Recent results on a study of exclusive processes of e^+e^- annihilation into hadrons below 2 GeV obtained at the SND detector are presented. The analyses are based on data collected at the VEPP-2000 collider. In particular, we present the measurements of the e^+e^-→ηγ , e^+e^-→π^+π^-π^0 , e^+e^-→ωπ^0 cross sections and the preliminary results on study of the e^+e^-→ηπ^+π^-2π^0 process. The results obtained are compared with previous measurements.

Recent results on a study of exclusive processes of annihilation into hadrons with the SND detector at the VEPP-2000 collider at center-of-mass energies below 2 GeV are presented. In particular, we discuss new data on the processes e^ + e^ - → nn̅ , pp̅ , ηγ , ωπ^0 , π^ + π^ - π^0 , and π^ + π^ - π^0π^0η .

The 𝑒+𝑒− → 𝑛𝑛 cross section was measured at energies from the threshold to 1908 MeV in the center of mass (c.m.). The experiment to measure the cross section has been carried out at the VEPP-2000 𝑒+𝑒− collider in 13 energy points. The SND detector is used to detect the produced neutron-antineutrons (𝑛𝑛) events. A special time measurement system on the calorimeter was used to select the time-delayed 𝑛𝑛 events. The measured 𝑒+𝑒− → 𝑛𝑛 cross section is 0.4–0.6 nb. The neutron effective timelike form factor in the energy range under study varies from 0.3 to 0.6.

The results of the search for the e & thorn;e- & thorn; e --* eta 0 gamma 0 gamma process in the center-of-mass energy range from 1.075 to 2 GeV are presented. We analyze data with an integral luminosity of 746 pb-1 - 1 accumulated with the SND detector at the e & thorn;e- & thorn; e - collider VEPP-2000 in 2010-2024. The eta 0 0-* eta pi 0 pi 0 0 pi 0 decay is used in the analysis, with the subsequent eta and pi 0 0 decays into gamma gamma. Upper limits not exceeding 13 pb have been set on the e & thorn;e- & thorn; e --* eta 0 gamma 0 gamma cross section at the 90% confidence level.

The cross section for the e + e --* K S K L process is measured in the center-of-mass energy range from 1000 to 1100 MeV in the experiment with the SND detector at the VEPP-2000 e + e - collider. The measurement is carried out in the K S-* 2,r0 0 decay mode. Data with an integrated luminosity of 20 pb-1 - 1 recorded in 2018 at 18 energy points are used in the analysis. The systematic uncertainty in the measured cross section at the maximum of the % resonance is 0.9%. The mass, width of the % meson, and the product of the branching fractions B ( %-* K S K L ) B ( %-* e + e - ) are determined from the fit to the cross section energy dependence.

Results are presented from measuring the e^+e^-→ nn̅ cross section and effective neutron timelike form factor. Data are collected in 2020–2021 at the VEPP-2000 e^+e^- collider in the 1891 to 2007 MeV center-of-mass range of energies. The general purpose nonmagnetic SND detector is used to detect neutron–antineutron events. The time-of-flight approach is used to select nn̅ events. The measured cross section is 0.4–0.6 nbn. The neutron form factor in the investigated range of energies varies from 0.3 to 0.2.

The e^+e^-→ηγ cross section is measured in the center-of-mass energy range from 1.07 to 2.00 GeV in the decay channel η→ 3π^0 , π^0→γγ . The data set with an integrated luminosity of 242 pb ^-1 accumulated in the experiment with the SND detector at the VEPP-2000 e^+e^- collider is analyzed.

The results of the search for the e^+e^-→η^'γ process in the center-of-mass energy range from 1.075 to 2 GeV are presented. We analyze data with an integral luminosity of 746 pb^-1 accumulated with the SND detector at the e^+e^- collider VEPP-2000 in 2010–2024. The η^'→ηπ^0π^0 decay is used in the analysis, with the subsequent η and π^0 decays into γγ. Upper limits not exceeding 13 pb have been set on the e^+e^-→η^'γ cross section at the 90% confidence level.

The process e^+e^-→π^+π^-π^0 was studied in the energy range between 1.075 and 1.975 GeV on the basis of data characterized by an integrated luminosity of about 70 pb ^-1 that were accumulated in an experiment with the Spherical Neutral Detector (SND) at the VEPP-2000 e^+e^- collider. The respective Dalitz distributions were analyzed within a model that includes ρ(770)π , ρ(1450)π , and ωπ^0 intermediate states. As a result, the energy dependences of the total cross section for the process e^+e^-→π^+π^-π^0 and the cross sections for the ρ(770)π and ρ(1450)π intermediate states were measured along with the phase shift of the ρ(770)π amplitude with respect to the ρ(1450)π and ωπ^0 amplitudes. For the first time, a simultaneous approximation of the cross sections for the processes e^+e^-→ρ(770)π and ρ(1450)π and the relative phase of the respective final states was performed on the basis of the vector-meson dominance model. The contributions of the ω , ϕ , ω(1420) , and ω(1650) resonances were taken into account. The approximation showed that the decay ω(1650)→π^+π^-π^0 proceeds predominantly through the ρ(1450)π intermediate state, while the decay ω(1420)→π^+π^-π^0 receives a dominant contribution from the ρ(770)π mechanism. The results of this study refined the results of earlier measurements with the SND setup.

The e^+e^-→ nn̅ cross section was measured at center of mass (c.m.) energies from the threshold to 1908 MeV. The experiment to measure the cross section has been carried out at the VEPP-2000 e^+e^- collider in 13 energy points. The SND detector is used to detect the produced neutron–antineutrons ( nn̅ ) events. A special time measurement system on the calorimeter was used to select the time-delayed nn̅ events. The measured e^+e^-→ nn̅ cross section is 0.4–0.6 nb. The neutron effective timelike form factor in the energy range under study varies from 0.3 to 0.6.

The process e+e−→ωπ0→π+π−π0π0 is studied in the center-of-mass energy region 1.05–2.00 GeV using data with an integral luminosity of about 35 pb−1 collected with the SND detector at the VEPP-2000 e+e− collider. In the energy range under study, the value of the measured Born cross section varies from 0.7 to 18 nb. The statistical uncertainty of the cross section is 2%–23%, while the systematic uncertainty is in the range of 3.0%–14.2%. The results are consistent with previous measurements but have better accuracy.4 MoreReceived 6 September 2023Accepted 1 November 2023DOI:https://doi.org/10.1103/PhysRevD.108.092012Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHadron productionLepton-lepton interactionsPhysical SystemsLight mesonsPropertiesForm factorsParticles & Fields

The e+e− → ηγ cross section is measured in the center-of-mass energy range from 1.07 to 2.00 GeV in the decay channel η → 3π0, π0 → γγ. The data set with an integrated luminosity of 242 pb−1accumulated in the experiment with the SND detector at the VEPP-2000 e+e− collider is analyzed.

In experiment at the VEPP-2000 e(+)e(-) collider the process e(+)e(-) -> n (n) over bar has been studied in the energy from the threshold up to 2 GeV. To identify n (n) over bar events the multichannel NaI(Tl) electromagnetic calorimeter of the SND detector was used. The measured e(+)e(-) -> n (n) over bar process cross section is from 0.6 to 0.3 nb. The effective neutron timelike form factor is derived and compared with the proton form factor. The ratio | G(E)|/|G(M)| of the neutron electric and magnetic form factor is obtained from the measured angular distribution and found to be between 1 and 2.

The process $e^+e^- \to \omega\pi^0 \to \pi^+\pi^-\pi^0\pi^0$ is studied in the center-of-mass energy region $1.05-2.00$ GeV using data with an integral luminosity of about 35 pb$^{-1}$ collected with the SND detector at the VEPP-2000 $e^+e^-$ collider. In the energy range under study, the value of the measured Born cross section varies from 0.7 to 18 nb. The statistical uncertainty of the cross section is $2-23$%, while the systematic uncertainty is in the range of $3.0-14.2$%. The results are consistent with previous measurements but have better accuracy.

In experiment at the VEPP-2000 e^ + e^ - collider the process e^ + e^ - → nn̅ has been studied in the energy from the threshold up to 2 GeV. To identify nn̅ events the multichannel NaI(Tl) electromagnetic calorimeter of the SND detector was used. The measured e^ + e^ - → nn̅ process cross section is from 0.6 to 0.3 nb. The effective neutron timelike form factor is derived and compared with the proton form factor. The ratio | G E |/| G M | of the neutron electric and magnetic form factor is obtained from the measured angular distribution and found to be between 1 and 2.

The process $e^+e^- \to \omega\pi^0 \to \pi^+\pi^-\pi^0\pi^0$ is studied in the center-of-mass energy region $1.05-2.00$ GeV using data with an integral luminosity of about 35 pb$^{-1}$ collected with the SND detector at the VEPP-2000 $e^+e^-$ collider. In the energy range under study, the value of the measured Born cross section varies from 0.7 to 18 nb. The statistical uncertainty of the cross section is $2-23$%, while the systematic uncertainty is in the range of $3.0-14.2$%. The results are consistent with previous measurements but have better accuracy.

The process $$e^+e^-\rightarrow \eta \eta \gamma $$ is studied in the center-of-mass energy range 1.17–2.00 GeV using data with an integrated luminosity of 201 pb $$^{-1}$$ collected by the SND detector at the VEPP-2000 $$e^+e^-$$ collider. The $$e^+e^-\rightarrow \eta \eta \gamma $$ cross section is measured for the first time. It is shown that the dominant mechanism of this reaction is the transition through the $$\phi \eta $$ intermediate state. Our result on the $$e^+e^-\rightarrow \eta \eta \gamma $$ cross section is consistent with the $$e^+e^-\rightarrow \phi \eta $$ measurement in the $$\phi \rightarrow K^+ K^-$$ mode. The search for radiative processes contributing to the $$e^+e^-\rightarrow \eta \eta \gamma $$ cross section is performed, and no significant signal is observed.

AbstractThe process $$e^+e^-\rightarrow n{\bar{n}}$$ e + e - → n n ¯ is studied in the experiment at the VEPP-2000 $$e^+e^-$$ e + e - collider with the SND detector. The technique of the time measurements in the multichannel NaI(Tl) electromagnetic calorimeter is used to select $$n{\bar{n}}$$ n n ¯ events. The value of the measured cross section in the center-of-mass energy range from 1.894 to 2 GeV varies from 0.5 to 0.35 nb. The effective neutron timelike form factor is derived from the measured cross section and compared with the proton form factor. The ratio of the neutron electric and magnetic form factors is obtained from the analysis of the antineutron polar angle distribution and found to be consistent with unity.