Hadron Spectroscopy at COMPASS

Hadron Spectroscopy at Overview and Analysis Methods Boris Grube for the Collaboration Physik-Department E18 Technische Universität München, Garching, Germany Future Directions in Spectroscopy Analysis JLab, Newport News, 18. Nov 2014 E 1 8

Outline 1 The experiment 2 Meson production in diffractive dissociation 3 Partial-wave analysis method 4 PWA of diffractively produced π η and π η final states 5 Scalar mesons in central production 2 Boris Grube, TU München Hadron Spectroscopy at

The Experiment at the CERN SPS Experimental Setup hep-ex/1410.1797, submitted to NIM A 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: lh 2, Ni, Pb, W > 1 PByte of data per year 4 Boris Grube, TU München Hadron Spectroscopy at

The Experiment at the CERN SPS Experimental Setup hep-ex/1410.1797, submitted to NIM A 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: lh 2, Ni, Pb, W > 1 PByte of data per year 4 Boris Grube, TU München Hadron Spectroscopy at

Production Processes Accessible at Diffractive dissociation of π, K, and p on various targets Quasi-real photoproduction in Coulomb field of nuclear targets beam P X h 1. h n p target p recoil Central production with π and p on proton target Muo-production with polarized beam and target 5 Boris Grube, TU München Hadron Spectroscopy at

Production Processes Accessible at Diffractive dissociation of π, K, and p on various targets beam P X h 1. h n Quasi-real photoproduction in Coulomb field of nuclear targets beam γ ( ) X h 1. h n p target p recoil Z Z Central production with π and p on proton target Muo-production with polarized beam and target 5 Boris Grube, TU München Hadron Spectroscopy at

Production Processes Accessible at Diffractive dissociation of π, K, and p on various targets beam P X h 1. h n Quasi-real photoproduction in Coulomb field of nuclear targets beam γ ( ) X h 1. h n p target p recoil Z Z Central production with π and p on proton target p beam p fast Muo-production with polarized beam and target µ µ 1 R, P R, P X 0 h 1. h n γ R, P X 0 h 1. h n p target p recoil N N 1 5 Boris Grube, TU München Hadron Spectroscopy at

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 190 GeV/c interaction predominantly via space-like pomeron exchange All final-state particles are measured 7 Boris Grube, TU München Hadron Spectroscopy at

Meson Production in Diffractive Dissociation beam P h 1. h n p target Exclusive measurement p recoil measured by RPD Clean data sample Reduced four-momentum transfer squared t t t min Range 0.1 < t < 1.0 (GeV/c) 2 p recoil Example: π π + π final state Number of Events / (166.7 MeV) 2.5 2 1.5 1 0.5 6 10 π p π π + π p ( 2008) Preliminary 0 0 20 40 60 80 100 120 140 160 180 200 Calculated Beam Energy (GeV) 8 Boris Grube, TU München Hadron Spectroscopy at

Meson Production in Diffractive Dissociation beam P h 1. h n p target Exclusive measurement p recoil measured by RPD Clean data sample Reduced four-momentum transfer squared t t t min Range 0.1 < t < 1.0 (GeV/c) 2 p recoil Example: π π + π final state Number of Events / (50 MeV) 0.5 0.4 0.3 0.2 0.1 6 10 π p π π + π p ( 2008) Preliminary 0 175 180 185 190 195 200 205 Calculated Beam Energy (GeV) 8 Boris Grube, TU München Hadron Spectroscopy at

Meson Production in Diffractive Dissociation beam P h 1. h n p target Exclusive measurement p recoil measured by RPD Clean data sample Reduced four-momentum transfer squared t t t min Range 0.1 < t < 1.0 (GeV/c) 2 p recoil Example: π π + π final state /c 2 ) 2 GeV 3 10 Number of Events (3.33 6 10 5 10 4 10 π p π π + π p ( 2008) Preliminary 0 0.2 0.4 0.6 0.8 1 1.2 2 Squared Four-Momentum Transfer t' (GeV /c 2 ) 8 Boris Grube, TU München Hadron Spectroscopy at

Meson Production in Diffractive Dissociation beam P h 1. h n p target p recoil Beam particle gets excited into intermediate resonance X X dissociates into n-body final state Rich spectrum of intermediate states X Disentanglement of all contributing X by partial-wave analysis (PWA) 9 Boris Grube, TU München Hadron Spectroscopy at

Meson Production in Diffractive Dissociation beam P X h 1. h n p target p recoil Beam particle gets excited into intermediate resonance X X dissociates into n-body final state Rich spectrum of intermediate states X Disentanglement of all contributing X by partial-wave analysis (PWA) 9 Boris Grube, TU München Hadron Spectroscopy at

Partial-Wave Analysis Method beam X P h 1. h n p target p recoil Ansatz: Factorization of production and decay σ(τ; m X ) rank r ɛ=±1 waves T r ɛ i i (m X ) Ai ɛ(τ; m X) 2 Transition amplitudes Ti r ɛ (m X ) contain interesting physics Decay amplitudes Ai ɛ(τ; m X) Describe kinematic τ distribution of partial waves Calculable using isobar model (for n > 2) and helicity formalism (Wigner D-functions) 11 Boris Grube, TU München Hadron Spectroscopy at

Partial-Wave Analysis Method beam X P h 1. h n p target p recoil Ansatz: Factorization of production and decay σ(τ; m X ) rank r ɛ=±1 waves T r ɛ i i (m X ) Ai ɛ(τ; m X) 2 Parity conservation taken into account via reflectivity ɛ = ±1 Corresponds to naturality of exchange particle in high-energy limit Rank accounts for incoherences from Integration over large t range Excitation of target particle... For most analyses rank 1 is sufficient 12 Boris Grube, TU München Hadron Spectroscopy at

Partial-Wave Analysis Method beam X P h 1. h n p target p recoil Two-stage analysis σ(τ; m X ) rank r ɛ=±1 waves T r ɛ i i (m X ) Ai ɛ(τ; m X) 1 Determination of T r ɛ i (m X ) Independent unbinned maximum likelihood fits to τ distribution in narrow m X bins Take into account detector acceptance and efficiency No assumptions about resonance content of X 2 Extraction of resonances χ 2 fit of resonance model to spin-density matrix ϱij ɛ (m X) = rank T r ɛ r i (m X ) Tj r ɛ (m X ) 13 Boris Grube, TU München Hadron Spectroscopy at 2

PWA of π p π η ( ) p slow [arxiv:1408.4286] Reconstruction from exclusive π π + π γγ final state η π + π π 0 with π 0 γγ 116 000 π η events: dominated by a 2 (1320) η π + π η with η γγ 39 000 π η events: broad peak at 1.7 GeV/c 2 π η invariant mass 15 Boris Grube, TU München Hadron Spectroscopy at

PWA of π p π η ( ) p slow [arxiv:1408.4286] Angular distributions Polar angles of η ( ) measured in Gottfried-Jackson frame z GJ p beam and y GJ production-plane normal At high masses distributions strongly peaked at cos θ GJ = ±1 Asymmetry: backward going η favored, less pronounced for η PWA = decomposition into spherical harmonics cos θ GJ vs. π η mass cos θ GJ vs. π η mass 16 Boris Grube, TU München Hadron Spectroscopy at

PWA of π p π η ( ) p slow J PC M ɛ = 2 ++ 1 + Partial Wave [arxiv:1408.4286] π η final state π η final state Relate π η and π η partial-wave amplitudes for spin J Phase space and barrier factors (q = breakup momentum) Branching fraction ratio b of η and η into π π + γγ 17 Boris Grube, TU München Hadron Spectroscopy at

PWA of π p π η ( ) p slow J PC M ɛ = 2 ++ 1 + Partial Wave π η final state π η final state; π η scaled [arxiv:1408.4286] Relate π η and π η partial-wave amplitudes for spin J Phase space and barrier factors (q = breakup momentum) Branching fraction ratio b of η and η into π π + γγ [ ] T πη J+1/2 J (m) T πη J (m) = b q πη (m) q πη (m) 17 Boris Grube, TU München Hadron Spectroscopy at

PWA of π p π η ( ) p slow J PC M ɛ = 2 ++ 1 + [arxiv:1408.4286] Very similar even-spin waves Expected for nn resonances (OZI rule) Similar physical content also in non-resonant high-mass region 18 Boris Grube, TU München Hadron Spectroscopy at

PWA of π p π η ( ) p slow J PC M ɛ = 2 ++ 1 + 4 ++ 1 + [arxiv:1408.4286] Phase: 4 ++ 1 + 2 ++ 1 + Very similar even-spin waves Expected for nn resonances (OZI rule) Similar physical content also in non-resonant high-mass region 18 Boris Grube, TU München Hadron Spectroscopy at

PWA of π p π η ( ) p slow J PC M ɛ = 2 ++ 1 + Spin-exotic 1 + 1 + [arxiv:1408.4286] Phase: 2 ++ 1 + 1 + 1 + Intensities of odd waves very different Suppression in πη channel predicted for intermediate qqg state Different phase motion in 1.6 GeV/c 2 region 19 Boris Grube, TU München Hadron Spectroscopy at

PWA of π p π η (0) pslow [arxiv:1408.4286] Resonance Interpretation a2 (1320) and a4 (2040) parameters extracted reliably Parameter values consistent with 3π analyses Parameters of a2 0 and π1 not stable Depend strongly on parametrization of non-res. contribution 20 Boris Grube, TU München Hadron Spectroscopy at

PWA of π p π η ( ) p slow [arxiv:1408.4286] Summary After scaling by phase space and barrier factors Even waves with L = 2, 4, and 6 very similar in π η and π η Odd waves with L = 1, 3, and 5 (all spin-exotic) strongly suppressed in π η w.r.t. π η Data can be described in terms of Breit-Wigner resonances and non-resonant terms Models used in analyses of previous experiments yield comparable results Inclusion of a 4 (2040) requires description of high-mass regions in 1 + and 2 ++ waves Region is dominated by non-resonant contributions Results depend strongly on parametrization Resonance interpretation of 1 + wave requires Better understanding of resonance structure of 2 ++ and 4 ++ waves More realistic description of non-resonant contributions 21 Boris Grube, TU München Hadron Spectroscopy at

Scalar Mesons in Central Production Search for glueball candidates Lattice QCD simulations predict lightest glueballs to be scalars Glueball would appear as supernumerous state Strong mixing with conventional scalar mesons expected Difficult to disentangle Central Exclusive Production Pomeron-Pomeron fusion well-suited to search for glueballs Isoscalar mesons produced at central rapidities Scalar mesons dominant in this channel Gluon-rich environment At energies: contributions from other Regge exchanges 23 Boris Grube, TU München Hadron Spectroscopy at

K + K Central Exclusive Production p beam p fast R, P R, P X 0 [J ǫ M ] [L] K + K p target p recoil Suppression of diffractive background by cut p(p fast ) > 140 GeV/c p fast K invariant mass Rapidity in CM frame 24 Boris Grube, TU München Hadron Spectroscopy at

K + K Central Exclusive Production p beam p fast R, P R, P X 0 [J ǫ M ] [L] K + K p target p recoil Suppression of diffractive background by cut p(p fast ) > 140 GeV/c p fast K invariant mass K + K invariant mass 24 Boris Grube, TU München Hadron Spectroscopy at

Fit of K + K Mass Dependence Fit model: Relativistic Breit-Wigner resonances S 0 : f 0(1370), f 0 (1500), f 0 (1710) D 0 : f 2(1270), f 2 (1525) Exponentially damped coherent background terms 25 Boris Grube, TU München Hadron Spectroscopy at

Fit of K + K Mass Dependence Comparison with WA102 WA102 PL B453 (1999) 305 450 GeV/c p beam Fit of wave intensities only 26 Boris Grube, TU München Hadron Spectroscopy at

PWA of p p p fast K + K p recoil Summary Outlook Order of magnitude larger data samples for K + K and π + π CP than previous experiments Background from p diffraction mostly at larger meson-pair masses Some irreducible contribution from RP and RR processes Observation of ρ(770) (in π + π ) and φ(1020) (in K + K ) Simplistic resonance model is able to describe data Breit-Wigner amplitudes Phenomenological parametrization of non-resonant terms Improved model needed to reliably extract resonance parameters and yields Better separation of non-pp contributions by performing analysis in bins of the two four-momentum transfers t 1,2 More realistic amplitudes for mass-dependent fit Combined fit of available channels (π + π, π 0 π 0, K + K, K 0 SK 0 S,... ) 27 Boris Grube, TU München Hadron Spectroscopy at

But wait... There s More! Other diffractively produced channels Pion beam: π π + π, π π 0 π 0, π ηη, π π 0 ω, KKπ, KKππ, π π + π π + π,... Kaon beam: K π + π Proton beam: baryon resonances Pomeron-induced E.g. p p pπ + π p recoil Other production reactions Primakoff reaction Radiative coupling of a 2 (1320) and π 2 (1670) EPJA 50 (2014) 79 is a unique experiment to study light-quark hadron spectroscopy 28 Boris Grube, TU München Hadron Spectroscopy at

But wait... There s More! Other diffractively produced channels Pion beam: π π + π, π π 0 π 0, π ηη, π π 0 ω, KKπ, KKππ, π π + π π + π,... Kaon beam: K π + π Proton beam: baryon resonances Pomeron-induced E.g. p p pπ + π p recoil p beam p target P X + 50 10 6 pπ + π events p π + p recoil Other production reactions Primakoff reaction Radiative coupling of a 2 (1320) and π 2 (1670) EPJA 50 (2014) 79 is a unique experiment to study light-quark hadron spectroscopy 28 Boris Grube, TU München Hadron Spectroscopy at

But wait... There s More! Other diffractively produced channels Pion beam: π π + π, π π 0 π 0, π ηη, π π 0 ω, KKπ, KKππ, π π + π π + π,... Kaon beam: K π + π Proton beam: baryon resonances Pomeron-induced E.g. p p pπ + π p recoil Other production reactions Primakoff reaction Radiative coupling of a 2 (1320) and π 2 (1670) EPJA 50 (2014) 79 beam X γ ( ) Z π π + π invariant mass π + Z is a unique experiment to study light-quark hadron spectroscopy 28 Boris Grube, TU München Hadron Spectroscopy at