Parton densities and dipole cross-sections at small x in large nuclei

Unintegrated gluon densities in nuclei, dipole-nucleus cross-sections and quark densities are numerically investigated in the high-color limit, with the scattering on a heavy nucleus exactly described by the sum of fan diagrams of BFKL pomerons. The initial condition for the evolution in rapidity is quickly forgotten, and the gluon density presents a “supersaturation” pattern, as previous studies indicated. Both dipole-nucleus cross-sections and quark densities present the expected saturation features. Identifying the position in transverse momentum l of the maximum of the gluon distribution with the saturation momentum , at large rapidities all distributions depend on only one variable, or . Received: 4 April 2001 / Revised version: 24 April 2001 / Published online: 8 June 2001     

Cross-section sum rules and inequalities in a quark model for light and heavy flavour productions

The model for inclusive processes is reformulated to consider the production of heavy flavours (c, b andt) and higher order flavour exchange effects. Predictions are made in terms of sum rules and inequalities for various inclusive cross-sections. Plausible parametrization of flavour symmetry breaking is also suggested.     

Nuclear effects on the extraction of neutron structure functions

Nuclear effects due to the presence of spin-1 isosinglet 6-quark clusters in the deuteron on the extraction of both spin-independent and spin-dependent neutron structure functions are investigated. The x-dependences of the 6-quark cluster structure functions are estimated by using a perturbative QCD (pQCD) dimensional counting rules. Within this framework, the ratio of the deuteron structure function to the free nucleon structure function can be well described. We find that the nuclear effect on the extracted spin-dependent neutron structure function is very different from that on the spin-independent neutron structure function. The effect enhances the Bjorken sum by about 10%, whereas it decreases the Gottfried sum by about 7%. The formalism for calculating nuclear effects is further used to evaluate the spin-dependent structure function of the He nucleus and a good self-consistent check is obtained. Received: 1 December 2000 / Revised version: 15 February 2001 / Published online: 25 April 2001     

Cumulative particle production and percolation of strings

Calculations of the production rate of particles with x > 1 in nuclear collisions due to the interaction of colour strings are presented. Momentum and colour sum rules are used to determine the fragmentation functions of fused strings. Mechanisms of the string interaction are considered with total and partial overlapping in the transverse plane. The results reveal a strong dependence of the chosen mechanism. In the percolation scenario with partial overlapping the x-dependence of the production rates agrees well with the existing data. The magnitude of the rates for production is in agreement with experiment. However the rates for the protons are substantially below the data. Received: 28 November 2001 / Revised version: 15 May 2002 / Published online: 12 July 2002     

Structure function of the nucleus in the perturbative QCD with N_c\rightarrow\infty (BFKL pomeron fan diagrams)

The equation for the sum of BFKL pomeron fan diagrams is rederived by a direct summation and solved numerically for rapidities . At high rapidities, , the resulting cross-sections for the scattering of a longitudinally polarized pair on the nucleus cease to depend on its transverse dimension and tend to a constant limit of , which corresponds to scattering of a color dipole on a black disk. Thus the unitarity is restored and the singularity in the j plane is reduced to a simple pole at . The nuclear structure function at small x behaves as . The gluon density found has a soliton-like form in the log k space: its form is close to Gaussian, independent of rapidity, the center moving towards higher log k with a nearly constant velocity as the rapidity increases. Received: 26 January 2000 / Published online: 8 June 2000     

Inclusive production of antihyperons in nC-interactions

The inclusive production cross-sections of , , and in neutron-carbon interactions at GeV mean energy of neutrons have been measured. The parameters n and b of differential cross-section parametrisation , where x_F is the Feynman variable, p_t is the transverse momentum, have been obtained. The kinematic region of validity of the parametrisation has been defined. Results are compared with published experimental data. Received: 7 June 2002 / Revised version: 23 January 2003 / Published online: 3 March 2003     

The contribution of off-shell gluons to the longitudinal structure function F<Subscript>L</Subscript>

We present the results for the structure function for a gluon target having a non-zero transverse momentum square at order . The results of a double convolution (with respect to the Bjorken variable x and the transverse momentum) of the perturbative part and the unintegrated gluon densities are compared with recent experimental data for at low x values and with the predictions of other approaches. Received: 21 August 2002 / Revised version: 31 October 2002 Published online: 24 January 2003     

Fragmentation of relativistic oxygen nuclei in interactions with a proton

The data on investigation of inelastic interactions of ¹⁶O nuclei in a hydrogen bubble chamber at an incident momentum of 3.25 A GeV/c are presented. Separate characteristics as fragments isotope composition and topological cross-sections of fragmentation channels are given. The processes of formation of light fragments and unstable nuclei, and the break-up of the ¹⁶O nucleus into multicharge fragments are investigated. A comparison between the experimental data and the calculations of the cascade fragmentation evaporation model (CFEM) is made. The observed singularities of the interactions point out the important role of the nucleus α-cluster structure in the formation of the final products. Received: 9 January 2001 / Accepted: 13 June 2001     

Measurement of neutral and charged current cross-sections in positron-proton collisions at large momentum transfer

The inclusive single and double differential cross-sections for neutral and charged current processes with four-momentum transfer squared between 150 and 30 000 GeV and with Bjorken x between 0.0032 and 0.65 are measured in collisions. The data were taken with the H1 detector at HERA between 1994 and 1997, and they correspond to an integrated luminosity of . The evolution of the parton densities of the proton is tested, yielding no significant deviation from the prediction of perturbative QCD. The proton structure function is determined. An extraction of the u and d quark distributions at high x is presented. At high electroweak effects of the heavy bosons and are observed and found to be consistent with Standard Model expectation. Received: 27 August 1999 / Published online: 25 February 2000     

Photoproduction of top quarks in peripheral heavy ion collisions

In relativistic heavy ion collisions, top quarks can be produced by photon-gluon fusion when a photon from the Weizs?cker-Williams virtual photon field of one nucleus interacts with a gluon in the other nucleus. Photoproduction with heavy ions at the Large Hadron Collider (LHC) will be the first accessible non-hadronic top production channel. We calculate the photoproduction cross sections, pair mass and top quark rapidity distributions in peripheral lead-lead and oxygen-oxygen collisions. The cross sections are sensitive to the top quark charge and the large- gluon distribution in the nucleus. We find a cross section of 15 pb in oxygen-oxygen collisions, leading to 210 pairs in a one month ( s) LHC run. In pA collisions, the rate is higher, 1100 pairs per month for pO. A comparison of the AA and pA data might allow for a study of gluon shadowing at high Q . Received: 18 May 2001 / Revised version: 13 June 2001 / Published online: 24 August 2001     

Heavy quark production at the TESLA collider and its sensitivity to the gluon content in the photon

Heavy quark production is studied at the TESLA high energy linear collider (LC) both in its nominal and photon collider (PC) mode. Leading order cross-sections are calculated for the production of heavy quarks, , at high transverse momenta. The sensitivity of this process to the gluon content in the photon is studied. Received: 26 November 2001 / Published online: 21 June 2002     

Flavour-changing effects on e+e-→Hbs̄,Hb̄s in the MSSM

Flavour-changing effects originating from the exchange of scalar particles in the processes e⁺e^-→H^xbs̄,H^xb̄s, with H^x≡h⁰,H⁰,A⁰, are investigated in the minimal supersymmetric standard model with non-minimal flavour violation at the one-loop level. The dominating SUSY-QCD contributions with squark–gluino loops are calculated and discussed. We consider the SUSY scenario with non-minimal flavour mixing in the down-type squark-mass matrix. The flavour-changing cross sections are derived, and we discuss the dependence on the MSSM parameters and the strength of flavour mixing. The values for the cross section can reach 10^-4 pb for the production of the heavy Higgs boson H⁰ or A⁰, and only 10^-7 pb for the light Higgs boson h⁰. Non-decoupling behaviour occurs for both h⁰,H⁰ production in the case of a common heavy SUSY mass scale.     

MRST2001: partons and $\alpha_S$ from precise deep inelastic scattering and Tevatron jet data

We use all the available new precise data for deep inelastic and related hard scattering processes to perform NLO global parton analyses. These new data allow an improved determination of partons and, in particular, the inclusion of the recent measurements of the structure functions at HERA and of the inclusive jets at the Tevatron help to determine the gluon distribution and better than ever before. We find a somewhat smaller gluon at low x than previous determinations and that . Received: 17 October 2001 / Revised version: 29 October 2001 / Published online: 20 December 2001     

Hadronization corrections to helicity components of the fragmentation function

In the hadronic decays of , gluon emission leads to the appearance of the longitudinal component of the fragmentation function, . Measurement of and the transverse component, , could thus provide insight into the gluon fragmentation function. However, hadronization corrections at low x can be significant. Here we present a method of accounting for such corrections, using the Jetset event generator as illustration. Received: 11 April 2001 / Revised version: 4 May 2001 / Published online: 29 June 2001     

The improved nuclear parton distributions

In this paper we propose an improvement of the EKS nuclear parton distributions for the small x region of high energy processes, where the perturbative high parton density effects cannot be disregarded. We analyze the behavior of the ratios and and verify that at small x they are strongly modified when compared to the EKS predictions. The implications of our results for heavy ion collisions in RHIC and LHC are discussed. Received: 18 January 2001 / Revised version: 6 March 2001 / Published online: 3 May 2001     

Resonance-reggeon and parton-hadron duality in strong interactions

By using the concept of duality between direct channel resonances and Regge exchanges we relate the small- and large-x behavior of the structure functions. We show that even a single resonance exhibits Bjorken scaling at large Q². Received: 30 October 2001 / Accepted: 16 November 2001