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تسجيل مستخدم جديدRecent results on the nucleon resonance spectrum and structure from the CLAS detector
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The CLAS detector at Jefferson Lab has provided the dominant part of all available worldwide data on exclusive meson electroproduction off protons in the resonance region. New results on the $gamma_{v}pN^*$ transition amplitudes (electrocouplings) are available from analyses of the CLAS data and will be presented. Their impact on understanding of hadron structure will be discussed emphasizing the credible access to the dressed quark mass function that has been achieved for the first time by a combined analysis of the experimental results on the electromagnetic nucleon elastic and $N rightarrow N^*$ transition form factors. We will also discuss further convincing evidences for a new baryon state $N^{, }(1720)3/2^+$ found in a combined analysis of charged double pion photo- and electroproduction cross sections off the protons.
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Studies of the nucleon resonance electroexcitation amplitudes in a wide range of photon virtualities offer unique information on many facets of strong QCD behind the generation of all prominent excited nucleon states. Advances in the evaluation of re
sonance electroexcitation amplitudes from the data measured with the CLAS detector and the future extension of these studies with the CLAS12 detector at Jefferson Lab are presented. For the first time, analyses of $pi^0p$, $pi^+n$, $eta p$, and $pi^+pi^-p$ electroproduction off proton channels have provided electroexcitation amplitudes of most resonances in the mass range up to 1.8 GeV and at photon virtualities $Q^2 < 5$~GeV$^2$.Studies of the resonance electroexcitation amplitudes revealed the $N^*$ structure as a complex interplay between the inner core of three dressed quarks and the external meson-baryon cloud. The successful description of the $Delta(1232)3/2^+$ and $N(1440)1/2^+$ electrocouplings achieved within the Dyson-Schwinger Equation approach under a traceable connection to the QCD Lagrangian and supported by the novel light front quark model demonstrated the relevance of dressed quarks with dynamically generated masses as an active structural component in baryons. Future experiments with the CLAS12 detector will offer insight into the structure of all prominent resonances at the highest photon virtualities, $Q^2 < 12$~GeV$^2$, ever achieved in exclusive reactions, thus addressing the most challenging problems of the Standard Model on the nature of hadron mass, quark-gluon confinement, and the emergence of nucleon resonance structures from QCD. A search for new states of hadronic matter, the so-called hybrid-baryons with glue as a structural component, will complete the long term efforts on the resonance spectrum exploration.
Studies of the spectrum of hadrons and their structure in experiments with electromagnetic probes offer unique insight into many facets of the strong interaction in the regime of large quark-gluon running coupling, {it i.e.} the regime of strong QCD.
The experimental program within Hall~B at Jefferson Laboratory based on data acquired with the CLAS spectrometer using electron and photon beams with energies up to 6~GeV has already considerably extended the scope of research in hadron physics in joint efforts between experiment and phenomenological data analysis. Impressive progress in relating the hadron structure observables inferred from the data to the strong QCD mechanisms underlying hadron mass generation has been achieved in the past decade. These results will be considerably extended with data from the experimental program with the new CLAS12 spectrometer that has begun data taking using electron beams with energies up to 11~GeV. With this extended kinematic reach the structure of nucleon resonances will be probed at the highest photon virtualities ever achieved in the studies of exclusive electroproduction, which will allow for the exploration of the distance scale where $>$98% of light hadron mass emerges from QCD in the transition of the strong interaction from the regime of quark-gluon confinement to perturbative QCD.
The analysis of the nine 1-fold differential cross sections for the $gamma_{r,v} p to pi^+pi^-p$ photo- and electroproduction reactions obtained with the CLAS detector at Jefferson Laboratory was carried out with the goal to establish the contributin
g resonances in the mass range from 1.6~GeV to 1.8~GeV. In order to describe the photo- and electroproduction data with $Q^2$-independent resonance masses and hadronic decay widths in the $Q^2$ range below 1.5~GeV$^2$, it was found that an $N(1720)3/2^+$ state is required in addition to the already well-established nucleon resonances. This work demonstrates that the combined studies of $pi^+pi^-p$ photo- and electroproduction data are vital for the observation of this resonance. The contributions from the $N(1720)3/2^+$ state and the already established $N(1720)3/2^+$ state with a mass of 1.745~GeV are well separated by their different hadronic decays to the $pi Delta$ and $rho p$ final states and the different $Q^2$-evolution of their photo-/electroexcitation amplitudes. The $N(1720)3/2^+$ state is the first recently established baryon resonance for which the results on the $Q^2$-evolution of the photo-/electrocouplings have become available. These results are important for the exploration of the nature of the ``missing baryon resonances.
The results on the photo- and electroexcitation amplitudes of most nucleon resonances in the mass range up to 2.0 GeV determined from the CLAS experimental data on exclusive $pi^+pi^-p$ photo-/electroproduction off protons in collaboration between th
e Jefferson Lab and Moscow State University are presented. The first and only available results on electroexcitation amplitudes from CLAS in a wide range of photon virtualities $Q^2$ $<$ 5.0 GeV$^2$ revealed the nucleon resonance structure as a complex interplay between the inner core of three dressed quarks and external meson-baryon cloud. These results shed light on the strong QCD dynamics which underlines the generation of excited nucleon states of different structural features from confined quarks and gluons. The future prospects of these studies in the new era of experiments with the CLAS12 detector, which started successfully in Spring of 2018, are outlined.
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