| 1. |
- Bluhm, Marcus, et al.
(författare)
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Dynamics of critical fluctuations : Theory - phenomenology - heavy-ion collisions
- 2020
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Ingår i: Nuclear Physics A. - : ELSEVIER. - 0375-9474 .- 1873-1554. ; 1003
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Tidskriftsartikel (refereegranskat)abstract
- This report summarizes the presentations and discussions during the Rapid Reaction Task Force "Dynamics of critical fluctuations: Theory - phenomenology - heavy-ion collisions", which was organized by the ExtreMe Matter Institute EMMI and held at GSI, Darmstadt, Germany in April 2019. We address the current understanding of the dynamics of critical fluctuations in QCD and their measurement in heavy-ion collision experiments. In addition, we outline what might be learned from studying correlations in other physical systems, such as cold atomic gases.
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| 2. |
- Terschlüsen, Carla
(författare)
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Theoretical Studies of Hadronic Reactions with Vector Mesons
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aiming at a systematic inclusion of pseudoscalar and vector mesons as active degrees of freedom in an effective Lagrangian, studies have been performed in this thesis concerning the foundations of such an effective Lagrangian as well as tree-level and beyond-tree-level calculations. Hereby, vector mesons are described by antisymmetric tensor fields.First, an existing power counting scheme for both pseudoscalar and vector mesons is extended to include the pseudoscalar-meson singlet in a systematic way. Based on this, tree-level calculations are carried out which are in good agreement with the available experimental data and several processes are predicted. In particular, the ω-π0 transition form factor is in better agreement with experimental data than the prediction done in the vector-meson-dominance model. Furthermore, a Lagrangian with vector mesons is used together with the leading contributions of chiral perturbation theory in order to calculate tree-level reactions in the sector of odd intrinsic parity. It turns out that both the Lagrangian with vector mesons and the Lagrangian of chiral perturbation theory are needed to describe experimental data.Additionally, a feasibility check for one-loop calculations with pseudoscalar and vector mesons in the loop is performed. Thereby, only a limited number of interaction terms in the Lagrangian with vector mesons is used. The results are used to both renormalise the low-energy constants of chiral perturbation theory up to chiral order Q4 and to determine the influence of loops with vector mesons on masses and decay constants of pseudoscalar mesons.
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| 3. |
- Adlarson, Patrik, et al.
(författare)
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Exclusive measurement of the eta -> pi(+) pi(-) gamma decay
- 2012
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Ingår i: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 707:2, s. 243-249
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Tidskriftsartikel (refereegranskat)abstract
- An exclusive measurement of the decay eta -> pi(+) pi(-) gamma has been performed at the WASA facility at COSY. The eta mesons were produced in the fusion reaction pd -> He-3 X at a proton beam momentum of 1.7 GeV/c. Efficiency corrected differential distributions have been extracted based on 13 960 +/- 140 events after background subtraction. The measured pion angular distribution is consistent with a relative p-wave of the two-pion system, whereas the measured photon energy spectrum was found at variance with the simplest gauge invariant matrix element of eta -> pi(+) pi(-) gamma. A parameterization of the data can be achieved by the additional inclusion of the empirical pion vector form factor multiplied by a first-order polynomial in the squared invariant mass of the pi(+) pi(-) system.
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| 4. |
- Adlarson, Patrik, et al.
(författare)
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Measurement of the ω → π+π−π0 Dalitz plot distribution : The WASA-at-COSY Collaboration
- 2017
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Ingår i: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 770, s. 418-425
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Tidskriftsartikel (refereegranskat)abstract
- Using the production reactions pd -> He-3 omega and pp -> pp omega, the Dalitz plot distribution for the omega -> pi(+)pi(-)pi(0)decay is studied with the WASA detector at COSY, based on a combined data sample of (4.408 +/- 0.042) x 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 phi. For the first time, a deviation from pure P-wave phase space is observed with a significance of 4.1 sigma. The deviation is parametrised by a linear term 1+2 alpha Z, with alpha determined to be +0.147 +/- 0.036, consistent with the expectations of rho-meson-type final-state interactions of the P-wave pion pairs. (C) 2017 The Author. Published by Elsevier B.V.
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| 5. |
- Alvarado, F., et al.
(författare)
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Electroweak structure of the nucleon
- 2024
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Ingår i: Nuovo cimento della societa italiana de fisica. C, Geophysics and space physics. - : SOCIETÀ ITALIANA DI FISICA. - 1124-1896 .- 1826-9885. ; 47:4
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Tidskriftsartikel (refereegranskat)abstract
- The nucleon form factors (FFs) are studied in relativistic chiral perturbation theory (ChPT) in two flavors with explicit Δ(1232) degrees of freedom. For the electromagnetic isovector form factors we also employ dispersion theory to account for ρ-dominated isovector ππ interaction and its quark-mass dependence in the t -channel nonperturbatively and beyond NLO in ChPT. With this framework we explore how LQCD data are described in both the Q2 and Mπ dimensions simultaneously. Furthermore, we have performed an NNLO calculation of the nucleon axial form factor, extracting relevant low -energy constants (LECs) from a combined set of recent LQCD results from different collaborations.
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| 6. |
- Alvarado, Fernando, et al.
(författare)
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Light quark mass dependence of nucleon electromagnetic form factors in dispersively modified chiral perturbation theory
- 2023
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Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 108:11
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Tidskriftsartikel (refereegranskat)abstract
- The nucleon isovector electromagnetic form factors are calculated up to next-to-next-to-leading order by combining relativistic chiral perturbation theory (ChPT) of pion, nucleon, and Δ(1232) with dispersion theory. We specifically address the light-quark mass dependence of the form factors, achieving a good description of recent lattice QCD results over a range of Q2≲0.6 GeV2 and Mπ≲350 MeV. For the Dirac form factor, the combination of ChPT and dispersion theory outperforms the pure dispersive and pure ChPT descriptions. For the Pauli form factor, the combined calculation leads to results comparable to the purely dispersive ones. The anomalous magnetic moment and the Dirac and Pauli radii are extracted.
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| 7. |
- Amano, Taiju, et al.
(författare)
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Sum rule for the partial decay rates of bottom hadrons based on the dynamical supersymmetry of the (s)over-bar quark and the ud diquark
- 2022
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Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 105:5
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the weak decays of (B) over bar (0)(s) and Lambda(b) to charm hadrons based on the dynamical supersymmetry between the (s) over bar quark and the ud diquark. We derive a new sum rule relating the decay rates of the processes (B) over bar (0)(s) -> Ds+P-, (B) over bar (0)(s) -> Ds*+P-, and Lambda(b) -> Lambda P-c(-) where P- is a negatively charged meson, such as pi(-) and K-. It is found that the observed decay rates satisfy the sum rule very well. This implies that the supersymmetry between the (s) over bar quark and the ud diquark is also seen in the wave functions of the heavy hadrons and suggests that the ud diquark can be regarded as a valid effective constituent for heavy hadrons.
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| 8. |
- Aoyama, T., et al.
(författare)
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The anomalous magnetic moment of the muon in the Standard Model
- 2020
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Ingår i: Physics reports. - : Elsevier BV. - 0370-1573 .- 1873-6270. ; 887, s. 1-166
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Forskningsöversikt (refereegranskat)abstract
- We review the present status of the Standard Model calculation of the anomalous magnetic moment of the muon. This is performed in a perturbative expansion in the fine-structure constant α and is broken down into pure QED, electroweak, and hadronic contributions. The pure QED contribution is by far the largest and has been evaluated up to and including O(α5) with negligible numerical uncertainty. The electroweak contribution is suppressed by (mμ/MW)2 and only shows up at the level of the seventh significant digit. It has been evaluated up to two loops and is known to better than one percent. Hadronic contributions are the most difficult to calculate and are responsible for almost all of the theoretical uncertainty. The leading hadronic contribution appears at O(α2) and is due to hadronic vacuum polarization, whereas at O(α3) the hadronic light-by-light scattering contribution appears. Given the low characteristic scale of this observable, these contributions have to be calculated with nonperturbative methods, in particular, dispersion relations and the lattice approach to QCD. The largest part of this review is dedicated to a detailed account of recent efforts to improve the calculation of these two contributions with either a data-driven, dispersive approach, or a first-principle, lattice-QCD approach. The final result reads aμSM = 116 591 810(43) x 10-11 and is smaller than the Brookhaven measurement by 3.7 σ. The experimental uncertainty will soon be reduced by up to a factor four by the new experiment currently running at Fermilab, and also by the future J-PARC experiment. This and the prospects to further reduce the theoretical uncertainty in the near future - which are also discussed here - make this quantity one of the most promising places to look for evidence of new physics.
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| 9. |
- Bashkanov, M., et al.
(författare)
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WHAT IS THE NATURE OF THE ABC EFFECT?
- 2011
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Ingår i: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 26:3-4, s. 627-629
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Tidskriftsartikel (refereegranskat)abstract
- The ABC effect - an intriguing low-mass enhancement in the pi pi invariant mass spectrum - is known from inclusive measurements of two-pion production in nuclear fusion reactions. Exclusive measurements have been carried out at CELSIUS/WASA and WASA-at-COSY for the fusion reactions leading to d, (3)He and (4)He. They all reveal this effect to be of isoscalar nature and associated with a narrow Lorentzian-shaped structure in the total cross section. The latter could point to the formation of an isoscalar resonance, which couples to the pn channel as well as to an intermediate Delta Delta state.
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| 10. |
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| 11. |
- Berlowski, Marcin, et al.
(författare)
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SEARCH FOR EXOTIC EFFECTS IN eta MESON LEPTONIC DECAYS
- 2011
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Ingår i: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 26:3-4, s. 630-631
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Tidskriftsartikel (refereegranskat)abstract
- The search for the decay eta -> e(+)e(-) was carried out using the WASA detector at the COSY storage ring. Data were taken during a two week experimental run in Sept-Nov 2008 in which about 10(7) eta mesons were collected from the reaction of pp -> pp eta at energy of 1.4 GeV. Background studies were performed for several different reactions. The performance of the WASA detector for the measurement of electron-positron pairs based on the single Dalitz decay of the eta meson was studied. We also identified a statistically significant data sample of other more frequent leptonic decays.
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| 12. |
- Bertilsson, Magnus, et al.
(författare)
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Goldberger-Treiman relation and Wu-type experiment in the decuplet sector
- 2024
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Ingår i: Physical Review D. - : American Physical Society. - 2470-0010 .- 2470-0029. ; 109:3
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Tidskriftsartikel (refereegranskat)abstract
- The leading-order chiral Lagrangian for the baryon octet and decuplet states coupled to Goldstone bosons and external sources contains six low-energy constants. Five of them are fairly well-known from phenomenology, but the sixth one is practically unknown. This coupling constant provides the strength of the (p-wave) coupling of Goldstone bosons to decuplet states. Its size and even sign are under debate. Quark models and quantum chromodynamics (QCD) for a large number of colors provide predictions, but some recent phenomenological analyses even suggest an opposite sign for the Delta -pion coupling. The Goldberger-Treiman relation connects this coupling constant to the axial charge of the Delta baryon. This suggests a Wu-type experiment to determine the unknown low-energy constant. While this is not feasible in the Delta sector because of the large hadronic width of the Delta, there is a flavor symmetry related process that is accessible; the weak semileptonic decay of the Omega baryon to a spin 3/2 cascade baryon. A broad research program is suggested that can pin down at least the rough size and the sign of the last unknown low-energy constant of the leading-order Lagrangian. It encompasses experimental measurements, in particular the forward-backward asymmetry of the semileptonic decay, together with a determination of the quark-mass dependences using lattice QCD for the narrow decuplet states and chiral perturbation theory to extrapolate to the Delta sector. Besides discussing the strategy of the research program, the present work provides a feasibility check based on a simple leading-order calculation.
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| 13. |
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| 14. |
- Buchheim, T., et al.
(författare)
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Chiral-partner D mesons in a heat bath within QCD sum rules
- 2018
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Ingår i: Journal of Physics G. - : IOP PUBLISHING LTD. - 0954-3899 .- 1361-6471. ; 45:8
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Tidskriftsartikel (refereegranskat)abstract
- Utilizing QCD sum rules, we extract the temperature dependences of the spectral properties of the pseudo-scalar and scalar D mesons regarded as chiral partners. Besides the masses also decay constants are analyzed as the D meson yields in heavy-ion collisions may be sensitive to their altered decay properties in an ambient strongly interacting medium. Our findings are (i) a decreasing scalar D meson mass for growing temperatures while its pseudo-scalar partner meson seems hardly affected, which is in qualitative agreement with hadronic model calculations; (ii) inferring an equally weak temperature dependence of the pseudo-scalar D meson decay properties the decreasing residua and decay constants of the scalar particle point towards partial chiral restoration. As a bonus of our analysis in the pseudo-scalar sector we determine the pseudoscalar decay constant at vanishing temperature. Due to the connection to particular leptonic branching fractions this decay constant is of great interest allowing for the determination of the off-diagonal CKM matrix element vertical bar V-cd vertical bar at zero temperature.
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| 15. |
- Cannarozzo, Simone
(författare)
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Isomeric yield ratio studies in nuclear reactions and alpha-particle induced fission of Thorium
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Despite decades of research since the initial discovery of nuclear fission, numerous unresolved questions still persist. It is known empirically that fission fragments emerge with high angular momentum. The mechanism responsible for the generation of the large angular momenta observed is one of these open questions. Since the characteristics of fission fragments are not directly measurable, experimentally accessible observables are used to derive the angular momenta using nuclear model codes. One of these observables is the yield ratio between fission products produced in different isomeric states, i.e., metastable energy levels of the same nucleus.In this thesis, a study of the level density models implemented in the nuclear model code TALYS is presented. Simulated and experimental isomeric yield ratios of a large number of nuclear reactions is compared. The results show a bias in the models that favours the population of the high-spin states and that this can be produced by the overestimation of the spin width distribution. The reason for this study is to improve the models then used in the angular momentum calculation. Moreover, the isomeric yield ratio measurement of twenty-one FFs is presented. The measurement was performed using the JYFLTRAP system at the University of Jyväskylä. The fission fragments were produced by the 32 MeV alpha-particle induced fission of 232Th. The analysis process, involving different identification and correction methods, and preliminary results are presented.
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| 16. |
- Danilkin, I. V., et al.
(författare)
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Photon-fusion reactions from the chiral Lagrangian with dynamical light vector mesons
- 2013
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 73:4, s. 2358-
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Tidskriftsartikel (refereegranskat)abstract
- We study the reactions gamma gamma -> pi(0)pi(0), pi(+)pi(-), K-0(K) over bar (0), K+K-, eta eta and pi(0)eta based on a chiral Lagrangian with dynamical light vector mesons as formulated within the hadrogenesis conjecture. At present our chiral Lagrangian contains five unknown parameters that are relevant for the photon-fusion reactions. They parameterize the strength of interaction terms involving two vector meson fields. These parameters are fitted to photon-fusion data gamma gamma -> pi(0)pi(0), pi(+)pi(-), pi(0)eta and to the decay eta -> pi(0)gamma gamma In order to derive gauge invariant reaction amplitudes in the resonance region constraints from maximal analyticity and exact coupled-channel unitarity are used. Our results are in good agreement with the existing experimental data from threshold up to about 0.9 GeV for the two-pion final states. The a(0) meson in the pi(0)eta channel is dynamically generated and an accurate reproduction of the gamma gamma -> pi(0)eta data is achieved up to 1.2 GeV. Based on our parameter sets we predict the gamma gamma -> K-0(K) over bar (0), K+K-, eta eta cross sections.
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| 17. |
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| 18. |
- Ekstedt, Andreas, et al.
(författare)
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Nucleon parton distributions from hadronic quantum fluctuations
- 2019
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Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 100:3
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Tidskriftsartikel (refereegranskat)abstract
- A physical model is presented for the nonperturbative parton distributions in the nucleon. This is based on quantum fluctuations of the nucleon into baryon-meson pairs convoluted with Gaussian momentum distributions of partons in hadrons. The hadronic fluctuations, here developed in terms of hadronic chiral perturbation theory, occur with high probability and generate sea quarks as well as dynamical effects also for valence quarks and gluons. The resulting parton momentum distributions f(x,Q20) at low momentum transfers are evolved with conventional Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equations from perturbative QCD to larger scales. This provides parton density functions f(x,Q2) for the gluon and all quark flavors with only five physics-motivated parameters. By tuning these parameters, experimental data on deep-inelastic structure functions can be reproduced and interpreted. The contribution to sea quarks from hadronic fluctuations explains the observed asymmetry between ¯u and ¯d in the proton. The strange-quark sea is strongly suppressed at low Q2, as observed.
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| 19. |
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| 20. |
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| 21. |
- Föhl, K, et al.
(författare)
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The WASA focussing light guide disc DIRC
- 2012
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Ingår i: Journal of Instrumentation. - 1748-0221. ; 7, s. C01002-
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Tidskriftsartikel (refereegranskat)abstract
- Two Disc DIRC prototypes have been designed and built and are shortly being tested with proton test beams. Different in design details, both aim to provide the WASA-at-COSY experiment with a particle velocity measurement that improves the missing mass resolution. This paper shows the difference in concept between these two designs and also compares to designs that have been proposed for PANDA.
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| 22. |
- Ghaderi, Hazhar
(författare)
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The interplay between quark and hadronic degrees of freedom and the structure of the proton
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We study the low-energy sector of the strong interaction which is the least understood part of the Standard Model, the theory that describes the interactions of all known particles. The ideal particles for this study are the proton and the neutron, collectively called the nucleon. They make up the nucleus of all the atoms of our world and understanding them has been of high priority ever since their discovery. We show that one cannot neglect the effects of other hadrons, such as neutrons and pions when studying the proton. A large part of the proton's hadronic wavefunction is shown to consist of the wavefunctions of other hadrons. In other words, when probing the proton there is a sizeable probability that one is probing some other hadron surrounding the proton as a quantum fluctuation.The nucleon itself consists of elementary particles known as quarks and gluons, collectively called partons. Exactly how the properties of these partons make up the properties of the nucleon has been the subject of active research ever since their discovery. Two main issues are the flavor asymmetry of the proton sea and the spin structure of the nucleon. To address these questions we study the interplay between the partonic and hadronic degrees of freedom. We introduce a model based on a convolution between hadronic quantum fluctuations as described by chiral perturbation theory, and partonic degrees of freedom motivated by a physical model of the nucleon having only few physically constrained parameters.We present the hadronic distribution functions and the parton distribution functions. The results are in agreement with a large set of experimental data. These include the structure functions of the proton and the neutron. Agreement with the sum rules of the spin structure functions offers new insight into the spin structure of the nucleon.
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| 23. |
- Granados, Carlos, et al.
(författare)
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The electromagnetic Sigma-to-Lambda hyperon transition form factors at low energies
- 2017
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Ingår i: European Physical Journal A. - : Springer Science and Business Media LLC. - 1434-6001 .- 1434-601X. ; 53:6
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Tidskriftsartikel (refereegranskat)abstract
- 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).
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| 24. |
- Hejny, Volker, et al.
(författare)
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CHARGE SYMMETRY BREAKING IN dd -> (4)He pi(0) WITH WASA-at-COSY
- 2011
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Ingår i: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 26:3-4, s. 595-597
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Tidskriftsartikel (refereegranskat)abstract
- Charge symmetry breaking effects can be used as an experimental tool to study quark mass effects. Using recent high precision data from TRIUMF and IUCF an international theory collaboration is working on a consistent description within the framework of chiral pertubation theory. The WASA-at-COSY collaboration has started an experimental program extending the studies on the reaction dd -> (4)He pi(0) towards higher excess energies in order to provide necessary information on the contribution of p-waves. First preliminary results on the total cross section at Q = 60 MeV are presented.
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| 25. |
- Hilger, T., et al.
(författare)
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Chiral QCD sum rules for open-charm mesons
- 2011
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Ingår i: Physical Review C. Nuclear Physics. - 0556-2813 .- 1089-490X. ; 84:4, s. 045202-
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Tidskriftsartikel (refereegranskat)abstract
- QCD sum rules for chiral partners in the open-charm meson sector are presented at nonzero baryon net density or temperature. We focus on the differences between pseudo-scalar and scalar as well as vector and axial-vector D mesons and derive the corresponding Weinberg-type sum rules. This allows for the identification of such QCD condensates which drive the nondegeneracy of chiral partners in the lowest order of the strong coupling alpha(s) and which, therefore, may serve as "order parameters" for chiral restoration (or elements thereof).
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| 26. |
- Hilger, T., et al.
(författare)
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Four-quark condensates in open-charm chiral QCD sum rules
- 2012
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Ingår i: Progress in Particle and Nuclear Physics. - : Elsevier BV. - 0146-6410 .- 1873-2224. ; 67:2, s. 188-193
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Forskningsöversikt (refereegranskat)abstract
- Recently, in Hilger et al. (2011) [1] QCD sum rules for chiral partners in the open-charm meson sector have been presented at nonzero baryon net density or temperature up to and including mass dimension 5. Referring to this, details concerning the cancelation of infrared divergences are presented and important technical and conceptional ingredients for an incorporation of four-quark condensates beyond factorization and of other mass dimension 6 condensates are collected.
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| 27. |
- Hilger, T., et al.
(författare)
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The impact of chirally odd condensates on the rho meson
- 2012
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Ingår i: Physics Letters B. - : Elsevier BV. - 0370-2693 .- 1873-2445. ; 709:3, s. 200-206
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Tidskriftsartikel (refereegranskat)abstract
- Based on QCD sum rules we explore the consequences of a scenario for the rho meson, where the chiral symmetry breaking condensates are set to zero whereas the chirally symmetric condensates remain at their vacuum values. This clean-cut scenario causes a lowering of the rho spectral moment by about 120 MeV. The complementarity of mass shift and broadening is discussed. A simple parametrization of the rho spectral function leads to a width of about 280 MeV if no shift of the peak position is assumed.
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| 28. |
- Hodana, Malcorzata, et al.
(författare)
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STUDY OF THE eta -> gamma e(+)e(-) DECAY WITH THE WASA-at-COSY
- 2011
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Ingår i: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 26:3-4, s. 650-651
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Tidskriftsartikel (refereegranskat)abstract
- In October 2008, the WASA-at-COSY collected more than 10(7) events for the pd -> (3)He eta reaction. The aim is the determination of the invariant mass of the lepton pairs created in the Dalitz decay eta -> gamma e(+)e(-). The e(+)e(-) invariant mass spectrum is directly related to the distribution of the four-momentum squared of the virtual photon from the eta -> gamma gamma* process and hence it allows for the study of the transition form factor which in turn reflects the structure of the decaying meson.
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| 29. |
- Hoferichter, Martin, et al.
(författare)
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Dispersion relation for hadronic light-by-light scattering : pion pole
- 2018
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Ingår i: Journal of High Energy Physics (JHEP). - 1126-6708 .- 1029-8479. ; :10
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Tidskriftsartikel (refereegranskat)abstract
- The pion-pole contribution to hadronic light-by-light scattering in the anomalous magnetic moment of the muon (g − 2)μ is fully determined by the doubly-virtual pion transition form factor. Although this crucial input quantity is, in principle, directly accessible in experiment, a complete measurement covering all kinematic regions relevant for (g −2)μ is not realistic in the foreseeable future. Here, we report in detail on a reconstruction from available data, both space- and time-like, using a dispersive representation that accounts for all the low-lying singularities, reproduces the correct high- and low-energy limits, and proves convenient for the evaluation of the (g − 2)μ loop integral. We concentrate on the systematics of the fit to e+e− → 3π data, which are key in constraining the isoscalar dependence, as well as the matching to the asymptotic limits. In particular, we provide a detailed account of the pion transition form factor at low energies in the time- and space-like region, including the error estimates underlying our final result for the pion-pole contribution, aμπ0-pole = 62.6+3.0-2.5 x 10-11, and demonstrate how forthcoming singly-virtual measurements will further reduce its uncertainty.
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| 30. |
- Hoferichter, M., et al.
(författare)
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Dispersive analysis of the pion transition form factor
- 2014
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; 74:11, s. 3180-
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the pion transition form factor using dispersion theory. We calculate the singly-virtual form factor in the time-like region based on data for the e(+) e(-) -> 3 pi cross section, generalizing previous studies on omega, phi -> 3 pi decays and gamma pi -> pi pi scattering, and verify our result by comparing to e(+) e(-) -> pi(0)gamma. data. We perform the analytic continuation to the space-like region, predicting the poorly-constrained space-like transition form factor below 1GeV, and extract the slope of the form factor at vanishing momentum transfer a(pi) = (30.7 +/- 0.6) x10(-3). We derive the dispersive formalism necessary for the extension of these results to the doubly-virtual case, as required for the pion-pole contribution to hadronic light-by-light scattering in the anomalous magnetic moment of the muon.
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| 31. |
- Hoferichter, Martin, et al.
(författare)
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Pion-Pole Contribution to Hadronic Light-By-Light Scattering in the Anomalous Magnetic Moment of the Muon
- 2018
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 121:11
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Tidskriftsartikel (refereegranskat)abstract
- The pi(0) pole constitutes the lowest-lying singularity of the hadronic light-by-light (HLBL) tensor, and thus, it provides the leading contribution in a dispersive approach to HLBL scattering in the anomalous magnetic moment of the muon (g - 2)(mu). It is unambiguously defined in terms of the doubly virtual pion transition form factor, which in principle, can be accessed in its entirety by experiment. We demonstrate that, in the absence of a direct measurement, the full spacelike doubly virtual form factor can be reconstructed very accurately based on existing data for e(+)e(-) -> 3 pi, e(+)e(-)-> e(+)e(-)pi(0), and the pi(0) -> gamma gamma decay width. We derive a representation that incorporates all the low-lying singularities of the form factor, matches correctly onto the asymptotic behavior expected from perturbative QCD, and is suitable for the evaluation of the (g - 2)(mu) loop integral. The resulting value, a(mu)(pi 0-pole) = 62.6(-2.5)(+3.0) x 10(-11), for the first time, represents a complete data-driven determination of the pion-pole contribution with fully controlled uncertainty estimates. In particular, we show that already improved singly virtual measurements alone would allow one to further reduce the uncertainty in a(mu)(pi 0-pole).
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| 32. |
- Holmberg, Måns, et al.
(författare)
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Low-energy axial-vector transitions from decuplet to octet baryons
- 2019
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Ingår i: Physical Review D. - : AMER PHYSICAL SOC. - 2470-0010 .- 2470-0029. ; 100:11
-
Tidskriftsartikel (refereegranskat)abstract
- Axial-vector transitions of decuplet to octet baryons are parametrized at low energies guided by a complete and minimal chiral Lagrangian up to next-to-leading order. It is pointed out that beyond the well-known leading-order term, there is only one contribution at next-to-leading order. This contribution is flavor symmetric. Therefore the corresponding low-energy constant can be determined in any strangeness sector. As functions of this low-energy constant, we calculate the decay widths and Dalitz distributions for the decays of decuplet baryons to octet baryons, pions, and photons and for the weak decay of the Omega baryon to a cascade baryon, an electron, and an antineutrino.
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| 33. |
- Holmberg, Måns, et al.
(författare)
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The relativistic chiral Lagrangian for decuplet and octet baryons at next-to-leading order
- 2018
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Ingår i: European Physical Journal A. - : SPRINGER. - 1434-6001 .- 1434-601X. ; 54:6
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Tidskriftsartikel (refereegranskat)abstract
- A complete and minimal relativistic Lagrangian is constructed at next-to-leading order for SU(3) chiral perturbation theory in the presence of baryon octet and baryon decuplet states. The Lagrangian has 13 terms for the pure decuplet sector, 6 terms for the transition sector from baryon octet to decuplet and (as already known from the literature) 16 terms for the pure octet sector. The minimal field content of 25 of these terms is meson-baryon four-point interactions. 3 terms give rise to the mass splitting for baryon octet and decuplet states, respectively. 2 terms give rise to overall mass shifts. 4 terms provide anomalous magnetic moments and a decuplet-to-octet magnetic transition moment. 1 term leads to an axial vector transition moment. It is shown that meson-baryon three-point coupling constants come in at leading order whereas no additional one appears in the minimal Lagrangian at next-to-leading order. Those low-energy constants that give rise to mass splitting and magnetic moments, respectively, are determined. Predictions are provided for radiative decays of decuplet to octet baryons.
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| 34. |
- Husek, Tomas, et al.
(författare)
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Radiative corrections for the decay Sigma(0) -> Lambda e(+)e(-)
- 2020
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Ingår i: European Physical Journal C. - : SPRINGER. - 1434-6044 .- 1434-6052. ; 80:3
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Tidskriftsartikel (refereegranskat)abstract
- Electromagnetic form factors serve to explore the intrinsic structure of nucleons and their strangeness partners. With electron scattering at low energies the electromagnetic moments and radii of nucleons can be deduced. The corresponding experiments for hyperons are limited because of the unstable nature of the hyperons. Only for one process this turns to an advantage: the decay of the neutral Sigma hyperon to a Lambda hyperon and a real or virtual photon. Due to limited phase space the effects caused by the Sigma-to-Lambda transition form factors compete with the QED radiative corrections for the decay sigma 0 -> e+e-. These QED corrections are addressed in the present work, evaluated beyond the soft-photon approximation, i.e., over the whole range of the Dalitz plot and with no restrictions on the energy of the radiative photon.
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| 35. |
- Husek, Tomas, et al.
(författare)
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Radiative corrections to the η(') Dalitz decays
- 2018
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Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 97:9
-
Tidskriftsartikel (refereegranskat)abstract
- We provide the complete set of radiative corrections to the Dalitz decays eta(()'()) -> l(+)l(-)gamma beyond the softphoton approximation, i. e., over the whole range of the Dalitz plot and with no restrictions on the energy of a radiative photon. The corrections inevitably depend on the eta(()'()) -> gamma*gamma(*) transition form factors. For the singly virtual transition form factor appearing, e.g., in the bremsstrahlung correction, recent dispersive calculations are used. For the one-photon-irreducible contribution at the one-loop level (for the doubly virtual form factor), we use a vector-meson-dominance-inspired model while taking into account the eta-eta' mixing.
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| 36. |
- Husek, Tomas, et al.
(författare)
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Two-hadron saturation for the pseudoscalar-vector€-vector correlator and phenomenological applications
- 2015
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Ingår i: European Physical Journal C. - : Springer Science and Business Media LLC. - 1434-6044 .- 1434-6052. ; C75:12
-
Tidskriftsartikel (refereegranskat)abstract
- The pseudoscalar-vector-vector correlator is constructed using two mesonmultiplets in the vector and two in the pseudoscalar channel. The parameters are constrained by the operator product expansion at leading order where two or all three momenta are considered as large. Demanding in addition the Brodsky-Lepage limit one obtains (in the chiral limit) a pion-vector-vector (pi VV) correlator with only one free parameter. The singly virtual pion transition form factor F-pi 0 gamma gamma* and the decay width of omega -> pi(0)gamma. are independent of this parameter and can serve as cross-checks of the results. The free parameter is determined from a fit of the omega-pi transition form factor F-pi 0 omega gamma*. The resulting pi VV correlator is used to calculate the decay widths omega -> pi(0) e(+) e(-) and omega -> pi(0) mu(+) mu(-) and finally the widths of the rare decay pi(0) -> e(+) e(-) and of the Dalitz decay pi(0) -> e(+) e(-) gamma(.) Incorporating radiative QED corrections the calculations of pi(0) decays are compared to the KTeV results. We find a deviation of 2 sigma or less for the rare pion decay.
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| 37. |
- Jany, Anna, et al.
(författare)
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STUDIES OF MESON PRODUCTION AND DECAYS IN pd -> (3)He X WITH WASA-at-COSY
- 2011
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Ingår i: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 26:3-4, s. 652-653
-
Tidskriftsartikel (refereegranskat)abstract
- Studies of meson production in the pd -> (3)He X reaction, where X = eta, eta' and omega, and their decays are investigated with the WASA-at-COSY facility. The angular distribution of the eta in the center of mass of the (3)He-eta system, have been measured at an excess energy of Q = 61 MeV over the full angular range. The anisotropy expected at the relatively large Q is observed.
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| 38. |
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| 39. |
- Jany, Benedykt R., et al.
(författare)
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LEADING MODES OF 3 pi(0) PRODUCTION IN PROTON-PROTON COLLISIONS AT INCIDENT PROTON MOMENTUM 3.35 GeV/c
- 2011
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Ingår i: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 26:3-4, s. 654-655
-
Tidskriftsartikel (refereegranskat)abstract
- For the first time pp -> pp3 pi(0) has been measured at an incident proton momentum of P(b)=3.35GeV/c. The total cross section has been determined. The reaction mechanism is studied by a Dalitz plot analyses, which indicate that the leading production and decay mechanism is closely related to the simultaneous excitation of Delta(1232) and N*(1440) baryon resonances.
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| 40. |
- Junker, Olov, et al.
(författare)
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Electromagnetic form factors of the transition from the spin-3/2 Σ to the Λ hyperon
- 2020
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Ingår i: Physical Review C. Nuclear Physics. - : American Physical Society (APS). - 0556-2813 .- 1089-490X. ; 101:1
-
Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 41. |
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| 42. |
- Leupold, Jochen, et al.
(författare)
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Fast chemical shift mapping with multiecho balanced SSFP
- 2006
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Ingår i: Magma. - : Springer Science and Business Media LLC. - 1352-8661. ; 19:5, s. 267-273
-
Tidskriftsartikel (refereegranskat)abstract
- Object: A method is proposed that provides spectroscopic images with high spatial resolution and moderate spectral resolution at very short total data acquisition times. Materials and methods: Balanced steady-state free precession (bSSFP, TrueFISP, FIESTA, b-FFE) is combined with a multiecho readout gradient and frequency-sensitive reconstruction such as Fourier reconstruction known from echo-planar spectroscopic imaging (EPSI) or matrix inversion. Balanced SSFP imaging requires short repetition times to minimize banding artefacts, thereby restricting the achievable frequency resolution. Results: Two-dimensional (2D) high-resolution spectroscopic images were produced of three H-1 resonances (water, acetone and fat) on phantoms and water/fat separation in vivo within 1-2 s. Additionally, fast P-31 spectroscopic images were acquired from a phantom consisting of two resonances within 195 ms. Conclusion: Frequency-sensitive reconstruction of multiecho bSSFP data can provide spectroscopic images with high spatial and temporal resolution while the frequency resolution is moderate at around 100 Hz. The method can also separate more than three resonances, allowing for hetero-nuclei metabolite mapping, for example C-13 and P-31.
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| 43. |
- Leupold, Jochen, et al.
(författare)
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Fast multiecho balanced SSFP metabolite mapping of H-1 and hyperpolarized C-13 compounds
- 2009
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Ingår i: Magma. - : Springer Science and Business Media LLC. - 1352-8661. ; 22:4, s. 251-256
-
Tidskriftsartikel (refereegranskat)abstract
- To investigate the feasibility of multiecho balanced steady-state free precession (bSSFP)-based fast chemical shift mapping hyperpolarized C-13 metabolites. The overall goal was to reduce total imaging time and to increase spatial resolution compared to common chemical shift imaging (CSI). A multiecho bSSFP sequence in combination with an iterative reconstruction algorithm was implemented. H-1 experiments were performed on phantoms and on a human volunteer in order to investigate the feasibility of the method on a system with metabolite maps that are known beforehand. C-13 experiments were performed in vivo on pigs, where CSI images were acquired also for comparison. Chemical shift images of three and four distinct H-1 resonance frequencies as well as chemical shift images of up to five hyperpolarized C-13 metabolites were successfully obtained. Fast metabolite mapping based on multiecho balanced SSFP in combination with an iterative reconstruction approach could successfully separate several H-1 resonances and hyperpolarized C-13 metabolites.
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| 44. |
- Leupold, Stefan
(författare)
-
Interactions of light mesons with photons
- 2014
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Ingår i: Meson 2014 - 13th International Workshop on Production, Properties and Interaction of Mesons. - : EDP Sciences.
-
Konferensbidrag (refereegranskat)abstract
- Interactions between hadrons and photons constitute an important tool to explore the intrinsic structure of hadrons. At the same time, high-precision determinations of electromagnetic processes, as in particular g - 2 of the muon, constitute a promising tool to look for the impact of beyond-standard-model physics. Here hadronic contributions constitute the main uncertainty for the standard-model prediction. In this talk results for electromagnetic transition form factors and two-gamma amplitudes are presented.
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| 45. |
- Leupold, Stefan
(författare)
-
The nucleon as a test case to calculate vector-isovector form factors at low energies
- 2018
-
Ingår i: European Physical Journal A. - : SPRINGER. - 1434-6001 .- 1434-601X. ; 54:1
-
Tidskriftsartikel (refereegranskat)abstract
- Extending a recent suggestion for hyperon form factors to the nucleon case, dispersion theory is used to relate the low-energy vector-isovector form factors of the nucleon to the pion vector form factor. The additionally required input, i.e. the pion-nucleon scattering amplitudes are determined from relativistic next-to-leading-order (NLO) baryon chiral perturbation theory including the nucleons and optionally the Delta baryons. Two methods to include pion rescattering are compared: a) solving the Muskhelishvili-OmnSs (MO) equation and b) using an N/D approach. It turns out that the results differ strongly from each other. Furthermore the results are compared to a fully dispersive calculation of the (subthreshold) pion-nucleon amplitudes based on Roy-Steiner (RS) equations. In full agreement with the findings from the hyperon sector it turns out that the inclusion of Delta baryons is not an option but a necessity to obtain reasonable results. The magnetic isovector form factor depends strongly on a low-energy constant of the NLO Lagrangian. If it is adjusted such that the corresponding magnetic radius is reproduced, then the results for the corresponding pion-nucleon scattering amplitude (based on the MO equation) agree very well with the RS results. Also in the electric sector the Delta degrees of freedom are needed to obtain the correct order of magnitude for the isovector charge and the corresponding electric radius. Yet quantitative agreement is not achieved. If the subtraction constant that appears in the solution of the MO equation is not taken from nucleon+Delta chiral perturbation theory but adjusted such that the electric radius is reproduced, then one obtains also in this sector a pion-nucleon scattering amplitude that agrees well with the RS results.
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| 46. |
- Leupold, Stefan, et al.
(författare)
-
Towards a dispersive determination of the pion transition form factor
- 2018
-
Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 2100-014X. ; 166
-
Tidskriftsartikel (refereegranskat)abstract
- We start with a brief motivation why the pion transition form factor is interesting and, in particular, how it is related to the high-precision standard-model calculation of the gyromagnetic ratio of the muon. Then we report on the current status of our ongoing project to calculate the pion transition form factor using dispersion theory. Finally we present and discuss a wish list of experimental data that would help to improve the input for our calculations and/or to cross-check our results.
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| 47. |
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| 48. |
- Leupold, Stefan, et al.
(författare)
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Vector-meson dominance revisited
- 2012
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Ingår i: EPJ Web of Conferences. - : EDP Sciences. - 2100-014X. ; 37, s. 05008-
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 49. |
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| 50. |
- Michler, Frank, et al.
(författare)
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Off-equilibrium photon production during the chiral phase transition
- 2013
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Ingår i: Annals of Physics. - : Elsevier BV. - 0003-4916 .- 1096-035X. ; 336, s. 331-393
-
Tidskriftsartikel (refereegranskat)abstract
- In the early stage of ultrarelativistic heavy-ion collisions chiral symmetry is restored temporarily. During this so-called chiral phase transition, the quark masses change from their constituent to their bare values. This mass shift leads to the spontaneous non-perturbative creation of quark-antiquark pairs, which effectively contributes to the formation of the quark-gluon plasma. We investigate the photon production induced by this creation process. We provide an approach that eliminates possible unphysical contributions from the vacuum polarization and renders the resulting photon spectra integrable in the ultraviolet domain. The off-equilibrium photon numbers are of quadratic order in the perturbative coupling constants while a thermal production is only of quartic order. Quantitatively, we find, however, that for the most physical mass-shift scenarios and for photon momenta larger than 1 GeV the off-equilibrium processes contribute less photons than the thermal processes.
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