Lowest Order QCD Research Articles

JET CONVERSIONS IN QUARK-GLUON PLASMA

In addition to loosing energy, a quark or gluon jet traversing through a quark-gluon plasma can also be converted to a gluon or quark jet through scattering with the thermal quarks and gluons. Their conversion rates due to two-body elastic [Formula: see text] and inelastic [Formula: see text] scattering have been evaluated in the lowest order in QCD. Including both energy loss and conversions of quark and gluon jets in the expanding quark-gluon plasma produced in relativistic heavy ion collisions, a net conversion of quark jets to gluon jets has been found. This reduces the difference between the nuclear modification factors for quark and gluon jets in heavy ion collisions and thus enhances the ratios of high transverse momentum protons and antiprotons to pions that are produced from the fragmentation of these jets. To account for the observed similar ratios in central Au + Au and p + p collisions at same energy requires, however, a much larger net quark to gluon jet conversion rate than that given by the lowest-order QCD, indicating the importance of higher-order processes and the strongly coupling nature of the quark-gluon plasma in describing the propagation of jets in the quark-gluon plasma.

Contribution of inverse gluon emission to QCD corrections to the Drell-Yan process for experiments at the large hadron collider (LHC)

The contributions of inverse gluon emission to the lowest order QCD corrections to the Drell-Yan process for future experiments at LHC are calculated. The use of fully differential cross sections makes it possible to apply readily the results of these calculations for experimental purposes (in correcting data from future experiments at LHC). It is shown analytically that the present results are independent of the quark mass. A numerical analysis of respective radiative effects is performed by means of the READY FORTRAN code with allowance for the experimental cuts used at the Compact Muon Solenoid (CMS) detector.

Effects of top-quark compositeness on Higgs boson production at the LHC

Motivated by the possibility that the right-handed top-quark (t_R) is composite, we discuss the effects of dimension-six operators on the Higgs boson production at the LHC. When t_R is the only composite particle among the Standard Model (SM) particles, the (V+A)\otimes (V+A) type four-top-quark contact interaction is expected to have the largest coefficient among the dimension-six operators, according to the Naive Dimensional Analysis (NDA). We find that, to lowest order in QCD and other SM interactions, the cross section of the SM Higgs boson production via gluon fusion does not receive corrections from one insertion of the new contact interaction vertex. We also discuss the effects of other dimension-six operators whose coefficients are expected to be the second and the third largest from NDA. We find that the operator which consists of two t_R's and two SM Higgs boson doublets can recognizably change the Higgs boson production cross section from the SM prediction if the cut-off scale is \sim 1TeV.

Jet flavor conversions in the quark-gluon plasma

A quark or gluon jet traversing through a quark-gluon plasma can be converted to a gluon or quark jet through scattering with the thermal quarks and gluons in the quark-gluon plasma. Their conversion rates due to two-body elastic \(q({\bar q})g\rightarrow gq(\bar q)\) and inelastic \(q{\bar q}\leftrightarrow gg\) scattering have recently been evaluated in the lowest order in QCD. Including both energy loss and conversions of quark and gluon jets in the expanding quark-gluon plasma produced in relativistic heavy ion collisions, a net conversion of quark jets to gluon jets has been found. This reduces the difference between the nuclear modification factors for quark and gluon jets in heavy ion collisions and thus enhances the ratios of high transverse momentum protons and antiprotons to pions that are produced from the fragmentation of these jets. To account for the observed similar ratios in central Au + Au and p + p collisions at same energy requires, however, a much larger net quark to gluon jet conversion rate than that given by the lowest-order QCD, indicating the importance of higher-order processes and the strongly coupling properties of the quark-gluon plasma in describing the propagation of jets in the quark-gluon plasma.

Heavy quark three-body collisional energy loss in QGP

Heavy quark drag coefficients due to three-body elastic scattering in a quark–gluon plasma have been evaluated in the lowest order in QCD. They are found to have similar values as those due to two-body elastic scattering, and both are larger than those due to two-body radiative scattering if the momenta of charm and bottom quarks are below about 4 and 9 GeV/c, respectively. Including three-body elastic scattering increases the momentum degradation of heavy quarks in QGP, bringing the nuclear modification factor for electrons from the decays of produced heavy mesons in Au+Au collisions at closer to the measured one.

Suppression of Heavy Quarks in Heavy-ion Collisions

Abstract Heavy quark drag coefficients in a quark-gluon plasma due to three-body elastic scattering have recently been approximately evaluated in the lowest order in QCD. For both charm and bottom quarks, their values are found to be comparable to those from two-body elastic scattering. In the Fokker-Planck approach based on a schematic expanding fireball model, its effect on the transverse momentum spectra of heavy quarks produced from Au+Au collisions at center of mass energy s NN = 200 has been studied, and the results show that adding contributions from heavy quark three-body scattering to those due to two-body elastic and radiative scattering brings the nuclear modification factor for electrons from heavy meson decays closer to available experimental data.

Drell-Yan lepton angular distribution at small transverse momentum

We investigate the dependence of the Drell-Yan cross section on lepton polar and azimuthal angles, as generated by the lowest-order QCD annihilation and Compton processes. We focus in particular on the azimuthal-angular distributions, which are of the form cos(phi) and cos(2phi). At small transverse momentum q_T of the lepton pair, q_T << Q, with Q the pair mass, these terms are known to be suppressed relative to the phi-independent part of the Drell-Yan cross section by one or two powers of the transverse momentum. Nonetheless, as we show, like the phi-independent part they are subject to large logarithmic corrections, whose precise form however depends on the reference frame chosen. These logarithmic contributions ultimately require resummation to all orders in the strong coupling. We discuss the potential effects of resummation on the various angular terms in the cross section and on the Lam-Tung relation.

Equation of state of deconfined matter within a dynamical quasiparticle description

A simple quasiparticle model, motivated by lowest-order perturbative QCD, is proposed. It is applied to interpret the lattice QCD equation of state. A reasonable reproduction of the lattice data is obtained. In contrast to existing quasiparticle models, the present model is formulated in dynamical rather than thermodynamical terms, and is easily applicable to a system with finite baryon density. In particular, the model simulates the confinement property.

Scalar and pseudoscalar Higgs boson plus one jet production at the CERN LHC and Fermilab Tevatron

The production of the standard model Higgs boson (H) plus one jet is compared with that of the lightest scalar Higgs boson ${(h}^{0})$ plus one jet and that of the pseudoscalar Higgs boson ${(A}^{0})$ plus one jet. The latter particles belong to the minimal supersymmetric model. We include both top and bottom quark loops to lowest order in QCD and investigate the limits of small quark mass and infinite quark mass. We give results for both the CERN Large Hadron Collider and the Fermilab Tevatron.

Studies of W + jets production with the CDF detector

We have studied the W + ≥ n jets process in Tevatron Run II experiment. This is the first result for the CDF Run II experiment. The data used correspond to a total integrated luminosity of 72 pb–1 taken from March 2002 through January 2003. The lowest order QCD predictions have been tested with a new prescription of the parton-jet matching, which allows to avoid a theoretical ambiguity of the collinear/infrared enhancement in calculation. We found a good agreement between data and theory in the typical kinematic distributions. The number of events for each inclusive samples up to 3 jets are also compared with Monte Carlo calculations. PACS: 13.87.Ce Jets Production in large-Q2 scattering

Extraction of polarized gluon distributions from large-p light hadron pair production

We propose a new formula for extracting the polarized gluon distribution from the large-$p_T$ light hadron pair production for semi-inclusive processes in polarized deep inelastic scattering. In general, a large-$p_T$ hadron pair is produced via photon-gluon fusion(PGF) and QCD Compton at the lowest order of QCD. The PGF gives us a direct information on $\Delta g$ in the nucleon, while QCD Compton becomes background to the signal process for extracting $\Delta g$. We show that the contribution from this background, i.e. the QCD Compton process, can be removed by using symmetry relation among fragmentation functions and taking an appropriate combination of various light hadron pair production processes, and thus the double spin asymmetry can be described in terms of $\Delta g/g$ alone.

An estimate of higher twist at small $x_{\mathrmB}$ and low $Q^2$ based upon a Saturation Model

We investigate the influence of higher twist corrections to deep inelastic structure functions in the low- $Q^2$ and small-x HERA region. We review the general features of the lowest-order QCD diagrams which contribute to twist-4 at small-x, in particular the sign structure of longitudinal and transverse structure functions which offers the possibility of strong cancellations in $F_2$ . For a numerical analysis we perform a twist analysis of the saturation model which has been very successful both in describing the structure function and the DIS diffractive cross section at HERA. As the main conclusion, twist 4 corrections are not small in $F_L$ or $F_T$ but in $F_2=F_L + F_T$ they almost cancel. This indicates the limitation on the use of the DGLAP formalism at small x and $Q^2$ . We point out that $F_L$ analysis needs a large twist-4 correction. We also indicate the region of validity of the twist expansion.

Diffractive photo- and leptoproduction of vector mesons $\rho$, $\rho'$ and $\rho''$

We calculate diffractive photo- and leptoproduction of $\rho$ -, $\rho'$ - and $\rho''$ -mesons. The incoming photon dissociates into a $q\bar{q}$ -dipole which scatters on the nucleon and transforms into a vector meson state. The scattering amplitude is calculated in non-perturbative QCD with the model of the stochastic vacuum. Assuming that the physical $\rho'$ - and $\rho''$ -mesons are mixed states of an active 2S-excitation and some residual hybrid state which cannot be produced diffractively in lowest order QCD, we obtain good agreement with the data, especially the markedly different spectrum in the $\pi^+\pi^-$ -invariant mass for photoproduction and $e^+e^-$ -annihilation.

The collision energy dependence of dijet cross sections as a probe of BFKL physics

The dependence of the subprocess cross section for dijet production at fixed transverse momentum on the (large) rapidity separation Delta y of the dijets can be used to test for `BFKL physics', i.e. the presence of higher--order (alphas log(Delta y)^n contributions. Unfortunately in practice these subtle effects are masked by the additional, stronger dependence arising from the parton distributions. We propose a simple ratio test using two different collider energies n which the parton distribution dependence largely cancels. Such a ratio does distinguish between the `asymptotic' analytic BFKL and lowest-order QCD predictions. However, when subasymptotic effects from overall energy-momentum conservation are included, the BFKL predictions for present collider energies change qualitatively. In particular, the real gluon emission contributions to the BFKL cross section are suppressed. The proposed ratio therefore provides an interesting laboratory for studying the interplay of leading-logarithm and kinematic effects.

On the twist-2 and twist-3 contributions to the spin-dependent electroweak structure functions

The twist-2 and twist-3 contributions of the polarized deep-inelastic structure functions are calculated both for neutral and charged current interactions using the operator product expansion in lowest order in QCD. The relations between the different structure functions are determined. New integral relations are derived between the twist-2 contributions of the structure functions g 3 ( x, Q 2) and g 5 ( x, Q 2) and between combinations of the twist-3 contributions to the structure functions g 2( x, Q 2) and g 3 ( x, Q 2). The sum rules for polarized deep-inelastic scattering are discussed in detail.

Elastic vector-meson production at HERA

We show that the lowest-order QCD calculation in a simple model of elastic vector-meson production does reproduce correctly the ratios of cross sections for ϱ, Φ and J ψ , both in photoproduction and in high- Q 2 quasi-elastic scattering. The dependence of the slopes on the mass of the vector meson is reproduced as well. We examine the lower-energy data, and find that the energy dependence of the cross section does not depend on Q 2, but may depend on m V .

Confinement: Understanding the relation between the Wilson loop and dual theories of long distance Yang-Mills theory.

In this paper we express the velocity-dependent, spin-dependent heavy quark potential ${V}_{q\overline{q}}$ in QCD in terms of a Wilson loop $W(\ensuremath{\Gamma})$ determined by pure Yang-Mills theory. We use an effective dual theory of long-distance Yang-Mills theory to calculate $W(\ensuremath{\Gamma})$ for large loops, i.e., for loops of size $R&gt;{R}_{\mathrm{FT}}$. [${R}_{\mathrm{FT}}$ is the flux tube radius, fixed by the value of the Higgs (monopole) mass of the dual theory, which is a concrete realization of the Mandelstam-'t Hooft dual superconductor mechanism of confinement.] We replace $W(\ensuremath{\Gamma})$ by ${W}_{\mathrm{eff}}(\ensuremath{\Gamma})$, given by a functional integral over the dual variables, which for $R&gt;{R}_{\mathrm{FT}}$ can be evaluated by a semiclassical expansion, since the dual theory is weakly coupled at these distances. The classical approximation gives the leading contribution to ${W}_{\mathrm{eff}}(\ensuremath{\Gamma})$ and yields a velocity-dependent heavy quark potential that for large $R$ becomes linear in $R$, and that for small $R$ approaches lowest-order perturbative QCD. This latter fact means that these results should remain applicable down to distances where radiative corrections giving rise to a running coupling constant become important. The spin dependence of the potential at long range as well as at short range reflects the vector coupling of quarks in QCD combined with the dual treatment of long-distance Yang-Mills theory. The methods developed here should be applicable to any realization of the dual superconductor mechanism. They give an expression determining ${W}_{\mathrm{eff}}(\ensuremath{\Gamma})$ independent of the classical approximation, but semiclassical corrections due to fluctuations of the flux tube are not worked out in this paper. Taking these into account should lead to an effective string theory free from the conformal anomaly.

Massive quasiparticle model of the SU(3) gluon plasma.

Recent SU(3) gauge field lattice data for the equation of state are interpreted by a quasiparticle model with effective thermal gluon masses. The model is motivated by lowest-order perturbative QCD and describes very well the data. The proposed quasiparticle approach can be applied to study color excitations in the nonperturbative regime. As an example we estimate the temperature dependence of the Debye screening mass and find that it declines sharply when approaching the confinement temperature from above, while the thermal mass continuously rises.

Twist-2 contributions to polarized structure functions and new sum rules

The twist-2 contributions to the polarized structure functions in deep inelastic lepton-hadron scattering are calculated including the exchange of weak bosons and using both the operator product expansion and the covariant parton model. A new relation between two structure functions leading to a sequence of new sum rules is found. The light quark mass corrections to the structure functions are derived in lowest order QCD.

Particle production in a hadron collider rapidity gap: the Higgs case

Production of rare particles within rapidity gaps has been proposed as a background-free signal for the detection of new physics at hadron colliders. No complete formalism accounts for such processes yet. We study a simple lowest-order QCD model for their description. Concentrating on Higgs production, we show that the calculation of the cross section pp → ppH can be embedded into existing models which successfully account for diffractive data. We extend those models to take into account single- and double-diffractive cross sections pp → HX 1 X 2 with a gap between the fragments X 1 and X 2. Using conservative scenarios, we evaluate the uncertainties in our calculation, and study the dependence of the cross section on the gap width. We predict that Higgs production within a gap of 4 units of rapidity is about 0.3 pb for a 100 GeV Higgs at the Tevatron, and almost 2 pb for a 400 GeV Higgs within a gap of 6 units at the LHC with √ s = 14 TeV.