Photon 2001

PREFACE.

COMMITTEE.

CONTENTS.

The hadronic structure function of the photon is measured as a function of Bjorken x and of the factorisation scale Q² using data taken by the OPAL detector at LEP. Previous OPAL measurements of the x dependence of are extended to an average Q² of 767 GeV². The Q² evolution of is studied for 11.9 < 〈Q²〉 < 1051 GeV². As predicted by QCD, the data show positive scaling violations in . Several parameterisations of are in agreement with the measurements whereas the quark-parton model prediction fails to describe the data.

The photon structure function has been studied in the Q² range from 3 GeV² to 2600 GeV². The data correspond to the integrated luminosity of 78 pb^-1 and 548 pb^-1, collected by the DELPHI detector during the 1994-1995 and 1998-2000 LEP runs respectively. Experimental distributions are compared with predictions of different generators. The is estimated from the fit of simulated events to the data and compared with theoretical expectations based on different models. A result for Q² evolution of the photon structure function in the different x intervals has been obtained.

Charm production is studied in deep-inelastic electron-photon scattering using OPAL data at e⁺e^- centre-of-mass energies from 183 to 209 GeV. Charm quarks are identified by exclusive reconstruction of D* mesons. The cross-section of D* production is measured in a restricted kinematic region, and then extrapolated to the total charm production cross-section and the charm structure function of the photon. For x > 0.1 the measurement is well described by Monte Carlo models and perturbative QCD calculations but for x < 0.1 the predictions are lower than the data both in the directly measured region and after the extrapolation.

Preliminary results of a new analysis ¹ of the real photon LO parton distributions are presented. Analysis is based on the GRV'92 ² approach but uses all new available experimental data and takes in consideration heavy charmed mesonic states which may appear in the photon structure evolution.

Inclusive jet cross sections for the reaction e⁺ p → jet + X for Q² < 1 GeV² have been measured with the H1 detector at HERA. Differential cross sections as a function of η_jet and E_Tjet are in good agreement with NLO QCD calculations using different sets of photon parton density functions as input. Dijet cross sections of jets with high E_T were measured by both the ZEUS and H1 experiments in photoproduction. Comparisons of ZEUS data to NLO QCD calculations show that the theory underestimates the data for low x_γ, indicating possible inadequacies in the photon structure parametrisations. H1 data are in good agreement with the predictions of NLO QCD. The measurements were, however, made in different kinematic regions.

Triple differential dijet cross-sections in e^±p interactions measured with the H1 and ZEUS detectors at HERA are presented. The data are compared to Monte Carlo simulations which differ in their assumptions about photon structure and parton evolution. Effects of the resolved processes of longitudinally polarized virtual photons at HERA are investigated for the first time.

The dependence of the virtual-photon structure on its virtuality, Q², has been studied for the first time in events containing dijets and a charm particle. The ratio of dijet cross sections at low relative to high as a function of Q² is presented and compared to the predictions of LO pQCD. The ratio does not change significantly with Q², in marked contrast to previous measurements which did not require the presence of a charm particle.

The status of and ongoing developments in the measurements and theoretical studies of the structure of the photon have been presented at the PHOTON 2001 conference in Ascona. The results presented in the structure function session are briefly summarised.

The reactions e⁺e^- → e⁺e^-π⁰ X and are studied using data collected at LEP with the L3 detector at centre-of-mass energies between 189 and 202 GeV. Inclusive differential cross sections are measured as a function of the particle transverse momentum p_t and the pseudo-rapidity. For p_t ≤ 1.5 GeV, the π⁰ and differential cross sections are described by an exponential, typical of soft hadronic processes. For p_t ≥ 1.5 GeV, the cross sections show the presence of perturbative QCD processes, described by a power-law. The data are compared to NLO QCD calculations.

Fragmentation measurements are presented for diffractive and non-diffractive deep inelastic ep scattering data in the Breit frame of reference. The average charged multiplicity in the current hemisphere, < n >, is shown to compare well with DIS at low β and with e⁺e^- at high β. The evolution of the peak and width of the current hemisphere fragmentation functions for charged particles is studied as a function of photon virtuality, Q, and is found to agree with results obtained in non-diffractive deep inelastic scattering.

Measurements of meson and Λ baryon production in γp interactions containing at least two high transverse energy jets at ZEUS using an integrated luminosity of 38.64 pb^-1 are presented and compared with the predictions of leading order (LO) Monte Carlo models. Strange hadron production provides detailed information on both the hadronization mechanism and hard sub-process for interactions containing photon and proton hadronic remnants in the final state.

Di-jet production is studied in collisions of quasi-real photons radiated by the LEP beams at e⁺e^- centre-of-mass energies from 189 to 209 GeV. The data have been taken with the OPAL detector. Jets are reconstructed using a k_⊥-clustering algorithm. The inclusive di-jet cross-section is measured as a function of the mean transverse energy of the two jets, and as a function of x_γ for different regions of . Furthermore the inclusive di-jet cross-section as a function of │η_jet│ and │Δη_jet│ is presented, where η_jet is the jet pseudo-rapidity. Different regions of the -space are explored to study and control the influence of a possible underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.

Preliminary results on fully-inclusive differential cross sections of dijet production for jets with E_T ≥ 5GeV in two-photon collisions as measured with the DELPHI detector at between 192 and 202 GeV are presented.

The production of prompt photons has been studied in the collisions of quasi-real photons at LEP. The OPAL data taken at e⁺e^- centre-of-mass energies between 183 and 209 GeV have been used. Differential cross-sections for isolated prompt photons in the kinematic range of transverse momentum and pseudorapidity │η^γ │ < 1 are compared to the leading order predictions of PYTHIA.

The substructure of jets produced in an exclusive and a charm-induced dijet sample in photoproduction and in charged and neutral current interactions has been studied with the ZEUS detector at HERA. Jets were identified using the longitudinally invariant k_T cluster algorithm in the laboratory frame. The substructure of jets has been studied in terms of the jet shape and subjet multiplicity. Comparisons between the dijet sample and the quark-induced samples allow an extraction of the jet substructure for gluons. Leading-logarithm parton-shower Monte Carlo calculations give a good description of the differences between quark- and gluon-initiated jets. In neutral current interactions, the measurements have been compared to next-to-leading-order QCD calculations which are used to make a determination of the strong coupling constant, α_s.

The process e⁺e^- → e⁺e^-Z/γ* is studied with the OPAL detector at LEP at a centre of mass energy of . The cross-section times the branching ratio of the Z/γ* decaying into hadrons is measured within Lorentz invariant kinematic limits to be (1.2±0.3±0.1) pb for invariant masses of the hadronic system between 5 GeV and 60 GeV and (0.7±0.2±0.1) pb for hadronic masses above 60 GeV. The differential cross-sections of the Mandelstam variables , , and are measured and compared with the predictions from the Monte Carlo generators grc4f and PYTHIA. From this, based on a factorisation ansatz, the total and differential cross-sections for the subprocess eγ → eZ/γ* are derived.

We have investigated a general systematic method for combining next-to-leading order QCD calculations with parton shower algorithms. We implemented this method for the case of inclusive jet production in the laboratory reference frame in ep scattering and calculated inclusive jet cross-sections and jet shape distributions. We find good agreement between our calculations and data from the ZEUS collaboration. In addition, we started implementing the same method for the case of dijet production in the Breit reference frame.

The importance of the interference terms and contribution due to the longitudinally polarised virtual photon in the semi-inclusive ep collision is discussed. The numerical results for the unpolarized Compton process at the Born level are shown.

Recent jet results from the HERA collaborations H1 and ZEUS are reviewed. Topics covered are soft perturbative QCD studies in photoproduction multi-jet events, multi-jet cross-section measurements in deep-inelastic scattering, determinations of the strong coupling parameter α_s and the gluon density and jet measurements at the highest photon virtualities Q² accessible at HERA. In most cases NLO QCD calculations deliver a satisfactory description of the data. The determinations of α_s using jets at HERA have achieved a precision similar to the world average.

Current theoretical framework for the calculation of heavy quark production in γγ collisions is reviewed. The importance of including direct photon contributions up to the order and of proper choice of renormalization and factorization scales in the evaluation of is emphasized.

The inclusive production of D^*+ is measured by DELPHI in photon-photon collisions at LEP-II energies. The measured cross sections are compatible with the QCD calculations having the contributions from the resolved processes sensitive to the gluon density in photon. The total cross section of the charm quark production in two-photon collisions at LEP-II energies is estimated.

Inclusive J/ψ production in photon-photon collisions has been observed by the DELPHI collaboration at LEP II beam energies. A clean signal from the reaction γγ → J/ψ + X is seen. Number of observed events, N(J/ψ → μ⁺μ^-) = 36 ± 7 for the integrated luminosity 617 pb^-1, yielding a cross section of σ(J/ψ → μ⁺μ^-) = 25.2 ± 10.2 pb. Based on a study of the event shapes of different types of γγ processes in the PYTHIA program, we conclude that (74±22)% of the observed J/ψ events are due to the 'resolved' photons, the dominant contribution of which is evidently a single color-octet gluon within the photon.

Measurements of charm and bottom quarks production in two-photon collisions at LEP are presented. The cross section of b production is in excess of the QCD prediction by a factor of three.

The application of k_t - factorization, supplemented with the CCFM small-x evolution equation, to heavy quark production is discussed. Differential cross sections of production and also inelastic J/ψ production as measured at HERA are compared to the hadron level CCFM Monte Carlo generator CASCADE, using the unintegrated gluon density obtained within the CCFM evolution approach from a fit to HERA F₂ data.

D^*± production in deep inelastic scattering at HERA has been measured by H1 and ZEUS using 19 pb^-1 of e⁺p data. Total and differential cross sections are compared to QCD calculations. A first measurement of semi-leptonically decaying charm at HERA is presented. ZEUS has preliminary results on D^*± production in 16.7 pb^-1 of e^-p data and 65.2 pb^-1 of e⁺p data. Charm production in diffractive ep scattering has been observed and results are compared to models of diffractive interactions.

The production of bottom quarks, tagged by their semi-leptonic decay, has been studied in ep collisions at HERA with the ZEUS and H1 detectors. Results are reported from both experiments on the total and differential bottom cross-sections.

Heavy-flavoured jets have been studied in photoproduction at HERA. This includes the first measurement of dijet angular distributions in D* photoproduction and the ratio of the vector/(vector + pseudoscalar) production rate for charm mesons.

Precise knowledge of R(s) = σ(e⁺e^- → hadrons)/σ(e⁺e^- → μ⁺μ^-) is important in a variety of problems in particle physics. While at high energies R(s) can be calculated within the framework of QCD, at low energies it has to be measured in experiment. In 1992-2000 the data with integrated luminosity of 70 pb^-1 were collected in the energy range with the CMD-2 and SND detectors at the VEPP-2M collider in Novosibirsk. The status of the R(s) measurements by these groups will be presented.

The reaction e⁺e^- → e⁺e^-γ* γ* → e⁺e^- hadrons for quasi-real photons is studied at LEP by L3 and OPAL collaborations from up to 202 GeV. Results on the total cross sections σ(e⁺e^- → e⁺e^- hadrons) and σ(γγ → hadrons) are obtained for the two-photon centre-of-mass energies 5 GeV ≤ W_γγ ≤ 185 GeV. The total cross section of two real photons can be described by a Regge parametrisation and compared to the expectations of different theoretical models.

The multihadron production in the reaction e⁺e^- → ⁺e^- + hadrons has been studied for the first time at LEP2 energies with both scattered e⁺ and e^- detected at very low Q², measured by the DELPHI VSAT. A reasonable agreement between data and full simulation is demonstrated and the total γγ hadronic cross-section is estimated for the γγ center of mass energy up to 100 GeV.

For a consistent description of small x processes in the BFKL approach one needs to know impact factors of colliding particles with the same accuracy as the kernel of the BFKL equation. The kernel is known now in the next-to-leading order (NLO), so that the problem of determination of the impact factors in the NLO became urgent. The results obtained up to now for the impact factors of highly virtual photons are briefly reviewed.

The interaction of virtual photons has been investigated using double tagged γ*γ*-events with hadronic final states, taken by the ALEPH experiment at e⁺e^- center-of-mass energies . The measured cross section is compared to the PYTHIA and the PHOT02 Monte Carlo Model, a NLO QCD calculation and BFKL calculations. The NLO QCD prediction is largely consistent with the data. The LO-BFKL predictions are ruled out by the data while NLO-BFKL calculations are in resonable agreement with the measurement.

Double-tag events in two-photon collisions are studied using the L3 detector at LEP with centre-of-mass energies from to . The cross-sections of e⁺e^- → e⁺e^- hadrons and γ* γ* → hadrons are given as a function of the photon virtualities, and , of the two-photon mass, W_γγ, and of the variable for an average photon virtuality 〈Q²〉 = 16 GeV². The results are in agreement with NLO calculations for the QPM process in the interval 2 ≤ Y ≤ 5. An excess is observed in the interval 5 ≤ Y ≤ 7, corresponding to W_γγ greater than 40 GeV. The PHOJET Monte Carlo describes the data reasonably well.

The interaction of virtual photons is investigated using the reaction e⁺e^- → e⁺e^- hadrons based on data taken by the OPAL experiment at e⁺e^- centre-of-mass energies , for W > 5 GeV and at an average Q² of 17.9 GeV². The measured cross sections are compared to the predictions of the Quark Parton Model (QPM), to the leading order (LO) QCD Monte Carlo model PHOJET, to the NLO prediction for the reaction , and to BFKL calculations. PHOJET, NLO , QPM describe the data reasonably well, whereas the cross section predicted by a LO BFKL calculation is too large.

The high-energy behaviour of the total cross section for highly virtual photons, as predicted by the BFKL equation at next-to-leading order in QCD, is presented. The NLO BFKL predictions, improved by BLM optimal scale setting, are in excellent agreement with recent OPAL and L3 data at CERN LEP2.

Cross section measurements made at HERA for the reactions γp → ωp and γp → ωπ⁰X, and on searches for the reactions γp → π⁰ N*, γp → f₂(1270)X, and , where N* denotes an excited nucleon state, are presented. The mean γp centre-of-mass energies were 〈W〉 = 200 GeV and 215 GeV, respectively. Cross sections for the Pomeron-mediated reactions were determined in agreement with previous measurements and phenomenological expectations. In contrast, Odderon induced processes have not been observed; upper limits on cross sections are below predictions from a non-perturbative QCD model.

Spin-dependent ρ⁰ production has been studied at the HERMES experiment at HERA. Recent results on the cross section and the spin density matrix elements of diffractive exclusive ρ⁰ production are compared to world data and theoretical predictions. The double-spin asymmetry of exclusive diffractive ρ⁰ production is found to be consistent with the theoretical expectation based on the Generalized Vector Meson Dominance description of both ρ⁰ production and inclusive DIS.

Recent measurements of the diffractive cross section in deep-inelastic scattering (DIS) at HERA are presented. The data are used to investigate the factorisation properties of diffractive DIS and to examine its quantum chromodynamic (QCD) structure.

New results on leading baryon production at HERA are presented for cross sections in photoproduction, in deep inelastic scattering and in the range in between these two kinematic regions. They are interpreted in terms of a Regge parameterization and, for the data with medium to high photon virtuality, in terms of structure functions. Vertex factorization is tested for these leading baryon data, as well as for events with a dijet system or a ρ meson in the final state.

Ultra-peripheral heavy ion collisions involve long range electromagnetic interactions at impact parameters larger than twice the nuclear radius, where no nucleon-nucleon collisions occur. The first observation of coherent ρ⁰ production with and without accompanying nuclear breakup, AuAu → Au* Au* ρ⁰ and AuAu → AuAuρ⁰ respectively, and the observation of e⁺e^- pair production AuAu → Au* Au* e⁺e^- are presented by the STAR collaboration. The transverse momentum spectra are peaked at low p_T, showing the coherent coupling to the nuclei. A clear ρ⁰ signal is observed in the two pion invariant mass spectrum.

We examine the energy dependence of total cross-sections for photon processes and discuss the QCD contribution to the rising behaviour.

The QED reaction e⁺e^- → e⁺e^-l⁺l^- is a dominant channel at LEP2. It has been studied with the L3 detector for l = μ using an untagged data sample collected at . The cross sections are measured as a function of the two-photon invariant mass for 3 ≤ W_γγ ≤ 40 GeV. Good agreement is found between these measurement and the O(α⁴) QED expectations.

The production of τ pairs in γγ collisions is studied with the L3 detector at LEP. Data were collected at for a total integrated luminosity of 608.1 pb^-1. An exclusive decay channel is considered, with τ^± → e^± → ν_τν_e and τ^∓ → ρ^∓ν_τ, with ρ^∓ → π^∓π⁰. The cross section σ (e⁺e^- → e⁺e^-τ⁺τ^-) is compared to QED calculations. The cross section of τ prduction is measured in γγ collisions for the first time at LEP 2.

The OPAL detector at LEP has been used to study the exclusive production of proton antiproton pairs in the collisions of two quasi-real photons using data taken at and 189 GeV. The results here presented are for γγ invariant masses, W, in the range 2.15 GeV < W < 3.95 GeV. The cross-section measurements are compared with previous data and with recent analytic calculations based on the quark-diquark model predictions.

Preliminary result of the proton-antiproton pair production in two-photon collisions at Belle experiment is presented. With high luminosity and good particle identifications measurements of W_γγ and angular dependence cross sections are performed at two-photon center-of-mass energies between 2.00 and 4.00 GeV. Uncertainties due to estimations of trigger and acceptance might have introduced large systematic uncertainty in our cross section calculations. 31.6 fb^-1 of data taken with Belle detector does reduce statistical uncertainty for events within 2.00 < W < 4.00 GeV region, where both nonperturbative and perturbative effects are present. This result shows that B factory is an ideal place for QCD study.

Baryon pair production in two-photon collisions is studied with the L3 detector at LEP using data collected at e⁺e^- center of mass energies from 91 GeV to 209 GeV with an integrated luminosity of 844 pb^-1. The four processes , , and are identified. The cross section for these processes as a function of the γγ center of mass energy is measured for the first time at LEP and results are compared to quark-diquark model predictions.

The reaction γγ → π⁺π^-π⁰ has been studied with the L3 detector at LEP. The data sample corresponds to a total integrated luminosity of 682.6 pb^-1 collected at the centre-of-mass energies from 183 GeV to 209 GeV. Preliminary results of the full energy dependent partial wave analysis in the mass region 0.65 - 2.15 GeV are presented. The reaction is dominated by the a₂(1320) formation. Other signals are also discussed.

Resonance formation in two-photon collisions in the π⁺π^-π⁰ final state was investigated for the a₂(1320) meson and a resonance-like structure at 1750 MeV/c². The a₂(1750) meson is observed in the ρπ and f₂π channels. The spin-parity parameters were determined by fitting the data to Monte Carlo predictions and found to be predominantly in the J^P = 2⁺ helicity 2 state. The mass and total width measured are 1740 ± 10 ± 10 MeV/c² and 290 ± 30 ± 20 MeV, respectively. The radiative width measured is 0.27 ± 0.02 ± 0.04 keV.

The possibility to constrain the meson distribution amplitude from γ* γ* → π, η, η′ transitions is investigated. It is shown that for a large range in the two photon virtualities the transition form factors are essentially independent of the distribution amplitudes. This in turn entails parameter-free predictions of QCD.

I review the present status of lattice calculations of properties of "gluon-rich" hadrons and comment on future prospects, in view of planned experiments.

Crystal Barrel data for proton-antiproton annihilation in flight at 900 MeV/c are presented. The channels , π⁰π⁰η and π⁰ηη are used to search for isoscalar 0⁺⁺ and 2⁺⁺ mesons in the mass range 1500 – 2000 MeV. Both 3π⁰ and π⁰ηη data sets require an isoscalar 2⁺⁺ resonance decaying into π⁰π⁰ and ηη with mass M = (1867±46) MeV and width Γ = (385±58) MeV. The analysis of π⁰π⁰η leads to an isovector 2⁺⁺ state decaying into π⁰η with mass M = (1698 ± 44) MeV and width Γ = (265 ± 55) MeV. The 3π⁰ and π⁰ηη data do not show any f₀(1710). This adds supportive evidence that this meson is mainly .

We perform simultaneous calculations of the radiative decays of scalar mesons f₀(980) → γγ, a₀(980) → γγ, vector meson φ(1020) → γf₀(980), γa₀(980), γπ⁰, γη, γη′ and tensor mesons a₂(1320) → γγ, f₂(1270) → γγ, f₂(1525) → γγ assuming all these states to be dominantly the ones¹. A good description of the considered radiative decays is reached by using almost the same radial wave functions for scalar and tensor mesons that supports the idea for the f₀(980), a₀(980) and a₂(1320), f₂(1270), f₂(1525) to belong to the same P-wave multiplet.

We have measured production and χ_c2 production in two-photon collisions with the Belle detector at the KEKB e⁺e^- collider. The invariant-mass spectra for γγ → K⁺K^- and processes are measured, and resonant structures found in the 1.7 - 2.3 GeV region are discussed. Production of the χ_c0 has been observed in the channel. The production of χ_c2 is measured in the γJ/ψ decay channel, and a preliminary result for its two-photon decay width is obtained.

KLOE has analysed ~ 16.6 pb^-1 of total 30 pb^-1 collected at DAΦNE by the end of 2000. The preliminary results for the branching ratios BR(φ → f₀(980)γ) = (23.7 ± 0.6_stat) × 10^-5 and BR(φ → a₀(980)γ) = (5.8 ± 0.5_stat) × 10^-5 lead to a ratio BR(φ → f₀(980)γ)/BR(φ → a₀(980)γ) = 4.1 ± 0.4 _stat with a systematic error which will not exceed 10%. The ratio BR(φ → η′γ)/BR(φ → ηγ) has been measured to about 10% accuracy leading to a very accurate determination of the mixing angle and to the most accurate determination of BR(φ → η′γ) to date, BR(φ → η′γ) = (6.8 ± 0.6 ± 0.5) · 10^-5.

The ηπ⁺π^- and final states in two-photon collisions are studied with the L3 detector at LEP. The f₁(1285) meson is observed in both channels. The η(1440) and f₁(1420) states are observed in the channel. The Q² dependences of their production are studied. The f₁(1285) γγ-coupling width, , is evaluated to be 3.5±0.6 (stat.) ±0.5 (sys.) keV. The branching fraction for f₁(1285) → a₀(980)π is estimated. is evaluated to be 199 ± 52 eV.

Two measurements of the cross-sections for the two-photon production of charged kaon and pion pairs at LEP2, based on data taken by ALEPH and DELPHI experiments are presented. The cross-sections have been measured for the invariant masses of the two-photon system above 2 GeV and for meson scattering polar angles, θ*, in the photon-photon centre-of-mass system in the region │cos θ*│ < 0.6. The results are compared to the QCD-based prediction by Brodsky and Lepage.

I present the first observation of the χ_c0 in photon-photon collisions and measurements of the two-photon widths of the χ_c0 and χ_c2. These results were obtained with the CLEO detector at the Cornell e⁺e^- collider CESR and are based upon an integrated luminosity of 12.7 fb^-1 obtained at centre-of-mass energies near the ϒ(4S). The X_c have been detected in their decays into π⁺π^-π⁺π^-. We obtained and . Using the world average values for we found Γ_γγ(χ_c0) = 3.76 ± 0.65(stat) ± 0.41(syst) ± 1.69(br)keV and Γ_γγ(χ_c2) = 0.53 ± 0.15(stat) ± 0.06(syst) ± 0.22(br)keV. The two-photon width of the χ_c2 agrees with the world average within the large uncertainty. We avoid the 40% branching fraction uncertainties and cancel the systematic uncertainties in the measurements by measuring the ratio Γ_γγ(χ_c0)/Γ_γγ(χ_c2) and find 7.4 ± 2.4(stat) ± 0.5(syst) ± 0.9(br). Non-perturbative factors in the QCD calculation cancel in the ratio so as a result the ratio has sensitivity to the value of α_s. The measurements are in excellent agreement with expectations from QCD.

A search for the pseudoscalar meson η_b was performed at LEP II energies with an integrated luminosity of 700 pb^-1. The search, done for the decay modes into 4 and 6 charged particles yielded 0 and 1 candidates, respectively. Upper limits on Γ_γγ(η)×BR for both modes of 57 eV and 128 eV were obtained with corresponding limits of 17% and 38% on branching ratios BR(η_b → 4 charged particles) and BR(η_b → 6 charged particles) at a confidence level of 95%. The candidate has a mass of 9.30 ± 0.04 GeV.

We discuss the measured partial width of the pseudoscalar charmonium state, η_c, into two photons. Predictions from potential models are examined and compared with experimental values. Including radiative corrections, it is found that present measurements are compatible both with a QCD type potential and with a static Coulomb potential, with α_s evaluated at two loops. Results are also compared with those from J/ψ data through the NRQCD model.

The CLEO Collaboration searched for the radiative decay ϒ(1S) → γη′ in 1.45 × 10⁶ ϒ(1S) decays. We found no candidate events and set an upper limit of 1.6 × 10^-5 at 90%CL. This limit, which is significantly smaller than the previous upper limit, is compared to other radiative ϒ and J/ψ decays and to theoretical predictions.

The CLEO collaboration and CESR accelerator physicists are proposing to convert CESR to operate in the charm threshold region. The achievable CESR luminosities and the excellent capabilities of the CLEO III detector would open a new frontier of exploration of weak and strong interaction physics. The principal goals of this program include: searches for glue-rich exotic states, searches for new physics, and challenging lattice QCD calculations with precise experimental data.

The production of excited charm mesons has been observed with the ZEUS detector at HERA. Neutral orbitally excited P-wave charm mesons have been reconstructed in the D^*±π^∓ final state and the charm-strange meson was found in the final state. The fraction of D^*± mesons originating from the decays of these mesons has been calculated and converted to the rate of c quarks hadronising as the excited charm mesons. A search for radially excited charm mesons in the D^*±π⁺π^- final state has also been performed. The results are compared with those obtained in e⁺e^- annihilations.

A very remarkable number of new results in the study of resonances and exclusive channels has been presented at this conference giving fundamental information in the understanding of strong interactions at low energies. The first results from the new high luminosity colliders are impressive and a lot of activity in this field is foreseen for the future. The most relevant issues are summarized and discussed in this paper.

Photon colliders (γγ, γe) are based on backward Compton scattering of laser light off the high energy electrons in linear colliders. Recently the Technical Design Report of the linear collider TESLA has been published. In this paper physics program, possible parameters and some technical aspects are discussed.

I discuss problems considered recently and some to be studied in the future: Phases of ππ scattering, resonances in e⁺e^- → e⁺e^- π⁺π^-, , , possible CP violation with t–quarks in . Possible discovery of the odderon in diffraction type process γp → π⁺π^-X. Breaking of quark–hadron duality. Weighted structure functions in DIS. Study of possible strong interaction in the Higgs sector via eγ → eW⁺W^-.

We analyze the structure of the Coulomb and unitarity corrections to the single e⁺e^- pair production as well as the cross section σ_n for the multiple pair production in collision of ultra-relativistic nuclei. In the external field approximation we consider the probability of one pair production at fixed impact parameter ρ between colliding nuclei. We obtain the analytical result for this probability at ρ ≫ 1/m_e. The energy dependence of this probability as well as that of σ_n differ essentially from widely cited but incorrect results. We present the Coulomb and unitary corrections to the total cross section of one pair production at the RHIC and LHC colliders.

We study possibilities to measure the triple anomalous W-boson couplings to photon in the eγ → Wν process via its lepton decay channel with the simplest signature. We found that in the study of the quadruple momentum λ one can limit himself small region in phase space. The way to find this region is proposed. The obtained estimates for λ at TESLA project are roughly twice better then contemporary ones for e⁺e^- mode. For anomalous magnetic momentum the discussed mode gives no improvements as compared e⁺e^- mode.

Several aspects of tagging two-photon interactions at the LHC, as separation from pomeron-pomeron events or control of the tagging efficiency, are shortly discussed.

We present an exact computation of the amplitudes of the process γγ → 4 leptons corresponding to diagrams with the exchange of a space-like photon. We have developed a Monte-Carlo generator and we show some relevant distributions for this process. A rate of four detected muons at a γγ collider is also evaluated.

We consider interacting electromagnetic and electron-positron fields as a non-metrized space-time 4-manifold. The Dirac and Maxwell equations is found to be a relationships expressing topological and metric proprieties of this manifold. A new equation for the weak interaction is proposed that explains geometrical mechanism of CP-violation.

The measurement of direct photon production will give important input to QCD studies at LHC, especially in providing constraints on parton densities in the proton in an as yet uncovered kinematical region. In addition, measurements of the strong coupling constant should be possible.

This article summarizes the experimental and theoretical results presented and discussed at the International Conference on the Structure and Interactions of the Photon, PHOTON 2001, in Ascona.

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