Near-threshold production of theηmeson via the quasifreepn→pnηreaction

Abstract

Total cross sections for the quasifree pn → pnη reaction in the range from the kinematical threshold up to 20 MeV excess energy have been determined. At threshold they exceed corresponding cross sections for the pp → ppη reaction by a factor of about 3 in contrast to the factor of 6 established for higher excess energies. To large extent, the observed decrease of the ratio σ (pn → pnη)/σ (pp → ppη) toward threshold may be assigned to the different energy dependence of the proton-proton and proton-neutron final-state interactions. The experiment was conducted using a proton beam of the cooler synchrotron COSY and a cluster jet deuteron target. The proton-neutron reactions were tagged by the spectator proton whose momentum was measured for each event. Protons and neutron outgoing from the pn → pnη reaction have been registered by means of the COSY-11 facility, an apparatus dedicated for threshold meson production.

η′ and η mesons with connection to anomalous glue

“…The near-threshold η meson production in nucleonnucleon collisions has been investigated extensively in the CELSIUS, COSY and SATURNE facilities. The results determined by different experiments for the total (Bergdolt et al, 1993;Calen et al, 1996;Chiavassa et al, 1994;Hibou et al, 1998;Moskal et al, , 2010Smyrski et al, 2000) and differential (Abdel-Bary et al, 2003;Moskal et al, , 2010Petren et al, 2010) crosssections for the pp → ppη and for the quasi-free pn → pnη reactions (Calen et al, 1996(Calen et al, , 1998Moskal et al, 2009) are consistent within the estimated uncertainties. In the different experiments η mesons could be produced up to excess energy E of 92 MeV at CELSIUS, 502 MeV at COSY and 593 MeV at SATURNE.…”

“…Measurements of the isospin dependence of η meson production in proton-nucleon collisions revealed that the total cross-section for the quasi-free pn → pnη reaction exceeds the corresponding cross section for pp → ppη by a factor of about three at threshold and by factor of six at higher excess energies between about 25 and 100 MeV (Calen et al, 1998;Moskal et al, 2009). The strong isospin dependence tells us there must be a significant isovector exchange contribution at work in the protonnucleon collisions.…”

“…Using the quasi-free proton-neutron interaction (Moskal et al, 2006) COSY-11 placed an upper bound on σ(pn → pnη ) and the ratio R η = σ(pn → pnη )/σ(pp → ppη ) (Klaja et al, 2010a). For excess energy between 8-24 MeV the upper limit of R η was observed to be consistently one standard deviation below the corresponding ratio for η production (Moskal et al, 2009). In the theoretical limit that η production proceeds entirely through gluonic excitation in the intermediate state this ratio would go to one.…”

Study of theηmeson production with the polarised proton beam

“…More constraints to the theoretical models [5,6,[20][21][22][23][24][25] have been deduced from the determination of the dependence on the isospin of the colliding nucleons [26]. The experiments performed by the WASA/PROMICE collaboration [26] revealed a strong isospin dependence confirmed at threshold by the COSY-11 group [27,28]. All together, the confrontation of predictions based upon different scenarios, involving exchanges of various mesons, with the so far determined unpolarised observables and with the first results on the analysing power indicate the dominance of π exchange in the production process [3].…”

Spin Dependence of η Meson Production in Proton-Proton Collisions Close to Threshold

“…Previous studies by the CELSIUS [10][11][12][13], COSY [14][15][16][17][18], and SATURNE [19][20][21] experiments of the total and differential cross sections for η meson production in pp and pn collisions revealed that the η meson is predominantly produced via the excitation of one of the nucleons to the S 11 current via exchange of virtual mesons with the subsequent decay into the proton-η pair. This conclusion was obtained from the observation of a large η production cross section relative to the η 0 meson production and the isotropic angular distribution of the η mesons in the center-of-mass (c.m.)…”