Study of the reaction π − p → η’π 0 n at the VES spectrometer. Yu. Guz (VES collaboration*)

1. Study of the reaction π−p →η’π0n at the VES spectrometer. Yu. Guz (VES collaboration*) * VES collaboration: D. Amelin, Yu. Gavrilov, Yu. Guz, V. Dorofeev, R. Dzhelyadin, A.Ivashin, I.Kachaev, V.Kabachenko, A.Karyukhin, Yu.Khokhlov, A.Konoplyannikov, V.Konstantinov, V. Kostyuhin, V. Matveev, V. Nikolaenko, A. Ostankov, B.Polyakov, D.Ryabchikov, A.Solodkov, O.Solovianov, E.Starchenko, A.Zaitsev, A.Zenin.

2. The final states ηπandη’πare mainly interesting for the P-waves, which have exotic quantum numbers JPC=1−+ . They can be produced in π− beam in diffraction-like (η π−, η’π−) and charge exchange (η π0, η’π0) reactions. Three of them were studied before: • π– A →ηπ–A: (VES, E852) a broad bump in the P+ wave (positive exchange naturality); allows resonance interpretation: π1–(1400). • π– A →ηπ0A: (GAMS/NA12, E852) a broad bump of significant intensity in the P+ wave; resonance interpretation is problematic. • π– A →η’π–A: (VES, E852) the P+ wave is dominant, contains a broad bump in the P+ wave; resonance interpretation is possible: π1–(1600).

3. The P-wave states in η8πandη0πare essentially different: in η0πit is SU(3)f-octet, while in η8πit can belong only to higher SU(3) representation, Therefore the π1(1600) is SU(3)f –octet state (can be a hybrid), and π1(1400) is not. (S.U.Chung, E.Klempt, J.G.Körner, Eur.Phys.J 2002). • in the reaction π–p →η’π0 nthe π10(1600) production in charge exchange process can be studied. • The events of the reaction π– p→η’π0 n were selected in the decay mode η’ →π+π–η,η →γγ,π0 →γγ. • The reaction π– p →ηπ0 nwas also studied, for normalizations and comparisons, in the decay mode η →π+π–π0,π0→γγ.

4. The VES setup Works on secondary π− beam (25-45 GeV) of the U-70 accelerator (IHEP Protvino) Measures charged particles and photons in final state: π–(K–)A → nπ(K)+m γA’ VES

5. η’π0: event selection • Selection criteria: • 2 tracks, for π+ and π–, and 4 photons; • the tracks are not electrons: |p|/Ecal<0.8 • 0.105<M(γ1γ2)<0.165 MeV/c2 (π0) ; • 0.478< M(γ3γ4)<0.618 MeV/c2 (η) ; • the 4γ should not satisfy the π0π0 hypothesis; • 0.940<M(π+π–η)<0.976 MeV/c2 (η’) ; • the side bands around η’ were used for the background evaluation and subtraction • 1955 “signal” and 809 “background” events were selected for the analysis.

6. η’π0: distributions • The a2(1320) peak is seen in the η’π0mass spectrum • The t-distribution peaks at 0

7. ηπ0: event selection • Selection criteria: • 2 tracks, for π+ and π–, and 4 photons; • the tracks are not electrons: |p|/Ecal<0.8 • 0.105<M(γ1γ2), M(γ3γ4)<0.165 MeV/c2 (π0) ; • 0.528<M(π+π–π0)<0.568 MeV/c2 (η) ; • the side bands around η were used for the background evaluation and subtraction • 18547 “signal” and 8436 “background” events were selected for the analysis.

8. ηπ0: distributions • a0(980) and a2(1320) are seen in the mass spectrum • asymmetry in the cosθGJ distribution • The φTY distribution shows significant fraction of positive exchange naturality waves • Agrees with previous studies of the ηπ0system *. * • D.Alde et al (GAMS collaboration), Phys. At. Nucl. Vol. 62 No. 3 (1999), p 421 ; • A.R.Dzierba et al, Phys. Rev. D67, 094015 (2003)

9. a2(1320)inηπ0and η’π0 The mass distributions are acceptance corrected – agrees with previous measurements (VES,E852)

10. ηπ0: PWA The PWA of the ηπ0 system, with S0, P0, P-, D0, D- waves in the unnatural parity exchange sector and P+ and D+ in the natural parity exchange sector, was performed. In agreement with results of similar analysis of GAMS and E852: a0(980) in the S0 wave a2(1320) in the D+ wave broad bump in P+ Arg(D+/P+): only phase variation corresponding to the a2(1320) production

11. Comparison of the mass spectra of η’π0 and η’π− π1(1600) a2(1320) η’π–: dominated by the π1(1600) production, a2(1320) is less η’π0:π1(1600) is absent, only a2(1320) is visible

12. η’π–: PWA π1(1600) The exotic wave is dominant in the natural parity exchange sector (P+ wave). It forms a bump at ~1.6 GeV/c2, π1(1600). It is most intensive in the mass range above a2(1320), at1.45<M<1.9 GeV/c2. G.M.Beladidze et al (VES collaboration), Phys.Lett.B313(1993) 276

13. ηπ0at 1.45<M<1.9 GeV/c2 • These spectra show that significant P+ is present and positive exchange naturality waves dominate: • significant asymmetry in cosθGJ • φTY distribution is almost sin2 • JPC=1-+ is produced mainly in the P+ wave (ρ-exchange)

14. η’π0at 1.45<M<1.9 GeV/c2 • The φTY spectrum is almost flat; • some asymmetry in cosθGJ • JPC=1-+ is not produced in the P+ wave (π1(1600) not coupled to ρπ)

15. PWA in “large bin” (1.45…1.9 GeV): η’π0 The PWA of both ηπ0 and η’π0was performed for events in the whole interval of 1.45…1.9 GeV : no rapid phase variations expected. The waves S0, P0, P-, D0, D-, P+, D+were included into the analysis. • Here η’π0is shown. • The S0, P0 and D0 waves are significant • P+ and D+ are small

16. PWA in “large bin” (1.45…1.9 GeV): ηπ0 In ηπ0the P+ and D+ aredominant.

17. 1-+ in ηπ0and η’π0 The ratio of matrix elements squared for the P+ production in ηπ0 and η’π0at 1.45...1.9 GeV: P+ wave in η(η’)π0 systems is mainly η8π, i.e. not octet state in ηπ–/ η’π–it is opposite: the P+ wave is mainly octet (G.M.Beladidze et al, 1993)

18. 1-+ production in ρ-exchange The a2(1320) gives main contribution to the D+ wave in ηπ0; let us suppose that the P+ wave in η’π0 contains only π1(1600). Both are produced via ρ-exchange. Comparing their production intensities, we can conclude that the coupling of π1(1600) to ρπ is weak. In the assumption of the same dependence of the couplings of a2 and π1 to ρπ on the ρ virtuality: which gives

19. π1(1600): known decay modes π1(1600)was seen in η’π, f1(1285)π and b1(1235)π, with relative intensities of approx. 1 : 1.1±0.3 : 1.0±0.3. It was not seen in f2π and K*K. The η’π decay mode is big: certainly BR(π1(1600) → η’π) > 0.1 This gives BR(π1(1600) → ρπ) < 0.03 The suppression of the decay into ρπ is predicted in many models for 1−+ hybrids  does not contradict to the hypothesis that π1(1600) is a hybrid.

20. Direct search of π1(1600) → ρπ - the results are ambiguous: π1(1600) peak was observed in ρπ in the PWA of the reaction π– A →π+π–π–A, in the wave JPMη = 1−1+ Its intensity depends on the PWA model: it is high when the waves are assumed coherent, and is much lower when the coherency is not assumed. (mainly leakage from π2(1670) ?) Even in the latter case, the fit gives for π1(1600)ρπ : η’π ≈ 1.5±0.5 : 1 (A.Zaitsev, contribution to HADRON 97) π+π−π− total intensity 1−1+ ρπ, tight model Possibilities for the bump in 1−1+ρπ “loose model” : still mainly leakage a physical object different fromπ1(1600) of η’π– or π1(1600) branchings are not constant (this is not a true resonance) 1−1+ ρπ, loose model

21. Conclusion • The reactions π−p → η’π0 n and π−p → ηπ0 n were studied at the VES experiment. • The ratio of branchings for the a20 meson was determined: Ra2=0.047±0.018, in agreement with measurements in the η’π– and ηπ–final states. • The P waves were studied in η’π0 and ηπ0systemsin the region of π1(1600) meson, 1.45…1.9 GeV. It was found that in this mass interval in the η’π0system the P+ wave production is small, and, in particular, much smaller than in the ηπ0system. This means that π1(1600) is not produced in ρ-exchange. The model-dependent constraint is obtained: BR(π1(1600) → ρπ) < 0.03.