Upper limit on the η → π+π− branching fraction with the KLOE experiment

Abstract

Based on an integrated luminosity of 1.61 fb−1e+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.

Results and perspectives on hadron physics at KLOE/KLOE-2

“…This sample has been combined with the one of the previous KLOE analysis [6] giving: Br ( → + − ) < 4.4 × 10 −6 . Considering instead the full KLOE-KLOE-2 statistics the expected UL on the branching fraction is: Br ( → + − ) < 2.7 × 10 −6 at 90% CL [2].…”

“…A branching fraction of 1.2 × 10 −15 is foreseen allowing additional CP violation phases in the extended Higgs sector. The observation of a branching fraction larger than the above mentioned would be a signal of new sources of CP violation and could be useful to clarify the origin of the observed baryon asymmetry in the Universe (BAU) [2]. The best upper limit (UL) on this branching ratio has been published by the KLOE collaboration analyzing a data set of 350 pb −1 , Br( → + − ) < 1.3 × 10 −5 at 90% confidence level (CL) [6].…”

“…Recently a new limit has been set also by the LHCb Collaboration by using a statistics of ∼ 3.0 fb −1 , Br( → + − ) < 1.6×10 −5 at 90% confidence level (CL) [7]. A new analysis has been performed by the KLOE Collaboration on the search of the → + − decay based on an integrated luminosity of 1.61 fb −1 , of data collected in years 2004 and 2005 [2].…”

“…In November 2014, a new data-taking campaign, referred to as KLOE-2 experiment, has started and has been concluded in March 2018 after collecting a data set of 5.5 fb −1 . The KLOE detector [2] consisted of a large cylindrical drift chamber (DC), surrounded by lead-scintillating fibers electromagnetic calorimeter (EMC) inserted in a 0.52 T magnetic field. Different upgrades to the KLOE detector have been applied in occasion of the KLOE-2 run.…”

“…In this paper recent results on hadron physics obtained with the KLOE data are reported (Sec. 2,3,4) while the studies ongoing at KLOE-2 are discussed in the Sec.5.…”