Perturbative QCD Model Research Articles

Energy and centrality dependence of rapidity densities at RHIC energies.

The energy and centrality dependence of the charged multiplicity per participant nucleon is shown to be able to differentiate between final state saturation and fixed scale perturbative QCD models of initial entropy production in high-energy heavy-ion collisions. The energy dependence is shown to test the nuclear enhancement of the minijet component of the initial conditions, while the centrality dependence provides a key test of whether gluon saturation is reached at RHIC energies. The HIJING model predicts that the rapidity density per participant increases with centrality, while the saturation model prediction is essentially independent of centrality.

Quark structure ofΛfromΛpolarization inZdecays

The flavor and spin structure for the quark distributions of the $\ensuremath{\Lambda}$ baryon is studied in a perturbative QCD (PQCD) analysis and in the SU(6) quark-diquark model, and then applied to calculate the $\ensuremath{\Lambda}$ polarization of semi-inclusive $\ensuremath{\Lambda}$ production in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation near the Z pole. It is found that the quark-diquark model gives a very good description of the available experimental data. The PQCD model can also give a good description of the data by taking into account the suppression of quark helicities compared to the naive SU(6) quark model spin distributions. Further information is required for a clean distinction between different predictions concerning the flavor and spin structure of the $\ensuremath{\Lambda}.$

The quark–gluon string model: soft and semihard hadronic processes

The basic principles of the quark–gluon string model, a perturbative QCD approach to describing hadronic processes, are presented. The relation between the s -channel topological 1/N expansion of the hadron–hadron scattering amplitude and the t -channel expansion in Regge poles is demonstrated, where N is the number of quark flavors or colors. This approach is used to analyze soft hadronic processes. The attempts to extend the region of applicability of this model to describe the inclusive spectra of resonances and also semihard hadronic processes are briefly reviewed. In particular, a new modification of the model in which Pomeron exchange is treated as the exchange of two nonperturbative gluons with dynamically generated mass is discussed. Soft and semihard processes, and also charmed-particle production in hadron collisions, are analyzed in this version of the model. The advantages and defects of this approach compared to other perturbative QCDmodels are discussed.

Spin dependence of high energy proton scattering

Motivated by the need for an absolute polarimeter to determine the beam polarization for the forthcoming RHIC spin program, we study the spin dependence of the proton-proton elastic scattering amplitudes at high energy and small momentum transfer.We examine experimental evidence for the existence of an asymptotic part of the helicity-flip amplitude phi_5 which is not negligible relative to the largely imaginary average non-flip amplitude phi_+. We discuss theoretical estimates of r_5, essentially the ratio of phi_5 to phi_+, based upon extrapolation of low and medium energy Regge phenomenological results to high energies, models based on a hybrid of perturbative QCD and non-relativistic quark models, and models based on eikonalization techniques. We also apply the model-independent methods of analyticity and unitarity.The preponderence of evidence at available energy indicates that r_5 is small, probably less than 10%. The best available experimental limit comes from Fermilab E704:those data indicate that |r_5|<15%. These bounds are important because rigorous methods allow much larger values. In contradiction to a widely-held prejudice that r_5 decreases with energy, general principles allow it to grow as fast as ln(s) asymptotically, and some models show an even faster growth in the RHIC range. One needs a more precise measurement of r_5 or to bound it to be smaller than 5% in order to use the classical Coulomb-nuclear interference technique for RHIC polarimetry. As part of this study, we demonstrate the surprising result that proton-proton elastic scattering is self-analysing, in the sense that all the helicity amplitudes can, in principle, be determined experimentally at small momentum transfer without a knowledge of the magnitude of the beam and target polarization.

Measurement of elastic ϒ photoproduction at HERA

The photoproduction reaction γp→ μ +μ −p has been studied in ep interactions using the ZEUS detector at HERA. The data sample corresponds to an integrated luminosity of 43.2 pb −1 . The ϒ meson has been observed in photoproduction for the first time. The sum of the products of the elastic ϒ(1 S), ϒ(2 S), ϒ(3 S) photoproduction cross sections with their respective branching ratios is determined to be 13.3±6.0( stat.) +2.7 −2.3( syst.) pb at a mean photon-proton centre of mass energy of 120 GeV. The cross section is above the prediction of a perturbative QCD model.

QCD description of diffractiveρmeson electroproduction

We critically review the QCD predictions for the cross sections sigma_L and sigma_T for diffractive rho meson electroproduction in longitudinally and transversely polarised states in the HERA energy region. We show that both perturbative and non-perturbative approaches which involve convolution with the rho meson wave function predict values of sigma_T which fall-off too quickly with increasing Q^2, in comparison with the data. We present a perturbative QCD model based on the open production of light qqbar pairs and parton-hadron duality, which describes all features of the data for rho electroproduction at high Q^2 and, in particular, predicts a satisfactory Q^2 behaviour of sigma_L/sigma_T. We find that precise measurements of the latter can give valuable information on the Q^2 behaviour of the gluon distribution at small x.

Perturbative QCD estimation of the Bc exclusive nonleptonic decays

We argue that the B c exclusive nonleptonic decays are perturbatively calculable. Working in a perturbative QCD model of Brodsky et al., we pay attention to the estimate of the partial widths of the modes B c → J Ψπ + , J Ψϱ + , etc., which serve to identify B c mesons and determine the B c mass and life time. Our estimations are found to be lower than the other predictions. We note that the spectator ansatz might be misleading in dealing with B c weak decays. So the results will be potentially important at LHC to measure the B c meson and to test various models of weak decays of heavy mesons.

Perturbative QCD estimation of the [formula omitted] branching ratio

Working in a perturbative QCD model approach, we obtain the essential form factor of the radiative transition B → V ∗γ and estimate the branching ratio BR(B → K ∗γ) . The results are determined by a parameter expressing the momentum distribution in the B-meson wave function. Our estimations are compared to other theoretical predictions as well as to experimental data.

Compact stars: Neutron stars or quark stars or hybrid stars?

Using results from energy calculations of neutron matter, we construct various neutron star equations of state. From these equations of state, together with the Tolman-Oppenheimer-Volkoff equations, we calculate quantities such as pressure, mass density, mass energy density, total mass, radius, and moment of inertia for neutron stars. Comparison is made with calculations based on other nuclear potentials and nuclear energy calculations, and our results are in reasonable agreement with results from observational data. We also calculate mass, radius, moment of inertia and surface gravitational red shift of “quark stars” described by three models: a non-interacting Fermi gas model, an asymptotic MIT bag model, and a perturbative QCD model. We also give results of phase transition calculations, and comment on possible observable differences between neutron stars and quark stars. Finally, properties of “hybrid stars” consisting of a core of strange quark matter surrounded by ordinary neutron matter are investigated. We discuss star models based on phenomenological equations of state including a phase transition between the hadronic phase and the quark-gluon plasma. For certain parameters, these equations of state support the existence of hybrid stars. The identification of such objects could provide information on the properties of strange quark matter.

POSSIBLE DYNAMICAL ORIGINS OF INTERMITTENCY IN HIGH ENERGY COLLISIONS

This is a brief review of the dynamical origins of intermittency. For elementary processes involving leptons and hadrons, we discuss toy models, perturbative QCD, Monte-Carlo and other models. For complex processes involving nuclear collisions we review theoretical ideas on hadronic fluctuations as consequences of phase transition.

Hadroproduction ofJ/? at largep T and its relevance to a signal of quark gluon plasma

Using the parton fusion model of perturbative QCD, we compute the production cross sections ofJ/ψ at largep T inpp,h(=π,p)A andAA collisions. The quark structure functions which we use are parametrised in a model-dependent manner to incorporate the well-known EMC-effect, whereas our nuclear gluon densities are as yet untested predictions of the model. Comparison of our computations withpp data and πA data suggests that in the respective kinematic ranges the perturbative QCD model works reasonably well and the predicted nuclear gluon density is a good description of the data. Having thus tested the ingredients of our computations, we compute the ratios ofJ/ψ production in the central regions ofAA,pA andpp as a function of thep T of theJ/ψ. Our results show a remarkable qualitative similarity to the proposed signature of the quark-gluon plasma, making its identification by this signature a fully quantitative task.

Large p TJ/ ψ production in pp, πA and AA collisions

Using a perturbative QCD model, we compute the production cross sections of J/ ψ at large pip in pp, πa and AA collisions. Comparison of our computations with pp and πa data from CERN and Fermilab suggests that i) the model works well for p T ⩾ 1.5 GeV at s = 20 GeV and ii) our chosen nuclear gluon density is a good description of data. Our results for the ratio of J/ ψ production in the central region of AA collisions to that in pp collisions exhibit a qualitatively similar pattern to that of the proposed signature of quark gluon plasma.

Meson decays in an effective perturbative QCD model

Abstract In a continuation of the examination of an effective perturbative QCD approach to some low-energy phenomena of nuclear and particle physics, we consider the decays of mesons into two other mesons. We include S-state and P-state (excited) mesons of the standard quark model description of mesons. Simple effective propagators are used for the colored vector (e.g. gluon-like) boson and the results are compared to experiment.

Strangeness production and evolution in quark gluon plasma

The formation and evolution of strange quarks in quark gluon plasma is studied assuming perturbative QCD and qualitative models of plasma phase expansion. Chemical equilibrium abundance characteristic of the hottest and densest phase of nuclear collisions is proven to survive the process of expansion and cooling of the plasma.

Measurement of the dijet cross section in 400-GeV/c pp interactions.

The invariant cross section for production of jet pairs in 400-GeV/c pp interactions has been measured as a function of p/sub T/ in the p/sub T/ range 4 to 9 GeV/c. The results are in good agreement with predictions of a perturbative QCD model. Details of the experiment and the procedures used to extract the jet signal are given.

Improved perturbative QCD estimates for the hadroproduction of heavy flavors.

Conservative estimates of heavy-flavor production are made within a parton perturbative QCD model, and lead to sizable cross sections. In particular, the integrated cross sections sigma/sub t/(p/sub T/>2 GeV) for M/sub t/ = 20 GeV top-quark production change from 2 ..mu..b at ..sqrt..s = 540 GeV to 14 ..mu..b at ..sqrt..s = 8000 GeV. The masses of the heavy quarks are taken into account in the kinematics and the evolution equations. The cross section sigma(p/sub T/>2 GeV) for heavy-quark production is in good approximation equal to (1/M/sup 2/)f(ln(s/Mp/sub T/))Vertical Bar p/sub T/ = 2 GeV, where f(x) is a quadratic polynomial of x for sufficiently high energy and masses (4 GeV/sup 2/<15 GeV) electrons from such decays are calculated for the CERN collider.

Measurement of the Dijet Cross Section in 400-GeVppCollisions

The invariant cross section for production of jet pairs in 400-GeV/c pp interactions has been measured as a function of p{tau} in the p{tau} range 4 to 9 GeV/c. The results are in good agreement with predictions of perturbative QCD models. 20 refs., 3 figs., 1 tab.

Hadronization by color bremsstrahlung

Particle production in the central plateau of hadronic final states is calculated in a perturbative QCD model. We assume that the controlling process is bremsstrahlung of gluons. We find important differences between the spectrum produced in soft hadronic collisions and the spectrum in e+e annihilation. Describing soft hadronic collisions with the Low-Nussinov model, we find that the central plateau is independent of total energy, but depends strongly on the momentum transfer in the soft-gluon exchange. In contrast, the quark jets from e+e annihilation have a central plateau growing with energy. The strong momentum dependence in the hadron-induced plateau arises from the cancellations in the amplitude associated with the gauge invariance of the theory.

Diffractive photoproduction of strangeness and charm-overlap of QCD and Regge models

We suggest an overlap between the perturbative QCD and Regge models for thediffractive photoproduction of strangeness and possibly charm, and support it by explicit comparison between the model predictions. This enables us to estimate some of the QCD model parameters-most notably the gluon x distribution over the region 0 ⩽ x ⩽ 0.1.

Transverse momentum of charged hadrons observed in deep inelastic muon scattering

The transverse momenta of charged hadrons produced in high energy muon-proton scattering have been studied. The average squared transverse momentum 〈 p 2 ⊥〉 shows a strong dependence on z = E h/ v characteristic of intrinsic momentum effects and a significant rise as a function of s = W 2. The W 2, q 2, x and z dependences of the data are compared with the predictions of a perturbative QCD model.