Observation of a fine structure in e+e− → hadrons production at the nucleon-antinucleon threshold

R.R Akhmetshin,I.B Logashenko,D.N Grigoriev,A.L Erofeev,D.N Shemyakin,G.P Razuvaev,V.Sh Banzarov,A.A Grebenuk,N.M Ryskulov,A.E Ryzhenenkov,A.V Bragin,V.S Okhapkin,N.S Bashtovoy,A.N Amirkhanov,S.S Gribanov,A.E Kuzmenko,S.E Gayazov,L.B Epshteyn,A.V Otboev,F.V Ignatov,A Talyshev ,Aleksei Kozyrev ,A Bondar ,P Krokovny ,I M Zemlyansky ,A N Kirpotin ,K Mikhailov ,V Shebalin ,A I Senchenko ,P Yu Shatunov ,S V Karpov ,D E Berkaev ,E A Kozyrev ,A Korobov ,A Sibidanov ,В Ф Казанин ,А.а Рубан ,S Eidelman ,D B Shwartz ,С С Толмачев ,Y Pestov ,А.і Воробйов ,A V Anisenkov ,G V Fedotovich ,В Л Иванов ,A Kuzmin ,I A Koop ,В.м Титов ,D Epifanov ,V Aulchenko ,Yu M Shatunov ,E P Solodov ,B Shwartz ,P Lukin ,А С Попов ,Yu V Yudin

doi:10.1016/j.physletb.2019.05.032

Abstract

A study of hadron production at the nucleon-antinucleon threshold has been performed with the CMD-3 detector at the VEPP-2000 e+e− collider. The very fast rise with about 1 MeV width has been observed in the e+e−→pp¯ cross section. A sharp drop in the e+e−→3(π+π−) cross section has been confirmed and found to have a less than 2 MeV width, in agreement with the observed fast rise of the e+e−→pp¯ cross section. For the first time a similar sharp drop is demonstrated in the e+e−→K+K−π+π− cross section. The behavior of the e+e−→3(π+π−),K+K−π+π− cross sections cannot be explained by an interference of any resonance amplitude with continuum, therefore this phenomenon cannot be due to a narrow near-threshold resonance. No such structure has been observed in the e+e−→2(π+π−) cross section.

Highlights

Production of six pions in e+e− annihilation, studied at DM2 [1,2,3], showed a “dip” in the cross section at about 1.9 GeV, confirmed later by the Fermilab E687 experiment in photoproduction [4,5], and with a much larger effective integrated luminosity at BaBar [6] using initial-state radiation (ISR)
In this paper we present the analysis of 50 pb−1 of integrated luminosity collected with the CMD-3 detector [18] at 29 c.m. energy points at the VEPP-2000 collider with the upgraded injection complex [19,20,21,22]
The energy position of the “dip” in the e+e− → K + K −π +π − cross sections, observed for the first time, is consistent with the nnproduction threshold, while that for the e+e− → 3(π +π −) reaction is close to the p pthreshold

Summary

Introduction

Production of six pions in e+e− annihilation, studied at DM2 [1,2,3], showed a “dip” in the cross section at about 1.9 GeV, confirmed later by the Fermilab E687 experiment in photoproduction [4,5], and with a much larger effective integrated luminosity. A measurement of the CMD-3 Collaboration [8] confirmed these observations and demonstrated that the drop in the e+e− → 3(π +π −) cross section occurred in the narrow energy range of less than 10 MeV width. For the new data we have reproduced all steps for selection of four charged tracks, pion-kaon separation procedure, and the calculation of the efficiency and radiative corrections. The same overall statistical accuracy is achieved since the scan around the N Nthreshold is performed with large integrated luminosity that allows us to select about 1500 signal events per energy point. At each energy a visible cross section is calculated as the number of selected events divided by the detection efficiency and integrated luminosity. We estimate the systematic uncertainty as about 10%, dominated by the uncertainty in the efficiency calculation: a special study was performed to estimate data-MC difference in the reconstruction efficiency

The N Nthreshold region

Conclusion