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Electromagnetic and Spin Properties of Hyperons
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Nuclear Physics.
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
Abstract [en]

The study of hyperons improves our understanding of nature in multiple ways. The intrinsic properties of hadrons are not fully understood at a fundamental level. Hence, the electromagnetic properties of hyperons provide complementary information to that of the nucleon and therefore allow for a broader investigation of the structure of matter. Electromagnetic form factors have been extensively studied for the nucleon, and are now addressed also in the hyperon sector. In this thesis, the electromagnetic Σ-Λ hyperon transition form factors have been determined at low energies, in a pioneering framework that makes use of dispersion relations to combine theoretical and experimental input. Subsequently this analysis has been extended to the decuplet spin-3/2 Σ-Λ transition. Assuming that these transition form factors are saturated by a two-pion inelasticity, their imaginary part can be written in terms of the pion vector form factor and a pion-hyperon scattering amplitude. Chiral perturbation theory at next-to-leading order has been used to calculate the latter, while the pion vector form factor and the related pion phase-shift are known from measurements.

The spin properties of hyperons can be resolved through their weak decays. The angular distribution of their decay products displays both polarization parameters and decay asymmetry parameters. These originate from processes characterized by an interplay of partial waves andare useful for different purposes. The polarization is sensitive to the production mechanism; in the p¯p case, it can be used to probe the strong interaction in the non-perturbative regime while in the e+e case it contains the relative phase between complex electromagnetic form factors. Decay asymmetry parameters, on the other hand, are used to construct observables that test CP violation. Additional sources of CP violation must be found to solve the problem of the baryon asymmetry in our universe. This work provides a systematic method to retrieve both polarization and decay asymmetry parameters directly from the angular distribution of the hyperon decay products.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. , p. 69
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1939
Keywords [en]
Electromagnetic form factors, hyperon physics, dispersion relations, QCD, chiral perturbation theory, CP violation
National Category
Subatomic Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:uu:diva-406648ISBN: 978-91-513-0956-9 (print)OAI: oai:DiVA.org:uu-406648DiVA, id: diva2:1425382
Public defence
2020-06-12, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2020-05-18 Created: 2020-04-21 Last updated: 2020-05-18
List of papers
1. The electromagnetic Sigma-to-Lambda hyperon transition form factors at low energies
Open this publication in new window or tab >>The electromagnetic Sigma-to-Lambda hyperon transition form factors at low energies
2017 (English)In: European Physical Journal A, ISSN 1434-6001, E-ISSN 1434-601X, Vol. 53, no 6, article id 117Article in journal (Refereed) Published
Abstract [en]

Using dispersion theory the low-energy electromagnetic form factors for the transition of a Sigma to a Lambda hyperon are related to the pion vector form factor. The additionally required input, i.e. the two-pion-Sigma-Lambda amplitudes are determined from relativistic next-to-leading-order (NLO) baryon chiral perturbation theory including the baryons from the octet and optionally from the decuplet. Pion rescattering is again taken into account by dispersion theory. It turns out that the inclusion of decuplet baryons is not an option but a necessity to obtain reasonable results. The electric transition form factor remains very small in the whole low-energy region. The magnetic transition form factor depends strongly on one not very well determined low-energy constant of the NLO Lagrangian. One obtains reasonable predictive power if this low-energy constant is determined from a measurement of the magnetic transition radius. Such a measurement can be performed at the future Facility for Antiproton and Ion Research (FAIR).

National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-326492 (URN)10.1140/epja/i2017-12324-4 (DOI)000402987600001 ()
Available from: 2017-08-22 Created: 2017-08-22 Last updated: 2020-04-21Bibliographically approved
2. Extraction of Polarization Parameters in the ‾pp → Ω̄Ω Reaction
Open this publication in new window or tab >>Extraction of Polarization Parameters in the ‾pp → Ω̄Ω Reaction
2018 (English)In: FAIRNESS 2017: Fair Next Generation Scientists 2017, 2018, article id UNSP 012019Conference paper, Published paper (Refereed)
Abstract [en]

A method to extract the polarization of Omega hyperons produced via the strong interaction is presented. Assuming they are spin 3/2 particles, the corresponding spin density matrix can be written in terms of seven non-zero polarization parameters, all retrievable from the angular distribution of the decay products. Moreover by considering the full decay chain Omega ->Lambda K -> p pi K the magnitude of the asymmetry parameters beta Omega and gamma Omega can be obtained. This method, applied here to the specific Omega case, can be generalized to any weakly decaying hyperon and is perfectly suited for the PANDA experiment where hyperon-antihyperon pairs will be copiously produced in proton-antiproton collisions. The aim is to take a step forward towards the understanding of the mechanism that reigns strangeness production in these processes.

Series
Journal of Physics Conference Series, ISSN 1742-6588, E-ISSN 1742-6596 ; 1024
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-367238 (URN)10.1088/1742-6596/1024/1/012019 (DOI)000436344100019 ()
Conference
5th Facility-for-Antiproton-and-Ion-Research (FAIR) NExt Generation ScientistS Conference (FAIRNESS), MAY 28-JUN 03, 2017, Sitges, SPAIN
Funder
Swedish Research Council
Available from: 2018-12-12 Created: 2018-12-12 Last updated: 2020-04-21Bibliographically approved
3. Constraining P and CP violation in the main decay of the neutral Sigma hyperon
Open this publication in new window or tab >>Constraining P and CP violation in the main decay of the neutral Sigma hyperon
2019 (English)In: Physics Letters B, ISSN 0370-2693, E-ISSN 1873-2445, Phys. Lett., Vol. 788, p. 535-541Article in journal (Refereed) Published
Abstract [en]

On general grounds based on quantum field theory the decay amplitude for Sigma(0) -> Lambda gamma consists of a parity conserving magnetic and a parity violating electric dipole transition moment. Because of the subsequent self-analyzing weak decay of the Lambda hyperon the interference between magnetic and electric dipole transition moment leads to an asymmetry in the angular distribution. Comparing the decay distributions for the Sigma(0) hyperon and its antiparticle gives access to possible C and CP violation. Based on flavor SU(3) symmetry the present upper limit on the neutron electric dipole moment can be translated to an upper limit for the angular asymmetry. It turns out to be far below any experimental resolution that one can expect in the foreseeable future. Thus any true observation of a CP violating angular asymmetry would constitute physics beyond the standard model, even if extended by a CP violating QCD theta-vacuumangle term.

Keywords
Radiative decays of hyperons, Electric dipole moment, CP violation
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-370074 (URN)10.1016/j.physletb.2018.09.065 (DOI)000455364400069 ()
Available from: 2018-12-18 Created: 2018-12-18 Last updated: 2020-04-21Bibliographically approved
4. Polarization observables in e(+) e(-) annihilation to a baryon-antibaryon pair
Open this publication in new window or tab >>Polarization observables in e(+) e(-) annihilation to a baryon-antibaryon pair
Show others...
2019 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 99, no 5, article id 056008Article in journal (Refereed) Published
Abstract [en]

Using the helicity formalism of Jacob and Wick, we derive spin density matrices of baryon antibaryon pairs produced in e(+)e(-) annihilation. We consider the production of pairs with spins 1/2 + (1/2) over bar, 1/2 + (3/2) over bar (+c.c.) and 3/2 + (3/2) over bar. We provide modular expressions to include chains of weak hadronic two-body decays of the produced hyperons. The expressions are suitable for the analysis of high statistics data from J/psi and psi(2S) ecays at e(+) e(-) colliders, by fits to the fully differential angular distributions of the measured particles. We illustrate the method by examples, such as the inclusive measurement of the e(+) e(-) -> psi(2S) -> Omega(-)(Omega) over bar (+) process where one decay chain Omega(-) -> AK(-) followed by A -> p pi(-) is considered. Finally, we show that the inclusive angular distributions can be used to test spin assignment of the produced baryons.

National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-381076 (URN)10.1103/PhysRevD.99.056008 (DOI)000461907100009 ()
Available from: 2019-04-23 Created: 2019-04-23 Last updated: 2020-04-21Bibliographically approved
5. Electromagnetic form factors of the transition from the spin-3/2 Σ to the Λ hyperon
Open this publication in new window or tab >>Electromagnetic form factors of the transition from the spin-3/2 Σ to the Λ hyperon
2020 (English)In: Physical Review C. Nuclear Physics, ISSN 0556-2813, E-ISSN 1089-490X, Vol. 101, no 1, article id 015206Article in journal (Refereed) Published
Abstract [en]

The three electromagnetic form factors for the transition from a spin-3/2 Σ* hyperon to the ground-state Λ hyperon are studied. At low energies, combinations of the transition form factors can be deduced from Dalitz decays of the  Σ* hyperon to Λ plus an electron-positron pair. It is pointed out how more information can be obtained with the help of the self-analyzing weak decay of the Λ. In particular, it is shown that these transitionform factors are complex quantities already in this kinematical region. Such measurements are feasible at hyperon factories such as, for instance, the Facility for Antiproton and Ion Research (FAIR). At higher energies, the transition form factors can be measured in electron-positron collisions. The transition form factors are related to decay distributions and differential cross sections. Using dispersion theory, the low-energy electromagneticform factors for the  Σ*-to-Λ transition are related to the pion vector form factor. The additionally required input, i.e., the two-pion–Σ*–Λ amplitudes, is determined from relativistic next-to-leading-order (NLO) baryon chiralperturbation theory, including the baryons from the octet and the decuplet. A poorly known NLO parameter is fixed to the experimental value of the Σ*→Λ γ decay width. Pion rescattering is taken into account by using dispersion theory and solving a Muskhelishvili-Omnès equation. Subtracted and unsubtracted dispersion relations are discussed. However, in view of the fact that the transition form factors are complex quantities, the current data situation does not allow for a full determination of the subtraction constants. To reduce the numberof free parameters, unsubtracted dispersion relations are used to make predictions for the transition form factors in the low-energy space- and timelike regions.

Keywords
hyperon form factors, dispersion relation, effective field theory
National Category
Subatomic Physics
Identifiers
urn:nbn:se:uu:diva-408713 (URN)10.1103/PhysRevC.101.015206 (DOI)
Available from: 2020-04-13 Created: 2020-04-13 Last updated: 2020-04-21Bibliographically approved

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