Average Gluon Research Articles

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].

Contribution of orbital angular momentum to the nucleon spin

We have calculated the orbital angular momentum of quarks and gluons in the nucleon. The calculations are carried out in the next to leading order utilizing the so-called valon model. It is found that the average quark orbital angular momentum is positive, but small, and the average gluon orbital angular momentum is negative and large. We also report on some regularities about the total angular momentum of the quarks and the gluon, as well as on the orbital angular momentum of the separate partons. We have also provided partonic angular momentum, [Formula: see text] as a function of [Formula: see text].

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

The Weighted GMD Model for multiplicity distributions

A new distribution, the Weighted GMD (WGMD) is obtained from the Generalised Multiplicity Distribution (GMD), describing charged-particle multiplicity distributions as the hadronisation products of quark and gluon branching with fluctuations in the initial gluon numbers produced from the collision. The WGMD is shown to describe charged-particle multiplicity distributions in pp collisions at the Large Hadron Collider (LHC), and the average initial gluon number is obtained for Poisson distributed gluon multiplicities.

Jets in multiparticle production in and beyond geometry of proton–proton collisions at the LHC

Experimental findings by CMS on properties of jets and underlying events at high multiplicities in proton–proton interactions at 7 TeV are interpreted as an indication of increasing role of central collisions with small impact parameters. The value of the average impact parameter of the pp collisions as a function of the soft hadron multiplicity is estimated. We find an indication that the rates of different hard processes observed by CMS and ALICE universally depend on the underlying event charged-particle multiplicity until it becomes four times higher than average. It is shown that the increase of the overlap area of colliding protons is not sufficient to explain the rate of jet production in events with charged-particle multiplicity that is more than three times higher than average. New mechanisms are necessary, like interaction of protons in rare configurations of higher than average gluon density. Such mechanisms are not included in the present Monte Carlo event generators. Further studies are proposed.

Average gluon and quark jet multiplicities at higher orders

We develop a new formalism for computing and including both the perturbative and nonperturbative QCD contributions to the scale evolution of average gluon and quark jet multiplicities. The new method is motivated by recent progress in timelike small-x resummation obtained in the MS¯ factorization scheme. We obtain next-to-next-to-leading-logarithmic (NNLL) resummed expressions, which represent generalizations of previous analytic results. Our expressions depend on two nonperturbative parameters with clear and simple physical interpretations. A global fit of these two quantities to all available experimental data sets that are compatible with regard to the jet algorithms demonstrates by its goodness how our results solve a longstanding problem of QCD. We show that the statistical and theoretical uncertainties both do not exceed 5% for scales above 10 GeV. We finally propose to use the jet multiplicity data as a new way to extract the strong-coupling constant. Including all the available theoretical input within our approach, we obtain αs(5)(Mz)=0.1199±0.0026 in the MS¯ scheme in an approximation equivalent to next-to-next-to-leading order enhanced by the resummations of lnx terms through the NNLL level and of lnQ2 terms by the renormalization group, in excellent agreement with the present world average.

Leading and next-to-leading order gluon polarization in the nucleon and longitudinal double spin asymmetries from open charm muoproduction

The gluon polarisation in the nucleon was measured using open charm production by scattering 160 GeV/c polarised muons off longitudinally polarised protons or deuterons. The data were taken by the COMPASS collaboration between 2002 and 2007. A detailed account is given of the analysis method that includes the application of neural networks. Several decay channels of D^0 mesons are investigated. Longitudinal spin asymmetries of the D meson production cross-sections are extracted in bins of D^0 transverse momentum and energy. At leading order QCD accuracy the average gluon polarisation is determined as (Delta g/G)^LO=-0.06 +/- 0.21 (stat.) +/- 0.08 (syst.) at the scale <mu^2> ~13 (GeV/c)^2 and an average gluon momentum fraction <x>~ 0.11. For the first time, the average gluon polarisation is also obtained at next-to-leading order QCD accuracy as (Delta g/G)^NLO = -0.13 +/- 0.15 (stat.) +/- 0.15 (syst.) at the scale <mu^2> ~ 13 (\GeV/c)^2 and <x> ~ 0.20.

Gluon polarisation in the nucleon and longitudinal double spin asymmetries from open charm muoproduction

The gluon polarisation in the nucleon has been determined by detecting charm production via D0 meson decay to charged K and π in polarised muon scattering off a longitudinally polarised deuteron target. The data were taken by the COMPASS Collaboration at CERN between 2002 and 2006 and correspond to an integrated luminosity of 2.8 fb−1. The dominant underlying process of charm production is the photon–gluon fusion to a cc¯ pair. A leading order QCD approach gives an average gluon polarisation of 〈Δg/g〉x=−0.49±0.27(stat)±0.11(syst) at a scale μ2≈13 (GeV/c)2 and at an average gluon momentum fraction 〈x〉≈0.11. The longitudinal cross-section asymmetry for D0 production is presented in bins of the transverse momentum and the energy of the D0 meson.

Jet tomography of Au+Au reactions including multi-gluon fluctuations

Jet tomography is the analysis of the attenuation pattern of high transverse momentum hadrons to determine certain line integral transforms of the density profile of the QCD matter produced in ultra-relativistic nuclear collisions. In this Letter, we calculate the distortion of jet tomography due to multi-gluon fluctuations within the GLV radiative energy loss formalism. We find that fluctuations of the average gluon number, 〈 N g 〉∼3 for RHIC initial conditions, reduce the attenuation of pions by approximately a factor Z≈0.4–0.5. Therefore the plasma density inferred from jet tomography without fluctuations must be enhanced by a factor 1/ Z∼2.

Gluon structure functions in nuclei from the quark cluster model

The quark cluster model has been previously shown to give a good description of the EMC effect and Drell-Yan dimuon pair production on nuclear targets at all values of the momentum fractionx. Here, we extend the quark cluster model in order to predictRG(x), the ratio of gluon structure functions in nuclei and nucleons. We find an increase in the average gluon momentum fraction as a function of the nucleon numberA, which agrees with recent results of the New Muon Collaboration (NMC) experiment.

Parton spin asymmetries in polarized hadrons at high energies

The predictions of perturbative QCD for the evolution of quark and gluon distributions for polarized hadrons, in general, and for polarized protons, in particular, are developed. Analytical and numerical studies suggest that quark and gluon polarizations increase with increasing energy at fixed Feynman x. Moreover, the average gluon asymmetry grows as ln Q 2 receiving contributions from both quark and gluon asymmetries at lower energy. This has important implications for the growth of orbital angular momenta of partons and presents a challenge to model building. A new sum rule for a particular moment of the quark and gluon asymmetries is derived. We conclude that more work is needed to determine how useful polarized proton beams might be in the search for and exploration of new physics in lepton-proton, p p and pp colliders.

Quark jet as a trigger for gluon jet in deep inelastic muon scattering

We calculate, to first order in QCD, the probability for a fast hadron observed at laboratory angle θ h from the virtual photon directionq, to originate from the quark of a quark-gluon final state. Given this probability, the average gluon energy and angle with respect toq are then calculated. This approach provides a method for gluon jet direction in forthcoming experiments.