γγ Physics and Transition Form Factors with KLOE/KLOE2

The KLOE results on the measurement of the transition form factors of the η and π0 mesons in φDalitz decays are presented. Also we report in the determination of the Γ(η→ γγ) in γγ collisions, for which KLOE has delivered the most precise measurement up to the date. The prospects for γγ physics of the upgraded detector KLOE-2, which is taking data aiming to collect more than 5 fb−1 by March 2018, are reviewed.


Introduction
The measurement of the Transition Form Factors (TFFs) of pseudo-scalar mesons both in time-like and space-like regions of momentum transfer has been of interest in KLOE-2 and also a main item in the KLOE-2 physics program [1]. The TFFs describe the coupling of the mesons to photon thus providing information about the nature and structure of mesons. More recently the interest on TFFs has grown since they have an essential contribution to the calculation of the hadronic Light-by-Light (LbL)scattering contribution to the anomalous momentum of the muon [2]. Any experimental results both in time-like and space-like q 2 will help to constrain the models used in the calculations. In the case of time-like q 2 TFFs is interesting to measure them in Dalitz decays, for which KLOE has already provided very precise measurements in φ → ηe + e − and φ → π 0 e + e − . KLOE-2 program is also focused in another interesting item, this is γγ physics, by studying reactions such as e + e − → e + e − γ * γ * → e + e − X, where X is a final state with even charge conjugation. The expected number of events is given as a function of the γγ energy in: where L int is the integrated luminosity, σ γγ→X the γγ cross section and dF dW γγ corresponds to the luminosity function in Fig. 1. In DAΦNE, which is operated at the φ center of mass, √ s ∼ M φ , the final states accessible are either single pseudo-scalar X = η, π 0 , or double pion production, X = ππ. Via the cross section, σ γγ→X , we can access the radiative width Γ(X → γγ) of the pseudo-scalar meson and the TFF, F(q 2 1 , q 2 2 ), for space-like q 2 . √ s = 1 GeV. The new upgraded KLOE-2 detector started to take data in November of 2014, with the aim of collecting more than 5 fb −1 at the end of March 2018. In the meanwhile it has collected about 3.9 pb −1 . During this period DAΦNE peak luminosity has been of 2.2 × 10 32 cm −2 s −1 , and the integrated daily luminosity has been of about 10 pb −1 .

KLOE-2 Detector Upgrade
After the installation of a new interaction scheme in DAΦNE in 2008, aiming for higher luminosity, the KLOE detector was prepared for a new data-taking campaign, known now as KLOE-2. Following the start of the DAΦNE commissioning for the KLOE-2 data taking in 2010, the first detector upgrade was the installation of a tagger system for scattered electrons and γγ physics. The tagger system [3] of KLOE-2 consists of two different devices: (1) the Low Energy Tagger (LET) and (2) the High Energy Tagger (HET), referring to the energy of the detected electrons or positron. In 2013 an Inner Tracker [4] made of four layers of cylindrical triple GEM detectors was installed between the interaction point (IP) and the Drift Chamber, to improve the vertex reconstruction for decay vertices close to the IP. Moreover, two crystal calorimeters (CCALT) have been added to cover the acceptance for photons and e ± from the IP down to 10 • [5]. Finally, the DAΦNE focusing quadrupoles, placed inside the KLOE-2 detector, have been instrumented with tungsten and scintillating tile calorimeters (QCALT) [6]. Scattered e ± with E > 400 MeV escape the beam-pipe after the first bending dipole of DAΦNE. In this case the trajectories of the scattered particles are strongly correlated with their energies. The HET was conceived to detect these e ± . Thus, it can be also used as spectrometer, giving a fast feedback on the machine operation. The HET is made of two scintillator hodoscopes readout by PMTs and symmetrically placed at 11 m from the interaction point, see

γγ physics without taggers
The two-photon width of the η meson has been measured by using KLOE data. For this measurement events from e + e − → e + e − η where η → π + π − π 0 and η → π 0 π 0 π 0 . In the absence of taggers the scatter leptons are not detected. To avoid the large background from φ decays, the data collected off-peak at √ s ∼ 1 GeV was used for the analysis, corresponding to an integrated luminosity of about 240 pb −1 . In Fig. 3, the transverse momentum of the π + π − γγ and the squared missing mass is presented. For the 6γ system the squared missing mass and the longitudinal momentum are shown in Fig. 4. From the two dimensional fits of these distributions we obtain the cross sections σ(e + e − → e + e − η) = (34.5 ± 2.5 ± 1.3) pb and σ(e + e − → e + e − η) = (32.0 ± 1.5 ± 0.9) pb for charged and neutral decays of η, respectively. Combining both results, we obtained the cross section σ(e + e − → e + e − η) = (32.7 ± 1.3 ± 0.7) pb, from which we can extract the most precise measurement of the two-photon width: Γ(η → γγ) = (520 ± 20 ± 13) eV, up to date. Figure 3: Left: distribution of the transverse momentum of the π + π − system. Right: distribution of the squared missing mass. The contribution of the signal is blue, e + e − → ηγ is red, e + e − → ωπ 0 is black, e + e − → e + e − γ is green, e + e − → K + K − is blue and e + e − → K S K L is purple.

γγ → π 0 with taggers at KLOE-2
The measurement of the radiative width of the π 0 is an important test of the strong interaction dynamics at low energies. It has been calculated in Chiral Pertubation Theory with an 1.4% uncertainty, Γ(π 0 → γγ) = (8.09 ± 0.11) eV [7] and the most precise experiment measurement up to now it is given by the PrimEx Collaboration, which is based in the Primakoff effect, Γ(π 0 → γγ) = (7.82 ± 0.14 ± 0.17)) eV [8]. Nonetheless, the measurement using the Primakoff effect suffers from some model dependence owing to the conversions in the nucleus field. The π 0 width can be measured at KLOE-2 by using a different process selecting e + e − → e + e − π 0 with quasi-real photons (q 2 0). The event selection requires that the scattered leptons are detected in the HET stations and the two photons from π 0 decay are registered in the calorimeter. In agreement with Monte Carlo simulation, the double HET coincidence efficiency is 1.4%, hence with a cross section of σ(π 0 → γγ) = 0.28 nb, we can expect 2000 events/fb −1 , which allows to reach 1% accuracy in Γ(π 0 → γγ) with the 5 fb −1 expected with KLOE-2. On top of this, the π 0 γ * γ TFF, with a quasi-real photon and a virtual one, can be measured by selecting events where where one lepton is detected in the HET (|q 2 | ≈ 0) and the other one in the calorimeter, at large angle. This way we can investigate an unexplored q 2 region (|q 2 | < 0.1 GeV 2 ), which is important to constrain the TFF parameterizations, see
From the event counting the branching ratio BR(φ → ηe + e − ) = (1.075 ± 0.007 ± .038) × 10 −4 [22] is obtained. Fitting the invariant mass e + e − distribution to the parameterization of ref. [19], by using the one-pole formula for the TFF, we extract the slope Λ −2 φ = (1.17 ± 0.10 0.07 −0.11 ) GeV −2 [22], a factor of 5 times more precise than previous result. This value is consistent with the VMD prediction. The TFF as a function of the e + e − invariant mass is shown in Fig. 9.

Conclusions
The large data sample of light mesons available at KLOE provides important results on decay dynamics and Transition Form Factors, together with limits on new physics, giving the most precise : TFF as a function of the e + e − invariant mass of φ → ηe + e − measurements for TFF for the reactions φ → ηe + e − and φ → π 0 e + e − . The KLOE-2 data-taking, with the upgraded detector, is in progress, with the goal to collect 5 fb −1 by the end of March 2018. A rich program of measurements has been proposed [1]. Due to the new taggers, a special significance has the γγ production of pseudo-scalar mesons, that can help to shed light on some of the still puzzling questions.