Study of Narrow Mesons near Their Thresholds

Abstract

A novel form of mass spectrometer has been used to measure the masses, widths, and cross sections of the g, w , 2!(958), and @mesons near their respective thresholds in the reaction a-+ p -missing mass + n . The incident momentum is varied in small steps through the threshold while neutrons of a given momentum are detected near zero degrees. The lower limit of the c.m. momentum P* at which measurements have been made ranges from about 50 MeV/c at the @ t o about 30 MeV/c at the 7. A somewhat low value for the w mass, 782.3 1-0.6 MeV, is found. The width of the 2! is < 1.9 MeV (95% confidence level). All four mesons show evidence of S-wave production, with values of u/P* of 21.2 1-1.8, 0.35 i0.03, and 0.2910.06 pb/(MeV/c) for the q, A ! , and @, respectively. A rapid rise in the w cross section appears to be modified by a final-state interaction. The effect of this rise can probably be seen in some Sli pion-nucleon phase-shift solutions. Evidence is also presented of a sudden drop in the s i n -mass spectrum just above the threshold for the production of a KiKpair. The paper includes a comprehensive discussion of the method and of the details of the spectrometer.-

Production of η and η′ mesons on nucleons and nuclei

“…In all cases the s-d interference drops out in the evaluation of |f | 2 . The agreement with the selected older data [127,128,130,133] is reasonable. Also shown in Fig.…”

“…In contrast to the photoproduction case, most of the measurements of the near-threshold π − p → ηn differential cross section were carried out many years ago and some inconsistencies are apparent in the data base [127,128,129,130,131,132,133,134]. There are, however, two more recent experiments where the η meson was reliably identified through its 2γ or 6γ decay mode [135,136].…”

“…(b) Average amplitude-squared of the π − p → ηn reaction showing the results of Ref. [135] (blue stars) and [136] (red crosses) as well as a selection of older data [127,128,130,133] (green circles). The solid (black) line represents the contribution from the N * (1535) isobar with parameters taken from Ref.…”

“…Apart from early bubble chamber work [138,139], there were counter measurements of the π − p → nη cross sections near threshold [130]. [135] and 730 MeV/c (red crosses) [136].…”

Near-Threshold Production ofωMesons in thepp→ppωReaction

“…We here report measurements of the ω production cross section in the range 3.8 ≤ Q ≤ 30 MeV. Though somewhat below the DISTO Q-value, these data suggest that the OZI enhancement is little different to that seen in π − p → nω/φ [5].…”