{'fraction_non_alphanumeric': 0.04797563142530778, 'fraction_numerical': 0.02398781571265389, 'mean_word_length': 3.7642079806529627, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 1, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 21, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Hadron Physics at KLOE and KLOE-2

The KLOE Collaboration completed the full data taking on March 2006, acquiring 2.5 fb ^ -1 at the peak of the \phi and other 240 pb ^ -1 off-peak. A new Collaboration is working on a new project, called KLOE-2, to refine and extend the KLOE physics program. We present here some preliminary and published results from the KLOE Collaboration on the pseudoscalar \eta meson and the study of \gamma \gamma processes, that are among the main points of the KLOE-2 physics program.


Introduction
The KLOE experiment has collected 2.5 f b −1 at the peak of the φ resonance at the e + e − collider DAΦNE in Frascati. KLOE has performed several precision measurements, here we present the preliminary analysis of the η → π + π − γ decay channel to study box anomaly and the recently published branching ratio measurement of η → e + e − e + e − decay channel, never observed before. Pseudoscalar production at the φ-factory associated to internal conversion of the photon into a lepton pair allows the measurement of the form factor F(q 2 1 = M(φ) 2 , q 2 2 > 0) of pseudoscalar mesons in the kinematical region of interest for the VMD model: a preliminary study of φ → ηe + e − is based on 739 pb −1 , using the η → π + π − π 0 final state. From a sample of 240 pb −1 taken off the φ resonance, a preliminary analysis of the e + e − → e + e − η process, without e ± tagging in the final state has been performed. The same data set has been used to search for the f 0 (600) produced in γγ interactions via the reaction e + e − → e + e − π 0 π 0 . The KLOE detector is being upgraded with small angle tagging devices, to detect both high and low energy e ± in e + e − → e + e − X events. The inner tracker and small angle calorimeters are scheduled to be installed in a subsequent step, providing wider acceptance for both charged particles and photons. This is the new KLOE-2 project [1]: the detector is successfully rolled in the new DAΦNE interaction region, with a new beam crossing scheme allowing for a reduced beam size and increased luminosity. The main goal of KLOE-2 is to collect an integrated luminosity of about 20 f b −1 in 2-3 years in order to refine and extend the KLOE physics programme.

The Pseudoscalar η meson
The decays η, η ′ → π + π − γ are supposed to get contribution from the anomaly accounted by the Wess Zumino Witten term into the Chiral Perturbation Theory Lagrangian [2]. Those anomalous processes, known as box anomalies, proceed via a vector meson resonant contribution (VDM) and maybe via a direct term. The presence of this direct term affects the partial width value in the case of the η → π + π − γ and the dipion invariant mass distribution, in the case of η ′ → π + π − γ. A comparison of the experimental M π + π − spectra and partial width for η, η ′ meson with theoretical predictions is mandatory to clarify the role of non-resonant contribution to the processes. The η → π + π − γ decay has been measured in 1970 by Gormley [4]. Theoretical papers trying to combine the two measurements found discrepancies in data treatment and problems in obtaining consistent results [2]. In 2007 CLEO Coll. has published the measurement Γ η→π + π − γ /Γ η→π + π − π 0 = 0.175 ± 0.007 ± 0.006, based on 859 η → π + π − γ events [5], which is more then 3σ below the old measurements. KLOE result [6], obtained using 558 pb −1 , gives Γ η→π + π − γ /Γ η→π + π − π 0 = 0.1838 ± 0.0005 stat ± 0.0030 syst , in agreement with the latest CLEO evaluation, providing strong evidence in favour of the box anomaly/direct term.
The knowledge of the η meson coupling to virtual photons is important for calculation of anomalous magnetic moment of the muon, because the pseudoscalar exchange is the major contribution to the hadron light-by-light scattering [1]. In the η → e + e − e + e − process we have conversion decays, which offer the possibility to precisely measure the virtual photon 4-momentum, via the e + and e − 4-momenta and we are directly sensitive to the η meson transition form factor because there are no hadrons among the decay products. The first theoretical evaluation dates from 1967 [7] and predicts a branching ratio BR(η → e + e − e + e − ) = 2.59 × 10 −5 . Double lepton-antilepton η decays have been searched by CMD-2 and WASA, obtaining upper limits at level of the theoretical expectation. KLOE has published the first observation of the η → e + e − e + e − decay, analysing 1.7 f b −1 and identifing 362 ± 29 events which results in a branching ratio of (2.4 ± 0.2 stat+bkg ± 0.1 syst ) × 10 −5 , in agreement with theoretical predictions [8]. Vector-meson-dominance assumption provides good description of photon coupling to hadrons, and, implementing systematic corrections to standard VMD, it correctly describes the ω → π 0 µ + µ − experimental results too. In this framework deviation from standard VMD for the φ → ηe + e − decay spectrum is predicted. The only existing data available come from SND experiment, which has measured the M ee invariant mass distribution with 213 events [9]. KLOE has selected 7000 φ → ηe + e − with η → π + π − π 0 using a sample of 739 pb −1 . Preliminary fit to the M ee using decay parametrization from [10] and F(q 2 ) as from [11], indicates the possibility to reach a 5% error on form factor slope.

Gamma-gamma Physics
The coupling of photons to scalar and pseudoscalar mesons brings information on meson's quark structure and can be measured directly in e + e − colliders via the reaction e + e − → e + e − γ * γ * → e + e − X. Using the Weizsäcker-Williams approximation [12] to understand main qualitative features of the process, when no cuts are applied to the final state leptons, it is possible to evaluate the event yields: N eeX = L ee dF dW γγ σ γγ→X (W γγ )dW γγ the L ee is the integrated luminosity, W γγ is the mass of the γ * γ * and dF/dW γγ the two photons flux function, defined as follows: dF Single π 0 or η production is accessible and this allows to improve determination of two photon decay width of these meson. In particular KLOE is looking for e + e − → e + e − η with η → π + π − π 0 final state: in a preliminary analysis of 240 pb −1 off-peak data about 600 events from η meson, produced in γγ interactions have been disentangled, versus other processes, with a statistical accuracy on Γ γγ comparable with existing measurements. The same off-peak data have been analysed to search for e + e − → e + e − η with η → π 0 π 0 π 0 final state. The question concerning σ/ f 0 (600) meson has been debated for a long time. An indirect evidence comes from φ → π 0 π 0 γ KLOE analysis [13]. The e + e − → e + e − π 0 π 0 process is a clean electromagnetic probe to investigate the question, because it is expected to be plainly affected by σ contribution. Our preliminary analysis on the off-peak data, shows a clear enhancement over estimated backgrounds at low M 4γ ; see  Figure 1: Preliminary spectrum of 4-photons invariant mass with KLOE: dots are data, black line is total MC background, lightblue is K S K L decays, blue is e + e − → ωπ 0 and magenta is φ → f 0 γ ; a clear evidence of e + e − → e + e − π 0 π 0 is given by the excess events at low M 4γ invariant mass is visible and study of differential cross section together with the understanding of the σ → ππ contribution are in progress.
Due to large background from e + e − → γγ(γ), information from e ± taggers already installed at KLOE-2, will be crucial in the analysis of new data to look for the production of σ.
The KLOE experiment with 2.5 f b −1 integrated luminosity at the peak of the φ resonance at the e + e − collider DAΦNE, has published several interesting results. In the next future a new data-taking campaign will be realized by KLOE-2 at the upgraded DAΦNE, with the aim to collect about 20 f b −1 in order to refine and extend the KLOE physics program.