Origin of the structures observed ine+e−annihilation into multipion states around thep¯pthreshold

Abstract

We analyze the origin of the structures observed in the reactions e + e − → 3(π + π − ), 2(π + π − π 0 ), ωπ + π − π 0 , and e + e − → 2(π + π − )π 0 around the antiproton-proton (pp) threshold. We calculate the contribution of the two-step process e + e − →N N → multipions to the total reaction amplitude. The amplitude for e + e − →N N is constrained from near-threshold data on the e + e − →pp cross section and the one forN N → multipions can be likewise fixed from available experimental information, for all those 5π and 6π states. The resulting amplitude for e + e − → multipions turns out to be large enough to play a role for the considered e + e − annihilation channels and, in three of the four reactions, even allows us to reproduce the data quantitatively near theN N threshold. The structures seen in the experiments emerges then as a threshold effect due to the opening of theN N channel.

Muon g-2 and Hadronic Vacuum Polarization: Recent Developments

“…shows the case when all pions are charged with the obvious dip around the NN threshold [21]. Such behavior of the cross section has been observed before at BaBar [22] as well as in much earlier e + e − and photoproduction measurements, therefore suggesting a probable threshold effect due to the opening of the NN channel [23]. Near the NN threshold the behavior of the cross section for the 2π + 2π − 2π 0 final state (see Fig.…”

Muon anomalous magnetic moment and hadronic vacuum polarization: Recent developments

“…8 shows the case when all pions are charged with the obvious dip around the NN threshold [19]. Such behavior of the cross section has been observed before at BaBar [20] and even in much earlier e + e − and photoproduction measurements and is probably a threshold effect due to the opening of the NN channel [21]. The behavior of the cross section for the 2π + 2π − 2π 0 final state (the middle plot in Fig.…”

Study of the process e+e−→π+π−η using initial state radiation

“…Alternatively, the change of the cross section slope near 1.9 GeV may be interpreted without inclusion of a fourth resonance, as a threshold effect due to the opening of the nucleon-antinucleon production channel. Structures near the nucleon-antinucleon threshold are observed in the e þ e − → 3ðπ þ π − Þ and 2ðπ þ π − π 0 Þ cross sections [31,32] as well as in the η 0 π þ π − mass spectrum in the decay J=ψ → γη 0 π þ π − [33]. A slope change near 1.9 GeV is seen in the [34].…”