download report - Sapienza

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