Low energy QCD predictions from non perturbative methods

We show that by using lattice results about the euclidean ?-propagator, it is possible to give theoretical predictions on the low energy behavior of thee ⁺ e ^?→had. cross section. Furthermore, we present a comparison of perturbative QCD results (plus certain sum rules corrections) with non perturbative results from the lattice in the intermediate energy region.     

QCD and short range nuclear phenomena

We have found essential QCD diagrams for the leading particle production in the high energy processes. The high momentum component of the deuteron (nucleus) wave function is estimated in terms of perturbative QCD. Reasonable agreement of the formulae obtained with the observed cross section of the deep inelastic reaction e + D → e + X at x > 1, and the observed momentum dependence of the high energy reactions h + A → fast backward, p, π + X indicate that the nuclear core and cumulative particle production are actually QCD phenomena. The crucial importance of the deep inelastic 1A scattering at x > 1 for the investigation of the short range nuclear structure is explained.     

Factorization and infrared properties of non-perturbative contributions to DIS structure functions

In this paper we present a new derivation of QCD factorization. We deduce the k _T and collinear factorizations for the DIS structure functions by consecutive reductions of a more general theoretical construction. We begin by studying the amplitude of forward Compton scattering off a hadron target, representing this amplitude as a set of convolutions of two blobs connected by the simplest, two-parton intermediate states. Each blob in the convolutions can contain both the perturbative and non-perturbative contributions. We formulate conditions for separating the perturbative and non-perturbative contributions and attributing them to the different blobs. After that the convolutions correspond to QCD factorization. Then we reduce this totally unintegrated (basic) factorization first to k _T -factorization and finally to collinear factorization. In order to yield a finite expression for the Compton amplitude, the integration over the loop momentum in the basic factorization must be free of both ultraviolet and infrared singularities. This obvious mathematical requirement leads to theoretical restrictions on the non-perturbative contributions (parton distributions) to the Compton amplitude and the DIS structure functions related to the Compton amplitude through the Optical Theorem. In particular, our analysis excludes the use of the singular factors x ^−a (with a>0) in the fits for the quark and gluon distributions because such factors contradict the integrability of the basic convolutions for the Compton amplitude. This restriction is valid for all DIS structure functions in the framework of both k _T -factorization and collinear factorization if we attribute the perturbative contributions only to the upper blob. The restrictions on the non-perturbative contributions obtained in the present paper can easily be extended to other QCD processes where the factorization is exploited.     

QCD predictions for γp collisions with a large pT real photon in the final state

The processes γp → γ + jet + X and γp → π⁰ + jet + X are discussed in detail using perturbative QCD. The possibilities of experimental tests are also discussed.     

The role of the pion cloud in the interpretation of the valence light-cone wavefunction of the nucleon

The pion cloud renormalises the light-cone wavefunction of the nucleon which is measured in hard, exclusive photon-nucleon reactions. We discuss the leading twist contributions to high-energy exclusive reactions taking into account both the pion cloud and perturbative QCD physics. The nucleon’s electromagnetic form-factor at high Q² is proportional to the bare nucleon probability Z and the cross-sections for hard (real at large angle or deeply virtual) Compton scattering are proportional to Z². Our present knowledge of the pion-nucleon system is consistent with Z = 0.7 ± 0.2. If we apply just perturbative QCD to extract a light-cone wavefunction directly from these hard exclusive cross-sections, then the light-cone wavefunction that we extract measures the three valence quarks partially screened by the pion cloud of the nucleon. We discuss how this pion cloud renormalisation effect might be understood at the quark level in terms of the (in-)stability of the perturbative Dirac vacuum in low energy QCD.     

B→f0(980)(π,η') decays in the PQCD approach

Based on the assumption of a two-quark structure of the scalar meson f0(980), we calculate the branching ratios and CP-violating asymmetries for the four B→f0(980)π and B→f0(980)η(') decays by employing the perturbative QCD (Pqcd) factorization approach. The leading order Pqcd predictions for branching ratios are, Br(B-→f0(980)π-)～2.5×10-6, Br(B0→f0(980)πo)～2.6×10-7, Br(B0→f0(980)η)～ 2.5×10-7 and Br(B0→f0(980)η')～6.7×10-7, which are consistent with both the QCD factorization predictions and the experimental upper limits.     

Pair production of pions with symmetric momenta in the range 0.5⩽pT⩽2.0 GeV/c In 70 GeV p-p collisions

The invariant cross-section slope of the pp→π⁺π^?+X process as a function of p_T is found to have a break near 1 GeV/c. Fitting the cross section by a sum of two exponents gives the values of powers (12.3±0.9)(GeV/c)^?1 and (8.7±0.6)(GeV/c)^?1. The experimental points at p_T?1 GeV/c are significantly higher than predictions based on hard scattering models such as QCD and CIM.     

Asymptotic properties of regge trajectories and elastic pseudoscalar-meson scattering on nucleons at high energies

A phenomenological Regge-eikonal model featuring nonlinear monotonic parametrizations of vacuum Regge trajectories, where their asymptotic behavior in the perturbative sector is taken explicitly into account, is proposed for describing the elastic diffractive scattering of light pseudoscalar mesons on nucleons. In analyzing available experimental data on angular distributions, it is shown that, at collision energies in the region √s > 13 GeV, the diffraction pattern of the processes π ^± p → π ^± p and K ^± p → K ^± p at low momentum transfers can be described qualitatively by using the same phenomenological approximations to vacuum Regge trajectories as in the case of nucleon-nucleon scattering. This fact is indicative of the possibility of explicitly relating Regge phenomenology of various hadron-hadron processes to fundamental results obtained within QCD.     

Theoretical Description and Measurement of the Pion–Photon Transition Form Factor

Detailed predictions for the scaled pion–photon transition form factor are given, derived with the method of light-cone sum rules and using pion distribution amplitudes with two and three Gegenbauer coefficients obtained from QCD sum rules with nonlocal condensates. These predictions agree well with all experimental data that are compatible with QCD scaling (and collinear factorization), but disagree with the high-Q ² data of the BaBar Collaboration that grow with the momentum. A good agreement of our predictions with results obtained from AdS/QCD models and Dyson–Schwinger computations is found.     

Direct photon production beyond leading order in QCD

The significance of the quark-quark scattering process (quark+quark→quark+quark+photon) for the production of large-q_T real photons is discussed in the framework of perturbative QCD. To extract the finite contribution of this process to the differential cross section dσ/dy d²q_T (hadron 1+hadron 2→photon+anything) we define the gluon distribution and the quark-to-photon fragmentation function beyond the leading approximation. The calculations are performed consistently in the dimensional regularization scheme. Our numerical estimates show the resulting finite qq→qqγ contribution to be a small (order α_s/2π) correction in comparison with the basic QCD subprocesses.     

Large-angle compton scattering in perturbative QCD

The processes γp→γp and γn→γn are studied in perturbative QCD for large scattering angles. The unpolarized cross sections, as well as the cross sections for polarized beams and targets, are given for various choices of the baryon wavefunction. The predictions for the unpolarized cross section of γp→γp are compared with the available data. Possible future experimental verifications of the theory are discussed.     

Study of scalar meson a0(980) from B→a0(980)π decays

In this paper, we calculate the branching ratios and the direct CP-violating asymmetries for decays B0 →a00(980)π0, a0+(980)π-, a0-(980)π and B- →a00(980)π-, a0-(980)π0 by employing the perturbative QCD (pQCD) factorization approach at the leading order. We found that (a) the pQCD predictions for the branching ratios are around (0.4 - 2.8) × 10-6, consistent with currently available experimental upper limits; (b) the CP asymmetries of B0→ao(980)π0 and B-→a0-(980)π0 decays can be large, about (70-80)% for α = 100°.     

Applications of Mueller theory of two-particle inclusive processes

Assuming factorization, and extracting inclusive Reggeon vertices from one-particle inclusive data, we predict Regge corrections to scaling for several two-particle inclusive processes. The data on p + P → π^? + π^? + X, K⁺ + p → π^? + π^? + X, K^? + p → π⁺ + π^? + X, and π⁺ + p → π^? + π^? + X agree with our predictions.     

The non-resonant reaction γγ→K s K ±π∓ in perturbative QCD

The non-resonant reaction γγ→K _S K _π is studied in perturbative QCD. In order to reduce the required computer time the meson wave functions are approximated by sums of delta functions. Total cross sections, energy and angular distributions are presented.     

Elastic and transition form factors of light pseudoscalar mesons from QCD sum rules

We revisit F _π(Q ²) and F _Pγ(Q ²), P = π, η, η′, making use of the local-duality (LD) version of QCD sum rules. We give arguments that the LD sum rule provides reliable predictions for these form factors at Q ² ≥ 5–6 GeV², the accuracy of the method increasing with Q ² in this region. For the pion elastic form factor, the well-measured data at small Q ² give a hint that the LD limit may be reached already at relatively low values of momentum transfers, Q ² ≈ 4–8 GeV²; we therefore conclude that large deviations from LD in the region Q ² = 20–50 GeV² seem very unlikely. The data on the (η, η′) → γγ* form factors meet the expectations from the LD model. However, the BaBar results for the π ⁰ → γγ* form factor imply a violation of LD growing with Q ² even at Q ² ≈ 40 GeV², at odds with the η, η′ case and with the general properties expected for the LD sum rule.     

B→πη('), η(')η(') decays and NLO contributions in the pQCD approach

By employing the perturbative QCD (pQCD) factorization approach, we calculate the full leading and the partial next-to-leading order (NLO) contributions to the seven B→πη(') and η(')η(') decays. For B+B→π+η(') decays, the pQCD predictions for their decay rates agree very well with the data after the inclusion of the small NLO contributions. For neutral decays, the pQCD predictions are also consistent with the experimental upper limits and can be tested by the LHC experiments. The measured value of ACPdir(π±η)=-19±7% can also be accommodated by the pQCD approach.     

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.     

An overview of recent DVCS results at HERMES

The HERMES experiment at DESY, Hamburg, collected unique data on Deeply Virtual Compton Scattering (DVCS) utilizing the HERA polarized electron or positron beams with an energy of 27.6 GeV and longitudinally or transversely polarized or unpolarized gas targets (H, D or heavier nuclei). For the last two years of HERA running, a recoil detector was installed to improve the selection of DVCS events by direct measurement of the recoil protons. Recent HERMES results on DVCS off the hydrogen target and on associated processes ep → epπ⁰γ and ep → enπ⁺γ in the Δ-resonance region obtained with the recoil detector are presented.     

Exclusive processes in quantum chromodynamics: Evolution equations for hadronic wavefunctions and the form factors of mesons

The predictions of quantum chromodynamics for meson form factors at large momentum transfer are given. Evolution equations are derived which determine the structure of hadronic wavefunctions at short distances from their form at large distances. The eigenvalues of the evolution equations appear as exponents in anomalous logarithm corrections to the nominal power law of form factors determined by dimensional counting. The results lead to detailed tests of the spin and scaling structure of QCD at short distances. The predictions for the charged pion, kaon and rho form factors and the γ → π⁰ transition form factor of the photon are absolutely normalized at asymptotic momentum transfer.