Two-jet Cross Section Research Articles

Two-loop anomalous dimension for the resummation of non-global observables

The soft radiation emitted in jet cross sections can resolve the directions and colors of individual hard partons, leading to a complicated pattern of logarithmically enhanced terms in the perturbative series. Starting from a factorization theorem and solving the renormalization group equations for its ingredients, these large logarithms can be resummed. In this paper we extract the two-loop anomalous dimension governing the resummation of subleading logarithms in jet cross sections and other non-global observables. This anomalous dimension can be obtained by considering soft limits of hard amplitudes, but the presence of collinear singularities in intermediate expressions makes its extraction delicate. As a consistency check, we use our results to predict the known subleading non-global logarithms in the two-jet cross section.

Tree-level contribution toB¯→Xdγusing fragmentation functions

We evaluate the most important tree-level contributions connected with the b-> u \bar{u} d gamma transition to the inclusive radiative decay \bar{B}-> X_d gamma using fragmentation functions. In this framework the singularities arising from collinear photon emission from the light quarks (u, \bar{u} and d) can be absorbed into the (bare) quark-to-photon fragmentation function. We use as input the fragmentation function extracted by the ALEPH group from the two-jet cross section measured at LEP, where one of the jets is required to contain a photon. To get the quark-to-photon fragmentation function at the fragmentation scale \mu_F \sim m_b, we use the evolution equation, which we solve numerically. We then calculate the (integrated) photon energy spectrum for b-> u \bar{u} d gamma related to the operators P^u_{1,2}. For comparison, we also give the corresponding results when using nonzero (constituent) masses for the light quarks.

Measurement of the ratio of three-jet to two-jet cross sections in [formula omitted] collisions at [formula omitted

We present a measurement of the ratio of multijet cross sections in pp¯ collisions at s=1.96 TeV at the Fermilab Tevatron Collider. The measurement is based on a data set corresponding to an integrated luminosity of 0.7 fb−1 collected with the D0 detector. The ratio of the inclusive three-jet to two-jet cross sections, R3/2, has been measured as a function of the jet transverse momenta. The data are compared to QCD predictions in different approximations. Popular tunes of the pythia event generator do not agree with the data, while sherpa provides a reasonable description of the data. A perturbative QCD prediction in next-to-leading order in the strong coupling constant, corrected for non-perturbative effects, gives a good description of the data.

NNLO corrections to two-jet observables in electron-positron annihilation

I report on a numerical program, which can be used to calculate any infra-red safe two-jet observable in electron-positron annihilation to next-to-next-to-leading order in the strong coupling constant alpha_s. The calculation is based on the subtraction method. The result for the two-jet cross section is compared to the literature.

Real radiation at next-to-next-to-leading order in QCD: e+e- --> 2 jets through O(alpha2s).

We present a calculation of the differential two-jet cross section in e(+)e(-) annihilation through next-to-next-to-leading order (NNLO) in the strong coupling constant alpha(s). The calculation is performed using a new method for dealing with real radiation suggested recently by us. For the first time, the two-jet event rate is computed directly, without any reference to the inclusive cross section e(+)e(-)-->hadrons. We also calculate the energy distribution of the leading jet in e(+)e(-)-->2 jets and find significant modifications of the shape of this distribution at NNLO.

Mapping the proton unintegrated gluon distribution in dijets correlations in real and virtual photoproduction at HERA

We discuss how the dijet azimuthal correlations in DIS and real photoproduction at HERA probe the differential (unintegrated) gluon distribution in the proton. We find a strong dependence of the azimuthal correlation pattern on Bjorken- x, photon virtuality and the cut on the jet transverse momenta. A rise of the azimuthal decorrelations is observed with decreasing Bjorken- x due to the interplay of perturbative and nonperturbative effects. We predict a strong rise of the same-side jet rate with photon energy for real photoproduction. We discuss conditions for the correlation function to be dominated by hard perturbative gluons and ways of constraining the size of the nonperturbative soft component. We make some predictions for the THERA energy range. The analysis of the energy dependence of the isolated jet and two-jet cross sections in photoproduction would be a new way to study the not yet well constrained unintegrated gluon distributions and to explore the onset of the pQCD regime.

Inclusive Jet Production in γp and γγ Processes: Direct and Resolved Photon Cross Sections in Next-To-Leading Order QCD

The production of jets in low Q2 ep scattering (photoproduction) and in low Q2 ey+ey− scattering (γγ scattering) allows for testing perturbative QCD and for measuring the proton and photon structure functions. This requires exact theoretical predictions for one- and two-jet cross sections. We describe the theoretical formalism, giving sufficient details, for calculating the direct and resolved processes in γp and γγ reactions in next-to-leading order QCD. We present the complete analytical results for the Born terms, the virtual, and the real corrections. To separate singular and regular regions of phase space we use the phase space slicing method with an invariant mass cut-off. In this way, all soft and collinear singularities are either canceled or absorbed into the structure functions. Using a flexible Monte Carlo program, we evaluate the cross sections numerically and perform various tests and comparisons with other calculations. We consider the scale dependence of our results and compare them to data from the experiments H1 and ZEUS at HERA and OPAL at LEP.

Inclusive Single- and Dijet Rates in Next-to-Leading Order QCD for γ*p and γ*γ Collisions

We present one- and two-jet inclusive cross sections for γ*γ scattering and virtual photoproduction in ep collisions. The hard cross sections are calculated in next-to-leading order QCD. Soft and collinear singularities are extracted using the phase-space-slicing method. The initial state singularity of the virtual photon depends logarithmically its’ virtuality. This logarithm is large and has to be absorbed into the parton distribution function of the virtual photon. We define for this purpose an \({\overline MS}\) factorization scheme similar to the real photon case. We numerically study the dependence of the inclusive cross sections on the transverse energies and rapidities of the outgoing jets and on the photon virtuality. The ratio of the resolved to the direct cross section in ep collisions is compared to ZEUS data.

JetViP 1.1: Calculating one- and two-jet cross sections with virtual photons in NLO QCD

JetViP is a computer program for the calculation of inclusive single- and dijet cross sections in eP- and eγ-scattering in NLO QCD. The virtuality of the photon, radiated by the incoming electron, can be chosen in a continuous range, reaching from photoproduction into deep inelastic scattering. The various contributions to the full jet cross section, including the resolved photon contributions, are implemented. The calculation is based on the phase-space-slicing method.

Jet cross sections in polarized photon–hadron collisions

We present a computation of one- and two-jet cross sections in polarized photon–hadron collisions, which is accurate to next-to-leading order in QCD. Our results can be used to compute photoproduction cross sections in electron–proton scattering. To this purpose, we investigate the structure of the polarized Weizsäcker–Williams function, where we include a universal, non-logarithmic term, neglected in the literature. We construct a Monte Carlo code, within the framework of the subtraction method, and we use it to study the phenomenology of jet production in the energy range relevant to HERA. In particular, we investigate the perturbative stability of our results, and we discuss the possibility of constraining polarized parton densities of the proton and the photon using jet data.

Next-to-leading order jet cross sections in polarized hadronic collisions

We present a next-to-leading order computation in QCD of one-jet and two-jet cross sections in polarized hadronic collisions. Our results are obtained in the framework of a general formalism that deals with soft and collinear singularities using the subtraction method. We construct a Monte Carlo programme that generates events at the partonic level. We use this code to give phenomenological predictions for pp collisions at √ S = 500 GeV, relevant for the spin physics programme at RHIC. The possibility of using jet data to constrain the poorly known polarized parton densities is examined.

The two-jet cross section at high Q2

We discuss the reliability of NLO QCD predictions for the two-jet cross section at high Q2. If the standard cone algorithm is used in the lab frame, the cross section is divergent. If instead the k algorithm is used, the NLO corrections and scale dependence are small.

Inclusive two-jet production at HERA: direct and resolved cross sections in next-to-leading order QCD

We have calculated inclusive two-jet cross sections in next-to-leading order QCD for low Q2ep collisions superimposing direct and resolved contributions. Infrared and collinear singularities in the virtual and real contributions are cancelled with the phase space slicing method. Various inclusive two-jet distributions have been computed. The results are compared with recent data from the ZEUS collaboration at HERA.

Dependence of Jet Photoproduction on Scales in Higher Order QCD Corrections at HERA

The dependencies of two-jet cross section on renormalization scale and factorization scales of resolved photon and proton are investigated up to next-to-leading order (NLO) QCD. It is shown that the NLO cross section is quite sensitive to the choice of the renormalization scale and the factorization scales of resolved photon and proton in large invariant mass and large negative rapidity of the two-jet system.

Inclusive one- and two-jet cross sections in γγ reactions at e+e− colliders

We have calculated inclusive one- and two jet production in photon-photon collisions superimposing direct, single-resolved and double-resolved cross sections for center of mass energies of the LEP1, LEP2 and NLC range. The direct and single-resolved cross sections are calculated up to next-to-leading order. The double-resolved two-jet cross section is calculated only in LO with a k factor estimated from the NLO one jet cross section. Various differential cross sections as functions of transverse momenta and rapidities of the jets are evaluated.

Dijet production at HERA in next-to-leading order

Two-jet cross sections in deep inelastic scattering at HERA are calculated in next-to-leading order. The QCD corrections are implemented in a new ep → n jets event generator, MEPJET, which allows to analyze arbitrary jet definition schemes and general cuts in terms of parton 4-momenta. First results are presented for the JADE, the cone and the k T schemes. For the W-scheme, disagreement with previous results and large radiative corrections and recombination scheme dependencies are traced to a common origin.

Photoproduction of jets at HERA

We review the theory of jet production in low- ep collisions including next-to-leading-order corrections for the direct and resolved contributions. Inclusive one-jet and two-jet cross sections are considered. The theoretical results are compared with experimental results from the ZEUS and H1 Collaborations at HERA.

Inclusive two-jet cross sections in γ-γ processes at e+e− colliders

We have calculated inclusive one- and two-jet production in photon-photon collisions superimposing direct, single resolved and double resolved cross sections. The direct and single resolved cross sections are calculated up to next-to-leading order. The double resolved two-jet cross section is estimated from leading order with a k factor taken from the single-jet prediction in next-to-leading order. The results are compared with recent experimental data from the TOPAZ and AMY Collaborations at TRISTAN. Good agreement is found between experiment and the theoretical results.

Inclusive dijet production at HERA: direct photon cross sections in next-to-leading order QCD

We have calculated inclusive two-jet cross sections in next-to-leading order QCD for direct photoproduction in low $Q^2$ $ep$ collisions at HERA. Infrared and collinear singularities in real and virtual contributions are cancelled with the phase space slicing method. Analytical formulas for the different contributions giving the dependence on the slicing parameter are presented. Various one- and two-jet distributions have been computed demonstrating the flexibility of the method.

Probing quark fragmentation functions for [formula omitted] baryon production in unpolarized e+e− annihilation

We study the measurement of the quark fragmentation functions for spin- 1 2 baryon production ( Λ and Λ , in particular) in unpolarized e + e − annihilation. The spin-dependent fragmentation functions g ̂ 1(z) and h ̂ 1(z) can be probed in the process as a result of quark-antiquark spin correlation and the weak decay of the baryons. The relevant cross section is expressed as a product of the two-jet cross section, the fragmentation functions, and the differential width of the hyperon decay.