Working Group on Goldstone Boson Product ion and Decay

312 Johan Bijnens et al. 1 I n t r o d u c t i o n Chiral symmetry plays a prominent role in low energy hadronic physics. The workshop was basically fully devoted to theoretical and experimental progress in this area. In our working group we discussed some topics not covered by the other three working groups (Bernstein and Kaiser (1997), Meif~ner and Sevior (1997), Drechsel (1997)) and in the various plenary talks. The subject of chiral symmetry and the modern way to extract its consequences, Chiral Perturbation Theory (CHPT), was introduced by Jtirg Gasser (Gasser (1997)). In this working group we did not discuss the extension to a small quark condensate (Stern (1997) and references therein). The plan of this contribution follows closely the summary as presented by the convener J. Bijnens. In particular some of the contributions to this working group are discussed in his plenary talk (Bijnens (1997)) and will not be repeated here. On the first day we discussed the experiments done by WASA/PROMICE at Celsius in Uppsala (A. Kupsz) on meson productions and their future ~/decay program, as well as new z/decay results and future programs (B. Nefkens) and the relevant CHPT aspects including the anomaly (J. Bijnens). The subject of anomalies was also covered experimentally (M. Moinester). The present status of theory for the anomalous magnetic moment of the muon and its relevance for beyond the standard model effects were covered. The main remaining uncertainty in the prediction remains the low-energy hadronic crosssection (E. Pallante). The subject of chiral sum rules in the V V and A A sector has recently had a boost by the two-loop calculations in CHPT (J. Kambor). Further progress at this order in CHPT was evident in the entire workshop. The topics covered here were 7r --~ eu 7 (P. Talavera) and various subtleties in the general p6 calculations as well as first results on the infinity structure at p6 (G. Colangelo). Beyond CHPT there is the discussion of the generalization of the LippmanSchwinger approach and the Inverse Amplitude Method applied to mesonmeson scattering (J. PelAez) and the at tempts to calculate relevant higher order terms using various models, double VMD or not was a recurring theme in both 7/and K decays and the muon anomalous magnetic moment. Experimental results in K decays and future prospects were reviewed (B. Renk) as well as the KLOE program at DA4~NE (G. Colangelo). Theoretical developments were discussed in the both the main decay modes (G. Isidori) as in various rare ones (G. D'Ambrosio, F. Gabbiani). This report is organized as follows in Sect. 2 we discuss various r /and 7r production experiments and future plans here. This also includes the anomalous process 77r --~ 7rTr. In Sect. 3 we discuss the parts mentioned above relevant for ~ and K decays. Here we also review the uncertainty on the decay 7r ° --* "y'y. Sect. 4 contains the discussion of the sum rules for g 2 and V V and A A as well as the meson-meson scattering by the IAM. The last section, Sect. 5, covers the progress in the p6 generating functional. Working Group on Goldstone Boson Production and Decay 313 The topics discussed in this working group but covered in the plenary talk by J. Bijnens (Bijnens (1997)) are: radiative 7r + decay and the light-by-light contribution to the muon anomalous magnetic moment. 2 P r o d u c t i o n I 2.1 H a d r o n i c p r o d u c t i o n o f 7r a n d r/ In Uppsala, an experimental programme for near threshold 77 and 7r meson production in proton-nucleon interactions is carried out. The experiments are done at the CELSIUS accelerator storage ring using an internal cluster gasjet target and the WASA/PROMICE detector capable of measuring forward going charged particles and decays photons from the neutral mesons (Cal~n et al. (1996)). The photon detection allows for measurements very close to threshold when the outgoing protons or deuterons escape undetected in the beam pipe. At higher energies, a kinematically complete reconstruction of the events is possible. An example is the near-threshold measurement of the reaction pp --* ppTr ° tha t has been measured at seven different excess energies, from 0.5 MeV to 14 MeV in the centre-of-mass system (Bondar et al. (1995)). The experiment gives added weight to an interpretation that the reaction near threshold is sensitive to the short range components of the nucleon-nucleon interaction. The understanding of the cross section magnitude remains a challenge for C H P T (van Kolck (1997)). Production of r/mesons in proton-nucleon collisions has been studied using both hydrogen and deuterium as targets. By exploiting the neutron Fermi momentum in the deuteron, the excitation functions for the reactions p n pn~) and p n ~ dr~ have been measured for CM excess energies from 15 MeV to 115 MeV (Fig. 1). The measured energy dependence of the cross section for the p n ~ dr~ reaction is the first firm evidence of the importance of the S n (1535) nucleon resonance in the reaction mechanism (Cal~n et al. (1997a)). The cross section for the reaction p n --~ pnr/was found to be about six times larger than that of the reaction p p ---* PPU. Recently, the measurements of the p n --~ dr~ were extended down to the threshold by using the CELSIUS bending magnets as a spectrometer to identify deuterons at 0 degree. The cross section shows an enhancement at the threshold (Cairn et al. (1997b)) which might be signal of a quasi-bound dr/ state suggested by Ueda (1991). Windowless thin internal targets offer unique possibilities to detect specta tor protons. The way for systematical studies o f p + n reactions at CELSIUS is provided by placing semiconductor detectors inside the scattering chamber. An experiment which develops this technique has started recently. The WASA/PROMICE setup will be dismounted and a new central detector with close to 47r coverage for photons and charged particles will be 314 Johan Bijnens et al.