arXiv subjects

V. A. Nikonov

Publications and source records attributed to V. A. Nikonov.

At least 19 recordsLinked to original sources

arXiv2025

Polarization observables in double neutral pion photoproduction

Measurements of target asymmetries and double-polarization observables for the reaction $γp\to pπ^0π^0$ are reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility (Bonn University) using the Bonn frozen-spin butanol (C$_4$H$_9$OH) target, which provided transversely polarized protons. Linearly polarized photons were produced via bremsstrahlung off a diamond crystal. The data cover the photon energy range from $E_γ$=650 MeV to $E_γ$=2600 MeV and nearly the complete angular range. The results have been included in the BnGa partial wave analysis. Experimental results and the fit agree very well. Observed systematic differences in the branching ratios for decays of $N^*$ and $Δ^*$ resonances are attributed to the internal structure of these excited nucleon states. Resonances which can be assigned to SU(6)$\times$O(3) two-oscillator configurations show larger branching ratios to intermediate states with non-zero intrinsic orbital angular momenta than resonances assigned to one-oscillator configurations.

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arXiv2025

Measurement of single- and double-polarization observables in the photoproduction of $π^+π^-$~meson pairs off the proton using CLAS at Jefferson Laboratory

The photoproduction of $π^+π^-$ meson pairs off the proton has been studied in the reaction $γp\to p\,π^+π^-$ using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target (FROST) in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the beam and target asymmetries, $I^{s,c}$ and $P_{x,y}$, have been measured along with the beam-target double-polarization observables, $P^{s,c}_{x,y}$, using a transversely polarized target with center-of-mass energies ranging from 1.51 GeV up to 2.04 GeV. These data and additional $ππ$ photoproduction observables from CLAS and experiments elsewhere were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from $s$-channel resonance production are observed in addition to $t$-channel exchange processes. The data indicate significant contributions from $N^\ast$ and $Δ^\ast$ resonances in the third and fourth resonance regions.

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arXiv2023

Measurement of the $γn\rightarrow K^0Σ^0$ differential cross section over the $K^*$ threshold

The differential cross section for the quasi-free photoproduction reaction $γn\rightarrow K^0Σ^0$ was measured at BGOOD at ELSA from threshold to a centre-of-mass energy of 2400 MeV. Close to threshold the results are consistent with existing data and are in agreement with partial wave analysis solutions over the full measured energy range, with a large coupling to the $Δ$(1900)1/2- evident. This is the first dataset covering the K* threshold region, where there are model predictions of dynamically generated vector meson-baryon resonance contributions.

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arXiv2019

Hyperon II: Properties of excited hyperons

We report properties of $Λ$ and $Σ$ hyperon resonances formed in $K^-$ induced reactions. Special emphasis is laid on the analysis of the three-body final states $2π^0Λ$ and $2π^0Σ$ and of the quasi-two-body final states $πΛ(1520)$, $\bar KΔ(1232)$, $πΣ(1385)$, $\bar K^*N$, and $ωΛ$. We give pole positions of $Λ$ and $Σ$ hyperon resonances and transition residues from the $K^-p$ initial to various final states as well as Breit-Wigner masses and widths and decay branching ratios. Twenty resonances and "bumps" reported in the Review of Particle Physics are not required in our fits, evidence for five new resonances is reported. The observed mass spectrum is compared to the spectrum calculated in the Bonn quark model. Three spin doublets, six $Λ$ hyperons, are tentatively assigned to the SU(3) singlet system.

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arXiv2019

Hyperon I: Partial wave amplitudes for $K^-p$ scattering

Early data on $K^-$ induced reactions off protons are collected and used in a coupled-channel partial wave analysis (PWA). Data which had been published in the form of Legendre coefficients are included in the PWA. In a {\it primary} fit using 3* and 4* resonances only, we observe some significant discrepancies with the data. In a systematic search for new $Λ$ and $Σ$ hyperon resonances, additional candidates are found. The significance of the known and of the additional resonances is evaluated. Seventeen resonances listed with 1* or 2* and one resonance listed with 3* in the Review of Particle Properties cannot be confirmed, five new hyperons are suggested. The partial-wave amplitudes deduced in this analysis are compared to those from other analyses.

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arXiv2019

Hyperon I: Study of the $Λ(1405)$

Low-energy data on the three charge states in $γp \to K^+(Σπ)$ from CLAS at JLab, on $K^-p\to π^0π^0Λ$ and $π^0π^0Σ$ from the Crystal Ball at BNL, bubble chamber data on $K^-p\toπ^-π^+π^{\pm}Σ^{\mp}$, low-energy total cross sections on $K^-$ induced reactions, and data on the $K^-p$ atom are fitted with the BnGa partial-wave-analysis program. We find that the data can be fitted well with just one isoscalar spin-1/2 negative-parity pole, the $Λ(1405)$, and background contributions.

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arXiv2019

First Measurements of the Double-Polarization Observables $F$, $P$, and $H$ in $ω$ Photoproduction off Transversely Polarized Protons in the $N^\ast$ Resonance Region

First measurements of double-polarization observables in $ω$ photoproduction off the proton are presented using transverse target polarization and data from the CEBAF Large Acceptance Spectrometer (CLAS) FROST experiment at Jefferson Lab. The beam-target asymmetry $F$ has been measured using circularly polarized, tagged photons in the energy range 1200 - 2700 MeV, and the beam-target asymmetries $H$ and $P$ have been measured using linearly polarized tagged photons in the energy range 1200 - 2000 MeV. These measurements significantly increase the database on polarization observables. The results are included in two partial-wave analyses and reveal significant contributions from several nucleon ($N^\ast$) resonances. In particular, contributions from new $N^\ast$ resonances listed in the Review of Particle Properties are observed, which aid in reaching the goal of mapping out the nucleon resonance spectrum.

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arXiv2018

Beam-Target Helicity Asymmetry $E$ in $K^{0}Λ$ and $K^{0}Σ^0$ Photoproduction on the Neutron

We report the first measurements of the $E$ beam-target helicity asymmetry for the $\vecγ \vec{n} \to K^{0}Λ$, and $K^{0}Σ^{0}$ channels in the energy range 1.70$\leq W\leq$2.34 GeV. The CLAS system at Jefferson Lab uses a circularly polarized photon beam and a target consisting of longitudinally polarized solid molecular hydrogen deuteride with low background contamination for the measurements. The multivariate analysis method boosted decision trees was used to isolate the reactions of interest. Comparisons with predictions from the KaonMAID, SAID, and Bonn-Gatchina models are presented. These results will help separate the isospin $I=0$ and $I=1$ photo-coupling transition amplitudes in pseudoscalar meson photoproduction.

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arXiv2018

Measurement of the beam asymmetry $Σ$ and the target asymmetry $T$ in the photoproduction of $ω$ mesons off the proton using CLAS at Jefferson Laboratory

The photoproduction of $ω$ mesons off the proton has been studied in the reaction $γp\to p\,ω$ using the CEBAF Large Acceptance Spectrometer (CLAS) and the frozen-spin target (FROST) in Hall B at the Thomas Jefferson National Accelerator Facility. For the first time, the target asymmetry, $T$, has been measured in photoproduction from the decay $ω\toπ^+π^-π^0$, using a transversely-polarized target with energies ranging from just above the reaction threshold up to 2.8 GeV. Significant non-zero values are observed for these asymmetries, reaching about 30-40% in the third-resonance region. New measurements for the photon-beam asymmetry, $Σ$, are also presented, which agree well with previous CLAS results and extend the world database up to 2.1 GeV. These data and additional $ω$-photoproduction observables from CLAS were included in a partial-wave analysis within the Bonn-Gatchina framework. Significant contributions from $s$-channel resonance production were found in addition to $t$-channel exchange processes.

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arXiv2018

Proton-$η^\prime$ interactions at threshold

Recent data on photoproduction of $η'$ mesons off protons have been included in the data base for the Bonn-Gatchina partial wave analysis. The real and imaginary parts of the $S$-wave $η^\prime p\to η^\prime p$ scattering amplitude in the threshold region were fit to yield the $η^\prime p$ scattering length and the interaction range. This new analysis found $|a_{η^\prime p}|= (0.403\pm 0.015\pm 0.060)$ fm and a phase $ϕ=(87\pm2)^\circ$, while the range parameter is not well-constrained. The striking behavior of the GRAAL data on the beam asymmetry in the threshold region suggests that a narrow proton-$η'$ resonance might exist. However, the scattering length was found to be relatively insensitive to the possible existence of this narrow resonance.

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arXiv2018

Measurement of the helicity asymmetry $E$ in $ω\toπ^+π^-π^0$ photoproduction

The double-polarization observable $E$ was studied for the reaction $γp\to pω$ using the CEBAF Large Acceptance Spectrometer (CLAS) in Hall B at the Thomas Jefferson National Accelerator Facility and the longitudinally-polarized frozen-spin target (FROST). The observable was measured from the charged decay mode of the meson, $ω\toπ^+π^-π^0$, using a circularly-polarized tagged-photon beam with energies ranging from the $ω$ threshold at 1.1 to 2.3 GeV. A partial-wave analysis within the Bonn-Gatchina framework found dominant contributions from the $3/2^+$ partial wave near threshold, which is identified with the sub-threshold $N(1720)\,3/2^+$ nucleon resonance. To describe the entire data set, which consisted of $ω$ differential cross sections and a large variety of polarization observables, further contributions from other nucleon resonances were found to be necessary. With respect to non-resonant mechanisms, $π$ exchange in the $t$-channel was found to remain small across the analyzed energy range, while pomeron $t$-channel exchange gradually grew from the reaction threshold to dominate all other contributions above $W \approx 2$ GeV.

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arXiv2017

Strong evidence for nucleon resonances near 1900\,MeV

Data on the reaction $γp\to K^+Λ$ from the CLAS experiments are used to derive the leading multipoles, $E_{0+}$, $M_{1-}$, $E_{1+}$, and $M_{1+}$, from the production threshold to 2180\,MeV in 24 slices of the invariant mass. The four multipoles are determined without any constraints. The multipoles are fitted using a multichannel $L+P$ model which allows us to search for singularities and to extract the positions of poles on the complex energy plane in an almost model-independent method. The multipoles are also used as additional constraints in an energy-dependent analysis of a large body of pion and photo-induced reactions within the Bonn-Gatchina (BnGa) partial wave analysis. The study confirms the existence of poles due to nucleon resonances with spin-parity $J^P = 1/2^-; 1/2^+$, and $3/2^+$ in the region at about 1.9\,GeV.

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arXiv2017

$N^*$ resonances from $KΛ$ amplitudes in sliced bins in energy

The two reactions $γp\to K^+Λ$ and $π^-p\to K^0Λ$ are analyzed to determine the leading photoproduction multipoles and the pion-induced partial wave amplitudes in slices of the invariant mass. The multipoles and the partial-wave amplitudes are simultaneously fitted in a multichannel Laurent+Pietarinen model (L+P model), which determines the poles in the complex energy plane on the second Riemann sheet close to the physical axes. The results from the L+P fit are compared with the results of an energy-dependent fit based on the Bonn-Gatchina (BnGa) approach. The study confirms the existence of several poles due to nucleon resonances in the region at about 1.9\,GeV with quantum numbers $J^P = 1/2^+$, $3/2^+, 1/2^-, 3/2^-, 5/2^-$.

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arXiv2017

$N^*\to N η^\prime$ decays from photoproduction of $η^\prime$-mesons off protons

A study of the partial-wave content of the $γp\to η^\prime p$ reaction in the fourth resonance region is presented, which has been prompted by new measurements of polarization observables for that process. Using the Bonn-Gatchina partial-wave formalism, the incorporation of new data indicates that the $N(1895)1/2^-$, $N(1900)3/2^+$, $N(2100)1/2^+$, and $N(2120)3/2^-$ are the most significant contributors to the photoproduction process. New results for the branching ratios of the decays of these more prominent resonances to $Nη^\prime$ final states are provided; such branches have not been indicated in the most recent edition of the Review of Particle Properties. Based on the analysis performed here, predictions for the helicity asymmetry $E$ for the $γp\to η^\prime p$ reaction are presented.

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arXiv2017

Measurement of the Differential and Total Cross Sections of the $γd \to K^0 Λ (p)$ Reaction within the Resonance Region

We report the first measurement of differential and total cross sections for the $γd \to K^0Λ(p)$ reaction, using data from the CLAS detector at the Thomas Jefferson National Accelerator Facility. Data collected during two separate experimental runs were studied with photon-energy coverage 0.8 - 3.6 GeV and 0.5 - 2.6 GeV, respectively. The two measurements are consistent giving confidence in the method and determination of systematic uncertainties. The cross sections are compared with predictions from the KAON-MAID theoretical model (without kaon exchange), which deviate from the data at higher W and at forward kaon angles. These data, along with previously published cross sections for $K^+ Λ$ photoproduction, provide essential constraints on the nucleon resonance spectrum. A first partial wave analysis has been performed that describes the data without the introduction of new resonances.

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arXiv2017

Photon beam asymmetry $Σ$ in the reaction $\vecγ p \to p ω$ for $E_γ$ = 1.152 to 1.876 GeV

Photon beam asymmetry $Σ$ measurements for $ω$ photoproduction in the reaction $\vecγ p \to ωp$ are reported for photon energies from 1.152 to 1.876 GeV. Data were taken using a linearly-polarized tagged photon beam, a cryogenic hydrogen target, and the CLAS spectrometer in Hall B at Jefferson Lab. The measurements obtained markedly increase the size of the database for this observable, extend coverage to higher energies, and resolve discrepancies in previously published data. Comparisons of these new results with predictions from a chiral-quark-based model and from a dynamical coupled-channels model indicate the importance of interferences between $t$-channel meson exchange and $s$- and $u$-channel contributions, underscoring sensitivity to the nucleon resonances included in those descriptions. Comparisons with the Bonn-Gatchina partial-wave analysis indicate the $Σ$ data reported here help to fix the magnitudes of the interference terms between the leading amplitudes in that calculation (Pomeron exchange and the resonant portion of the $J^P=3/2^+$ partial wave), as well as the resonant portions of the smaller partial waves with $J^P$= $1/2^-$, $3/2^-$, and $5/2^+$.

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arXiv2017

The Beam-Target Helicity Asymmetry for $\vecγ \vec{n} \rightarrow π^- p$ in the {\bf{$N^*$} Resonance Region

We report the first beam-target double-polarization asymmetries in the $γ+ n(p) \rightarrow π^- + p(p)$ reaction spanning the nucleon resonance region from invariant mass $W$= $1500$ to $2300$ MeV. Circularly polarized photons and longitudinally polarized deuterons in $H\!D$ have been used with the CLAS detector at Jefferson Lab. The exclusive final state has been extracted using three very different analyses that show excellent agreement, and these have been used to deduce the {\it{E}} polarization observable for an effective neutron target. These results have been incorporated into new partial wave analyses, and have led to significant revisions for several $γnN^*$ resonance photo-couplings.

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arXiv2017

Study of the proton-proton collisions at 1683 MeV/c

The new data on the elastic $pp$ and single pion production reaction $pp\to pn π^{+}$ taken at the incident proton momentum 1683 MeV/c are presented. The data on the $pp\to pn π^{+}$ reaction are compared with predictions from the OPE model. To extract contributions of the leading partial waves the single pion production data are analyzed in the framework of the event-by-event maximum likelihood method together with the data measured earlier. The analysis allows us to obtain the assessment with a better precision the contributions from different initial partial waves.

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