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Here, extending our study of the vector meson-nucleon scattering lengths, we are focusing on the 𝜌-meson case using recent CLAS threshold data for the reaction 𝛾⁢𝑝 →𝜌⁢𝑝 within the meson–baryon reaction model. The total 𝜎_𝑡⁡(𝛾⁢𝑝 →𝜌⁢𝑝) and 𝜎_𝑡⁡(𝛾⁢𝑝 →𝜔⁢𝑝) cross sections are close to the momentum of the vector meson in CM 𝑞 =0.2 GeV/c. Then the 𝜌⁢𝑝 photoproduction cross section grows rapidly. Our result for 𝜌⁢𝑁 scattering length is a factor of 4 smaller than the size of the hadron and the phenomenological determination of the 𝜔 nucleon scattering length using threshold photoproduction cross sections. The observed difference betweenmore » 𝜌⁢𝑁 and 𝜔⁢𝑁 scattering lengths is of interest for further understanding within hadron structure theory.« less

The electroexcitation amplitudes or $$\gamma_vpN^*$$ electrocouplings of the $N(1440)1/2^+$, $N(1520)3/2^-$, and $$\Delta(1600)3/2^+$$ resonances were obtained for the first time from the $$ep \to e'\pi^+\pi^-p'$$ differential cross sections measured with the CLAS detector at Jefferson Lab within the range of invariant mass $$W$$ of the final state hadrons from 1.4–1.7~GeV for photon virtualities $Q^2$ from 2.0--5.0~GeV$^2$. A good description of the nine independent one-fold differential $$\gamma_v p\to \pi^+\pi^-p'$$ cross sections achieved within the data-driven Jefferson Lab-Moscow State University (JM) meson-baryon reaction model in each bin of ($$W$$,$Q^2$) allows for separation of the resonant and non-resonant contributions. The electrocouplings were determined inmore » the fits of the $$\pi^+\pi^-p$$ cross sections within three overlapping $$W$$ intervals with a substantial contribution from each of the three resonances listed above. Consistent results on the electrocouplings extracted from the data in these $$W$$ intervals provide evidence for their reliable extraction. These studies extend information on the electrocouplings of the $N(1440)1/2^+$ and $N(1520)3/2^-$ available from this channel over a broader range of $Q^2$. The electrocouplings of the $$\Delta(1600)3/2^+$$, which decays preferentially into $$\pi\pi N$$ final states, have been determined for the first time. Here, the reliable extraction of the electrocouplings for these states is also supported by the description of the $$\pi^+\pi^-p$$ differential cross sections with $Q^2$-independent masses and total/partial hadronic decay widths into the $$\pi\Delta$$ and $$\rho p$$ final states.« less

The first results on the σ_LT' structure function in exclusive π⁰p electroproduction at invariant masses of the final state of 1.5 GeV < W < 1.8 GeV and in the range of photon virtualities 0.4 GeV² < Q² < 1.0 GeV² were obtained from data on beam spin asymmetries and differential cross sections measured with the CLAS detector at Jefferson Lab. The Legendre moments deter- mined from the σ_LT' structure function have demonstrated sensitivity to the contributions from the nucleon resonances in the second and third resonance regions. These new data on the beam spin asymmetries in π⁰p electroproduction extendmore » the opportunities for the extraction of the nucleon resonance electroexcitation amplitudes in the mass range above 1.6 GeV.« less

The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion π0 into two photons (γγ), leading to its unusually short lifetime. Here, we precisely measured the decay width Γ of the π⁰→ γγ process. The differential cross sections for π0 photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding Γ(π⁰→ γγ)=7.798±0.056(stat.)±0.109(syst.) eV, where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weightedmore » average of Γ(π⁰→ γγ)=7.802±0.052(stat.)±0.105(syst.) eV. Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.« less