Feasibility study of measuring CP symmetry violation via eta->4pi decay using WASA-at-COSY detector
Author: Tomasz Bednarski
Supervisor: prof. Paweł Moskal
abstract
Defence year: 2011
It is known that the Standard Model does not describe all phenomena related to subatomic particles. This thesis presents feasibility studies of the measurement of eta meson decay using WASA-at-COSY detector which tests predictions of the Standard Model. The aim of this thesis is to estimate the time of measurement for which the current branching ratio limit of eta -> 4pi0 decay can be improved. In order to estimate the time of measurement Monte Carlo simulations were performed. Kinematics of the pp -> ppeta -> pp4pi0 -> pp8gamma reactions were simulated using PLUTO program. Next, by means of the GEANT3 program the response of the WASA-at-COSY detector was simulated for each particle. Received signals were analysed in RootSorter software package based on ROOT. Studies of the reaction with many gamma quanta in the exit channel required the investigation of WASA-at-COSY calorimeter functioning. Tests of a cluster building algorithm with emphasis on merging and splitting of detected signals were done. The obtained result shows that only about 3% of eta -> 4p0 decays can be properly reconstructed. The identification of the eta -> 4pi0 -> 8gamma decay was based on the invariant mass method for both pi0 and eta mesons. As a result, the resolution of the invariant mass determination of the eta meson was established to sigma = 31 MeV/c2. The acceptance of the WASA-at-COSY detector for pp -> ppeta -> pp4pi0 -> pp8gamma reaction was determined to Aeta=(Neta detected) / (Neta produced) = 1.5%. The time of measurement for obtaining a statistical precision equal to the current branching ratio limit for Tbeam = 1.3 GeV is about 13 hours. It was also established that the lower the excess energy in the eta production is the shorter the time of the measurement needed.
Feasibility study of eta-mesic nuclei production by means of the WASA-at-COSY and COSY-TOF facilities
Author: Magdalena Skurzok
Supervisor: prof. Paweł Moskal
abstract
Defence year: 2010
Despite the fact that existence of eta-mesic nuclei in which the eta meson might be bound with the light nucleus by means of the strong interaction was postulated already in 1986, it is still not experimentally confirmed. Discovering of this new kind of an exotic nuclear matter is very important as it might allow for better understanding of eta meson structure and its interaction with nucleons. The search of the eta-helium bound states is carried out at the COSY accelerator in the Research Center Juelich in Germany, by means of the WASA detection system. The search are conducted with high statistic and high acceptance for the free production of the 4He-eta bound states. It is also considered to search for eta-tritium in quasi free reaction which might be realised with COSY-TOF facility. In this thesis the results of the Monte Carlo simulations of the eta-helium bound states and eta-tritium bound state are presented and discussed. The acceptances of the WASA-at-COSY and COSY-TOF detectors for the free and quasi-free eta-mesic nuclei production reactions were determined, respectively. Furthermore acceptances were compared for three different models of nucleon momentum distribution inside atomic nuclei and three different values of width of a considered bound states. In case of COSY-TOF detector it was established that the most effcient measurement of quasi-free dd --> p_sp (T-eta)_bs --> p_sp dp pi- reaction can be done at beam momentum of p_beam =3.1GeV/c.
Feasibility study of the eta-prime -> pi+pi-pi0 decay using WASA-at-COSY apparatus
Author: Marcin Zieliński
Supervisor: prof. Paweł Moskal
abstract
Defence year: 2008
One of the objectives of the vast physics programme of the recently comissioned WASA-at-COSY facility is the study of fundamental symmetries via the measurements of the eta and eta-prime mesons decays. Especially interesting are isospin violating hadronic precesses of these mesons into 3pi systems driven by the term of QCD Lagrangian which depends on the mass difference of the u and d quarks. When an eta or an eta-prime meson is created in the hadronic reaction signals from such decays may be significantly obscured by the prompt production of pi mesons. In this thesis we present the estimation of the upper limit of the background due to prompt pion production for the eta-prime ->3pi0 and eta-prime->pi+pi-pi decays. Using the data from proton-proton collisions measured by the COSY-11 group we have extracted differential cross sections for the multimeson production with the invariant mass corresponding to the mass of the eta-prime meson. Based on these results and on parametrizations of the total cross sections for the eta-prime meson as well as parametrization of the upper limit for the prompt pi+pi-pi0 production in the collisions of protons we discuss in details the feasibility of a measurement of the eta-prime meson decay into 3pi channels with the WASA-at-COSY facility. Based on the chiral unitary approach the value of the branching ratio BR(eta-prime->pi+pi-pi0) was recently predicted to be about 1%. We show that the WASA-at-COSY has a potential to verify this result empirically.
Poszukiwanie jąder eta-mezonowych helu w reakcji pd -> dppi0 za pomocą układu detekcyjnego WASA-at-COSY
Author: Aleksander Khreptak
Supervisor: prof. Paweł Moskal
Auxilliary supervisor: dr Magdalena Skurzok
abstract
Defence year: 2021
Rozprawa doktorska poświęcona jest eksperymentalnemu badaniu możliwości powstania silnie związanego układu mezonu eta i jądra atomowego, tzw. jądra mezonowego. Pomiar reakcji pd -> dppi0 wykonano w 2014 roku przy użyciu układu detekcyjnego WASA-at-COSY w Centrum Badawczym Juelich w Niemczech. Dane zbierano podczas powolnej i ciągłej zmiany pędu wiązki w zakresie od 1.426 do 1.635 GeV/c, co odpowiada zakresowi energii dostępnej od -70 do +30 MeV. Aby wyznaczyć funkcję wzbudzenia dla tego procesu zidentyfikowano wszystkie emitowane cząstki oraz zastosowano odpowiednie kryteria selekcji wybrane na podstawie wyników symulacji Monte Carlo reakcji powstania jądra mezonowego 3He-eta i jego rozpadu poprzez wzbudzenie rezonansu N*(1535). W funkcji wzbudzenia nie zaobserwowano wąskiej struktury rezonansowej, która mogłaby zostać zinterpretowana jako sygnatura istnienia eta-mezonowego jądra helu. Dlatego też wyznaczono górną granicę całkowitego przekroju czynnego na produkcję stanu związanego 3He-eta i jego rozpad w procesie pd -> (3He-eta)związany -> dppi0. Górna granica na poziomie ufności 90% waha się od 13 do 24 nb dla zakresów energii wiązania od 0 do 40 MeV oraz szerokości od 5 do 50 MeV.
Search for eta-mesic 3He in non-mesonic final states
Author: Oleksandr Rundel
Supervisor: prof. Paweł Moskal
Auxilliary supervisor: dr Magdalena Skurzok
abstract
Defence year: 2019
The eta-mesic nucleus that is the bound state of a nucleus and eta meson has been postulated theoretically in 1986 but has not been observed in the experiments yet.
In May-June 2014, the experiment devoted to the search for the bound state of the eta meson and 3He nucleus has been performed at COSY accelerator in Research Center Jülich in Germany with WASA-at-COSY facility. The excitation functions for pd->3He+2gamma, pd->3He+6gamma, pd->ppp+pi-, pd->ppn+pi0, pd->dn+pi-, pd->dp+p0, pd->pd, and pd->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 3He-eta excess energy range from -70 to +30 MeV.
This dissertation is devoted to the search for eta mesic 3He nucleus in the non-mesonic decay channels: pd->3He+2gamma and pd->3He+6gamma reactions. The excitation functions for these processes were obtained after identification of all outgoing particles and selection of events with conditions based on the results of Monte Carlo simulations of direct decay of eta meson bound in the 3He nucleus. The integrated luminosity dependence on the excess energy, used for the excitation function normalization, was calculated based on pd->3He+eta and pd->ppn_{spectator} reactions.
The analysis of the obtained excitation functions for the pd->3He+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.
Determination of the analysing power for the vec(p)p -> ppeta reaction using WASA-at-COSY detector
Author: Iryna Schätti-Ozerianska
Supervisor: prof. Paweł Moskal
Auxilliary supervisor: dr Marcin Zieliński
abstract
Defence year: 2016
The eta meson production process can be studied via measurements of the analyzing power, Ay, which may be understood as a measure of the relative deviation between the differential cross section with and without polarized beam. Spin-dependent observables such as cross sections and analyzing powers have been determined only for a small number of excess energies and with very high statistical uncertainty. Therefore, measurements of the reaction vec(p)p -> ppeta was performed at the WASA-at-COSY detector in a fixed-target experiment with beam momenta of 2026 MeV/c and 2188 MeV/c. These correspond to excess energies of 15 MeV and 72 MeV, respectively. The eta meson was identified by the techniques of missing mass and invariant mass. The invariant mass technique was mainly used to reduce background events, while the missing mass technique provided an accurate value of the number of the eta mesons as a function of the emission angle.
The angular distribution of the analyzing power was compared with existing theoretical models. Predictions of all existing models strongly disagree with the experimental results determined in the framework of this thesis.
A comparison of the obtained Ay angular distribution with a series of associated Legendre polynomials revealed negligible contribution of the Sd partial wave at Q =15 MeV. However, at Q=72 MeV, a significant interference of the Ps and Pp partial waves was observed.
Search for eta-mesic helium via dd -> 3Henpi0 reaction by means of the WASA-at-COSY facility
Author: Magdalena Skurzok
Supervisor: prof. Paweł Moskal
abstract
Defence year: 2016
This dissertation is about the search for 4He-? bound state in dd?3Hen?0 reaction with the WASA detector installed in Forschungszentrum Jülich. The experiment was carried out using a deuteron COSY beam and deuteron pellet target. The beam momentum varied continuously in each of acceleration cycle from 2.127 GeV/c to 2.422 GeV/c which corresponds to a range of excess energy Q?(-70,30)MeV.
The excitation function for the considered process was determined after identification of all outgoing particles and the application of the selection conditions based on Monte Carlo simulations of ?-mesic helium production and its decay via excitation of the N? resonance. No narrow structure of the ?-mesic helium was observed in the excitation function. The upper limit of the total cross section for the dd?(4He-?)bound?3Hen?0 process was determined on the 90% confidence level. It varies from 21 to 36 nb for the bound state width ranging from 5 MeV to 50 MeV, respectively. However, an indication for a broad state was observed. The kinematic region, where we expect the evidence of the signal from the bound state, cannot be fully described only by the combination of the considered background processes. In contrast, the experimental excitation function is very well fitted by the background contributions for the region where the signal is not expected.
Test of charge conjugation invariance in eta->pi(0)e(+)e(-) and eta->pi(+)pi(-)pi(0) decays
Author: Marcin Zieliński
Supervisor: prof. Paweł Moskal
abstract
Defence year: 2012
Charge conjugation C is one of the fundamental symmetries in nature which
transforms particles into antiparticles. This symmetry was studied in weak interaction
where it is fully violated, but it is poorly known in the strong and electromagnetic interactions.
Therefore, it is important to test this symmetry accurately for a better understanding of the nature
of the strong interaction and for the understanding of the significantly larger abundance of matter
over antimatter in the Universe.
To this end, in this thesis we investigated nd ???+?-?0 and ???0e+e- decays, which might violate charge conjugation symmetry.
The violation of C symmetry in nd ???+?-?0 process could manifest
itself as an asymmetry between energy spectra of charged pions, and can be studied using event density
distribution on Dalitz plot.
The ???0e+e- decay is forbidden by C symmetry in the first order of electromagnetic
interaction, and can only proceed by emission of two virtual photons with the branching ratio
on a level of 10^{-8}, as predicted in the framework of the Standard Model.
Therefore, observation of a larger branching ratio could indicate a
mechanism involving first order electromagnetic interaction violating charge conjugation.
Both decays were investigated by means of the WASA-at-COSY detector operating at the COSY synchrotron
at the Forchungszentrum Juelich in Germany. The ? meson was produced via pp?pp?
reaction at the proton beam momentum of 2.14 GeV/c which corresponds to kinetic energy of 1.4 GeV.
The measurement was done at the turn of October and November in the year 2008. In total around
5x10^7 ? mesons were collected during the two weeks of data taking.
The tagging of the ? meson was done by means of the missing mass technique
and the decay products were identified by the invariant mass reconstruction.
As a result of the analysis conducted in the framework of this thesis a Dalitz Plot distribution
for the and ???+?-?0 decay was obtained. From this distribution we extracted
asymmetry parameters sensitive to C symmetry violation for different isospin values of the final state
and we have established that all are consistent with zero within the obtained accuracy.
For the ???0e+e- decay we have not observe signal and thus we estimated an upper limit
for the branching ratio. The established upper limit amounts to BR(???0e+e-) < 3.7x10^{-5}
at the 90% confidence level. This result is more precise than previously obtained in other experiments.
We intend to continue the research, and thanks to the 20 times higher statistics of already collected data by WASA-at-COSY, the upper limit will be improved significantly.
Study of the eta -> g e+ e- decay using WASA-at-COSY detector system
Author: Małgorzata Hodana
Supervisor: prof. Paweł Moskal
abstract
Defence year: 2012
Since the eta meson is a short-lived neutral particle, it is not possible to investigate its structure via classical methods of particle scattering. To learn about its quark wave function, one studies those decay processes of this meson in which a pair of photons is produced, where at least one of them is virtual. The virtual photons have a non-zero mass and convert into lepton-antilepton pairs. Therefore, information about the quarks' spatial distribution inside the meson can be extracted from the lepton-antilepton invariant mass distributions by comparing the empirical results with QED predictions, which are based on the assumption that the meson is a point-like particle. The deviation from the expected behavior in the leptonic mass spectrum exposes the inner structure of the meson. This deviation is characterized by a form factor.
The corresponding data were collected for the pd --> 3He eta reaction at a proton beam momentum of 1.69 GeV/c. The experiment was performed using the WASA-at-COSY detector in November 2008.
525 +/- 26 events of the eta --> g e+ e- decay channel were reconstructed. The analysis chain led to the determination of the value of the slope parameter, bp, equal to (2.27 +/- 0.73stat. +/- 0.46sys.) GeV-2, where the systematical uncertainty should be treated as an upper limit only.
The estimation of the charge radius of the eta meson has been made resulting with the value of < r(eta)^2 >^(1/2) = (0.68 +/- 0.02) fm.
@Version including minor corrections according to the comments of the referees
Search for eta-mesic 4He with the WASA-at-COSY detector
Author: Wojciech Krzemień
Supervisor: prof. Jerzy Smyrski
abstract
Defence year: 2011
We performed a search for the 4He-eta bound state via exclusive measurement of the excitation function for the dd->3Heppi- reaction, where the outgoing p-pi- pair originates from the conversion of the eta meson on a neutron inside the 4He nucleus. The measurements were performed at the Cooler Synchrotron COSY-Juelich with the WASA-at-COSY detection system.The internal deuteron beam of COSY was scattered on a pellet-type deuteron target. The data were taken during a slow acceleration of the beam from 2.185 GeV/c to 2.400 GeV/c crossing the kinematical threshold for the eta production in the dd->4He-eta reaction at 2.336 GeV/c. The corresponding excess energy in the 4He-eta system varied from -51.4 MeV to 22 MeV. Events corresponding to decays of the eta-mesic 4He were selected using cuts on the 3He momentum, p and pi- kinetic energies and the relative p-pi- angle in the center of mass system. The range of the applied cuts was inferred from simulations of decay of the 4He-eta bound state proceeding via excitation the N* resonance. The integrated luminosity in the experiment was determined using the dd->3He-n reaction and the relative normalization of the points of the dd->3Heppi- excitation function was based on the quasi-elastic proton-proton scattering. No signal of the 4He-eta bound state was observed in the excitation function. An upper limit for the cross-section for the bound state formation and decay in the process dd->(4He-eta)_bound->3Heppi- determined for the bound state width of 10, 20 and 30 MeV equals to 28, 32 and 41 nb, respectively. The measured angular and momentum distributions of the reaction products are close to those simulated under the assumption of uniform phase-space distribution.
Study of the eta meson decay into e+ e- pi+ pi- using WASA-at-COSY detector system
Author: Michał Janusz
Supervisor: prof. Paweł Moskal
abstract
Defence year: 2010
