A. Goswami

Measurements of branching ratios for

The WASA-at-COSY experiment has collected 3×107 events with η mesons produced via the reaction pd→He3η at T=1.0GeV. Using this data set, we evaluate the branching ratios of the decays η→π+π-γ, η→e+e-γ, η→π+π-e+e-, and η→e+e-e+e-. The branching ratios are normalized to the η→π+π-π0 decay. In addition an upper limit on a CP-violating asymmetry in η→π+π-e+e- is extracted.

\eta

Abstract.We report on a high-statistics measurement of the most basic double-pionic fusion reaction

decays into charged particles

\overrightarrow{n} p\rightarrow d\pi^{0}\pi^{0}

Measurement of the (n)over-right-arrowp -g d pi(0) pi(0) reaction with polarized beam in the region of the d(*)(2380) resonance

over the energy region of the d*(2380) resonance by use of a polarized deuteron beam and observing the double fusion reaction in the quasifree scattering mode. The measurements were performed with the WASA detector setup at COSY. The data reveal substantial analyzing powers and confirm conclusions about the d* resonance obtained from unpolarized measurements. We also confirm the previous unpolarized data obtained under complementary kinematic conditions.

An Isotensor Dibaryon in the p p → p p π + π − Reaction?

Exclusive measurements of the quasifree pp→ppπ^{+}π^{-} 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 Δ(1232)Δ(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 ΔN dibaryon resonance with I(J^{P})=2(1^{+}) produced associatedly with a pion is able to overcome these deficiencies.

Measurement of the \omega \rightarrow \pi ^{+}\pi ^{-}\pi ^{0} Dalitz plot distribution

Abstract Using the production reactions p d → He 3 ω and p p → p p ω , the Dalitz plot distribution for the ω → π + π − π 0 decay is studied with the WASA detector at COSY, based on a combined data sample of ( 4.408 ± 0.042 ) × 10 4 events. The Dalitz plot density is parametrised by a product of the P-wave phase space and a polynomial expansion in the normalised polar Dalitz plot variables Z and ϕ. For the first time, a deviation from pure P-wave phase space is observed with a significance of 4.1σ. The deviation is parametrised by a linear term 1 + 2 α Z , with α determined to be + 0.147 ± 0.036 , consistent with the expectations of ρ-meson-type final-state interactions of the P-wave pion pairs.

Search for eta-mesic 4He in the dd->3He n pi0 and dd->3He p pi- reactions with the WASA-at-COSY facility

The search for 4He-eta bound states was performed with the WASA-at-COSY facility via the measurement of the excitation function for the dd->3He n pi0 and dd->3He p pi- processes. The beam momentum was varied continuously between 2.127 GeV/c and 2.422 GeV/c, corresponding to the excess energy for the dd->4He eta reaction ranging from Q=-70 MeV to Q=30 MeV. The luminosity was determined based on the dd->3He n reaction and quasi-free proton-proton scattering via dd->pp n_spectator n_spectator reactions. The excitation functions determined independently for the measured reactions do not reveal a structure which could be interpreted as a narrow mesic nucleus. Therefore, the upper limits of the total cross sections for the bound state production and decay in dd->(4He-eta)_bound->3He n pi0 and dd->(4He-eta)_bound->3He p pi- processes were determined taking into account the isospin relation between both the channels considered. The results of the analysis depend on the assumptions of the N* momentum distribution in the anticipated mesic-4He. Assuming as in the previous works, that this is identical with the distribution of nucleons bound with 20 MeV in 4He, we determined that (for the mesic bound state width in the range from 5 MeV to 50 MeV) the upper limits at 90% confidence level are about 3 nb and about 6 nb for npi0 and ppi- channels, respectively. However, based on the recent theoretical findings of the N*(1535) momentum distribution in the N*-3He nucleus bound by 3.6 MeV, we find that the WASA-at-COSY detector acceptance decreases and hence the corresponding upper limits are 5 nb and 10 nb for npi0 and ppi- channels respectively.

Importance of d-wave contributions in the charge symmetry breaking reaction dd\rightarrow ^{4}He\pi^{0}

Abstract This letter reports a first quantitative analysis of the contribution of higher partial waves in the charge symmetry breaking reaction d d → 4 He π 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 σ / d Ω = a + b cos 2 ⁡ θ ⁎ , where θ ⁎ 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 ) + 0.32 − 0.8 ( 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 Δ isobar to the d d → 4 He π 0 reaction and is, therefore, crucial for a quantitative understanding of quark mass effects in nuclear production reactions.

Backward single-pion production in the pd→3Heπ0 reaction with WASA-at-COSY

Abstract.New data on the production of single neutral pions in the

Backward single-pion production in the

pd\rightarrow {}^{3}\mathrm{He} \pi^{0}