcome of jet dynamics at recollimation zone. We obtained the $\gamma$-ray light-curves in three different energy bands, namely, 100--300 MeV, 300 MeV--1 GeV and 1--3 GeV from observations by the \emph{Fermi} Large Area Telescope (LAT). We also use the \emph{Swift}--XRT flux from 0.3--10 keV obtained from archival data. We supplement these with the 0.07--0.4 TeV observations with MAGIC telescope, available in the literature. The detection of source at very high energy (VHE, $E>100$ GeV) with a differential photon spectral index of $2.7\pm0.3$ and the rapid variability associated with it suggests that the emission arises from a compact region located beyond the broad line emitting region. The plausible $\gamma$-ray emission mechanism can then be inverse Compton scattering of IR photons from obscuring torus. Further, the decay time of LAT flare cannot be explained by considering simple radiative loss mechanisms. Hence, to interpret the LAT light curves, we develop a model where the broadband emission originates from a compact region, arising plausibly from the compression of jet matter at the recollimation zone. The flare is then expressed as an outcome of jet deceleration probably associated with this focusing effect. The parameters of the model are further constrained by reproducing the broadband spectral energy distribution of the source obtained during the flare episode. Our study suggests that the particle energy density exceeds magnetic energy density by a large factor which in turn may cause rapid expansion of the emission region. However, near equipartition can be achieved towards the end of LAT flare during which the compact emission region would have expanded to the size of jet cross-section.

Impact dynamics of granular jets with noncircular cross sections

Using high-speed photography, we investigate two distinct regimes of the impact dynamics of granular jets with noncircular cross sections. In the steady-state regime, we observe the formation of thin granular sheets with anisotropic shapes and show that the degree of anisotropy increases with the aspect ratio of the jet's cross section. Our results illustrate the liquidlike behavior of granular materials during impact and demonstrate that a collective hydrodynamic flow emerges from strongly interacting discrete particles. We discuss the analogy between our experiments and those from the Relativistic Heavy Ion Collider, where similar anisotropic ejecta from a quark-gluon plasma have been observed in heavy-ion impact.

Next-to-leading order QCD corrections to five jet production at the LHC

We present theoretical predictions for five jet production in proton-proton collisions at next-to-leading order accuracy in QCD. Inclusive as well as differential observables are studied for collision energies of 7 and 8 TeV. In general the next-to-leading order corrections stabilize the theoretical predictions with respect to scale variations. In case of the inclusive jet cross sections, we compare with experimental data where possible and find reasonable agreement. We observe that the four-to-three and five-to-four jet ratios show better perturbative convergence than the known three-to-two ratio and are promising candidates for future alpha_s measurements. Furthermore, we present a detailed analysis of uncertainties related to parton distribution functions. The full colour virtual matrix elements used in the computation were obtained with the NJet package, a publicly available library for the evaluation of one-loop amplitudes in massless QCD.

The BFKL equation, Mueller-Navelet jets and single-valued harmonic polylogarithms

We introduce a generating function for the coefficients of the leading logarithmic BFKL Green's function in transverse-momentum space, order by order in alpha_s, in terms of single-valued harmonic polylogarithms. As an application, we exhibit fully analytic azimuthal-angle and transverse-momentum distributions for Mueller-Navelet jet cross sections at each order in alpha_s. We also provide a generating function for the total cross section valid to any number of loops.

The HERA Proton

The almost 1 fb −1 of ep data collected by the H1 and ZEUS collider exper- iments at HERA allows for a precise determination of the proton's parton distribution functions (PDFs). Measurements used to constrain the PDFs — inclusive and jet cross sections, charm contribution to the F2 proton structure function, F c¯ c 2 — are presented herein. The measurement process itself includes cataloguing the sensitivity of the cross sections to the various sources of correlated systematic uncertainties. In the jet measure- ment, correlations of a statistical nature are also quantified and catalogued. These corre- lations provide a basis to combine measurements of the same physical observable across different time periods, experiments and measurement methodology. The subsequent PDF fitting procedure also takes into account such correlations. The resulting HERAPDF1.5 set based on inclusive data as well as PDF sets derived from inclusive plus charm data are presented togeteher with their predictions for pp cross sections at the LHC.

Investigation of unsteady processes, flow properties, and tonal acoustic radiation of a swirling jet

The results of an investigation of the unsteady flow structure in a turbulent swirling jet obtained using the PIV technology are presented. The greater part of the measurements is carried out at the swirl intensity W0 ≈ 1.7. A part of the data is obtained under other conditions of the swirl jet outflow. To establish the relation between disturbances of different types the phase averaging technique is employed with the pressure fluctuation in the acoustic field of the jet taken as a reference signal. The flow structure is numerically calculated. The results of the investigation show that a quasisteady inhomogeneity observable in the jet flow executes rotational motion relative to the mean flowfield in the jet cross-section, or “precession”. It causes disturbances in the flow ejected by the jet, which transform into acoustic disturbances far away from the jet. The frequencies of the dynamic disturbances near the jet and the acoustic disturbances far away from it coincide with the precession frequency.

Precision QCD measurements at HERA

A review of recent experimental results on perturbative QCD from the HERA experiments H1 and ZEUS is presented. All inclusive deep inelastic cross sections mea- sured by the H1 and ZEUS collaborations in neutral and charged current unpolarised ep scattering are combined. They span six orders of magnitude in negative four-momentum- transfer squared, Q 2 , and in Bjorken x. This data set is used as the sole input to NLO and NNLO QCD analyses to determine new sets of parton distributions, HERAPDF2.0, with small experimental uncertainties and an estimate of model and parametrisation un- certainties. Also shown are new results on inclusive jet, dijet and trijet differential cross sections measured in neutral current deep inelastic scattering. The precision jet data is used to extract the strong coupling αs at NLO with small experimental errors.

Hard And Soft QCD Physics In ATLAS

Hard and soft QCD results using proton-proton collisions recorded with the ATLAS detector at the LHC are reported. Charged-particle distributions and forward- backward correlations have been studied in low-luminosity minimum bias data taken at centre-of-mass energies of √ s = 0.9, 2.36 and 7 TeV. Recent measurements on under- lying event characteristics using charged-particle jets are also presented. The results are tested against various phenomenological soft QCD models implemented in Monte-Carlo generators. A summary of hard QCD measurements involving high transverse momen- tum jets is also given. Inclusive jet and dijet cross-sections have been measured at a centre-of-mass energy of 7 TeV and are compared to expectations based on NLO pQCD calculations corrected for non-perturbative effects as well as to NLO Monte Carlo predic- tions. Recent studies exploiting jet substructure techniques to identify hadronic decays of boosted massive particles are reported.

MOJAVE. X. PARSEC-SCALE JET ORIENTATION VARIATIONS AND SUPERLUMINAL MOTION IN ACTIVE GALACTIC NUCLEI

ABSTRACT We describe the parsec-scale kinematics of 200 active galactic nucleus (AGN) jets based on 15 GHz Very Long Baseline Array (VLBA) data obtained between 1994 August 31 and 2011 May 1. We present new VLBA 15 GHz images of these and 59 additional AGNs from the MOJAVE and 2 cm Survey programs. Nearly all of the 60 most heavily observed jets show significant changes in their innermost position angle over a 12–16 yr interval, ranging from 10° to 150° on the sky, corresponding to intrinsic variations of ∼0.°5 to ∼2°. The BL Lac jets show smaller variations than quasars. Roughly half of the heavily observed jets show systematic position angle trends with time, and 20 show indications of oscillatory behavior. The time spans of the data sets are too short compared to the fitted periods (5–12 yr), however, to reliably establish periodicity. The rapid changes and large jumps in position angle seen in many cases suggest that the superluminal AGN jet features occupy only a portion of the entire jet cross section and may be energized portions of thin instability structures within the jet. We have derived vector proper motions for 887 moving features in 200 jets having at least five VLBA epochs. For 557 well-sampled features, there are sufficient data to additionally study possible accelerations. We find that the moving features are generally non-ballistic, with 70% of the well-sampled features showing either significant accelerations or non-radial motions. Inward motions are rare (2% of all features), are slow (<0.1 mas yr−1), are more prevalent in BL Lac jets, and are typically found within 1 mas of the unresolved core feature. There is a general trend of increasing apparent speed with distance down the jet for both radio galaxies and BL Lac objects. In most jets, the speeds of the features cluster around a characteristic value, yet there is a considerable dispersion in the distribution. Orientation variations within the jet cannot fully account for the dispersion, implying that the features have a range of Lorentz factor and/or pattern speed. Very slow pattern speed features are rare, comprising only 4% of the sample, and are more prevalent in radio galaxy and BL Lac jets. We confirm a previously reported upper envelope to the distribution of speed versus beamed luminosity for moving jet features. Below 1026 W Hz−1 there is a fall-off in maximum speed with decreasing 15 GHz radio luminosity. The general shape of the envelope implies that the most intrinsically powerful AGN jets have a wide range of Lorentz factors up to ∼40, while intrinsically weak jets are only mildly relativistic.

NNLOPS simulation of Higgs boson production

Abstract We detail a simulation of Higgs boson production via gluon fusion, accurate at next-to-next-to-leading order in the strong coupling, including matching to a parton shower, yielding a fully exclusive, hadron-level description of the final-state. The approach relies on the Powheg method for merging the NLO Higgs plus jet cross-section with the parton shower, and on the Minlo method to simultaneously achieve NLO accuracy for inclusive Higgs boson production. The NNLO accuracy is reached by a reweighting procedure making use of the Hnnlo program.

Jet mass spectra in Higgs boson plus one jet at next-to-next-to-leading logarithmic order

The invariant mass of a jet is a benchmark variable describing the structure of jets at the LHC. We calculate the jet mass spectrum for Higgs plus one jet at the LHC at next-to-next-to-leading logarithmic (NNLL) order using a factorization formula. At this order, the cross section becomes sensitive to perturbation theory at the soft m_jet^2/p_T^jet scale. Our calculation is exclusive and uses the 1-jettiness global event shape to implement a veto on additional jets. The dominant dependence on the jet veto is removed by normalizing the spectrum, leaving residual dependence from non-global logarithms depending on the ratio of the jet mass and jet veto variables. For our exclusive jet cross section these non-global logarithms are parametrically smaller than in the inclusive case, allowing us to obtain a complete NNLL result. Results for the dependence of the jet mass spectrum on the kinematics, jet algorithm, and jet size R are given. Using individual partonic channels we illustrate the difference between the jet mass spectra for quark and gluon jets. We also study the effect of hadronization and underlying event on the jet mass in PYTHIA. To highlight the similarity of inclusive and exclusive jet mass spectra, a comparison to LHC data is presented.