High-Energy Photoproduction ofρ′(1600)

Abstract

A resonance is observed in the diffractively photoproduced two-pion final state with a mass of ~ 1600 MeV and a width of ~ 300 MeV. The four-pion distribution is characterized by a width of ~ 600 MeV and peaks around 1500 MeV.The p'(1600) meson has been observed as an enhancement in the four-pion final state both in e + e~ collisions 1 and in photoproduction experiments. 2 "" 5 Estimates of the mass vary from 1430 to 1620 MeV, while estimates of the width vary from 310 to 650 MeV. A nonresonance interpretation of the photoproduction data as a purely kinematic enhancement in the pmi production amplitude also appears to describe these data adequately. 6 Indications of the existence of the two-pion decay mode have come from two photoproduction experiments 5 ' 7 and from a phase-shift analysis of the reaction n~p -+ir + Ti~n. 8 It has been assumed that the branching ratio of p'(1600) into two pions is small, although no published measurement of this number exists.We report here on a study of high-mass twoand four-pion final states photoproduced in the wide-band neutral beam at Fermilab. 9 Our expo-sure consisted of approximately 6 xlO 11 photons with energies greater than 50 GeV incident on a 2-cm-long (5% of a radiation length) scintillator target. The products of the interactions in the target were detected in the multiparticle spectrometer system described in Ref. 9. Two Cherenkov counters with pion thresholds of 6 and 12 GeV were used for particle indentification. A large lead-glass array detected y rays accompanying the charged particles. Two-pion events were recorded under a trigger designed to accept e + e~ pairs. The trigger had a finite efficiency for 7r + 7r pairs; however, uncertainties in the calculation of this efficiency result in a relatively large uncertainty in the absolute normalization for 77 + 7T states. Four-pion events were recorded under a trigger requiring at least three tracks in the spectrometer and a minimum energy deposition of 40 GeV in the hadron calorics)

Observation ofπ+π−π+π−photoproduction in ultraperipheral heavy-ion collisions atAbelev1, Aggarwal2, Ahammed3 et al. 2010Phys. Rev. C

“…In previous photoproduction experiments using carbon targets, the ρ was seen not only in the π + π − π + π − decay mode, but also in π + π − final states [11]. We do not observe a significant ρ signal in the high-mass region of the m π + π − spectrum, as shown in Fig.…”

“…The corresponding upper limit of the ratio is R < 2.5% at 90% confidence level. This is an indication that, in the process measured here, R is smaller than the ratio of the total ρ cross sections in the two-and four-pion channel on a carbon target that was measured to be 6.6 ± 3.4% [11].…”

“…Figure 5 shows the measured π + π − π + π − invariant mass spectrum, which exhibits a broad peak around 1540 MeV/c 2 , indicating resonant ρ production similar to what was seen in fixed-target photoproduction experiments [6][7][8][9][10][11]. This assumes that the peak is dominated by spin states with quantum numbers J P C = 1 −− .…”

“…The OMEGA spectrometer measured photoproduction on proton targets at energies E γ of up to 70 GeV [10]. The heaviest target nucleus used so far to study diffractive two-and four-pion photoproduction was carbon with photon energies between 50 and 200 GeV [11]. These experiments observe a broad structure in the four-pion invariant mass distribution at masses ranging from 1430 ± 50 MeV/c 2 [6] to 1570 ± 60 MeV/c 2 [8] and with widths between 340 ± 60 MeV/c 2 [8] and 850 ± 200 MeV/c 2 [7] that the PDG assigns to the ρ 0 (1700).…”

Semiinclusive Production of Light Mesons by High-Energy Photoproduction

“…The optical theorem relates the imaginary part of the forward elastic scattering amplitude to the total cross-section, giving: The cross-section for the diffractively photoproduced p(770) has been found to be approximately flat over a large range, while the </>(1020) cross-section is found to increase (as shown in Figure 4 [23,24,25,26,27,28,29]). The cross-sections from various experiments are given in Figure 4 and in Table 2 [23,26,30,31,32,33).…”