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Scientific Report 2007-2009<br />
Particle physics<br />
P20. Study of light hadron production and decay with the KLOE<br />
experiment<br />
The KLOE experiment carried out at the Frascati<br />
ϕ−factory DAΦNE has completed its first period of<br />
data-taking with an integrated luminosity of about 2.5<br />
fb −1 collected at the peak of the ϕ(1020) resonance. This<br />
sample corresponds to about 8 × 10 9 ϕ mesons. Large<br />
samples of light hadrons have been produced, through<br />
the radiative decays ϕ → Mγ, where M can be a scalar<br />
or pseudoscalar meson. Since the ϕ is a nearly pure s¯s<br />
state, these decays are unique probes of the properties<br />
and of the internal structure of the light hadrons.<br />
Events/4 MeV<br />
450<br />
400 (η→ γγ)<br />
350<br />
300<br />
250<br />
200<br />
150<br />
100<br />
50<br />
0<br />
650 700 750 800 850 900 950 10001050<br />
M ηπ<br />
(MeV)<br />
Figure 1: a 0(980) shape from the ηπ 0 invariant mass [1].<br />
It is not yet well understood whether the light<br />
scalar mesons (σ(600), f 0 (980), a 0 (980)) are ordinary<br />
q¯q mesons, qq¯q¯q states, or bound states of a K and a<br />
¯K mesons. The scalar production dominates the radiative<br />
decays of the ϕ to two pseudoscalars, and the<br />
decay rates are expected to be sensitive to the internal<br />
structure of those resonances. Precise measurements of<br />
the branching ratios of ϕ → f 0 (980)γ → π 0 π 0 γ and of<br />
ϕ → a 0 (980)γ → ηπ 0 γ have been performed and the<br />
parameters of the scalar resonances have been extracted<br />
respectively from a fit of the Dalitz plot or of the invariant<br />
mass distribution of the two pseudoscalars [1] (see<br />
Fig.1). The f 0 (980) production has also been studied<br />
in e + e − → π + π − γ events, in which a small signal from<br />
e + e − → f 0 (980)γ → π + π − γ has been extracted from<br />
a large background due to Initial State Radiation and<br />
Final State Radiation processes. We obtained branching<br />
ratios of the order of 10 −4 and large couplings of the<br />
scalars to the ϕ meson: these results favour the qq¯q¯q hypothesis,<br />
in which one of the pair is s¯s, for the structure<br />
of the light scalar mesons. Moreover from the analysis of<br />
the Dalitz plot of the ϕ → π 0 π 0 γ we obtained an indication<br />
of the presence of the decay σ(600) → π 0 π 0 . We set<br />
also an upper limit to the branching ratio of ϕ → K 0 ¯K0 γ<br />
[2], which is expected to be dominated by the production<br />
of f 0 /a 0 in the intermediate state.<br />
About 10 8 η and ∼ 5 × 10 5 η ′ mesons have been produced<br />
during the first KLOE data-taking. These samples<br />
have been used to study rare η decays [3] and the<br />
η − η ′ mixing angle, φ P , and to investigate the η ′ structure,<br />
looking for a possible gluonium content in its wave<br />
fuction[4]. We measured the ratio Γ(ϕ → η ′ γ)/Γ(ϕ →<br />
ηγ) = (4.77 ± 0.09 ± 0.19) × 10 −3 and we fit our results<br />
together with other measurements of Vector → Pseudoscalar<br />
+ γ and Pseudoscalar → Vector + γ decay<br />
rates obtaining the result shown in Fig.2, which indicates<br />
a gluonium content in η ′ (Z G ) different from zero<br />
at three standard deviation level.<br />
Z 2 G<br />
0.5<br />
0.45<br />
0.4<br />
0.35<br />
0.3<br />
0.25<br />
0.2<br />
0.15<br />
0.1<br />
0.05<br />
Γ(φ→η'γ)/Γ(φ→ηγ)<br />
Γ(η'→ωγ)/Γ(ω→π 0 γ)<br />
Γ(φ→ηγ)/Γ(ω→π 0 γ)<br />
Γ(η'→γγ)/Γ(π 0 →γγ)<br />
Γ(η'→ργ)/Γ(ω→π 0 γ)<br />
0<br />
30 32 34 36 38 40 42 44 46 48 50<br />
ϕ P<br />
degree<br />
Figure 2: Fit result in the plane Z G vs the η − η ′ mixing<br />
angle: the black ellipse represents the 68% C.L. region [4].<br />
A new data-taking is starting at DAΦNE with<br />
increased luminosity with the KLOE detector upgraded<br />
with the insertion of electron taggers for γγ physics.<br />
We plan to continue the study of the light hadrons by<br />
looking for γγ → σ(600) in e + e − → e + e − ππ events and<br />
by measuring the two-photon decay widths of π 0 , η, η ′<br />
and their transition form factors to γγ.<br />
References<br />
1. F. Ambrosino, et al., Phys. Lett. B 681, 5 (2009).<br />
2. F. Ambrosino, et al., Phys. Lett. B 679, 10 (2009).<br />
3. F. Ambrosino, et al., Phys. Lett. B 675, 283 (2009).<br />
4. F. Ambrosino, et al., JHEP 07, 105 (2009).<br />
Authors<br />
C. Bini, V. Bocci 1 , A. De Santis, G. De Zorzi, A. Di<br />
Domenico, S. Fiore, P. Franzini, P. Gauzzi, F. Lacava, E.<br />
Pasqualucci 1 , M. Testa, P. Valente 1<br />
http://www.roma1.infn.it/exp/kloe/<br />
<strong>Sapienza</strong> Università di Roma 127 Dipartimento di Fisica