Search for a new light boson in meson decays

The subject of the presented work lies in the field of experimental particle physics. The main topic is the study of e+ e− pairs from η meson decays. The data sample used in this work was collected by the WASA-at-COSY collaboration in proton-proton collisions at 1.4 GeV kinetic beam energy. The experiment took place in 2012 at Forschungzentrum Jülich in Germany at the COSY storage ring. An internal proton beam interacted with a pellet target of frozen hydrogen. We implemented a set of selection criteria in order to extract the η → e+ e− γ event candidates. This is a rare electromagnetic decay of the η meson with branching ratio equal to 6.9·10−3. The resulting set of events served as the basis for three analyses. First, we extracted the η transition form factor that is a function depending on the inner quark and gluon structure of the meson. We implemented a specific method to reduce the contribution of background channels from direct pion production. The second analysis was the search for a narrow structure on the e+ e− invariant mass in the selected sample of η → e+ e− γ candidates. Many theoretical models and some astrophysical and particle physics measurements suggest the existence of a new boson, also called the dark photon, that couples to both dark and to Standard Model particles. This particle would decay to e+ e− pairs of well-defined mass and therefore could be detected by looking for narrow peaks in the e+ e− invariant mass spectra. Since no statistically significant signal was observed, we set an upper limit on the coupling parameter ε2. The third objective of this work was to select a sample of η → e+ e− candidates. This is a very rare decay and therefore sensitive to physics beyond the Standard Model. No signal from η → e+ e− was observed, therefore we were able to set an upper limit on the branching ratio for this decay

Initial Stages 2021

Studies of the phase diagram of the strongly interacting matter is one of the hottest topics in high energy physics. While the transition from hadronic matter to Quark-Gluon Plasma (onset of deconfinement) is now rather uncontroversial, the difficulties in obtaining a unique and quantitative prediction of the QGP signal within the QCD theoretical framework remain and the scientific community still awaits the confirmation of an experimental observation of the critical endpoint of the transition line. In this talk, we will briefly report some recent results obtained by the NA61/SHINE Collaboration on those important issues. In particular, we will present the study of the proton-proton momenta intermittency phenomenon as a possible approach to the search of the QCD critical point. Up to now, an indication of second-order fluctuations has been observed in the freeze-out state of central Si+Si collisions at the NA49 Super Proton Synchrotron (SPS) experiment with beam energy √sNN=17.2 GeV. As its direct successor, the NA61/SHINE experiment is carrying on this search with the ability to scan the QCD phase diagram by varying the colliding nuclei (system size) and the collision energy

On the relation between

A compilation of the experimental $$p+p$$ data of neutral and charged kaon production has revealed a discrepancy between the observed $$K^{0}_{s}$$ yield and the the average number of produced charged kaons $$K^{0}_{s}=(K^{+}+K^{-})/2$$. This widespread relation holds only for a colliding system that is an uniform population w.r. to the isospin (i.e. that consists of an equal number of all members of a given isospin multiplet). This is not the case for $$p+p$$ collisions therefore one should not expect that it will describe the data accurately. A much better agreement is obtained with a model based on the quark structure of the participating hadrons

Dielectron pairs from eta meson decays at WASA detector

We present the results of the analysis of eta -> e(+)e(-)gamma and eta -> e(+)e(-) decays. The experimental data were collected in proton-proton collisions at incident proton kinetic energy 1.4 GeV using the WASA detector and the COSY storage ring. We describe the extraction procedure of the eta meson transition form factor, based on a sample of around 10(8)eta mesons, and show an attempt to search for physics beyond the Standard Model that led to the setting of an upper limit on the coupling between photons and hypothetical dark bosons. We also provide an estimate of the branching ratio upper limit for the very rare eta -> e(+)e(-) decay

The cosmic ray detector for the NICA collider

Multi-Purpose Detector (MPD) is a main part of a new Ion Collider fAcility (NICA) located in Dubna, Russia. To increase MPD functionality, it was proposed to add an additional muon trigger system for off-beam calibration of the MPD sub-detectors and for rejection of cosmic ray background during experiments. The system could also be very useful for astrophysical observations of cosmic showers initiated by high energy primary particles. This article describes the main goals of MCORD detector and the early stage of MCORD design, based on plastic scintillators with silicon photomultiplier photodetectors (SiPM) for scintillation readout and electronic system based on MicroTCA standard

The cosmic ray detector for the NICA collider

Multi-Purpose Detector (MPD) is a main part of a new Ion Collider fAcility (NICA) located in Dubna, Russia. To increase MPD functionality, it was proposed to add an additional muon trigger system for off-beam calibration of the MPD sub-detectors and for rejection of cosmic ray background during experiments. The system could also be very useful for astrophysical observations of cosmic showers initiated by high energy primary particles. This article describes the main goals of MCORD detector and the early stage of MCORD design, based on plastic scintillators with silicon photomultiplier photodetectors (SiPM) for scintillation readout and electronic system based on MicroTCA standard

Search for η mesic 3He with the WASA-at-COSY facility in the pd -> 3He2γ and pd -> 3He6γ reaction

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

Measurement of the np -> np pi(0)pi(0) reaction in search for the recently observed d* (2380) resonance

Exclusive measurements of the quasi-free np --> np pi(0)pi(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 root 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 Faldt and Wilkin as well with those of Albadajedo and Oset. The ABC effect, i.e. the low-mass enhancement in the pi(0)pi(0)-invariant mass spectrum, is found to be very modest - if present at all, which might pose a problem to some of its interpretations

Measurement of the np -> np pi(0)pi(0) reaction in search for the recently observed d* (2380) resonance

Exclusive measurements of the quasi-free np --> np pi(0)pi(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 root 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 Faldt and Wilkin as well with those of Albadajedo and Oset. The ABC effect, i.e. the low-mass enhancement in the pi(0)pi(0)-invariant mass spectrum, is found to be very modest - if present at all, which might pose a problem to some of its interpretations

Measurement of the np -> np pi(0)pi(0) reaction in search for the recently observed d* (2380) resonance

Exclusive measurements of the quasi-free np --> np pi(0)pi(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 root 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 Faldt and Wilkin as well with those of Albadajedo and Oset. The ABC effect, i.e. the low-mass enhancement in the pi(0)pi(0)-invariant mass spectrum, is found to be very modest - if present at all, which might pose a problem to some of its interpretations