η-meson production in proton-proton collisions at excess energies of40and72MeV

Abstract

The production of η mesons in proton-proton collisions has been studied with the WASA detector using internal pellet targets in the CELSIUS storage ring at the The Svedberg Laboratory in Uppsala. Data were taken at two beam energies, 1360 MeV and 1445 MeV, corresponding to CM excess energies of 40 and 72 MeV, respectively. The η was detected via its 2γ decay in a near-4π electromagnetic calorimeter, whereas the protons were measured by a combination of straw chambers and plastic scintillator planes in the forward direction. The measurements were kinematically complete. The analysis yielded 69•10 3 events at 1360 MeV and 93•10 3 events at 1445 MeV, with a background contribution of less than 5%. Data were acceptance-corrected using a parametrization of a matrix element which includes all states up to two units of total angular momentum. The final state interaction between protons in the 1 S 0 state was included by a momentum-dependent enhancement factor in the relevant amplitudes. Angular distributions of the final state, invariant mass spectra and Dalitz plots are presented. The cos θ * η-distribution is found to be anisotropic with its maximum at 90 • at both energies. From the parametrization it is inferred that this is due to interference between the Ss and Sd final states. A significant contribution from the Pp final state is also needed to describe data.

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.…”

The legacy of the experimental hadron physics programme at COSY

“…Near threshold we can expect A y dσ/dΩ to be a linear combination of sin θ η and sin 2θ η . Taking the unpolarized angular distribution at this energy from the earlier WASA measurement at CELSIUS [154] shown in Fig. 46, the A y data are well described by…”

“…(4.4). This assumes that the data are dominated by S-wave pp final states but this is in conflict with the differential data at Q = 72 MeV, where the valley along the diagonal of the Dalitz plots shows strong evidence for the production of P p or higher waves [154]. This suggests that the deviations from the curve in Fig.…”

“…At such high excess energies only data at small η angles were accessible and, putting a further cut of cos θ η > 0.95, a preliminary cross section of (4.3 ± 0.8) nb at 55 MeV could be extracted, where only the statistical error is quoted. The CELSIUS-WASA experiment [154] was not very sensitive to this kinematic region but the ANKE value does raise questions regarding the restricted amplitude analysis used at CELSIUS to extrapolate into this region.…”

“…The COSY-TOF experiment also produced angular distributions of both the η and the pp relative momentum [155] and the η differential cross section at 41 MeV is shown in Fig. 46, where it is compared with CELSIUS-WASA measurements at 40 MeV and 72 MeV [154]. The shape of the distribution seems to be better defined by the CELSIUS experiment.…”