Moments of the virtual photon structure function

The photon structure function is a useful testing ground for QCD. It is perturbatively computable apart from a contribution from what is usually called the hadronic component of the photon. There have been many proposals concerning this nonperturbative part of the real photon structure function. By studying moments of the virtual photon structure function, we explore the extent to which proposed nonperturbative contributions can be identified experimentally.     

Factorization scheme and parton distributions in the polarized virtual photon target

We investigate spin-dependent parton distributions in the polarized virtual photon target in perturbative QCD up to the next-to-leading order (NLO). In the case , where is the mass squared of the probe (target) photon, the parton distributions can be predicted completely up to NLO, but they are factorization-scheme dependent. We analyze the parton distributions in six different factorization schemes and discuss their scheme dependence. We study, in particular, the QCD and QED axial anomaly effects on the first moments of the parton distributions to see the interplay between the axial anomalies and factorization schemes. We also show that the factorization-scheme dependence is characterized by the large-x behaviors of the quark distributions in the virtual photon. The gluon distribution is predicted to be the same up to NLO among the six factorization schemes examined. In particular, the first moment of the gluon distribution is found to be factorization-scheme independent up to NLO. Received: 24 January 2001 / Published online: 18 May 2001     

Gluon evolution at low x and the longitudinal structure function

We obtain an approximate analytical form of the gluon distribution using the GLAP equation with a factorization ansatz, and test its validity by comparing it with that of Glück, Reya and Vogt at low x regime. We also present calculations of the longitudinal structure functions.     

Photoproduction of jets and the virtual structure of the photon

We compute the ratio between the direct and the resolved photon components of single jet and dijet production in ep collisions for the kinematical range covered by the most recent ZEUS data. We analyse the phenomenological consequences of different models for the structure of virtual photons in these observables and compare them with the available data. We also comment on the correlation between the so-called $x_{αmma}^{? obs}$ and the ‘true’ x_γ, that can be inferred from the data.     

Predictions for high-energy real and virtual photon–photon scattering from color dipole BFKL–Regge factorization

High-energy virtual photon–virtual photon scattering can be viewed as an interaction of small size color dipoles from the beam and target photons, which makes scattering at high energies (LEP, LEP200 and NLC) an indispensable probe of the short distance properties of the QCD pomeron exchange. Based on the color dipole representation, we investigate the consequences for the scattering of the incorporation of asymptotic freedom into the BFKL equation which makes the QCD pomeron a series of isolated poles in the angular momentum plane. The emerging color dipole BFKL–Regge factorization allows us to relate in a model-independent way the contributions of each BFKL pole to scattering and DIS off protons. Numerical predictions based on our early works on the color dipole BFKL phenomenology of DIS on protons are in good agreement with the experimental data on the photon structure function and the most recent data on the cross section from the OPAL and L3 experiments at LEP200. We discuss the role of non-perturbative dynamics and predict a pronounced effect of the Regge-factorization breaking due to large unfactorizable non-perturbative corrections to the perturbative vacuum exchange. We comment on the salient features of the BFKL–Regge expansion for scattering including the issue of the decoupling of subleading BFKL poles and the soft plus rightmost hard BFKL pole dominance. Received: 9 January 2001 / Revised version: 25 September 2001 / Published online: 7 December 2001     

Spin-dependent structure functions of real and virtual photons

The implications of the positivity constraint, , on the presently unknown spin–dependent structure function of real and virtual photons are studied at scales where longitudinally polarized photons dominate physically relevant cross sections. In particular it is shown how to implement the physical constraints of positivity and continuity at in NLO calculations which afford a nontrivial choice of suitable (DIS) factorization schemes related to and and appropriate boundary conditions for the polarized parton distributions of real and virtual photons. The predictions of two extreme ‘maximal’ and ‘minimal’ saturation scenarios are presented and compared with results obtained within the framework of a simple quark ‘box’ calculation expected to yield reasonable estimates in the not too small regions of x and . Received: 16 March 2001 / Published online: 18 May 2001     

Parton distributions in the photon

We discuss in detail the photon structure function beyond the leading logarithm approximation. Of special concern is the factorization scheme and the hadronic input; we show how to naturally absorb large terms due to the factorization scheme in a modified hadronic component. The effect of the charm quark mass threshold is also discussed in relation to the phenomenology. A comparison with data shows that the modified hadronic component can be reasonably described by a VDM-type input.URA 14-36 du CNRS, associée à l'Ecole Normale Supérieure de Lyon, et au Laboratoire d'Annecy-le-Vieux de Physique des Particules     

Gluon jet-like structure in the upsilon decay

The properties of hadronic jets in e⁺ e^– annihilation in quantum chromodynamics have been studied. Motivated by the observation of the hadronic jets (arising from quark jets), the partial cross sections from the two-gluon jet-like events are calculated perturbatively. Estimates for the jet angular radius and its energy dependence are discussed.The author would like to thank Dr. Mubarak Ahmad for his guidance and encouragement.     

Three photon virtual annihilation contributions to positronium hyperfine structure

Three photon virtual annihilation contributions to the positronium hyperfine interval are calculated to order α⁶m(α⁴R_∞). The result is obtained in analytic form and leads to the small value of -0.91 MHz for the contribution to the ground state energy difference from this source.     

Electroproduction cross section of large-E$_{\bot}$ hadrons at NLOand virtual photon structure function

We calculate higher order corrections to the resolved component of the electroproduction cross section of large- hadrons. The parton distributions in the virtual photon are studied in detail and a NLO parametrization of the latter is proposed. The contribution of the resolved component to the forward production of large- hadrons is calculated and its connection with the BFKL cross section is discussed.Received: 31 July 2004, Revised: 10 September 2004, Published online: 26 November 2004     

Strong coupling constant from the photon structure function

We extract the value of the strong coupling constant alpha(s) from a single-parameter pointlike fit to the photon structure function F(gamma)(2) at large x and Q(2) and from a first five-parameter full (pointlike and hadronic) fit to the complete F(gamma)(2)data set taken at PETRA, TRISTAN, and LEP. In next-to-leading order and the MS renormalization and factorization schemes, we obtain alpha(s)(m(Z))=0.1183+/-0.0050(expt)(+0.0029)(-0.0028)(theor) (pointlike) and alpha(s)(m(Z))=0.1198+/-0.0028(expt)(+0.0034)(-0.0046)(theor) (pointlike and hadronic). We demonstrate that the data taken at LEP have reduced the experimental error by about a factor of 2, so that a competitive determination of alpha(s) from F(gamma)(2) is now possible.     

Gluon helicity distributions from hyperon production in proton-proton collisions

We point out a sensitive way of analyzing the gluon polarization inside a polarized proton using hyperon production at largep _T inPP collisions, by using initial and final state polarizations.     

Second-order QCD analysis of the photon structure function

The QCD predictions for the photon structure function are reexamined with particular emphasis on the small-x behavior. A simple parametrization of the real photon structure function, free of 1/x singularity, is derived. The structure function is found to be sensitive at small x to the non-perturbatively calculable constant term in the n = 2 moment, and we show that the problem of a negative structure function can be solved on the basis of the knowledge of this single non-perturbative parameter.     

Generalized parton distributions of the photon

We present a first calculation of the generalized parton distributions of the photon (both polarized and unpolarized) using overlaps of light-front wave functions at leading order in α and zeroth order in αs; for non-zero transverse momentum transfer and zero skewness. We present the novel parton content of the photon in transverse position space.