Project A: Study of Light Mesons in Two-Photon Interaction
Fritz-Herbert Heinsius, Institut für Experimentalphysik I, Ruhr-Universität Bochum
Motivation: Identify type of bound states
QCD predicts bound states: bosons
• 
• 
• 
• 
• 
Mesons: qq
Hybrids: qqg
Glueballs: gg
Tetraquarks: qqqq
Molecules: bound state of
two mesons
Identify quark/gluonic content by
studying the production mechanisms
Problem
•  Mesons, hybrids, glueballs and tetraquarks
well established
with same quantum numbers can mix
•  Direct identification not possible
not unambiguously
identified
Identification by
à Partial wave
analysis
•  Mass
•  Width
•  Quantum numbers
•  Partial decay widths
Solution
•  Study different production mechanisms
•  QED coupling to two photons (Project A)
•  QCD coupling to gluons (Project B)
Ratio of coupling depends on the content
Simple case
Isosinglet mesons
4
Γ(R → γγ ) 9eQ # α &
=
% (
Γ(R → gg)
2 $ αS '
2
F. Close, G. Farrar, and Z. Li, Phys. Rev. D55 (1997) 5749-5766.
Model for scalar mesons/tetraquarks without mixing
Identification
would be simple
qq
qqqq
M.R. Pennington, Prog.Theor.Phys.Suppl.168 (2007) 143
Methods and tools
e-
J = 0, 2, 3,.. C=+1 possible
J = 1 strongly suppressed
e+
γ
h
γ
h
e-
Compare two-photon decay width with gluon rich
production in charmonium decays (Project B)
Compare mechanisms to X, Y, Z in the charmonium
sector (Projects C, D)
Work program: Channels of special interest
Two photon interaction
e+
Identify quark and gluonic degrees of freedom
Background: e+e- annihilation
only: JPC=1-e
+
h
γ
e-
0 < Q2 < 0.01 GeV2
e+e− → e+e−γγ → e+e− KK
a0(980), f0(980), f2(1270), a2(1320), f0(1370),
a0(1450), f0(1500), f’2(1525), f2(1640), a2(1700),
f0(1710), f2(1810)
e+e− → e+e−γγ → e+e− KK π
h
η(1405), η(1475), η2(1645), π2(1670)
η (1475) → K S0 K ±π ∓
0.01 < Q2 < 0.12 GeV2
Example:
γγàη(1405)/η(1475) at L3
Blue: observed in γγ-interaction
+ −
+ −
+ −
e e → e e γγ → e e ηππ
→ K + K −π 0
η (1405) → K S0 K ±π ∓ / K + K −π 0
→ ηπ +π − / ηπ 0 π 0
η’, η(1295), η(1405), η2(1645), X(1835), η2(1870)
L3, JHEP 03 (2007) 018
e± tagging for BESIII
€
cos θ < 0.93
•  no tag (quasi-real photons)
•  single tag
•  double tag
•  Study production of above and new resonances
•  Compare to production in charmonium decays and with
theory predictions
Common experimental
Existing data sets at BESIII
methods
•  Particle identification
•  Partial wave analysis
•  Data in the 3.8 – 4.6 GeV
energy region
•  Total 8.3 fb-1
and further data taking …