Recent Results on Spectroscopy from COMPASS

Recent Results on Spectroscopy from COMPASS Boris Grube Physik-Department E18 Technische Universität München, Garching, Germany Hadron 15 16. September 15, Newport News, VA E COMPASS 1 8

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 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

The COMPASS Experiment at the CERN SPS Experimental Setup NIMA 779 (15) 69 Spectroscopy program Explore light-meson spectrum for 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 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states 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 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states 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 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Meson Production in Diffractive Dissociation 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 5 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Meson Production in Diffractive Dissociation 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).4. 6 1 6 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS 18 19 [GeV] E beam

Meson Production in Diffractive Dissociation 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 / (1 6 1 5 1 4 1 3 1 1 1 1 1 3 4 5 t' [(GeV/c) ] 6 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Meson Production in Diffractive Dissociation 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) 7 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Meson Production in Diffractive Dissociation 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) 7 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Partial-Wave Analysis Method 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 8 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Partial-Wave Analysis Method 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 9 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states 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 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 11 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 11 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 11 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 1 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 1 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 1 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Even-Spin Waves PLB 74 (15) 33 J PC = 4 ++ ++ Phase: 4 ++ ++ Similar even-spin waves Intermediate states couple to same final-state flavour content Similar physical content also in nonresonant high-mass region 13 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS π η final state; π η scaled

Even-Spin Waves PLB 74 (15) 33 J PC = 4 ++ ++ Phase: 4 ++ ++ 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) % 13 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS π η final state; π η scaled

J PC = 1 + Spin-Exotic Wave PLB 74 (15) 33 Spin-exotic J PC = 1 + ++ Phase: 1 + ++ 1 + intensities very different Suppression in πη channel predicted for intermediate qqg state Different phase motion in 1.6 GeV/c region 14 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS π η final state; π η scaled

J PC = 1 + Spin-Exotic Wave PLB 74 (15) 33 Spin-exotic J PC = 1 + ++ Phase: 1 + ++ 1 + resonance interpretation requires better understanding of ++ wave Nonresonant contributions 14 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS π η final state; π η scaled

J PC = 1 + Spin-Exotic Wave PLB 74 (15) 33 Spin-exotic J PC = 1 + ++ beam Phase: 1 + ++ Multi-Regge exchange, e.g. η p target a 1 + resonance interpretation requires better understanding of ++ wave Nonresonant contributions p recoil 14 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS P π η final state; π η scaled

Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states 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 15 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Partial-Wave Analysis: π π + π Final State [arxiv:159.99] beam P X π + p target p recoil Events / (5 MeV/c ).4.3. 6 1 a 1 (16) a (13) π (167).1.5 1 1.5.5 m [GeV/c 3π ] Strong π + π correlations in X π π + π decay 16 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Partial-Wave Analysis: π π + π Final State [arxiv:159.99] beam P X π + Events / (5 MeV/c ).4.3..1 6 1 a 1 (16) p target a (13) π (167).5 1 1.5.5 m [GeV/c 3π ] [(GeV/c ) ] + m π π 1.5 1.5 p recoil m 1318 MeV/c < 1 MeV/c 3π ρ(77).5 1 1.5 m [(GeV/c ) π π ] + 8 7 6 5 4 3 1 Strong π + π correlations in X π π + π decay 16 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Partial-Wave Analysis: π π + π Final State [arxiv:159.99] beam P X π + p target p recoil Events / (5 MeV/c ).4.3..1 6 1 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) 35 3 5 15 1 5.5 1 1.5.5 m [GeV/c 3π ] 1 m [(GeV/c ) π π ] + Strong π + π correlations in X π π + π decay 16 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Partial-Wave Analysis: π π + π Final State [arxiv:159.99] beam P X 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 17 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Partial-Wave Analysis: π π + π Final State beam X [J PC M ǫ ] [L] P Isobar Bachelor 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 17 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Partial-Wave Analysis: π π + π Final State beam X [J PC M ǫ ] [L] P Isobar Bachelor 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 ) 1.5 6 1 p recoil ρ(77) f (98) f (17) ρ (169) 3.5 1 1.5 m [GeV/c π π ] + 17 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 / (5 4 1 3 1 1 <.85 GeV/c.8 < m 3π..4.6.8 1 t' [(GeV/c) ] 16 < m 3π < 165 MeV/c (GeV/c) ) 3 1 Events / (5 5 1 4 1 3 1 < 1.65 GeV/c 1.6 < m 3π..4.6.8 1 t' [(GeV/c) ] 18 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 ) 3 1 6 4.1 < t' <.1 (GeV/c).5 1 1.5.5 m [GeV/c 3π ].44 < t < 1.(GeV/c) Events / (5 MeV/c ) 4 3 1 3 1.44 < t' < 1. (GeV/c).5 1 1.5.5 m [GeV/c 3π ] 18 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum 1 ++ + ρ(77)πs: a 1 (16) ++ 1 + ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ).4.3..1 6 1 a 1 (16) a (13) π (167).5 1 1.5.5 m [GeV/c 3π ] 19 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum 1 ++ + ρ(77)πs: a 1 (16) ++ 1 + ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ).4.3..1 6 1 a 1 (16) a (13) π (167).5 1 1.5.5 m [GeV/c 3π ] Intensity / ( MeV/c ) 6 1 1.5 3.7% 1.5 + + + 1 ρ(77) π S.1 < t' < 1. (GeV/c).5 1 1.5.5 m [GeV/c 3π ] 19 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum 1 ++ + ρ(77)πs: a 1 (16) ++ 1 + ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ) Intensity / ( MeV/c ).4.3..1 6 1 a 1 (16) a (13) π (167).5 1 1.5.5 m [GeV/c 3π ] 6 1 + + + 1 ρ(77) π D 1 7.7%.1 < t' < 1. (GeV/c).5 Intensity / ( MeV/c ) 6 1 1.5 3.7% 1.5 + + + 1 ρ(77) π S.1 < t' < 1. (GeV/c).5 1 1.5.5 m [GeV/c 3π ].5 1 1.5.5 m [GeV/c 3π ] 19 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum 1 ++ + ρ(77)πs: a 1 (16) ++ 1 + ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ) Intensity / ( MeV/c ).4.3..1 6 1 a 1 (16) a (13) π (167).5 1 1.5.5 m [GeV/c 3π ] 6 1 + + + 1 ρ(77) π D 1 7.7%.1 < t' < 1. (GeV/c).5 Intensity / ( MeV/c ) Intensity / ( MeV/c ) 6 1 1.5 3.7% 1.5 + + + 1 ρ(77) π S.1 < t' < 1. (GeV/c).5 1 1.5.5 m [GeV/c 3π ] + + 6 1 f (17) π S 6.7%.1 < t' < 1. (GeV/c).3..1.5 1 1.5.5 m [GeV/c 3π ].5 1 1.5.5 m [GeV/c 3π ] 19 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

PWA of π p π π + π p recoil : Major Waves [arxiv:159.99] π π + π invariant mass spectrum 1 ++ + ρ(77)πs: a 1 (16) ++ 1 + ρ(77)πd: a (13) + + f (17)πS: π (167) Events / (5 MeV/c ) Intensity / ( MeV/c ).4.3..1 6 1 a 1 (16) a (13) π (167).5 1 1.5.5 m [GeV/c 3π ] + + + 6 1 1 ρ(77) π D 1 7.7%.1 < t' < 1. (GeV/c) In total 88 partial waves.5 Intensity / ( MeV/c ) Intensity / ( MeV/c ) 6 1 1.5 3.7% 1.5 + + + 1 ρ(77) π S.1 < t' < 1. (GeV/c).5 1 1.5.5 m [GeV/c 3π ] + + 6 1 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 6.3..1.5 1 1.5.5 m [GeV/c 3π ].5 1 1.5.5 m [GeV/c 3π ] 19 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

PWA of π p π π + π p recoil : Selected Small Waves [arxiv:159.99] 4 ++ 1 + ρ(77)πg a 4 (4) + + f (98)πS π(18) Intensity / ( MeV/c ) 3 1 3.8% 1 + + + 4 1 ρ(77) π G.1 < t' < 1. (GeV/c) 1 ++ + f (98)πP Unexpected peak around 1.4 GeV/c Small intensity:.3% Similar signal in π π π.5 1 1.5.5 m [GeV/c 3π ] Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

PWA of π p π π + π p recoil : Selected Small Waves [arxiv:159.99] 4 ++ 1 + ρ(77)πg a 4 (4) + + f (98)πS π(18) Intensity / ( MeV/c ) 3 1 3.8% 1 + + + 4 1 ρ(77) π G.1 < t' < 1. (GeV/c) Intensity / ( MeV/c ) 3 1.4% 1 5 + + f (98) π S.1 < t' < 1. (GeV/c) 1 ++ + f (98)πP Unexpected peak around 1.4 GeV/c Small intensity:.3% Similar signal in π π π.5 1 1.5.5 m [GeV/c 3π ].5 1 1.5.5 m [GeV/c 3π ] Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

PWA of π p π π + π p recoil : Selected Small Waves 4 ++ 1 + ρ(77)πg a 4 (4) + + f (98)πS π(18) 1 ++ + f (98)πP Unexpected peak around 1.4 GeV/c Small intensity:.3% Similar signal in π π π Intensity / ( MeV/c ) 15 1 5 3 1.3% ++ + 1 [arxiv:159.99] (98) π P.1 < t' < 1. (GeV/c).5 1 1.5.5 m [GeV/c 3π ] Boris Grube, TU München Recent Results on Spectroscopy from COMPASS f

PWA of π p π π + π p recoil : Selected Small Waves 4 ++ 1 + ρ(77)πg a 4 (4) + + f (98)πS π(18) 1 ++ + 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) 4 ++ + 1 f (98) π P π π π, π π π + (scaled).1 < t' < 1. GeV /c (incoherent sum) 18 Preliminary 16 14 1 1 8 6 4.6.8 1 1. 1.4 1.6 1.8..4 (GeV/c ) Boris Grube, TU München Recent Results on Spectroscopy from COMPASS m (3π)

Is Peak in 1 ++ + 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 J PC = ++ isobar amplitude from data Drastic reduction of model bias Caveat: significant increase in number of fit parameters 1 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

ππ S-Wave Amplitude in J PC = 1 ++ 3π 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 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

ππ S-Wave Amplitude in J PC = 1 ++ 3π Wave Im(T) [(Events/(GeV/c )) 1/] 4 1.75 1..995 ++ + 1 [arxiv:159.99] [ππ].95 ++ π P.194 < t' <.36 (GeV/c) 1.38 < m < 1.4 GeV/c 3π.965.99 4 6 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 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

ππ S-Wave Amplitude in J PC = 1 ++ 3π Wave Intensity / (4 MeV/c ) 1 1 5 3 ++ + 1 [ππ] [arxiv:159.99] ++ π P.194 < t' <.36 (GeV/c).96 < m < 1. GeV/c π π +.5 1 1.5.5 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 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

ππ S-Wave Amplitude in J PC = 1 ++ 3π Wave Intensity / ( MeV/c ) 15 1 3 1.3% ++ + 1 f (98) π P.1 < t' < 1. (GeV/c) Intensity / (4 MeV/c ) 1 1 5 3 ++ + 1 [ππ] [arxiv:159.99] ++ π P.194 < t' <.36 (GeV/c).96 < m < 1. GeV/c π π + 5.5 1 1.5.5 m [GeV/c 3π ].5 1 1.5.5 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 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

ππ S-Wave Amplitude in J PC = + 3π Wave [arxiv:159.99] Im(T) [(Events/(GeV/c )) 1/] 6 4 + + [ππ] ++ π S.194 < t' <.36 (GeV/c) 1.78 < m < 1.8 GeV/c 3π.95.965 1.4 1.6 1.54 1.75.995 6 4 Re(T) [(Events/(GeV/c )) 1/ ] Coupling of π(18) to f (98)π and f (15)π decay modes See talk 1E by F. Krinner (Thu, 8:55) 3 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Resonance-Model Fit PRL 115 (15) 81 Coherent sum of resonant (Breit-Wigner) and nonresonant terms 4 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Resonance-Model Fit PRL 115 (15) 81 Intensity / ( MeV/c ) 1 5 15 1 3 () ++ + 1 f (98) π P.1 < t' < 1. (GeV/c) (1) Model curve () a 1 (14) resonance (3) Non-resonant term 5 (1) (3) 1 1. 1.4 1.6 1.8. m [GeV/c 3π ] 1 ++ peak consistent with Breit-Wigner resonance a 1 (14): M = 1414 +15 13 MeV/c Γ = 153 +8 3 MeV/c 5 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Resonance-Model Fit PRL 115 (15) 81 Intensity / ( MeV/c ) 15 1 5 3 1 ++ + 4 1 ρ(77) π G 5.1 < t' < 1. (GeV/c) (1) Model curve () a 4 (4) resonance (3) Non-resonant term (1) () (3) 1 1. 1.4 1.6 1.8. m [GeV/c 3π ] 1 ++ peak consistent with Breit-Wigner resonance a 1 (14): M = 1414 +15 13 MeV/c Γ = 153 +8 3 MeV/c Intensity / ( MeV/c ) Phase [deg] 1 5 15 1 5 1 1 3 () (1) ++ + 1 f (98) π P.1 < t' < 1. (GeV/c) (1) Model curve () a 1 (14) resonance (3) Non-resonant term (3) 1 1. 1.4 1.6 1.8. m [GeV/c 3π ] ++ + + + + 1 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) 1 1. 1.4 1.6 1.8. m [GeV/c 3π ] 5 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 6 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

a 1 (14) PRL 115 (15) 81 Several proposed explanations Two-quark-tetraquark mixed state Tetraquark with mixed flavor symmetry [Wang, arxiv:141.1134] [Chen et al., PRD 91 (15) 94] Two-channel unitarized Deck amplitude + direct a 1 (16) production See talk 1B4 by E. Berger (Mon, 5:3) [Basdevant and Berger, PRL 114 (15) 191 and arxiv:151.4643] Singularity (branching point) in triangle diagram See talk 5A1 by B. Ketzer (Thu, 8:3) [Mikhasenko et al., PRD 91 (15) 9415] 7 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

a 1 (14) PRL 115 (15) 81 Several proposed explanations Two-quark-tetraquark mixed state Tetraquark with mixed flavor symmetry [Wang, arxiv:141.1134] [Chen et al., PRD 91 (15) 94] Two-channel unitarized Deck amplitude + direct a 1 (16) production See talk 1B4 by E. Berger (Mon, 5:3) [Basdevant and Berger, PRL 114 (15) 191 and arxiv:151.4643] Singularity (branching point) in triangle diagram See talk 5A1 by B. Ketzer (Thu, 8:3) [Mikhasenko et al., PRD 91 (15) 9415] K (89) a 1 (16) K K + f (98) π + 7 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Spin-Exotic J PC = 1 + Signal in (3π) PWA Broad intensity bump Similar in both channels π π π π π + π scaled 8 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 9 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Spin-Exotic J PC = 1 + Signal in π π + π PWA Model for Nonresonant Component Deck effect beam Isobar 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 3 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 31 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Spin-Exotic J PC = 1 + Signal in π π + π PWA Relative Phase w.r.t. 1 ++ + ρ(77)πs Wave Slow phase 6 motion in 1.6 GeV/c region independent of t 3 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

Outline 1 Introduction Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced π η and π η final states 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 33 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 34 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS

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 See talk 1E1 by A. Austregesilo (Thu, 8:3) πγ scattering using Primakoff reactions on heavy targets See talk 6C4 by M. Krämer (Thu, 11:45) 35 Boris Grube, TU München Recent Results on Spectroscopy from COMPASS