In order to connect predictions for the baryon spectrum from quark models or lattice calculations to experimental data, coupled-channel frameworks are especially suited. In those approaches a simultaneous partial-wave analysis of multiple reactions with different initial and final states is performed.
I will present recent results from the Juelich-Bonn dynamical coupled-channel approach,...
The study of the spectrum and structure of excited nucleon (N∗
) states via the electroproduction of exclusive reactions is an important avenue for exploring the nature of the non-perturbative strong interaction. The $\gamma_vpN^*$ electrocouplings of N∗ states in the mass range below 1.8 GeV have been determined from analyses of CLAS $\pi N$, $\eta N$, and $\pi\pi N$ data at beam energies up...
The study of the nucleon structure and of its excited states
is a powerful tool in order to gain a better understanding
of the dynamics inside the nucleon and of the non-perturbative regime of QCD.
The different observables accessible using polarized photon
beams and/or polarized nucleon targets play an essential role
in this experimental research due to their enhanced sensitivity...
CLAS baryon spectroscopy programme
M. Dugger* (for the CLAS Collaboration)
Arizona State University,
College of Integrated Sciences and Arts
Mesa, AZ 85212-6415, USA
E-mail: dugger@jlab.org
One of the major thrusts in hadronic physics is to more fully
understand the internal structure of baryonic matter. The challenges
presented in understanding baryonic structure are large, in...
The Mainz Microtron (MAMI), operated by the Institute for Nuclear Physics at the Gutenberg University Mainz, provides unpolarized and polarized electron beams of up to 1604 MeV energy. These beams have been used in the multi-spectrometer facility (called A1) to probe kaon electro-production reactions including studies of hypernuclei. In parallel, studies of photo-induced kaon production...
The last two decades have witnessed the discovery of a myriad of new and unexpected hadrons. Hadron spectroscopy provides direct physical measurements that shed light on the non-perturbative behavior of quantum chromodynamics (QCD) and the new pentaquark states observed by LHCb offer unique insights into the QCD dynamics in hadron structures. In this talk, some of the main experimental...
A good understanding of the spectrum and the properties of baryon resonances requires a detailed study of the excited states and their decays. To extract contributing resonances from data, cross sections and polarization observables must be determined and further investigated by partial wave analysis. Multi-meson final states are particularly important at high energies, where resonances are...
The discoveries of the pentaquark, $P_C$, states and $XYZ$ mesons in the charmed quark sector initiated a new epoch in hadron physics. The existence of exotic multi-quark states beyond the conventional $\textit{q$\overline{q}$}$ and $\textit{qqq}$ systems has obviously been realised. Such states could manifest as single colour bound objects, or evolve from meson-baryon and meson-meson...
Hadron electromagnetic form factors describe the intrinsic dynamics of the charge and magnetic distributions in composite particles. They are experimentally accessible through (un)polarized cross sections measurements and angular distributions in the crossing symmetry related reactions : electron-hadron elastic scattering and electron-positron annihilation into hadron-antihadron (and its...
New models for photo- and electroproduction of kaons on the proton were constructed [1,2,3] utilizing new experimental data from LEPS, GRAAL, and particularly CLAS collaborations. The higher spin nucleon (spin-3/2 and spin-5/2) and hyperon (spin-3/2) resonances were included using a consistent formalism and they were found to play an important role in the data description. In these analyses,...
Recently structures in invariant mass distributions and excitation energy spectra have been attributed to triangular singularities as discussed in e.g., [1,2] and in the review by Guo et al. [3]. These singularities emerge under specific kinematic conditions when new reaction channels open up. It will be shown that a triangular singularity associated with the opening of the $\gamma p...
A very important contribution of dilepton production in pion-nucleon collisions comes from the Dalitz decay of s-channel baryon resonances, R->Ne+e-. These dileptons originate from a virtual photon with small timelike squared four-momentum q^2, therefore the study of this reaction gives access to the electromagnetic interaction of baryon resonances in a kinematical domain inaccessible...
The exclusive double pion electromagnetic production is an important tool for the study of N and 𝛥 excitations and for the search of missing baryonic resonances. As far as photoproduction is concerned, the two pion channel represents the dominant contribution to the total cross section especially in the second resonant region, therefore the formation of resonances coupling directly to the 𝛾N...
Unlike stable hadrons, whose production yield in relativistic heavy-ion collisions is established by the temperature at the chemical freeze-out, hadron resonances are subject to final-state interactions occurring in the late hadron-gas phase of the collision after the chemical freeze-out. Processes such as the rescattering of the decay products and the regeneration are competing out of...
The study of electromagnetic transitions opens a window into the very nature of the strong interaction. And, indeed, such a study of how a ground-state nucleon transitions to an excited state, over a broad range of $q^2$, will provide keen insight into the evolution of how dynamically-generated masses emerge from the asymptotically-free, nearly massless quarks of perturbative QCD as well as...
We describe the application of a model-independent reconstruction method to experimental data in order to identify complex poles of overlapping resonances. The algorithm is based on the Schlessinger Point Method where data points are interpolated using a continued-fraction expression. Statistical uncertainties of the experimental data are propagated with resampling. In order to demonstrate the...
During the past several decades a large quantity of high-quality mesonic photo- and electroproduction data of have been measured at electromagnetic facilities worldwide. By contrast, meson-beam data for these same final states are mostly outdated, largely of poorer quality, or even non-existent. Thus existing meson beam results provide inadequate input to interpret, analyze, and exploit the...
In this contribution a summary of the highlights of the recent experimental findings on strangeness production and dynamics will be presented.
Results obtained by different experimental collaborations spanning in
a large range in centre-of-mass energy and a variety of collision systems will be discussed. The talk does not aim at being a complete review, but rather at connecting the...
A rigorous identification of physical states from scattering experiments is possible by tracing the pole origin of the observed peaks. The identification becomes nontrivial if a peak appears very close to a two-hadron threshold. In this work we discuss how one can utilize a neural network to help map the observed peaks with the nature of S-matrix pole. Specifically, we can teach a deep neural...
In 2010, high-precision studies of muonic hydrogen found notably smaller values for the charge radius than earlier results that have been extracted from elastic electron-scattering data and through the spectroscopy of atomic hydrogen. The MUon Scattering Experiment (MUSE) at the Paul Scherrer Institute (PSI) has been developed to address this so-called proton-radius puzzle. The experiment will...
Light-flavor hadrons constitute the bulk of particle production in ultrarelativistic hadron-hadron collisions at the LHC. The study of their production yields, differential in transverse momentum and multiplicity, is fundamental to constrain hadron production models and to investigate the hadronization process. In this context, hadronic resonances are particularly interesting tools since they...
At J-PARC, we built a large acceptance time-projection chamber, named Hyp-TPC, for various studies on baryons. Especially, E45 experiment will study exotic nucleon/Delta resonances via $p(\pi,2\pi)$ reactions. We will also introduce the other experiments with Hyp-TPC, namely:
- H-dibaryon search (E42)
- Exotic hyperon resonance search (E72)
- $\Sigma N$ cusp study with $d(K^-,\pi^-)$ reaction
The electric ($\alpha_{\text{E1}}$) and magnetic ($\beta_{\text{M1}}$) scalar polarizabilities describe the response of the nucleon to an applied electric or magnetic field. They are not only fundamental properties related to the internal structure and dynamics of the nucleon, but they are important also in other areas of physics, such as atomic structure.
The A2@MAMI Collaboration has...
High-energy electrons and photons are a remarkably clean probe of hadronic matter, essentially providing a microscope for examining atomic nuclei and the strong nuclear force. One of the most striking phenomena of Quantum Chromodynamics (QCD) is the formation of the nucleon out of massless gluons and almost massless quarks. This system of confined quarks and gluons serves as the basic...
The idea of dressing quark-model states in a coupled-channel analysis to describe scattering data has been around for decades. What’s new are formalisms able to bring these descriptions to the finite-volume of lattice QCD where calculations of the excitation spectrum provide new constraints. This combination of lattice QCD and experiment demands that we reconsider our preconceived notions...
Why nucleon resonance excitations play a unique role in leading the path towards a comprehensive QCD theory will be laid out and illustrated by recent exclusive meson electroproduction results off free and bound nucleons. Near- and potential long-term achievements will highlight upcoming opportunities.
Not long ago, we have developed and implemented a novel (Jülich-Bonn-Washington) model for pion electroproduction off the proton. Based on phenomenological (Jülich-Bonn) model, it incorporated constraints from the photoproduction and scattering data. Going to non-zero virtuality of the photon (Q^2>0) it allows now to address the abundant (O(10^5)) electroproduction data.
In this talk we...
Accelerator experiments have reported the exotic hadrons that cannot be understood in the ordinary hadron picture, describing baryons as $qqq$ and mesons as $q\bar{q}$. The experimental observations indicate that the exotic hadrons have multiquark structures such as a compact multiquark, e.g. $qq\bar{q}\bar{q}$ and $qqqq\bar{q}$, and a hadron composite state called hadronic molecule. There...
The High Acceptance Di-Electron Spectrometer (HADES) [1], installed at GSI Helmholtzzentrum in Darmstadt, was designed for spectroscopy of positron-electron pairs with excellent purity and mass resolution. The experimental program of HADES focuses on two main goals: (I) measurements of dielectron emission of a compressed baryonic matter formed in 1-2 AGeV heavy-ion collisions and investigate...
Diquarks are often used as QCD effective degrees of freedom to describe nucleons and other baryons as well as exotic hadrons. Phenomenologically the splittings of the four possible diquark operators, grouped into three channels dubbed "good", "bad" and "not-even-bad", can be estimated from the experimentally observed spectrum in principle. Yet, despite the concept of diquarks being very old...
The pion-nucleus reaction is an important source of information about hadronic matter. At incident momenta below 2 GeV/c, it gives access in a very unique way to the properties of baryonic resonances in the nuclear medium. While the region of the Δ(1232) resonance, corresponding to incident pion beam momenta of about 300 MeV/c, was studied in detail in the past, only very scarce measurements...
The photo-induced reactions are complementary to hadron-induced reactions in the study of excited baryons.
In particular, GeV photon beam can provide information on highly excited hadrons.
In the LEPS experiment at SPring-8, hadron photoproduction reactions have been studied using linearly polarized photons from laser Compton scattering up to 2.9 GeV. The LEPS experiment measured production...
Recent Belle II results on charmed baryon lifetimes are presented. In addition prospects for other baryon analyses expected to be performed using Belle II data in the future are reviewed.
The GlueX experiment at Jefferson Lab studies the spectrum of hadrons in photoproduction on a LH2 target. Its almost hermetic detector configuration is optimized to measure both charged and neutral final state particles with good resolutions. This allows GlueX to measure a wide range of different reactions, including those with strangeness. In this talk we are going to present our ongoing...
Electromagnetic dipole moments of fundamental particles provide powerful probes for physics within and beyond the Standard Model. For the case of short-lived particles these have not been experimentally accessible to date due to the difficulties imposed by their short lifetimes. Novel experimental techniques have been developed to allow a unique program of direct measurements of electric and...
In the past two decades there has been tremendous progress in the theoretical and experimental investigation of multiquark states, which has expanded our understanding of what a “hadron” is. Experimental evidence suggests that Nature does not only form “conventional” hadrons such as mesons as quark-antiquark states and baryons as three-quark states, but also more exotic combinations such as...
Photoproduction of a neutral pion, omega, or eta meson on the proton has been experimentally studied in their neutral decay modes at the LEPS2 BGOegg experiment with incident photon energies ranging from 1.3 GeV to 2.4 GeV. Differential cross sections, photon beam asymmetries, and spin density matrix elements are measured with high statistics and a wide angular region by using a large...
I will review recent progress in lattice QCD calculations of the hadron spectrum, with a focus on baryonic resonances. The talk will highlight formal progress in extracting various multi-hadron amplitudes as well as recent numerical results. In the presentation I will highlight key challenges faced by lattice calculations, e.g. due to the restriction to a Euclidean finite-volume spacetime. I...
I report on a pilot lattice simulation to study hadronic parity violation with lattice QCD, in particular
to calculate the $P$-odd long-range pion-nucleon coupling $h_\pi^1$.
I discuss the implementation of a recently proposed new approach, where the parity-violating Lagrangian
is mapped to a $P$-conserving one, based on the PCAC relation, and the coupling $h_\pi^1$
is extracted from...
The experimental investigation of the low-energy negatively charged kaons interaction with the nuclear matter is very important to understand the strength of the K-nuclei interaction and to provide essential input for understanding of the non-perturbative QCD in the strangeness sector. It has strong consequences in various sectors of physics, like nuclear and particle physics as well as ...
Hadrons are strongly interacting systems whose dynamics is driven by complex intercommunication between quarks and gluons. The theory of strong interaction, Quantum ChromoDynamics (QCD) , is supposed to describe all particles, however, due to numerical complexity we are still far away from reaching this goal. In such a situation, experimental knowledge about existing resonances becomes...
Our research contributes to a larger experimental program that seeks to shed light on the evolving status of the proton spectrum. Determining the hadronic spectrum is a complicated task due to the high number of excited states of the nucleon, all of which have large widths causing resonances to overlap. Also, these resonances may decay into a multitude of decay channels.
In this talk, we...
The observation that neutrinos can oscillate from one flavour to another suggests that these elementary particles have a very small but non-zero mass. The weak cross sections that occur in the neutrino oscillation experiments are parametrized among other things by the nucleon axial and vector form factors, which can be computed via lattice QCD simulations. In this talk, I will highlight the...
We present results for the heavy baryon spectrum for ground and excited states with quantum numbers $J^P = 1/2^+$ and $3/2^+$ using functional methods in QCD. To this end, we reduce the three-quark Faddeev equations to two-body equations via the quark-diquark approach, where the baryons are treated as bound states of quarks and effective diquarks. The resulting Bethe-Salpeter equation amounts...
The FROST experiment at Jefferson Lab used the CLAS detector in Hall B with the intention of performing a complete and over-determined measurement of the polarisation observables associated with strangeness photoproduction, in combination with data from previous JLab experiments as part of the N* program. This was achieved by utilising the FROST polarised target in conjunction with polarised...
QCD-motivated models for hadrons predict a wide variety of multi-quark states beyond ordinary mesons and baryons, known as exotic states.
The first observation of a heavy exotic state by Belle in 2003 has triggered a huge experimental effort, and the last 20 years have marked a turning point in the field.
To date, states composed of four and five valence quark have been observed and their...
Hadron spectroscopy is a field of considerable interest for validating predictions of the Heavy Quark effective theory and calculations of Lattice QCD at low energies. LHCb has been the main player in the field, having observed more than 50 new hadrons, both conventional and exotic. This talk will present new exciting results released by LHCb recently. On the one hand, for conventional...
In this talk we will give a general overview on recent results from several groups
on the spectrum and properties of light baryons as obtained in the framework of
Dyson-Schwinger and Bethe-Salpeter equations.
We will discuss the spectrum of light baryons with focus on the comparison with
quark model expectations and explain the importance of relativistic components
in the wave...
The talk will provide an overview of the newly-obtained knowledge about the light-baryon spectrum, as well as of the energy-dependent partial-wave analysis models which have been used to attain said knowledge. Some details will be provided on how these models can be classified, in particular with regard to their incorporation of S-Matrix constraints. An outlook will be given at the end of the...
Exploring the low energy region of QCD requires a detailed study of the nucleon resonances and their decays. Analysing the excitation spectrum of (quasi-) free nucleons is essential to establish a baseline for comparisons with in-medium modifications. Of particular interest are the mixed-charged channels of double-pion photoproduction, as they are sensitive to sequential decays of $\Delta$...
We shall present a recent calculation of \gamma^{(*)}p->N(1535) transition form factors, based upon a continuum Schwinger method approach which employs a Poincaré-covariant Faddeev equation to describe baryons as composite states. Although limited to a symmetry-preserving contact interaction model of QCD, the results herein shown serve as benchmarks for future more sophisticated calculations;...
A new approach has been developed to energy-dependent, single-channel partial wave analysis which does not require constructing and solving elaborate theoretical model of analyzed two body reaction, but uses general principles of analyticity instead. Standard approach of obtaining energy dependent two body partial waves (multipoles) was to create a theoretical model, solve it, fit the free...
Although many more cascade resonances than established are expected from quark models and QCD calculations, experimental evidence of most of them have been rather weak. With increased beam energy and luminosity now available at CLAS12, it is possible to search for excited cascades using quasi-real photoproduction as well as electroproduction using the CLAS12 spectrometer and probe their...
We study the two step sequential one pion production mechanism, $np(I = 0) \toπ^− pp$, followed by the fusion reaction $pp \to π^+ d$, in order to describe the $np \to π^+ π^− d$ reaction with $π^+ π^−$ in $I = 0$, where a narrow peak, so far identified with a “d(2380)” dibaryon, has been observed. We find that the second step $pp \to π^+ d$ is driven by a triangle singularity that determines...
Hadrons are strongly interacting particles composed of quarks and gluons and described by Quantum Chromodynamics (QCD). Their internal structure can be described in terms of structure functions that encode, for example, the momentum and spin distributions of their constituents. Parton distribution functions (PDFs), for example, describe the quark and gluon momentum distributions inside a...
