Pair Production In Two-photon Collisions Research Articles

High-statistics study of KFormula pair production in two-photon collisions

We report a high-statistics measurement of the differential cross section of the process gamma gamma --> K^0_S K^0_S in the range 1.05 GeV <= W <= 4.00 GeV, where W is the center-of-mass energy of the colliding photons, using 972 fb^{-1} of data collected with the Belle detector at the KEKB asymmetric-energy e^+ e^- collider operated at and near the Upsilon-resonance region. The differential cross section is fitted by parameterized S-, D_0-, D_2-, G_0- and G_2-wave amplitudes. In the D_2 wave, the f_2(1270), a_2(1320) and f_2'(1525) are dominant and a resonance, the f_2(2200), is also present. The f_0(1710) and possibly the f_0(2500) are seen in the S wave. The mass, total width and product of the two-photon partial decay width and decay branching fraction to the K bar{K} state Gamma_{gamma gamma}B(K bar{K}) are extracted for the f_2'(1525), f_0(1710), f_2(2200) and f_0(2500). The destructive interference between the f_2(1270) and a_2(1320) is confirmed by measuring their relative phase. The parameters of the charmonium states chi_{c0} and chi_{c2} are updated. Possible contributions from the chi_{c0}(2P) and chi_{c2}(2P) states are discussed. A new upper limit for the branching fraction of the P- and CP-violating decay channel eta_c --> K^0_S K^0_S is reported. The detailed behavior of the cross section is updated and compared with QCD-based calculations.

High-statistics study of neutral-pion pair production in two-photon collisions

The differential cross sections for the process gamma gamma -> pi(0)pi(0) have been measured in the kinematic range 0.6 GeV < W < 4.1 GeV, vertical bar cos theta* vertical bar < 0.8 in energy and pion scattering angle, respectively, in the gamma gamma center-of-mass system. The results are based on a 223 fb(-1) data sample collected with the Belle detector at the KEKB e(+)e(-) collider. The differential cross sections are fitted in the energy region 1.7 GeV < W < 2.5 GeV to confirm the two-photon production of two pions in the G wave. In the higher-energy region, we observe production of the chi(c0) charmonium state and obtain the product of its two-photon decay width and branching fraction to pi(0)pi(0). We also compare the observed angular dependence and ratios of cross sections for neutral-pion and charged-pion pair production to QCD models. The energy and angular dependence above 3.1 GeV are compatible with those measured in the pi(+)pi(-) channel, and in addition we find that the cross section ratio, sigma(pi(0)pi(0))/sigma(pi(+)pi(-)), is 0.32 +/- 0.03 +/- 0.05 on average in the 3.1-4.1 GeV region.

High-statistics measurement of neutral-pion pair production in two-photon collisions

We present a high-statistics measurement of differential cross sections and the total cross section for the process γγ→π0π0 in the kinematic range 0.6 GeV≤W≤4.0 GeV and |cosθ*|≤0.8, where W and θ* are the energy and pion scattering angle, respectively, in the γγ center-of-mass system. Differential cross sections are fitted to obtain information on S, D0, D2, G0, and G2 waves. The G waves are important above W≃1.6 GeV. The general behavior of partial waves is studied by fitting differential cross sections in a simple parametrization where amplitudes contain resonant contributions and smooth background. The D2 wave is dominated by the f2(1270) meson whose parameters are consistent with the current world averages. The D0 wave contains a f2(1270) component, whose fraction is fitted. For the S wave, the f0(980) parameters are found to be consistent with the values determined from our recent π+π− data. In addition to the f0(980), the S wave prefers to have another resonance-like contribution whose parameters are obtained.5 MoreReceived 22 May 2008DOI:https://doi.org/10.1103/PhysRevD.78.052004©2008 American Physical Society

Vector meson pair production in two-photon collisions near threshold

The cross sections for several $\gamma \gamma \to VV$ processes exhibit strong enhancements near threshold, where $V$ denotes a vector meson. The pattern of these enhancements is not well understood; for example, $\gamma \gamma \to \rho^0 \rho^0$ shows an appreciable peak, while $\gamma \gamma \to \rho^+ \rho^-$ does not. Some possible mechanisms for this behavior are discussed. Tests are proposed involving production of systems containing heavier quarks, e.g., through the reaction $\gamma \gamma \to J/\psi \rho^0$. The importance of modeling $\ell^+ \ell^- \pi^+ \pi^-$ angular distributions in decays of threshold $J/\psi \rho^0$ enhancements is illustrated by comparing the expected distributions for S-wave decays of scalar particles and P-wave decays of pseudoscalar particles.

Baryon-antibaryon pair production in two-photon collisions at LEP

Abstract Baryon pair production in two-photon collisions is studied with the L3 detector at LEP. The following reactions are identified: γγ → p p , γ → Λ Λ , γ → Σ 0 Σ 0 and γγ → Λ GS 0 + Σ 0 Λ . The analyses are based on data collected at e+e− center-of-mass energies from 91 GeV to 208 GeV, corresponding to an integrated luminosity of 844 pb−1. Their cross section is measured as a function of the the two-photon center-of-mass energy and the production polar angle. The results are compared to the predictions of the quark-diquark and handbag models.

Proton–antiproton pair production in two-photon collisions at LEP

The reaction e+e−→e+e−pp̄ is studied with the L3 detector at LEP. The analysis is based on data collected at e+e− center-of-mass energies from 183 GeV to 209 GeV, corresponding to an integrated luminosity of 667 pb−1. The γγ→pp̄ differential cross section is measured in the range of the two-photon center-of-mass energy from 2.1 to 4.5 GeV. The results are compared to the predictions of the three-quark and quark–diquark models.

Hard exclusive baryon-antibaryon production in two-photon collisions

We study baryon pair production in two-photon collisions, gamma gamma -> B Bbar, within perturbative quantum chromodynamics, treating baryons as quark-diquark systems. We extend previous work within the same approach by treating constituent-mass effects systematically by means of an expansion in the small parameter (mass/photon energy). Our approach enables us to give a consistent description of the cross sections for all octet baryon channels. Adopting the model parameters from foregoing work, we are able to reproduce the most recent large-momentum-transfer data from LEP for the p pbar, Lambda Lambdabar, and Sigma^0 Sigma^0bar channels in a quite satisfactory way. We also briefly address the crossed process for the proton channel, gamma p -> gamma p.

Λ and Σ0 pair production in two-photon collisions at LEP

Strange baryon pair production in two-photon collisions is studied with the L3 detector at LEP. The analysis is based on data collected at e +e − centre-of-mass energies from 91 GeV to 208 GeV, corresponding to an integrated luminosity of 844 pb −1. The processes γγ→Λ Λ ̄ and γγ→Σ 0 Σ 0 are identified. Their cross sections as a function of the γγ centre-of-mass energy are measured and results are compared to predictions of the quark–diquark model.

Perturbative limit of the two-pion light-cone distribution amplitude

We consider pion pair production in two-photon collisions ${\ensuremath{\gamma}}^{*}\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\gamma}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ at large photon virtuality ${Q}^{2}$ and center-of-mass energy $\sqrt{s}$ in the hard-scattering picture. In the limit where s is large but $s\ensuremath{\ll}{Q}^{2}$ we derive an expression for the two-pion light-cone distribution in terms of the conventional pion distribution amplitudes. Our result reproduces the well-known scaling behavior, helicity selection rule and symmetry relations in this regime.

Exclusive heavy-meson pair production at large recoil.

The Brodsky-Lepage (BL) formalism in perturbative QCD is applied to estimate the validity of the heavy-quark effective theory and the high energy scaling laws to the exclusive heavy-meson pair production in two-photon collisions as well as [ital e][sup +][ital e][sup [minus]] annihilation. In addition to the ordinary heavy mesons such as [ital B] and [ital D], the heavy mesons of which both quark masses are larger than the typical strong interaction scale [Lambda][sub QCD] are considered. Explicit forms of transition amplitudes are presented for the heavy mesons with two heavy quarks in the peaking approximation and the modifications for the ordinary heavy-meson production are described. The result in the heavy-quark limit is consistent with the heavy-quark effective theory only in a certain range of center of mass energy. In the high energy limit, the transition amplitudes turn out to comply with the high energy scaling laws. The common features and different points of the [ital e][sup +][ital e][sup [minus]] and [gamma][gamma] cases are described in detail. The limitation of the BL formalism and the heavy-quark effective theory is discussed.

Perturbative QCD analysis of pion and kaon form factors and pair production in photon-photon collisions using a frozen coupling constant.

Within the framework of leading-order perturbative QCD and using a frozen coupling constant, we calculate the pion and kaon form factors and the cross section of pion and kaon pair production in two-photon collisions. We use the same frozen coupling constant as taken in the nucleon Dirac-form-factor analysis and find that the results for the {pi} and {ital K} form factors, the reactions {gamma}{gamma}{r arrow}{pi}{sup +}{pi}{sup {minus}},{ital K}{sup +}{ital K}{sup {minus}}, and the proton Dirac form factor are in fair agreement with the available experimental data. The cutoff value of the frozen coupling constant used in our analysis is consistent with the theoretical estimation presented by Cornwall.

W+W−pair production in two-photon collisions and the magnetic moment of theW±bosons

We consider the process $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{W}^{+}{W}^{\ensuremath{-}}$ for arbitrary values of the ${W}^{\ifmmode\pm\else\textpm\fi{}}$ boson magnetic moment ${\ensuremath{\mu}}_{W}=(\frac{e}{2{M}_{W}})(1+\ensuremath{\kappa})$. The angular distribution, at sufficiently high energy, is extremely sensitive to the magnetic-moment parameter $\ensuremath{\kappa}$, with the standard value $\ensuremath{\kappa}=1$ clearly distinguished from other values. This process is then inserted into ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{+}{e}^{\ensuremath{-}}{W}^{+}{W}^{\ensuremath{-}}$, using the equivalent-photon approximation, and we evaluate the angular distribution measured in the ${W}^{+}{W}^{\ensuremath{-}}$ center-of-mass frame. The sensitivity to $\ensuremath{\kappa}$ can be retained only if one puts a lower cutoff on the ${W}^{\ifmmode\pm\else\textpm\fi{}}$ boson energies.