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The KLOE-2 experiment at the upgraded e+e− DAΦNE collider of the INFN Laboratori Nazionali di Frascati completed its data taking campaign at the end of March 2018, collecting 5.5 fb−1 at the center of mass energy corrisponding to the mass of the ϕ-meson. Together with the data set of its predecessor KLOE, the acquired data sample of 8 fb−1 corresponds to 2.4×1010 ϕ-meson produced: the largest sample ever collected at the ϕ(1020) at e+e− colliders. KLOE-2 Collaboration activities are now focused on data reconstruction and analysis, continuing the KLOE long-standing tradition of flavour physics precision measurements in the kaon sector, to probe CKM unitarity and search for dark force mediator. Latest results on KS rare decays will be presented and discussed in the framework of Flavour Physics and CP Violation tests, focusing on the measurement of KS semileptonic branching ratios, using 1.7 fb−1 KLOE data, and the search for the pure CP-violating KS → 3π0 decay.

The possibility to accelerate electron beams to ultra-relativistic velocities over short distances by using plasma-based technology holds the potential for a revolution in the field of particle accelerators 1 – 4 . The compact nature of plasma-based accelerators would allow the realization of table-top machines capable of driving a free-electron laser (FEL) 5 , a formidable tool to investigate matter at the sub-atomic level by generating coherent light pulses with sub-ångström wavelengths and sub-femtosecond durations 6 , 7 . So far, however, the high-energy electron beams required to operate FELs had to be obtained through the use of conventional large-size radio-frequency (RF) accelerators, bound to a sizeable footprint as a result of their limited accelerating fields. Here we report the experimental evidence of FEL lasing by a compact (3-cm) particle-beam-driven plasma accelerator. The accelerated beams are completely characterized in the six-dimensional phase space and have high quality, comparable with state-of-the-art accelerators 8 . This allowed the observation of narrow-band amplified radiation in the infrared range with typical exponential growth of its intensity over six consecutive undulators. This proof-of-principle experiment represents a fundamental milestone in the use of plasma-based accelerators, contributing to the development of next-generation compact facilities for user-oriented applications 9 . Using a compact, particle-beam-driven plasma-based accelerator to accelerate high-quality electron beams that are completely characterized in the six-dimensional phase space, free-electron lasing  is observed with narrow-band amplified radiation in the infrared range.

A bstract The ratio R = Γ( K S → πeν ) / Γ( K S → π + π − ) has been measured with a sample of 300 million K S mesons produced in ϕ → K L K S decays recorded by the KLOE experiment at the DAΦNE e + e − collider. K S → πeν events are selected by a boosted decision tree built with kinematic variables and time-of-flight measurements. Data control samples of K L → πeν decays are used to evaluate signal selection efficiencies. With 49647 ± 316 signal events we measure R = (1 . 0421 ± 0 . 0066 stat ± 0 . 0075 syst ) × 10 − 3 . The combination with our previous measurement gives R = (1 . 0338 ± 0 . 0054 stat ± 0 . 0064 syst ) × 10 − 3 . From this value we derive the branching fraction B ( K S → πeν ) = (7 . 153 ± 0 . 037 stat ± 0 . 044 syst ) × 10 − 4 and f + (0) |V us | = 0 . 2170 ± 0 . 009.

The Free-Electron Laser facility of the EuPRAXIA@SPARC_LAB infrastructure is driven by an electron beam with 1 GeV energy, produced by an X-band normal conducting linear accelerator followed by a plasma wakefield acceleration stage. The AQUA beamline aims at delivering variable polarization photons in the 3-4 nm wavelength range by means of out-of-vacuum APPLE-X permanent magnet undulators with 18 mm period length. The main radiator is composed by an array of ten APPLE-X 2 m-long modules. The current AQUA design is investigated and discussed taking into account effects on the FEL performance from off-axis injection as well as from modelling of the resistive wall wakefields in the foreseen vacuum chamber.

A bstract The quantum interference between the decays of entangled neutral kaons is studied in the process ϕ → K S K L → π + π − π + π − , which exhibits the characteristic Einstein-Podolsky-Rosen correlations that prevent both kaons to decay into π + π − at the same time. This constitutes a very powerful tool for testing at the utmost precision the quantum coherence of the entangled kaon pair state, and to search for tiny decoherence and CPT violation effects, which may be justified in a quantum gravity framework. The analysed data sample was collected with the KLOE detector at DAΦNE, the Frascati ϕ -factory, and corresponds to an integrated luminosity of about 1.7 fb − 1 , i.e. to about 1 . 7 × 10 9 ϕ → K S K L decays produced. From the fit of the observed ∆ t distribution, being ∆ t the difference of the kaon decay times, the decoherence and CPT violation parameters of various phenomenological models are measured with a largely improved accuracy with respect to previous analyses. The results are consistent with no deviation from quantum mechanics and CPT symmetry, while for some parameters the precision reaches the interesting level at which — in the most optimistic scenarios — quantum gravity effects might show up. They provide the most stringent limits up to date on the considered models.

A bstract Using 1.63 fb −1 of integrated luminosity collected by the KLOE experiment about 7 × 10 4 K S → π ± e ∓ ν decays have been reconstructed. The measured value of the charge asymmetry for this decay is A S = (−4.9 ± 5.7 stat ± 2.6 syst ) × 10 −3 , which is almost twice more precise than the previous KLOE result. The combination of these two measurements gives A S = (−3.8 ± 5.0 stat ± 2.6 syst ) × 10 −3 and, together with the asymmetry of the K L semileptonic decay, provides significant tests of the CPT symmetry. The obtained results are in agreement with CPT invariance.

A bstract Based on an integrated luminosity of 1.61 fb − 1 e + e − collision data collected with the KLOE detector at DAΦNE, the Frascati ϕ -factory, a search for the P - and CP -violating decay η → π + π − has been performed. Radiative ϕ → ηγ decay is exploited to access the η mesons. No signal is observed in the π + π − invariant mass spectrum, and the upper limit on the branching fraction at 90% confidence level is determined to be ℬ( η → π + π − ) < 4 . 9 × 10 − 6 , which is approximately three times smaller than the previous KLOE result. From the combination of these two measurements we get ℬ( η → π + π − ) < 4 . 4 × 10 − 6 at 90% confidence level.

A bstract We present a measurement of the doubly radiative decay η → π 0 γγ based on a sample of 82 million η mesons produced in the e + e − → ϕ → ηγ process at the Frascati ϕ -factory DAΦNE. From the data analysis, 1246 ± 133 signal events were observed. By normalising the signal to the well-known η → 3 π 0 decay the branching fraction is measured to be (0 . 98 ± 0 . 11 stat ± 0 . 14 syst ) × 10 − 4 . This result agrees with a preliminary KLOE measurement, but is a factor of two smaller than the current world average. Results for d Γ( η → π 0 γγ ) /dM 2 ( γγ ) are also presented and compared with the latest theoretical predictions.

The three precision measurements of the cross section σ(e+e− → π+π−γ(γ)) using initial state radiation by the KLOE collaboration provide an important input for the prediction of the hadronic contribution to the anomalous magnetic moment of the muon. These measurements are correlated for both statistical and systematic uncertainties and, therefore, the simultaneous use of these measurements requires covariance matrices that fully describe the correlations. We present the construction of these covariance matrices and use them to determine a combined KLOE measurement for σ(e+e− → π+π−γ(γ)). We find, from this combination, a two-pion contribution to the muon magnetic anomaly in the energy range 0.10 < s < 0.95 GeV2 of aμπ+π−=489.8±1.7stat±4.8sys×10−10.Data vectors and covariance matrices are available at http://www.lnf.infn.it/kloe/ppg/ppg_2017/ppg_2017.html.