Mesons In Photon-photon Collisions Research Articles

Quantum field theory and the new phase of the study of hadron production mechanisms caused by high experimental accuracy

Modern experiment makes it possible to solve many problems of hadron physics both rather old and new ones. In particular, during implementation of the project, mechanisms of production of the light scalar mesons in photon-photon collisions are established; i.e., the question raised thirty years ago is solved.

Inclusive production of D $^{\ast\pm}$ mesons in photon-photon collisions at $\sqrt{s}_{\rm ee}=183$ and 189 GeV and a first measurement of $F^\gamma _{2,{\rm c}}$

The inclusive production of ${{\rm D}^{\ast\pm}}$ mesons in photon-photon collisions has been measured using the OPAL detector at LEP at $\mboxe^+\mboxe^-$ centre-of-mass energies $\sqrt{s_{\rm ee}}$ of 183 and 189 GeV. The ${{\rm D}^{\ast+}}$ mesons are reconstructed in their decay to ${\rm D}^0\pi^+$ with the ${\rm D}^0$ observed in the two decay modes ${\rm K}^-\pi^+$ and ${\rm K}^-\pi^+\pi^-\pi^+$ . After background subtraction, $100.4\pm12.6 (\rm stat)$ ${\rm D}^{\ast\pm}$ mesons have been selected in events without observed scattered beam electron (“anti-tagged”) and $29.8 \pm 5.9 ({\rm stat}){{\rm D}^{\ast\pm}}$ mesons in events where one beam electron is scattered into the detector (“single-tagged”). Direct and single-resolved events are studied separately. Differential cross-sections ${\rm d}\sigma/{\rm d}p_{\rm T}^{{\rm D}^{\ast}}$ and ${\rm d} \sigma/{\rm d} \vert \eta^{{\rm D^{\ast}}}\vert$ as functions of the ${{\rm D}^{\ast\pm}}$ transverse momentum $p^{\rm D^{\ast}}_{\rm T}$ and pseudorapidity $\eta^{{\rm D}^{\ast}}$ are presented in the kinematic region $2{\rm GeV} < p^{\rm D^{\ast}}_{\rm T} < 12{\rm GeV}$ and $|\eta^{{\rm D}^{\ast}}|< 1.5$ . They are compared to next-to-leading order (NLO) perturbative QCD calculations. The total cross-section for the process $\mboxe^+\mboxe^- \to \mboxe^+\mboxe^- \mbox c\overline{\mbox c}$ where the charm quarks are produced in the collision of two quasi-real photons is measured to be $\sigma({\rm e}^+{\rm e}^- \to{\rm e}^+{\rm e}^-{\rm c}\bar{{\rm c}}) = 963\pm 110 ({\rm stat})\pm 86({\rm sys})\pm 224 ({\rm extrapolation}){\rm pb}$ . A first measurement of the charm structure function $F^\gamma _{2,{\rm c}}$ of the photon is performed in the kinematic range $0.0014 < x < 0.87$ and $5 {\rm GeV}^2 < Q^2 < 100{\rm GeV}^2$ , and the result is compared to a NLO perturbative QCD calculation.

Production of D ∗ mesons in photon-photon collisions at √ see = 183 GeV and 189 GeV using the OPAL detector at LEP

The inclusive production of D ∗± mesons in photon-photon collisions has been measured using the OPAL detector at LEP at e +e − centre-of-mass energies √ s ee of 183 and 189 GeV. The D ∗+ mesons are reconstructed in their decay to D 0 π + with the D 0 observed in the two decay modes K − π + and K − π + π − π +. After background subtraction, 121 ± 14 (stat.) D ∗± events have been selected. Jets are reconstructed using a cone jet finding algorithm to separate direct and single-resolved events. Differential cross-sections dσ/ dp T D ∗ and dσ/ dvbη D ∗ vb as functions of the D ∗± transverse momentum p T D ∗ and pseudorapidity η D ∗ are presented in the kinematic region 2 < p T D ∗ < 12 GeV and vbη D ∗ vb < 1.5 . They are compared to next-to-leading order (NLO) perturbative QCD calculations. The total cross-section for the process e +e − → e +e −c c in the anti-tagged mode is measured to be σ (e +e − → e +e −c c ) = (945 ± 239) pb with a direct contribution of σ (e +e − → e +e −c c ) dir = (392 ± 117) pb and a single-resolved contribution of σ (e +e − → e +e −c c ) res = (553 ± 184) pb.

Production of χc2 mesons in photon-photon collisions at LEP

We present an observation at LEP of the production of χ c2 mesons in the collisions of two quasi-real photons using the OPAL detector. The χ c2 mesons are reconstructed in the decay channel χ c2 →J/ψ γ→ℓ + ℓ − γ (with ℓ = e, μ) using all data taken at e +e − centre-of-mass energies of 91 and 183 GeV, corresponding to integrated luminosities of 167 and 55 pb −1 respectively. The two-photon width of the χ c2 is determined to be Γ( χ c2→ γγ)=1.76±0.47±0.37±0.15 keV, where the first error is statistical, the second is systematic and the third comes from branching ratio uncertainties.

Measurement of the D ∗± cross section in two photon collisions at LEP

The inclusive production of D ∗± mesons in photon-photon collisions has been measured by the Aleph experiment at LEP with a beam energy of 45 GeV. The D ∗+ are detected in their decay to D 0 π + with the D 0 observed in three separate decay modes: (1) K − π +, (2) K − π + π 0 and (3) K − π + π − π +, and analagously for the D ∗− modes. A total of 33 events was observed from an integrated luminosity of 73 pb −1 which corresponds to a cross section for Σ( e +e − → e +e −D ∗±X ) of 155 ± 33 ± 21 pb. This result is compatible with both the direct production γγ → c c in the Born approximation and with a more complete calculation which includes both radiative QCD corrections and contributions in which one of the photons is first resolved into its quark and gluon constituents. The shapes of distributions for events containing a D ∗+ are found to be better described by the latter.

Production mechanisms for Bc mesons in photon-photon collisions

Using photon-photon collisions as a particularly transparent study case we investigate the production mechanisms for B c mesons. In the nonrelativistic approximation and to O( a 2 a s 2) it is shown that recombination of b- and c-quarks dominates by far over b and c fragmentation. This dominance persists up to the highest accessible transverse momenta and leads to distributions in energy which differ completely from the spectra expected on the basis of the fragmentation functions. For processes in which a b b- pair is radiated from a primary c-quark, the fragmentation description is found to be inadequate. We anticipate important implications of these results for hadronic production of heavy quark resonances. Using realistic photon spectra we predict two-photon production rates for B c and B c ∗ at present and future e + e −-machines.

Semihard production of neutral pseudoscalar and tensor mesons in photon-photon collisions

We investigate the semihard production of neutral pseudoscalar and tensor mesons in high-energy γγ collisions (M = P = π 0, η, η′ or M = T = a 2, f 2, f′ 2). We deal with the exclusive γγ → MM′ or semi-exclusive γγ → MX reactions (X is the hadron jet with not too large mass). The considered transfer momenta are small in comparison with the photon energies and they are large in comparison with the confinement scale. The amplitudes of these processes are determined by the odderon exchange, i.e. three-gluon exchange in the lowest order of perturbative QCD. The cross sections are calculated in this approximation. The possibility of measurements at LEP and at future γγ colliders is discussed.

Inclusive D∗± production in photon-photon collisions

Abstract The TPC/Two-Gamma Collaboration has measured the inclusive cross section for production of charmed D∗± mesons in photon-photon collisions. The reaction utilized was e+e-→e+e-D∗±X, with D∗±→DOπ+-, DO→K-+π ±, and either zero or one outgoing e± detected. The result, σ(e + e - → e + e - D ∗± X) = 74±26±19 pb , is in agreement with the quark parton mo del prediction for e+e- → e+e-c c , combined with a Lund model for the hadronization of the charmed quarks.

Observation of charmed mesons in photon-photon collisions

Abstract The inclusive production of D∗± mesons in single tagged photon-photon collisions is investigated using the JADE detector at PETRA. D∗± mesons are reconstructed through their decay into D0+π± where the D0 decays via D0→Kππ0. The event rate and topology are compared to the expectations of c quark production in the quark-parton model: γγ→c c .

Exclusive production of heavy mesons in photon-photon collisions: The double scattering mechanism

We present two mechanisms which can compete with constituent interchange in the exclusive production at wide angle of heavy mesons in photon-photon collisions. We show that they have the same energy behaviour as constituent interchange, and that their amplitudes are free from large logarithms. We exploit this to calculate the double-scattering diagrams numerically. We find that while constituent interchange dominates at 90°, double scattering becomes dominant for angles less than about 45°.

Diffractive production of vector mesons in high-energy photon-photon collisions

We examine here the energy dependence of the production of vector mesons in photon-photon collisions based on a model applied earlier to the calculation of diffractive photoproduction cross sections with a derived form of the vector-dominance model. We observe here that production of heavy vector mesons is considerably suppressed.

Production of unusual mesons in photon-photon collisions

The cross-section for the production of unusual mesons, such as (bc̄) or (b̄c), in photon-photon collisions is obtained in a high energy approximation. For a 100 + 100 GeV e +e − collider, the cross section is estimated to be about one unit of R for (bc̄) and (b̄c).