Jet fragmentation function moments in heavy ion collisions

The nature of a jet’s fragmentation in heavy-ion collisions has the potential to cast light on the mechanism of jet quenching. However, the presence of the huge underlying event complicates the reconstruction of the jet fragmentation function as a function of the momentum fraction z of hadrons in the jet. Here we propose the use of moments of the fragmentation function. These quantities appear to be as sensitive to quenching modifications as the fragmentation function directly in z. We show that they are amenable to background subtraction using the same jet-area-based techniques proposed in the past for jet p _t ’s. Furthermore, complications due to correlations between background-fluctuation contributions to the jet’s p _t and to its particle content are easily corrected for.     

Process dependence of the target jet remnants in the photoproduction of large transverse momentum jets

We use the quark recombination model of Das and Hwa to predict the inclusive meson spectra (π⁺, π^?,K ⁺,K ^?) in the target fragmentation region for processes in which a large transverse momentum jet is produced by a nearly real photon. We find that the antiparticle ratios of such target jet mesons are sensitive to the type of process which has produced the largep _t jet. By comparing the ratios found in photoproduction to those in deep inelastic scattering we point out that it is in principle possible to identify the presence of the Bethe-Heitler photoproduction subprocess. We conclude that the target jet remnants can contribute to separating the subprocesses responsible for the photoproduction of largep _t jets.     

Spin asymmetries for large-pt jet production in two-photon processes

Relying on the hard scattering formalism we estimate cross sections and spin asymmetries for large-p_t jet production from two-photon processes in collisions of electrons and positrons of definite helicity states. We use distribution functions of polarized partons in polarized electrons obtained in a modified leading logarithmic approximation as well as spin-dependent parton cross sections according to lowest-order QCD calculations. Detecting at least one forward or backward hadronic jet, topologies of jets can be observed different from the 2 large p_t production.     

Scaling and fragmentation distributions of beam remnants in high Et proton-proton collisions at √s =27.4 GeV

Charged particle trajectories have been reconstructed in pp collisions triggered by transverse energies (E_t) ranging from 1 GeV to 20 GeV. The forward fragmentation function of negative particles in the beam jet is found to scale with E_t in variable x=2p_L/√s−E_t, and the slope is near to that found for small jet triggers in an earlier experiment. Above about E_t=8 GeV, positive particles scale and for 0.5<x<1.0 the ratio of positives to negatives is 1.73±0.2. The forward fragmentation functions at high E_t do not show the behavior expected from the fragmentation of diquarks and are more like that from quarks.Multiplicity and energy flow in various forward polar angle regions are also presented; they are compared to the beam jet predictions of ISAJET and found to be greater at wide angles and less at small.     

A study of jets in electron positron annihilation into hadrons in the energy range 3.1 to 9.5 GeV

From an analysis of data taken with the detector PLUTO and the DORIS storage rings we have obtained evidence for jet structure in e⁺e^? annihilation with hadrons. Results for mean sphericity, mean thrust, the angular dependence of the jet axis and for 〈p_t〉 and 〈p_∥〉 with respect to the jet axis are presented. At 9.4 GeV we also discuss the angular dependence of the charged and neutral deposited energy with respect to the jet axis.     

Dominant structures in jet streams

A Hamiltonian version has been formulated for the model of axisymmetric equally rotating jet streams with a free boundary. In the framework of this approach, dominant structures, i.e., structure elements appearing in strongly disturbed jet streams at the preturbulent stage of their decay, are studied. It has been shown that compactons, i.e., solution with a compact support, can be such dominant structures. Analysis of the mechanism of the instability of compactons shows the possibility of collapse, which occurs almost without deformation of their shape but leads to the intensification of the vortex sheet at the boundary according to the law (t ₀ ? t)^?1, where t ₀ is the collapse time.     

Jet algorithms in electron–positron annihilation: perturbative higher order predictions

This article gives results on several jet algorithms in electron–positron annihilation: Considered are the exclusive sequential recombination algorithms Durham, Geneva, Jade-E0 and Cambridge, which are typically used in electron–positron annihilation. In addition also inclusive jet algorithms are studied. Results are provided for the inclusive sequential recombination algorithms Durham, Aachen and anti-k _t , as well as the infrared-safe cone algorithm SISCone. The results are obtained in perturbative QCD and are N³LO for the two-jet rates, NNLO for the three-jet rates, NLO for the four-jet rates and LO for the five-jet rates.     

Measurement of the single-jet invariant cross section at Fermilab

In an experiment performed at Fermilab we have studied the production of high p_t hadron jets from 400 GeV/c pp interactions. A large solid-angle, towered calorimeter was used to trigger and reconstruct the jet events. We report results for inclusive single-jet production and compare those results with QCD predictions and results obtained at the ISR and the SPS Collider.     

On the applicability of finite order perturbative QCD calculations of jet rates ine + e − annihilation

We have studied the measurement of α _s using exact second order QCD matrix element calculations of jet rates ine ⁺ e ^?→hadrons. We investigate the dependence of α _s (M _Z ² ), measured by experiments at theZ ⁰ resonance, on the choice of renormalisation scale in terms of the functional form of the three-jet matrix element. We find that only a restricted domain of the jet resolution parametery _c , $\Lambda _{\overline {MS} } $ , and scale gives a perturbation series which is well-behaved to second order, and that this domain is different for the various jet algorithms in current use.     

Current and target jets produced in high energy neutrino-deuterium interactions

In an analysis of ≈ 7000 charged current events produced in high energy neutrino-deuterium interactions, we observe that the struck quark jet has a wider spread in P_t than that from the recoiling diquark jet for events with hadronic energy W > 9 GeV. This suggests that the effect signals the onset of gluon emission from the single quark.     

Quark fragmentation and trigger side momentum distributions in high-p T processes

It has been widely argued that the experimental evidence concerning the momentum accompanying highp T triggers is a grave problem for models which take the trigger hadron to be a quark fragment. It is claimed that the trigger hadron takes much too large a fraction (z _c ) of the jet momentum for the trigger side jet to be a quark. The jet momentum is not directly measured, but deduced from the derivative of the momentum (p _x ) accompanying the trigger with respect to the trigger transverse momentum-p _T ^t . This argument is shown to be unsafe. Using both an approximate analytic approach to illustrate the physics and subsequently a full numerical computation it is proved that the deduction of the fractional momentum accompanying the trigger, 1/z _c ?1, fromdp _x /dp _T ^t is not correct. Further we show that models—specifically that of Feynman and Field—which do take the trigger to be a quark fragment are essentially in agreement with the data on trigger side momentum distributions. A surprising prediction of our analysis is thatp _x should be approximately constant forp _T ^t ≧6GeV/c.     

The effect of hadronic rindler horizon on hadronization process

In this study by extending the recent suggested mechanisms to hadronization processes, the information loss for QCD matter in hadronic Rindler horizon is found. We notice that for all finite values of quark and gluon energies, all information from all hadronization processes experiences some degree of loss. Then the effect of hedonic Rindler horizon on three jet cross section is explored. It is found that the three jet cross section is rising at y_cut =?0.0002 exhibits a turn-over at moderate value of y_cut =?0.01 and then rapidly decreases as y_cut increases. This model is consistent with OPAL data. Finally, different channels for producing Higgs boson near hadronic Rindler horizon are studied. It is shown that the cross section of Higgs boson produced via gluon fusion and quark interaction near a single Hadronic Rindler Horizon is much larger for higher center of mass energies. This is because an increase in the energy of hadronic Rindler horizon raises the temperature, thus intensifying the thermal radiation of QCD matter.     

Low-pt jets and the rise with energy of the inelastic cross section

The contribution of QCD jets to the rise with energy of the inelastic cross section is discussed quantitatively and found to be large. It is seen that the inclusive jet yield is the fastest growing component of the total cross section. The dependence of this yield upon the rapidity cuts, the choice of parton densities, the QCD scale Q² and the transverse momentum cutoff p_cut are examined. At higher energies, multiple parton scattering processes are seen to be non-negligible. Extrapolations of the low-p_t jet yield to the Tevatron are presented also.     

Instability of potential jet streams

A Hamiltonian version has been formulated for the model of a potential jet stream of a homogeneous incompressible fluid with a free boundary. In the framework of this model, instability regimes have been analyzed. It has been shown that self-similar solutions with a compact support can be dominant structures. Analysis of the instability mechanism shows that two collapse scenarios are possible. The first scenario occurs without the deformation of the shape and leads to an intensification of the vortex sheet according to the law (t ₀ ? t)^?1, where t ₀ is the collapse time. The second scenario leads to the formation of a singularity for the surface shape and to a decrease in the intensity of the vortex sheet according to the laws (t ₀ ? t)^?1/5 and (t ₀ ? t)^1/5, respectively. The integral collapse criterion has been found.     

Transverse momentum of charged hadrons produced in ν and\bar v deuterium charged current interactions

The transverse momentum properties of charged hadrons produced in charged current ν and \(\bar v\) deuterium interactions are studied. The variation of <P _t ² > withW ²,Q ²,x _F andz ² is analyzed. A study of transverse momentum balance shows that soft gluon emission and some quark primordial transverse momentum with <k _t ² >?(0.4 GeV/c)² are needed to balance highp _t particles. Distributions ofp _tin andP _tout in and out of the event plane are presented. A three jet structure is observed for events withW ²>50 (GeV²) and with a highp _t track, which is compatible with models taking into account hard gluon emission.     

The semihard processes γγ→ΨX, γγ→ΨΨ, γγ→ϱΨ

The cross sections for Ψ-production in the semihard collisions are calculated at s ⪢ |t|, m_Ψ² and at not too large a jet effective mass m_X²≲m_Ψ² + |t| for the process γγ→ΨX. We found these cross sections in the lowest nontrivial QCD order (two-gluon exchange in the t-channel): σ(γγ→ΨX) ≈ 10³⁵ cm², σ(γγ→ΨΨ)=2×10⁻³⁶ cm². The polarization of Ψ in these processes is transversal (contrary to the longitudinal one of light mesons ϱ⁰, ω, ϕ in similar processes).     

Bifurcation to infinitely many sinks

This paper considers one parameter families of diffeomorphisms {F _t } in two dimensions which have a curve of dissipative saddle periodic pointsP _t , i.e.F _t ⁿ (P _t )=P _t and |detDF _t ⁿ (P _t )|<1. The family is also assumed to create new homoclinic intersections of the stable and unstable manifolds ofP _t as the parameter varies throught ₀. Gavirlov and Silnikov proved that if the new homoclinic intersections are created nondegenerately att ₀, then there is an infinite cascade of periodic sinks, i.e. there are parameter valuest _n accumulating att ₀ for which there is a sink of periodn [GS2, Sect. 4]. We show that this result is true for real analytic diffeomorphisms even if the homoclinic intersection is created degenerately. We give computer evidence to show that this latter result is probably applicable to the Hénon map forA near 1.392 andB equal ?0.3. Newhouse proved a related result which showed the existence of infinitely many periodic sinks for a single diffeomorphism which is a perturbation of a diffeomorphism with a nondegenerate homoclinic tangency. We give the main geometric ideas of the proof of this theorem. We also give a variation of a key lemma to show that the result is true for a fixed one parameter family which creates a nondegenerate tangency. Thus under the nondegeneracy assumption, not only is there a cascade of sinks proved by Gavrilov and Silnikov, but also a single parameter valuet* with infinitely many sinks.     

Mean-Field Behavior for the Survival Probability and the Percolation Point-to-Surface Connectivity

We consider the critical survival probability (up to timet) for oriented percolation and the contact process, and the point-to-surface (of the ball of radiust) connectivity for critical percolation. Let θ_t denote both quantities. We prove in a unified fashion that, if θ_t exhibits a power law and both the two-point function and its certain restricted version exhibit the same mean-field behavior, then θ_t=t^-1 for the time-oriented models with d > 4 and θ_t=t^-2 for percolation with d > 7.     

Quantumchromodynamic corrections for ΔF=2 processes in the presence of a heavy top quark

We calculate the QCD corrections to the ΔS=2 and ΔB=2 transition amplitudes for the casem _t ?m _w and extrapolate the results tom _t ?m _w , where they join smoothly with the low mass calculations,m _t ?m _w . We present explicit formulas for the corrections with numerical results summarized in tables and figures and compare them with earlier calculations. We arrive at a QCD corrected effective Hamiltonian which must hold for all ranges ofm _t .     

Corona effect in <Emphasis Type="Italic">AA</Emphasis> collisions at the LHC

Following our earlier finding based on RHIC data on the dominant jet production from nucleus corona region, we reconsider this effect in nucleus–nucleus collisions at the LHC energies. Our hypothesis was based on experimental data, which raised the idea of a finite formation time for the produced medium. At the RHIC energy and in low-density corona region, this time reaches about 2 fm/c. Following this hypothesis, the nuclear modification factor R _AA at high p _t should be independent on particle momentum, and the azimuthal anisotropy of high p _t particles, v ₂, should be finite. A separate prediction held that, at the LHC energy, the formation time in the corona region should be about 1 fm/c. New LHC data show that R _AA is not flat and is rising with p _t . We add to our original hypothesis an assumption that a fast parton traversing the produced medium loses the fixed portion of its energy. A shift of about 7 GeV from the original power law p ^?6 production cross section in pp explains well all the observed R _AA dependencies. The shift of about 7 GeV is also valid at the RHIC energy. We also show that the observed at the LHC dependence of v ₂ at high p _t and our previous predictions agree.