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Exclusive tensor meson photoproduction

V. Mathieu, A. Pilloni, M. Albaladejo, Ł. Bibrzycki, A. Celentano, C. Fernández-Ramírez, and A. P. Szczepaniak (Joint Physics Analysis Center)
Phys. Rev. D 102, 014003 – Published 6 July 2020
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  • INTRODUCTION
  • VECTOR EXCHANGES
  • UNNATURAL EXCHANGES AND COMPARISON WITH…
  • POLARIZATION OBSERVABLES
  • CONCLUSIONS
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    We study tensor meson photoproduction outside of the resonance region, at beam energies of a few GeVs. We build a model based on Regge theory that includes the leading vector and axial exchanges. We consider two determinations of the unknown helicity couplings and fit to the recent a2 photoproduction data from CLAS. Both choices give a similar description of the a2 cross section but result in different predictions for the parity asymmetries and the f2 photoproduction cross section. We conclude that new measurements of f2 photoproduction in the forward region are needed to pin down the correct production mechanism. We also extend our predictions to the 8.5 GeV beam energy, where current experiments are running.

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    • Received 9 May 2020
    • Accepted 17 June 2020

    DOI:https://doi.org/10.1103/PhysRevD.102.014003

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Diffractive productionLeptonic, semileptonic & radiative decays
    Particles & Fields

    Authors & Affiliations

    V. Mathieu1,*, A. Pilloni2,3,†, M. Albaladejo4, Ł. Bibrzycki4,5,6, A. Celentano3, C. Fernández-Ramírez7, and A. P. Szczepaniak4,5,8 (Joint Physics Analysis Center)

    • 1Departamento de Física Teórica, Universidad Complutense de Madrid and IPARCOS, 28040 Madrid, Spain
    • 2European Centre for Theoretical Studies in Nuclear Physics and related Areas (ECT*) and Fondazione Bruno Kessler, Villazzano (Trento), I-38123, Italy
    • 3INFN Sezione di Genova, Genova I-16146, Italy
    • 4Theory Center, Thomas Jefferson National Accelerator Facility, Newport News, Virginia 23606, USA
    • 5Physics Department, Indiana University, Bloomington, Indiana 47405, USA
    • 6Institute of Computer Science, Pedagogical University of Cracow, 30-084 Kraków, Poland
    • 7Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México, Ciudad de México 04510, Mexico
    • 8Center for Exploration of Energy and Matter, Indiana University, Bloomington, Indiana 47403, USA

    • *vmathieu@ucm.es
    • †pillaus@jlab.org

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    Issue

    Vol. 102, Iss. 1 — 1 July 2020

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    Images

    • Figure 1
      Figure 1

      Factorization of the tensor meson T photoproduction amplitude via the Regge exchange E=V, A.

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    • Figure 2
      Figure 2

      Predictions for a2(1320) photoproduction differential cross section at Eγ=4 (blue lines) and 5 GeV (red lines). In the top panels we show the results for the minimal model, in the bottom ones the tensor meson dominance. The left plots feature the wrong-signature zero at t=−0.55  GeV2. In the right plots, we modify the ρ helicity-flip bottom coupling G2ρ→1α(t)G2ρ to fill in the zero, as explained in the text. The overall coupling is determined from the ωππ width. Data points from CLAS [6].

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    • Figure 3
      Figure 3

      Fit to a2 photoproduction at Eγ=4 and 5 GeV. The minimal model (left panel) and TMD model (right panel) are fitted to the CLAS data [6]. The solid lines show the full models, which includes both vector and axial exchanges. The strengths of vectors and axials is fitted to data. The contribution of axials is shown separately with dashed lines. The systematic uncertainties of [6] are reported in the bands on top and have not been considered in the fit.

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    • Figure 4
      Figure 4

      Differential cross sections of a2 (top) and f2 (bottom), for different beam energies. The minimal model is shown in the left panel, the TMD in the right ones. The strengths of vectors and axials are fitted to the a2 data only. The results are shown in Table 3. The error bands show the 1σ confidence interval which results from the statistical uncertainty of the fit. The a2 data are taken from CLAS [6], and the extraction of the f2 data from the partial wave analysis of π+π− by CLAS [7] is described in Appendix pp3.

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    • Figure 5
      Figure 5

      Parity asymmetry Pσ in a2 and f2 photoproduction, for different beam energies. The minimal model is shown in the left panel, the TMD in the right ones. The strengths of vectors and axials are fitted to the a2 differential cross section data only. The results are shown in Table 3. The error bands show the 1σ confidence interval which results from the statistical uncertainty of the fit.

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    • Figure 6
      Figure 6

      Fit to the CLAS D-wave data on π+π− photoproduction, as discussed in the text. Data are averaged over the four beam energy bins, Eγ=3.0–3.8  GeV. Mass and width of f2 are fitted independently in each t bin (red curve) or constrained to be the same (green curve). In dashed lines we show the separate contributions of f2 and of the linear background. The strength of the f2 looks constant in t, while the strength and shape of background change dramatically.

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    • Figure 7
      Figure 7

      Differential cross section of f2. A 40% systematic error is shown. We compare with the CLAS data points from Fig. 24 of [7] (blue lines). We remind that CLAS points were obtained by integrating the mππ bins in the [1090,1400]  MeV range, that roughly corresponds to [mf2−Γf2,mf2+23Γf2]. Some f2 signal is lost, and a substantial background is included. Moreover, the branching ratio B(f2→ππ) is not included. The red and green points correspond to the different extractions, namely if the f2 mass and width are fitted independently or not in the different t bins. These two results are consistent within error. Red and green points are slightly shifted horizontally to ease the reading.

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