First observation of γγ → ωω

The reaction γγ → 2π⁺ 2π⁻ 2π⁰ has been studied using the ARGUS detector at the e⁺e⁻ storage ring DORIS II at DESY. Production of ω mesons is observed and, in particular, the reaction γγ → ωω is seen for the first time. The cross section for γγ → ωω has an enhancement at ∼ 1.9 GeV/c² of about 10 nb. The cross sections for γγ → 2π⁺ 2π⁻ 2π⁰ and γγ → ωπ⁺ π⁻ π⁰ are also given.     

Lightest scalar resonances and the dynamics of the γγ → ππ reactions

The high-statistics Belle data on the γγ → π⁺π^? and γγ → π⁰π⁰ reactions have been jointly analyzed. The main dynamical mechanisms of these reactions for energies below 1.5 GeV have been revealed. It has been shown that the direct coupling constants of the σ(600) and f ₀(980) resonances with a γγ pair are small and that the σ(600) → γγ and f ₀(980) → γγ decays are four-quark transitions due primarily to π⁺π^? and K ⁺ K ^?-loop mechanisms, respectively. The role of the chiral shielding of the σ(600) resonance is emphasized. The widths of the f ₀(980) → γγ and σ(600) → γγ decays averaged over the resonance mass distributions, as well as the width of the f ₂(1270) → γγ decay, are estimated as \(\langle \Gamma _{f_0 \to \gamma \gamma } \rangle _{\pi \pi } \approx 0.19 keV, \langle \Gamma _{\sigma \to \gamma \gamma } \rangle _{\pi \pi } \approx 0.45 keV\), and \(\Gamma _{f_2 \to \gamma \gamma } (m_{f_2 }^2 ) \approx 3.8 keV\).     

Charged Top-Pion Production at e^＋e^- and γγ Colliders

We study the charged top-pion in the topcolor assisted technicolor model（TC2）, and calculate the production of charged top-pion at e^＋ e^- and γγ colliders. At an e^＋ e^- collider, charged top-pion can be produced via the processes e^＋e^- → t^bπt^-（^tbπt^＋）. At a γγ collider, it can be produced via the processes γγ →t^bπt^-（t^-bπt^＋）. The cross section can reach to a few of fb depending on the mass of the charged top-pion.     

Meson pair production in γγ scattering and implications for scalar mesons

We describe a simple analytic unitary model which provides with a parametrization of low energy (from threshold up to 1.4GeV) two photon production of ππ andK \(\bar K\) pairs in term of a few parameters related to the γγ width of the scalar and tensor resonances. It can be useful for analysing data because of the large continuum found in γγπ⁺π^? production besides thef ₀ (1,270) peak in recent experiments. We show that the present data already provides a strong evidence for the existence of a broad low mass scalar state, the old σ or ε (600) with a γγ width of 5 to 9KeV, and that it will be difficult to get more information on the controversialS ^* parameters from ππ pair production only. In thef ₀ range we explain the downwards shift of the π⁺π^? peak but we expect the π⁰π⁰ peak around 1.27–1.29 GeV which could raise the question either of systematic errors in mass calibration of crystal Ball experiment or of the precise value of thef ₀ mass.     

Distinguishing ‘Higgs’ spin hypotheses using γγ and WW ∗ decays

A multi-step setup for heavy-flavor studies in high-energy nucleus-nucleus (AA) collisions—addressing within a comprehensive framework the initial $Q\overline{Q}$ production, the propagation in the hot medium until decoupling and the final hadronization and decays—is presented. The initial hard production of $Q\overline{Q}$ pairs is simulated using the POWHEG pQCD event generator, interfaced with the PYTHIA parton shower. Outcomes of the calculations are compared to experimental data in pp collisions and are used as a validated benchmark for the study of medium effects. In the AA case, the propagation of the heavy quarks in the medium is described in a framework provided by the relativistic Langevin equation. For the latter, different choices of transport coefficients are explored (either provided by a perturbative calculation or extracted from lattice-QCD simulations) and the corresponding numerical results are compared to experimental data from RHIC and the LHC. In particular, outcomes for the nuclear modification factor R _AA and for the elliptic flow v ₂ of D/B mesons, heavy-flavor electrons and non-prompt J/ψ’s are displayed.     

Production of Top-Pions Associated with W Boson at Next Generation γγ Colliders

In the context of topcolor-assisted technicolor (TC2) model, we study the charged and neutral top-pions production process γγ→W⁺ Π_t^-Π_t⁰. We find that the production cross section is larger than that of the process γγ→W⁺H^-H in the minimal supersymmetric standard model. With reasonable values of the parameters in the TC2 model, the cross section can reach the level of a few fb. Furthermore, the flavor-changing (FC) decay mode Π_t⁰ → t\bar{c} is the best channel to detect theneutral top-pion due to the clean SM background. With a large number of events and the clean background, the neutral top-pion should be observable at future linear colliders operating in γγ mode at the TeV energy scale.     

Production of Top-Pions Associated with W Boson at Next Generation γγ Colliders

In the context of topcolor-assisted technicolor （TC2） model, we study the charged and neutral top-pions production process γγ →W＋ ∏t-∏t^0. We find that the production cross section is larger than that of the process γγ→ W＋ H-H in the minimal supersymmetric standard model. With reasonable values of the parameters in the TC2 model, the cross section can reach the level of a few fb. Furthermore, the flavor-changing （FC） decay mode ∏t^0 → te^- is the best channel to detect the neutral top-pion due to the clean SM background. With a large number of events and the clean background, the neutral top-pion should be observable at future linear colliders operating in γγ mode at the TeV energy scale.     

Study of hard scattering processes in multihadron production from γγ collisions at LEP

The production of multihadronic states in collisions at LEP has been studied with the DELPHI detector. The analyzed data correspond to an integrated luminosity of about 32pb^–1, collected in the LEP runs of 1990–1992. Minimum bias data and a sample of events with jets at highp _T have been selected under the requirement that no scattered electron or positron is observed. The two data sets have been compared to Monte Carlo predictions. The non-perturbative contribution described by the vector meson dominance Model and direct production from pointlike photons described by the quark parton model were found to be insufficient to reproduce the data. It has been necessary to include a third interaction component, which is due to perburbative hard scattering of the partonic constituents of the photon. Several parametrisations of the quark and gluon densities of the photon have been tested. The interplay with the cut in jet transverse momentum, which is necessary for the separation of the perturbative and non-perturbative regions, is discussed. The data favour parametrisations with rather soft partonic content of the photon.     

Measurement of η' → π+π−γ in γγ collisions

The production of the pseudoscalar meson η'(958) is observed in the reaction e⁺e−→ e⁺e⁻η′→e⁺e⁻π⁺π⁻γ with the ARGUS detector at DESY. We measure the product Γ_γγ(η')Br(η′ →ϱγ) to be 1.13±0.04±0.13 keV. Using the known branching ratio s, we calculate Γ_γγ(η′) to be 3.76±0.13±0.47 keV and Γ_η' to be 203±32 keV.     

Pair production of neutral Higgs bosons from the left-right twin Higgs model via γγ collisions

The left-right twin Higgs (LRTH) model predicts the existence of the neutral Higgs bosons (h, Φ⁰), which can be produced in pairs (Φ⁰Φ⁰, hh, Φ⁰h) via γγ collisions at the next generation e⁺e^- International Linear Collider (ILC). Our numerical results show that the production cross section of the neutral Higgs boson pair Φ⁰Φ⁰ can reach 8.8 fb. The subprocess γγ→Φ⁰Φ⁰ might be used to test the LRTH model in future ILC experiments.     

Pair production of neutral Higgs bosons from the left-right twin Higgs model via γγ collisions

The left-right twin Higgs (LRTH) model predicts the existence of the neutral Higgs bosons (h, φ<'0>), which can be produced in pairs (φ<'0>φ<'0>, hh, φ<'0>h) via γγ collisions at the next generation e<'+>e<'-> International Linear Collider (ILC). Our numerical results show that the production cross section of the neutral Higgs boson pair φ<'0>φ<'0> can reach 8.8 fb. The subprocess γγ→φ<'0>φ<'0> might be used to test the LRTH model in future ILC experiments.     

Inclusive one- and two-jet cross sections in γγ reactions at e+e− colliders

We have calculated inclusive one- and two jet production in photon-photon collisions superimposing direct, single-resolved and double-resolved cross sections for center of mass energies of the LEP1, LEP2 and NLC range. The direct and single-resolved cross sections are calculated up to next-to-leading order. The double-resolved two-jet cross section is calculated only in LO with a k factor estimated from the NLO one jet cross section. Various differential cross sections as functions of transverse momenta and rapidities of the jets are evaluated.     

Observation of exclusive γγ production in pp¯ collisions at √s=1.96 TeV

We have observed exclusive γγ production in proton-antiproton collisions at √s=1.96 TeV, using data from 1.11±0.07 fb(-1) integrated luminosity taken by the Run II Collider Detector at Fermilab. We selected events with two electromagnetic showers, each with transverse energy E(T)>2.5 GeV and pseudorapidity |η|<1.0, with no other particles detected in -7.4<η<+7.4. The two showers have similar E(T) and azimuthal angle separation Δφ~π; 34 events have two charged particle tracks, consistent with the QED process ppˉ→p+e(+)e(-)+pˉ by two-photon exchange, while 43 events have no charged tracks. The number of these events that are exclusive π(0)π(0) is consistent with zero and is <15 at 95% C.L. The cross section for ppˉ→p+γγ+pˉ with |η(γ)|<1.0 and E(T)(γ)>2.5 GeV is 2.48(-0.35)(+0.40)(stat)(-0.51)(+0.40)(syst) pb.     

Inclusive Jet Production in γp and γγ Processes: Direct and Resolved Photon Cross Sections in Next-To-Leading Order QCD

The production of jets in low Q² ep scattering (photoproduction) and in low Q² ey⁺ey⁻ scattering (γγ scattering) allows for testing perturbative QCD and for measuring the proton and photon structure functions. This requires exact theoretical predictions for one- and two-jet cross sections. We describe the theoretical formalism, giving sufficient details, for calculating the direct and resolved processes in γp and γγ reactions in next-to-leading order QCD. We present the complete analytical results for the Born terms, the virtual, and the real corrections. To separate singular and regular regions of phase space we use the phase space slicing method with an invariant mass cut-off. In this way, all soft and collinear singularities are either canceled or absorbed into the structure functions. Using a flexible Monte Carlo program, we evaluate the cross sections numerically and perform various tests and comparisons with other calculations. We consider the scale dependence of our results and compare them to data from the experiments H1 and ZEUS at HERA and OPAL at LEP.     

Resonance structure in the γγ invariant mass spectrum in pC, dC, and dCu interactions

Along with ?? ⁰ and ?? mesons, a resonance structure in the invariant mass spectrum of two photons atM _??? = 360 ± 7 ± 9 MeV is observed in dC interactions at momentum 2.75 GeV/c per nucleon. This resonance structure is not observed in pC collisions at the beam momentum 5.5 GeV/c. The result obtained in the reaction dC is confirmed by the second experiment carried out on the deuteron beam at momentum 3.83 GeV/c per nucleon with a copper target. Some other checks of the observed effect are presented.     

Associated production of a neutral top-Higgs with a heavy-quark pair in the γγ collisions at ILC

We have studied the associated production processes of a neutral top-Higgs in the topcolor assisted technicolor model with a pair of heavy quarks in γγ collisions at the International Linear Collider (ILC). We find that the cross section for ttht in γγ collisions is at the level of a few fb with the c.m. energy √s=1000 GeV, which is consistent with the results of the cross section of ttH in the standard model and the cross section of tth in the minimal supersymmetric standard model. It should be clear that hundreds of to thousands of ht per year can be produced at the ILC. This process of γγ→ttht is really interesting in testing the standard model and searching the signs of technicolor.     

Associated production of a neutral top-Higgs with a heavy-quark pair in the γγ collisions at ILC

We have studied the associated production processes of a neutral top-Higgs in the topcolor assisted technicolor model with a pair of heavy quarks in γγ collisions at the International Linear Collider （ILC）. We find that the cross section for ttht in γγ collisions is at the level of a few fb with the c.m. energy √s=1000 GeV, which is consistent with the results of the cross section of ttH in the standard model and the cross section of tth in the minimal supersymmetric standard model. It should be clear that hundreds of to thousands of ht per year can be produced at the ILC. This process of γγ→ttht is really interesting in testing the standard model and searching the signs of technicolor.     

QCD corrections to the reaction PP → γγ + X

It is shown that QCD corrections taking into account both collinear and infrared gluons attain a magnitude of the order of 100% in the reaction PP r + X. The value of the K factor is calculated: K1.5–1.6 withk _T 10 (GeV/sec)². Perturbation theory is applicable, since the large constants are exponential and are practically completely taken into account by the K factor.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 82–87, July, 1985.The authors thank A. V. Efremov, A. V. Kadyushkin, and D. V. Shirkov for discussions of the results obtained.