We report on the experimental search for the bound state of an eta meson and He-3 nucleus performed using the WASA-at-COSY detector setup. In order to search for the eta-mesic nucleus decay, the pd ...-> (3)He2(gamma) and pd -> (3)He6 gamma channels have been analysed. These reactions manifest the direct decay of the ti meson bound in a He-3 nucleus. This non-mesonic decay channel has been considered for the first time. When taking into account only statistical errors, the obtained excitation functions reveal a slight indication for a possible bound state signal corresponding to a He-3-eta nucleus width Gamma above 20 MeV and binding energy B-s between 0 and 15 MeV. However, the determined cross sections are consistent with zero in the range of the systematic uncertainty. Therefore, as final result we estimate only the upper limit for the cross section of the eta-mesic He-3 nucleus formation followed by the eta meson decay which varies between 2 nb and 15 nb depending on possible bound state parameters.
We report on the experimental search for the bound state of an η meson and He3 nucleus performed using the WASA-at-COSY detector setup. In order to search for the η-mesic nucleus decay, the pd→3He2γ ...and pd→3He6γ channels have been analysed. These reactions manifest the direct decay of the η meson bound in a He3 nucleus. This non-mesonic decay channel has been considered for the first time. When taking into account only statistical errors, the obtained excitation functions reveal a slight indication for a possible bound state signal corresponding to a 3He-η nucleus width Γ above 20 MeV and binding energy Bs between 0 and 15 MeV. However, the determined cross sections are consistent with zero in the range of the systematic uncertainty. Therefore, as final result we estimate only the upper limit for the cross section of the η-mesic He3 nucleus formation followed by the η meson decay which varies between 2 nb and 15 nb depending on possible bound state parameters.
We present a search for axion-like particles, \(a\), produced in photon-proton collisions at a center-of-mass energy of approximately 4 GeV, focusing on the scenario where the \(a\)-gluon coupling is ...dominant. The search uses \(a\to\gamma\gamma\) and \(a\to\pi^+\pi^-\pi^0\) decays, and a data sample corresponding to an integrated luminosity of 168 pb\(^{-1}\) collected with the GlueX detector. The search for \(a\to\gamma\gamma\) decays is performed in the mass range of \(180 < m_a < 480\) MeV, while the search for \(a\to\pi^+\pi^-\pi^0\) decays explores the \(600 < m_a < 720\) MeV region. No evidence for a signal is found, and 90% confidence-level exclusion limits are placed on the \(a\)-gluon coupling strength. These constraints are the most stringent to date over much of the mass ranges considered.
The quasifree pp -> pp pi(+)pi(-) reaction has been measured by means of pd collisions at T-p = 1.2 GeV using the WASA detector setup at COSY enabling exclusive and kinematically complete ...measurements. Total and differential cross sections have been extracted for the energy region T-p = 1.08-1.36 GeV (root s = 2.35-2.46 GeV) covering thus the regions of N* (1440 ) and Delta(1232)Delta(1232) resonance excitations. Calculations describing these excitations by t-channel meson exchange as well as isospin relations based on the pp -> pp pi(0)pi(0) data underpredict substantially the measured total cross section. The calculations are also at variance with specific experimental differential cross sections. An isotensor Delta N dibaryon resonance with I(J(P)) = 2(1(+) ) produced associatedly with a pion is able to overcome these deficiencies. Such a dibaryon was predicted by Dyson and Xuong Phys. Rev. Lett. 13, 815 (1964) and more recently calculated by A. Gal and H. Garcilazo Nucl. Phys. A 928, 73 (2014).
Exclusive measurements of the quasifree pp -> pp pi(+)pi(-) reaction have been carried out at WASA@ COSY by means of pd collisions at T-p = 1.2 GeV. Total and differential cross sections have been ...extracted covering the energy region T-p = 1.08-1.36 GeV, which is the region of N* (1440) and Delta(123)Delta(1232) resonance excitations. Calculations describing these excitations by t-channel meson exchange are at variance with the measured differential cross sections and underpredict substantially the experimental total cross section. An isotensor Delta N dibaryon resonance with I(J(P)) = 2(1(+)) produced associatedly with a pion is able to overcome these deficiencies.
This letter reports a first quantitative analysis of the contribution of higher partial waves in the charge symmetry breaking reaction dd -> He-4 pi(0) using the WASA-at-COSY detector setup at an ...excess energy of Q = 60 MeV. The determined differential cross section can be parametrized as d sigma/d Omega = a + b cos(2) theta*, where theta* is the production angle of the pion in the center-of-mass coordinate system, and the results for the parameters are a = (1.55 +/- 0.46(stat)(-0.8)(+0.32)(syst)) pb/sr and b = (13.1 +/- 2.1(stat)(-2.7)(+1.0)(syst)) pb/sr. The data are compatible with vanishing p-waves and a sizable d-wave contribution. This finding should strongly constrain the contribution of the A isobar to the dd -> He-4 pi(0) reaction and is, therefore, crucial for a quantitative understanding of quark mass effects in nuclear production reactions.
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