Multiple Gluon Emission Research Articles

Role of initial gluon emission in double J/ψ production at central rapidities

We consider the process of double $J/\ensuremath{\psi}$ production in $pp$ collisions at the LHC in the framework of the ${k}_{T}$-factorization approach. We focus on the gluon fragmentation mechanism which is related to multiple gluon emission in the initial state. The initial state emission is treated according to Catani-Ciafaloni-Fiorani-Marchesini evolution equation, and applies to both single and double parton scattering cases. We show the importance of fragmentation contributions to $J/\ensuremath{\psi}$ pair production and perform a comparison between theoretical predictions and the latest ATLAS data collected at $\sqrt{s}=8\text{ }\text{ }\mathrm{TeV}$. We find that the effects of multiple gluon emission are essential for both single- and multiparton interaction processes. Finally, we highlight the problem of correct choice of the factorization scale with respect to numerical stability of the calculations and the consistency with noncollinear evolution equations.

Radiative energy loss and radiative p ⊥-broadening of high-energy partons in QCD matter

We study the connection between radiative energy loss and radiative p_T-broadening of a high-energy quark or gluon passing through QCD matter. The generalized Baier-Dokshitzer-Mueller-Peigne-Schiff-Zakharov (BDMPS-Z) formalism is used to calculate energy loss due to multiple gluon emission. With L the length of the matter and l_0 the size of constituents of the matter we find a double logarithmic correction proportional to ln^2(L/l_0) to parton energy loss due to two-gluon emission. We also show that the radiative energy loss per unit length -dE/dz = alpha_s N_c < p_T^2 >/12 by carrying out a resummation of the double logarithmic terms. Here, the transverse momentum broadening <p_T^2> is obtained by resumming terms proportional to alpha_s ln^2(L/l_0) in Ref. [11]. Our result agrees with that by the renormalization of qhat proposed in Refs. [13,14].

Initial conditions for the modified evolution of fragmentation functions in the nuclear medium

Initial conditions are required to solve medium modified DGLAP (mDGLAP) evolution equations for modified fragmentation functions due to multiple scatterings and parton energy loss. Such initial conditions should in principle include energy loss for partons at scale $Q_0$ above which mDGLAP evolution equations can be applied. Several models for the initial condition motivated by induced gluon bremsstrahlung in perturbative QCD are used to calculate the modified fragmentation functions in nuclear medium and to extract the jet transport parameter $\hat q$ from fits to experimental data in deeply inelastic scattering (DIS) off nuclei. The model with a Poisson convolution of multiple gluon emissions is found to provide the overall best $\chi^2$/d.o.f. fit to the HERMES data and gives a value of $\hat q_0 \approx 0.020 \pm 0.005$ GeV$^2$/fm at the center of a large nucleus.

Perturbative versus non-perturbative aspects of jet quenching: in-medium breaking of color coherence

Abstract The quenching of jets (and high- p T particle spectra) observed in heavy-ion collisions is interpreted as due to the energy lost by hard partons crossing the Quark Gluon Plasma. Here we review recent efforts to include in its modeling important qualitative features of QCD, like the correlations in multiple gluon emissions and the color-flow pattern in parton branchings. In particular, the modification of color connections among the partons of a shower developing in the presence of a medium is a generic occurrence accompanying parton energy-loss. We show how this effect can leave its fingerprints at the hadronization stage, leading by itself to a softening of hadron spectra and to an enhanced production of soft particles in jet-fragmentation.

The λ-measure and the generalised dipoles in the Lund model

We demonstrate that the multiple gluon emission phase space in the dipole cascade model has a strong linear correlation with the number of gluons emitted. The number of gluons per unit available phase space at a certain resolution scale is found to be remarkably independent of the cms energy and global event properties like thrust, and even changes in the ordering variable or resolution scale. We show that the distribution of sizes of gluon–gluon dipoles in a parton cascade has stability properties which are sufficient to account for those of the phase space variable. We observe that certain more abstract entities, defined in the context of hadronisation and related to the gluon emission phase space, share those properties of colour dipoles and name them generalised dipoles. We also present an analytical model to qualitatively describe our findings.

Erratum: Multiple gluon effects in fermion-(anti)fermion scattering at and beyond CERN LHC energies [Phys. Rev. D52, 108 (1995)

We extend the methods of Yennie, Frautschi and Suura (YFS) to compute, via Monte Carlo methods, the effects of multiple gluon emission in the processes $q+q'\to q''+q'''+n(G)$, where $G$ is a soft gluon. We show explicitly that the infrared singularities in the respective simulations are canceled to all orders in $\alpha_s$. Some discussion of this result from the standpoint of confinement is given. More importantly, we present, for the first time ever, sample numerical Monte Carlo data on multiple soft gluon emission in the rigorously extended YFS framework. We find that such soft gluon effects must be taken into account for precise SSC/LHC physics simulations.

QCD radiation off heavy particles

We study QCD radiation in decay processes involving heavy particles. As input, the first-order gluon emission rate is calculated in a number of reactions, and comparisons of the energy flow patterns show a non-negligible process dependence. To proceed further, the QCD parton shower language offers a convenient approach to include multi-gluon emission effects, and to describe exclusive event properties. An existing shower algorithm is extended to take into account the process-dependent mass, spin and parity effects, as given by the matrix element calculations. This allows an improved description of multiple gluon emission effects off b and t quarks, and also off nonstandard particles like squarks and gluinos. Phenomenological applications are presented for bottom production at LEP, Higgs particle decay to heavy flavours, top production and decay at linear colliders, and some simple supersymmetric processes.

Multiple gluon effects in fermion-(anti)fermion scattering at and beyond CERN LHC energies.

We extend the methods of Yennie, Frautschi, and Suura (YFS) to compute, via Monte Carlo methods, the effects of multiple gluon emission in the processes q+(q\ifmmode\bar\else\textasciimacron\fi{})'\ensuremath{\rightarrow}q''+(q\ifmmode\bar\else\textasciimacron\fi{})'''+n(G), where G is a soft gluon. We show explicitly that the infrared singularities in the respective simulations are canceled to all orders in ${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$. Some discussion of this result from the standpoint of confinement is given. More importantly, we present, for the first time, sample numerical Monte Carlo data on multiple soft gluon emission in the rigorously extended YFS framework. We find that such soft gluon effects must be taken into account for precise physics simulations at and beyond CERN LHC energies.

QCD predictions for the transverse energy flow in deep-inelastic scattering in the DESY HERA small x regime.

The distribution of transverse energy, $E_T$, which accompanies deep-inelastic electron-proton scattering at small $x$, is predicted in the central region away from the current jet and proton remnants. We use BFKL dynamics, which arises from the summation of multiple gluon emissions at small $x$, to derive an analytic expression for the $E_T$ flow. One interesting feature is an $x^{-\epsilon}$ increase of the $E_T$ distribution with decreasing $x$, where $\epsilon = (3\alpha_s/\pi)2\log 2$. We perform a numerical study to examine the possibility of using characteristics of the $E_T$ distribution as a means of identifying BFKL dynamics at HERA.

Minijet corrections to Higgs-boson production.

We study higher order corrections to Higgs production with an associated jet at SSC energies, using the resummation of the leading logarithmic contributions to multiple gluon emissions due to Lipatov and collaborators. We find a considerable enhancement of Higgs production at large transverse momenta.

Fluctuations and anomalous dimensions in QCD cascades

A recently proposed multiplicity measure is used to study the properties of QCD cascades. We show that it is possible to define anomalous dimensions locally in rapidity space for the QCD cascade ine+e−-annihilation events and in deep inelastic lepton scattering. In this way it will be possible to differentiate between different suggested models for multiple gluon emission. We also show that the properties of an event are to a surprisingly large degree deterimined by the first one or two gluons. Thus e.g. the multiplicity fluctuations ine+e−-annihilation at theZ0-pole are to about 90% determined by the hardest gluon. This implies that it may be principally difficult to make a distinct separation between the hard perturbative phase and the soft hadronization phase.

QCD at collider energies.

We examine available experimental distributions of transverse energy and transverse momentum, obtained at the CERN pp-bar collider, in the context of quantum chromodynamics. We consider the following. (i) The hadronic transverse energy released during W/sup + -/ production. This hadronic transverse energy is made out of two components: a soft component which we parametrize using minimum-bias events and a semihard component which we calculate from QCD. (ii) The transverse momentum of the produced W/sup + -/. If the transverse momentum (or the transverse energy) results from a single gluon jet we use the formalism of Dokshitzer, Dyakonov, and Troyan, while if it results from multiple-gluon emission we use the formalism of Parisi and Petronzio. (iii) The relative transverse momentum of jets. While for W/sup + -/ production quarks play an essential role, jet production at moderate p/sub T/ and present energies is dominated by gluon-gluon scattering and therefore we can study the Sudakov form factor of the gluon. We suggest also how through a Hankel transform of experimental data we can have direct access to the Sudakov form factors of quarks and gluons.