Exclusive electroproduction of pion pairs

We investigate electroproduction of pion pairs on the nucleon in the framework of QCD factorization for hard exclusive processes. We extend previous analyses by taking the hard-scattering coefficients at next-to-leading order in α_s . The dynamics of the produced pion pair is described by two-pion distribution amplitudes, for which we perform a detailed theoretical and phenomenological analysis. In particular, we obtain constraints on these quantities by comparing our results with measurements of angular observables that are sensitive to the interference between two-pion production in the isoscalar and isovector channels.     

Power corrections to the process $\gamma^{*}\gamma \to \pi\pi$ in the running coupling method

Power-suppressed corrections coming from the end-point integration regions in the amplitude of the process at large Q ² and small squared center-of-mass energy W ² are calculated in the QCD hard-scattering approach where the amplitudes factorize in a hard perturbatively calculable part and a generalized distribution amplitude. The running coupling method and the technique of infrared renormalon calculus are applied to obtain Borel resummed expressions for the two main components of the process amplitude. Numerical estimates for these power corrections are presented. They are sizeable when Q ² < 10 GeV².Received: 29 March 2004, Revised: 11 May 2004, Published online: 1 October 2004     

Vector meson electroproduction at next-to-leading order

The process of light neutral vector meson electroproduction is studied in the framework of QCD factorization in which the amplitude factorizes in a convolution of the nonperturbative meson distribution amplitude and generalized parton densities with perturbatively calculable hard-scattering amplitudes. We derive a complete set of hard-scattering amplitudes at next-to-leading order (NLO) for the production of vector mesons, V=ρ⁰, ω, ?.     

The Physics of Exclusive Reactions in QCD: Theory and Phenomenology

The modern formulation of exclusive reactions within Quantum Chromodynamics is reviewed, the emphasis being placed on the pivotal ideas and methods pertaining to perturbative and non-perturbative topics. Specific problems, related to scale locality, infrared safety, gluonic radiative corrections (Sudakov effects), and the role of hadronic size effects (intrinsic transverse momentum), are studied. These issues are more precisely analyzed in terms of the essential mechanisms of momentum transfer to a hadron while remaining intact. Different factorization schemes are considered and the conceptual lacunas are pointed out. The quite technical subject of renormalization-group evolution is given a detailed account. By combining analytical and numerical algorithms, the one-gluon exchange nucleon evolution equation is diagonalized and next-to-leading eigenfunctions are calculated in terms of Appell polynomials. The corresponding anomalous dimensions of trilinear quark operators are found to form a degenerate system whose envelope shows logarithmic large-order behavior. Selected applications of this framework are presented, focusing on the helicity-conserving elastic form factors of the pion and the nucleon. The theoretical constraints imposed by QCD sum rules on the moments of nucleon distribution amplitudes are used to determine a whole spectrum of optional solutions. They organize themselves along an “orbit” characterized by a striking scaling relation between the form-factor ratio and the projection coefficient B₄ on to the corresponding eigensolution. The main reasons for the failure of the present theoretical predictions to match the experimental data are discussed and workable explanations are sketched.     

Pseudo scalar contributions to light-by-light correction of muon in AdS/QCD

We have performed a holographic calculation of the hadronic contributions to the anomalous magnetic moment of the muon, using the gauge/gravity duality. As a gravity dual model of QCD with three light flavors, we study a U(3)_L×U(3)_R flavor gauge theory in the five-dimensional AdS background with a hard-wall cutoff. The anomalous (electromagnetic) form factors for the pseudo scalars, π⁰, η and η^′, are obtained from the 5D Chern–Simons term of the gravity dual, which correctly reproduce the asymptotic behavior of the form factor, dictated by QCD. We find the total light-by-light contributions of pseudo scalars to the muon anomalous magnetic moment, , which is consistent with previous estimates, based on other approaches.     

Determination of the strange quark content of the nucleon from a next-to-leading-order QCD analysis of neutrino charm production

We present the first next-to-leading-order QCD analysis of neutrino charm production, using a sample of 6090 ^– and -induced opposite-sign dimuon events observed in the CCFR detector at the Fermilab Tevatron. We find that the nucleon strange quark content is suppressed with respect to the non-strange sea quarks by a factor =0.477 _–0.053 ^+0.063 , where the error includes statistical, systematic and QCD scale uncertainties. In contrast to previous leading order analyses, we find that the strange seax-dependence is similar to that of the non-strange sea, and that the measured charm quark mass,m _c =1.70±0.19 GeV/c², is larger and consistent with that determined in other processes. Further analysis finds that the difference inx-distributions betweenxs(x) and is small. A measurement of the Cabibbo-Kobayashi-Maskawa matrix element |V _cd |=0.232 _–0.020 ^+0.018 is also presented.     

The nucleon quark distribution amplitude and its influence on theQ 2-behaviour of nucleon form factors

In a re-analysis of nucleon form factors in perturbative quantum chromodynamics we examine the -dependence of the nucleon quark distribution amplitude by means of some model distribution amplitudes. We point out some problems that arise in the determination of the -evolution of the distribution amplitude. Our suggestions to cure these problems are discussed and resulting nucleon quark distribution amplitudes are presented. These give reasonable agreement with the available data for the form factors.Work is supported by Deutsche Forschungsgemeinschaft (Ga 153-13-1) and partially by NATO (0581/87)     

Subthreshold K− production in pion-nucleus reactions

The inclusive subthresholdK ^– mesons production in charged pion-induced reactions is analyzed with respect to the one-step production processes on the basis of an appropriate first collision model, which allows one to take into account the various forms of an internal nucleon momentum distribution. Free elementary production processes are described by the phase space calculations normalized to the corresponding total experimental cross sections. Simple parametrizations for the total cross sections of theK ^– production in pion-nucleon collisions are given. Primary-pion energy dependence of the inclusive cross sections for K^– production in ⁺ C– collisions is calculated. The influence of the uncertainties in the elementaryK ^– production cross sections close to the production threshold on theK ^– yield is explored.The authors acknowledge stimulating discussions with V. Koptev on the initial stage of this study.     

decays in Chiral Perturbation Theory

We study the decays K→ππ in one-loop two-flavour Chiral Perturbation Theory. We provide arguments why the calculation of the coefficient of the pionic chiral logarithm ℓ_M=M²logM² is unique and then perform the calculation. As a check we perform the reduction of the known three-flavour result. Our result can be used to perform the extrapolation to the physical pion mass of direct lattice QCD calculations of K→ππ at fixed m_s or . The underlying arguments are expected to be valid for heavier particles and other processes as well.     

The (spin) structure of the nucleon

A selection of new data obtained by the HERMES experiment at DESY is presented, which provides new insight into the QCD structure of the nucleon. Using polarized lepton beams and polarized targets, the spin-dependent structure function g ₁(x) has been determined for ¹ H, ² H and ³ He. By also observing one of the produced hadrons it has been possible to extract the polarization distribution of individual quark flavours in the nucleon as well. Further information on nucleon structure has been obtained by observing (almost) exclusive reactions, which can be interpreted in terms of the recently introduced generalized parton distributions (GPDs). As an example of such data measurements of both the beam-spin and beam-charge asymmetries resulting from deeply virtual Compton scattering (DVCS) are presented. By embedding the deep-inelastic scattering process in the nuclear environment additional information can be obtained on nucleon structure and some QCD effects. The potential of this technique is illustrated by showing new results on tagged structure functions and hadronization in nuclei.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.60.Hb Total and inclusive cross-sections (including deep-inelastic processes) - 13.88. + e Polarization in interactions and scattering - 14.20.Dh Protons and neutrons - 24.85. + p Quarks, gluons, and QCD in nuclei and nuclear processesG. van der Steenhoven: For the HERMES Collaboration     

Exclusive decay to double at next-to-leading order in

Within the non-relativistic QCD (NRQCD) factorization framework, we calculate the exclusive decay process η_b→J/ψJ/ψ to next-to-leading order in the strong coupling constant, and, at leading order, in the charm quark relative velocity. It is found that this new contribution to the amplitude is comparable in magnitude to the previously calculated relativistic correction piece, but differs by a phase of about 90°. Including this new contribution will increase the previous prediction of substantially, thus brightening the discovery potential of this clean hadronic decay channel of η_b in the forthcoming LHC experiment.     

Higher twist and higher order contributions to the pion electromagnetic form factor

The andO ( _S ² ) corrections to the leading term of the perturbative QCD calculation of the pion electromagnetic form factor are examined numerically. Both sets of terms provide significant corrections for values ofQ ² between 1 and 15 GeV²/c².Parts of this work were done while one of us (JP) was a visitor at the national INT, the University of Maryland and TRIUMF. He is grateful to those institutions for their hospitality. We have benefitted from discussions with A. Mueller and G. Sterman. This work is supported in part by the U.S. Department of Energy.     

A determination ofα s from the transverse momenta of hadrons produced in neutrino and antineutrino interactions

TheW andQ ² dependence of the transverse momentum of the forward-going hadrons in deep inelastic neutrino charged current interactions on neon is studied. The data were obtained by exposing BEBC filled with a Ne/H₂ mixture to the CERN SPS wide-band neutrino beam. The data on average transverse momentum are compared to a model which includes first order QCD, intrinsic quark momentum in the nucleon and Fermi motion, as well as a simple model for hadronization. The model is found to describe the data well. Values of _s =0.16±0.02(stat)±0.02(syst.)±0.03(frag.) at an averageQ ² of 15.4±0.4 GeV²/c² and (stat.) _–90 ⁺¹⁰⁰ (syst.) _–90 ⁺¹³⁰ , (frag.) MeV/v for three flavours are extracted.     

Nonlocal theory of single pion photoproduction

A nonlocal interaction theory is formulated for photoproduction processes by introducing a form function. An effective nonlocal, Lorentz invariant and gauge invariant Lagrangian density for the four-field interaction that gives rise to a production amplitude is constructed. The general structure of the form function is investigated by using some restrictions of the form function. For low energy ⁰ photoproduction an explicit form of the Fourier component of the form function is obtained. The physical model of the present formalism is to assume, similar to the strong absorption model, that the system in intermediate states is confined in a finite domain of space. For ⁰ production the linear dimension of this domain is obtained to be ₀ = 3·88F. It is important to observe that the extent of electromagnetic distribution in a nucleon is also nearly the size of ₀. This is believed to be the reason that in low energy pion photoproduction the effects of electromagnetic structure of a nucleon are irrelevant. The unpolarized differential and total cross sections are calculated for ⁰ production in helicity representation and the predictions are found to be in good agreement with experiments.     

Towards B→Vγ decays at NNLO in SCET

We compute NNLO ((α_s ²)) corrections to the hard-scattering kernels entering the QCD factorization formula for B→Vγ decays, where V is a light vector meson. We give complete NNLO results for the dipole operators Q₇ and Q₈, and partial results for Q₁ valid in the large-β₀ limit and neglecting the NNLO correction from hard-spectator scattering. Large perturbative logarithms in the hard-scattering kernels are identified and resummed using soft-collinear effective theory. We use our results to estimate the branching fractions for B→K^*γ and B_s→φγ decays at NNLO and compare them with the current experimental data.     

Color transparency effects in electron deuteron interactions at intermediateQ 2

High momentum transfer electrodisintegration of polarized and unpolarized deuterium targets,d(e, ep)n is studied. We show that the importance of final state interactions FSI, occuring when a knocked out nucleon interacts with the other nucleon, depends strongly on the momentum of the spectator nucleon. In particular, these FSI occur when the essential contributions to the scattering amplitude arise from internucleon distances 1.5 fm. But the absorption of the high momentum ^* may produce a point like configuration, which evolves with time. In this case, the final state interactions probe the point like configuration at the early stage of its evolution. If the point like configuration is still small after propagating about 1.5 fm, the FSI are suppressed. The result is that significant color transparency effects, which can either enhance or suppress computed cross sections, are predicted to occur forQ ²4GeV². We suggest searching for color transparency phenomenon by examining ratios of experimentally measured quantities. Possible theoretical uncertainties of the calculations, including those due to the deuteron wave function and relativistic effects, are found to be small.This work was supported in part by the USA — Israel Binational Science Foundation Grant No. 9200126 and by the US Department of Energy under Contract Nos. DE-FG02-93ER40771 and DE-FG06-88ER40427.     

Exclusive photoproduction of a heavy vector meson in QCD

The process of exclusive heavy vector meson photoproduction, , is studied in the framework of QCD factorization. The mass of the produced meson, or , provides a hard scale for the process. We demonstrate that, in the heavy quark limit and at the one-loop order in perturbation theory, the amplitude factorizes in a convolution of a perturbatively calculable hard-scattering amplitude with the generalized parton densities and the non-relativistic QCD matrix element . We evaluate the hard-scattering amplitude at one-loop order and compare the data with theoretical predictions using an available model for generalized parton distributions.Received: 22 January 2004, Published online: 23 March 2004     

A new determination of the γπ → ππ anomalous amplitude via πe<Superscript>-</Superscript> → π<Superscript>-</Superscript>e<Superscript>-</Superscript>π<Superscript>0</Superscript> data

We discuss the reaction π^-e^- → π^-e^-π⁰ with the purpose of obtaining information on the γπ → ππ anomalous amplitude _3π. We compare a full calculation at (p⁶) in chiral perturbation theory and various phenomenological predictions with the existing data of Amendolia et al. By integrating our theory results using Monte Carlo techniques, we obtain σ = 2.05 nb at (p⁶) and σ = 2.17 nb after including the dominant electromagnetic correction. Both results are in good agreement with the experimental cross-section of σ = (2.11±0.47) nb. On the basis of the ChPT results one would extract from the experimental cross-section as amplitudes _3π^(0)extr = (9.9±1.1) GeV^-3 and _3π^(0)extr = (9.6±1.1) GeV^-3, respectively, which have to be compared with the low-energy theorem _3π = e/(4π²F_π³) = 9.72GeV^-3. We emphasize the need for new data to allow for a comparison of experimental and theoretical distributions and to obtain _3π with smaller uncertainty.