Search for eta-mesic 3He in non-mesonic final states

In May-June 2014, the experiment devoted to the search for the bound state of the $\eta$ meson and $^3$$He$ nucleus has been performed at COSY accelerator in Research Center J\"ulich in Germany with WASA-at-COSY facility. The excitation functions for $pd\rightarrow^3$$He2\gamma$, $pd\rightarrow^3$$He6\gamma$, $pd\rightarrow ppp\pi^{-}$, $pd\rightarrow ppn\pi^{0}$, $pd\rightarrow dn\pi^{-}$, $pd\rightarrow dp\pi^{0}$, $pd\rightarrow pd$, and $pd\rightarrow ppn$ reactions have been measured in the vicinity of the $\eta$ meson production threshold. The experiment was carried out using COSY ramped proton beam and deuterium pellet target. The beam momentum varied continuously in the range of $1.426-1.635~GeV/c$ corresponding to $^3$$He-\eta$ excess energy range from $-70$ to $+30~MeV$. The analysis of the obtained excitation functions for the $pd\rightarrow^3$$He\eta$ indicate slightly the signal from the bound state for $\Gamma>20~MeV$ and $B\in[0;15]~MeV$. However, the observed indication is in the range of the systematic error. Therefore the final conclusion of this thesis is that no narrow structure that could be interpreted as $\eta$-mesic nucleus was observed in both excitation curves. Thus, the upper limit for the total cross section of the bound state formation was estimated assuming that the $\eta$ decay branching ratios in the bound state is the same as in free space. The upper limit at the $90\%$ confidence level varies from $2$ to $15~nb$ within the binding energy range from $0$ to $60~MeV$ and the width from $2.5$ to $40~MeV$.


Introduction
The η-mesic nuclei were postulated more than 30 years ago [1] and currently are one of the hottest topic in nuclear physics from experimental and theoretical point of view [2][3][4][5][6]. The experimental searches for η-mesic helium-3 have been performed by MAMI [7,8] and COSY11 [9,10] collaborations but no bound state was observed. In the experiment studying η meson photoproduction [7,8], a peak-like structure was observed though its interpretation is still a subject of discussion. In earlier experiments on hadronic η production [9,10], the upper limit for bound state creation cross section upper 70 nb was obtained.
Earlier, WASA-at-COSY collaboration has performed measurements in order to search for η mesic 4 He nucleus [11][12][13][14] and no bound state was observed. In current experiment [15], the statistics gathered is the largest one ever obtained for this experimental conditions. The expected result accuracy is of few nanobarns order of magnitude. In case if the bound state is observed, the width is expected to be larger then the binding energy [16].

Luminosity estimation
One of the most important questions in current experiment is data normalization. For this purpose we obtain luminosity analyzing reactions that have known cross sections. The luminosity as a function of the beam momentum is needed. This dependence can be essential because of small beam trajectory changes during each accelerator cycle that can influence the target overlapping [17]. Luminosity estimation is based on pd → 3 Heη and pd → ppn spectator reactions [18,19].
The registration efficiencies have been obtained from Monte Carlo simulations. For pd → 3 Heη reaction simulation, the cross sections and angular distributions have been taken * e-mail: alexrndl@gmail.com from references [20,23]. For quasielastic proton-proton scattering, the simulation was performed in the frame of spectator model analogously as in analysis described in ref. [21,22]. The distribution of Fermi motion momentum for nucleons in target deuteron was calculated using PARIS model [24] while the proton-proton scattering cross sections were taken from SAID database [25]. The obtained quasielastic scattering cross section was multiplied by a factor of 0.955 taking into account the shading effect [26].
The events from pd → 3 Heη and pd → ppn spectator reactions are identified by algorithms that were applied for raw data and Monte Carlo simulation results in order to obtain the number of experimental of events and to calculate the efficiency. The integrated luminosity in this experiment is calculated by the formula where N data and N MC are the number of events extracted from data and Monte Carlo simulation correspondingly, σ is the reaction's cross section, and is the efficiency. S trigger is the trigger's scaling factor set up during the measurements. S MC is the total number of generated Monte Carlo events.

Searching for a bound state
The pd → 3 Heγγ and pd → 3 He6γ channels are investigated to search for 3 He − η bound state. In these channels, the bound state would be visible in case of direct bound η decay without being absorbed by any nucleon: pd → ( 3 Heη) bound → 3 Heγγ and pd → ( 3 Heη) bound → 3 Heπ 0 π 0 π 0 → 3 He6γ. For efficiency estimation, the Monte Carlo simulation for bound state production and decay was performed. The distribution of 3 He − η relative momentum was calculated by S. Hirenzaki and H. Nagahiro [27]. The simulation was carried out with assumption that 3 He plays a role of spectator. Coupled η meson decay on two γ quanta is assumed to be isotropic in the η rest frame and the decay into three π 0 mesons was simulated assuming uniform phase space population. Each of π 0 decays into two γ quanta isotropically in π 0 rest frame.
The pd → 3 Heγγ and pd → 3 He6γ events identification is based on conditions applied for particle registration time, angles, and kinematic magnitudes such as invariant and missing mass. For pd → 3 Heγγ reaction the background processes efficiency was decreased to the level below 0.5% while the bound state decay acceptance is about 10% in the whole beam momentum range (Fig. 1).
In the excess energy region Q3 Heη < 10 MeV the acceptance for pd → 3 Heη reaction is neglectable because of small θ angle for 3 He emission direction. This property is useful for searching the bound state near the threshold.
Actually, the analysis is still in progress. In case if the bound state is observed, a peak on the excitation curve for both pd → 3 Heγγ and pd → 3 He6γ reactions will be obtained.