Meson cloud of the nucleon in polarized semi-inclusive deep-inelastic scattering

We investigate the possibility of identifying an explicit pionic component of the nucleon through measurements of polarized ⁺⁺ baryon fragments produced in deep-inelastic leptoproduction off polarized protons, which may help to identify the physical mechanism responsible for the breaking of the Gottfried sum rule. The pion-exchange model predicts highly correlated polarizations of the ⁺⁺ and target proton, in marked contrast with the competing diquark fragmentation process. Measurement of asymmetries in polarized production may also reveal the presence of a kaon cloud in the nucleon.Communicated by: W. Weise     

Transversity and T-odd PDFs from Drell–Yan processes with <Emphasis Type="Italic">pp</Emphasis>, <Emphasis Type="Italic">pD</Emphasis> and <Emphasis Type="Italic">DD</Emphasis> collisions

We estimate the single-spin asymmetries (SSA) which provide the access to transversity as well as to Boer–Mulders and Sivers PDFs via investigation of the single-polarized Drell–Yan (DY) processes with pp, pD and DD collisions available to RHIC, NICA, COMPASS, and J-PARC. The feasibility of these SSA is studied with the new generator of polarized DY events. The estimations performed demonstrate that there exist kinematical regions where SSA are presumably measurable. Most useful for PDFs extraction are the limiting kinematical ranges, where one can neglect the sea PDFs contributions which occur at large values of the Bjorken variable x. It is of interest that, contrary to the Sivers PDF, the transversity PDF is presumably accessible only in a particular kinematical region. Contrary to the option with the symmetric collider mode (RHIC, NICA), this is of importance for the COMPASS experiment and the future J-PARC facility, where the fixed-target mode is available.     

Gauge-invariant sub-structures of tree-level double-emission exact QCD spin amplitudes

In this note we discuss possible separations of exact, massive, tree-level spin amplitudes into gauge-invariant parts. We concentrate our attention on processes involving two quarks entering a color-neutral current and, thanks to the QCD interactions, two extra external gluons. We will search for forms compatible with parton-shower languages, without applying approximations or restrictions on phase space regions. Special emphasis will be put on the isolation of parts necessary for the construction of evolution kernels for individual splittings and to some degree for the running coupling constant as well. Our aim is to better understand the environment necessary to optimally match hard matrix elements with parton-shower algorithms. To avoid complications and ambiguities related to regularization schemes, we ignore, at this point, virtual corrections. Our representation is quite universal: any color-neutral current can be used; in particular, our approach is not restricted to vector currents only. This work is partially supported by RTN European Programme, MRTN-CT-2006-035505 (HEPTOOLS, Tools and Precision Calculations for Physics Discoveries at Colliders).     

Spin density matrix elements in exclusive ρ0 electroproduction on?1H and 2H targets at 27.5?GeV beam?energy

Spin Density Matrix Elements (SDMEs) describing the angular distribution of exclusive ρ ⁰ electroproduction and decay are determined in the HERMES experiment with 27.6 GeV beam energy and unpolarized hydrogen and deuterium targets. Eight (fifteen) SDMEs that are related (unrelated) to the longitudinal polarization of the beam are extracted in the kinematic region 1<Q ²<7 GeV², 3.0<W<6.3 GeV, and −t<0.4 GeV². Within the given experimental uncertainties, a hierarchy of relative sizes of helicity amplitudes is observed. Kinematic dependences of all SDMEs on Q ² and t are presented, as well as the longitudinal-to-transverse ρ ⁰ electroproduction cross-section ratio as a function of Q ². A small but statistically significant deviation from the hypothesis of s-channel helicity conservation is observed. An indication is seen of a contribution of unnatural-parity-exchange amplitudes; these amplitudes are naturally generated with a quark-exchange mechanism.     

Parton distributions in the virtual photon target up to NNLO in QCD

Parton distributions in the virtual photon target are investigated in perturbative QCD up to the next-to-next-to-leading order (NNLO). In the case Λ ²≪P ²≪Q ², where −Q ² (−P ²) is the mass squared of the probe (target) photon, parton distributions can be predicted completely up to the NNLO, but they are factorisation-scheme dependent. We analyse parton distributions in two different factorisation schemes, namely the and DIS _γ schemes, and we discuss their scheme dependence. We show that the factorisation-scheme dependence is characterised by the large-x behaviours of the quark distributions. The gluon distribution is predicted to be very small in absolute value except in the small-x region.     

On the applicability of perturbative QCD to exclusive processes at currently available momentum transfer

We re-examine the problems connected with the end-point dominance in the calculation of exclusive processes in perturbative QCD. In a re-analysis we construct nucleon quark distribution amplitudes from the respective moments obtained from a QCD sum rule approach. These functions lead to acceptable values for the e.m. Dirac form factorsF ₁ ^p,n of the nucleon if effective gluon masses of ca. 300–600 MeV are included into the hard-scattering amplitude. In addition we also find a reasonable Q₂ — dependence of the proton form factor. The results point at the importance of the end-point k-dependence of distribution amplitude and hard-scattering amplitude in the calculation of exclusive processes.Work is supported by Deutsche Forschungsgemeinschaft (Ga 153-13-1) and partially by NATO (0581/87)     

Novel sets of coupling expansion parameters for low-energy pQCD

In quantum theory, physical amplitudes are usually presented in the form of a Feynman perturbation series in powers of coupling constant α. However, it is known that these amplitudes are not regular functions at α = 0. For QCD, we propose new sets of expansion parameters w _k (α _s ) that reflect singularity at α _s = 0 and should be used instead of powers α _s ^k . Their explicit form is motivated by the so-called Analytic Perturbation Theory. These parameters reveal saturation in a strong coupling case at the level α _s ^eff (α _s 1) = w ₁(α _s 1) ∼ 0.5. They can be used for the quantitative analysis of divers low-energy amplitudes. We argue that this new picture with non-power sets of perturbation expansion parameters, as well as the saturation feature, is of a rather general nature. The text was submitted by the author in English. A preliminary version with the main results was published in [1].     

On the cross-section peaks for a heavy-quark bound state

We discuss the values of resonance peaks of the cross section of a heavy-quark bound state obtained by means of a Green function method applied to a Coulombic model and compare the result to the ϒ and J/ψ data.     

Medium-modified average multiplicity and multiplicity fluctuations in jets

The energy evolution of average multiplicities and multiplicity fluctuations in jets produced in heavy-ion collisions is investigated from a toy QCD-inspired model. In this model, we use modified splitting functions accounting for medium-enhanced radiation of gluons by a fast parton which propagates through the quark–gluon plasma. The leading contribution of the standard production of soft hadrons is enhanced by a factor while next-to-leading order (NLO) corrections are suppressed by , where the parameter N _s >1 accounts for the induced soft gluons in the medium. Our results for such global observables are cross-checked and compared with their limits in the vacuum.     

Glueballs and the superfluid phase of Two-Color QCD

We present the first results on scalar glueballs in cold, dense matter using lattice simulations of two-color QCD. The simulations are carried out on a 6³×12 lattice and use a standard hybrid molecular dynamics algorithm for staggered fermions for two values of quark mass. The glueball correlators are evaluated via a multi-step smearing procedure. The amplitude of the glueball correlator peaks in correspondence with the zero temperature chiral transition, μ _c=m _π /2, and the propagators change in a significant way in the superfluid phase, while the Polyakov loop is nearly insensitive to the transition. Standard analysis suggest that lowest mass in the 0⁺⁺ gluonic channel decreases in the superfluid phase, but these observations need to be confirmed on larger and more elongated lattices. These results indicate that a non-zero density induces non-trivial modifications of the gluonic medium.     

Semi numerical solution of non-singlet Dokshitzer–Gribov–Lipatov–Altarelli–Parisi evolution equation up to next-to-next-to-leading order at small x

The non-singlet structure functions have been obtained by solving Dokshitzer–Gribov–Lipatov–Altarelli–Parisi (DGLAP) evolution equations in next-next-to-leading order (NNLO) in the small-x limit. Here a Taylor series expansion has been used to solve the evolution equations and we obtain the semi numerical solution. Results are compared with the Fermi Lab experiment E665 data and New Muon Collaboration (NMC) data.     

On the temperature dependence of quarkonium correlators

I discuss the temperature dependence of quarkonium correlators calculated in lattice QCD. I show that the dominant source of the temperature dependence comes from the zero-mode contribution, while the temperature dependence associated with the melting of bound states is quite small. I study the zero-mode contribution quantitatively for various quark masses and show that it is well described by a quasi-particle model with temperature-dependent heavy quark mass. As a byproduct, an estimate of the medium dependence of the heavy-quark mass is obtained.     

QCD sum rules with finite masses

The concept of QCD sum rules is extended to bound states composed of particles with finite mass such as scalar quarks or strange quarks. It turns out that mass corrections become important in this context. The number of relevant corrections is analyzed in a systematic discussion of the IR- and UV-divergencies, leading in general to a finite number of corrections. The results are demonstrated for a system of two massless quarks and two heavy scalar quarks.We wish to thank Dr. Lech Mankiewicz for very helpful discussions. This work was supported by DFG (G. Hess program).     

Multi-jet events in the k T -factorisation scheme

A Markovian Monte Carlo algorithm for multi-parton production in the high-energy limit is proposed and the matching with unintegrated parton densities is discussed.     

The neutron/proton ratio of fast nucleon emission in antiproton annihilation on nuclei

The measurement of fast protons and neutrons emitted after antiproton annihilation at rest on²³⁸U and⁶³Cu reveals a large neutron/proton ratioR. Its value for⁶³Cu is larger than expected from the conventional model of intranuclear pion rescattering. A value ofR essentially constant over the whole range of mass number, from¹²C up to²³⁸U, is also announced by the experimentalists. It is shown that, on the contrary, the conventional scheme predicts a regular increase ofR with mass number. Alternative explanations of the effect are looked for. Within the usual scheme, an excess of negative pions, leading to more emitted neutrons, is not compatible with final pion multiplicities. The influence of meson resonances on the neutron/proton ratio is considered, as well as the possible occurrence of annihilations on two nucleons. They cannot warrant an increase ofR at low mass targets sufficient to explain a constant value over the whole range of target masses.     

Determining the uncertainty on the total heavy-flavor cross section

We assess the theoretical uncertainties on the total heavy-quark cross section. We discuss the importance of the quark mass, the choice of the scale, the number of light flavors and the parton densities on the estimate of the uncertainty. At first glance, the uncertainty bands on the total charm cross sections obtained by integrating the FONLL inclusive cross section and by integrating the partonic total cross sections appear to be incompatible. We explain how this apparent difference arises and describe how the two results can be reconciled. The small mass of the charm quark amplifies the effect of varying the other parameters in the calculation, making the uncertainty on the total charm cross section difficult to quantify. On the other hand, the bottom-quark total cross section is under much better theoretical control, and differences between the two approaches are small.     

Markovian MC simulation of QCD evolution at NLO level with minimum <Emphasis Type="Italic">k</Emphasis><Subscript><Emphasis Type="Italic">T</Emphasis></Subscript>

We present two Monte Carlo algorithms of the Markovian type which solve the modified QCD evolution equations at NLO level. The modifications with respect to the standard DGLAP evolution concern the argument of the strong coupling constant, α _S. We first analyze the z-dependent argument and then the k _T -dependent one. The evolution time variable is identified with the rapidity. The two algorithms are tested to 0.05% precision level. We find that the NLO corrections in the evolution of the parton momentum distributions with k _T -dependent coupling constant are of the order of 10 to 20%, and in the small-x region even up to 30%, with respect to the LO contributions. The project is partly supported by the EU grant MTKD-CT-2004-510126, realized in a partnership with the CERN Physics Department and by the Polish Ministry of Science and Information Society Technologies grant No. 620/E-77/6.PR UE/DIE 188/2005-2008.