COMPASS 1 8 The COMPASS Experiment at the CERN SPS Experimental - PowerPoint PPT Presentation

Meson Spectroscopy at COMPASS Boris Grube Physik-Department E18 Technische Universität München, Garching, Germany MESON2016 07. June 2016, Kraków E COMPASS 1 8

The COMPASS Experiment at the CERN SPS Experimental Setup [NIMA 779 (2015) 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) 2 Boris Grube, TU München Meson Spectroscopy at COMPASS

The COMPASS Experiment at the CERN SPS Experimental Setup [NIMA 779 (2015) 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 2008-09, 2012 190 GeV/ c secondary hadron beams h − beam: 97 % π − , 2 % K − , 1 % p h + beam: 75 % p , 24 % π + , 1 % K + Various targets: ℓ H 2 , Ni, Pb, W 2 Boris Grube, TU München Meson Spectroscopy at COMPASS

The COMPASS Experiment at the CERN SPS Experimental Setup [NIMA 779 (2015) 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 2008-09, 2012 190 GeV/ c secondary hadron beams h − beam: 97 % π − , 2 % K − , 1 % p h + beam: 75 % p , 24 % π + , 1 % K + Various targets: ℓ H 2 , Ni, Pb, W 2 Boris Grube, TU München Meson Spectroscopy at COMPASS

The COMPASS Experiment at the CERN SPS Experimental Setup [NIMA 779 (2015) 69] Spectroscopy program Explore light-meson spectrum for m � 2 GeV/ c 2 Search for states beyond the constituent quark model Precision measurement of known resonances Hadron spectroscopy 2008-09, 2012 190 GeV/ c secondary hadron beams h − beam: 97 % π − , 2 % K − , 1 % p h + beam: 75 % p , 24 % π + , 1 % K + Various targets: ℓ H 2 , Ni, Pb, W 2 Boris Grube, TU München Meson Spectroscopy at COMPASS

Outline Introduction 1 Meson production in diffractive dissociation Partial-wave analysis method PWA of diffractively produced 3 π final states 2 Observation of a new narrow axial-vector meson a 1 ( 1420 ) J PC = 1 − + spin-exotic partial wave Conclusions and outlook 3 3 Boris Grube, TU München Meson Spectroscopy at COMPASS

Meson Production in Diffractive Dissociation h 1 π ´ π ´ π ´ π ´ beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Soft scattering of beam particle off target Production of n forward-going hadrons Target particle stays intact At 190 GeV/ c , interaction dominated by space-like pomeron exchange 4 Boris Grube, TU München Meson Spectroscopy at COMPASS

Meson Production in Diffractive Dissociation h 1 π ´ π ´ π ´ π ´ beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Example: π − π + π − final state Exclusive measurement × 6 10 Events / (50 MeV) Clean data samples Reduced four-momentum transfer squared t ′ ≡ | t | − | t | min 0.4 Analyzed range: 0.1 < t ′ < 1.0 ( GeV/ c ) 2 0.2 0 180 190 200 [ ] E GeV beam 5 Boris Grube, TU München Meson Spectroscopy at COMPASS

Meson Production in Diffractive Dissociation h 1 π ´ π ´ π ´ π ´ beam beam beam beam . . . h n P Ó t 1 p target p target p target p target p recoil p recoil p recoil p recoil Example: π − π + π − final state Exclusive measurement ) 2 ) Clean data samples c (GeV/ 6 10 Reduced four-momentum transfer 5 squared t ′ ≡ | t | − | t | min 10 2 − 10 4 ( 10 Events / Analyzed range: 0.1 < t ′ < 1.0 ( GeV/ c ) 2 3 10 2 10 10 1 0 1 2 3 4 5 [ 2 ] t' (GeV/ c ) 5 Boris Grube, TU München Meson Spectroscopy at COMPASS

Meson Production in Diffractive Dissociation h 1 π ´ π ´ π ´ π ´ beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Excitation of beam particle 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) 6 Boris Grube, TU München Meson Spectroscopy at COMPASS

Meson Production in Diffractive Dissociation h 1 π ´ π ´ π ´ π ´ X ´ beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Excitation of beam particle 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) 6 Boris Grube, TU München Meson Spectroscopy at COMPASS

Meson Production in Diffractive Dissociation h 1 π ´ π ´ π ´ π ´ X ´ beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Excitation of beam particle 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) 6 Boris Grube, TU München Meson Spectroscopy at COMPASS

Partial-Wave Analysis Method h 1 π ´ π ´ π ´ π ´ T X ´ A beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Ansatz: Factorization of production and decay 2 � waves � I ( τ ; m X ) = ∑ ǫ = ± 1 � ∑ T ǫ i ( m X ) A ǫ i ( τ ; m X ) � � i � Transition amplitudes T ǫ i ( m X ) = ⇒ interesting physics Decay amplitudes A ǫ i ( τ ; m X ) Describe kinematic distribution of partial waves Calculated using isobar model (for n > 2) and helicity formalism (Wigner D -functions) ǫ = ± 1: naturalities of exchange particle 190 GeV/ c beam momentum = ⇒ pomeron ( ǫ = + 1) dominates 7 Boris Grube, TU München Meson Spectroscopy at COMPASS

Partial-Wave Analysis Method h 1 π ´ π ´ π ´ π ´ T X ´ A beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Ansatz: Factorization of production and decay 2 � waves � I ( τ ; m X ) = ∑ ǫ = ± 1 � ∑ T ǫ i ( m X ) A ǫ i ( τ ; m X ) � � i � Transition amplitudes T ǫ i ( m X ) = ⇒ interesting physics Decay amplitudes A ǫ i ( τ ; m X ) Describe kinematic distribution of partial waves Calculated using isobar model (for n > 2) and helicity formalism (Wigner D -functions) ǫ = ± 1: naturalities of exchange particle 190 GeV/ c beam momentum = ⇒ pomeron ( ǫ = + 1) dominates 7 Boris Grube, TU München Meson Spectroscopy at COMPASS

Partial-Wave Analysis Method h 1 π ´ π ´ π ´ π ´ T X ´ A beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Ansatz: Factorization of production and decay 2 � waves � I ( τ ; m X ) = ∑ ǫ = ± 1 � ∑ T ǫ i ( m X ) A ǫ i ( τ ; m X ) � � i � Transition amplitudes T ǫ i ( m X ) = ⇒ interesting physics Decay amplitudes A ǫ i ( τ ; m X ) Describe kinematic distribution of partial waves Calculated using isobar model (for n > 2) and helicity formalism (Wigner D -functions) ǫ = ± 1: naturalities of exchange particle 190 GeV/ c beam momentum = ⇒ pomeron ( ǫ = + 1) dominates 7 Boris Grube, TU München Meson Spectroscopy at COMPASS

Partial-Wave Analysis Method h 1 π ´ π ´ π ´ π ´ T X ´ A beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Two-step analysis 2 � waves � I ( τ ; m X ) = ∑ ǫ = ± 1 � ∑ T ǫ i ( m X ) A ǫ i ( τ ; m X ) � � i � Determination of m X dependence of spin-density matrix 1 i ( m X ) T ǫ ∗ ̺ ǫ ij ( m X ) = T ǫ ( m X ) j Independent maximum likelihood fits to measured τ distributions in narrow bins of m X Fits take into account detection efficiency No assumptions about resonance content of X Extraction of resonances 2 χ 2 fit of resonance model to spin-density (sub)matrix 8 Boris Grube, TU München Meson Spectroscopy at COMPASS

Partial-Wave Analysis Method h 1 π ´ π ´ π ´ π ´ T X ´ A beam beam beam beam . . . h n P p target p target p target p target p recoil p recoil p recoil p recoil Two-step analysis 2 � waves � I ( τ ; m X ) = ∑ ǫ = ± 1 � ∑ T ǫ i ( m X ) A ǫ i ( τ ; m X ) � � i � Determination of m X dependence of spin-density matrix 1 i ( m X ) T ǫ ∗ ̺ ǫ ij ( m X ) = T ǫ ( m X ) j Independent maximum likelihood fits to measured τ distributions in narrow bins of m X Fits take into account detection efficiency No assumptions about resonance content of X Extraction of resonances 2 χ 2 fit of resonance model to spin-density (sub)matrix 8 Boris Grube, TU München Meson Spectroscopy at COMPASS

Partial-Wave Analysis: π − π + π − Final State [arXiv:1509.00992] π ´ π ´ π ´ π ´ π ´ X ´ beam beam beam beam π + π ´ P p target p target p target p target p recoil p recoil p recoil p recoil X − decay via π + π − resonances = “ isobars ” 9 Boris Grube, TU München Meson Spectroscopy at COMPASS