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.

Recently, a γγ collider based on the existing 17.5 GeV linac of the European XFEL has been proposed. High-energy photons will be generated by Compton scattering of laser photons with a wavelength of 0.5–1 m on electrons. Such a photon collider covers the range of invariant masses W_γγ <12 Ge V/c^2 . The physics program includes spectroscopy of C -even resonances ( c -, b -quarkonia, 4-quark states, glueballs) in various J^P states. Variable circular and linear polarizations will help in determining the quantum numbers. In this paper, we present a summary of measured and predicted two-photon widths of various resonances in the mass region 3–12 Ge V/c^2 and investigate the experimental possibility of observing these heavy two-photon resonances under the conditions of a large multi-hadron background. Registration of all final particles is assumed. The minimum values of _γγ(W) are obtained at which resonances can be detected at a 5σ confidence level in 1 year of operation.

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 of elastic scattering of photons (γγ→ γγ, light-by-light scattering) has attracted significant interest in recent years as a loop-induced process sensitive to all charged particles. To date, this process has been studied through Delbrück scattering, photon splitting in the nuclear Coulomb field, and ultra-peripheral heavy-ion collisions at the LHC. Future precision measurements are anticipated at high-luminosity e^+e^- colliders (SuperKEKb, FCC, CEPC, ILC, CLIC), their e^-e^- options, and γγ colliders based on the backscattering of laser photons. In this paper, we show that background QED processes severely limit the study of elastic light-by-light scattering at e^+e^- colliders. For the e^+e^- case, the main background arises from e^+e^- annihilation into two photons within the detector acceptance after the initial electron and positron emit hard ISR photons at small angles. Since only two photons with a small total transverse momentum are registered in the detector in both cases, this process effectively mimics the γγ→ γγ signal with a significantly larger cross section at high invariant masses. Other relevant background QED processes for both types of collisions are also comprehensively analyzed and discussed.

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.

The process e^+e^-→ nn̅ is studied in the experiment at the VEPP-2000 e^+e^- collider with the SND detector. The technique of the time measurements in the multichannel NaI(Tl) electromagnetic calorimeter is used to select nn̅ 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.

Preliminary results of the SND experiment at the VEPP-2000 e + e − collider are presented. Data corresponding to about 40 pb −1 of integrated luminosity were col- lected in the e + e − c.m. energy range from 1.05 to 2.00 GeV. The cross sections for e + e − ! !� 0 , � + � − 4� 0 , p ¯ p, n¯ n have been measured.

The $${{e}^{ + }}{{e}^{ - }} \to n\bar {n}$$ cross section has been measured in the experiment at the VEPP-2000 $${{e}^{ + }}{{e}^{ - }}$$ collider with the SND detector. The technique of the time measurements in the multichannel NaI(Tl) electromagnetic calorimeter is used to select $$n\bar {n}$$ events. The value of the measured cross section in the energy range from the threshold up to 2 GeV is about 0.5 nb, which corresponds to the value of the neutron effective timelike formfactor ~0.3.

We review here recent results on [Formula: see text] annihilation to hadrons below 2 GeV obtained with the SND detector at the VEPP-2000 collider. Among others we report cross sections and dynamics properties for the [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text].

In the experiment with the SND detector at the VEPP-2000 e^+e^- collider, events of the process e^+e^-→ nn̅ were separated via measuring, in each counter of the multichannel calorimeter based on 1640 NaI(Tl) crystals, the time of delay of the signal from nonrelativistic antineutrons. The time resolution for events of the process e^+e^-→γγ was 0.8 ns. The measured time spectrum of delays of the signal from antineutrons in the calorimeter at the c.m. energy of 1902 MeV agrees with the results of calculations.