meson spectroscopy and partial wave analysis at clas

R. De Vita Meson Spectroscopy and Partial Wave Analysis at CLAS ECT* Trento, January 27 th 2011

Meson Spectroscopy and Partial Wave Analysis at CLAS

Raffaella De VitaINFN – Genova

for the CLAS Collaboration

Workshop on Amplitude Analysis in Hadron Spectroscopy

ECT* Trento, ItalyJanuary 24-28 2011


Meson Production at CLASStudy of meson properties and production represents a large fraction of the physics program at CLAS

• vector meson production at large momentum transfer

• vector meson modification in the nuclear medium

• meson spectroscopy and search for exotics

• …


The first CLAS result

Study of photoproduction at large momentum transfer

• POMERON 2- gluons • Nucleon wave function• Correlations between quarks• u-exchange around -tmax

g p p p K+ K-

E.Anciant et al. PRL 85 4682 (2000)


Meson Production at CLASStudy of meson properties and production represents a large fraction of the physics program at CLAS

• vector meson production at large momentum transfer

• vector meson modification in the nuclear medium

• meson spectroscopy and search for exotics

• … • Search for exotic mesons• Study of scalar mesons• Study of strangeonia• …

• Reveal the nature of the mass of hadrons• Identify the relevant degrees of freedom• Understand the origin of confinement and the

role of gluons• …

R. De Vita Meson Spectroscopy at CLAS and CLAS12: the present and the future MENU2010, Williamsburg, 2 June 2010

Jefferson Lab

A B C

Continuous Electron Beam Accelerator Facility

E: 0.75 –6 GeV Imax: 200mA RF: 1499 MHz Duty Cycle: 100% s(E)/E: 2.5x10-5

Polarization: 80% Simultaneous

distribution to 3 experimental Halls

Injector

LIN

AC LINAC

Experimental Halls

R. De Vita Meson Spectroscopy at CLAS and CLAS12: the present and the future MENU2010, Williamsburg, 2 June 2010

The CLAS detector at JLab


CEBAF Large Acceptance Spectrometer

Cherenkov Counter e/ separation, 256 PMTs

Time of FlightPlastic Scintillator, 684 PMTs

Drift Chamber35,000 cells

Target + g start counter e mini-torus

Electromagnetic Calorimeterlead/plastic scintillator, 1296 PMTs

Torus Magnet6 Superconductive Coils


Hall-B Photon Tagger• Photon beam produced from the

primary electron beam via Bremsstrahlung

• Gold and diamond radiator for In/Coherent Bremsstrahlung

• Energy coverage: 0.2-0.95 E0• Efficiency ~ 80%• Energy Resolution ~ 10-3

• Timing Resolution ~100 ps

Maximum photon energy of 5.7 GeV- Wmax 3.4 GeV

Beam intensity 107 g/s


Search for the Exotic π1(1600)• Candidate for an exotic hybrid

meson• Should belong to JPC=1-+ multiplet• Observed in pion beam

experiments and pp annihilation but with controversial results

• Observed by VES initially in ηπ and then in other decays

• Strongest signal observed by E852 in ρπ, f2π, and others

• E852 results not confirmed by a later analysis of a larger data sample


Search for the Exotic π1(1600)• Candidate for an exotic hybrid

meson• Should belong to JPC=1-+ multiplet• Observed in pion beam

experiments and pp annihilation but with controversial results

• Observed by VES initially in ηπ and then in other decays

• Strongest signal observed by E852 in ρπ, f2π, and others

• E852 results not confirmed by a later analysis of a larger data sample

• Recently positive results reported by COMPASS


Search for the π1(1600) at CLASFirst results from a photoproduction experiment:

• Bremsstrahlung photon beam produced from 5.744 GeV electrons

• Photon flux of 5×107 γ/s• Liquid-hydrogen target

• Search in the γpnπ+π+π-

• Final state selected detecting the 3 pions and using the missing mass technique

• Selection of low –t region and forward-going pions to reduce baryon-resonance background

M. Nozar et al. (CLAS Collaboration),PRL 102, 102002 (2009)


PWA for the 3π channel• Full PWA of selected 83000 events

from G6C data set• PWA based on isobar model

developed from E852 tools• Choice of waves based on observed

dominance of ρ and f2 in the ππ invariant mass

• Different sets of waves used for low mass an high mass portion of the 3 π spectrum

• Spin-density matrix reduced to Rank-2 matrix because of limited statistics

• Unbinned log-likelihood fit with different sets of waves

Final wave set:

13 waves

31 waves


Results from the 3π channel

CLAS-g6C

Possible distortion/leakages because of limited wave set forced by low statistics

Results are consistent with expectations from previous measurements

Demonstrate that PWAs in CLAS are feasible Higher statistics data samples are necessary

for thorough study

Dominance of the 2++ state a2(1320): clear signal at correct mass and width

a2(1320) cross section (0.81±0.25) μb in agreement with previous measurements

No-evidence of exotic 1-+ state π1(1600), upper limit on c.s. of 13.5 nb (95%CL)


Photoproduction of Pion Pairs Study of π+π- production on the

proton and study of scalar mesons

• Bremsstrahlung photon beam: 1.6-3.8 GeV

• 40 cm long liquid hydrogen target• ~7∙109 triggers• Integrated Luminosity ~ 80 pb-1

• Proton and π+ detected in CLAS• Reactionπ γppπ+π- isolated via

missing mass• Analysis focused on high energy (3.0-

3.8 GeV) and low –t (0.4-1.0 GeV2) region


Photoproduction of Pion Pairs1) Extraction of Moments of the Di-Pion Angular Distribution

• Moments can be expressed as bilinear combination of partial wave amplitudes

+…

• Partial Waves are parameterized using Dispersion Relations in term of known phase shifts and unknown coefficients and fitted to the experimental moments

• Partial Wave cross sections are derived and can be compared with models

Short range (QCD) production

Meson formation

e.g. 2 system

2) Evaluation of Partial Waves from Experimental Moments

• Moments are expanded on a set of basis function and weighted with Monte Carlo events to account for detector acceptance and efficiency

• Moments are fitted to the data using an unbinned likelihood procedure


Moments of the Di-Pion Angular Distribution

3.2 GeV < Eg < 3.4 GeV 0.5 GeV2 < -t < 0.6 GeV2

M. Battaglieri et al. (CLAS Collaboration), PRD 80, 072005


Partial Wave Amplitudes M(+-) spectrum below 1.5 GeV:

• P-wave: r meson• D-wave: f2(1270)• S-wave: s, f0(980) and f0(1370)

Moments of the 2-pion angular distribution extracted via likelihood fit of data

Partial Wave fitted to experimental moments

gpp+-

Known states well reproduced, e.g. r(770)

P wave

D wave

S wave

r(770)

f0(980)

f2(1270)

First observation of f0(980) in photoproduction

3.4 GeV < Eg < 3.6 GeV0.5 GeV2 < -t < 0.6 GeV2

M. Battaglieri et al. (CLAS Collaboration), PRL 102, 102001


New High Statistics Photon run: g12Search for new forms of hadronic matter in photoproduction on the proton:

• Data taking completed in 2008• Photon Energy up to 5.5 GeV• More than 26 billion triggers (2-prong + 3-prong)• Total Luminosity: 68 pb-1 (~50 pb-1 for E> 4.4 GeV) • 10 times more statistics than previous CLAS run in this energy range• Data processing completed and physics analysis in progress

Several exclusive channels are being analyzed:γp π+π+π-(n)γp pπ+π-ηγp pK+K- (η/π)γp K+K+ X*- (1530)γp π+K+K-(n)γp e+e-p...

Meson Spectroscopy• Search for exotic mesons• Study of Strangeonia• …

Baryon Spectroscopy• Cascades• …


G12: search for the π1(1400)D. Schott (Florida International U.)

g

p Δ++

X

p

η

π-

π+π

γp➝Δ++ηπ-• All particles detected• By requiring a Δ++ exchanged particles is

constrained to be a pion:➝ X resonance is constrained to be m=1 state➝ From VMD π:ρ exchanges are 9:1

• Structure corresponding to known states are clearly visible in πη mass spectrum

• PWA started !!

PRELIMINARY

PRELIMINARY


G12: search for the π1(1600)C. Bookwalter (Florida State U.)

g

p

X

n

π+

π-

π+

π,ρ

γp➝nπ+π+π-

• 3 pions detected and neutron reconstructed via missing mass

• Low –t region and forward-going pions selected to suppress baryons-resonance background

• ∼850 000 events after all selection cuts

PRELIMINARY


G12: search for the π1(1600)C. Bookwalter (Florida State U.)

g

p

X

n

π+

π-

π+

π,ρ

γp➝nπ+π+π-

• 3 pions detected and neutron reconstructed via missing mass

• Low –t region and forward-going pions selected to suppress baryons-resonance background

• ∼850 000 events after all selection cuts

PWA in progress..

PRELIMINARY


Study of strangeonia• Mesons containing ss pairs• Can be regular states in the quark model or hybrids with/without

exotic quantum numbers• Experimental data still quite sparse, only 7 of the 22 states expected

below 2.2 GeV are widely accepted • Model prediction for width and decays available• Experimental search would require measurement of many different

final states

State Γtot Γi

KK KK* ηφ K*K*, KK1,(2)η'φ, …

φ(1019) Γth=2.5 MeVΓexp=4.26 MeV

φ(1680) Γth=378 MeVΓexp=150 MeV

Γth=89 MeV Γth=245 MeV Γth=44 MeV

φ(2050) Γth=378 MeV Γth=20 MeV Γth=21 MeV Γth=337 MeV

3P0 model, Barnes, Black and Page (2002)


G12: study of strangeoniaM. Saini (Florida State U.)

• Several different final states are being investigated:KK, KK*, ηφ ,…

γp p➝ φη ➝pK+K-η

PRELIMINARYPWA started


Coherent Meson Production on 4He

Strongest evidence of JPC=1-+ 1(1400) exotic meson -p nh0 in E852-Brookhaven

Search for a resonance in P-wave in 0h and 0h’ Known (non-exotic) resonances can be used as a

benchmark (e.g. JPC=2++ a2(1232))

Eliminate s-channel resonance background

Simper PWA: S=I=0 target acts as spin and parity filter for final state mesons Production cross section expected ~ e-bt |A FA(t)|2 low -t kinematic

Meson spectroscopy on 4Heg4He 4He0 h g4He 4He0 h’

gp p+-

r D++


Coherent Production of π0η, π0η’ on 4He

➔ Only C=-1 ω-exchanges is allowed, Natural Parity Exchange (NPE)

➔ The helicity of the produced state will be that of the incoming photon, λ=λγ, production of π0η (π0η’) in S-wave is forbidden

220 +- ++

AAA

dd

C

s

),(Im212 0

2/1max

0 1

+ +

+ LL

L

L

DLLA

;0;00 -AA

),(Im212 101

2/1max

1

+ +

+ LL

L

DLLA

• Resonances decaying to π0η (π0η’) in a P wave is exotic (JPC = 1-+) because of the quantum numbers of the final state (I=1, C=+1, G=-1, and P=(-1)L )

• Scattering off a spin and isospin zero target works as a spin and parity filter

s 22

2

2111 sinsin)1cos5(

27

21cos53 --+

+++ FDPdd

C

Test for above assumptions


EG6: Meson spectroscopy in coherent production on 4He

• search for exotics in h, h’ final states• 6 GeV electron beam on target on high pressure gas target• scattered electron at “0” degrees quasi-real photo-production• recoiling nucleus detected in Radial TPC• hadronic final state detected in CLAS• data taking completed in fall 2009• analysis in progress

Analysis of existing CLAS data shows clear peaks associated to known mesons in

ep(e’)p00

ep(e’)p0h

The Technique works!!

a0(980)f2(1270)f0(980)




Radial TPC with 7atm. He4 Target

Solenoid for forward-focusing of Moeller electrons and bending of recoil nucleus in the TPC

PbWO4 calorimeter for improved photon acceptance at forward angles




RTPC calibration using elastic events on 4He


SummaryRich physics program in meson spectroscopy with CLAS:• Meson spectrum investigated in photoproduction to determine properties of

regular states and search for unconventional configurations• Large statistics data sets are now available• Several decay modes have been/are being investigated• Different PWA frameworks have been developed:

• Isobar model• Moments + Dispersion relations

• First results published, several new analyses in progress...• Present program will be extended with CLAS12 using quasi-real

photoproduction

Better understanding of hadrons structure Progress in understanding confinement in QCD and the role of constituent

quarks and gluons in the description of the non-perturbative regime


Hybrids and Exotics

Normal meson:qq state

m=0, PC=(-1)S+1

qq

Hybrid meson:qqg

gluonic field contribute to quantum numbers

PC≠(-1)S+1

qq

Excitation of the flux tube leads to a new spectrum of hadrons that can have exotic quantum numbers JPC = 0+- , 1-+ , 2+- ...

Lattice QCD calculations predict masses around 2 GeV, a range that can be explored at JLab

Hybrids (qqg) are the ideal system to study qq interaction and the role of gluonsThe existence is not prohibited by QCD but not yet firmly established. A possibility to identify unambiguously a meson as an hybrid state is to look for exotic quantum numbers

Normal mesons (qq) are classified according to their JPC where P=(-1)L+1 C=(-1)L+S

JPC= 0-+ (π,K,η,η’)1-- (ρ,K*,ω,Φ)1+- (b1,K1,h1,h1’)...

S1

S2

L

Some combinations of JPC are not allowed for conventional qq systems but can exist for “unconventional” states as hybrids


Search for Exotics in Photoproduction

Pion BeamQuark spins anti-alignedJPC = 1-- , 1++

Quark spinsalready alignedJPC = 0+- , 1-+ , 2+-

Photon Beam

regular mesons @ Eg = 5GeVX = a2

Exotic meson @ Eg = 8GeVX = 1(1600)

Photoproduction: exotic JPC are more likely produced by S=1 probe

Few data (so far) but expected similar production rate as regular mesons

Production rate for exotics is expected to be comparable to regular mesons

A. Szczepaniak and M. Swat, Phys. Lett. B516 (2001) 72

Signals for exotic hybrids have been reported by various experiments but evidence is not yet fully convincing: more data are needed and experiments with different production modes (different probes) are crucial

meson spectroscopy and partial wave analysis at clas

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