Volume 175, :umber 3 PHYSI[CS LETTERS B 7 August 198~ A SEARCH FOR NARROW STATES IN ANTINEUTRON-PROTON TOTAL AND ANNIHILATION CROSS SECTIONS NEAR NN THRESHOLD Brookhaven ~-Houston :-Pennsyivania State ~-Rice 2 T. ARMSTRONG ~, C. CHU ~, J0 CLEMENT ~, C. ELtNON ~, M. FURIC b K. HARTMAN ~, A. HICKS ~, E. HUNGERFORD u, T. IKJSHIMOTO ~, J. KRUK ~, R. LEWIS ~, D. LOWENSTEIN d, W. LOCHSTET ~, B. MAYES b, B. MOSS *, Go MUTCHLER ~, L. PINSKY u G.A. SMITH ~, L. TANG b W. von WITSCH ~ and Y. XUE u "Departmen~ of Physics, 7"hePennsytvarJa Slate Universit.y, University Park, PA 16802, USA ~'Department of Physics; University ~f ltous~on, tlouston, TX 77004, USA c 7: ~'. Bonnet Nuclear Physics Labora~oo,, Rice University, Houston, TX 77001, USA AGS Department, Brookhaven Nal[onat Laboratory, Upton, NY l ~973, USA Received 29 May ~ 986 The tip total and annihilation cross sections have been measured from near NN %hreshoid ( 1880 MeV) ~o ~, 940 MeV w?,h RMS resoiudon ranging from 0.08 MeV (1880 MeV) to 6.7 MeV (t940 MeV). No significam narrow meson structures were seen, with 90% CL upper ii.m. its of 40- ! 80 mb-MeV on ~F for states with width less ~han our resolution. Combined with increasing unitarity bounds on ~ as one approaches threshold, ~hese iimits confi~e widths of possiNe predicted states below t900 MeV to ~ess than I MeV. Daring the past ten years much experimental ef- fort has been devoted to the search for na~ow (F< 10 MeV) NN states above t~r,~sno~d. Far instance, evi- dence for a s.:P.acture in the total and annihiiation cross sections has been reposed in severa~ 15p for- mation experiments in the S(I936) region [1-6]. However, other experiments have found negative re- sults [% 12 I. Such states have been predicted to ex- ist by both potential model's [13,14] and quark modeis[ 15-18 ] based on q2qa baryonium states. D~e to problems asssociated with low intensities from conventionai I3 sources and energy loss and Coulomb scaUe~ng in low-energy ~ beams, cross s~tions have no~ been published below the ~ 1900 MeV mass° Independently, we have pursued this proNem by developir~g a source of.Ws for measuring tip cross sections for incident momenta !00-500 MeV/c (t 880-1940 MeV mass)~ This method allows Work supposed by ~,he US Depa~ment of Energy. 2 Work supposed in part by the US Depa~ment of Energy, Work supported in part by the US Nadona! Science Foundation. 0370-2693/86/$ 0350 © Elsevier Science Publishers B.V. (North-Ho~ia~'~d Physics Pubiis~ng Division) especiaI!y good resolution due to time-of-flight tech- niques employed to measure the fi momentum. For instance, at 1880 (t 900) MeV the mass resolution is 0°08 (1.4) MeV RMS. Hence, in the heretofore une×plored mass region of 1880-t 900 MeV, we have excellent sensitivity to narrow staten. Furthermore, the ~p system is pure I= I. This is especiaIly impor- tant, as the potentiaI modeh [ 14] predict a ciaster~ ing of.[= i states clone "~othreshold. The experiment was performed in the low-energy separated beam (LESB!I) m; the BrookA'~,aven Na- tiona! Laboratory AGS. The apparatus is shown in fig. !. Antiprotons were identified by tiw, e-of-flight (TOF) between a scintiilation hodoscope (BH, not shown) 3.89 m upstream of scintillation counter $i, along with pulse height (PH) discrimination in coumers S 1 and $2. The beam imensity was typically 2500 .p/s and 5× I0 s ~-/s, with iess ~han i% pion contamination at the trigger love1. The direction of the beam immediately in front of the de~ector was monitored by two (x, y) propo~iona! chambers (PWCI,2). A collimator around the beam efimi- 383