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Volker D. Burkert

Publications and source records attributed to Volker D. Burkert.

At least 19 recordsLinked to original sources
arXiv2026

Direct Bayesian Inference of Helicity Amplitudes from Detector-Level Scattering Data

Helicity amplitudes give the most complete description of a variety of scattering reactions used in studies of strong interactions, but they cannot be measured directly: experiments record bilinear combinations of them folded through a detector response, and conventional analyses recover them through multiple stages that introduce discrete ambiguities and require a separate extraction of the absolute cross sections. Here we replace that chain with a single Bayesian inference that determines the experimentally identifiable amplitude parameters directly from detector-level measurements. A score-based diffusion model, trained on a forward simulator, provides the full posterior in each kinematic bin, with the detector response carried by the forward model and positivity of the spin-density matrix guaranteed by the parameterization. The observable amplitude content in electroproduction of final particles grows with polarization of beams and targets, providing additional sensitivity to underlying phases. In simulation, the posterior achieves empirical coverage at or above the nominal level and delivers the angular observables and the separated contributions from longitudinal and transverse photons with their correlations retained. It transfers without retraining to a realistic detector response absent from training and remains reliable in the weakly constrained nucleon-helicity-flip sector, where per-bin likelihood maximization degrades. The result is a general framework for a broad class of inverse problems, phase retrieval, quantum-state tomography, and partial-wave analysis are further instances. Its core requires only a forward simulation of the complete measurement process, instrumental effects are handled within one statistically consistent posterior, applicable across exclusive vector-meson programs at Jefferson Lab, COMPASS, HERMES, and the future Electron-Ion Collider.

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arXiv2022

50 Years of Quantum Chromodynamics

This paper presents a comprehensive review of both the theory and experimental successes of Quantum Chromodynamics, starting with its emergence as a well defined theory in 1972-73 and following developments and results up to the present day. Topics include a review of the earliest theoretical and experimental foundations; the fundamental constants of QCD; an introductory discussion of lattice QCD, the only known method for obtaining exact predictions from QCD; methods for approximating QCD, with special focus on effective field theories; QCD under extreme conditions; measurements and predictions of meson and baryon states; a special discussion of the structure of the nucleon; techniques for study of QCD at high energy, including treatment of jets and showers; measurements at colliders; weak decays and quark mixing; and a section on the future, which discusses new experimental facilities or upgrades currently funded. The paper is intended to provide a broad background for Ph.D. students and postdocs starting their career. Some contributions include personal accounts of how the ideas or experiments were developed.

hep-ph↗
arXiv2022

Nucleon resonances and transition form factors

This is a contribution to the review 50 Years of Quantum Chromodynamics edited by F. Gross and E. Klempt, to be published in Journal EPJC. This contribution reviews the nucleon resonance transition form factors determined from meson electro-production experiments at electron accelerator facilities, i.e. this contribution focuses on space-like transition form factors and amplitudes. Comparisons are made when available to LQCD and to approaches with traceable links to strong QCD and to advanced quark model calculations

hep-ph↗
arXiv2021

Determination of two-photon exchange via $e^+p/e^-p$ Scattering with CLAS12

The proton elastic form factor ratio shows a discrepancy between measurements using the Rosenbluth technique in unpolarized beam and target experiments and measurements using polarization degrees of freedom. The proposed explanation of this discrepancy is uncorrected hard two-photon exchange (TPE), a type of radiative correction that is conventionally neglected. The effect size and agreement with theoretical predictions has been tested recently by three experiments. While the results support the existence of a small two-photon exchange effect, they cannot establish that theoretical treatments are valid. At larger momentum transfers, theory remains untested. This proposal aims to measure two-photon exchange over an extended and so far largely untested $Q^2$ and $\varepsilon$ range with high precision using the {\tt CLAS12} experiment. Such data are crucial to clearly confirm or rule out TPE as the driver for the discrepancy as well as test several theoretical approaches, believed valid in different parts of the tested $Q^2$ range.

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arXiv2019

N$^*$ Experiments and what they tell us about Strong QCD Physics

I give an overview on experimental studies of the spectrum and the structure of the excited states of the nucleon and what we can learn about their internal structure. One focus is on the efforts to obtain a more complete picture of the light-quark baryon excitation spectrum employing electromagnetic beams that will allow us to draw some conclusions on the symmetries underlying the spectrum. For the higher mass excitations, the full employment of coupled channel approaches is essential when searching for new excited states in the large amounts of data already accumulated in different channels involving a variety of polarization observables. The other focus is on the study of transition form factors and helicity amplitudes and their dependences on $Q^2$, especially on some of the more prominent resonances, especially $Δ(1232)\frac{3}{2}^+$, $N(1440)\frac{1}{2}^+$, and negative parity states $N(1535)\frac{1}{2}^-$, and $N(1675)\frac{5}{2}^-$. These were obtained in pion and eta electroproduction experiments off proton targets and have already led to further insights in the active degrees-of-freedom as a function of the distance scale involved.

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arXiv2018

N* Experiments and their Impact on Strong QCD Physics

I give a brief overview of experimental studies of the spectrum and the structure of the excited states of the nucleon and what we learn about their internal structure. The focus is on the effort to obtain a more complete picture of the light-quark baryon excitation spectrum employing electromagnetic beams, and on the study of the transition form factors and helicity amplitudes and their dependence on the magnitude of the photon virtuality $Q^2$, especially for some of the most prominent resonances. The results were obtained in pion and eta electroproduction experiments off proton targets. They strengthen the connection of experiment and new results from modeling sQCD in DSE and Light Cone SR approaches. They also point to the nature of these states as 3-quark excitations at the core.

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arXiv2018

Positrons at JLab - Advancing Nuclear Science in Hall B

In this talk I address two high impact physics programs that require the use of polarized and unpolarized positron beams in addition to using electron beams of the same energy. First, I address what will be gained from using positron beams in addition to electron beams in the extraction of the Compton Form Factors (CFFs) and generalized parton distributions (GPDs) from Deeply Virtual Compton Scattering (DVCS) on a proton target. As a second high impact science program I discuss an experimental scenario using unpolarized positrons to measure elastic scattering on protons in an effort to determine definitively the 2-photon exchange contributions in order to resolve a longstanding discrepancy in the determination of the proton's electric and magnetic form factors.

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arXiv2017

Roper resonance -- solution to the fifty year puzzle

For half a century, the Roper resonance has defied understanding. Discovered in 1963, it appears to be an exact copy of the proton except that its mass is 50% greater. The mass is the first problem: it is difficult to explain with any theoretical tool that can validly be used to study quantum chromodynamics [QCD]. In the last decade, a new challenge has appeared, viz. precise information on the proton-to-Roper electroproduction transition form factors, reaching $Q^2\approx 4.5\,$GeV$^2$. This scale probes the domain within which hard valence-quark degrees-of-freedom could be expected to determine form factor behavior. Hence, with this new data the Roper resonance becomes a problem for strong-QCD [sQCD]. An explanation of how and where the Roper resonance fits into the emerging spectrum of hadrons cannot rest on a description of its mass alone. Instead, it must combine this with a detailed understanding of the Roper's structure and how that is revealed in the transition form factors. Furthermore, it must unify all this with a similarly complete picture of the proton. This is a prodigious task, but a ten-year international effort, drawing together experimentalists and theorists, has presented a solution to the puzzle. Namely, the Roper is at heart the proton's first radial excitation, consisting of a dressed-quark core augmented by a meson cloud that reduces the core mass by approximately 20% and materially alters its electroproduction form factors on $Q^2<2m_N^2$, where $m_N$ is the proton's mass. We describe the experimental motivations and developments which enabled electroproduction data to be procured within a domain that is unambiguously the purview of sQCD, thereby providing a real challenge and opportunity for modern theory; and survey the developments in reaction models and QCD theory that have enabled this conclusion to be drawn about the nature of the Roper resonance.

nucl-ex↗
arXiv2017

12 GeV CEBAF - The Physics and Experiments

In this talk the role of spin and polarization is discussed in experimental search for new excited baryon states and in the study of the internal quark-gluon structure of the proton and neutron. Also the perspective of spin physics at the 12 GeV CEBAF electron accelerator is discussed and what we hope to learn about fundamental properties of hadrons such as their multi-dimensional structure through the momentum and spatial imaging, and about the forces on the quarks in the proton and how quark confinement may be realized through the spatial distribution of such forces.

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arXiv2016

Spectrum and Structure of Excited Baryons with CLAS

In this contribution we discuss recent results in light quark baryon spectroscopy involving CLAS data and higher level analysis results from the partial wave analysis by the Bonn-Gatchina group. New baryon states were discovered largely based on the open strangeness production channels $γp \to K^+ Λ$ and $γp \to K^+ Σ^0$. The data illustrate the great potential of the kaon-hyperon channel in the discovery of higher mass baryon resonances in s-channel production. Other channels with discovery potential, such as $γp \to p ω$ and $γp \to ϕp$ are also discussed. In the second part I will demonstrate on data the sensitivity of meson electroproduction to expose the active degrees of freedom underlying resonance transitions as a function of the probed distance scale. For several of the prominent excited states in the lower mass range the short distance behavior is described by a core of three dressed-quarks with running quark mass, and meson-baryon contributions make up significant parts of the excitation strength at large distances. Finally, an outlook is given of baryon resonance physics at the 12 GeV CEBAF electron accelerator.

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arXiv2016

Nucleon Resonance Physics

Recent results of meson photo-production at the existing electron machines with polarized real photon beams and the measurement of polarization observables of the final state baryons have provided high precision data that led to the discovery of new excited nucleon and $Δ$ states using multi-channel partial wave analyses procedures. The internal structure of several prominent excited states has been revealed employing meson electroproduction processes. On the theoretical front, lattice QCD is now predicting the baryon spectrum with very similar characteristics as the constituent quark model, and continuum QCD, such as is represented in the Dyson-Schwinger Equations approach and in light front relativistic quark models, describes the non-perturbative behavior of resonance excitations at photon virtuality of $Q^2 > 1.5GeV^2$. In this talk I discuss the need to continue a vigorous program of nucleon spectroscopy and the study of the internal structure of excited states as a way to reveal the effective degrees of freedom underlying the excited states and their dependence on the distance scale probed.

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arXiv2014

Status of some P-wave baryon resonances and importance of inelastic channels

We analyze the current status of three P-wave baryon states $N(1710){1/ 2}^+$, $N(1900){3/2}^+$, and $Δ(1600){3/2}^+$ as given in the Review of Particles Physics (RPP). Since the evidence for a particle's existence is linked to its RPP "star" rating, we discuss its subjective present definition. We also present the accumulating evidence supporting these states and give our new "star" rating recommendations.

nucl-ex↗
arXiv2013

Study of excited nucleons and their structure

Recent advances in the study of excited nucleons are discussed. Much of the progress has been achieved due to the availability of high precision meson production data in the photoproduction and electroproduction sectors, the development of multi-channel partial wave analysis techniques, and advances in Lattice QCD with predictions of the full excitation spectrum.

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arXiv2012

Evidence of new nucleon resonances from electromagnetic meson production

The study of nucleon resonances in electromagnetic meson production with the CLAS detector is discussed. The electromagnetic interaction is complementary to pion scattering in the exploration of the nucleon excitation spectrum. Higher mass states often decouple from the $Nπ$ channel and are not seen in $πN \to πN$. Photoproduction of mesons, such as $KΛ$, $ωp$ and $η^\prime p$ may be more sensitive to many of these states. The CLAS detector, combined with the use of energy-tagged polarized photons and polarized electrons, as well as polarized targets and the measurement of recoil polarization, are the tools needed for a comprehensive nucleon resonance program. Some of the recently published high statistics data sets had significant impact on further clarifying the nucleon excitation spectrum.

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arXiv2012

The JLab 12GeV Upgrade and the Initial Science Program

An overview is presented of the upgrade of JLab's cw electron accelerator from a maximum beam energy of currently 6 GeV to 12 GeV. Construction of the 12 GeV upgrade project has started in 2008. A broad experimental program has been developed to map the nucleon's intrinsic correlated spin and momentum distribution through measurements of deeply exclusive and semi-inclusive processes, and to probe the quark and gluon confinement by studying the spectrum of mesons with exotic quantum numbers. Other programs include the forward parton distribution function at large xB, the quark and gluon polarized distribution functions, the measurements of electromagnetic form factors of the nucleon ground state and of nucleon resonance transitions at high Q2, and the exploration of physics beyond the Standard Model in high precision parity violating processes. The 12 GeV electron beam is also well suited to explore quark hadronization properties using the nucleus as a laboratory.

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arXiv2011

Nucleon Resonance Electrocouplings from the CLAS Data on Exclusive Meson Electroproduction off Protons

$γ_{v}NN^*$ transition helicity amplitudes (electrocouplings) of several prominent excited proton states are determined for the first time in independent analyses of $π^+n$, $π^0p$, and $π^+π^-p$ electroproduction off protons. Analysis of $π^+π^-p$ electroproduction has extended considerably information on electrocouplings of high lying N* states, which decay preferentially to the $Nππ$ final states.

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arXiv2010

Meson Production and Baryon Resonances at CLAS

I give a brief overview of the exploration of baryon properties in meson photo- and electroproduction. These processes provide ample information for the study of electromagnetic couplings of baryon resonances and to search for states, yet to be discovered. The CLAS detector, combined with the use of energy-tagged polarized photons and polarized electrons, as well as polarized targets and the measurement of recoil polarization, provide the tools for a comprehensive nucleon resonance program. I briefly present the status of this program, prospects for the next few years, and plans for the Jefferson Lab 12 GeV upgrade.

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