arXiv subjects

A. Pyszniak

Publications and source records attributed to A. Pyszniak.

At least 19 recordsLinked to original sources

arXiv2020

Isoscalar Single-Pion Production in the Region of Roper and $d^*(2380)$ Resonances

Exclusive measurements of the quasi-free $pn \to ppπ^-$ and $pp \to ppπ^0$ reactions have been performed by means of $pd$ collisions at $T_p$ = 1.2 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region $T_p = 0.95 - 1.3$ GeV ($\sqrt s$ = 2.3 - 2.46 GeV), which includes the regions of $Δ(1232)$, $N^*(1440)$ and $d^*(2380)$ resonance excitations. From these measurements the isoscalar single-pion production has been extracted, for which data existed so far only below $T_p$ = 1 GeV. We observe a substantial increase of this cross section above 1 GeV, which can be related to the Roper resonance $N^*(1440)$, the strength of which shows up isolated from the $Δ$ resonance in the isoscalar $(Nπ)_{I=0}$ invariant-mass spectrum. No evidence for a decay of the dibaryon resonance $d^*(2380)$ into the isoscalar $(NNπ)_{I=0}$ channel is found. An upper limit of 90 $μ$b (90 $\%$ C.L.) corresponding to a branching ratio of 5 $\%$ has been deduced.

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arXiv2020

Differential Cross Sections for Neutron-Proton Scattering in the Region of the $d^*(2380)$ Dibaryon Resonance

Differential cross sections have been extracted from exclusive and kinematically complete high-statistics measurements of quasifree polarized $\vec{n}p$ scattering performed in the energy region of the $d^*(2380)$ dibaryon resonance covering the the range of beam energies $T_n$ = 0.98 - 1.29 GeV ($\sqrt s$ = 2.32 - 2.44 GeV). The experiment was carried out with the WASA-at-COSY setup having a polarized deuteron beam impinged on the hydrogen pellet target and utilizing the quasifree process $dp \to np + p_{spectator}$. That way the $np$ differential cross section $σ(Θ)$ was measured over a large angular range. The obtained angular distributions complement the corresponding analyzing power $A_y(Θ)$ measurements published previously. A SAID partial-wave analysis incorporating the new data strengthens the finding of a resonance pole in the coupled $^3D_3 - ^3G_3$ waves.

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arXiv2018

Backward single-pion production in the $\boldsymbol{pd\rightarrow {}^3\textrm{He}\,π^0}$ reaction with WASA-at-COSY

New data on the production of single neutral pions in the $pd\rightarrow{}^3\textrm{He}\,π^0$ reaction are presented. For fifteen proton beam momenta between $p_p=1.60\;\textrm{GeV}/c$ and $p_p=1.74\;\textrm{GeV}/c$, differential cross sections are determined over a large fraction of the backward hemisphere. Since the only previous systematic measurements of single-pion production at these energies were made in collinear kinematics, the present work constitutes a significant extension of the current knowledge on this reaction. Even this far above the production threshold, significant changes are found in the behaviour of the angular distributions over small intervals in beam momentum.

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arXiv2018

Importance of d-wave contributions in the charge symmetry breaking reaction $dd \to {}^4\text{He}π^0$

This letter reports a first quantitative analysis of the contribution of higher partial waves in the charge symmetry breaking reaction $dd \to {}^4\text{He}π^0$ using the WASA-at-COSY detector setup at an excess energy of $Q = 60$ MeV. The determined differential cross section can be parametrized as $\text{d}σ/\text{d}Ω= a + b\cos^{2}θ^*$, where $θ^*$ is the production angle of the pion in the center-of-mass coordinate system, and the results for the parameters are $a = \left(1.55 \pm 0.46 (\text{stat}) ^{+0.32}_{-0.8} (\text{syst}) \right)$ pb/sr and $b = \left(13.1 \pm 2.1 (\text{stat}) ^{+1.0}_{-2.7} (\text{syst})\right)$ 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 $dd \to {}^4\text{He}π^0$ reaction and is, therefore, crucial for a quantitative understanding of quark mass effects in nuclear production reactions.

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arXiv2018

Search for $C$ violation in the decay $η\rightarrowπ^0+e^++e^-$ with WASA-at-COSY

We report on the investigation of the rare decay $η\rightarrowπ^0+\text{e}^++\text{e}^-$ which is of interest to study both $C$ violation in the electromagnetic interaction and to search for contributions from physics beyond the Standard Model, since the allowed decay via a two-photon intermediate state is strongly suppressed. The experiment has been performed using the WASA-at-COSY installation, located at the COSY accelerator of the Forschungszentrum Jülich, Germany. In total $3\times10^7$ events of the reaction $\text{p}+\text{d}\rightarrow{^3\text{He}}+η$ have been recorded at an excess energy of $Q = 59.8\,\textrm{MeV}$. Based on this data set the $C$ parity violating decay $η\rightarrowπ^0+γ^*\rightarrowπ^0+\textrm{e}^++\textrm{e}^-$ via a single-photon intermediate state has been searched for, resulting in new upper limits of $Γ\left(η\rightarrowπ^0+\textrm{e}^++\textrm{e}^-\right)/ Γ\left(η\rightarrowπ^++π^-+π^0\right) < 3.28\times10^{-5}$ and $Γ\left(η\rightarrowπ^0+\textrm{e}^++\textrm{e}^-\right)/ Γ\left(η\rightarrow\textrm{all}\right) < 7.5\times10^{-6}\ (\textrm{CL} = 90\,\%)$, respectively.

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arXiv2018

Total and differential cross sections of $η$-production in proton-deuteron fusion for excess energies between $Q_η=13\;\text{MeV}$ and $Q_η=81\;\text{MeV}$

New data on both total and differential cross sections of the production of $η$ mesons in proton-deuteron fusion to ${}^3\text{He}\,η$ in the excess energy region $13.6\;\text{MeV}\leq Q_η\leq 80.9\;\text{MeV}$ are presented. These data have been obtained with the WASA-at-COSY detector setup located at the Forschungszentrum Jülich, using a proton beam at 15 different beam momenta between $p_p = 1.60\;\text{GeV}/c$ and $p_p = 1.74\;\text{GeV}/c$. While significant structure of the total cross section is observed in the energy region $20\;\text{MeV}\lesssim Q_η\lesssim 60\;\text{MeV}$, a previously reported sharp variation around $Q_η\approx 50\;\text{MeV}$ cannot be confirmed. Angular distributions show the typical forward-peaking that was reported elsewhere. For the first time, it is possible to study the development of these angular distributions with rising excess energy over a large interval.

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arXiv2017

Spin dependence of η meson production in proton-proton collisions close to threshold

Taking advantage of the high acceptance and axial symmetry of the WASA-at-COSY detector, and the high degree of the polarized proton beam of COSY, the reaction pp{\to} ppη has been measured close to threshold to explore the analyzing power Ay. The angular distribution of Ay is determined with the precision improved by more than one order of magnitude with respect to previous results allowing a first accurate comparison with theoretical predictions. The determined analyzing power is consistent with zero for an excess energy of Q = 15 MeV signaling s wave production with no evidence for higher partial waves. At Q = 72 MeV the data reveals strong interference of P s and P p partial waves and cancellation of (P p) and Ss*Sd contributions. These results rule out the presently available theoretical predictions for the production mechanism of the η meson.

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arXiv2016

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.

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arXiv2016

Measurement of the $ω\to π^+ π^- π^0$ Dalitz plot distribution

Using the production reactions $pd\to {}^3\mbox{He}\,ω$ and $pp\to ppω$, the Dalitz plot distribution for the $ω\to π^+ π^- π^0$ decay is studied with the WASA detector at COSY, based on a combined data sample of $ (4.408\pm 0.042) \times 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 $1+2αZ$, with $α$ determined to be $+0.147\pm0.036$, consistent with the expectations of $ρ$-meson-type final-state interactions of the $P$-wave pion pairs.

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arXiv2016

Feasibility study for the measurement of $πN$ TDAs at PANDA in $\bar{p}p\to J/ψπ^0$

The exclusive charmonium production process in $\bar{p}p$ annihilation with an associated $π^0$ meson $\bar{p}p\to J/ψπ^0$ is studied in the framework of QCD collinear factorization. The feasibility of measuring this reaction through the $J/ψ\to e^+e^-$ decay channel with the PANDA (AntiProton ANnihilation at DArmstadt) experiment is investigated. Simulations on signal reconstruction efficiency as well as the background rejection from various sources including the $\bar{p}p\toπ^+π^-π^0$ and $\bar{p}p\to J/ψπ^0π^0$ reactions are performed with PandaRoot, the simulation and analysis software framework of the PANDA experiment. It is shown that the measurement can be done at PANDA with significant constraining power under the assumption of an integrated luminosity attainable in four to five months of data taking at the maximum design luminosity.

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arXiv2016

Feasibility studies of time-like proton electromagnetic form factors at PANDA at FAIR

Simulation results for future measurements of electromagnetic proton form factors at \PANDA (FAIR) within the PandaRoot software framework are reported. The statistical precision with which the proton form factors can be determined is estimated. The signal channel $\bar p p \to e^+ e^-$ is studied on the basis of two different but consistent procedures. The suppression of the main background channel, $\textit{i.e.}$ $\bar p p \to π^+ π^-$, is studied. Furthermore, the background versus signal efficiency, statistical and systematical uncertainties on the extracted proton form factors are evaluated using two different procedures. The results are consistent with those of a previous simulation study using an older, simplified framework. However, a slightly better precision is achieved in the PandaRoot study in a large range of momentum transfer, assuming the nominal beam conditions and detector performance.

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arXiv2016

Measurement of the $\vec{n}p \to dπ^0π^0$ Reaction with Polarized Beam in the Region of the $d^*(2380)$ Resonance

We report on a high-statistics measurement of the most basic double pionic fusion reaction $\vec{n}p \to dπ^0π^0$ 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.

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arXiv2015

Measurements of Branching Ratios for $η$ Decays into Charged Particles

The WASA-at-COSY experiment has collected $3\times10^{7}$ events with $η$-mesons produced via the reaction $pd\rightarrow{^{3}\textrm{He}}η$ at $\textrm{T} = 1.0 \textrm{GeV}$. Using this data set, we evaluate the branching ratios of the decays $η\toπ^{+}π^{-}γ$, $η\to e^{+}e^{-}γ$, $η\toπ^{+}π^{-}e^{+}e^{-}$ and $η\to e^{+}e^{-}e^{+}e^{-}$. The branching ratios are normalized to the $η\rightarrowπ^{+}π^{-}π^{0}$ decay. In addition an upper limit on a $CP$-violating asymmetry in $η\rightarrow π^{+}π^{-}e^{+}e^{-}$ is extracted.

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arXiv2014

Charge Symmetry Breaking in dd->4Heπ0 with WASA-at-COSY

Charge symmetry breaking (CSB) observables are a suitable experimental tool to examine effects induced by quark masses on the nuclear level. Previous high precision data from TRIUMF and IUCF are currently used to develop a consistent description of CSB within the framework of chiral perturbation theory. In this work the experimental studies on the reaction dd->4Heπ0 have been extended towards higher excess energies in order to provide information on the contribution of p-waves in the final state. For this, an exclusive measurement has been carried out at a beam momentum of p=1.2 GeV/c using the WASA-at-COSY facility. The total cross section amounts to sigma(tot) = (118 +- 18(stat) +- 13(sys) +- 8(ext)) pb and first data on the differential cross section are consistent with s-wave pion production.

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arXiv2014

Measurement of the $np \to npπ^0π^0$ Reaction in Search for the Recently Observed $d^*(2380)$ Resonance

Exclusive measurements of the quasi-free $np \to npπ^0π^0$ reaction have been performed by means of $dp$ collisions at $T_d$ = 2.27 GeV using the WASA detector setup at COSY. Total and differential cross sections have been obtained covering the energy region $\sqrt s$ = (2.35 - 2.46) GeV, which includes the region of the ABC effect and its associated $d^*(2380)$ resonance. Adding the $d^*$ resonance amplitude to that for the conventional processes leads to a reasonable description of the data. The observed resonance effect in the total cross section is in agreement with the predictions of Fäldt and Wilkin as well Albadajedo and Oset. The ABC effect, {\it i.e.} the low-mass enhancement in the $π^0π^0$-invariant mass spectrum, is found to be very modest - if present at all, which might pose a problem to some of its interpretations.

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arXiv2014

ABC Effect and Resonance Structure in the Double-Pionic Fusion to $^3$He

Exclusive and kinematically complete measurements of the double pionic fusion to $^3$He have been performed in the energy region of the so-called ABC effect, which denotes a pronounced low-mass enhancement in the $ππ$-invariant mass spectrum. The experiments were carried out with the WASA detector setup at COSY. Similar to the observations in the basic $pn \to d π^0π^0$ reaction and in the $dd \to ^4$He$π^0π^0$ reaction, the data reveal a correlation between the ABC effect and a resonance-like energy dependence in the total cross section. Differential cross sections are well described by the hypothesis of $d^*$ resonance formation during the reaction process in addition to the conventional $t$-channel $ΔΔ$ mechanism. The deduced $d^*$ resonance width can be understood from collision broadening due to Fermi motion of the nucleons in initial and final nuclei.

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arXiv2014

Neutron-Proton Scattering in the Context of the $d^*$(2380) Resonance

New data on quasifree polarized neutron-proton scattering, in the region of the recently observed $d^*$ resonance structure, have been obtained by exclusive and kinematically complete high-statistics measurements with WASA at COSY. This paper details the determination of the beam polarization, checks of the quasifree character of the scattering process, on all obtained $A_y$ angular distributions and on the new partial-wave analysis, which includes the new data producing a resonance pole in the $^3D_3$-$^3G_3$ coupled partial waves at ($2380\pm10 - i40\pm5$) MeV -- in accordance with the $d^*$ dibaryon resonance hypothesis. The effect of the new partial-wave solution on the description of total and differential cross section data as well as specific combinations of spin-correlation and spin-transfer observables available from COSY-ANKE measurements at $T_d$ = 2.27 GeV is discussed.

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arXiv2014

Cross section ratio and angular distributions of the reaction p + d -> 3He + eta at 48.8 MeV and 59.8 MeV excess energy

We present new data for angular distributions and on the cross section ratio of the p + d -> 3He + eta reaction at excess energies of Q = 48.8 MeV and Q = 59.8 MeV. The data have been obtained at the WASA-at-COSY experiment (Forschungszentrum Jülich) using a proton beam and a deuterium pellet target. While the shape of obtained angular distributions show only a slow variation with the energy, the new results indicate a distinct and unexpected total cross section fluctuation between Q = 20 MeV and Q = 60 MeV, which might indicate the variation of the production mechanism within this energy interval.

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