Heavy Quark Potential and Quarkonium Spectrum in Nonperturbative pNRQCD

The charmonium and bottomonium mass spectra are investigated systematically in potential nonrelativistic QCD with the heavy quark potential computed by lattice QCD simulations nonperturbatively. The potential consists of a static potential and relativistic corrections classified in powers of the inverse of heavy quark mass m, and the effects of the O(1/m) correction, the O(1/m ²) spin–orbit and spin–tensor corrections on the mass spectra are examined systematically. The pattern of the mass spectra is found to be in fairly good agreement with the experimental data, in which the O(1/m) correction gives an important contribution.     

Decay constants of pseudoscalar and vector mesons with improved holographic wavefunction

We calculate the decay constants of light and heavy-light pseudoscalar and vector mesons with improved soft-wall holographic wavefuntions,which take into account the effects of both quark masses and dynamical spins.We find that the predicted decay constants,especially for the ratio f V/f P,based on light-front holographic QCD,can be significantly improved,once the dynamical spin effects are taken into account by introducing the helicity-dependent wavefunctions.We also perform detailed χ~2 analyses for the holographic parameters(i.e.the mass-scale parameterκ and the quark masses),by confronting our predictions with the data for the charged-meson decay constants and the meson spectra.The fitted values for these parameters are generally in agreement with those obtained by fitting to the Regge trajectories.At the same time,most of our results for the decay constants and their ratios agree with the data as well as the predictions based on lattice QCD and QCD sum rule approaches,with only a few exceptions observed.     

Method of vacuum correlation functions: Results and prospects

Basic results obtained within the QCD method of vacuum correlation functions over the past 20 years in the context of investigations into strong-interaction physics at the Institute of Theoretical and Experimental Physics (ITEP, Moscow) are formulated Emphasis is placed primarily on the prospects of the general theory developed within QCD by employing both nonperturbative and perturbative methods. On the basis of ab initio arguments, it is shown that the lowest two field correlation functions play a dominant role in QCD dynamics. A quantitative theory of confinement and deconfinement, as well as of the spectra of light and heavy quarkonia, glueballs, and hybrids, is given in terms of these two correlation functions. Perturbation theory in a nonperturbative vacuum (background perturbation theory) plays a significant role, not possessing drawbacks of conventional perturbation theory and leading to the infrared freezing of the coupling constant α _s.     

The effect of 1Q2 and αs corrections on tests of QCD

We discuss in detail the use of the structure function F₃(x, Q²) of deep-inelastic neutrino scattering for testing quantum chromodynamics. QCD is entirely consistent with all data. However, we show that higher-twist (order $\text{1}\text{Q}{}^2$) contributions, which are commonly neglected, can have a dramatic impact on interpretation of this result. At present the data are not accurate enough to determine the magnitudes of these $\text{1}\text{Q}{}^2$ contributions within the context of QCD. Furthermore, the possible presence of higher-twist terms makes it impossible to unambiguously detect the logarithmic Q² dependence and anomalous dimensions which distinguish QCD from hypothetical alternative theories. As a result, more precise data with higher Q² are needed to provide definitive tests of QCD. The corrections of second-order in α_s introduce fewer complications for testing QCD, and provide a useful context for understanding critical ambiguities in the definitions of α_s and Λ.     

Vacuum pressure effects in low-p⊥ hadronic spectra

Nonperturbative fluctuations in the QCD vacuum create a “bag”, which shows itself in the low-p_⊥ spectra of secondaries by the suppression of the transverse collective expansion, so that the spectra are nearly thermal, in agreement with the data.     

Energy flow and energy correlations in deep inelastic scattering

We study two examples of infrared-safe quantities in deep inelastic scattering: the flow of energy in a given angular range and the energy-energy angular pattern. We derive expressions for these quantities in perturbative QCD, to order α_s, and show explicitly their infrared safety. Our formulas for the angular energy flow and the energy-energy angular pattern depend only on well-defined QCD factors and on the deep inelastic structure functions. To gauge whether or not these perturbative QCD results are applicable to present day data, we estimate in a simple model the effects of hadronization and primordial parton transverse momentum. In general these non-perturbative effects mask the resulting QCD pattern at present energies, but vanish more rapidly at higher energies than the QCD effects. However, we point out two examples where it may be possible to test the perturbative QCD predictions with available data. One of them involves studying the x-dependence of the azimuthal asymmetry of the energy flow. The other involves studying the angular width of the energy-energy correlation function.     

Heavy quark correlations ine + e − annihilations

In heavy quark jets the quark mass acts as a regulator of collinear singularities, making the quark momentum an infra-red safe variable in perturbative QCD. This allows a direct comparison of measured heavy hadron momentum spectra with perturbative calculations. We exploit the factorisation of heavy quark fragmentation to derive QCD predictions for momentum correlations between heavy hadrons produced ine ⁺ e ^? annihilations. We study the practical feasibility and model sensitivity of our approach using Monte Carlo simulations. Higher order perturbative corrections and contributions from non-perturbative effects are found to be at the level of 10%.     

Asymmetries in Largep T photoproduction and the polarized gluon structure function

QCD predictions for asymmetries in largep _T polarized photon reactions on a polarized target are given. Taking cross-section differences is shown to yield new tests of QCD. Double inclusive cross section measurements provide a direct way to measure the distribution function of polarized gluons in the proton.     

Higher moments of net-proton multiplicity distributions

Higher moments of event-by-event net-proton multiplicity distributions have been applied to search for the QCD critical point. Model results are used to provide a baseline for this search. The measured moment products, ??? ² and S?? of net-proton distributions, which are directly connected to the thermodynamical baryon number susceptibility ratio in Lattice QCD and Hadron Resonance Gas (HRG) model, are compared to the transport and thermal model results. We argue that a non-monotonic dependence of ??? ² and S?? as a function of beam energy can be used to search for the QCD critical point.     

Limiting soft particle emission in e + e −, hadronic and nuclear collisions

In e ⁺ e ^? collisions the particle spectra at low momenta reflect the properties of the underlying “soft” QCD gluon bremsstrahlung: the particle density, in the limit p→0, becomes independent of the incoming energy $\sqrt{s}$ and directly proportional to the colour factors C _A , C _F for primary gluons or quarks respectively. We find that experimental data from the pp and nuclear reactions reveal the same behaviour: in the limit p _T→0 the invariant particle spectra become independent of the collision energy, and their intensities in e ⁺ e ^?, pp and nuclear reactions are compatible with the expected colour factors C _F : C _A : (N _part/2)C _A for N _part nucleons, participating in the interaction. Coherent soft gluon bremsstrahlung is, therefore, suggested to be the dominant QCD mechanism for the soft particle production in all these reactions. These “soft” particles probe the very early stage of hadron formation in the collision. Future measurements at the LHC will provide crucial tests on the contributions from possible incoherent multi-component processes.     

QCD estimates for associated charm production by weak neutral currents

We give quantitative estimates for associated charm production in neutrino and antineutrino induced neutral current interactions, based on (a) quantum chromodynamics (QCD) and (b) the quark parton model (QPM) and a phenomenological generalization thereof. We emphasize the need for a precise measurement of the ratio σ(v_μN→v_μe⁺X)/σ(v_μN→μ^?X) and the corresponding ratio for antineutrinos, as these can provide clean tests of certain characteristic features of QCD. Bounds are obtained for single charm production by charm-changing neutral currents.     

Testing QCD scaling violations in the HERA energy range

High energy electron-proton colliders open new kinematical domains for the investigation of scaling violations in nucleon structure functions. Focussing on HERA as a definite study case we explore the precision which can be expected in determinations of the QCD scale parameter Λ and the running coupling constant α _s (Q ²). Our results provide a quantitative basis for discussions of the possibilities and requirements of QCD tests in inclusive deep inelastic scattering at HERA energies.     

On high energy leptoproduction

The particle distributions and the event structures in high energy leptoproduction are considered in a model where perturbative QCD is used to compute the cross sections to order α _s and the Lund jet model is used for the soft hadronization process. Since complete events are generated by a Monte Carlo program not only single-particle spectra can be studied but also various correlations, thus making a more detailed comparison between theory and experiment possible. The model is found to be in agreement with data from the European Muon Collaboration. We indicate how a one-particle trigger can be used to study an enriched sample of two forward jet events, and exhibit a useful experimental test for the existence of such events with the properties predicted by perturbative QCD. We also indicate how to gain further information on the confinement and soft fragmentation mechanisms.     

In-Medium η′ Mass Reduction

In this talk, we present the reduction of the mass of η′-meson at finite temperature which leads to the enhancement of η′ contributions to the dilepton spectra from relativistic heavy ion collisions. QCD low energy theorem, together with the Witten–Veneziano formula, provides the relation of the η′ mass with gluon condensates.     

A fit of upsilon and charmonium spectra

We show that upsilon and charmonium spectra can be fitted by a common potential of the form A+Br^α, with α close to 0.1. In particular the mass difference of γ? and γ is reproduced within 10 MeV. In this way it is confirmed that for the bb? and cc? systems the relevant partof the potential is flavour-independent and approximately though not exactly logarithmic. At the same time the claims, according to which the short-distance part of the potential could be shown to agree with the predictions of QCD by a comparison of charmonium and upsilon spectra, are put in serious doubt.     

Momentum distributions int\bar t production and decay near threshold

We apply the Green function formalism for \(t - \bar t\) production and decay near threshold in a study of the effects due to the momentum dependent width for such a system. We point out that these effects are likely to be much smaller than expected from the reduction of the available phase space. The Lippmann-Schwinger equation for the QCD chromostatic potential is solved numerically forS partial wave. We compare the results on the total cross section, top quark intrinsic momentum distributions and on the energy spectra ofW bosons from top quark decays with those obtained for the constant width.     

Adiabatic potentials and spectra of heavy hybrid mesons

Using the QCD string approach, adiabatic potentials and spectra of bb hybrid mesons are calculated. The results are compared with lattice studies.     

QCD at lowQ 2—A correspondence relation for moments of structure functions

The precocious validity of QCD predictions in deep inelastic lepton nucleon scattering ande ⁺ e ^? annihilation is interpreted as a signal for an underlying “correspondence principle” relating perturbative and nonperturbative physics on theQ ² average. Correspondence relations for nonsinglet moments of deep inelastic structure functions are formulated, discussed and successfully tested against experiment. The relations provide an independent determination of the QCD ?-parameter from lowQ ² data in perfect agreement with results from largeQ ² analyses.