Spectroscopy Results from COMPASS. Jan Friedrich. Physik-Department, TU München on behalf of the COMPASS collaboration

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1 Spectroscopy Results from COMPASS Jan Friedrich Physik-Department, TU München on behalf of the COMPASS collaboration 11th European Research Conference on "Electromagnetic Interactions with Nucleons and Nuclei" EINN15, 5. November Πάφος,Κύπρος

2 The COMPASS Experiment at the CERN SPS Experimental Setup NIMA 779 (15) 69 Fixed-target experiment Two-stage spectrometer Large acceptance over wide kinematic range Electromagnetic and hadronic calorimeters Beam and final-state particle ID (CEDARs, RICH) Jan Friedrich, TU München Spectroscopy Results from COMPASS

3 The COMPASS Experiment at the CERN SPS Experimental Setup NIMA 779 (15) 69 Fixed-target experiment Two-stage spectrometer Large acceptance over wide kinematic range Electromagnetic and hadronic calorimeters Beam and final-state particle ID (CEDARs, RICH) Hadron spectroscopy 8-9, 1 19 GeV/c secondary hadron beams h beam: 97 % π, % K, 1 % p h + beam: 75 % p, 4 % π +, 1 % K + Various targets: lh, Ni, Pb, W Jan Friedrich, TU München Spectroscopy Results from COMPASS

4 The COMPASS Experiment at the CERN SPS Experimental Setup NIMA 779 (15) 69 Fixed-target experiment Two-stage spectrometer Large acceptance over wide kinematic range Electromagnetic and hadronic calorimeters Beam and final-state particle ID (CEDARs, RICH) Hadron spectroscopy 8-9, 1 19 GeV/c secondary hadron beams h beam: 97 % π, % K, 1 % p h + beam: 75 % p, 4 % π +, 1 % K + Various targets: lh, Ni, Pb, W Jan Friedrich, TU München Spectroscopy Results from COMPASS

5 The COMPASS Experiment at the CERN SPS Experimental Setup NIMA 779 (15) 69 Spectroscopy program Explore light-meson spectrum, extending to m GeV/c Search for states beyond the constituent quark model Precision measurement of known resonances Hadron spectroscopy 8-9, 1 19 GeV/c secondary hadron beams h beam: 97 % π, % K, 1 % p h + beam: 75 % p, 4 % π +, 1 % K + Various targets: lh, Ni, Pb, W Jan Friedrich, TU München Spectroscopy Results from COMPASS

6 Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states Even partial waves similar Exotic 1 + much stronger in π η 3 PWA of diffractively produced 3π final states Observation of a new narrow axial-vector meson a 1 (14) J PC = 1 + spin-exotic partial wave 4 Conclusions and outlook 3 Jan Friedrich, TU München Spectroscopy Results from COMPASS

7 Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states Even partial waves similar Exotic 1 + much stronger in π η 3 PWA of diffractively produced 3π final states Observation of a new narrow axial-vector meson a 1 (14) J PC = 1 + spin-exotic partial wave 4 Conclusions and outlook 4 Jan Friedrich, TU München Spectroscopy Results from COMPASS

8 Meson Production in Diffractive Dissociation π beam P h 1. h n p target p recoil Beam particle gets excited into intermediate resonances X X dissociate into n-body final state Rich spectrum of intermediate states X Disentanglement of all contributing X by partial-wave analysis (PWA) 5 Jan Friedrich, TU München Spectroscopy Results from COMPASS

9 Meson Production in Diffractive Dissociation π beam P X h 1. h n p target p recoil Beam particle gets excited into intermediate resonances X X dissociate into n-body final state Rich spectrum of intermediate states X Disentanglement of all contributing X by partial-wave analysis (PWA) 5 Jan Friedrich, TU München Spectroscopy Results from COMPASS

10 Meson Production in Diffractive Dissociation π beam P X h 1. h n p target p recoil Beam particle gets excited into intermediate resonances X X dissociate into n-body final state Rich spectrum of intermediate states X Disentanglement of all contributing X by partial-wave analysis (PWA) 5 Jan Friedrich, TU München Spectroscopy Results from COMPASS

11 Partial-Wave Analysis Method π beam X P h 1. h n p target p recoil Two-stage analysis σ(τ; m X ) ǫ=±1 waves i Ti ǫ(m X) Ai ǫ(τ; m X) 1 Determination of m X dependence of spin-density matrix ǫij (m X) = Ti ǫ(m X) Tj ǫ (m X ) Independent maximum likelihood fits to τ distributions in narrow m X bins Take into account detection efficiency No assumptions about resonance content of X Extraction of resonances χ fit of resonance model to spin-density (sub)matrix 6 Jan Friedrich, TU München Spectroscopy Results from COMPASS

12 Partial-Wave Analysis Method π beam X P h 1. h n p target p recoil Two-stage analysis σ(τ; m X ) ǫ=±1 waves i Ti ǫ(m X) Ai ǫ(τ; m X) 1 Determination of m X dependence of spin-density matrix ǫij (m X) = Ti ǫ(m X) Tj ǫ (m X ) Independent maximum likelihood fits to τ distributions in narrow m X bins Take into account detection efficiency No assumptions about resonance content of X Extraction of resonances χ fit of resonance model to spin-density (sub)matrix 6 Jan Friedrich, TU München Spectroscopy Results from COMPASS

13 Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states Even partial waves similar Exotic 1 + much stronger in π η 3 PWA of diffractively produced 3π final states Observation of a new narrow axial-vector meson a 1 (14) J PC = 1 + spin-exotic partial wave 4 Conclusions and outlook 7 Jan Friedrich, TU München Spectroscopy Results from COMPASS

14 PWA of π p π η ( ) p recoil PLB 74 (15) 33 Odd-spin waves: spin-exotic quantum numbers Disputed J PC = 1 + resonance signals π 1 (14) in πη and π 1 (16) in πη Comparison of πη and πη : information about flavor structure Reconstruction from exclusive π π + π γγ final state η π + π π with π γγ η π + π η with η γγ 8 Jan Friedrich, TU München Spectroscopy Results from COMPASS

15 PWA of π p π η ( ) p recoil PLB 74 (15) 33 Odd-spin waves: spin-exotic quantum numbers Disputed J PC = 1 + resonance signals π 1 (14) in πη and π 1 (16) in πη Comparison of πη and πη : information about flavor structure Reconstruction from exclusive π π + π γγ final state η π + π π with π γγ η π + π η with η γγ π η invariant mass 8 Jan Friedrich, TU München Spectroscopy Results from COMPASS

16 PWA of π p π η ( ) p recoil PLB 74 (15) 33 Odd-spin waves: spin-exotic quantum numbers Disputed J PC = 1 + resonance signals π 1 (14) in πη and π 1 (16) in πη Comparison of πη and πη : information about flavor structure Reconstruction from exclusive π π + π γγ final state η π + π π with π γγ η π + π η with η γγ π η invariant mass π η invariant mass 8 Jan Friedrich, TU München Spectroscopy Results from COMPASS

17 Comparison of J PC = ++ Partial Waves PLB 74 (15) 33 Quark-line picture for n = (u, d) and pointlike resonances π η and π η partial-wave intensities for spin J related by Different phase space and barrier factors Branching fraction ratio b of η and η into π π + γγ [ ] J+1 N πη q J (m) πη (m) b q πη N πη (m) J (m) q = breakup momentum 9 Jan Friedrich, TU München Spectroscopy Results from COMPASS

18 Comparison of J PC = ++ Partial Waves PLB 74 (15) 33 Quark-line picture for n = (u, d) and pointlike resonances π η and π η partial-wave intensities for spin J related by Different phase space and barrier factors Branching fraction ratio b of η and η into π π + γγ [ ] J+1 N πη q J (m) πη (m) b q πη N πη (m) J (m) q = breakup momentum π η final state π η final state 9 Jan Friedrich, TU München Spectroscopy Results from COMPASS

19 Comparison of J PC = ++ Partial Waves PLB 74 (15) 33 Quark-line picture for n = (u, d) and pointlike resonances π η and π η partial-wave intensities for spin J related by Different phase space and barrier factors Branching fraction ratio b of η and η into π π + γγ [ ] J+1 N πη q J (m) πη (m) b q πη N πη (m) J (m) q = breakup momentum π η final state π η final state; π η scaled 9 Jan Friedrich, TU München Spectroscopy Results from COMPASS

20 Even-Spin Waves PLB 74 (15) 33 J PC = Phase: Similar even-spin waves 1 Jan Friedrich, TU München Spectroscopy Results from COMPASS Intermediate states couple to same final-state flavour content Similar physical content also in nonresonant high-mass region π η final state; π η scaled

21 Even-Spin Waves PLB 74 (15) 33 J PC = Phase: Resonance-model fit (Breit-Wigner) N(a πη ) N(a πη) = (5±) % First-time measurement of N(a 4 πη ) N(a 4 πη) = (3±7) % 1 Jan Friedrich, TU München Spectroscopy Results from COMPASS π η final state; π η scaled

22 J PC = 1 + Spin-Exotic Wave PLB 74 (15) 33 Spin-exotic J PC = Phase: intensities very different Suppression in πη channel predicted for intermediate qqg state Different phase motion in 1.6 GeV/c region 11 Jan Friedrich, TU München Spectroscopy Results from COMPASS π η final state; π η scaled

23 J PC = 1 + Spin-Exotic Wave PLB 74 (15) 33 Spin-exotic J PC = Phase: resonance interpretation requires better understanding of ++ wave Nonresonant contributions 11 Jan Friedrich, TU München Spectroscopy Results from COMPASS π η final state; π η scaled

24 J PC = 1 + Spin-Exotic Wave PLB 74 (15) 33 Spin-exotic J PC = π beam Phase: Multi-Regge exchange, e.g. η p target 11 Jan Friedrich, TU München Spectroscopy Results from COMPASS a 1 + resonance interpretation requires better understanding π of P ++ wave Nonresonant contributions p recoil π η final state; π η scaled

25 Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states Even partial waves similar Exotic 1 + much stronger in π η 3 PWA of diffractively produced 3π final states Observation of a new narrow axial-vector meson a 1 (14) J PC = 1 + spin-exotic partial wave 4 Conclusions and outlook 1 Jan Friedrich, TU München Spectroscopy Results from COMPASS

26 Partial-Wave Analysis: π π + π Final State [arxiv:159.99] π beam P X π π + π p target p recoil Events / (5 MeV/c ) a 1 (16) a (13) π (167) m [GeV/c 3π ] Strong π + π correlations in X π π + π decay 13 Jan Friedrich, TU München Spectroscopy Results from COMPASS

27 Partial-Wave Analysis: π π + π Final State [arxiv:159.99] π beam P X π π + π Events / (5 MeV/c ) a 1 (16) p target a (13) π (167) m [GeV/c 3π ] [(GeV/c ) ] + m π π p recoil m 1318 MeV/c < 1 MeV/c 3π ρ(77) m [(GeV/c ) π π ] Strong π + π correlations in X π π + π decay 13 Jan Friedrich, TU München Spectroscopy Results from COMPASS

28 Partial-Wave Analysis: π π + π Final State [arxiv:159.99] π beam P X π π + π p target p recoil Events / (5 MeV/c ) a 1 (16) a (13) π (167) [(GeV/c ) ] + m π π 1 m 167 MeV/c < 1 MeV/c 3π ρ(77) f (98) f (17) m [GeV/c 3π ] 1 m [(GeV/c ) π π ] + Strong π + π correlations in X π π + π decay 13 Jan Friedrich, TU München Spectroscopy Results from COMPASS

29 Partial-Wave Analysis: π π + π Final State [arxiv:159.99] π beam P X π π + π p target p recoil Events / (5 MeV/c ) a 1 (16) a (13) π (167) [(GeV/c ) ] + m π π 1 m 167 MeV/c < 1 MeV/c 3π ρ(77) f (98) f (17) m [GeV/c 3π ] 1 m [(GeV/c ) π π ] + Strong π + π correlations in X π π + π decay 13 Jan Friedrich, TU München Spectroscopy Results from COMPASS

30 Partial-Wave Analysis: π π + π Final State [arxiv:159.99] π beam P X π π + π p target p recoil Isobar model X decays via intermediate π + π resonance = isobar [ππ] S J PC = ++ ρ(77) 1 f (98) ++ f (17) ++ f (15) ++ ρ 3 (169) 3 PWA requires precise knowledge of isobar π + π amplitude 14 Jan Friedrich, TU München Spectroscopy Results from COMPASS

31 Partial-Wave Analysis: π π + π Final State π beam X [J PC M ǫ ] [L] P Isobar Bachelor π π + π [arxiv:159.99] p target p recoil Isobar model X decays via intermediate π + π resonance = isobar [ππ] S J PC = ++ ρ(77) 1 f (98) ++ f (17) ++ f (15) ++ ρ 3 (169) 3 PWA requires precise knowledge of isobar π + π amplitude 14 Jan Friedrich, TU München Spectroscopy Results from COMPASS

32 Partial-Wave Analysis: π π + π Final State π beam X [J PC M ǫ ] [L] P Isobar Bachelor π π + π [arxiv:159.99] Isobar model p target X decays via intermediate π + π resonance = isobar [ππ] S J PC = ++ ρ(77) 1 f (98) ++ f (17) ++ f (15) ++ ρ 3 (169) 3 PWA requires precise knowledge of isobar π + π amplitude Entries / (5 MeV/c ) p recoil ρ(77) f (98) f (17) ρ (169) m [GeV/c π π ] + 14 Jan Friedrich, TU München Spectroscopy Results from COMPASS

33 PWA of π p (3π) p recoil Two Data Sets 1 π π + π (5 M events) Crosscheck with π π π (3.5 M events) Very different acceptance Isobars separated by isospin I = 1 isobars: π π I = isobars: π π Complex correlation of m 3π and t Two-dimensional PWA in bins of t and m 3π π π + π : 11 t bins π π π : 8 t bins Better disentanglement of resonant and nonresonant contributions 15 Jan Friedrich, TU München Spectroscopy Results from COMPASS

34 PWA of π p (3π) p recoil Two Data Sets 1 π π + π (5 M events) Crosscheck with π π π (3.5 M events) Very different acceptance Isobars separated by isospin I = 1 isobars: π π I = isobars: π π Complex correlation of m 3π and t Two-dimensional PWA in bins of t and m 3π π π + π : 11 t bins π π π : 8 t bins Better disentanglement of resonant and nonresonant contributions 8 < m 3π < 85 MeV/c (GeV/c) ) 3 1 Events / ( <.85 GeV/c.8 < m 3π t' [(GeV/c) ] 16 < m 3π < 165 MeV/c (GeV/c) ) 3 1 Events / ( < 1.65 GeV/c 1.6 < m 3π 15 Jan Friedrich, TU München Spectroscopy Results from COMPASS t' [(GeV/c) ]

35 PWA of π p (3π) p recoil Two Data Sets 1 π π + π (5 M events) Crosscheck with π π π (3.5 M events) Very different acceptance Isobars separated by isospin I = 1 isobars: π π I = isobars: π π Complex correlation of m 3π and t Two-dimensional PWA in bins of t and m 3π π π + π : 11 t bins π π π : 8 t bins Better disentanglement of resonant and nonresonant contributions.1 < t <.1(GeV/c) Events / (5 MeV/c ) < t' <.1 (GeV/c) m [GeV/c 3π ].44 < t < 1.(GeV/c) Events / (5 MeV/c ) < t' < 1. (GeV/c) 15 Jan Friedrich, TU München Spectroscopy Results from COMPASS m [GeV/c 3π ]

36 PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum ρ(77)πs: a 1 (16) ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ) a 1 (16) a (13) π (167) m [GeV/c 3π ] 16 Jan Friedrich, TU München Spectroscopy Results from COMPASS

37 PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum ρ(77)πs: a 1 (16) ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ) a 1 (16) a (13) π (167) m [GeV/c 3π ] Intensity / ( MeV/c ) % ρ(77) π S.1 < t' < 1. (GeV/c) m [GeV/c 3π ] 16 Jan Friedrich, TU München Spectroscopy Results from COMPASS

38 PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum ρ(77)πs: a 1 (16) ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ) Intensity / ( MeV/c ) a 1 (16) a (13) π (167) m [GeV/c 3π ] ρ(77) π D 1 7.7%.1 < t' < 1. (GeV/c).5 Intensity / ( MeV/c ) % ρ(77) π S.1 < t' < 1. (GeV/c) m [GeV/c 3π ] m [GeV/c 3π ] 16 Jan Friedrich, TU München Spectroscopy Results from COMPASS

39 PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum ρ(77)πs: a 1 (16) ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ) Intensity / ( MeV/c ) a 1 (16) a (13) π (167) m [GeV/c 3π ] ρ(77) π D 1 7.7%.1 < t' < 1. (GeV/c).5 Intensity / ( MeV/c ) Intensity / ( MeV/c ) % ρ(77) π S.1 < t' < 1. (GeV/c) m [GeV/c 3π ] f (17) π S 6.7%.1 < t' < 1. (GeV/c) m [GeV/c 3π ] m [GeV/c 3π ] 16 Jan Friedrich, TU München Spectroscopy Results from COMPASS

40 PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum ρ(77)πs: a 1 (16) ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ) Intensity / ( MeV/c ) a 1 (16) a (13) π (167) m [GeV/c 3π ] ρ(77) π D 1 7.7%.1 < t' < 1. (GeV/c) In total 88 partial waves.5 Intensity / ( MeV/c ) Intensity / ( MeV/c ) % ρ(77) π S.1 < t' < 1. (GeV/c) m [GeV/c 3π ] f (17) π S 6.7%.1 < t' < 1. (GeV/c) Largest wave set used so far for π π + π Spin J up to 6 Orbital angular momentum L up to m [GeV/c 3π ] m [GeV/c 3π ] 16 Jan Friedrich, TU München Spectroscopy Results from COMPASS

41 PWA of π p π π + π p recoil : Selected Small Waves [arxiv:159.99] ρ(77)πg a 4 (4) + + f (98)πS π(18) Intensity / ( MeV/c ) % ρ(77) π G.1 < t' < 1. (GeV/c) f (98)πP Unexpected peak around 1.4 GeV/c Small intensity:.3% Similar signal in π π π m [GeV/c 3π ] 17 Jan Friedrich, TU München Spectroscopy Results from COMPASS

42 PWA of π p π π + π p recoil : Selected Small Waves [arxiv:159.99] ρ(77)πg a 4 (4) + + f (98)πS π(18) Intensity / ( MeV/c ) % ρ(77) π G.1 < t' < 1. (GeV/c) Intensity / ( MeV/c ) 3 1.4% f (98) π S.1 < t' < 1. (GeV/c) f (98)πP Unexpected peak around 1.4 GeV/c Small intensity:.3% Similar signal in π π π m [GeV/c 3π ] m [GeV/c 3π ] 17 Jan Friedrich, TU München Spectroscopy Results from COMPASS

43 PWA of π p π π + π p recoil : Selected Small Waves ρ(77)πg a 4 (4) + + f (98)πS π(18) f (98)πP Unexpected peak around 1.4 GeV/c Small intensity:.3% Similar signal in π π π Intensity / ( MeV/c ) % [arxiv:159.99] (98) π P.1 < t' < 1. (GeV/c) m [GeV/c 3π ] 17 Jan Friedrich, TU München Spectroscopy Results from COMPASS f

44 PWA of π p π π + π p recoil : Selected Small Waves ρ(77)πg a 4 (4) + + f (98)πS π(18) f (98)πP π π π Unexpected peak around 1.4 GeV/c Small intensity:.3% Similar signal in π π π π π + π scaled for each plot ) intensity (per 4 MeV/c 3 1 COMPASS 8 (π p (3π) p) f (98) π P π π π, π π π + (scaled).1 < t' < 1. GeV /c (incoherent sum) 18 Preliminary (GeV/c ) 17 Jan Friedrich, TU München Spectroscopy Results from COMPASS m (3π)

45 Is Peak in f (98)πP Wave a Model Artifact? Novel analysis method (inspired by E791 analysis, PRD 73 (6) 34) Replace J PC = ++ isobar parametrizations by piece-wise constant amplitudes in m π + π bins Extract m 3π dependence of ++ isobar amplitude from data Drastic reduction of model bias Caveat: significant increase in number of fit parameters Result: the a 1 (14) signal is indep. on the f (98) description 18 Jan Friedrich, TU München Spectroscopy Results from COMPASS

46 Is Peak in f (98)πP Wave a Model Artifact? Novel analysis method (inspired by E791 analysis, PRD 73 (6) 34) Replace J PC = ++ isobar parametrizations by piece-wise constant amplitudes in m π + π bins Extract m 3π dependence of ++ isobar amplitude from data Drastic reduction of model bias Caveat: significant increase in number of fit parameters Result: the a 1 (14) signal is indep. on the f (98) description 18 Jan Friedrich, TU München Spectroscopy Results from COMPASS

47 Is Peak in f (98)πP Wave a Model Artifact? Novel analysis method (inspired by E791 analysis, PRD 73 (6) 34) Replace J PC = ++ isobar parametrizations by piece-wise constant amplitudes in m π + π bins Extract m 3π dependence of ++ isobar amplitude from data Drastic reduction of model bias Caveat: significant increase in number of fit parameters Result: the a 1 (14) signal is indep. on the f (98) description 18 Jan Friedrich, TU München Spectroscopy Results from COMPASS

48 Resonance-Model Fit PRL 115 (15) 81 Coherent sum of resonant (Breit-Wigner) and nonresonant terms 19 Jan Friedrich, TU München Spectroscopy Results from COMPASS

49 Resonance-Model Fit PRL 115 (15) 81 Coherent sum of resonant (Breit-Wigner) and nonresonant terms 19 Jan Friedrich, TU München Spectroscopy Results from COMPASS

50 Resonance-Model Fit PRL 115 (15) 81 Intensity / ( MeV/c ) () f (98) π P.1 < t' < 1. (GeV/c) (1) Model curve () a 1 (14) resonance (3) Non-resonant term 5 (1) (3) m [GeV/c 3π ] 1 ++ peak consistent with Breit-Wigner resonance a 1 (14): M = MeV/c Γ = MeV/c Jan Friedrich, TU München Spectroscopy Results from COMPASS

51 Resonance-Model Fit PRL 115 (15) 81 Intensity / ( MeV/c ) ρ(77) π G 5.1 < t' < 1. (GeV/c) (1) Model curve () a 4 (4) resonance (3) Non-resonant term (1) () (3) m [GeV/c 3π ] 1 ++ peak consistent with Breit-Wigner resonance a 1 (14): M = MeV/c Γ = MeV/c Intensity / ( MeV/c ) Phase [deg] () (1) f (98) π P.1 < t' < 1. (GeV/c) (1) Model curve () a 1 (14) resonance (3) Non-resonant term (3) m [GeV/c 3π ] f (98) π P 4 1 ρ(77) π G.1 < t' <.113 (GeV/c).164 < t' <.189 (GeV/c).449 < t' <.74 (GeV/c) m [GeV/c 3π ] Jan Friedrich, TU München Spectroscopy Results from COMPASS

52 Resonance-Model Fit PRL 115 (15) 81 Intensity / ( MeV/c ) ρ(77) π G 5.1 < t' < 1. (GeV/c) (1) Model curve () a 4 (4) resonance (3) Non-resonant term (1) () (3) m [GeV/c 3π ] 1 ++ peak consistent with Breit-Wigner resonance a 1 (14): M = MeV/c Γ = MeV/c Intensity / ( MeV/c ) Phase [deg] () (1) f (98) π P.1 < t' < 1. (GeV/c) (1) Model curve () a 1 (14) resonance (3) Non-resonant term (3) m [GeV/c 3π ] f (98) π P 4 1 ρ(77) π G.1 < t' <.113 (GeV/c).164 < t' <.189 (GeV/c).449 < t' <.74 (GeV/c) m [GeV/c 3π ] Jan Friedrich, TU München Spectroscopy Results from COMPASS

53 a 1 (14) PRL 115 (15) 81 Nature unclear No quark-model states expected at 1.4 GeV/c Ground state a 1 (16) very close and wider Seen only in f (98)π decay mode Isospin partner of narrow f 1 (14)? Suspiciously close to KK threshold 1 Jan Friedrich, TU München Spectroscopy Results from COMPASS

54 a 1 (14) PRL 115 (15) 81 Several proposed explanations Two-quark-tetraquark mixed state Tetraquark with mixed flavor symmetry [Wang, arxiv: ] [Chen et al., PRD 91 (15) 94] Two-channel unitarized Deck amplitude + direct a 1 (16) production [Basdevant and Berger, PRL 114 (15) 191 and arxiv: ] Singularity (branching point) in triangle diagram [Mikhasenko et al., PRD 91 (15) 9415] Jan Friedrich, TU München Spectroscopy Results from COMPASS

55 a 1 (14) PRL 115 (15) 81 Several proposed explanations Two-quark-tetraquark mixed state Tetraquark with mixed flavor symmetry [Wang, arxiv: ] [Chen et al., PRD 91 (15) 94] Two-channel unitarized Deck amplitude + direct a 1 (16) production [Basdevant and Berger, PRL 114 (15) 191 and arxiv: ] Singularity (branching point) in triangle diagram [Mikhasenko et al., PRD 91 (15) 9415] Jan Friedrich, TU München Spectroscopy Results from COMPASS

56 a 1 (14) PRL 115 (15) 81 Several proposed explanations Two-quark-tetraquark mixed state Tetraquark with mixed flavor symmetry [Wang, arxiv: ] [Chen et al., PRD 91 (15) 94] Two-channel unitarized Deck amplitude + direct a 1 (16) production [Basdevant and Berger, PRL 114 (15) 191 and arxiv: ] Singularity (branching point) in triangle diagram [Mikhasenko et al., PRD 91 (15) 9415] K (89) π a 1 (16) K K + f (98) π + π Jan Friedrich, TU München Spectroscopy Results from COMPASS

57 Spin-Exotic J PC = 1 + Signal in (3π) PWA Broad intensity bump Similar in both channels π π π π π + π scaled 3 Jan Friedrich, TU München Spectroscopy Results from COMPASS

58 Spin-Exotic J PC = 1 + Signal in π π + π PWA Drastic Change of Mass Spectrum with t Low t.1(gev/c) High t.8(gev/c) Dominant nonresonant contribution Needs to be better understood in order to extract resonance content 4 Jan Friedrich, TU München Spectroscopy Results from COMPASS

59 Spin-Exotic J PC = 1 + Signal in π π + π PWA Model for Nonresonant Component Deck effect π π beam Isobar π π + π P p target p recoil MC pseudodata generated according to model of Deck amplitude Analyzed like real data based on ACCMOR, NPB 18 (1981) 69 5 Jan Friedrich, TU München Spectroscopy Results from COMPASS

60 Spin-Exotic J PC = 1 + Signal in π π + π PWA Deck-Model for Nonresonant Component Low t.1(gev/c) High t.8(gev/c) Deck MC scaled to t -summed intensity Similar mass spectrum at low t Different shape at high t 6 Jan Friedrich, TU München Spectroscopy Results from COMPASS

61 Spin-Exotic J PC = 1 + Signal in π π + π PWA Relative Phase w.r.t ρ(77)πs Wave Slow phase 6 motion in 1.6 GeV/c region independent of t 7 Jan Friedrich, TU München Spectroscopy Results from COMPASS

62 Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states Even partial waves similar Exotic 1 + much stronger in π η 3 PWA of diffractively produced 3π final states Observation of a new narrow axial-vector meson a 1 (14) J PC = 1 + spin-exotic partial wave 4 Conclusions and outlook 8 Jan Friedrich, TU München Spectroscopy Results from COMPASS

63 Conclusions and Outlook Precise data on pion diffraction PWA reliably extracts even very small signals New axial-vector state a 1 (14) in (3π) final states Novel analysis schemes: PWA in bins of t Better separation of resonant and nonresonant contribution Extraction of ππ S-wave amplitude from π π + π system Study dependence on 3π source Study rescattering effects Extension to higher ππ waves 9 Jan Friedrich, TU München Spectroscopy Results from COMPASS

64 Conclusions and Outlook Nonresonant contributions play important role Limit extraction of resonance parameters First studies using Deck models Extraction of nonresonant contributions from data Collaboration with JPAC: Veneziano amplitudes + finite-energy sum rules Other ongoing analyses Pion diffraction into π ηη, π π ω, KKπ, KKππ,... Kaon diffraction into K π + π Central-production reactions πγ scattering using Primakoff reactions on heavy targets 3 Jan Friedrich, TU München Spectroscopy Results from COMPASS

65 Conclusions and Outlook Nonresonant contributions play important role Limit extraction of resonance parameters First studies using Deck models Extraction of nonresonant contributions from data Collaboration with JPAC: Veneziano amplitudes + finite-energy sum rules Other ongoing analyses Pion diffraction into π ηη, π π ω, KKπ, KKππ,... Kaon diffraction into K π + π Central-production reactions πγ scattering using Primakoff reactions on heavy targets 3 Jan Friedrich, TU München Spectroscopy Results from COMPASS

66 Outline 5 Backup slides 31 Jan Friedrich, TU München Spectroscopy Results from COMPASS

67 Meson Production in Diffractive Dissociation π beam P h 1. h n p target p recoil Soft scattering of beam particle off target Production of n forward-going hadrons Target particle stays intact At 19 GeV/c, interaction dominated by space-like pomeron exchange All final-state particles are measured 3 Jan Friedrich, TU München Spectroscopy Results from COMPASS

68 Meson Production in Diffractive Dissociation π beam P h 1. h n p target Exclusive measurement Clean data sample Reduced four-momentum transfer squared t t t min Analyzed range:.1 < t < 1.(GeV/c) p recoil Example: π π + π final state Events / (5 MeV) Jan Friedrich, TU München Spectroscopy Results from COMPASS [GeV] E beam

69 Meson Production in Diffractive Dissociation π beam P Ó t 1 h 1. h n p target Exclusive measurement Clean data sample Reduced four-momentum transfer squared t t t min Analyzed range:.1 < t < 1.(GeV/c) p recoil Example: π π + π final state (GeV/c) ) Events / ( Jan Friedrich, TU München Spectroscopy Results from COMPASS t' [(GeV/c) ]

70 Partial-Wave Analysis Method π beam X P h 1. h n p target p recoil Ansatz: Factorization of production and decay Ti ǫ(m X) Ai ǫ(τ; m X) σ(τ; m X ) ǫ=±1 waves i Transition amplitudes T ǫ i (m X) contain interesting physics Decay amplitudes A ǫ i (τ; m X) Describe kinematic τ distribution of partial waves Calculable using isobar model (for n > ) and helicity formalism (Wigner D-functions) ǫ = ±1: naturalities of exchange particle 19 GeV/c beam momentum = pomeron (ǫ = +1) dominates 34 Jan Friedrich, TU München Spectroscopy Results from COMPASS

71 Partial-Wave Analysis Method π beam X P h 1. h n p target p recoil Ansatz: Factorization of production and decay Ti ǫ(m X) Ai ǫ(τ; m X) σ(τ; m X ) ǫ=±1 waves i Transition amplitudes T ǫ i (m X) contain interesting physics Decay amplitudes A ǫ i (τ; m X) Describe kinematic τ distribution of partial waves Calculable using isobar model (for n > ) and helicity formalism (Wigner D-functions) ǫ = ±1: naturalities of exchange particle 19 GeV/c beam momentum = pomeron (ǫ = +1) dominates 34 Jan Friedrich, TU München Spectroscopy Results from COMPASS

72 Partial-Wave Analysis Method π beam X P h 1. h n p target p recoil Ansatz: Factorization of production and decay Ti ǫ(m X) Ai ǫ(τ; m X) σ(τ; m X ) ǫ=±1 waves i Transition amplitudes T ǫ i (m X) contain interesting physics Decay amplitudes A ǫ i (τ; m X) Describe kinematic τ distribution of partial waves Calculable using isobar model (for n > ) and helicity formalism (Wigner D-functions) ǫ = ±1: naturalities of exchange particle 19 GeV/c beam momentum = pomeron (ǫ = +1) dominates 34 Jan Friedrich, TU München Spectroscopy Results from COMPASS

73 ππ S-Wave Amplitude in J PC = π Wave [arxiv:159.99] Correlation of 3π intensity around 1.4 GeV/c with f (98) f (98) semicircle in Argand diagram Confirms that f (98)π signal is not an artifact of isobar parametrization 35 Jan Friedrich, TU München Spectroscopy Results from COMPASS

74 ππ S-Wave Amplitude in J PC = π Wave [arxiv:159.99] Correlation of 3π intensity around 1.4 GeV/c with f (98) f (98) semicircle in Argand diagram Confirms that f (98)π signal is not an artifact of isobar parametrization 35 Jan Friedrich, TU München Spectroscopy Results from COMPASS

75 ππ S-Wave Amplitude in J PC = π Wave Im(T) [(Events/(GeV/c )) 1/] [arxiv:159.99] [ππ] π P.194 < t' <.36 (GeV/c) 1.38 < m < 1.4 GeV/c 3π Re(T) [(Events/(GeV/c )) 1/ ] Correlation of 3π intensity around 1.4 GeV/c with f (98) f (98) semicircle in Argand diagram Confirms that f (98)π signal is not an artifact of isobar parametrization 35 Jan Friedrich, TU München Spectroscopy Results from COMPASS

76 ππ S-Wave Amplitude in J PC = π Wave Intensity / (4 MeV/c ) [ππ] [arxiv:159.99] ++ π P.194 < t' <.36 (GeV/c).96 < m < 1. GeV/c π π m [GeV/c 3π ] Correlation of 3π intensity around 1.4 GeV/c with f (98) f (98) semicircle in Argand diagram Confirms that f (98)π signal is not an artifact of isobar parametrization 35 Jan Friedrich, TU München Spectroscopy Results from COMPASS

77 ππ S-Wave Amplitude in J PC = π Wave Intensity / ( MeV/c ) % f (98) π P.1 < t' < 1. (GeV/c) Intensity / (4 MeV/c ) [ππ] [arxiv:159.99] ++ π P.194 < t' <.36 (GeV/c).96 < m < 1. GeV/c π π m [GeV/c 3π ] m [GeV/c 3π ] Correlation of 3π intensity around 1.4 GeV/c with f (98) f (98) semicircle in Argand diagram Confirms that f (98)π signal is not an artifact of isobar parametrization 35 Jan Friedrich, TU München Spectroscopy Results from COMPASS

78 ππ S-Wave Amplitude in J PC = + 3π Wave [arxiv:159.99] Im(T) [(Events/(GeV/c )) 1/] [ππ] ++ π S.194 < t' <.36 (GeV/c) 1.78 < m < 1.8 GeV/c 3π Re(T) [(Events/(GeV/c )) 1/ ] Coupling of π(18) to f (98)π and f (15)π decay modes 36 Jan Friedrich, TU München Spectroscopy Results from COMPASS